I am quite certain that diesel engines are likely to be the future of light aviation.
Firstly, the reliability of a properly designed diesel engine is excellent. They have fewer working parts than spark ignition engines, having no magnetos or other ignition components to fail, as someone already pointed out. Provided the fuel pump keeps pumping properly, there is little to go wrong. Ask any truck or taxi driver. Most diesel engines clock up phenomenal mileages without problems.
Fuel consumption is excellent because of much better volumetric efficiency than petrol engines. Diesel engines do not have a throttle in the inlet so that a full charge of air is induced on each stroke; instead the power output is controlled by varying the output of the fuel injection pump. The very high compression ratios required to ignite the fuel/air mixture (over 20:1) also results in a more efficient thermodynamic cycle and low specific fuel consumption.
Diesels are not nearly as critical as petrol engines over fuel quality. They can run on anything from kerosene to re-cycled vegetable cooking oil! (However, I don't think that aviation engines would be certificated if they left a smell of fish and chips at the airport). The types of fuel generally used are not prone to vapour lock, unlike gasoline based fuels.
The problem of fuel waxing can affect the fuel filters of road-going vehicles in low temperatures. This however need not be a problem because the fuel used can be AVTUR, i.e. jet/turbine fuel. Even if a DERV type fuel is used, the formula can be adjusted to resist waxing as it is in European countries. Another remedy is to design in a fuel/oil heat exchanger as used in many modern turbine engines. These channel fuel en route to the engine around the oil filter casing, which heats the fuel aiding combustion or in this case also preventing waxing. This obviously also cools the engine oil as an added benefit!
The throttle response issue is not such a great problem as in cars because aircraft engines driving a propellor are required to operate over a much smaller RPM range. Diesel engines provide good torque at low speeds and are ideal for use in aircraft, as they are in marine applications. In any case, turbosupercharging overcomes the response and power output issues.
(Edit: One important thing I forgot to mention. Because there is no carburettor and no requirement to externally vapourise the fuel, not surprisingly there is no possibility of carburettor icing which is a significant safety advantage to light aircraft).
Ever seen or driven one of the latest Merc or BMW high performance diesel saloons? It is hard to believe they aren't petrol powered because all the disadvantages have been overcome. Even from the outside, they don't even sound like diesels any more.
I recently rode an experimental diesel powered motorbike. The engine, based on an industrial engine, was warranted for 3000 hours between oil changes. Work that out as far as how many miles you can travel between service intervals. Answer: at 60 mph you could go 180,000 miles! The manufacturer of the engine has apparently acknowledged that some industrial users removed the oil filters altogether so they didn't even have to bother changing them, and even this didn't result in reliability problems!
Well, I'd certainly buy one, as you have probably guessed by now!
ShyT
[This message has been edited by ShyTorque (edited 08 June 2001).]