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 (avtur) is worldwide available and can be used in diesel engines. This fuel can also be made of renewable sources (biomass) which contribute to a cleaner environment.

In the United Kingdom there are a number of aircraft diesel engine manufacturers active: Wilksch Airmotive, Diesel Air Limited and Powerplant Developments. We will shine a light on each to see what they have to offer to the experimental aircraft market.

UK Manufacturers

Diesel engines 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 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 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. Usually lightweight turbo's are used to gain power at altitude or 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. The third manufacturer went with an inverted, inline three and four cylinder two stroke diesel with turbo and supercharger. Which creates a narrow but rather high engine, this raised thrust line helps keeping the propeller away from the ground.

Powerplant Developments Ltd - Weslake Air Services - Jade Aire

A relatively newcomer on the aircraft diesel engine market. Engine design is 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 the engine runs at 4000 RPM and the propeller at 2500 RPM. This increases power for the same engine weight.

More information about the engine can be read in these 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 in: outline drawing 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 are promising. End of 2008 will see the Gemini 125, an turbocharged version of the Gemini 100 suitable for aircraft like the Murphy Elite, RV-9 and more. Do keep an eye on these manufacturers, the future of aircraft diesel engines is really promising!

Wilksch Airmotive Ltd.

Wilksch Airmotive Diesel Engine

Wilksch has been developing their range of engines since the mid 90's when they flew a proof of concept engine in a J-3 Cub. This was a two cylinder model and capable of 80 bhp. Late 1999 they had their three cylinder model flying in a Europa aircraft, this is the 120 bhp engine and is also used in a Thorp, Murphy Elite and RV-9. They also have a three cylinder 100 bhp model flying in a Pietenpol Aircamper.

Development is still going strong and a four cylinder 160 bhp is under test. We expect that more power is possible in these engines and that the current 120/160 bhp limitation could be raised to at least 130/170 bhp with proper engine cooling design. Engines have a inlet port and exhaust valves and this is perfect for exact engine timing and tuning. We also have a diagram showing the Wilksch Diesel engine operating principle.

For engine startup a supercharger is fitted supported by a large truck size turbo for normal engine operations. Provisions for a hydraulic propellor control is available. The alternator is installed on the back of the engine and driven by gears. The engine features an intercooler and glycol/water cooling system.

Diesel Air Ltd.

Once working together with Wilksch, they decided to pursue a different engine design. The DAIR-100 is of the opposed piston like the JUMO design and has two cylinders with two pistons each acting toward each other.

From the site: "The twin cylinder opposed piston engine has two pistons in each cylinder with the combustion chamber formed between the crowns of the pistons. It eliminates the need for a cylinder head, camshaft and associated valves and has in effect two half length crankshafts linked by a gear train driving a centrally mounted propeller. The cylinders are ported at each end with one ring of ports opened by the air piston supplying the fresh air charge, and the other opened by the exhaust piston and connected to the exhaust system."

The engine has flown in a Luscombe aircraft, power boats and on an Airship with good results. Power developed is 100 bhp without turbo charger, with a turbo power is increased by 20%. They sell the engine in licence through other manufacturers. Dieseltech sells this engine as the Powerlite-100.