WW2 aero engine fuel consumption comparison

[spicmart]

The title says it. Anyone has a list?

 

[Shortround6]

No list.
It is sort of complicated and yet it is not.

Most allied engines used fuel for partial cooling (or to compensate for poor fuel distribution?) and could be run lean at lower power settings (cruise) and rich at higher power settings.
Rich power settings could increase fuel consumption by around 50% per hp/hr.
German and some other fuel injected engines used similar amounts of fuel (10-15% ?) per hp/hr from cruise settings to full power.

Just about everybody's engines would give around low/mid .40s lbs per hp/hour in cruise. Merlins sometimes hit 0.50lb/hp/hr and that was among the worst of the big engines.
Some of the American radials could hit in the low 0.70s in lbs/hp/hr at full power.
In most data sheets for engines they give the cruise consumption.

As can be seen this means fuel consumption is very power dependent. An R-2800 is going to suck up a lot more fuel than an R-1830 just because it is making more power. But they may both be running at 0.46-0.48 lbs/hp/hr in cruise and depending on last spark plug change could swap who was most economic.

If you know the amount of power being used and you know if it was running lean or rich (or German fuel injection?) you can come up with educated guesses.

The Famous economical Sakae engine in the early Zero is a sort of special case but it's economy was only at certain power setting/s.
Cruise at 180kts was 16.4 US gallons an hour (power unknown/ altitude unknown) but at 190 kts it jumped to 24.04 US gal/hr and at 200kts it moved to 26.15 US g/hr. At full power the engine needed 91.14 g/h and at 950hp that is around .574 lbs/hp/hr.
The later 2 speed engines lost the 180kt cruise or rather the 180kt cruise jumped to 22 US gal per hour. Rated power of 1100hp (low gear/ 1100hp/ 115gph)) might have required .627lbs/hp/hr.
Here is where things get tricky, In high gear at 6000 meters the engine may have been making about the same power in the cylinders and burning the same amount of fuel (or close) but the propeller was only getting 980hp because the extra 120 hp was driving the supercharger in high gear.
Making a big assumption (you judge) but if the Sakae 21 engine was using the same 115 US gallons at 6000meters in high gear to get it's 980hp the fuel consumption was now 0.704lb/hp/hr.

Trying to sort through even a fraction of the engines to get the the consumptions for different powers at different altitudes is going to be a lot of work.
The engine spec sheets are advertising figures. Best case, your actual mileage may vary

 

[don4331]

You want to add confusion - 2 identical P-38s fly the same mission, so same hp/hr - one uses 100s of gallons less fuel than the other.

So, the pilot/flight engineer has huge impact on aero engine fuel consumption.

 

[ThomasP]

If you can find the bench test sfc data for the individual engines you could maybe put a table/list together that would be of some use in comparing the theoretical thermal and mechanical efficiencies of the engines. But in the real world - when installed in the various aircraft - the same values would have very little application.

Having said the above, engines with higher compression ratios (say 7:1) are usually slightly more fuel efficient than those with lower compression ratios (say 6:1). If you have 2 engines of the same type and put them both in the same airframe, with everything else remaining the same, the 7:1 CR engine variant should have a greater range - at the same weak mixture cruise horsepower. At higher power it will depend on where the mixture has to begin richening up - the 7:1 CR engine will theoretically encounter detonation before the 6:1 CR engine and will have to begin using more petrol to help cool the mixture. In war-time practical use at most efficient cruise, if we look at the DB601A (6.7:1 CR) vs the Merlin III (6:1 CR) we will in real use situations see a lower sfc/BHP in favor of the DB601A - with the DB601A's at ~.46 lb/BHP-hr vs the Merlin III's ~.48 lb/BHP-hr. Why the DB601A did not manage an even lower sfc I do not know - theoretically it should be slightly better (maybe ~.44 lbs/BHP-hr) due to the difference in CR, and possibly even better due to the use of fuel injection.

In war-time practice the Sakae 12 engine (7.2:1 CR) used on the A6M2 managed ~.48 lb/BHP-hr for most efficient cruise, same as the Merlin. As Shortround6 points out above when running at at full power of 950 BHP it had an sfc of ~.574 lbs/BHP-hr, while the Merlin ran at about .59 lbs/BHP-hr for an output of 990 BHP in climbing power.

The enormous range difference between the A6M2 and the Hurricane (even when both cruised at the same speed and altitude) was due to the much larger fuel load of the A6M2 and the lower BHP required to maintain the cruise speed. The lower BHP required for the same cruise speed was the result of a lower CD for the A6M2.

The diesel engines used in aircraft had a significantly lower sfc than the petrol engines, usually around .38-.40 lbs/BHP-hr (I think).

