# Supercharger vs Turbocharger



## Jenisch (Jan 5, 2012)

Hello, 

I know the supercharger used the engine crankshaft to compress air, at the cost of more friction horsepower. If I understand correctly, aircraft such as the P-51 used multiple speed and stage superchargers in order to not suffer much with FHP.

The turbocharger, or turbo supercharger, was a combination of a supercharger and a turbine that used exhaust gases to be powered and compress more air. The turbine would play the main part, and FHP would be less. My understanding of the turbo is that it was more heavier and therefore intended for aircraft flying high with extremely powerful (and heavier) engines, in order to obtain full use of the horsepower in the thin air of high altitudes.

Can someone explain me if I'm wrong regarding the advantages of both?


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## davebender (Jan 5, 2012)

A turbocharger would cause some restriction to exhaust gas flow. How much does this reduce engine power?


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## Shortround6 (Jan 5, 2012)

About as much as flying at sea level.

General Motors published a booklet during WWII comparing the various supercharger set ups. They estimated a 1000hp engine at sea level would be good for 1080hp at 20,000ft *IF* you could maintain the same manifold pressure as the engine had at sea level without any additional power losses(can't be done in real life) due to decrease back pressure. US turbo set-ups had the waste-gates wide open at sea level and added very little additional back pressure to the engine. In fact the original turbo controls used on US aircraft used the back pressure to regulate the waste gate opening and thus the speed of the turbine and so the pressure going to the carburetor of the engine mounted supercharger.


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## davebender (Jan 5, 2012)

If I understand you correctly, turbo induced exhaust back pressure at high altitude is offset by lower back pressure as the air gets thinner.

On the other hand, supercharged engines should also benefit from lower back pressure as air gets thinner and they don't have a turbine blocking exhaust flow. So turbochargers must have a performance penalty even if relatively small.


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## Shortround6 (Jan 5, 2012)

turbocharged engine does have a performance "penalty" but it is not as great as a supercharged engine as altitude climbs. The US turbo set ups were supposed to maintain a constant pressure at the intake to the carburetor from sea level to the rated altitude of the turbo and a constant back pressure from sea level to the rated altitude. This constant pressure was supposed to be standard sea level pressure. This was the design goal, it was not really achieved in practice but they came close. the loss from back pressure was zero at sea level ( or darn close) since a non turboed engine would see about the same back pressure in it's exhaust manifolds. It is only as the aircraft climb that the none turboed plane sees any advantage due to lower back pressure. Of course the non turboed plane needs to use crankshaft power to run it's supercharger faster than the sea level speed used in in 2 speed super chargers or at the higher speeds of a a variable drive supercharger. in many cases the the higher speed of a two speed supercharger used roughly double the amount of power in used in low speed as did the supercharger on a DB601 when it went from maximum slip to minimum slip. Throw a second stage mechanical supercharger in and the power consumed by the superchargers could really add up. While a P&W R-2800 might suffer a "penalty of 160 hp at 20,000ft for using a turbo due to higher back pressure that seems pretty small compared to the 350 hp it takes to turn the auxiliary supercharger on a F4U or F6F at 20,000ft. 
This doesn't count the loss of exhaust thrust but some planes didn't make a lot of use of exhaust thrust, like the F4U-1 and even the F6f didn't use it really well. And exhaust thrust "power" varied with the speed of the plane. Very useful at high speed, a lot less useful when climbing.


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## GregP (Jan 5, 2012)

There are two types of supercargers, positive displaement and dynamic compressors.

A positive displacement type delivers about the same level of pressure increase at all speeds and a dynamic compressor gives more pressure increase as the speed rises. They can absorb as much as one third of the crankshaft power, but are very common where engine response is critical, such as top fuel dragsters, tractor pulls, etc. In many application, they are less efficient than turbochargers.

Positive displacement types include a roots type, a screw type, a sliding vane type,and a scroll type.

Dynamic compressors include the centrifugal type, the multi-stage axial flow type, and the pressure wave supercharger. All raidals had internal centrifugal supercargers, as did the Merlin, the Allison, and the Griffon.

They can all be belt driven, direct driven, gear driven, or chain driven.

In most applications, a turbocarger can make more power but will not give as good a throttle response (Recall the famous "turbo lag" in performance cars), and they required quantities of tungsten and other rare metals that many countries simply didn't have. 

In WWII, the best performance came from an engine with an internal supercarger (or a 2-speed or 2-stage supercharger) and a turbosupercarger as well, giving the best of both worlds. But, such a system is complicated, expensive and can be very difficult to troubnleshoot when something goes wrong.


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## tomo pauk (Jan 6, 2012)

Wasn't the turbo-lag an attribute of early turbo-equipped engines, back in 1980s? When driving in low RPM, engine was not supplying enough of exhaust gases for a turbine - the turbine being of non-variable geometry. So when a driver wants more engine power, the engine 'hessitates' until the exhaust gas pressure is built up sufficiently, so the turbine is accelerated.


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## Shortround6 (Jan 6, 2012)

Turbo-lag isn't really that important for WW II aircraft. While it might take a while for those big turbos to spool up, opening the throttle from cruise to full power also means the engine driven supercharger can cover for part of the time and with a 300-500 propeller acting as a flywheel instant rpm changes weren't going to happen anyway.


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## davparlr (Jan 6, 2012)

A tale of two engines.

A look at two R-2800 engines used toward the end of WW2, one supercharged and one turbo and supercharged, the R-2800-18W used in the F4U-4 and the R-2800-57, used in the P-47M/N.

Comparing weight w/accessories
-18W 2804 lbs (taken from F6F data)
-57 3277 lbs



Comparing hp to weight vs alt.

SL
-18W 2455 hp .84 hp/lb
-57 2800 hp .85

23k 
-18W 2080 hp .74 hp/lb
-57 2800 hp .85

30k
-18W 1760 hp .62 hp/lb
-57 2800 hp .85

I am sure there is more weight involved with the installation of the -57 engine with turbo than shown here, but I think this shows the trade off. The turbo is heavier and more complex with probably more maintenance issues. However, performance over altitude is impressive, and I am sure cannot be made up with exhaust thrust or back pressure issues.


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## wmaxt (Jan 6, 2012)

Turbo chargers are very good at several things one of the biggest is that a turbos can raise the apparent compression ratio which in turn makes more hp per lb of fuel. The difference between 9/1 and 10/1 compression is an increase in power and fuel efficiency of 10%. A turbo charger has more flexibility to provide additional boost that acts like added compression to the engine.

Mechanical superchargers are tied directly to the engine and provide "boost" based on the RPM of the engine/throttle opening. Turbos operate by using the heat expansion of the exhaust. 

At cruise a Merlin as set up in a P-51 had a fuel consumption of .60lbs/hp/hr at 2000RPM and 27"MAP producing 500HP 

A P-38 could by using 1600RPM and 31"MAP could produce 525HP at a fuel consumption of .46lbs/hp/hr.

The second thing Turbos are good at is a consistent power level an Allison rated at 1600HP at sea level can with a B-33 turbo maintain yhat power level to 25,000ft. A mechanical supercharger will have "steps" the power will be high then drop off as the air thins with altitude.


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## Vincenzo (Jan 6, 2012)

davparl -57 is a newest variant, i think is best compare a -21 with a -18


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## davebender (Jan 6, 2012)

Late war Daimler-Benz engines were typically more fuel efficient then the RR Merlin. 

How does a 1944 supercharged DB605AS engine compare to a 1944 turbocharged P-38 engine?


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## Denniss (Jan 6, 2012)

Most DB engines were tuned for 215-220 g/PSh specific fuel consumption on (max) continuous power settings. With max eco settings it was possible the got this down to 205-210 g/PSh


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## MikeGazdik (Jan 6, 2012)

wmaxt said:


> Turbo chargers are very good at several things one of the biggest is that a turbos can raise the apparent compression ratio which in turn makes more hp per lb of fuel. The difference between 9/1 and 10/1 compression is an increase in power and fuel efficiency of 10%. A turbo charger has more flexibility to provide additional boost that acts like added compression to the engine.
> 
> Mechanical superchargers are tied directly to the engine and provide "boost" based on the RPM of the engine/throttle opening. Turbos operate by using the heat expansion of the exhaust.
> 
> ...



Just a slight terminology correction. Forced induction, super or turbo, does not raise compression. It raises cylinder pressure. The engine still compresses the volume at the same rate, but more air and fuel is "forced" into the combustion chamber, which is still compressed at the same ratio. But because more mixture is compressed, it raised the cylinder pressure, which equals more power.


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## GregP (Jan 7, 2012)

Pounds per horseppower is not a fuel consumption number.

Maybe you were thinking of pounds per horsepower per hour?


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## davparlr (Jan 7, 2012)

Vincenzo said:


> davparl -57 is a newest variant, i think is best compare a -21 with a -18



I wanted to show turbo vs supercharger comparison with the last ww2 manifestations of the R2800. The P-47M/N was the contemporary of the F4U-4. The contemporary of the P-47D, with the -21 engine (I am not sure the -18 was used), would be the F4U-1 with the -8 engine, or the -21(water) with the -8(water). I think those comparisons would show a similar drop off of power with altitude. The comparison was meant to show the how the turbo-supercharger maintains hp with altitude as compared to the supercharged engine.


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## wmaxt (Jan 7, 2012)

Dinniss, That's why I said "apparent" compression ratio. As far as the engine is concerned its the same thing.

The down side of Turbo-superchargers is space/weight for a separate unit (the turbo) and the associated plumbing for exhaust and compressed air to the engine.

The second thing is that it takes pilots that understand how to use the flexibility of the turbo's capability.The P-38 suffered tremendously over northern Europe because pilots wer using poor techniques in engine management. High RPM and low MAP resulted in high fuel consumption and cold engines and turbochargers, when they went to full power the cold oil would not lubricate the engines/turbochargers (the turbo's have their own oil supply) and they failed. 

Also an important point the P-38 mixture had two settings Auto Lean and Auto Rich. Auto Lean is used up to 2300RPM and Auto Rich above that. The extra mixture setting gave the P-38 pilot the added flexibility to get both power and fuel economy. However if the throttle was maxed and the prop set to max power before the mixture was set to Auto Rich the engine would detonate and explode!


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## Shortround6 (Jan 7, 2012)

As far as the extra space/weight for the turbo charger is concerned we are comparing a turbocharged 2 stage supercharged airplane with inter cooler (or after cooler) to a mechanical two stage supercharged airplane with inter cooler. Comparing a two stage engine to a single stage engine makes little sense they have very different capabilities. The extra weight is just the turbine section of the turbo since you need the compressor section anyway even if you use a mechanical drive plus the weight and bulk of the exhaust pipes/ducts to get the exhaust to the turbine. Perhaps the turbo has to located further away from the engine than the mechanical for heat reasons and needs more ducting. 
The Merlin used a air/liquid after cooler instead of the the air to air inter cooler the US used and while this was somewhat more compact ( but see the space for the after cooler radiator) the Merlin made less power than the R-2800 and needed both less combustion air flow and less cooling airflow for the inter/after cooler. a 1600hp engine needeing roughly 2/3 the airflow of a 2400hp engine. The ducts can be smaller.


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## Vincenzo (Jan 7, 2012)

sorry i was confusing with engine variants

-57W data are for 72"? and that for 18W?


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## Siegfried (Jan 7, 2012)

Despite the recovery of exhaust energy to power the compressor this comes at a price.
1 The exhaust ducting and manifolding of the turbo adds considerable bulk and weight.
2 The energy in the exhaust was used very effectively in order to provide jet thrust: about 300lbs (135kg) jet thrust for a two stage Merlin and about 448lbs (200kg) for a Jumo 213E.
At altitude these propellor aircraft had jet thrust equal to 50% the thrust of a Derwent or Jumo 004 engine.

Adding all of these factors of greater bulk and lost jet thrust together and the turbo supercharger looks marginal for high speed aircraft where the jet thrust is of great import.

The turbo superchargers would appear to have an advantage at low speed cruise at high altitude but even here I would argue the advantage may not be as large as often assumed.

One clear advantage of the turbo is that it provides intrinsice flame damping and silencing. The P-51's Merlin was so noisy pilots found it very draining and fatiquing on long flights; the ducting and turbo of the PW R-2800 on a P_47 must have been an effective silencers. In a commercial aircraft the silencing effect of the turbo would have been of great import.


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## MikeGazdik (Jan 8, 2012)

wmaxt said:


> Dinniss, That's why I said "apparent" compression ratio. As far as the engine is concerned its the same thing.
> 
> The down side of Turbo-superchargers is space/weight for a separate unit (the turbo) and the associated plumbing for exhaust and compressed air to the engine.
> 
> ...




And you have hit on the "secret" to making the P-38 the weapon it could be. The technology was one thing, but training was another. The pilots that were properly trained made the Lightning the aircraft it should be. I think the turbosupercharged engine, particularly in the P-38, was _the_ escort fighter. The P-51 was economicaly the better answer but I feel the Lightning once fully developed with properly trained pilots was the better aircraft.


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## wmaxt (Jan 8, 2012)

Mike, IT was the escort fighter of WWII. The AAF did a study after the war and found that P-38 escorts provided the the best escort of the war losing fewer bombers than any other AAF fighter escort. However this has several things to keep in mind
1. The P-51 was freed from the bombers to follow the E/A allowing a greater bomber loss rate as a trade for a greater E/A destruction
2. The P-38 was identifiable from a greater distance so E/A often bypassed the escorted bombers for unescorted bombers.
3. P-38s had 98% close escort which is better for bombers and bad for "Scoring"

P-38's escorted bombers on 1,600 mi missions in mid '43 in the Med. and 1,700 - 1,800 mi missions to Singapore in '45 No other fighter could have done it.

The real question is a bit more simplicity for the mechanical supercharger or a bit more capability for the turbo.

Simplicity was the ticket for the ETO giving over 5,000 P-51s in less than 2 years to fight the Germans

In the PTO the added capability of the turbos allowed the P-38s to escort Mosquito's all the way to Singapore or the Dutch east Indies oil fields.


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## GregP (Jan 8, 2012)

Although not relly well understood, superchargers were MUCH better understood in WWII than turbochargers.

Sir Stanley Hooker was a supercharger genius and helped the merlin achieve what it did. Whish Allison had him on staff!

The turbochargers were OK, but not really well thought out in WWII. The P-39 installation was amateur at best.

From earlier, I never said turbo lag was a problem in WWII fighters, it was a general statement that turbo lag does not give as good a throttle response as a supercharger does, and that is true. Even the last turbochargerd Formula 1 cars were nowhere NEAR as driver friendly as the supercharged cars were.

Having both systems was heavy and complicated. I think the 2-speed, 2-stage supercharger was better for a WWII fighter, but the urbo guys will argue.

If I am not mistaken, the highest-flying WWII airplanes were both supercharged and turbosupercharged simulatneously.

The highest-flying aircraft of WWII were relatively experimental.

