WW2 aero engine fuel consumption comparison (1 Viewer)

[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

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.

 

[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.