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Direct Injection (DI) is believed to be one of the key strategies for maximizing the thermal efficiency of Spark Ignition (SI) engines and meeting the ever-tightening emissions regulations. This paper explores the use of propane and methane gas fuels in a 1.5 liter SI four cylinder gasoline engine with double over head camshafts, four valves per cylinder, and a centrally located DI injector. With DI technology, the injection timing can be set to avoid fuel bypassing the engine during valve overlap into the exhaust system prior to combustion. DI of fuel reduces the embedded air displacement effects of gaseous fuels and lowers the charge temperature. Injection timings and compression ratio are optimized for best performances at Wide Open Throttle (WOT) conditions when configured to achieve homogeneous charge at stoichiometry or run lean jet controlled stratified.

The gross emissions for the Australian vehicle fleet are forecast from estimates of population growth, vehicle ownership and vehicle in-service performance. These estimates are coupled with sensitivity analyses, to emissions deterioration and on-road performance and the impact of alternative fuels. It is concluded that more data on precontrol and high mileage controlled vehicles is needed to remove uncertainties in the projections. The results indicate that existing vehicle emission control standards will continue to give beneficial reductions in hydrocarbon emission levels through to 1995 and present levels will not be exceeded this century. Ambient ozone trends have fallen and are qualatatively in accord with reducing vehicle emissions.