Supercharger vs Turbocharger

[Jenisch]

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?

 

[davebender]

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

 

[Shortround6]

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.

 

[davebender]

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.

 

[Shortround6]

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.

 

[GregP]

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.

 

[tomo pauk]

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.

 

[Shortround6]

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.

 

[davparlr]

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.

 

[wmaxt]

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.

 

[Vincenzo]

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

 

[davebender]

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?

 

[Denniss]

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

 

[MikeGazdik]

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.

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.

 

[GregP]

Pounds per horseppower is not a fuel consumption number.

Maybe you were thinking of pounds per horsepower per hour?

 

[davparlr]

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.

 

[wmaxt]

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!

 

[Shortround6]

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.

 

[Vincenzo]

sorry i was confusing with engine variants

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

 

[Siegfried]

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.