One of the interesting technical stories from this summer’s AirVenture in Oshkosh was that Cessna has completed certification and introduced into the market a diesel cycle 182 aircraft, known as the Turbo Skylane JT-A.
The engine, produced by SMA, a division of the French company Safran, is an opposed, 4-cylinder air cooled engine laid out much like a O-360 Lycoming. The engine displaces 305 cubic inches or 5 liters and produces around 230 hp. It is turbocharged and has a mechanical injection system much like a truck diesel engine.
So does this spell the end of 100LL avgas engines?
Well, no, but I think they are going to find a significant market for this aircraft. For one, a lot of third world countries will buy them because on-spec 100LL is very difficult to impossible to find.
There is also significant operating cost savings for the diesel over a corresponding horsepower avgas engine. First, the Brake Specific Fuel Consumption (BSFC) rated in pounds of fuel burned per horsepower hour is about 30% to 35% better than a 100LL engine. Then, you factor in the significantly lower cost of Jet A vs. 100LL and your fuel cost could be reduced by as much as 50%. This will help offset the higher purchase price for the turbocharged diesel engine. This may be a factor for numerous domestic fleets and commercial operators that have a higher use factor for their aircraft.
So are there any negative factors for the diesel, other than higher initial cost? There are some minor points, like cold temperature starting. In a gasoline engine, we pre-heat to get the assembly expanded and the oil thin enough to flow to all points once the engine starts.
With a diesel cycle engine, cold temperature starting is dependent on not only cranking fast enough, but you need to build up enough heat of compression to ignite the intake charge. This will be significant in very cold climates and will reduce the service ceiling because of limitations on relight altitudes. The low temperature limit of Jet A is -40C compared to -58C for 100LL. This will be significant for flying in Alaska and in some northern climates. I know some people will use Jet A-1, which has a -47C freeze point, but Jet B, if available, would not be an option.
Probably the biggest concern with a Jet A diesel is misfueling. Many years ago, several manufacturers named their aircraft Turbo something or other like the Cessna Turbo-210. This caused a lot of problems in the field because people saw turbo and thought jet fuel. Here is just the opposite.
Another real concern is that, technically, this engine will be happy running on good old diesel fuel, and someone somewhere is going to try it and find the engine runs fine on a warm day. However, if they do not get the tank drained completely, some of the heavy ends could plug things up on a cool day or at high altitude. There are also concerns about microbial growth and surfactants that can cause problems, especially on low usage aircraft.
The bottom line is I congratulate SMA and Cessna on the new aircraft. They should do well in the sales department and the aircraft will serve their customers well. But there are some differences that need to be recognized and owners and pilots need to be trained to observe these differences.
A wise man once said, “A little learning is a dangerous thing. Drink deep or taste not that Pierian Spring.”