I always find it amusing when people think they can just put a car engine in an aeroplane and away you go. Has never really happened yet has it? Wonder why? The operating regimes are miles apart.
Well, it has actually. Take a look at Diamond Aircrafts DA40 and DA42 Twin. Powered by Thielert Centurion 1.7 Turbo Diesels which are modified Mercedes A-Class engines.
http://www.centurion-engines.com/index.htm
The DA42 Twin crossed the atlantic on 72 gals of fuel! If that isn't efficiency I don't know what is. Maybe that particular engine isn't powerful enough for even the smallest heli, but Jet A1 burning pistons must be the way forward for aircraft like the 22 or 300
It might make them vaguely affordable (nahh, who am I trying to kid)
fly safe