An attractive alternative to the standard aircraft engine are aircraft grade Diesel engines. Diesels have been used almost exclusively in industrial applications and larger less weight dependent vehicle applications, Diesel has recently been revived as a potential alternative for aircraft.
Diesel’s are different in a few ways than the typical 4 cycle engine. Instead of using an ignition source like a magneto or spark plug, it uses the heat generated on the compression stoke to ignite fuel in the cylinder. It avoids the problem of detonation or preignition altogether by introducing fuel only when it wants it burned, somewhere near top of dead center, right before it wants combustion. This eliminates the need for octane enhanced fuels like 100LL to deal with the problem of detonation. This also eliminates the compression barriers most gasoline engines experience. Aviation engines sort of delay this barrier up to a higher compression level by using the before mentioned higher octane fuels. This fact is a crucial part of the Diesels efficiency, because the larger the compression ratio the more efficient the engine. The power stroke produces all the power on an engine, the longer the power stroke the more time the piston has to extract energy from the expanding hot gases. However, the longer the power stroke the higher the compression.
The main limitation to compression in Diesal engines lies in the engine structurally being able to hold itself together, as well as the auxiliary component that have to also deal with high pressures. This naturally results in heavier more bulky engines meant to take the abuse. Up until recently this has been the primary challenge to aircraft use of Diesel. Bulk equals alot of wieght.
Typical Diesel Engines have an efficiency of up to about 41% but average above 30% of the initial chemical energy in the fuel. The flow looking something like this.
Fuel 46.4 MJ —->good diesel 35% efficient (46.4*.35) —-> Work output 16.24 MJ
That would be 225 additional miles on a standard gasoline tank, if we assumed Diesel had the same energy density as gasoline. This would be a 75% increase in efficiency. However it doesn’t have exactly the same density, gasoline has a slightly higher energy content per weight but Diesel has more energy per volume. Diesel has a 10% higher energy content per gallon than gasoline, further increasing a tanks range. Due to the nature of diesels, they can accept a variety of fuels, ranging from jet A to biodiesal, and even vegetable oil.
With the large amount of benefits Diesel engines offer, they are seriously being considered by major aircraft engine manufactures. Some engine manufactures have already begun producing diesels. Although teething problems have been reported as this new technology hits the market. Additional complexity and less fuel robustness has been traded off in some of these diesels to obtain weight specs closer to that of current aircraft engines.
One interesting question engineers might pose would be, “what kind of energy flow would a hyrbrid look like using a diesel generator instead of a gas powered one”