Pollution reduction in urban areas is a major driving force to upgrade mass transportation systems. Options to the urban planner include electric traction and combustion engine upgrade. Electric traction centralizes the emission source, usually removed from urban areas, but requires substantial capital costs and lead time for the transportation infrastructure. Engine emission improvement is possible through both fuel changes and engine upgrade. Natural gas engines are a viable option for clean-operating urban buses. In the near term, conversion of existing diesel bus engines to spark-ignited natural gas is an attractive solution in terms of capital costs and lead time. This paper contains the analysis required to transform diesel engines into spark-ignited natural gas engines. Experimental data are shown for both a turbocharged and a naturally aspirated conversion. Emission data are presented showing the natural gas conversion to meet present EEC emission requirements.

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