I believe the highest-flying were, in order:

1. Bristol 138: 62,407 feet; High-altitude research aircraft; Bristol Pegasus PE.VIS. Not really WWII, but higher than the rest and earlier, too.
2. Blohm und Voss BV.155: 57,305 feet, High-altitude-fighter; DB603A.
3. Henschel HS.130E: 50,712 feet; High-altitude recon; DB603B.
4. Focke-Wulf Ta-152H-1: 50,036 feet; fighter; MW-50 and GM-1 boost.
5. Aichi M6A Serian: 49,825 feet; submarine-launched seaplane; Aututa 32 V-12.

Extreme aircraft, not really friendly to maintenance or operational use.


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## Rick65 (Jan 9, 2012)

I am interested in the basis for the inclusion of the Seiran in your list as it seems contradictory to it's intended role and a quick internet search found multiple sources quoting a ceiling of 9900m or about 32,500ft.
However lots of internet information is recycled from a limited number of sources so if they are wrong so are all the rest.


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## Shortround6 (Jan 9, 2012)

GregP said:


> Although not relly well understood, superchargers were MUCH better understood in WWII than turbochargers.
> 
> Sir Stanley Hooker was a supercharger genius and helped the merlin achieve what it did. Whish Allison had him on staff!
> 
> ...



Just so we are clear here, The Merlin (and two stage Griffons) used two impellers ( superchargers ) on a single shaft driven from a 2 speed gearbox so that both impellers turn the same speed at the same time. The P&W two stage superchargers used two shafts. the "engine" supercharger was one speed while the auxiliary supercharger had 2 speeds AND a neutral. ALL United States service Turbo charged planes used a two stage set up, a single speed engine driven supercharger plus a variable speed turbine driven auxiliary supercharger (the turbo). The P-63 used an Allison with a single speed engine supercharger and a mechanical (hydraulic) variable speed auxiliary supercharger. To get the best performance (non racing) from a two stage system an intercooler of some sort must be used. With the two stage systems (of any kind) compressing the air 5-6 times the surrounding air at altitude the intake air temperature from the heating of compression can become the limiting factor on boost used. Only the P-63 did not use an inter cooler of some sort ( and Allison tried). The intercoolers were a limiting factor on the P-38 G and H models. 


GregP said:


> If I am not mistaken, the highest-flying WWII airplanes were both supercharged and turbosupercharged simulatneously.



Not sure what you mean by this. since ALL American service planes used both an engine driven and a turbocharger at the same time they are using them simultaneously.


GregP said:


> The highest-flying aircraft of WWII were relatively experimental.
> 
> I believe the highest-flying were, in order:
> 
> ...



1. The Bristol used a remote auxiliary supercharger driven by a shaft, I believe it was not clutched in until an altitude of over 30,000ft was reached? But no turbo. It did use a sizable intercooler however. see: http://www.airwar.ru/image/idop/xplane/b138/b138-4.jpg
2. Blohm und Voss BV.155 was turboed and kept an engine driven supercharger, those large housings out on the wings are the intercoolers which shows the problem of using enough superchargering (of any type) to operate at high altitudes. 
3. Henschel HS.130E, didn't use a turbo. It used a DB605 engine in the fuselage driving a large supercharger to supply air to the DB603 engines out on the wings. It worked but could hardly be called either light or compact.
4. Focke-Wulf Ta-152H-1 was supposed to use a 3 speed 2 stage supercharger and the GM-1 ( nitrous oxide) was a way of carrying supplemental oxygen in the plane to augment the air from the supercharger set up.


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## tomo pauk (Jan 9, 2012)

GregP said:


> Although not relly well understood, superchargers were MUCH better understood in WWII than turbochargers.
> 
> Sir Stanley Hooker was a supercharger genius and helped the merlin achieve what it did. Whish Allison had him on staff!
> 
> ...



Since just for 4 designs (B-17/24, P-38/47) it was produced some 180 thousands turboed engines in USA (plus what ever spares were produced for those), I'd reckon that turbo was a pretty mature stuff for the WW2. If Rolls-Royce P&W were superb in two-stage designs, that does not qualify the majority of mech-supercharged engines (= single-stage ones) their manufacturers as better than their turboed counterparts.
Formula I engine in turbo variation were never intended to be driver-friendly, but HP-monsters. IIRC the BMW design from the 1970s was featuring a turbo-charger as big as the 'naked' engine; the turbo-lag was huge, but so were 1300 HP from the 1500 cc.
But I agree that even in today's VTG turboed cars the lag can be felt, I drive one  Not that the lag matters to me.


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## davparlr (Jan 9, 2012)

Vincenzo said:


> sorry i was confusing with engine variants
> 
> -57W data are for 72"? and that for 18W?


I am not sure what the boost was for the 18W engine, but I don't think the Navy was using the 44-1 fuel so it most likely was not the same as the -57 engine. However, that was not the point, which was showing the difference turbos made. The F4U-4 lost 26% of its power, 620 hp, from SL to 30k, the P-47M/N lost zero. Difference increases as altitude goes up. At 35k F4U-4 loses 52% of its power (1230 hp), the P-47M/N loses only 7% (200 hp). As a further comparison, the noted high altitude Ta-152H loses 46% of its power (950 hp) at 35k, which helps explain why the P-47M is 20 mph faster than the Ta-152H at this altitude.


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## GregP (Jan 9, 2012)

The Serian was produced asboth a floatplane and as a landplane. One landplane was modified for high altitude. Yes, it was a prototype, but I could argue taht the Ta-152H was, too, since so few were produced. Main thing is, it happened during WWII. Naturally, all these altitude records have been eclipsed, even by drones, but during or before WWII, these planes flew what must be considered as the highest.

About the turbos, I KNOW the turbos were installed into bombers as well as the P-38's and P-47's. None of those installations were optimum and they were not at the cutting edge of power. I stand by mu statement taht turbocharging was not nearly as well understood as supercharging was in WWII, notwithstanding numerous small bomber engines taht were essentially turbo-normalized. That is, they were turbocharged so as to produce sea level rated pwoer at higher altitudes, not to give higher horsepower than otherwise possible.


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## Siegfried (Jan 9, 2012)

Shortround6 said:


> 3. Henschel HS.130E, didn't use a turbo. It used a DB605 engine in the fuselage driving a large supercharger to supply air to the DB603 engines out on the wings. It worked but could hardly be called either light or compact.



I think the fueselage engine was a DB605T, that is T for turbo, so turbo super charging was used, albeit indirectly.


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## wmaxt (Jan 9, 2012)

In the 1920's the Army Air Corps mandated that its aircraft be turbocharged, and experimentation on better turbo's began then. By the late 1930's turbo's were understood in themselves but practical experience was not really that great. The biggest problems were the metallurgy and the control of the related systems. On the whole Greg's assertion that turbo's or at least turbo's and their related systems were not well understood is true. The level of complexity required for their best usage was just being understood. Control of the later turbo's on the P-38 was much better than earlier versions. 

Mechanical superchargers had been researched by everyone else and were less technologically complex and pretty much perfected for aircraft by the late 1930's.

Supercharging as a whole was intended to maintain sea level performance at altitude, it wasn't until the later years of the war that higher boost was experimented with to boost performance over the sea level rating.


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## Shortround6 (Jan 9, 2012)

GregP said:


> About the turbos, I KNOW the turbos were installed into bombers as well as the P-38's and P-47's. None of those installations were optimum and they were not at the cutting edge of power. I stand by mu statement taht turbocharging was not nearly as well understood as supercharging was in WWII, notwithstanding numerous small bomber engines taht were essentially turbo-normalized. That is, they were turbocharged so as to produce sea level rated pwoer at higher altitudes, not to give higher horsepower than otherwise possible.



It is sort of yes and sort of no on the last part. The engine supercharger provided the boost at low altitude (sea level) over and above normal sea level pressure or power. While gear driven (or hydraulic ) could easily supply enough boost at sea level( SL) using a low ratio it took more power to turn a high ratio that could provide better power at altitude. In the late 30s engines with sigle speed superchargers were often described as ground boosted, medium supercharged or fully supercharged, depending on the gear ratio used on the supercharger. The ground boosted engine offered the most power at SL or take off because it's supercharger took the least power to drive, it heated then intake charge the least and allowed the engine to run with throttle plate/s wide open rather than part open like the altitude engines needed to do to keep from over boosting. The American turbo system allowed the use of ground boosted rated engines at high altitudes with little drop in performance. 
Let us not confuse the limits of the parent engine with the limits of the supercharger system. The P&W R-1830's development rather slowed down or practically stopped fairly early in WWII. P&W effort went into the R-2800 and the R-4360. Wright had to redesign the crankcase, crankshaft, rods, cylinders, cylinder hold down bolts and cylinder heads to get the R-1820 past 1300hp and get to 1425 HP. Allison went through a number of versions and at several points in time the engines in the P-38 could give more power than the equivelent engine/s in the P-39/P-40 because they could use a lower gear to drive the supercharger and depend on the turbo charger to make up and exceed the difference at altitude. I don't have my books in front of me at the moment but if memory serves an Allison with 8.80 gears was good for 1325 HP for take off and 1150 HP at 11,500-12,000 ft while a similar engine with 9.60 gears was good for 1200 HP at take off and 1150 HP at 15,000-15,500 ft. Some P-38 engines used 7.48 gears and had 1425 HP for take off and the turbo allowed this rating to be held much higher. So in a sense the turbo allowed higher power to be used. Air cooled engines didn't like over boosting as much as liquid cooled engines did. The R-2800s in the P-47M-N used a different cylinder and cylinder head with much different cooling abilities than earlier R-2800s.
It could take several hundred(300-400 for the bigger engines) horsepower to run a second stage super charger over and above the power needed to run the primary or 1st stage ( one closest to engine) and engine block,crank,cylinders,heads,etc needed to be able to stand up to the extra power even if it wasn't showing up as crankshaft or output power.


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## Shortround6 (Jan 10, 2012)

I think that the idea that "turbos" were not well understood is a mistake. The turbocharger is made up of two elements, the turbine that drives it and the compressor that actually does the supercharging. The compressor design wasn't much different than most people's mechanical drive compressors. In fact with General Electric supplying the compressor designs to both P&W and Wright until about 1937 and Allison making supercharger parts for both of them under subcontract to GE there may not have been much difference in design at all between them. Now it is true that by 1944/45 a lot had been learned by many countries about supercharger design compared to what was known in 1937 ( some 1930s German impellers look very strange to modern eyes) that is not the same thing.


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## wmaxt (Jan 10, 2012)

The principal, and operation of turbo's was thoroughly understood but just how to control them efficiently was just coming of age technologically. It took WWII to push the knowledge of turbochargers to their ultimate configuration.

Early P-38 turbo/waste gate controls worked but there were some operational and maintenance issues, by the J series very few problems were encountered and the systems were easier for the pilot to manage.

At wars end turbo compounding and related technologies were being tested. In the -90's turbocharging technology expanded greatly giving us much more responsive and efficient power than ever before. Today with electronics WWII designers could have only dreamed of and new materials and configurations are still changing the face of turbocharging.


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## Siegfried (Jan 11, 2012)

Shortround6 said:


> I think that the idea that "turbos" were not well understood is a mistake. The turbocharger is made up of two elements, the turbine that drives it and the compressor that actually does the supercharging. .



And the wastegate which needs to be controlled and its own ducting and exhaust. Diesels don't need one.


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## greybeard (Jan 11, 2012)

Turbosupercharger is definitely better than mechanically driven supercharger, especially for aircraft powerplants. Drawbacks, which are lost jet exhaust thrust, higher weight and so-called "turbo lag" are by far compensated by overall performance.

It's only a matter of available technology if one uses one or another.

During WWII only U.S.A. did have that technology and required materials available in quantity. Others like british and german were forced to apply only to jets.

GB


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## Shortround6 (Jan 11, 2012)

The Turbo lag doesn't really apply to aircraft. Aircraft seldom make multiple large throttle changes in a very short period of time. Both the propeller and the supercharger impellers act as rather substantial flywheels which limit rapid rpm change in any case. 
Modern turbo set ups should not be confused with WWII turbo set ups. In the post war era a number of civil aviation engines used gear driven superchargers (Lycoming, Continental, De havilland, and others) during the late 40s and 50s. By the early 60s almost none were left, they had been replaced by turbo superchargers much like the ones used in cars today. These were all single stage systems without intercoolers. 
The Turbo set ups of WW II were almost all (I may have over looked a few experimental set ups) two stage systems. The Turbos did impose penalties of weight, bulk and drag. Wither they were better or not depended on the altitude the plane was expected to operate at. There were cross over points, no one system was the best at all altitudes.


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## davparlr (Jan 11, 2012)

Siegfried said:


> Adding all of these factors of greater bulk and lost jet thrust together and the turbo supercharger looks marginal for high speed aircraft where the jet thrust is of great import.



If the Ta-152H with the Jumo 213E engine had been equipped with a P-47M type turbo supercharger, i.e., flat rated to 33k, instead of the GM-1 set up, its power available at 40k ft. would be approximately 1824 hp or about 900 hp more than the Ta-152H without GM-1. With GM-1, the turbo supercharged would have 744 hp more power. The numbers are probably even more favorable to the turbo since the 213 engine would not be powering the second stage compressor that the GM-1 version would have to do (see Shortround6 post #5), which would probably offset any jet thrust. Weight would probably not be a big issue since the GM-1 with is associated 85L of nitrous would not be carried. Bulk could be a factor, but, again, the GM-1 system with tank would not be carried. I suspect that the Germans understood this since it was trying to develop turbo engines, including the 213.

Can you imagine how the Ta-152H would perform it it had a flat rated engine to 33k, and over 90% SL power at 40k?


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## Shortround6 (Jan 13, 2012)

The advantages of the turbo-charger may have changed with both fuel and time.

For a hypothetical let's look at the Merlin in 1939. 

In MK III trim and with 87 octane fuel it was good for 880 HP for take off and kept increasing power until it hit 1030 HP at 16,250ft after which power fell off with altitude. As the MK X with a 2 speed supercharger it was good for 1075 HP for take off, 1130 HP at 5250 ft and 1010hp at 17,750 ft. Basically the 6.39 supercharger gear took much less power to drive, heated the air less and allowed a wider throttle opening than the 8.58 gears in the MK III while the 8.75 gears in the high gear setting of the MK X offered only slightly better power at altitude. Now we know that the basic engine would stand up to 1130 HP if not actually between 1200-1300hp with higher boost. 
If equipped with a turbo as an auxiliary supercharger the Merlin could have used an even lower gear that would have allowed between 1130-1200hp at sea level for take-off and kept that power all the way to 24-25,000 ft. Obviously some aircraft would have benefitted from that kind of power increase. 
However the down sides would be an increase of approximately 325-425lbs of power plant weight and the need for 10-13 cubic feet of volume in the aircraft per engine. A bomber may have been able to house such an installation much better than a small fighter like the Spitfire. The further from the engine the turbo goes the more volume is needed by the ducts leading to and fro. The more volume needed the larger (heavier) the fuselage and the more drag. In the Spitfire the space right behind the engine is taken by the fuel, which has to be on or close to the CG so moving it has limited options. 
Once 100 octane fuel became available the existing supercharger was able to use more boost down low (although it didn't change the power at over 16,250 ft) which helped the low altitude performance with no extra weight or drag. 
We now get into the exhaust thrust or power. 
I don't have the figures for the MK III or X but the figures for the MK XX are available. This somewhat more powerful engine is credited with the following in a Hurricane II.
At 15,000ft and 48.24in boost with a back pressure of 23.2in and a charge flow of 140.5pm/min the ejector HP was figured at 86.5hp at 325mph.
At 20,000 ft and 48.24 in boost and a back pressure of 22.3 in and a charge flow of 144.0lb/min the ejector HP was figured at 113 at 335 mph.
At 20,0000 ft and 50.67 in of boost and a back pressure of 23.2 in and a charge flow of 151.0 lb/min the ejector HP was 126.8 at 340 mph.
At 25,000ft and 42.12in of boost and a back pressure of 19.6in and a charge flow of 129.1lb/min the ejector HP was 107.2 at a speed of 330mph.
At 30,000ft and 34.30in of boost and a a back pressure of 16.3in and a a charge flow of 107.2lb/min the ejector HP was 89 at 317 mph. 

