Aircraft diesel engine

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.

Early diesel aircraft

The first successful diesel engine developed specifically for aircraft was the Packard Radial Diesel of 1928-29, which was laid out in the familiar air-cooled radial format similar to Wright and Pratt & Whitney designs. The first successful flight of a diesel powered aircraft was made September 18, 1928 in a Stinson model SM-IDX "Detroiter," registration number X7654 (presenlty owned by Greg Herrick and based near Minneapolis, Minnesota). Contemporary with the Packard Radial, the ill-fated R101 airship also used diesel engines, but these had been modified by Beardmore from an engine originally intended for railcar use.

Entering service in the early 1930's, the two-stroke Junkers Jumo 205 opposed piston engine became far more widely used than previous aero diesels. 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 LZ 129 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 (later Dassault Aviation), whose MB203 bomber prototype used Clerget diesels of radial design.

Interest in diesel engines in the postwar period was sporadic; having worked on a derivative of the Junkers Jumo 205 before the Second World War, called the 'Culverin', Napier & Son took up diesel aero-engines again. The British Air Ministry supported the development of the 3,000 hp Napier Nomad which was exceptionally efficient in terms of brake specific fuel consumption 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, diesel-powered aircraft virtually disappeared. The near-death of the general aviation market in the 1990s saw a massive decline in the development of any new aircraft engine types.

Modern developments

Several factors have emerged to change this equation. First, a number of new manufacturers of general aviation aircraft developing new designs have emerged. Second, in Europe in particular, avgas has become very expensive. Third, 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 application.

Germany

The first manufacturer to produce a certified design for the general aviation market is Thielert, located in the small town of Lichtenstein. 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.

Thielert users include Austrian aircraft firm Diamond Aircraft Industries which 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.

In May 2008, Thielert announced on their company website that they were insolvent and were looking for new investors. Legal disputes have been reported between Diamond and Thielert over costs of spares and contractual issues , with Diamond apparently developing their own diesel power unit, also based on the automotive Mercedes engine, rather than attempting a financial rescue of Thielert.

France

SMA engines, located in Bourges, 150 km south of Paris have designed a four-stroke, air-cooled, turbo-diesel aircraft engine from the ground up, the SR305-230. SMA's engineering team came from Renault Sport (Formula 1). The 230 hp, 305 cubic inch (5.0 liter) jet fuel engine first obtained European certification in April 2001, followed by US FAA certification in July 2002. It is now certified as retrofit on several Cessna 182 models in Europe and the USA, and Maule is working toward certification of the M-9-230.

USA

Interest in diesel aircraft in the USA has been more limited with the lower taxes on fuel there. However doubts about the future availability of Avgas has raised awareness of diesel alternatives, and Indus Aviation unveiled its latest aircraft at Sun 'n Fun 2008. This is a proof of concept aircraft built by a team led by Aldo Sibi, who successfully installed and tested a British-made Wilksch Airmotive Ltd WAM-100 (see section on Experimental Engines) in a Thorp Thorpedo T-211D Light-sport aircraft. It was built in less than 30 days and successfully flew to Lakeland, Florida. Further development and testing has been announced, and it is expected to attain pre-production prototype status by Oshkosh 2008. From there Indus plans on ASTM certification for the airframe, and making the aircraft and engine available worldwide.

Experimental Engines

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 100 Hp twin-cylinder (therefore four-piston), two-stroke opposed piston engine inspired by the original Junkers design. Their engine has flown in test aircraft and airship installations. Unlike the Junkers it is made for horizontal installation with a central output shaft for the geared cranks, the overall installed shape thereby approximately resembling a four stroke flat-four engine.
• 'Powerplant Developments, a British company developing a 100 Hp opposed piston engine called the Gemini 100 that resembles the Dair engine and uses the Junkers twin-crank principle, again for horizontal installation with a central output shaft for the geared cranks. However, the Gemini 100 is a three-cylinder (therefore six-piston) engine. Like Dair, Powerplant Developments claim to be using Weslake Air Services for production. They have recently announced that Tecnam will test a prototype with the Gemini engine.
• Wilksch Airmotive Ltd, a British company who are developing/producing a 120 Hp 3 cylinder (WAM-120) two-stroke diesel and are working on a 4 cylinder 160 Hp design (WAM-160). In 2007 Wilksch claimed that they had completed multiple tests on the WAM-100 LSA in accordance with ASTM F 2538 - the WAM-100 LSA is a de-rated WAM-120. Wilksch originally showed a 2-cylinder prototype alongside the 3- and 4-cylinder models.
• 'Vulcan Aircraft Engines', an American company currently developing the Raptor 105 diesel engine. It is a four-stroke inline turbo charged engine.
• DeltaHawk Engines, an American company currently developing V-4, 160, 180 and 200 horsepower designs. Also using a two-stroke, ported design, they have also flown a prototype engine in a pusher configuration Velocity aircraft and are claiming delivery of non-certified engines since 2005 and hope to achieve certification in 2008. DeltaHawk engines have a dry oil sump so can run in any orientation, Upright, Inverted or Vertical Shaft by changing the location of the oil scavenge port. They can also run counter-rotation for installation in twins.
• Eco-Motors, a company with sites in Germany and France, which developed an 100HP aircraft engine based on a small turbo charged automotive diesel.
• 'Zoche', a German company developing two-stroke diesel radial engines for general aviation.

Dair, Wilksch and Zoche at least have had considerable problems bringing their prototype designs into production, with delays running into several years; as of August 2008 none of these manufacturers could supply engines to customers. The Dair-powered Airship is no longer registered by the CAA.

External links

• http://www.wilksch.com/ Wilksch Airmotive Ltd homepage
• http://www.oldengine.org/members/diesel/Duxford/avrange.htm Diesel Engines in Aviation
• http://www.experimentalaircraft.info/ 'Experimental Aircraft Info', a website dedicated to building experimental aircraft and diesel engines in aircraft.