MMM its very simple, auto engines do not repeat do not work in aircraft. look around do your recreach and you will find that this is so true. 1st a prop will not work at 8000 rpm which now will require a gearbox that will have to take that rpm down to around the 2500 rpm level and increase the toque on that gearbox will now make it extrmely heavy. VW engines that are used are already quite light being air cooled. the main problem with diesels is the vibration and that the impluse from the firing is not smooth this now causes vibration threw the whole of the engine and back into the airframe. look at how light a gas turbine gear box is, high rpm but light loads and next to no vibration. and at the end of it all if the engine stops you cant just walk to the shops can you when the electrics stop doing there little things that they do. the cheapest part of running an aeroplane is the fuel !