As the others point out up-thread, the application is as or more important than the theoretical sfc of the engine.

 

[don4331]

Why the DB601A did not manage an even lower sfc I do not know - theoretically it should be slightly better (maybe ~.44 lbs/BHP-hr) due to the difference in CR, and possibly even better due to the use of fuel injection.

Wouldn't the inefficiency of the fluid drive for the supercharger explain much of the difference between the DB601 and the Allied engines?

When at cruise rpm/altitude, those engines with geared drive should be set so the throttle is wide open - so the supercharger isn't fighting the carb. While the DB fluid drive was very efficient, it can't match geared drive.

The fluid coupling does ensure the supercharger isn't fighting a closed throttle (below critical altitude) at full power. So, even if it is slightly less efficient drive the increased supercharger efficiency more than makes up for it. Engineering trade offs....​

 

[Shortround6]

Wouldn't the inefficiency of the fluid drive for the supercharger explain much of the difference between the DB601 and the Allied engines?

There is a crap load of other stuff.
Early Allisons used up about 100hp just turning the engine over, measured with electric motors. Piston and ring friction, bearing friction, oil pumps and so on.
Late model Allisons were using up closer to 200hp in friction, including new rings to hold higher boost pressures and even strong valve springs creating more friction on the cam shafts.
Piston and ring friction is a large part of the internal friction and trying to compare the DB601 and Allison/Merlin gets hard real quick. The DB601 uses bigger cylinders with more surface area and while it often turns at lower rpm the longer stroke means higher piston speed and since friction goes up with the square of the speed ........??????
Now how smooth are cylinders, what are the side loads, what is the surface area of the actual piston rings and what is the tension the rings are operating at and...........and......and................and...Efficacy of the supercharger drive is near the tail end of things to look at.

Somebody once claimed that the Allison was 8% (?)more efficient than a Merlin because of it's higher compression ratio. It may be true, But it is more than just nominal compression. If you use valve timing with longer duration and/or overlap the actual compression is lower than the nominal compression.

Actual power needed to turn the supercharger is all over the place. Power for a particular supercharger goes up with the square of the speed of the impeller.

 

[ThomasP]

Hey don4331,
re

Wouldn't the inefficiency of the fluid drive for the supercharger explain much of the difference between the DB601 and the Allied engines?

When at cruise rpm/altitude, those engines with geared drive should be set so the throttle is wide open - so the supercharger isn't fighting the carb. While the DB fluid drive was very efficient, it can't match geared drive.

The fluid coupling does ensure the supercharger isn't fighting a closed throttle (below critical altitude) at full power. So, even if it is slightly less efficient drive the increased supercharger efficiency more than makes up for it. Engineering trade offs....​

Hmmm, there's a thought. I do not know how much difference that would make - but it would make some difference. Good catch!

 

[AerialTorpedoDude69]

I've read that there were a number of innovations in engine design during the pre-war and war period relating to engine efficiency. Does anyone know if these are true?

• Four valve engines were significantly more efficient than two valve engines. I think some inline engines were four valve but no radials.
• Four stroke engines were more fuel efficient than two stroke. (I don't know of any four two-stroke aero engines.)
• Long stroke engines are more fuel efficient than short stroke.
• Fuel injection is supposedly more efficient than carb (but the timings are difficult to get right without electronics).
• Large cylinders
• Low RPMs
• EDIT: Higher compression ratios are more economical than lower one ( T ThomasP mentioned this earlier)
• EDIT: Water injection improves fuel economy apparently.

While there are no comprehensive fuel economy charts for aero engines, if you could find an engine that had most of the features listed above, you'd probably have the most efficient engine, although that's only if what I've read is true. I doubt direct injection engines were any more efficient due to more primitive timing control of the era.

EDIT: It turns out that direct injection wastes less fuel (according to one source) by squirting fuel more directly into the combustion chamber, compared to carbureted engines, which loses some fuel during the fuel-air mixture. (I need a better source for this claim though.)

 

[pbehn]

I've read that there were a number of innovations in engine design during the pre-war and war period relating to engine efficiency. Does anyone know if these are true?

• Four valve engines were significantly more efficient than two valve engines. I think some inline engines were four valve but no radials.
• Four stroke engines were more fuel efficient than two stroke. (I don't know of any four-stroke aero engines.)
• Long stroke engines are more fuel efficient than short stroke.
• Fuel injection is supposedly more efficient than carb (but the timings are difficult to get right without electronics).
• Large cylinders
• Low RPMs

While there are no comprehensive fuel economy charts for aero engines, if you could find an engine that had most of the features listed above, you'd probably have the most efficient engine, although that's only if what I've read is true. I doubt direct injection engines were any more efficient due to more primitive timing control of the era.

