One of the many reasons for the slow progress in the certification of diesel engines for GA use, is the doubled affect of the cost. First you have to type certify the engine, and within that, the propeller, a hugely costly effort. Then, you have to certify that enigne propeller combination in each aircraft type, one by one. All of this on top of the immense development and design costs. There is just not enough need for the aircraft out there to justify this. I worked on and off for a year with a company who were planning to type certify the Subaru diesel for aviation use. I was just to costly, and complicated to document the quality control and traceability of an automotive engine, when the eninge manufacturer did not support this effort. The rather limted choices of suitable propellers for diesel engine's torque characteristics does not help the matter much either.
More recently I was involved in the STC approval of the DA42 back to gasoline engines. The DA42 diesel is a delightful and simple aircraft to fly, but challenges with the engines are well documented. The Lycoming IO-360 is tried and true. A gas gobbler, well, yes, compared to a diesel. But dollar for dollar to operate the gasoline engine is still cheaper overall, and pilots are continuously whining about the cost of flying.
If pilots and owners were saying, here industry, is gobs of money for developement of diesel powered aircraft, we'd be getting the new techology. Pilots and owners (with the exception of the DA42 and Thielert STC'd aircraft owners, are not) so here we are.
Of the six vehicles/machines I own, only the plane and the ATV are gasoline powered. The ATV used to be available diesel, but they discontinued it the year before I bought (I guess they had their reasons). The plane only sees MOGAS, so the saving of diesel would not really get the operating cost lower enough to be worth the up front cost. Gasoilne should be a fuel of the past for aircraft, but that just is not going to be the case for a very long time. The will just is not there for the development of long term economical diesel for aircraft. Here in North America, we can't even get car owners to embrace it!
Pilot DAR