It doesn't say, but perhaps something like compressed air? I seem to remember some LEGO toys used to have air hoses and pumps, didn't they? In theory if they hooked up enough of that it could work.
It obviously doesn't just "run on air" like we're thinking with air replacing gasoline in a combustion process, so it almost has to be something like compressed air being pumped in to make the pistons work and turn a driveshaft. That would be my guess.