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The last couple of years development in aircraft engines has been more or less concentrating on diesels. We have seen one off installations to fully developed 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).
Diesel engines are able to use JET fuel (AVtur). This fuel is available worldwide and can also be made of renewable sources (algae) which should contribute to a cleaner environment.
In the United Kingdom are a number of aircraft diesel engine manufacturers active, or at least they claim to be as it has been very quiet on their part.
They are: Wilksch Airmotive, Diesel Air Limited and Superior Air Parts Powerplant Developments. We will shine a light on each to see what they have to offer to the experimental aircraft market.
Diesels are inherently heavier than a comparable petrol or gas engine. This due to the design as acting forces (torque) are much higher inside the engine and as such it needs to be build stronger. But adding weight in an aircraft or engine can only be done if it can be put to good use. Diesel engine manufacturers are looking for ways to minimize weight by using sometimes unusual designs to maximize the power to weight ratio.
Two stroke diesels, compared to four stroke, deliver more power for their weight. This is good. Boxer or horizontally opposed type engines are lighter because the opposing piston/ cylinders balance each other out and there is no need to use extra balance weights in the engine for smoothness, which helps to keep the weight down.
Usually lightweight turbo's are used to gain power at altitude or to normalize the engine (keep the sea level pressure constant from sea level to a certain altitude without boosting the engine). No nonsense low weight mechanical fuel injection systems are simpler, weigh less and are more reliable than the heavier, common rail, dual FADEC and electrical system, high pressure fuel injection systems we see in modern computerized car diesels.
The manufacturers mentioned above use a combination of these principles in their engines and this has led to two different designs. Two manufacturers used the two piston per cylinder (acting toward each other) principle without valves like the German Junkers JUMO series, Rolls-Royce, Napier Deltic and Coventry Climax engines.
Formerly developed by Powerplant Developments Ltd - Weslake Air Services - Jade Aire but was acquired by Superior Air Parts in 2014.
At Sun and Fun Florida, US Superior Air Parts annouced they are manufacturing and selling the Gemini Diesel engines from PPD and plan to bring them to the market somewhere this year (2015). This engine is aimed at the LSA market for small two seaters and possible homebuilt aircraft. They expect to deliver the turbocharged Gemini 125 early 2016.
A relatively newcomer on the aircraft diesel market. The design consist of a three cylinder, horizontally opposed two pistons per cylinder. This engine, Gemini 100, has one cylinder and two pistons more than the DAIR-100. Developed power is 100 bhp at 2500 RPM.
In contrary to the DAIR-100 and the Wilksch engines, the propeller is driven through a reduction drive where both of the crankshafts runs at 4000 RPM and the propeller at 2500 RPM. This increases power for almost the same weight.
Some pre-production information about the engine can be read in these old Powerplant Development pdf files: Gemini 100 specifications, the FAQ's, Gemini 100 principle of operation and the Gemini 100 power curve.
Drawings of the engine, compared to an O-200 are here: outline comparison and engine outline drawing.
The Gemini 100 is the first of a range of engines where new metallurgy has been applied to proven technology and the results look to be very promising. In the near future we hope to see the Gemini 125, a turbocharged version of the Gemini 100 suitable for aircraft like the Murphy Elite, RV-9 and more. The Gemini 100 was at one point being tested in a Tecnam Aircraft by PPD in the UK.
More technical information from Superior on the Gemini 100/125 and Gemini engine family.