You may be thinking of LPG, which is a mix of propane (three carbon atoms) and butane (four carbon atoms) and it's a liquid under reasonably low pressures. LNG

You may be thinking of LPG, which is a mix of propane (three carbon atoms) and butane (four carbon atoms) and it's a liquid under reasonably low pressures. LNG is pure methane (one carbon atom) and you have to cool it to -163 degrees C to make it liquid. I don't know of any land vehicles that would use LNG as a fuel. Normally it's just transported in enormous LNG tankers by sea to import (regasification) terminals and then fed into the grid. The comparison with coal is quite apt, because one of the main selling points of LNG is that it displaces coal in terms of power generation and as an industrial feedstock. There's also CNG which is compressed natural gas, and that's (largely) methane but it's a pressurized gas not a liquid. That is sometimes used in heavy goods vehicles.