The smoke... what IS Robo powered by? Wood? Coal? Oil? Ethanol? Its clearly some combustible...
I'm guessing you just answered your own question: "SOME combustible". Not one particular kind, but any kind of combustible will do. At least this is how I see it.
Judging from the white exhaust, he's steam-powered - anything that will heat water to boiling will do. Wood, coal, oil, magitek fire core, anything. Probably a conventional combustible, though.
