A bi-fuel car is one that can run on two different fuels, most commonly they are petrol and liquified petroleum gas (LPG). Bi-fuel cars are based on standard petrol-engined models, but they have one fuel tank for each type of fuel and other modifications to enable the engine to switch between fuel sources. LPG fuel is cheaper to buy than petrol, so it can help reduce running costs, and it helps cars using it to emit less CO2 emissions.