Aircraft Diesel Engines

The last couple of years development in aircraft engines has been more or less concentrating on diesel engines. We have seen one off installations to fully developed engine production lines.

A number of companies are active on this market primarily due to major concern of long term availability and the relative high price of AVgas (Europe). Jet fuel is available worldwide, even in places where AVgas is not and has to be flown in.

Diesel engines are able to use JET fuel (avtur). This fuel is available everywhere and can also be made of renewable sources (biomass) which will contribute to a cleaner environment.

The engines weigh a little more compared to a gasoline engine, but advantages are found in a much longer service life and higher MTBO.

Last but not least: they also have an excellent specific fuel consumption compared to their AVgas cousins and as fuel is denser too, range of a diesel powered aircraft is improved. Or with equal range payload will increase.

Diesels in General

A diesel engine is a compression ignition engine and it draws in air and compresses it. Fuel is then injected into the combustion chamber (either direct or indirect) with injection pressures up to 2000 bar. Due to compression of air by the piston in the cylinder (compression ratios are in the range of 14:1 to 24:1), temperatures are very high (700 - 900 °C) and the fuel ignites almost instantly when injected.

There is no need for a carburettor, a throttle valve (no carb ice!) or a separate ignition system. Starting a diesel in a cold environment can be difficult, a form of preheat should be used. To implement this, diesel engines use a glow plug per cylinder to preheat the cold air before and after starting and thus help the combustion the first couple of minutes after a cold start.

Two principles

Diesels can be two or four stroke principles. In aviation you will find both types. Automotive diesels are almost exclusively four strokes but in marine applications the large propulsion engines are two strokes. Where in aviation you will find both types.

BMW Radial Diesel Engine

High torque at low RPM

Diesel fuel burns slower than gasoline therefore restricting the maximum RPM of the engine to around 4500 RPM. On the contrary, diesels deliver a very high torque at low RPM. This is ideal for propeller driven aircraft. One drawback is that due to higher compression and acting forces in the engine, these engines tend to be a bit heavier than a comparable gasoline engine. Two stroke diesels overcome this problem, because they have a power stroke for every revolution per cylinder, compared to a four stroke diesel (every other revolution per cylinder).
Aircraft diesel engines are usually the in line or flat four type but BMW and Packard (among others) once developed a radial diesel engine, see image.

Reliable design

Diesels are (compared to their gasoline types) basically simpler: they have no ignition system, are more reliable, durable, have more torque, use less fuel and have a higher thermal efficiency and denser fuel which gives us more range (about 9%) for the same volume of fuel. Diesels have been used in aircraft before WW-II (the JUMO series come to mind), but since then, development was very slow due to the development of the JET engine.
Until now.

Fuel system

As already said, due to the fuel injection method used there is no carburettor or associated throttle valve. Power is controlled by the amount of fuel injected by the high pressure fuel pump. This is a very reliable but also a very delicate piece of hardware, fuel must be filtered (below 70 micron) and fed through a water/fuel separator sometimes combined with a electric heater so that any water is dissolved in the fuel and can cause no blocking of filters due to ice formation.

With the high pressure fuel system and injector there is more fuel fed to the engine than it uses, this extra fuel is heated by the engine and return to the tank in use. Advantage is that warm fuel reduces the possibility of ice formation.

Turbo and Intercooler

As diesels will be heavier in construction compared to a gasoline engine, it pays to increase efficiency and power to weight ratio by adding a turbo or supercharger combined with an intercooler. Air compressed by the turbocharger heats up and is cooled to lower its density by an intercooler and the extra compressed air is then able to burn more fuel for the same cylinder volume. Increased power is the result.

Diesel knock

Older type of diesel engines (tractor type) have a characteristic sound: diesel knock. Especially when the engine was cold and at low rpm. This knock is basically the same as detonation in a gasoline engine, unstable combustion and high peak pressures. This knocking was one of the reasons that these older diesels were build quite heavy.

Conclusion

Modern light weight diesel engines have dealt with this typical diesel knock. They use indirect injection, where fuel is injected into a prechamber; two stage injectors, which prolong combustion; electronic engine control (FADEC), for exact timing considering engine conditions; compression ratio's that are not over 20:1 and better fuel air mixing all have led to almost no knock at all.

The engine may be somewhat heavier but this will translate into reliability, durability and piece of mind when flying over inhospitable terrain or large bodies of water.

The future will be bright for the aircraft diesel engine!