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 this text we spend some time with these engines as they are getting more and more common on the flight line and are accessible to aircraft builders too.

Diesels in General

Diesel engines use the compression ignition principle. Fuel is injected into the combustion chamber (either direct or indirect) under high pressure. Due to compression of air by the piston in the cylinder, temperatures are very high and the fuel ignites almost instantly when injected.
BMW Radial Diesel Engine Diesel fuel burns slower than gasoline therefore restricting the maximum RPM of the engine to around 4500 RPM. On the contrary, diesels deliver very high torque at low RPM. This is ideal for propeller driven aircraft. One drawback is that due to higher compression and acting forces in the engine, these engines tend to be a bit heavier than a comparable gasoline engine. Two stroke diesels overcome this problem, because they have a power stroke for every revolution per cylinder, in comparison with a four stroke diesel (every other revolution per cylinder).
Aircraft diesel engines are usually the in line or flat four type but BMW (among others) once developed a radial diesel engine, see image.

Diesels are (compared to their gasoline types) basically simpler, no ignition system, more reliable, durable, more torque, use less fuel, have a higher thermal efficiency and denser fuel which gives us more range (about 9%) for the same amount of fuel. Diesels have been used in aircraft before WWII (the JUMO series come to mind), since then, development was very slow due to the development of the JET engine.
Until now.

Two Stroke Diesels

A number of engines that are being developed are of the two stroke design. A two stroke diesel can not be compared to a two stroke gasoline engine, they use a mix of fuel and oil (1:50) to lube the engine and finally burn this mixture resulting in very polluting exhaust gasses.

German JUMO 205

Two stroke diesels have more in common with their four stroke cousins, only all of the engine strokes occur in one revolution: intake, compression, power and exhaust. That seems like a challenge, and it is. Timing is crucial here. Some engine designs use intake ports and exhaust valves, where as others have ports only and the pistons acting toward each other in pairs in the same cylinder, like the German JUMO diesels from WWII, see figure.
Read this tutorial (pdf) for a complete explanation on how they operate (from enginehistory.org).

Two stroke diesels must use some type of blower to start as their intake stroke is not capable of inhaling air (at this point the intake port is open but the piston is almost reaching or is at bottom dead point so there is no displacement of anything. The blower, usually a roots type, is engine driven and blows air under pressure into the cylinders. Most of these engines also have a turbocharger which takes it over from the roots blower when the engine is running, usually after reaching a certain RPM. And as RPM range is rather narrow (900 - 2700 RPM) the turbo can be perfectly matched to the engine requirements.

Engine design is simple: they have a low pressure fuel pump (common rail pumps are around 1800 bar), no electronics, no gearbox and a very reliable turbo. Just add oil, fuel and start. The battery is only needed to activate the glow plugs and start the engine.

The NASA has published the "Lightweight two stroke cycle aircraft diesel engine technology enablement program" in August 1985, it is dated but a very interesting document. Careful: this document is quite large, 116 pages (14Mb).

Four Stroke Diesels

This engine uses two revolutions to accomplish its task. Intake, compression in the first revolution and power and exhaust stroke in the second revolution, timing is less critical. Power is lower compared to its two stroke cousin, but that can be overcome by running it at a higher RPM and using a gearbox to reduce the RPM for the propeller.

Everything else being equal, compared with a two stroke diesel of the same displacement and RPM, the four stroke type will be heavier and have less power. A number engines are being developed are of four stroke design, they use a gear box (to get more power they need more RPM, two strokes are direct driven and have more power/torque at lower efficient propeller RPMs), usually have electronic engine management called FADEC (which must be dual for redundancy, a dual electrical system is also needed) and use common rail for higher injection pressures.

These engines are modern but very complex compared with the two stroke type. Weight is higher too. A very familiar engine manufacturer has developed a 135hp diesel weighing almost 150Kg (no fluids) without propeller. It is a modern engine which uses somewhat less fuel (10%-15%) than the two stroke. These engine installations are also quite complex, heavier, need dual electrical systems and a lot of hoses and radiators for cooling. Not exactly the engine for the first time builder.

Engine Manufacturers

Some companies designing and selling aircraft diesel engines are:

• Austro Engine GmbH, AE 300 2.0 170 bhp 560 Nm 4 stroke 2000 hr TBO. Based on a Mercedes car engine and developed with Diamond Aircraft for use in DA-40, DA-42 and DA-50 aircraft, launched mid 2008, flying since 2007. This engine is also based on a Mercedes Benz car engine and can be retrofitted in existing DA-40 and DA-42 replacing the TAE engines.
• Thielert Aircraft Engines GmbH, Centurion 1.7 (135 bhp) and 2.0 (135 bhp and 155 bhp) and Centurion 4.0 (350 bhp) 4 stroke, was used mainly in Diamond Aircraft DA-40/42, no longer in production due to bankruptcy. This engine is high maintenance and has very high cost due to design shortcuts and management decisions.
  Update 22 April 2008: Thielert in financial trouble investigations into fraud and criminal charges, source: Defence Industry Daily
  Update 4 May 2008: Cessna puts diesel airplane line on ice and TAE has a new general manager.
  Update 18 June 2008: Thielert is again producing Centurion 2.0 aircraft engines, expect 80 engines per month.
• Thielert Engine Owners must visit the website of the "The Thielert Engine Owner's Group".

• Deltahawk, 160 bhp, 180 bhp and 200 bhp, 2 stroke piston ported models, turbo and supercharged, flying
• Wilksch Airmotive, 120 bhp 312 Nm 0,49 lb/hp/hr, 3 cylinder 2 stroke, turbo and supercharged, (mass 100 Kg) and a 160 bhp 4 cylinder (mass 115 Kg), 120 bhp model flying and 160 bhp model almost ready
• Diesel Air Limited, DAIR-100, 90 bhp, 2 cylinder 4 pistons 2 stroke opposed based on JUMO design, model flying in airship, acquired by Howells Aero Engines Limited UK.
• Powerplant Developments Limited, Gemini-100, 100 bhp 285 Nm 0,39 lb/hp/hr dry mass 71 Kg, 3 cylinder 6 pistons 2 stroke opposed based on JUMO design, 125 bhp turbocharged expected end of 2008
• SMA, the SR305-230 engine, 230 bhp 4 cylinder, certified and STC available for retrofit
• Zoche, radial diesel 2 stroke, been designing for years (>15), nothing in production
• Raptor Turbo Diesel (Vulcan Aircraft Engines), 105 bhp 4 stroke 4 cylinder diesel with FADEC, dry mass is 81 Kg
• ECO Motors, 80 and 100 bhp 4 stroke 4 cylinder diesel with FADEC based on a car engine, dry mass is around 98 Kg

The future will be bright for the aircraft diesel engine!