The ejector jet velocity in FPM was 1395, 1695, 1788, 1840, and 1901 respectively. 

While the jet thrust is a product of the charge weight (mass) and jet velocity the jet horse power is the jet thrust adjusted for speed or efficiency. The closer the match to the jet speed the speed of the aircraft is the more "power" you get from the same thrust. 
A Spitfire will get the same power from the exhaust as a Hurricane II if both at are at the same speed and a altitude but the Spitfire will get more benefit as it exceeds the speeds the Hurricane can go. A bomber will get proportionately less power from the exhaust as it can not travel ( Mosquito excepted) as those higher speeds. 
The more charge weight you can stuff through an engine the higher the jet thrust goes, an engine running 60in of manifold pressure should have about 33% more jet thrust than the same engine at 45in of manifold pressure but this requires fuel that will keep the engine from self destructing at 60in of manifold pressure.


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## Siegfried (Jan 13, 2012)

davparlr said:


> If the Ta-152H with the Jumo 213E engine had been equipped with a P-47M type turbo supercharger, i.e., flat rated to 33k, instead of the GM-1 set up, its power available at 40k ft. would be approximately 1824 hp or about 900 hp more than the Ta-152H without GM-1. With GM-1, the turbo supercharged would have 744 hp more power. The numbers are probably even more favorable to the turbo since the 213 engine would not be powering the second stage compressor that the GM-1 version would have to do (see Shortround6 post #5), which would probably offset any jet thrust. Weight would probably not be a big issue since the GM-1 with is associated 85L of nitrous would not be carried. Bulk could be a factor, but, again, the GM-1 system with tank would not be carried. I suspect that the Germans understood this since it was trying to develop turbo engines, including the 213.
> Can you imagine how the Ta-152H would perform it it had a flat rated engine to 33k, and over 90% SL power at 40k?




FW did try to develop the FW 190C "Kanguruh' with turbo charger: the result shows how difficult it could be to integrate a turbo-supercharger:





It seems to have been rejected on grounds related to reliabillity which was probably linked in with the alloys used: which were stainless steels. (Sicromal, the alloy used, seems to have become usable in the BMW 801T and BMW 003 turbojet via cooling methods)

I'm not sure how feasible it would be to combine high boost with Nitrous Oxide. However Nitrous Oxide did not have to be used in large quantities nor was it bulky: it was injected at a rate about equal to the fuel consumption. The rate the Ta 152 consumed was either 50, 80 or 130 (of my imperfect memmory) grams per 100 grams of fuel. The idea for the Me 109 and FW 190 had been to provide a tank of about 130L which could be used for water methanol, extra fuel or cryogenic NOX depending on mission profile however TA 152 had greater capacity however plumbing and control issues prevented the widespread use of this multipurpose arrangment.

There seems no doubt the turbo charger did provide better performance, the German super high altitude fighter the BV.155 used one and was expected to exede the improvised Ta 152H. (though turbo is conspicuously absent on the R-4360 of the B-36B)

About the only German use of the turbo supercharger was in the BMW 801T which was used in a few Ju 88S-3 and Ju 388L, these engines had the turbosupercharger, engine, cowling and intercooler integrated into one package


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## Siegfried (Jan 13, 2012)

Shortround6 said:


> The more charge weight you can stuff through an engine the higher the jet thrust goes, an engine running 60in of manifold pressure should have about 33% more jet thrust than the same engine at 45in of manifold pressure but this requires fuel that will keep the engine from self destructing at 60in of manifold pressure.



I think it was better than that, the two stage superchargers would also be blasting their air into a low pressure ambient, which also improves thrust. Higher pressure should lead to *both* higher mass flow and higher exhaust gas velicity. The thrust is given by F =dm/dt x v. Jet thrust is also of greater value at higher speeds where mach effects reduce airscrew efficiency


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## Shortround6 (Jan 13, 2012)

Siegfried said:


> I think it was better than that, the two stage superchargers would also be blasting their air into a low pressure ambient, which also improves thrust. Higher pressure should lead to *both* higher mass flow and higher exhaust gas velicity. The thrust is given by F =dm/dt x v. Jet thrust is also of greater value at higher speeds where mach effects reduce airscrew efficiency



Yes, the thrust ( and power) goes up with the reduction in outside air pressure. But the charge weight is decreasing once the plane is above critical altitude. As in the examples I gave above, the Merlin XX engine dropped from a charge weight of 144lbs at 9lbs boost (or close to it) at 20,000ft and an jet speed of 1695fps to 129.1 lbs at 25,000ft at just over 6lbs boost but with a jet speed of 1840fps. the difference in jet HP was 113 to 107. or look at the difference between 15,000ft and 30,000ft a difference of just 2.5hp in jut HP even though the charge weight went from 140.5lb/min to 107.2lb/min. the jet exhaust speed went from 1395fpm to 1901. the same book does give the details for 35,000ft. The charge flow is down to 84.5lb/min and the exhaust jet speed is actually dropping a bit to 1890fps ( or instrument error?) but in any case the jet hp is down to 65. Power to the propeller is 568hp. 
power to the propeller for 15,000ft, 20,000ft (48.24in boost), 20,000ft (50.67in boost), 25,000ft, and 30,000ft are, respectively--- 1048, 1073, 1126, 960, and 778hp.

Please note that the combined power at 25,000ft is 1067.2hp which is quite a bit shy of what a turbo-supercharged engine would have capable of delivering. A Turboed MK XX would have been capable of 1280hp if not more at 25,000ft to the propeller. Wither the extra 20% in power is worth the extra 400-500lbs and the extra bulk and drag of the installation is the question. 
The two stage mechanical drive engines will not only get more Jet thrust from the higher charge weights but because they are operating at higher cylinder pressures and exhaust pressures, but then they also need bigger, heavier superchargers/intercoolers and ducts and drag that go with them. They are also using more fuel to drive the supercharger. The Merlin XX was using anywhere from a minimum of 132hp at 35,000ft to drive the supercharger to a maximum of 236hp at 20,000ft (and the 50.67boost) for it's single stage supercharger in high gear. A two stage Merlin would probably need in excess of 400hp to run it's supercharger set up.


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## wuzak (Jan 14, 2012)

A two stage supercharger should, in theory, require less hp to drive than a single stage supercharger with the same boost and mass flow. Of course in the case of the two stage Merlins they ran more boost, or a higher overal pressure ratio (that is, maintained the boost to a higher altitude).


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## wuzak (Jan 14, 2012)

Siegfried said:


> though turbo is conspicuously absent on the R-4360 of the B-36B



The B-36's R-4360s didn't have one turbo - they had two. They used two C-series turbochargers, similar to the ones used by the P-47.

The B-29 used 2 B-series (also used on P-38, B-17 and B-24).

The choice of turbocharger was based on engine size and/or power output.


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## davparlr (Jan 14, 2012)

Siegfried said:


> FW did try to develop the FW 190C "Kanguruh' with turbo charger: the result shows how difficult it could be to integrate a turbo-supercharger:


There is no doubt that the bulk of the turbo system is a major design issue, especially with a very trim design like the Ta-152. If it could have been made to work it would probably been impressive. The P-47 seems to have solved the mass problem with massive, and bulky, hp, kind of like using a sledge hammer to get a square peg into a round hole. It worked, though.



Shortround6 said:


> ....



Great info on jet hp, thanks.


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## Siegfried (Jan 14, 2012)

wuzak said:


> The B-36's R-4360s didn't have one turbo - they had two. They used two C-series turbochargers, similar to the ones used by the P-47.
> 
> The B-29 used 2 B-series (also used on P-38, B-17 and B-24).
> 
> The choice of turbocharger was based on engine size and/or power output.



Are you sure? The B-36A and B-36B seem to have had only mechanical superchargers: the R-4360-51VDT never entered service.


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## fastmongrel (Jan 14, 2012)

Siegfried said:


> And the wastegate which needs to be controlled and its own ducting and exhaust. Diesels don't need one.



I might have misunderstood you there but a turbo diesel definitely needs a wastegate. Without a wastegate a turbo is going to suffer damage wether its a petrol or diesel.


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## Siegfried (Jan 14, 2012)

Shortround6 said:


> Please note that the combined power at 25,000ft is 1067.2hp which is quite a bit shy of what a turbo-supercharged engine would have capable of delivering. A Turboed MK XX would have been capable of 1280hp if not more at 25,000ft to the propeller. Wither the extra 20% in power is worth the extra 400-500lbs and the extra bulk and drag of the installation is the question.



We also need to absorb the extra high altitude power through a larger propeller and that propeller is only 85% efficient.



Shortround6 said:


> two stage Merlin would probably need in excess of 400hp to run it's supercharger set up.



Worth about 300lbs thrust (130kg or 1300N) this is worth at 440mph 200m/s (from P = F x v) 260kW at the prop but this would require around 315kW at the shaft, say 450hp?


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## Shortround6 (Jan 14, 2012)

Siegfried said:


> Are you sure? The B-36A and B-36B seem to have had only mechanical superchargers: the R-4360-51VDT never entered service.



From Joe Baugher's web site.

B-36A
"Engines: Six Pratt Whitney R-4360-25 Wasp Major air cooled radial engines, each rated at 3250 hp for takeoff and 3000 hp at 40,000 feet. Performance: Maximum speed 345 mph at 31,600 feet."

B-36B "Six 3500 Pratt Whitney R-4360-41 Wasp Major air cooled radial engines. Performance: Maximum speed 381 mph at 34,500 feet"

There is no way that is done with mechanical superchargers.


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## wuzak (Jan 14, 2012)

Siegfried said:


> Are you sure? The B-36A and B-36B seem to have had only mechanical superchargers: the R-4360-51VDT never entered service.



Yes, I'm sure.

I was looking at a schematic of the B-36 engine installation, with its two turbos hanging beneath one end of the R-4360, when I typed that.


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## wuzak (Jan 14, 2012)

The B-50's R-4360s also used two C-series turbos, the XB-44 used a mechanically supercharged R-4360.


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## WJPearce (Jan 14, 2012)

It is my recollection (from things I have read) that the B-36 had two conventional turbochargers. I also recall that for high altitude cruise, all the exhaust would be routed through one turbo. Of course the 4360s still had their mechanical supercharger too.

The VDT 4360 did not enter service just as stated by Siegfried. I believe in its final form the VDT engine did not have a supercharger, and the throttle was basically controlled by restricting the output of the "turbo"; hence the name VDT - Variable Discharge Turbine.


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## wuzak (Jan 15, 2012)

WJPearce said:


> It is my recollection (from things I have read) that the B-36 had two conventional turbochargers. I also recall that for high altitude cruise, all the exhaust would be routed through one turbo. Of course the 4360s still had their mechanical supercharger too.
> 
> The VDT 4360 did not enter service just as stated by Siegfried. I believe in its final form the VDT engine did not have a supercharger, and the throttle was basically controlled by restricting the output of the "turbo"; hence the name VDT - Variable Discharge Turbine.



I think there were several variations on the VDT theme. Some certainly did not have an engine stage supercharger, but some may have. Some also had compounding - that is, the turbine in the supercharger also fed power back to the crank.

What killed teh VDT in the end was no suitable control system. The one that flew in a B-50 had to be constantly monitored and adjusted by the engineer. Imagine teh workload if teh engineer had to deal with 6 of them?


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## Siegfried (Jan 15, 2012)

wuzak said:


> Yes, I'm sure.
> 
> I was looking at a schematic of the B-36 engine installation, with its two turbos hanging beneath one end of the R-4360, when I typed that.



Which version? B-36A, B or D? or latter?


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## wuzak (Jan 15, 2012)

The diagram didn't specify which version.

But I believe they were all essentially the same.


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## wuzak (Jan 15, 2012)

Seems I was wrong....they used B-series superchargers:



> Each engine was also provided with two General Electric Model B-1 (or BH-1 on later models) exhaust driven turbosuperchargers arranged in parallel. The Primary purpose of the turbos was not to increase the power rating of the engine. Instead, they allowed the sea-level power rating to be maintained up to 35,000 feet, with a gradual degradation at altitudes above that.





> The right-hand turbosupercharger on each engine also provided air for cabin pressurization. The flight engineer had teh ability to select dual or single opertaion of the turbosuperchargers for each engine. When single mode was selected, all exhausts gases were passed through the right hand turbo - there was no option to select using the left turbo only.



From Dennis R. Jenkins, _Magnesium Overcast, The Story of the Convair B-36_

Again, no particular model was specified - perhaps because all production models used the same engine setup.


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## WJPearce (Jan 15, 2012)

wuzak said:


> I think there were several variations on the VDT theme. Some certainly did not have an engine stage supercharger, but some may have. Some also had compounding - that is, the turbine in the supercharger also fed power back to the crank.



You are 100% correct. A number of different configurations were mocked up but I do not know how many were actually run. Some VDTs did have the mechanical super and some were turbo compound. The one I mentioned I think was intended for the B-36. I just wanted to separate out the standard turbos used on the B-36 vs the VDT engine which was not used.

The Jenkins book has a better schematic for the turbos but this one will do. I'm not sure of the specific model but you can see that number 16 is the turbocharger and it says there are two (2). It says the engine is a R-4360-41, which I think were used on the B-36B and some Ds, and Es.
http://www.air-and-space.com/ficon/handbook%20GRB-36D-III%2010-11%20l.jpg


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## wuzak (Jan 15, 2012)

Nice picture. Illustrates the turbos well.


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## Gixxerman (Jan 15, 2012)

My limited understanding of the supercharger v turbocharger in aircraft was that whilst the supercharger provided sterling service and was stretched (thanks to multi-stage/speed inter-cooling) to meet just about everything asked of it in allied service, the turbocharger was the superior unit for larger aircraft and very high altitudes.

Germany in fact I think illustrates the limitations of the supercharger better than most, whilst they had a beautifully elegant design (the infinitely variable fluid coupled supecharger) - according to a Robbie Coltrane TV show on the DB engines - they simply didn't have large supplies of the fuels required to max out the supercharger, unlike the allies, in general service.
Hence the permanent high altitude issues almost all German planes had.

Of course jets rendered this moot it is quite obvious Germany was prepared to sacrifice 'otto' engine R&D resources for the jets.....and had things not collapsed so quickly who knows maybe their choice could have paid off, tactically at least even if the strategic picture would change little.

