I still say steam is the way to go...we could burn dried algae...or cow poops.
Big problem for steam is the amount of time it takes to warm it up and the energy lost after walking away from a warmed up engine. Parking lots in strip malls would have to be twice as large, since customers would take as much time warming up their engines as they do shopping (unless they left them running while making purchases.)
Do you really want to warm up your engine for 15 minutes to pick up a pizza?