By: Norm Goyer
The fact that 100LL aircraft fuel is going to be withdrawn from production due to its source of lead in the atmosphere has spurred a frantic search for a replacement. It seems that aircraft fuel using lead is the sole survivor and the environmentalists want it stopped. The pressure is growing and it appears that 100 LL will soon be history. The logical substitute is diesel fuel as this is also used by turbine powered aircraft and diesel powered support vehicles at all airports. So what’s the problem? Diesels have a very long history of not working very well in aircraft and pilots have a prejudice about their use because of the history of failures. It is also a fact that diesels engines are classified as “compression ignition” engines. So? Our Lycomings, Continntals and even the huge radials and V-12s currently installed in our aircraft fleet have moderate compression ratios that does not cause failure of connecting rods and for creating cracks in the case from the constant hammering of the diesel engines. In order to prevent this self destructive damage diesel engines have to have very rugged construction and more metal means more weight. Big trucks are not hampered by heavy engines but small aircraft are. Not too many years ago the FAA tackled this weight vs strength situation and their solution was not acceptable to aircraft owners. After a certain number of hours the diesel engine was to be removed from aircraft service and a new diesel installed. Sales of existing diesel engines came to a slowdown.
The use of diesel engines dates back to the dirigibles of World War I. The diesels use of minimum fuel and its anti fire properties sounded good to designers, in reality they didn’t work that well. Diesels never became popular after decades of attempts. The designers now claim that new developments in chemistry, metallurgy and engine design such as turbos and fuel injectors have solved the problems. I would not bet money on this, history is not with the diesel in airplanes.
The first manufacturer of aircraft diesels to produce a certified design for the general aviation market was Thielert, located in the small town of Lichtenstein in the German state of Saxony. 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 (Jet A-1). Their first engine, a 1.7 litres (100 cu in), 135 hp (101 kW) four-cylinder (based on the 1.7 turbo diesel Mercedes A-class power unit), was first certified in 2002. It is certified for retrofitting to Cessna 172s and Piper Cherokees which were originally equipped with the 160 hp (120 kW) Lycoming O-320 320 cubic inches (5.2 l) Avgas engine. Although the weight of the 135 hp (101 kW) Thielert Centurion 1.7 at around 136 kilograms (300 lb) is similar to that of the 160 hp (120 kW) 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. This is the engine that Redbird is putting into their restored Cessna 172. The problem with this unit is that the clutch in the gear box must be replaced very 300 hours with a rebuilt clutch from Germany. This will take at least two days labor to replace the clutch. Thielurt designer of the engine is now in jail for fraud. This engine has an extremely poor reputation.
Another Diesel engine which is under development for aircraft use is the SMA from France. This Renault engine made its mark in F-1 racing cars. 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 (170 kW), 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.
Interest in diesel aircraft in the United States has been more limited. However, doubt about the future availability of avgas has raised awareness of diesel alternatives. In March 2008, the Indus Aviation team led by Aldo Sibi (Director Of Production, Chief Mechanic and Head of Research and Development) prototyped the world’s first diesel powered Light Sport Aircraft, N211GD. This airplane was built and flown in 30 days. This novel aircraft, although a prototype, sparked huge interest in alternative fuels in the industry.
Europe has always loved diesels in their cars and in many of their airplanes. The pilots in the United States have trusted our Lycomings, Continentals and our auto engines for so many decades the switch is much more complicated.