Even turbo's only had a short period in the sun as turbo-prop engines rendered the piston engine obsolete and offered much better outputs altitude.


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## wuzak (Jan 15, 2012)

Gixxerman said:


> My limited understanding of the supercharger v turbocharger in aircraft was that whilst the supercharger provided sterling service and was stretched (thanks to multi-stage/speed inter-cooling) to meet just about everything asked of it in allied service, the turbocharger was the superior unit for larger aircraft and very high altitudes.
> 
> Germany in fact I think illustrates the limitations of the supercharger better than most, whilst they had a beautifully elegant design (the infinitely variable fluid coupled supecharger) - according to a Robbie Coltrane TV show on the DB engines - they simply didn't have large supplies of the fuels required to max out the supercharger, unlike the allies, in general service.
> Hence the permanent high altitude issues almost all German planes had.
> ...



The fuel limited the amount of boost that could be used, but was not the source of the high altitude issues.

All of the German engines that saw widespread service had single stage superchargers. With a single stage supercharger the pressure ratio that can be achieved is limited. At high altitudes their superchargers couldn't give enough boost to maintain power. There were several projects for 2 stage superchargers and turbocharged engines, but I don'tthink many, if any, saw service.


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## riacrato (Jan 16, 2012)

Jumo 213 E had two stage, 3 gears, supercharger and saw limited service near the end of the war in Fw 190 D-13, Ta 152 H, Ju 388 and I think some Ju 88 nightfighter versions.


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## Siegfried (Jan 16, 2012)

wuzak said:


> The fuel limited the amount of boost that could be used, but was not the source of the high altitude issues.
> 
> All of the German engines that saw widespread service had single stage superchargers. With a single stage supercharger the pressure ratio that can be achieved is limited. At high altitudes their superchargers couldn't give enough boost to maintain power. There were several projects for 2 stage superchargers and turbocharged engines, but I don'tthink many, if any, saw service.



The major German engines: BMW 801 and Jumo 211 had two speed superchargers by the time the war started while the Merlin, Allison had only one. 

The DB600 series was not so dependant on its supercharger: this engine used a compression ratio of up to 8.5:1 compared to levels below 6.4:1 for the Merlin.

High compression ratios provide for high power, high efficiencies. This means the engine needs less boost and so a single stage supercharger will provide sufficient altitude compensation.


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## wuzak (Jan 16, 2012)

Siegfried said:


> The major German engines: BMW 801 and Jumo 211 had two speed superchargers by the time the war started while the Merlin, Allison had only one.
> 
> The DB600 series was not so dependant on its supercharger: this engine used a compression ratio of up to 8.5:1 compared to levels below 6.4:1 for the Merlin.
> 
> High compression ratios provide for high power, high efficiencies. This means the engine needs less boost and so a single stage supercharger will provide sufficient altitude compensation.




Obviously it didn't, otherwise they would not have played with GM-1 and such like.

The high compression ratio means that at a given altitude the supercharger needs a lower pressure ratio to provide the air to the engine, but at higher alltitudes the pressure ratio will be beyond the supercharger's comfort zone, and it will take a lot more hp to drive than a 2 stage supercharger would.

As we have seen before, 2 speed supercharging does not in itself provide higher altitude performance. It may mean that there has to be less compromise in the supercharger settings, giving a higher altitude rating. But we do know that the BMW struggled for performance at altitude.


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## Shortround6 (Jan 16, 2012)

Siegfried said:


> The major German engines: BMW 801 and Jumo 211 had two speed superchargers by the time the war started while the Merlin, Allison had only one.



The Merlin X was announced at the 1938 Paris air show with a 2 speed supercharger. It was used in the Whitley bomber. 


Siegfried said:


> The DB600 series was not so dependant on its supercharger: this engine used a compression ratio of up to 8.5:1 compared to levels below 6.4:1 for the Merlin.
> 
> High compression ratios provide for high power, high efficiencies. This means the engine needs less boost and so a single stage supercharger will provide sufficient altitude compensation.



The Merlin always used a 6.0:1 compression ratio. While the higher compression ratios provide for higher efficiencies in all engines they only really provide higher power in un-supercharged engines. A higher compression gives you more power per unit of fuel/air burned but the higher compression ration ( for any particular fuel) limits the amount of boost that can be used and since the amount of boost governs the _TOTAL_ amount of fuel/air burned per power stroke the higher compression engine actually winds up being able to make less power. It was the German use of a large displacement slower turning engine OF THE SAME WEIGHT for the SAME POWER that allowed them to use lower boost to get the same power using the lower boost. The lower boost needed did provide higher critical altitude than the higher boost Merlin and Allison but only up to a certain point. Part of the reason the DB600 series was able to do "so well" at altitude was the fact that the 109 was one of the lighter fighter aircraft of WW II. Trying to use a DB600 series (not including the 603) in an 8-9,000lb fighter would have soon shown that it's altitude performance was wanting, at least up until the very last 605s. That was one reason for the GM 1 equipment. Supplemental oxygen for the engine becaseu the supercharger wasn't good enough. 

The Germans had used high compression ratio engines in WW I to maintain altitude performance in WW I with un-supercharged engines but that required gated throttles to prevent wide open throttle plates at low altitudes which would wreck the engines. As the Plane reached certain altitudes the the throttle lever was moved to a different slot which allowed a greater throttle opening.


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## Tante Ju (Jan 17, 2012)

Shortround6 said:


> Part of the reason the DB600 series was able to do "so well" at altitude was the fact that the 109 was one of the lighter fighter aircraft of WW II. Trying to use a DB600 series (not including the 603) in an 8-9,000lb fighter would have soon shown that it's altitude performance was wanting, at least up until the very last 605s.



He 177 was certainly no light but it is difficult to see how altitude performance was 'wanting'. I think you see thing from other side completely, you see, Bf 109 was one of the lighter fighter aircraft of WW II exact because DB 600 series was itself very light and compact.. the whole package. Good fuel consumption means less fuel needs carried for same range, and lack of two stage supercharger means no bulky intercooler, intercooler radiator needed, coolant requirement is less so small radiator will do. No wonder it was light.

There is decreasing dividients with increasing power via supercharge. More power is developed, yes, but more and more power is lost: cooling installation drag greater, weight is greater etc. 

Its easy to see costs of such system. For example Spitfire Mark type V and Mark Type IX are really same aircraft, save engine: Mark type IX has two stage supercharged engine, larger prop, intercooler, needs intercooler radiator, larger oil cooler, larger coolant cooler. Otherwise all same. Now V weights about 6500 lbs, IX about 7400 lbs. So you add 900 lbs or about 400 kg for to install two stage system. Probably closer to 1100 lbs if you consider that range was down by about 20 % too, so to sompensate you need to carry about 200 lbs more fuel, fuel installation.. Now two stage system makes much more power, butmuch of that power goes into nullify that 900 (1100) lbs extra weight.



> That was one reason for the GM 1 equipment. Supplemental oxygen for the engine becaseu the supercharger wasn't good enough.



Also I think because no supercharger was so good as GM 1.. and GM 1 was simply, easy added device to any aircraft where such performance was desire. Idea was to produce one engine tuned for generic altitudes, and add GM to these few planes requiring for more. Easier production, mainatiance.


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## tomo pauk (Jan 17, 2012)

Siegfried said:


> The major German engines: BMW 801 and Jumo 211 had two speed superchargers by the time the war started while the Merlin, Allison had only one.
> 
> ...



1. BMW-801 was in use 2 years after war started
2. If we really want to list down non-fighter, two-speed engines, than the R-1820, -1830, -2600 all were two-speed ones, with R-1830 even offering two-stage variant, unlike German engines
3. Merlin X is already covered by SR6


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## Tante Ju (Jan 17, 2012)

tomo pauk said:


> 1. BMW-801 was in use 2 years after war started
> 2. If we really want to list down non-fighter, two-speed engines, than the R-1820, -1830, -2600 all were two-speed ones, with R-1830 even offering two-stage variant, unlike German engines
> 3. Merlin X is already covered by SR6



DB 600 series was two speed from start, that is 1934 I think... 1938 in operation. Really two speed design is only give you flexibility between altitude range, it do not give *better* altitude performance. For example, Merlin XX was two speed design, Merlin 45 wasimplified Merlin XX in which only one speed was, but that was for high altitude, ie. max gear I know.


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## Juha (Jan 17, 2012)

Tante Ju said:


> lso I think because no supercharger was so good as GM 1.. and GM 1 was simply, easy added device to any aircraft where such performance was desire. Idea was to produce one engine tuned for generic altitudes, and add GM to these few planes requiring for more. Easier production, mainatiance.


 
People of course had different tastes, I'd not like to have an unprotected tank of highly explosive liquid in my fighter plane. And probably some others thought the same, after all GM 1 wasn't much used in 109s. And after the liquid was used only thing left was the weight of the system. Also all the problems in handling, storing and supplying explosive liguid. British also knew the system but IIRC they used it only in the rare high altitude Mossie XV, at least in fighters. IMHO the GM1 system was better suitted for recon planes, which tried to avoid combat than in fighters, which seeked combat.

Juha


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## Shortround6 (Jan 17, 2012)

tomo pauk said:


> 1. BMW-801 was in use 2 years after war started
> 2. If we really want to list down non-fighter, two-speed engines, than the R-1820, -1830, -2600 all were two-speed ones, with R-1830 even offering two-stage variant, unlike German engines
> 3. Merlin X is already covered by SR6



Lets not forget the Armstrong-Siddeley Tiger (first production engine with a two speed supercharger) and the Bristol Pegasus. The Bristol Hercules had two speeds by the time the BMW 801 came along.


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## Denniss (Jan 17, 2012)

Don't forget the 1936 Jumo 210D/E with two-speed supercharger, offering 680PS on take-off.


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## Shortround6 (Jan 17, 2012)

Tante Ju said:


> He 177 was certainly no light but it is difficult to see how altitude performance was 'wanting'.



Ok, I will bite on this one, Just how did the He 177 altitude performance compare to the B-17 or B-24. Operational use differed considerably from book figures. Service ceiling is usually the altitude at which a single plane (operating at _normal_gross weight can still climb at 100ft/min. Operational ceiling (much less commonly given) is the height at which a plane can still climb at 500ft/min. this is a much more useful limit as it covers the needs of a group of planes flying in formation. The Worst performing plane of the group may be in a position were it needs to go to maximum power in order to maintain it's formation position in a particular maneuver. Let us not forget that operational height could also be determined by weather (trying to fly under clouds) and by the fact that some systems may not have equaled the engines ability to operate at high altitudes. Crew heating (either cabin or flying suits) and oxygen may not have kept pace with engine development for prolonged high altitude operations. He 177s didn't do a lot of high altitude, large formation flying did they? 



Tante Ju said:


> I think you see thing from other side completely, you see, Bf 109 was one of the lighter fighter aircraft of WW II exact because DB 600 series was itself very light and compact.. the whole package. Good fuel consumption means less fuel needs carried for same range, and lack of two stage supercharger means no bulky intercooler, intercooler radiator needed, coolant requirement is less so small radiator will do. No wonder it was light.



I think that you too, are seeing things from one side only. The Early DB 600/601s were no lighter and not any more compact than the early Merlin or Allison. The last of the DB 605s also managed to pick up about 155kg of "dry" weight over the DB 601A so while they still may have been "compact" it was only about 60-100lbs lighter than a two stage Merlin not including accessories. A difference of 3-4% is not much to excited about. 



Tante Ju said:


> There is decreasing dividients with increasing power via supercharge. More power is developed, yes, but more and more power is lost: cooling installation drag greater, weight is greater etc.
> 
> Its easy to see costs of such system. For example Spitfire Mark type V and Mark Type IX are really same aircraft, save engine: Mark type IX has two stage supercharged engine, larger prop, intercooler, needs intercooler radiator, larger oil cooler, larger coolant cooler. Otherwise all same. Now V weights about 6500 lbs, IX about 7400 lbs. So you add 900 lbs or about 400 kg for to install two stage system. Probably closer to 1100 lbs if you consider that range was down by about 20 % too, so to sompensate you need to carry about 200 lbs more fuel, fuel installation.. Now two stage system makes much more power, butmuch of that power goes into nullify that 900 (1100) lbs extra weight.



Really? range was down 20% under the same conditions? what were they? or was the loss of range closer to 10%? 
Now what did they get for it? Compared to a Merlin III the the Merlin 61 got _double_ the horsepower at 30,000ft. Compared to a Merlin 45/46 they didn't get double the HP but a 50% increase. At certain altitudes of around 30,000ft and higher they got double the climb rate. Some times around 2 1/2 times the climb rate. Comparing at Spit V and a IX at 30,000ft gives the IX around 60-70mph more speed. It sure seems like there was at least some extra power to provide extra performance after nullifing _ALL_ that extra weight. 




Tante Ju said:


> Also I think because no supercharger was so good as GM 1.. and GM 1 was simply, easy added device to any aircraft where such performance was desire. Idea was to produce one engine tuned for generic altitudes, and add GM to these few planes requiring for more. Easier production, mainatiance.



Simple to add I will accept. maintenance I am not sure of. Extra weight and bulk rather depends on the aircraft and it's mission. Short range interceptor may have been fine with it. Which German Bomber was it that needed a 1400-1500lb installation (with N2O tanks full) that filled the rear half of the bomb-bay?


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## Siegfried (Jan 18, 2012)

wuzak said:


> Obviously it didn't, otherwise they would not have played with GM-1 and such like.
> 
> The high compression ratio means that at a given altitude the supercharger needs a lower pressure ratio to provide the air to the engine, but at higher alltitudes the pressure ratio will be beyond the supercharger's comfort zone, and it will take a lot more hp to drive than a 2 stage supercharger would.
> 
> As we have seen before, 2 speed supercharging does not in itself provide higher altitude performance. It may mean that there has to be less compromise in the supercharger settings, giving a higher altitude rating. But we do know that the BMW struggled for performance at altitude.



I would disagree with a fiar bit of what you've state about the DB600, Improvements in DB605 performance (aside from tuned scavenging and spark plugs) came about as a result of improvements in piston compression ratio and compressor fluid dynmamics, latter on still the supercharger was increased in size in the D and AS series engines but still remained a single stage. 1.98 ata boost, the maximum used by the 2000hp DB605DCM at CR of 8.5 is only 14 psig or 59 inches of mercury. To achieve somewhat lower power levels the Merlin needed 25psig (about 2.8 ata boost) for the same wieght and worse fuel consumption. Two stage compressors are not more efficient than single stage, they may draw less shaft power but this comes at the expense of intercooling which is basically throwing away energy into the intercooler for disposal by a bulky radiator of some kind.

Note I didn't say the DB605 with a single stage supercharger was better than engines with a two stage unit, I said it was far dependant on high supercharger compression ratios and provided sufficient performance; *it clearly still faded away slightly earlier but not at all by much especially the latter englarged supercharger versions*. The Merlin used its supercharger not to compensate for altitude but to overboost its engine while the DB605 did less so and it seems gained considerable fuel economy.

