Nothing to do with mopeds, but:
The guys who build our giant steam-powered electric generators worry about air gaps a lot. Parts expand as they heat up. And even if you compensate for that, air gets pulled along by the rotor and gets ram-compressed in the air gap, which causes even more heat where you don't want it. So besides having a huge fan inside the generator case, they fill the thing with the world's slipperiest gas, which is hydrogen. That also prevents oxidation of the electrical insulation in there.