Why are piston engines for aircraft rarely diesel powered?

Question

Why hasn't diesel caught on much for aircraft engines? The list of diesel aircraft engines on Wikipedia is very short.

The reason I see mentioned is their poorer power-to-weight ratios. (e.g. What are the advantages/disadvantages of diesel/Jet A-1 piston engine?)

I knew this was true in the past but is it true even today? Does anyone have values comparing the power-to-weight rations of typical diesel vs gasoline engines, say for automotive use?

Also, doesn't the better efficiency, especially at higher altitudes compensate for the poorer power-to-weight ratio by allowing lesser fuel and more payload?

50 years ago diesel engines for automobiles were uncommon too but in the last decade they have been steadily making headway in the car market. Do the same developments translate to the aircraft engine selection too?

Other than weight, are there any other disadvantages of diesel?

Answer 1

You are right, there are few modern aerodiesels around - so few in fact that we can go through the brief list one by one:

• Thielert: This is a development based on Mercedes-Benz car diesels, of which more than 1500 1.7 liter engines were built. Currently, a 2.0 liter engine is offered. The original Thielert company went bankrupt in 2008 and has been taken over by Continental.
• Austro-Engine, another conversion of a car diesel for aircraft use. It is the result of the disappointment of Thielert's main customer, Diamond Aircraft, with the record of Thielert's initial Centurion 1.7 engine.
• SMA:, a French turbocharged diesel with 5 liters displacement. Currently, around 100 of them have been delivered after almost 20 years of development work.
• Zoche: The product of a father-son team with many clever details, but a quarter century of development work has not yet resulted in an airworthiness certificate for their two-stroke radial diesels. While Zoche claims to be the victim of a conspiracy by the authorities, the accused complain that the engines could not keep their cylinder heads on for long enough.

What has held the diesels back for so long?

• Cost: Development in aviation piston engines has effectively stopped with the jet age. The existing designs were simply good enough until fuel prices went through the roof. This made diesels only attractive in niches, and as a consequence, production runs were far too small to justify the major development effort. Aviation engines run constantly at around 70% of their full power, so regular car engines could not be used without heavy modifications. As a consequence, a conversion kit can cost you $89,000!
• Weight and complexity: The higher compression ratio of 1:30 requires a heavier structure, and fuel injection, a gearbox and turbocharging push the part count well above that of a gasoline engine. Once you compare system mass (fuel plus engine), the diesel wins only at ranges which are now mostly flown by jets. Yes, fuel flow is less (10 gallons per hour for a diesel against 13.5 gallons per hour for a comparable gasoline engine), but will only help at ranges above a couple of hundred miles.
• Vibration! With that compression ratio, every ignition causes a three or four times heavier jolt to the engine and the aircraft structure. Not only the engine, but the whole aircraft structure needs to be designed with that in mind.

To be fair, a modern gasoline engine will also benefit from fuel injection, a gearbox and turbocharging, and Zoche initially did not use an electric system, relying instead on pressurized air for starting the engine (a pressure bottle loses much less of its stored energy when cooled to -20° than a battery, after all), but the practical aerodiesels are a lot more complex than the engines they are meant to replace, creating an obvious reliability disadvantage.

Thielert Centurion 1.7 in a Diamond Twin Star (picture source)

Answer 2

Although this has partially been covered in this question, it's different enough to warrant an answer.

One word: cost

In the vast majority of light piston aircraft the benefits of diesel are way lower than the costs in involved.

Diesel engines are heavy, this is because the diesel cycle relies on cylinder pressure rather than a spark to ignite the fuel-air mix. This means that diesel engine blocks have to be at least part steel, as aluminum is not strong enough to contain the pressures. All the aero-diesel engines I have read about have a hybrid steel-aluminum block, which is harder and more expensive to manufacture. The weight of the engine has the following impact on costs:

• You have to have a variable pitch prop in order to get the same amount of performance
• The extra weight up front means that you have to re-arrange the balance of the airplane, moving parts from the front to the aft
• Diesel fuel requires a complete re-working of the fuel system, so the wings have to be completely opened up for new tanks and the new piping. Some of this piping is to cycle warmed fuel through the tanks to prevent the fuel from gelling at low temperatures
• Although diesels do not require a spark they are heavily computerized, so they require steady electrical power to operate. 2 batteries are required for redundancy. More weight, more cost
• The aircraft has to have a STC (supplemental type conversion) in order to be legal. If your airplane doesn't have one you'd have to pay for it yourself, which costs tens of thousands to millions of dollars to do
• Your dashboard has to be modified for the new instruments and controls

So not only are the engines more expensive in the first place, fitting them is very expensive as well. Unless your airplane is going to fly a lot it would take decades to recoup the costs. Many of the other downsides (lack of Jet A1 fuel at many GA airfields, lack of engineering support for the engines, etc) would go away if it was cheaper to get a diesel installed.

Answer 3

The issue with weight to HP has been surpassed by many manufacturers already. There are 200hp diesel engines in the market, that are smaller and weigh less than its counterparts from Lycoming or continental. Actually, their frontal dimension is smaller than avgas engines, providing for the possibility of smaller and more aerodynamic cowlings resulting in less weight and higher performance. This engine can further be enhanced by using MT propellers that are lighter than traditional propellers and provide better climbing performance.

The cost of the Diesel engine will go down as did computers with more volume. If we consider that jet fuel can be 50% to 60% cheaper and some engines are 30% to 40% more efficient than avgas engines, it merits for new airplanes to consider the use of diesel and abandon the close to the 100-year obsolete technology of the avgas engines.

Furthermore, diesel engines lend them self for future use of alternate fuels and biodiesel that could be produced cheaper than biofuel for avgas-burning engines.

Until the electric technology for aviation does not reach a point of balance between speed and range, I consider that the future for us real pilots that have to cope with ever increasing costs of flying would benefit from lower operating costs if Diesel airplane would be available.