Stirling Engine based hybrid car
I'm not the only one to have thought of this, but as yet no real work has been done. How about a hybrid car powered with a Stirling Engine? (Not spelled Sterling, btw.)
The Stirling is more efficient than the internal combustion or diesel engine, and it's also a lot quieter. Sounds great, but it's not good for cars because it can't rev up quickly and it takes about 5 minutes to get the engine hot enough to run well. We want our cars to start the minute we put the key in.
A hybrid design (with enough batteries for 10-20 miles) solves this. You can get all the acceleration you need from the electric motors, and you can start driving right away, while heating up the Stirling "boiler". Then it kicks in to provide the power to run the car for the long haul. If you know the trip is short, no need to fire up the Stirling until the battery gets low.
The Stirling can burn anything. Gasoline, kerosene, diesel, vegetable oil, hydrogen, even wood! Yes, you could, in theory, be stuck by the side of the road out of gas, then go out with an axe to chop trees and refuel your car.
Well, almost. You want high-temperature burning for the best efficiency, and this would pollute and probably dirty your nice clean boiler. Right now the engines are expensive to machine but I suspect that could change.