Aircraft diesel engine

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The diesel engine has not been widely used as an aircraft engine. Its excellent specific fuel consumption has been outweighed by combination of its inherent disadvantages compared to gasoline-fuelled or turboprop engines and the accidents of history.

[edit] Early diesel aircraft

The first diesel aircraft engine was the Junkers Jumo 205, which used an opposed-piston two stroke design. Entering service, it was moderately successful in its use in the Blohm & Voss Ha 139 and even more so in airship use. A Mercedes-Benz diesel engine was also used in Zeppelins, including the infamous Hindenburg. This engine proved unsuitable in military applications and subsequent German aircraft engine development concentrated on gasoline and jet engines. Other manufacturers also experimented with diesel engines in this period, such as the French Bloch MB203 bomber prototype [1]. The Clerget diesels used were radial designs [2].

Interest in diesel engines in the postwar period was sporadic; the British Air Ministry supported the development of the 3,000 hp Napier Nomad. It was exceptionally efficient but judged too bulky and complex and cancelled in 1955. With fuel available cheaply and most research interest in turboprops and jets for high-speed airliners, interest in diesel-powered aircraft virtually disappeared. The near-death of the general aviation market saw a massive decline in interest in the development of any new aircraft types.

[edit] Modern developments

Several factors have emerged to change this equation. Firstly, a number of new manufacturers of general aviation aircraft developing new designs have emerged. Secondly, in Europe in particular, avgas has become very expensive. Thirdly, in several (particularly remote) locations, avgas is harder to obtain than diesel. Finally, automotive diesel technologies have improved greatly in recent years, offering higher power-to-weight ratios more suitable for aircraft applications.

The first manufacturer to produce a certified design for the general aviation market is 'Thielert GmbH', located at the small town of Lichtenstein, eastern Germany (not to be confused with the principality of Liechtenstein, between Switzerland and Austria). They produce four-stroke, liquid-cooled, geared, turbo-diesel aircraft engines based on Mercedes automotive designs which will run on both Diesel and Jet Aviation fuel (JetA1). Their first engine, A 1.7 litre, 135 hp four-cylinder (based on the 1.7 turbo diesel Mercedes A-class power unit) was first certified in 2002. It is certified for retrofit to Cessna 172s and Piper Cherokees which were originally equipped with the 160-hp Lycoming O-320 Avgas (petrol) engine. Although the weight of the 135 hp Thielert Centurion 1.7 at around 136 kg, is similar to that of the 160 hp Lycoming O-320, its displacement is less than a third of that of the Lycoming. It however achieves maximum power at 2300 prop rpm (3900 crank rpm) as opposed to 2700 for the petrol Lycoming.

The Austrian aircraft firm Diamond Aircraft Industries offers its single-engine Diamond DA40-TDI Star with a Thielert Centurion 1.7' engine and also the Twin Star with two. The Star offers low fuel consumption with a very fuel efficient figure of 15.1 l/h. Several hundred Thielert-powered airplanes are now flying, and the company has certified a 4.0-litre, V8, 310 HP version in 2005.

'Apex aircraft', formerly Robin also offers an aircraft (Ecoflyer) with the Thielert engine

Interest in diesel aircraft in the USA has been more limited with the lower taxes on fuel there.

A number of other manufacturers are currently developing experimental diesel engines, many using aircraft-specific designs rather than adapted automotive engines. Many are using two-stroke designs, with some opposed-piston layouts directly inspired by the original Junkers design. Examples include:

• 'Dair', a British company who are developing a twin-piston, two-stroke layout inspired by the original Junkers design. Their engine has flown in test aircraft and airship installations.
• DeltaHawk Engines, an American company currently developing V-4, 160 and 200 horsepower designs. Also using a two-stroke, ported design, they have also flown a prototype engine in an aircraft and are claiming delivery of non-certified engines in 2006 and hope to achieve certification in 2007.
• Eco-Motors LTD, a company with sites in Germany and France, which developed an 100HP aircraft engine based on a small turbo charged automototive diesel.
• 'Zoche', a German company developing two-stroke diesel radial engines for general aviation.