Your bullet point two I think you mixed up, almost all aero engines were four stroke, the only two stroke I know was the RR Crecy.

 

[Shortround6]

Four valve engines were significantly more efficient than two valve engines. I think some inline engines were four valve but no radials.

Efficient for fuel economy is different than efficient for power. Unless we are comparing fuel economy at full or nearly full throttle?

Long stroke engines are more fuel efficient than short stroke.

True the difference is minor since very few engines were short stroke. Longer stroke than bore was the normal.

Fuel injection is supposedly more efficient than carb (but the timings are difficult to get right without electronics).

Fuel injection was more efficient than carbs, but in cruising it was under 10% so????

Large cylinders

things get confusing here. Big cylinders are harder to fill with mixture. But at cruise you are not filling the cylinders anyway.

Low RPMs

Most aircraft engines used in WW II operated max rpm at between 2100-2200 to 3000rpm, a few oddballs aside (Napier Sabre was one) with cruise in proportion.

There may have been claims made post war by car makers about war time developments making their cars better than the competition but most of that was advertising BS.
Chrysler hemi was a great engine but the Hemi head dates back to about 1913 if not before. So have large shovel of salt ready for advertising claims.

 

[MiTasol]

I've read that there were a number of innovations in engine design during the pre-war and war period relating to engine efficiency. Does anyone know if these are true?

• Four valve engines were significantly more efficient than two valve engines. I think some inline engines were four valve but no radials.
• Four

Starting with the Jupiter (or maybe earlier) in the mid 1920s, all Bristol radials predating the move to sleeve valves had 4 valves per cylinder. This is why, in that time period, Bristol engines were licence built in more countries than any other engine.

 

[Shortround6]

There were a slew of reasons for going to 4 valve heads. Not the least of which was that with the alloys of the time and crappy cooling large 2 valve heads tended to warp their valves. Have the exhaust valve not seal lead to other problems real quick.
Another problem was that valve springs were not what they would be even in 1930. Engine companies took out full page ads if one of their engines managed a long distance flight without needing at least one replacement valve spring. Using small light valves helped the springs last longer compared to large heavy valves.
4 valve heads meant the larger valve seat area transferred heat better from the valve to the head than an equivalent 2 valve head.

It was a lot of these problems that the sleeve valve was suppose to solve. But a lot these problems were solved by other means (better metallurgy) before the Sleeve valve was ready for production.

 

[PStickney]

You want to add confusion - 2 identical P-38s fly the same mission, so same hp/hr - one uses 100s of gallons less fuel than the other.

So, the pilot/flight engineer has huge impact on aero engine fuel consumption.

If one of them is flying in Auto-Ritch, High RPMs, and low Manifold Pressure, and the other following the manual - Auto-Lean, Low RPMs, and High Manifold Pressure, that's the most likely outcome.

 

[MiTasol]

If one of them is flying in Auto-Ritch, High RPMs, and low Manifold Pressure, and the other following the manual - Auto-Lean, Low RPMs, and High Manifold Pressure, that's the most likely outcome.

The manual was actually wrong for a very long time. Lockheed said do x and the USAAF said do Y. Lockheed were correct but it took a long while for the message to get through.

 

[Simon Thomas]

From "Principles of aircraft propulsion machinery" by Israel Katz.

 

[AerialTorpedoDude69]

S Shortround6 the poster is asking for fuel economy. So my guess is that they're asking about fuel economy in cruise.

I also forgot to list high compression ratio engines as being more efficient. I think that high octane engines are also more efficient but for the same reason that high compression ratio engines require high octane fuels.

Going off what Simon Thomas posted, the only engine that combines all the features that we're talking about that might stand a chance against the Lycoming (which wasn't a WW2 design IIRC) in fuel economy per HP, would be the DB 603 which was long stroke, high compression, direct fuel injected, and four valve. According to Wikipedia it had the following specific fuel consumption:

0.288 kg/(kW·h) (0.474 lb/(hp·h))

But I wouldn't know how to find the 603's fuel consumption for cruise.

 

[tomo pauk]

But I wouldn't know how to find the 603's fuel consumption for cruise.

Perhaps a good start - a table from the manual for the 603A:

Fuel injection is supposedly more efficient than carb (but the timings are difficult to get right without electronics).

British have shoehorned the fuel injection system from the Jumo 211 to the Merlin in 1940, and immediately noticed the lowered consumption figures on the test bench. Also, DB 601A (fuel injected) have had lowered consumption that the carburated DB 600.

FWIW, the Alfa Romeo 33 1.7L with fuel injection was more powerful and more frugal than mine Alfa 33 1.5 TI with two twin carbs.

