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Optimized 1997 model year Caterpillar C10 dual-fuel natural gas engines certified to the California Air Resources Board's Alternative Low NOx 2.5 gram/brake horsepower-hour emission standard were demonstrated in three commuter buses over a 12-month period, in Santa Barbara, California. The project evaluated the retrofit costs and process, performance, reliability, fuel economy, operating costs, and emissions of the three C-10 dual-fuel natural gas engines compared to a standard C-10 diesel engine. Chassis dynamometer tests using the U.S. EPA Urban Dynamometer Drive Schedule, the Central Business District (West Virginia University version) and the 55-mph Steady State cycles were conducted to characterize in-use emissions of the dual-fuel engines for the commuter bus application. During 94,000 combined service miles, performance, reliability and durability of the dual fuel buses were similar to the diesel control.

Emissions from trucks and buses equipped with Caterpillar dual-fuel natural gas (DFNG) engines were measured at two chassis dynamometer facilities: the West Virginia University (WVU) Transportable Emissions Laboratory and the Los Angeles Metropolitan Transportation Authority (LA MTA). Emissions were measured over four different driving cycles. The average emissions from the trucks and buses using DFNG engines operating in dual-fuel mode showed the same trends in all tests - reduced oxides of nitrogen (NOx) and particulate matter (PM) emissions and increased hydrocarbon and carbon monoxide (CO) emissions - when compared to similar diesel trucks and buses. The extent of NOx reduction was dependent on the type of test cycle used.