Interesting Discusssion
Comparision with auto engines is flawed. Yes, auto engines have shown great advances in technology, but look at the vast difference in the market size to spread the developement costs over.
Some of the technology on the auto engine doesn't operate quite so well in the aviation environment. Show me one auto conversion that has worked reliably in an aircraft? I don't count the Theirlet as successful yet, they have their issues.
Auto engines don't go to 100% output every time they operate and then run a 75% of rated power for hours on end.
Yes, your standard Lycoming or Continental isn't as smooth as your twin overhead cam 4 valve per cylinder electronic fuel injected chariot. The main reason is because the aero engine is a large capacity engine, this a product of the need for high power at low RPM. Why not run a gear box you say? Take a look at the reliability (low TBO) of gear boxes generally on aero engines of less than 9 cylinders.
A Lycoming or Continental may be old technology, genuine failures are rare and they generally go to or past TBO.
They don't stop when the gear is retracted either as happened to a Theirlet equipped aircraft recently. I know there were contributing factors, my point is they are simple and work well.
I understand that Lycoming has plans for a diesel engine but I suspect that until avgas prices get to much higher prices in the USA that progress wont be fast.