As far as the BMW 801 was converned, it used CR of around 6.5:1 as did allied engines, the aircraft did quite well at altitude besting Sabre Typhoons and Griffon XII Spitfires and P-40's. its supercharger was improved and so it could remained competitive to 25,000ft which met most needs.

The DB605 was eventually developed as the DB605L (with two stage supercharger for the Me 109K-14) and also the bigger DB603L and LA of the Ta 152C, however these engines didn't use intercoolers and hardly gained weight. An earlier DB605 derivative

The BMW 801 also was developed as a turbo version, the BMW801T and Q which had outstanding performance, while there was also a BMW 801F with apparently a two stage three speed supercharger for the BMW 801F (for Fw 190A-10),

However I would argue that the DB605 didn't need a two stage supercharger due to a minimal gain in performance.

GM-1 was used for very specialised reconaisance missions and Mosquito intercepts, some USAAF P-51s also used NOX in an attempt to chase Me 262.

For various reasons the Germans fell behined in engine power throughout 1943 but caught up in 1944, however lack of two stage superchargers is a not the cause of this lag.


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## Siegfried (Jan 18, 2012)

Juha said:


> People of course had different tastes, I'd not like to have an unprotected tank of highly explosive liquid in my fighter plane. Juha



It wasn't so bad, earlier versions were compressed NOX. Latter cryogenic versions were rendered combat safe (relatively speaking) by various methods.


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## Tante Ju (Jan 18, 2012)

Siegfried said:


> It wasn't so bad, earlier versions were compressed NOX. Latter cryogenic versions were rendered combat safe (relatively speaking) by various methods.



Used in GM-1 was NO2.. also known "Haha" gas. Completely harmless and non-flammable. Juha is simply complete wrong, and I am not sure why he would have problem with 100 liters of lets say "highly explosive" (which is not) gm1 if he has no problem with on other hand 1000 liters of highly explosive aviation fuel... regular carried of course by all plane.


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## Juha (Jan 18, 2012)

Tante Ju said:


> Used in GM-1 was NO2.. also known "Haha" gas. Completely harmless and non-flammable. Juha is simply complete wrong, and I am not sure why he would have problem with 100 liters of lets say "highly explosive" (which is not) gm1 if he has no problem with on other hand 1000 liters of highly explosive aviation fuel... regular carried of course by all plane.



Hello Tante Ju
Maybe you should read more, first of all it os N20, 2 really should be ½ line lower, and if you read instructions of its use as narcotic, smoking is absolutely forbidden in the spaces it is used, guess why, and why the N2O cylinders must be absolutely clean before filling. You might also look on page 100 in Fernández-Sommerau's Messerschmitt Bf 109 Recognition Manual, or simply think why fertillers with high Nitrogen component are good base material for IEDs. And all burning needs oxygen.

Fuel tanks in 41-45 were usually self-sealing, that's why they were not so dangerous than high pressure tank with potentially explosive liquid gas which could self-ignite when got in contact with oil, grease, rubber etc.

Juha


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## wuzak (Jan 19, 2012)

Siegfried said:


> I would disagree with a fiar bit of what you've state about the DB600, Improvements in DB605 performance (aside from tuned scavenging and spark plugs) came about as a result of improvements in piston compression ratio and compressor fluid dynmamics, latter on still the supercharger was increased in size in the D and AS series engines but still remained a single stage. 1.98 ata boost, the maximum used by the 2000hp DB605DCM at CR of 8.5 is only 14 psig or 59 inches of mercury. To achieve somewhat lower power levels the Merlin needed 25psig (about 2.8 ata boost) for the same wieght and worse fuel consumption. Two stage compressors are not more efficient than single stage, they may draw less shaft power but this comes at the expense of intercooling which is basically throwing away energy into the intercooler for disposal by a bulky radiator of some kind.



Merlin 66 +25psi, PN100/150 fuel, 2000hp @ 5,250ft. In 1944.

Two stage superchargers are more efficient than single stage superchargers, particularly for higher pressure ratios, which were required at higher altitudes.

The single stage supercharger will heat the air more for a given pressure ratio. The Merlin two stage engines used intercooling (cooling passageways in the supercharger housing) and aftercooling - the big water to air radiator mounted at the top of the back of the engine.

As the German engines used higher compression ratios they needed less pressure ratios from their superchargers. This less heating of the air and less need for ADI/aftercooling. Though, the MW50 system used in several German engines was ADI.

The intercooler prevented detonation by providing lower temperatures for the air to the engine, and ADI was not required.




Siegfried said:


> Note I didn't say the DB605 with a single stage supercharger was better than engines with a two stage unit, I said it was far dependant on high supercharger compression ratios and provided sufficient performance; *it clearly still faded away slightly earlier but not at all by much especially the latter englarged supercharger versions*. The Merlin used its supercharger not to compensate for altitude but to overboost its engine while the DB605 did less so and it seems gained considerable fuel economy.



All superchargers on WW2 aircraft were used to compensate for altitude. In the Merlin's case they did add extra boost.

Note that a single stage Merlin is equally capable of +18psi or +25psi boost (providing it has the necessary upgraded components) as the two stage engines. It will not be able to maintain the boost at the same altitudes, though.

The two stage engine was developed specifically for the high altitude Wellington that was under development. It was Lord Hives (or Rolls-Royce) that came up with the idea of sticking one in a Spitfire.

The two stage Merlins make more power at the higher altitudes - somewhere above 20,000ft. Unless they are tuned for low level work (like the Merlin 66).




Siegfried said:


> The DB605 was eventually developed as the DB605L (with two stage supercharger for the Me 109K-14) and also the bigger DB603L and LA of the Ta 152C, however these engines didn't use intercoolers and hardly gained weight.



The DB605L used MW50 - which is a form of ADI. The purpose of MW50 is exactly the same as the inter/aftercooler. The difference is that once it is used it is gone. Rolls-Royce could have done a similar thing, but chose not to.




Siegfried said:


> As far as the BMW 801 was converned, it used CR of around 6.5:1 as did allied engines, the aircraft did quite well at altitude besting Sabre Typhoons and Griffon XII Spitfires and P-40's. its supercharger was improved and so it could remained competitive to 25,000ft which met most needs.



Sabres, MkXII Spitfires and P-40s all used single stage engines. The XII was developed as a quick way to fight Fw190s.

Most Rolls-Royce engines used 6.0:1 compression ratios. Some Allisons had 6.5:1, many later ones had 6.0:1.

25,000ft may have met "most" needs - but couldn't combat PR Mossies and late mark bombers zipping across the sky at 30,000ft+.




Siegfried said:


> The BMW 801 also was developed as a turbo version, the BMW801T and Q which had outstanding performance, while there was also a BMW 801F with apparently a two stage three speed supercharger for the BMW 801F (for Fw 190A-10),
> 
> However I would argue that the DB605 didn't need a two stage supercharger due to a minimal gain in performance.



The turbo version is a two stage system of another type.

If there was aminimal gain in performance why did they persist in developing 2 stage systems? The answer is they didn't want improved performance, they wanted improved altitude performance.




Siegfried said:


> GM-1 was used for very specialised reconaisance missions and Mosquito intercepts, some USAAF P-51s also used NOX in an attempt to chase Me 262.



Why did they use GM-1 for chasing Mossies? Perhaps because they needed better altitude performance?





Siegfried said:


> For various reasons the Germans fell behined in engine power throughout 1943 but caught up in 1944, however lack of two stage superchargers is a not the cause of this lag.



Fuel grade may be one reason. 

But the higher octane fuels in allied aircraft allowed more boost at lower altitudes. That is, the critical altitude of the engine became lower, but would make the same hp at the critical altitude of the engine using the lower octane fuel.

So, higher octane fuels didn't allow for higher altitude performance. That had to be gained by work on the supercharger.


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## Tante Ju (Jan 19, 2012)

Juha said:


> Hello Tante Ju
> Maybe you should read more, first of all it os N20, 2 really should be ½ line lower, and if you read instructions of its use as narcotic, smoking is absolutely forbidden in the spaces it is used, guess why, and why the N2O cylinders must be absolutely clean before filling.



Because medical tanks actually contain a mix of N2O and 50-70% oxigene? Its oxigine that is flammable. Also please read sign:







Again, I am sorry you are not right in this matter.N2O Nitrogen oxide is complete unflammable, read up please. It is only flammable when it decomposes at high temperature to oxygene and nitrogene, ie. when its already in the engine, doing the thing it is supposed to do.

Nitrogen dioxide, NO2 on other hand is flammable, but it is not what is in GM-1. Perhaps this is root of confusion.



> You might also look on page 100 in Fernández-Sommerau's Messerschmitt Bf 109 Recognition Manual,



I do not have book. What does it say?



> or simply think why fertillers with high Nitrogen component are good base material for IEDs. And all burning needs oxygen.



Chemistry simply does not work this way.. Compund has different properties than individual element. Nitrogen in fertilzer is good for explosive. Hydrogene is explosive, oxygene needs for burning. By logic it follows that if I mix fertilizer with water (H2O) I get some highly explosive stuff, right..? No, I actually don't.. I get just a stinky goo.



> Fuel tanks in 41-45 were usually self-sealing, that's why they were not so dangerous than high pressure tank with potentially explosive liquid gas which could self-ignite when got in contact with oil, grease, rubber etc.



Nitrous oxide (N2O) is not potentially explosive, its completely non-flammable especially not in liqued form. Second you seem to mix it bit: "high pressure tank with potentially explosive liquid gas" - no such thing was ever used by German..

There was (i believe, steel) high pressure bottles, which I believe stored Nitrous oxide (GM-1) in gas form, similiar to oxygene bottles that were on any and every plane.
Then there was low pressure, insulated tank, this was simple aluminium I believe. It stored cooled Nitrous oxide gas that because of relative low temperature in liquid form, so no pressurization needed.

Now, thinking about bit, GM-1 (N2O, Haha, "Nitrous") was probably not preferred on Allied engines because it is more problem on carburrator engines, more risk of backfire. Not really on German engines, all of which were direct fuel injected.


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## Juha (Jan 19, 2012)

Tante Ju said:


> Because medical tanks actually contain a mix of N2O and 50-70% oxigene? Its oxigine that is flammable. Also please read sign:



Hello Tante Ju
At least in Finland they sell it 100% pure, when given to the patient, it had to be mixed so that the mixture incl at least 21% oxygen. It is inflameable but "Oxidant. Strongly supports combustion. May react violently with combustible materials." as it stands in the data sheet.



Tante Ju said:


> Nitrogen dioxide, NO2 on other hand is flammable, but it is not what is in GM-1. Perhaps this is root of confusion.



Now read my previous message, I know it is N2O.




Tante Ju said:


> I do not have book. What does it say?



"Potentially explosive, the gas demanded very careful handling" Now in that I made a mistake, it isn't highly explosive only potentially explosive.



Tante Ju said:


> Chemistry simply does not work this way.. Compund has different properties than individual element. Nitrogen in fertilzer is good for explosive. Hydrogene is explosive, oxygene needs for burning. By logic it follows that if I mix fertilizer with water (H2O) I get some highly explosive stuff, right..? No, I actually don't.. I get just a stinky goo.



Frankly, I have had lectures on how to make IEDs and because of obvious reasons would not go into details, but I'd say what you wrote isn't entirely correct.




Tante Ju said:


> Nitrous oxide (N2O) is not potentially explosive, its completely non-flammable especially not in liqued form. Second you seem to mix it bit: "high pressure tank with potentially explosive liquid gas" - no such thing was ever used by German..
> 
> There was (i believe, steel) high pressure bottles, which I believe stored Nitrous oxide (GM-1) in gas form, similiar to oxygene bottles that were on any and every plane.
> Then there was low pressure, insulated tank, this was simple aluminium I believe. It stored cooled Nitrous oxide gas that because of relative low temperature in liquid form, so no pressurization needed.



In that you are probably right, there were 3 systems, in 2 the gas was stored in 4 or 8 cylinders/bottles and might have pressuried, the 4 bottles system was behind the pilot and was protected by a steel plate, but the 3rd system, used in 109G-5, it was in a insulated unprotected light alloy tank behind the pilot.

Juha


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## davparlr (Jan 19, 2012)

Tante Ju said:


> r
> Nitrous oxide (N2O) is not potentially explosive, its completely non-flammable especially not in liqued form. Second you seem to mix it bit: "high pressure tank with potentially explosive liquid gas" - no such thing was ever used by German..


 
I am not so sure of this non-flammable designation. Liquid O2 is also non-flammable. Only, it makes everything else burn. Just ask the Apollo astronauts. I always considered the liquid oxygen on the aircraft as far more dangerous than jet fuel. Now I really don't know much about Nitrous, but if it is an oxidizer, it could be very dangerous. What did the yellow tag say on the bottle?


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## Siegfried (Jan 19, 2012)

Juha said:


> Fuel tanks in 41-45 were usually self-sealing, that's why they were not so dangerous than high pressure tank with potentially explosive liquid gas which could self-ignite when got in contact with oil, grease, rubber etc. Juha



It's not high pressure effectively zero pressure, its cryogenic. N2O is weakly cryogenic. As I am overseas I don't have my library, however there is a reference to the N2O system being made safe in "junkers aircraft and engines" by Anthony Kay in reference to the Ju 88S-1. It's a non inflamable liquid that could be protected by some kind of a self sealing system. Obviously as an oxidiser it needs to be kept away from flamable materials.


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## Juha (Jan 19, 2012)

Hello davparlr
the yellow tag normally means Oxidizing substances.

Juha


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## Juha (Jan 19, 2012)

Siegfried said:


> It's not high pressure effectively zero pressure, its cryogenic. N2O is weakly cryogenic.



Yes, I corrected that in my message #77



Siegfried said:


> ... however there is a reference to the N2O system being made safe. It's a non inflamable liquid that could be protected by some kind of a self sealing system.



Yes but it is as the data sheet of it says ""Oxidant. Strongly supports combustion. May react violently with combustible materials." The last part made self-sealing difficult maybe impossible, at least with the materials Germans used to self-seal their fuel tanks and according to Fernández-Sommerau it wasn't self-sealed, at least not in the 3rd system used in 109G-5 and I really doubt that the 2 earlier system were self-sealing.

Juha


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## wmaxt (Jan 19, 2012)

Nitrous itself is not flammable.

Nitrous does three things, 

First - it cools the Air/Fuel charge allowing more (denser charge) to be stuffed into the combustion chamber and 

Second - the cooling reduces detonation 

Third - it adds oxygen - EVERY Nitrous system adds extra fuel in proportion to the oxygen in the Nitrous charge to get maximum power.


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## Shortround6 (Jan 22, 2012)

Siegfried said:


> I would disagree with a fiar bit of what you've state about the DB600, Improvements in DB605 performance (aside from tuned scavenging and spark plugs) came about as a result of improvements in piston compression ratio and compressor fluid dynmamics, latter on still the supercharger was increased in size in the D and AS series engines but still remained a single stage. 1.98 ata boost, the maximum used by the 2000hp DB605DCM at CR of 8.5 is only 14 psig or 59 inches of mercury. To achieve somewhat lower power levels the Merlin needed 25psig (about 2.8 ata boost) for the same wieght and worse fuel consumption. Two stage compressors are not more efficient than single stage, they may draw less shaft power but this comes at the expense of intercooling which is basically throwing away energy into the intercooler for disposal by a bulky radiator of some kind.



