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Some GA aircraft, notably the DA40/DA42, are equipped with piston engines that use Jet A-1 and/or automotive-grade Diesel. An example of such engine is the Austro Engine AE300:

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What are the advantages/disadvantages of these engines and why are they not more common?

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Advantages

  1. Compression-ignition engines have higher compression ratio which leads to being more efficient. So the aircraft range improves and on longer flights payload capacity may improve as well. Fuel consumption is usually about 30% lower.
  2. The higher efficiency combined with the fact that Jet-A fuel is cheaper mean they have lower operating cost. Especially since in Europe the leaded avgas is heavily taxed to discourage it's use for environmental concerns.
  3. They should be a bit more reliable. A diesel engine needs high pressure fuel pump, but apparently there are less problems with those than there are with spark-plugs.
  4. They are somewhat easier to operate, since they don't have separate throttle and mixture controls.
  5. They don't suffer the problems associated with incorrect settings of those controls, namely knocking/detonation and pre-ignition.
  6. Turbo-charging them does not carry the risk of pre-ignition and most of them are turbo-charged, so the performance does not decline as fast with density altitude.
  7. There is lower risk of fire since jet/diesel fuel is less flammable (has higher ignition temperature).

Disadvantages

  1. They are heavier for the same power, because they need to be made from stronger material due to the higher compression ratio and because they need to have larger cylinder volume because of lower maximal rpm. On longer flights the reduction in fuel weight often makes up for the heavier engine.
  2. Turbo-charging comes with specific operating procedures unfamiliar to those used to normally aspirated engines and a slight lag in thrust lever (it's not a throttle) response.
  3. As others mentioned, small airports may not have jet fuel yet. This is probably better in Europe where the pressure to phase out 100LL avgas is stronger. Some engines also use automobile diesel fuel or even either jet or diesel fuel as diesel engines are less picky about what they burn.
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Modern aero-diesel engines are far more technically advanced than the majority of avgas powered engines as they are very new designs. Most avgas engines are normally aspirated, most of the time carbureted, and have manually controlled fuel to air mixtures, which is ancient engine technology. Aero-diesels have turbochargers, electronic fuel-injection, and computer-controlled mixture which gives them better performance and efficiency. Prop pitch is also usually computer-controlled, giving the pilot a single-lever control.

So there are no real disadvantages to the engines themselves, the disadvantages are more around the logistics and start-up costs. There are few mechanics who can work on them, and few aircraft with STCs which allow them to be retrofitted, so if you want one chances are you can't have one. To retrofit (if permitted) requires substantial modifications to fuel tanks, fuel delivery systems, electrical systems, and more, so it's not a cheap option.

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weight can be a notable disadvantage of some "aero-diesel" designs (though I understand this has improved quite a bit, with a least one engine claiming a weight savings over its avgas counterpart) –  voretaq7 Apr 22 at 5:18
    
This is true, diesel engines have higher pressures than gasoline engines, therefore are made from steel rather than aluminum. Many aero-diesels started life as automobile engine blocks but newer designs have been designed from the start for aircraft use and have been able to shave of weight. –  GdD Apr 22 at 10:12
    
@voretaq7: The linked engine itself is very slightly heavier than it's gasoline counterpart. The weight savings come from the fact that you have to carry less fuel. –  Jan Hudec Apr 23 at 9:40
    
In a way it is silly, aero-diesels have advantages, but many of them come from the fact that Jet-A1 is cheaper in many locations. If we built an engine around aviation-grade gasoline with the same technology we could probably do away with the lead and have as good or better performance than aero-diesels, but they'd still be more expensive to operate because of taxes. –  GdD Apr 23 at 10:27
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Advantage: AvGas is heavily taxed in Europe. This is primarily an envy tax, because it brings in less than the cost of its administration, but the non-flying majority feels good by "punishing the millionaires who waste their money on flying". AvGas costs currently about 2.6 - 3 € per liter in Germany, for example.

Jet fuel cannot be taxed in the same way because arbitrage is too easy. Airlines would simply stop refueling within Europe. At prices of around 1 € at large airports the difference is substantial. With a DA-40 you might not get the same price that airlines get, but a sizable difference remains.

Disadvantage: At pure GA airports, you will have a hard time to get Jet fuel at all. There are very few piston engines which can use Jet A-1 and are rated for aviation. Car engines cannot be used without heavy modifications, because they are not designed for continuous operation at 60% or 70% of their maximum performance. Due to the higher internal pressure, the engines tend to be heavier and produce more vibrations. However, since the specific fuel consumption of a Diesel engine is lower, the total system mass for ranges in excess of 1000 km should be lower.

The GA market is simply too small to support the design of new engines. We have to use what was created half a century ago. It's a shame, but it has been shown several times in the last two decades - there were a number of attempts to create GA diesel engines, with very little to show.

In the 1930's Junkers made a range of very interesting Diesel piston engines (see here) which powered a range of aircraft designed for very long ranges. They were used for transatlantic mail service (see here and here for examples).

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Energy content of Jet fuel is only higher per volume. Per mass it is lower. However diesel engines have significantly better thermodynamic efficiency due to the much higher compression ratio. –  Jan Hudec Apr 23 at 9:24
    
@Jan Hudec: Thanks for pointing this out, the density of Jet-A1 is between 0.78 and 0.84 kg/l and that of AvGas 0.69 - 0.76 kg/l. If you are space limited, switching to a Diesel helps a lot; if you are mass limited, less so. But still, the SFC is 250 g/KWh for AvGas and 200 g/KWh for Diesel engines. Using mean values for the density, this translates into 0.246 l/kWh for Diesels and 0.345 l/kWh for AvGas engines. –  Peter Kämpf Apr 23 at 10:23
    
@David Richerby: You're right "stop refueling" is what I meant. Airliner ranges are big enough today to make refueling stops a thing of the past. I really want to express that in case Europe levies a hefty aviation fuel tax, airlines will still service European airports, but do so with enough fuel in the tanks to cease to refuel there. –  Peter Kämpf Apr 23 at 12:44
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