A series hybrid generator-pac. Their engine of choice at the moment? A 48hp Wankel that is used in a racing go-kart!
I said 2 years ago watch for the return of the Wankel, I was spot on. Again a Omnivorous engine, quiet and smooth, the question is can they get the fuel specifics (BSFC) down to were it is do-able.
The materials are their, expensive as all get up, unless a ceramic breakthrough occurs and if it did, my guess we wouldn't know because it would be used on don't ask what I do for a living stuff for quite sometime....
If you look around the world, I think we will see more European type turbo diesels.
If the battery pack or supercaps are taking care of peak power demands, a turbo diesel small enough to fit in a lawn tractor could power a car that handled like a Corvette.