Seventy cents of natural gas possesses the same amount of energy as one dollar of gasoline. CNG also produces low tailpipe emissions, no evaporative emissions, and low refining energy. Unfortunately, however, CNG requires bulky gas cylinders and higher cost vehicles. CNG vehicles must be tested and inspected annually for corrosion, pressure, and possible gas leaks. Propane is a by-product of gasoline, but it can also be extracted from natural gas.
Propane offers slow evaporative emissions and virtually complete combustion.
Safety valves should also discharge to the atmosphere and not to enclosed spaces.