[shovel]

I'm a big fan of external combustion as it does not require a transmission of any sort (most external combustion designs can be operated in either direction and produce maximum torque at 0 rpm) and as suggested, can be fueled by anything that generates heat - the same flash boiler heated by gasoline could just as well be heated by a solar oven, electric element, kerosene, flatulence, whatever you like.

It is worth noting that if people are given a relatively fuel-agnostic car it would become possible to dispose of nearly any waste oil in your boiler, and because it would function it would be rewarding to those who do so ( save money by buying less fuel, save effort by just dumping that random oil in the burner instead of taking it to the auto parts store for recycling) - burning random junk would certainly not be a good thing for our health or that of our atmosphere.