I just had to respond after reading Paul McBride’s response to Alex Rougier’s question concerning diesels in the June 9 edition (Why is it better to put a diesel engine on a GA aircraft?) I disagree with the answer you gave him, and here’s why:
First off, a diesel is inherently more efficient because of the higher compression (read expansion) ratio. During the power stroke more energy is extracted from the hot gasses because they are expanded further than in an S.I. (spark ignition) engine.
Concerning fuels, one of the principal advantages of diesel engines is that they will run on jet fuel, diesel, biodiesel, vegetable oil, etc.
Jet fuel is less expensive than avgas (even more true for diesel), and far more widely available worldwide, and just as available here in the U.S. except at the smallest of airports.
Diesels are a high torque, low rpm device. They are quite happy running at 1,500-2,000 rpm without gearing — just the range required for more quiet and efficient direct drive on GA aircraft.
Audi just ran their new R10 diesel race car and won at Le Mans. The two R10s were in the top three for most of the race. The R10 diesel runs at half the rpm of the other cars and produces monstrous amounts of torque. They were much quieter than the S.I. cars as a result, and inherently more reliable since the lower rpm imposes less stress on the reciprocating elements.
Diesels are renowned for their longevity. They aren’t as hard on turbochargers either. The Turbine Inlet Temperature (TIT) for a diesel is so much lower than that for an S.I. engine that it doesn’t even need to be monitored. Diesels, of course, don’t have ignition systems, which are probably the biggest source of unreliability on GA engines. An aircraft powered by a diesel could eventually have a significantly greater TBO than is currently available for S.I. engines.
Diesel fuel (and to a lesser degree jet fuel) does not have near the volatility that avgas does. When an avgas-fueled plane is parked on the ramp on a hot day, there is significant amount of evaporated fuel that escapes through the vents when the sun heats the tanks. There will be less of this with a diesel powered plane. This is significant considering the direction that government is heading.
The oil companies have to use a completely different infrastructure to handle 100LL as opposed to what they use for mogas. 100LL has more lead in it than leaded mogas used to have. The oil companies are precluded from mixing the two in transport or even running them through the same lines without purging because of the possibility of tainting mogas with lead. For the oil companies, avgas is a boutique item — it has relatively low demand but a high liability associated with it.
Finally, considering how the GA manufacturers have been plagued by the sharks that we call lawyers, it is not surprising that they would take their time in dipping the first toe in the water. However considering all the advantages that diesels have, I believe they stand to reduce liability costs by adopting diesel power.
As always, advances in aircraft design are led by engine design. As more engine manufacturers provide diesel products, we’re sure to see the GA world adopt them.
Kurt F. Hanke