<P> The air engine is an emission - free piston engine using compressed air as fuel . Unlike hydrogen, compressed air is about one - tenth as expensive as fossil fuel, making it an economically attractive alternative fuel . </P> <P> Propane is a cleaner burning, high performance fuel derived from multiple sources . It is known by many names including propane, LPG (liquified propane gas), LPA (liquid propane autogas), Autogas and others . Propane is a hydrocarbon fuel and is a member of the natural gas family . </P> <P> Propane as an automotive fuel shares many of the physical attributes of gasoline while reducing tailpipe emissions and well to wheel emissions overall . Propane is the number one alternative fuel in the world and offers an abundance of supply, liquid storage at low pressure, an excellent safety record and large cost savings when compared to traditional fuels . </P> <P> Propane delivers an octane rating between 104 and 112 depending on the composition of the butane / propane ratios of the mixture . Propane autogas in a liquid injection format captures the phase change from liquid to gas state within the cylinder of the combustion engine producing an "intercooler" effect, reducing the cylinder temperature and increasing air density . The resultant effect allows more advance on the ignition cycle and a more efficient engine combustion . </P>

List 5 benefits of using hydrogen as a fuel source