 

[Shortround6]

From "Principles of aircraft propulsion machinery" by Israel Katz.View attachment 787242

We have to believe the advertising departments and that 9 out of 11 engines (and the 'best' engine never flew) had identical fuel consumption.
I have waterfront vacation swamp land for sale in Florida and fertile farm land Arizona (just add water) also available.

early engine chart for a P-47.

Using the lines for minimum specific fuel consumption for this engine and assuming 6lbs per US gallon we get
0.45 lb/hp/hr at 5000ft/1700rpm/32in
0.442 lb/hp/hr at 15,000ft/1850rpm/31in (turbo is kicking in)
0.518 lb/hp/hr at 25,000ft/2150rpm/31in

Economical max power is
0.525 lb/hp/hr at 25000ft/2250rpm/32in

above this power needs rich mixture.
Max continuous
0.775 lb/hp/hr at 29000ft/2550rpm/42in

Max power, but not WEP
0.81 lb/hp/hr at 25000ft/2700rpm/52in


What is the SFC of the R-2800?

We can also do the math on the R-2800-8 in a Corsair

We can also get a good idea of what the turbocharger does by looking at the power levels and fuel consumption figures of the different altitudes of this 2 stage multi-speed supercharged version.

 

[AerialTorpedoDude69]

I just read something about continuous water injection improving efficiency:

Squirts of Water Can Boost Engine Performance, Fuel Economy by 13%

Bosch's WaterBoost system cools engines down to save on gas while increasing power

spectrum.ieee.org

The DB 603 never had MW50 injection as standard. There was only a few experimental and prototype engines that were modified to use it, likely because of mechanical issues adapting the technology to liquid-cooled engine. As far as I'm aware, MW-50 injection is relatively rare in inline engines but more common among radials. It appears that there is no perfect engine that combines all fuel efficient technologies of the era. But there are a few that might.

The Centaurus had a lot of variants and there may have been one with direct fuel injection. It certainly had MW50 injection, although not in the way that the Japanese used it for continuous operation above a certain throttle setting.

There are some radials that were designed specifically for fuel economy, such as the Ha 105-Toku stored at NASM. It is a Sakae that was modified for fuel economy but there's no other information on this modification.

Nakajima Ha 105 Toku, Radial 14 Engine

Type: Reciprocating, 14 cylinders, 2 rows, radial, air-cooledPower rating: 858 kW (1,150 hp) at 2,700 rpmDisplacement: 27.9 L (1,704 cu in.)Bore and Stroke: 130 mm (5.1 in.) x 150 mm (5.9 in.)Weight: 585 kg (1,289 lb)The Nakajima Ha 105 began development as a transitional engine

airandspace.si.edu

There's also the Nakajima Ha-107, Ha-117, and Ha 217 (or Ha 46, depending on your nomenclature), which were developments of the Nakajima Mamoru engine, an unusual four-valve per-cylinder radial. The Mamoru had large cylinders and longer strokes. A refinement of it probably would have added direct injection and continuous MW50 injection, but I can't find any information on these prototype engines other than what's available on Japanese Wikipedia.

 

[ThomasP]

Hmmm . . . at the risk of throwing a monkey wrench into the gears. How do we account for the different weights/densities of the fuels used for the sfc ratings? The Germans seemed to use liters/hour for rating their engine charts, while the US and UK used gallons/hour on their engine charts. But we seem to be resting our estimates (so far) on pounds/HP-hour, and this may (probably will?) give us a skewed view of what the sfc was vs what the values would be if we used [volume]/hour.

The reason this makes a significant difference is that - in general - the high aromatic content fuels used by the UK (pre- and early-war) and Germans weighed more than the low-aromatic gasoline used by the US and UK (war-time).

UK pre- & early-war__________ .62-.625 lbs/USgal (high aromatic)
US pre- & early-war__________ .60-.61 lbs/USgal (very-low to low aromatic)
German war-time____________ .62-.63 lbs/USgal (high aromatic)
US made war-time__________ .60 lbs/USgal (very-low aromatic and also the majority of the fuel used by the UK during the second half of the war)
France pre- & early-war______ not sure, but the number I have is about .625 lbs/USgal
Japan ?
Italy ?
Soviet ?

The numbers above should be representative of the US/UK/German fuels, but if anyone has authoritative data on the subject I would be very interested - particularly for France/Japan/Italy/Soviet fuels.

Incidentally, when I went through the MSDS sheets on today's automotive and aviation gasolines, I found that most of the US and EU fuels (regardless of country) are what would be considered low- to medium-aromatic, with most around 20-25% and weights of .620-.625 lbs/USgal - though I have run across a few significant exceptionally high (upto 40%) and low (<1%) aromatics among the individual manufacturer.