It has taken tens, hundreds of millions of years for organic detritus to transform into fossil fuels – coal, oil and gas. These are being used primarily as fuel or feedstock. The concern over non-renewability of fossil fuels comes from the onceonly application as fuel. There are many other fuels but only some of the liquids derived from fossil fuels can be used to fuel off-track mobility.
Off-track Mobility
In the beginning, power sources  were externally attached to vehicles; examples: palanquins, chariots. Boats were the exception where the power source was on board. Integration of fuel and conversion system into vehicles began with the invention of steam locomotives and steam-ships. The age of coal gave way to the age of oil. Today, except for nuclear-powered ships and submarines, and rockets with solid or liquid propellants, all vehicles carrying fuel and engine on board are powered by refined petroleum liquids. There are vehicles which are externally connected to power sources – electric trains for instance, but such vehicles can move only on fixed tracks.