I disagree that just because an engine is stone-reliable in a light sedan, it will be equally reliable in a helicopter. In the average auto, an engine is only run at full throttle about 11% of its life, and is making full power (maximum RPM and WOT) less than 5% of its life (if ever).
That is absolutely true Flingwing.
Even if those car engines could be certified in aviation use, i doubt that they would ever reach sort of TBO that for examble Lycoming has. Even if those engines are massproduced (cheap), a TBO of 300 hours and it would be quite hard to sell it to anyone. An average R22 should get it's engine changed twice a year. Nice.
Thielert's smaller diesel engine is based on Mercedes' 1,7 liter CDI engine. I don't know the situation nowadays but last year it had 600 hr TBO when installed in C172. So car engines can be certified, but it doesn't happen easily.
