Natural gas is my guess.

Either compressed natural gas, which has the disadvantage of weight (the tanks are heavy as faaaak)

or Liquified natural gas, which has the disadvantage of not lasting long (it must be stored at a very low temperature).

I don't know which one you are referring to, but I am sure that it is one of them.

LNG is fun, as there are highly insulated tanks that will store the fuel for a week or two before bleeding it out because of it evaporating when the temperature goes up.

Let me know which it is, though...
-=B-=