In model year 2006, three EPA-certified natural gas urban bus engine models were available. These engines are equipped with relatively simple emission control systems, including engine modifications, electronic controls, oxidation catalysts, and oxygen sensors. On average, they are certified at a 77% lower PM emission rate and a 47% lower NOX rate than the diesel baseline.
Alternative Fuels Study: A Report to Congress on Policy Options for Increasing the Use of Alternative Fuels in Transit Vehicles, U.S. Department of Transportation Federal Transit Administration