Improvements to the 605 over the 601 came from a number of things, 1. a 5.3% increase in displacement. 2. a 12-15% increase in rpm (2400-2500rpm to 2800rpm). 3. an 9% increase in boost (1.3 Ata to 1.42). Since some of these improvements compound it means that the increased valve overlap, increased compression ratio and improved compressor fluid dynamics are responsible for under 1/2 the improvement. 

With the later versions it was mostly the increased boost as can be seen by comparing the percentage increase in boost to the percentage increase in power. It is not exactly the same but it is close. The fact that some times (certain models) of the 605 used "over boost" to reach their power levels is also evident by looking at some of the charts on Kurfurst's site. By over boost I mean using more boost than can be sustained to the critical height of the previous model. 

Two stage compressors *ARE* more efficient than single stage compressors for a given level of boost. This was not new even in the late 30s, Mercedes used a two stage supercharger on their 1939 Grand Prix cars for that reason. comparing one supercharger to another at different levels of boost isn't really comparing the superchargers. 

This line is a real hoot (joke) "at the expense of intercooling which is basically throwing away energy into the intercooler for disposal by a bulky radiator of some kind"

While true in a very theoretical sense regarding the whole system as a "heat engine" for ultimate fuel efficiency from a practical, real world sense the intercooler is a necessity to get high power output. High fuel efficiency and high power from the size size/weight engine not really being possible at a given level of technology. Charge cooling does at least three things for an engine. Since the power limitation comes from oxygen that can be moved through an engine per unit of time (mechanical strength being ignored) the more air means the more power. We mean the mass or weight of air. At a given pressure say 1.8 ata (12 lbs boost) the actual amount of air is also governed by the temperature of the air. The lower the temperature the more dense the air is even at the same pressure and the more weight (mass) of air there is. Charge cooling means higher destiny air at the same pressure for more power. The second thing is the fact that at times the engine is material limited. Certain parts of the engine can only operate at certain high temperatures limits without quickly failing. Intake air temperature affects the temperature of the charge flowing through the engine at all points. Increase the intake charge temperature by 200 degrees and peak temperature in the cylinder will be 200 degrees higher and the exhaust temperature will be 200 degrees higher. using charge cooling can affect things like exhaust valve life. The third thing is that there is a limit to the amount of boost, cylinder compression and charge temperature that a particular engine (they vary a bit due to spark plug placement, exhaust valves, combustion chamber shape and cooling) will tolerated before getting to detonation levels with a particular grade of fuel. The lower the intake charge temperature the more boost and/or cylinder compression can be used. 
MW/50 is a form of charge cooling. The extra fuel sprayed into the supercharger intake of the BMW 801D did some charge cooling in addition to providing the extra fuel. GM-1 did some charge cooling in addition to providing the extra oxygen. Some late model Jumo 211s used an intercooler (or more properly an after cooler) on a single stage supercharger (perhaps the only single stage supercharger to use an inter/after cooler in WW II ?). For the Jumo 211 it was worth about 100 extra horsepower or several thousand ft of altitude. Of course it fit much better on bomber type aircraft than on small fighters. 

There are figures around for the Merlin XX, it's supercharger in high gear heated the intake charge about 148 degrees C over the incoming air. when providing 9lb of boost. It had a pressure ratio of around 3.5 to 1. Higher pressure ratio superchargers are going to heat the intake charge much more (the Merlin 61 could compress the air 5.1 times at 23,500ft) and such high supercharger ratios at high altitude would have been unusable due to detonation problems without some form of charge cooling. Please note that the WER ratings on the P&W R-2800 were achieved using BOTH intercoolers and water/alcohol (MW/50) injection. 


When assessing the supercharger set ups and what they achieved it is also good to remember in what order things happened and why. 


Siegfried said:


> Note I didn't say the DB605 with a single stage supercharger was better than engines with a two stage unit, I said it was far dependant on high supercharger compression ratios and provided sufficient performance; *it clearly still faded away slightly earlier but not at all by much especially the latter englarged supercharger versions*. The Merlin used its supercharger not to compensate for altitude but to overboost its engine while the DB605 did less so and it seems gained considerable fuel economy.



When assessing the supercharger set ups and what they achieved it is also good to remember in what order things happened and why. When the Merlin 61 was developed and introduced it's purpose was to improve altitude performance. Initial Boost was limited to 15lbs which was achievable with the single stage supercharger at low altitudes. Getting 15lbs of boost ( 60in manifold pressure) at 23,500 ft was NOT achievable with a single stage supercharger at that time. The Merlin needed a strengthened supercharger drive to get to 18lbs of boost and beyond and better fuel or water injection in order to use the higher than 18lbs boost at any altitude. The water injection was used in non flying experimental engines. It took several years to get to the 21 and 25lb boost levels even at low altitudes after the 2 stage supercharger was introduced. The P&W two stage engines were always about altitude performance as were the the American turbo installations, all two stage installations.


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## tomo pauk (Jan 22, 2012)

Another great post


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## wuzak (Jan 23, 2012)

Nice post SR.


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## Siegfried (Jan 23, 2012)

Shortround6 said:


> Improvements to the 605 over the 601 came from a number of things, 1. a 5.3% increase in displacement. 2. a 12-15% increase in rpm (2400-2500rpm to 2800rpm). 3. an 9% increase in boost (1.3 Ata to 1.42). Since some of these improvements compound it means that the increased valve overlap, increased compression ratio and improved compressor fluid dynamics are responsible for under 1/2 the improvement.




Multiply the displacement increase by RPM increase by boost increase one barely gets a 36% increase.

An increase of compression ratio from 6.9 (DB601) to 8.5 (DB605DC) however figures prominently.

Valve overlap and multipoint fuel injection can get 10% to 15% more power.

The DB605A and AS/ASM had a CR of 7.5 the latter DB605DB, DC 8.5.




Shortround6 said:


> With the later versions it was mostly the increased boost as can be seen by comparing the percentage increase in boost to the percentage increase in power. It is not exactly the same but it is close. The fact that some times (certain models) of the 605 used "over boost" to reach their power levels is also evident by looking at some of the charts on Kurfurst's site. By over boost I mean using more boost than can be sustained to the critical height of the previous model.



The DB605 series has a quirky proportionality of boost levels at sea level and power increase however at full power height it was about 150hp more.




Shortround6 said:


> Two stage compressors *ARE* more efficient than single stage compressors for a given level of boost. This was not new even in the late 30s, Mercedes used a two stage supercharger on their 1939 Grand Prix cars for that reason. comparing one supercharger to another at different levels of boost isn't really comparing the superchargers.



Two stage compressors are used when technology is inadaquet to produce the required compression in a single stage. 



Shortround6 said:


> This line is a real hoot (joke) "at the expense of intercooling which is basically throwing away energy into the intercooler for disposal by a bulky radiator of some kind"



It is true. Modern gas turbines are renowned for reducing the number of stages as part of efficiency improvements.

Of course it all gets down to detail design, but its clear the DDs engineering avoided dependance on high supercharger pressures and intercooling; so touting the Merlins intercooler supercharger setup rather ignores the point that it was in a way a fix to the limitations of carburation and throttle body injection.



Shortround6 said:


> While true in a very theoretical sense regarding the whole system as a "heat engine" for ultimate fuel efficiency from a practical, real world sense the intercooler is a necessity to get high power output.



From a practical point of view the DB series managed to operate high piston compression ratios and avoid the need for high pressure ratio superchargers and inter-coolers.

High compression ratios allow greater expansion of the hot gases and therefore more power and efficiency. There is more danger of pre ignition, however no energy is lost as in intercooling and while intercooling can clearly demonstate advantages it comes at a price as well.



Shortround6 said:


> High fuel efficiency and high power from the size size/weight engine not really being possible at a given level of technology.



The DB605DC managed 1850hp on C3 a 96/130 fuel without MW50 it weighed 745 kg.
The Merlin (66) seemed to need 100/150 fuel (really 110/150) to do the same it weighed 744kg.

The Daimler Benz engine seems to have been more fuel efficient, suggesting a smaller radiator as well.

Higher power levels (eg Merlin 100) resorted to 100/150 plus ADI.




Shortround6 said:


> Charge cooling does at least three things for an engine. Since the power limitation comes from oxygen that can be moved through an engine per unit of time (mechanical strength being ignored) the more air means the more power.



SNIP



Shortround6 said:


> There are figures around for the Merlin XX, it's supercharger in high gear heated the intake charge about 148 degrees C over the incoming air. when providing 9lb of boost. It had a pressure ratio of around 3.5 to 1. Higher pressure ratio superchargers are going to heat the intake charge much more (the Merlin 61 could compress the air 5.1 times at 23,500ft) and such high supercharger ratios at high altitude would have been unusable due to detonation problems without some form of charge cooling. Please note that the WER ratings on the P&W R-2800 were achieved using BOTH intercoolers and water/alcohol (MW/50) injection.
> 
> 
> When assessing the supercharger set ups and what they achieved it is also good to remember in what order things happened and why.



Again the boost level of 9 psi is equal to 1 ata plus 9/14 = 1.65, a level not introuced (1.7 ata) early 1944 on the DB engine.

The technology the DB605 used simply didn't need high boost levels nor did it need inter cooling.
The supercharger was simply less importation a factor to this engine.

The DB605L for the almost in service Me 109K-14 (and DB603LA) had two stage superchargers but did not need to use inter cooling




Shortround6 said:


> When assessing the supercharger set ups and what they achieved it is also good to remember in what order things happened and why. When the Merlin 61 was developed and introduced it's purpose was to improve altitude performance.



For the high altitude Wellington, supposedly.



Shortround6 said:


> Initial Boost was limited to 15lbs which was achievable with the single stage supercharger at low altitudes. Getting 15lbs of boost ( 60in manifold pressure) at 23,500 ft was NOT achievable with a single stage supercharger at that time.



1.8 ata boost 11 psig
1.98 boost 14 psig.
The DB605 could get away with a single stage supercharger.


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## Shortround6 (Jan 23, 2012)

Instead of discussing the advantages and disadvantages of the different systems you seem to want a Rolls Royce vs Daimler Benz argument. 



Siegfried said:


> Multiply the displacement increase by RPM increase by boost increase one barely gets a 36% increase.



That "barely gets a 36% increase" would have been good for for 1598hp at sea level for a DB 605A compared to a DB601Aa instead of the 1475. at 3700meters it should have been good for 1496hp (ok I am using HP instead of PS for convenience but all I am comparing here are DB engines) instead of about the 1400 hp/PS the 605A got at that altitude. Since the 605A did NOT display even a 36% total increase in power over the 601Aa it is rather hard to see how those 3 improvements didn't contribute significantly to the increase that was achieved. 



Siegfried said:


> An increase of compression ratio from 6.9 (DB601) to 8.5 (DB605DC) however figures prominently.



You do, of course, have a source to back that up?
There is no question that the DB series did increase form the 6.9 to 8.5 compression. However a chart from "Aircraft Power Plants" by Jones, Insley, Caldwell and Kohr (1926) shows that there is a 10% increase in power going from 5.4:1 to 7.0:1 compression ratio and another 5% increase going from 7.0:1 to 8.5:1. Similar charts from other books, like "Aircraft Engine Design" by Liston (1942) show similar even if not identical increases. There is usually a diminishing return in increasing compression, there is less of an increase going from 9.0 to 10.0 than in going from 6.0 to 7.0. They also show about a 40% increase in peak cylinder pressure for the same 7.0:1 to 8.5:1 change, they do show about a 5-6% improvement in fuel efficiency though. 




Siegfried said:


> Valve overlap and multipoint fuel injection can get 10% to 15% more power.



They can, the question is did they? maybe they did and maybe they didn't, I don't know, but we do know that the point fuel injection didn't provide the charge cooling that the evaporation of fuel in the supercharger of some carburetor or single point (throttle body?) injection systems did. Without a detailed analysis or report comparing both on ONE engine we are just guessing as to which was better. R-R figured that the Fuel evaporation was good for about 20-25 degrees C reduction in intake charge temperature in the Early Merlins. 





Siegfried said:


> The DB605 series has a quirky proportionality of boost levels at sea level and power increase however at full power height it was about 150hp more.



No more than some other engines that were throttle back at take-off or sea level compared to what they could do a several thousand meters. But the 150hp claim may be pushing things just a bit. From the chart for the DB605A on the Kurfurst site it seems it topped out at about 1525-1550 HP or so at 2100meters compared to the 1475 for take-off?

Looking a the 605AM compared to the 605A we get 1.42ata of boost at sea level for 1475hp and 1.8 ata (using C-3 fuel plus MW-50?) for 1800hp at sea level. Note how the power and boost track fairly closely? But with the same supercharger this high level of boost can only be maintained to 4000meters and by 5600meters or so and without MW-50 the power has fallen to the same level as a 605A. The use of MW-50 (charge cooling) is still worth about 100hp if I am reading the chart right. Please note that the C-3 fuel has allowed for over boosting at low altitudes but has done little or nothing for high altitude performance. 

With the bigger supercharger used on the 605AS DB gained several thousand meters of altitude. Or to put it another way it made around 200hp more at 8000meters than a 605A. But the bigger supercharger cost about 40hp at sea level at 1.42Ata. Granted with the bigger supercharger 'over boosting' at low altitude was quite possible subject to engine strength or temperature limits and the use of C-3 fuel and/or MW-50 (charge cooling). 




Siegfried said:


> Two stage compressors are used when technology is inadaquet to produce the required compression in a single stage.



While that is true and it is true that nobody was going to design a two stage Supercharger to get a pressure ratio of under 3 to 1 which could easily be done with a single stage supercharger (after 1940/41) it is also true that NOBODY had a single stage centrifugal compressor that could reach a 5 to 1 pressure ratio in WW II, either supercharger or jet engine compressor, in fact you would be hard put to find very many centrifugal compressors today that exceed 5 to 1. 



Siegfried said:


> Of course it all gets down to detail design, but its clear the DDs engineering avoided dependance on high supercharger pressures and intercooling; so touting the Merlins intercooler supercharger setup rather ignores the point that it was in a way a fix to the limitations of carburation and throttle body injection.



I think you are mixing cause and effect again. DBs engineering avoided dependence on high supercharger pressures by using a large displacement engine. An extra 25.5-32% of displacement can certainly allow for some different choices. In the 1930s when design and development of both engines started and they went into initial production the DB used 39 in of manifold pressure to the Merlin's 42in and the BMEP of the two engines were with a couple of percent of each other. The Merlin hung on as long as it did partially because of the better fuels. Had these fuels not been available perhaps more work would have been done on the Vulture or the Griffon been pushed along faster. Given the displacement difference the ONLY way for the Merlin to compete was to use higher boost pressure. Using point fuel injection would have helped mixture distribution problems but would have done little to solve the the displacement difference. A 5-10% increase doesn't cover the 32% displacement difference. 




Siegfried said:


> From a practical point of view the DB series managed to operate high piston compression ratios and avoid the need for high pressure ratio superchargers and inter-coolers.


