Diesel vs. petrol in aircraft

[Pasoleati]

I found an interesting chart comparing fuel efficiencies of actual aircraft. Please compare the Ju 86 with petrol and diesel engines. The difference is massive! The source is Shell Aviation News April 1939. Article by some Lufthansa director.

 

[Dagger]

The efficiency of a piston engine depends very much on its compression ratio:
the higher the CR the higher the efficiency and therefor the lower the fuel consumption.

However a gasoline engine with a higher CR requires a higher octane number gasoline to avoid knocking (detonation).
The article is from 1939 so ON were still rather low in those days.

A diesel engine can have a higher efficiency than a gasoline engine for the same CR. Moreover a diesel engine works on a different thermodynamic principle and cycle. It is fuel cetane number that matters, not octane number.
Diesel engines are usually heavier than gasoline engines.

You could research the CR of each engine in that table and plot it against the mentioned fuel consumption. That should give a trend, however the weight and aerodynamic shape of the plane also affect its fuel consumption.

 

[Pasoleati]

To be honest, in real world engines that is not so simple. For example, a Wright R-1820 G-series Cyclone running on 91 octane fuel and with a compression ratio of 6.45:1 has a cruising sfc of 190 g/hp/h whereas a "modern" Lycocrap AEIO-360 running on 100LL and with a compression ratio of 8.7:1 manages approx. 206 g/hp/h. A WW1 Austro-Daimler achieves about 226 g/hp/h with a CR of approximately 5.4:1 and WW1 fuels were perhaps 60 octane at best. So, CR is in practice a very poor stick of measure.

And weight has surprisingly little effect of speed and range. 1 % increase in flying weight has no measurable effect on speed (the difference is smaller than measuring errors).

Increasing CR is a very poor way to decrease sfc and/or increase power in SI engines as beyond 7:1 the improvement quite rapidly diminishes (the sfc curve flattens) while peak pressures increase rapidly.

 

[Nicknick]

well, compression ratio helps and it even beyond 7:1 you have a real gain (up to about 14- 16:1), but it only partially explains the better fuel economy of the Diesel. Diesel Engines run lean all the time which helps to reduce heat losses and their combustion speed is much greater at low rpm (unlike Otto engines it is almost constant, independent of rpm). Conventional gasoline engines need to be throttled at part load which reduces their effectivity dramatically. Also, they usually (especially air cooled aero engines) need to run rich at full power which is extremely inefficient.

As long as there is no mechanical limit, you can supercharge Diesels without delaying the combustion, because there is no danger of knocking.

The much higher cylinder preassure (compression ratio + boost) plus the additional injection equipment will make it heavier than Otto engines, so that it is quite demanding to design Aero Diesel which can replace a conventional Engine.

 

[JEDI]

Most modern diesels do not run on the Diesel cycle but are closer the Otto cycle. The pure Diesel meters the fuel to burn at constant pressure. Engine efficiency can be improved by injecting the fuel as quickly as possible to raise cylinder pressures and get the added power from the Otto cycle due to the higher peak pressure.

The greater heat content (BTUs) of the diesel fuel can also slew the efficiency numbers when comparing gallons. Comparing pounds in aviation use is more realistic but still favors the diesel as the heavier fuels have more carbon and therefore more BTUs per pound.

 

[Nicknick]

This is only partially true, since at high loads the injection start is delayed because of limitations by the maximum peak pressure. You will see a first peak pressure by compression followed by a second peak by the combustion. The late energy release is closer to the Diesel cycle than to the Otto cycle. At low loads and engine speeds, the Diesel is closer to the Otto cycle than any non hydrogene Otto engine.