USE OF NATURAL GAS AND LIQUEFIED PETROLEUM GAS IN HEAVY-DUTY ENGINES. LITERATURE REVIEW

Exhaust gas emissions from heavy-duty vehicles will be reduced in the 1990s to the level hardly achiecable with conventional diesel techniques. Alternative uses of natural gas and liquefied petroleum gas (LPG) and experiences from their use as the fuel of heavy-duty vehicles are reviewed in this note mainly with reference to literature and, to a minor extent, to own experiments. Natural gas and LPG are good engine fuels. As regards LPG, only propane can be used in Finland's climate. These gases are used nearly exclusively in spark-ignition engines, but can also be used in stationary engines as the main fuel of the diesel engine. The gratest problems relate to their rather low energy density and high storage pressure. The fuel tanks are pressure vessels and require a lot of space. The output of uncharged gas engines is higher than that of corresponding diesels, but lower than that of charged diesels. The spark-ignition engines of heavy vehicles are nearly without exception based on diesel engines. Thew simplest modification requires reduction in compression ratio and construction of an ignition system and gas feed. It is easy to change an otto engine modified from a diesel in a vehicle, as changes are required only in the motor. The cleanest exhaust gases are obtained from a gas engine equipped with a threeway catalyst. Very clean exhaust gases are also achieved with leanblend engines compared to those of a diesel. The exhaust gases of the gas engines contain a very low amount of particles, while, for example, an engine operating on a stoichiometric blend ratio produces an abundance of gaseous emission components, if the exhaust gases are not cleaned up catalytically. In an engine equipped with a threeway catalyst, natural gas and LPG are equal fuels with regard to the purity of exhaust gases.