Actually the high piston compression ratios prohibited the use of of high pressure ratio superchargers and without the high pressure ratio superchargers there was a lot less need for the inter coolers. Figure it out for yourself, Say you boost the piston compression ratio by 20%, you do not get 20% more power, you may get 5-10% more power but if you had kept the same piston compression ratio and increased the boost pressure by 20% you could burn 20% more fuel per power stroke. Even counting the extra power to drive the supercharger you come out ahead. 


Siegfried said:


> High compression ratios allow greater expansion of the hot gases and therefore more power and efficiency. There is more danger of pre ignition, however no energy is lost as in intercooling and while intercooling can clearly demonstate advantages it comes at a price as well.



the energy _LOST_ in the intercooler is pretty much theoretical as this energy is pretty much destructive to a high power engine. If you want to design a fuel economy special go ahead but the increased power from the denser charge has been proven over and over again (some German engines could pick up 4-5% in power just from using MW-50 WITH NO INCREASE IN BOOST). Speaking which doesn't MW-50 cause a loss of power as it reduces the heat (energy) in the intake charge? The Price of an inter cooler is weight, bulk and drag. But then lugging around 100-200liters if water/alcohol brings a few penalties too doesn't it? 




Siegfried said:


> The DB605DC managed 1850hp on C3 a 96/130 fuel without MW50 it weighed 745 kg.
> The Merlin (66) seemed to need 100/150 fuel (really 110/150) to do the same it weighed 744kg.



How about we go back to the Merlin 61 and the DB605AS? Merlin 61 does 1390hp at 7170meters. DB605AS does 1200PS at 8000meters 
Or the Merlin 71 vs the DB605DC? Merlin has 1475hp at 6742 meters compared to the DB605DC's 1550ps at 6000meters while using MW-50? 
And the Merlins don't need anything more than 100/130 fuel. 




Siegfried said:


> For the high altitude Wellington, supposedly.



You have proof of another explanation? 




Siegfried said:


> 1.8 ata boost 11 psig
> 1.98 boost 14 psig.
> The DB605 could get away with a single stage supercharger.



Yes it could, for low altitude work. And it only needed better fuel of it's own (late war C-3) and/or charge cooling (MW-50) to do it. 

there is nothing wrong with MW-50 (water/alcohol) Plenty of American aircraft used it. Just don't claim how superior one countrie's or companie's "technology" is for not using intercoolers whenthey were using an alternative form of charge cooling.


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## wuzak (Jan 24, 2012)

Siegfried said:


> It is true. Modern gas turbines are renowned for reducing the number of stages as part of efficiency improvements.



Is this so?

Have you any numbers to back that up?

FYI:
RR RB211 - 1 fan stage, 7 intermediate compressor stages, 6 high pressure compressor stages.
RR Trent - 1 fan stage, 8 intermediate compressor stages, 6 high pressure compressor stages.


Turbine manufacturers continue to work to improve the efficiency of the compressor stages, which means les work is required to drive the compressor stage, which means less power needs to be extracted in the turbine and therefore more thrust (or shaft hp).

If they have reached an overall pressure ratio with which they are satisfied it wouldn't surprise me if try to do it with fewer stages. Fewer stages mean higher pressure ratios per stage, and more heat generated per stage, unless there is a gain in efficiency of the compressor design.


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## Edgar Brooks (Jan 24, 2012)

--


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## Shortround6 (Jan 24, 2012)

ADI is anti-detonation injection. Just a different term for water (water/alcohol) injection /MW-50.


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## Juha (Jan 24, 2012)

Edgar Brooks said:


> What's ADI?


Anti-Detonant Injection, OK SR beats me


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## Juha (Jan 24, 2012)

Siegfried said:


> The DB605DC managed 1850hp on C3 a 96/130 fuel without MW50 it weighed 745 kg.
> The Merlin (66) seemed to need 100/150 fuel (really 110/150) to do the same it weighed 744kg.



In fact Merlin 66 produced appr 1980hp at sea level with 100/150 fuel and 25lb boost.

Juha


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## wuzak (Jan 24, 2012)

Juha said:


> In fact Merlin 66 produced appr 1980hp at sea level with 100/150 fuel and 25lb boost.
> 
> Juha



According to Lumsden the Merlin 66 with 100/150 fuel and +25psi boost made 2000hp @ 3000rpm at 5250ft and 1860hp @ 3000rpm @ 11,000ft.

On 100/130 fuel it was 1750hp @ 5250ft and +18psi, and 1625 @ 12,500ft, +18psi.


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## Edgar Brooks (Jan 24, 2012)

--


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## wuzak (Jan 24, 2012)

Siegfried said:


> Higher power levels (eg Merlin 100) resorted to 100/150 plus ADI.



I don't think any Rolls-Royce built Merlins ever entered service with ADI. Some Packard Merlins did.

Anyone know a definitive answer?

Rolls-Royce did play with ADI on the test bench, at least.

RM.17SM made 2620hp (corrected from 2640hp) @ 3150rpm with +36psi boost, special fuel (extra TEL) and ADI.

A development Merlin 66 ran at 2380hp @ 3300rpm and +30psi boost for a 15 minute "sprint" run in 1943.


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## wuzak (Jan 24, 2012)

Edgar Brooks said:


> Thank you (both) since it means I've been able to confirm my suspicion that the Merlin 100-series didn't have it, they just had "standard" fuel injection, which did sufficient cooling on its own.



Yes, the fuel injection was in the throttle body. Or, if not, in the supercharger impeller eye.


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## Siegfried (Jan 24, 2012)

Edgar Brooks said:


> Thank you (both) since it means I've been able to confirm my suspicion that the Merlin 100-series didn't have it, they just had "standard" fuel injection, which did sufficient cooling on its own.



Packard V-1650-9 (Merlin 100) of the P-511H used it.

North American P-51H Mustang
Specs of the P-51H-5-NA: 

One Packard Merlin V-1650-9 twelve-cylinder Vee liquid cooled engine rated at 1380 hp for takeoff and a a war emergency power of 2218 hp at 10,200 feet and 1900 hp at 20,000 feet with water injection.


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## wuzak (Jan 24, 2012)

Yes, but only Packard Merlins.


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## Edgar Brooks (Jan 24, 2012)

--


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## Tante Ju (Jan 24, 2012)

Shortround6 said:


> the energy _LOST_ in the intercooler is pretty much theoretical as this energy is pretty much destructive to a high power engine. If you want to design a fuel economy special go ahead but the increased power from the denser charge has been proven over and over again (some German engines could pick up 4-5% in power just from using MW-50 WITH NO INCREASE IN BOOST). Speaking which doesn't MW-50 cause a loss of power as it reduces the heat (energy) in the intake charge? The Price of an inter cooler is weight, bulk and drag. But then lugging around 100-200liters if water/alcohol brings a few penalties too doesn't it?



I dont think "100-200 liter" MW was carried. More like 70-80 liter.. also one big plus for MW that it also massive cools engine, as it evaporates in combustion chamber, so you do not only no intercoolar radiator needed, but also smaller coolant radiator will do. Without the added drag and bulk of using larger radiator to compete with increased engine temperatures during high power. So you can actually size radiators to be smaller and to cope just with engine temps at military power, and MW takes care of rest at high powers, high power I mean 5-700 HP more than normal.. This is significant benefit, that you do not have to size radiators for power of say in case of DB, 2000 HP but 1300 HP. It means less bulk added, but more important, much less drag.

So essence - MW is like charge cooling and evaporate cooling. Otherwise very learnable post, it was good to read!


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## Shortround6 (Jan 24, 2012)

I would like to thank everyone for the kind words.

Tante JU, I am trying to keep this discussion somewhat general about the different supercharger systems and their advantages and disadvantages, not turning it into a R-R vs DB arguement. Some of the American planes with water injection for their R-2800's carried as much as 25 US gallons of water/alcohol and a twin engine plane using MW-50/ADI/water injection would obviously carry more fluid than a smaller single engine plane. 
The water/alcohol mix had pretty much evaporated in the supercharger/intake manifolds as spraying the liquid into air that was (or soon would be) at several hundred degrees centigrade would have it turning to steam before it got to the cylinders. I am certainly not saying that it couldn't pick up even more heat and carry it out of the engine but it's primary purpose was to lower the intake charge temperature. Of course lowering the intake charge temperature also lowered peak cylinder temperatures and exhaust temperatures.


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## Juha (Jan 24, 2012)

Those 109s with DB605D family engine still needed a bigger oil cooler.

Juha


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## Shortround6 (Jan 25, 2012)

I think there are a few more things that need to be addressed. 



Siegfried said:


> Valve overlap and multipoint fuel injection can get 10% to 15% more power.



The DB 601 already used multipoint fuel injection so using it on the DB605 shouldn't really be responsible for any increase in power. 





Siegfried said:


> The Daimler Benz engine seems to have been more fuel efficient, suggesting a smaller radiator as well.



While the Daimler Benz engines were more fuel efficient jumping to the smaller radiators is a bit of a stretch. Maybe they were smaller but radiators also have to matched to the aircraft. A bomber using the same engine as a fighter might very well have to use larger radiators because of the more extended but slower flight speed high power periods of flight. Like a long slow climb to operating altitude. Engines also wound up with different amounts of heat rejection to their cooling and oil systems. An Allison and a Merlin of about the same power loose different percentages of heat to their oil system and cooling system from each other. At full power (or close to it) extra fuel (part of the rich condition) was used as an internal coolant. 




Siegfried said:


> Again the boost level of 9 psi is equal to 1 ata plus 9/14 = 1.65, a level not introuced (1.7 ata) early 1944 on the DB engine.



This smacks of making a virtue out of a necessity. Didn't the Germans have some initial problems using even 1.42 ata? like burned piston crowns and things? Trying to move to 1.7 ata without better fuel or charge cooling is just going to make things worse. 



Siegfried said:


> The technology the DB605 used simply didn't need high boost levels nor did it need inter cooling.
> The supercharger was simply less importation a factor to this engine.



There was no 'secret' or 'special' technology. It was a series of choices that were made that were know to all the major aircraft engine companies. In the 1930s with 80-87 octane fuel and for a given engine weight you can make a smaller, high revving engine (Merlin/Allison) or you can make a larger displacement, slower revving engine. The available fuel simply wouldn't allow much boost to be used and the detonation limits required trade-offs between cylinder compression and boost levels. Many engines used compression ratios of 6.5-7.0:1 but low boost(usually under 5lbs or so). Point Fuel Injection was known and experimented with by several companies, it was also expensive to make and maintain and some countries, to be honest, just didn't have the infrastructure (number of skilled subcontractors) to support it's adoption at the time. But the engine designers did have a pretty good idea of it's benefits. The negative "G" thing did kind of slip by them though. 

At sea level 75 in of manifold pressure (22.5lbs boost) or 2.5Ata is just 2.5 times the surrounding air pressure and can be achieved rather easily by most single stage compressors. The ability of the fuel to allow it or the engine to stand up to it were the limiting factors. The problem comes with altitude. An intake pressure of 54in (1.8Ata) at 6000meters needs a pressure ratio of 3.92 this was much harder for a single stage compressor and was frankly NOT possible before or during the early war years. An intake pressure of 60 in (15lbs/2.0Ata) at 23,500ft (Merlin 61) needed a pressure ratio of 5.1 to one and an intake pressure of 44 in (7lbs/1.46Ata) at 25,000ft needed 3.96 (early P-38).

By Not possible I mean getting that pressure ratio in a useable fashion. Not only do superchargers have pressure ratios but they have efficiency ratios. Usually for aircraft they tried to keep the efficiency up to .70 or above. Some of the late 1930s superchargers could NOT provide a pressure ratio of even 2.8 with an efficiency of over .70 . 

The efficiency is how much power is actually used to compress the air. If a supercharger took 100HP to give desired flow (mass and pressure) at .70 it means that 70HP is going to the work of compressing the air and the rest is going into the the friction in the supercharger drive and bearings AND INTO HEATING THE INTAKE CHARGE. Since the mechanical loses are only a few percent (at worst) that means almost 30HP is heating the intake air. Raising the pressure ratio at the expense of lower efficiency means a less dense intake charge (fewer lbs of air per cubic foot) and a hotter intake charge pushes the detonation limit. It also takes more power to drive the supercharger. Supercharger pressure ratio ( for a given supercharger) is fairly proportional to the impeller tip speed and power required goes up with the square of the tip speed. Changing from a 7.0 gear ratio to a 10.0 gear ratio doubles the power used by the supercharger. It also doubles the amount of heat absorbed by the intake charge over and above the heat of compression even if the efficiency of the supercharger stayed the same and they usually did not. Efficiency dropped a few points at the higher pressure ratios.

This why the push came for two stage superchargers in the pre- war and early war years. The existing single stage superchargers could not provide the altitude performance that was desired. 




Siegfried said:


> The DB605L for the almost in service Me 109K-14 (and DB603LA) had two stage superchargers but did not need to use inter cooling



But they did use MW-50 much like the 2 stage Allisons in the P-63 used ADI instead of an intercooler.


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## Shortround6 (Jan 25, 2012)

A few more notes on superchargers and charge cooling.

There is a rule of thumb of a 1% power loss or gain for every 10 degrees Farhenheit or 3-3.5% for every 20 degrees Celcius. Please remember that the Merlin XX should heat it's intake charge by about 148 degrees celcius in high gear. 

As a guideline there is chart showing the power needed to compress air to sea level pressure and the supercharger outlet temperature for a 1200 HP engine. This the power needed to maintain normal sea level pressure at the outlet of the supercharger and the temperature rise up to about 45,000ft. Obviously at sea level no power is required and the temperature is the normal 59F (15C). At 10,000 ft about 50-55 HP is needed and the out put temperature has risen to about 100 degrees F, despite the intake temperature falling to 24 degree F. At 20,000 ft the power needed is 105-110hp and the outlet temperature has climbed to over 150 degrees F despite the intake temp falling to to around 12 degrees below zero. At 30,000ft around 175-180hp are needed and output temperature has risen to about 200 degrees F compared to the inlet temperature of -48 degrees. An intercooler (or ADI system) that could lower the output temperature ( or inlet temperature of the next supercharger stage) would be worth about 10% in power just in higher charge density even if did nothing for allowing a higher boost to be used or eased the cooling requirements of the engine. 

Three were studies done to determine the factors affecting detonation in the cylinder. These included ( but not in actual order of importance);

1, Compression ratio
2, Inlet air pressure (manifold pressure/inlet to cylinder)
3, Inlet air temperature
4, Temperature of combustion chamber walls (cylinder head)
5, Spark advance
6, Engine Speed (faster is better)
7, Combustion chamber size and shape
8, Combustion chamber deposits

The chemistry of the fuel is also a factor. For example while leaded fuel and cat cracked unleaded track each other fairly well and run nearly parallel, fuel with a large amount of benzene starts at low temperatures with a better detonation resistance than gasoline but between 250-310 degrees inlet temperature it crosses over and has worse resistance to detonation. Or that while at low specific fuel consumption both straight run gasoline with 4cc. T.T.L. And Aromatic Fuel show a similar resistance to detonation as the mixture richens (lb/bhp/hr go up) the Aromatic fuel can show a 10% or greater allowance in BMEP before detonation starts.


