Supercharger vs Turbocharger

[Shortround6]

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.

 

[Juha]

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

Juha

 

[Shortround6]

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

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.

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.

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.

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.

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.

 

[Shortround6]

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.

 

[fastmongrel]

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.

 

[wuzak]

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

 

[wuzak]

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.

 

[riacrato]

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

 

[Siegfried]

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.

 

[Siegfried]

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.

 

[tyrodtom]

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

 

[Shortround6]

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?

 

[Siegfried]

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.

 

[Edgar Brooks]

--

 

[wuzak]

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?

 

[Edgar Brooks]

--

 

[Siegfried]

+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.
.
But they didn't, which makes speculation meaningless.

Where? (And I'm not being confrontational; I genuinely don't know where it was.)

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.

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.

 

[Edgar Brooks]

--

 

[Siegfried]

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.

 

[Milosh]

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.