Liquefied Petroleum Gas

Liquefied petroleum gas (LPG) is a mixture of commercial propane and butane. It is obtained either from the natural gas processing or during petroleum refining. Composition of LPG varies very widely from country to country depending on the use and demand of butane. LPG is a gas at normal ambient temperatures and pressures (the boiling point of propane and butane at atmospheric pressure is about - 45° C and -2° C, respectively). When subjected to pressure of 4 to 20 bars mixture of propane and butane becomes liquid. The pressure at which it becomes liquid at room temperature depends upon propane to butane ratio.  The LPG is usually  fed to the engine in gas phase.

• Most LPG vehicles operate on bi-fuel systems for operation either on gasoline or LPG. It is important as the number of LPG filling stations is usually small.
• One drawback with a bi-fuel system is that the engine is neither optimised on LPG nor on gasoline.
• Better cold start and warm-up characteristics due to its gaseous state compared to gasoline hence lower HC emissions.
• HC emissions from LPG vehicles have significantly lower potential of smog formation compared to gasoline and diesel fuels.
• Negligible PM emissions compared to diesel.
• Small reductions in CO compared to gasoline as no enrichment of mixtures during warm up or acceleration phase is required.
• No significant difference in NO_x emissions.
• Variation in propane/butane ratio in LPG poses problem as the octane number of the two main constituents; propane (RON is 112) and butane (RON is 94) is quite different.
• For bi-fuel vehicles specific technological development will be necessary to ensure compliance with the stringent emission standards.