While CNG, compressed natural gas, is mainly methane, LPG, liquefied petroleum gas, is the name of the hydrocarbon consisting of commercial propane and butane in liquid form. .
LPG becomes liquid at room temperature if moderately compressed and reverts to gas when the pressure is sufficiently reduced. Advantage is taken of this property to transport and store LPG in the liquid state, in which they are roughly 250 times as dense as they are when gases. . LPG liquefies under a very modest pressure of only 7 bars and, therefore, a lot of energy can be stored in a small space.
CNG on the other hand and unlike LPG, does not liquefy and, therefore, a pressure of around 200 bars has to be used to compress the gas and results in heavy and expensive tanks on the vehicle which are substantially larger than those required for LPG
Obviously LPG is safer as compared to CNG. However in terms of cost CNFG is cheaper. I uses both CNG and LPG are EQUALLY superior to petrol and diesel in terms of emissions.
LPG has my vot but being a poor Pakistani CNG is the choice.
Hope this helps