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## fastmongrel (Jan 25, 2012)

Wow Sr6 that is some brilliant work you have posted in this thread. Took me several reads but I think I understood it all in the end, if I owned a hat I would raise it to you.


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## wuzak (Jan 25, 2012)

Thanks again SR.

Looking at Kurfurst's site found these:

Kurfürst - DB 601, 603, 605 datasheets - DB 605 AM
Kurfürst - DB 601, 603, 605 datasheets - DB 605 DB/DC

For the DB605AM the power drops off markedly from 4000m/13,123ft?

Not being able to read German, but it would appear the critical altitude using 1.98 ata and MW50 for the DB605DC was 4900m/16,076ft and at 1.80 ata 6000m/19,685ft


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## wuzak (Jan 26, 2012)

Just read that the two stage versions of the Jumo 213 and 222 had intercoolers. They also used a lower compression ratio - about 6.5:1.


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## riacrato (Jan 26, 2012)

The Jumo 213 E-1 did, but the Jumo 213 F did not have an intercooler.


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## Siegfried (Jan 26, 2012)

riacrato said:


> The Jumo 213 E-1 did, but the Jumo 213 F did not have an intercooler.



Is there any basis for that. I'd always assumed that the Jumo 213F didn't have an intercooler so that it could fit in the FW 190D-13 airframe wherease the Jumo 213E required the Ta 152 but I doubt it was all that big. I suspect the 213F had an intercooler or rather aftercooler, perhaps smaller? The reason I say this is that some of the FW 190D series were to receive the 213E or even EB engine engine? The term intercooler a bit of an misnomer as these engines were aftercooled, a small amount of intercooling came out of cooling the housing of the compressors.


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## Siegfried (Jan 26, 2012)

wuzak said:


> Just read that the two stage versions of the Jumo 213 and 222 had intercoolers. They also used a lower compression ratio - about 6.5:1.



The format for power growth of the Jumo 213 was however heading in a different direction to the somewhat similar Griffon/Merlin high boost route. They were going the route of high RPM; 3250 rpm while the 2700+ hp Jumo 213J was turning over at 3750 RPM: approaching that of the Sabre, with poppet valves of course.

The Jumo 222A-3/B-3 and the Jumo 213E/F (letter designates rotation direction) were the same core engine the latter with what was reputedly a very effective two stage intercooled supercharger and were back on the production schedule towards the end of the war.

Its worth noting that the Ju 488 when equiped with the BMW 801TJ tturbo supercharged engine was expected to achieve a service ceiling of 51000ft while the Jumo 222E/F version, which was much faster and had much more powerfull engines was good for only about 46000ft, though at higher speed.

The Jumo 222A-1/B-1 and A-2/B-2 were the early versions whose production was controversially abandoned. An interesting aside is that the DB series used roller bearings at the big end wherease most engines used pressure lubricated sleeve/journal bearings whose use of copper and tin apparently imposed a severe restriction on German supply.


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## tyrodtom (Jan 26, 2012)

The rest of the world calls it a intercooler, and understands what that means perfectly well.


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## Shortround6 (Jan 26, 2012)

When dealing with large aircraft engines there are only so many routes to take. You can only make the cylinders so big for instance, before your start running into trouble. Since the speed of the flame front in the cylinder is pretty much fixed 6in is about the max diameter you can use ( or 6 and a fraction), 7 inches is out without going to 3 spark plugs per cylinder. The allowable piston speed rather fixes stroke and rpm. Most engines did not exceed 3000 FPM. The Merlin ran at 3000FPM, the Griffon only a bit more, it's longer stroke balanced by the 250 less rpm it turned. THe DB605 did 2940fpm at 2800rpm. 
With a V-12 you have a fixed number of cylinders, an upper limit on how big the cylinders can be (there is a reason the Russian AM-35/38 ran at 2350rpm with it's 190mm stroke). This leaves two options. 1, increase the RPM and deal with the friction and stresses in the reciprocating parts that go up with the square of the speed or increase the manifold boost and cram more air and fuel the same sized engine and deal with the detonation problem.
Or you can try more cylinders, like the radials. Big slow turning 14 or smaller cylinder higher rpm 18?


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## Siegfried (Jan 26, 2012)

Shortround6 said:


> When dealing with large aircraft engines there are only so many routes to take. You can only make the cylinders so big for instance, before your start running into trouble. Since the speed of the flame front in the cylinder is pretty much fixed 6in is about the max diameter you can use ( or 6 and a fraction), 7 inches is out without going to 3 spark plugs per cylinder. The allowable piston speed rather fixes stroke and rpm. Most engines did not exceed 3000 FPM. The Merlin ran at 3000FPM, the Griffon only a bit more, it's longer stroke balanced by the 250 less rpm it turned. THe DB605 did 2940fpm at 2800rpm.
> With a V-12 you have a fixed number of cylinders, an upper limit on how big the cylinders can be (there is a reason the Russian AM-35/38 ran at 2350rpm with it's 190mm stroke). This leaves two options. 1, increase the RPM and deal with the friction and stresses in the reciprocating parts that go up with the square of the speed or increase the manifold boost and cram more air and fuel the same sized engine and deal with the detonation problem.
> Or you can try more cylinders, like the radials. Big slow turning 14 or smaller cylinder higher rpm 18?



Dear Short-round,

Thanks for your work in this thread. The Merlin and DB601/605 will always tend be compared. The fact that the DB605 took a different approach also means it will be a canditate to compare. For the record I do believe the DB605 was a slightly better engine in consideration of the fact that it had to compete with the Merlin with the constraints of far lower grade octane fuels till 1944 when C3(96/130 octane) suplies increased relative to B4(87 octane). At a time the allies were transitioning from 100/130 to 100/150. The exception being around 41/42 when the DB601N series used a lower grade of C3 (then about 94/115 I believe) before being replaced by the DB601E which generated the same power on B4 (87 octane) using tuned port scavenging, variable length induction manifolds and a large valve overlap. There also seem to have been extreme constraints on engine length and fuel consumption due to the size limitations of the Me 109G/K airframe. Anyway, the point I originally made was that the DB600 series pushed engineering in the direction of higher compression ratios (as well as swept volumes using some engineering that used the cylinder sleeves as a head bolt and tuning/valve overlap). The superiority of Stanley Hookers superchargers is less relevant to the Daimler Benz engines which needed to generate less than 70% of the pressure to generate the same horsepower (DB605DCM at 1.98 ata (14.6 psig) on C3+MW50 versus Merlin 66 on 100/150 at (25 psig) required 2.75 ata probably at similar mass flow rates and that is at the end of the war. 

However, earlier DB engine barely needed sea level supercharging at all eg 1.3 ata is 4.5 psig a boost level that remained common till late 1943. That's a pressure ratio of 1.3 at sea level and 2.6 at around 20000ft. What did a Merlin use in 1942, about 9 or 12 which is a pressure ratio of 1.66 to 1.8 already.

Nevertheless we don't know for sure since the Merlin was never challenged by use of lower octane fuels; it's possible Rolls Royce might have made use of water injection or bigger inter coolers to compensate, it may even have abandoned the Merlin as too small and ramped up production of a Griffon in 1942 using a two speed (instead of single speed) supercharger to get the swept volume they needed. With less power going through the engine they might have worked to lighten the block. Britain did build a synthetic coal to fuel plant in case they needed to use the technology. (I've always wondered why a Spitfire XII with a two speed instead of single speed Griffon single stage supercharger wasn't produced)

Despite admiring the ingenuity of the DB engine, even the roller bearings probably saved in expensive sleeve bearing brass, Daimler Benz did mess up the lubrication system design (frothing of oil at high altitude that was not detected) that wasn't cured till an oil de-aerator was fitted. Possibly cost the Me 109 10 mph in top speed for a whole year.

The Jumo 213 series took an approach similar to the allied engines of low compression (though not as low) and higher pressure ratios, perhaps it could be regarded as an intermediate approach. It seems though that if Vanirs' posts are looked at he points out that the DB603LA (which took the high compression ratio two stage non inter-cooled approach) was superior to the Jumo 213E (which took the low compression ratio, higher boost inter-cooled approach). The rather large Jumo's approach to increasing power was to focus on increases RPM.

The only German turbo-supercharged engine that achieved a level of production seems to have been the BMW801TJ. Noteworthy is the way the turbine, supercharger, inter coolers were packaged into and integral unit rather than the opposite approach seen on the P-47 and P-38.

One interesting approach to increased power to be exploited on the BMW 802 engine was variable exhaust valve timing to get valve overlaps since variable inlet port length tunning used in the DB605 V12 is harder to achieve.


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## Edgar Brooks (Jan 26, 2012)

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## wuzak (Jan 26, 2012)

Edgar Brooks said:


> It was; no Griffon, in the Spitfire, was single-speed. Griffon 3, 4, 6, 26, 36 were all single-stage two-speed. In trials, the prototype DP845, with a Griffon RG 14 SM achieved 397mph at 14,200' in FS gear, and 384 at 2,600' in MS gear.



Were any Griffons single speed engines?


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## Edgar Brooks (Jan 27, 2012)

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## Siegfried (Jan 27, 2012)

Edgar Brooks said:


> +4 for maximum weak continuous flight, +7 for max, rich continuous, +9 max. climbing (limit 1 hour,) +12 take-off to 1000', +16 combat (max. 5 minutes.) This was the Mk.V with Merlin 45.
> 
> It was using 87 until mid-1940, and the fuel was still allowed for in the Mk.II in June 1943.
> .
> ...



The British synthetic fuel plant was at Billingham. It was demolished with gread difficulty a few years ago as the contractor did not know of the bomb proof concrete used:
GANSG - Oil from Coal and Other Synthetic Fuels

Also
Encyclopedia of 20th-century technology - Colin Hempstead, William E. Worthington - Google Books

The apparent modest altitude performance of the Mk XII with the single stage (an you say two speed) Griffon suggests to me that the philosophy used by Rolls-Royce made the engine rather dependant on high performance superchargers; which wasn't a problem as they knew how to develop these.

During the 1942/43 period the DB605 were distinguished by much higher pressure ratios in the Merlin versus the DB. The 1.3 ata to 1.42 ata used by the DB605 at this time is equal to 4.5 psig to 6 psig at a time the Merlin could accept 12-15 psig.


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## Edgar Brooks (Jan 27, 2012)

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## Siegfried (Jan 28, 2012)

Edgar Brooks said:


> Thanks, a friend lives in Billingham; I must remember to ask him about it.
> The single-stage Griffons were developed solely with the expectation of low-level use. The Spitfire XII was developed as a counter against low-flying Fw190s, and also proved ideal against the V1. The same type of engine was developed for the Seafire XV 17, which, likewise, were mostly used at low level.
> Boost levels, for the Griffon III or IV, in the XII, on 100 octane (only,) were +6, +7, + 9 +12, not very different from the Merlin 45; the Seafires, with the Griffon VI 100 octane, were +7 - +15.



It's fairly well known that Rolls Royce manipulated supercharger
settings such as gear ratios, impellor diamatersm and boost regulator
settings to optimise the Merlin and Griffon for different
altitudes. It would appear then that the Spitfire XII
was never given a larger diameter impellor or settings to raise its
high altitude performance. Possibly because the two stage
Spitfire was available. It quite a good idea, the Soviets would
have love clipped,croped and clapped Spitifres.


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## Milosh (Jan 28, 2012)

Every Griffon engine from the Mk 61 on had a 2 speed, 2 stage supercharger, except for the Mk 101, 102, 105, 121, and 122 which had a 2 speed, 3 stage supercharger.


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## cimmex (Jan 28, 2012)

Milosh said:


> Every Griffon engine from the Mk 61 on had a 2 speed, 2 stage supercharger, except for the Mk 101, 102, 105, 121, and 122 which had a 2 speed, 3 stage supercharger.



Are you sure about this, IMO it should be two stage, three speed.
cimmex


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## Milosh (Jan 28, 2012)

That is what 'Spitfire: The History' says, yet when I checked in White's WW2 engine book says 2 stage, 3 speed.

Yes, 2 stage, 3 speed sounds correct.


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## Siegfried (Jan 28, 2012)

cimmex said:


> Are you sure about this, IMO it should be two stage, three speed.
> cimmex



Only the Merlin 100 series had a 3 speed supercharger. The Merlin 60 and 70 etc series had only two stages.


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## wuzak (Jan 28, 2012)

Siegfried said:


> Only the Merlin 100 series had a 3 speed supercharger. The Merlin 60 and 70 etc series had only two stages.



Merlin 100 series wre 2 speed 2 stage engines.

IIRC only the Griffon received a 3 speed 2 stage supercharger in some versions.


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## Edgar Brooks (Jan 30, 2012)

Siegfried said:


> . It would appear then that the Spitfire XII
> was never given a larger diameter impellor or settings to raise its
> high altitude performance. Possibly because the two stage
> Spitfire was available. It quite a good idea, the Soviets would
> have love clipped,croped and clapped Spitifres.


The XII was only ever intended for low-level, hence the clipped wings (they had a very damaging effect on the rate of climb and service ceiling.)
I've been reading the files on this "cropped clapped" business, and cropping the impeller blades was great at low-level, but dreadful at height; it was eventually recommended that cropping should be kept to the 50-series Merlin, and there was a 50M 55M, with just such a mod.
As you say, there was little point in tinkering with the XII, with a longer nose and extra radiator needed for a two-stage Griffon, so it was better to wait for the XIV 21.


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## Shortround6 (Feb 6, 2012)

A couple of notes about compression ratios and then the Merlin superchargers.

An example of changing the compression of an engine a little else can be found in the American Ranger six cylinder L-440 engine. At the end of it's life it was available in 4 versions.

6.0 compression-65 octane fuel=175hp
6.2 compression-73 octane fuel=180hp
6.8 compression-80 octane fuel=190hp 
7.5 compression-87 octane fuel=200hp

A 25% increase in compression giving a 14.3% increase in power. A 25% increase in manifold pressure in an engine "should" give a 25% increase in power, minus what it takes to drive the supercharger.

Merlin's seem to have 5 or 6 different superchargers, not including different gear ratios or drives. 
1. The supercharger used on the Merlin III and others including the 2 speed Merlin X.
2. The Hooker modified supercharger on the Merlin XX and 45. Also used on other marks.
2a. The same as above with a cropped impeller.
3. A modified supercharger using a larger impeller (10.85in vs 10.25in) with circular arc rotating guide vanes and a modified diffuser. This was good for several thousand more ft of altitude and used in the Merlin 46, 47, 50A and 56.
4. The famous two stage supercharger. Which required a strengthened drive for the higher pressures. It had an 11.5 in first stage rotor.
5. A modified two stage supercharger with a 12 in first stage impeller, modified guide vanes and a modified diffuser. It's first application was in the Merlin 65 but the two types overlapped some in MK numbers. Different gear ratios were used to adapt the supercharger/engine to different altitudes. A little used version was the Merlin 110-114A series which used the newer supercharger with the original gear ratios to give 1325hp at 27,250 ft with 18lbs of boost ( no ram?).


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