Air pollutant emissions of light-duty vehicles operating on various natural gas compositions

Interest is growing in the expanded use of natural gas as an alternative to conventional liquid fuels for use in automotive internal combustion engines. This study investigates the implications of natural gas composition on the gaseous pollutants, fuel economy, and the engine power output of two light-duty vehicles (a 2002 Ford Crown Victoria and a 2006 Honda Civic GX) operated over different driving cycles. The results of the study show that for modern light-duty NGVs, fuel properties have a clear and direct impact on fuel economy and some emissions components, such as CO2 and NMHC, but not for other emission components, such as THC, NOx, and CO. The gases with the higher energy contents provided better fuel economy on a volumetric basis and some higher power levels. Changes in CO2 depended on the fuel/cycle/vehicle, but, generally, blends with heavier hydrocarbons and lower H/C ratios, had higher CO2 emissions. NMHC emissions were very low, in general, but did increase for the two fuels with the highest levels of heavier hydrocarbons for both vehicles. THC emissions showed higher emissions for the fuel with the higher levels of methane for the Crown Victoria, but no trends for the Honda. CO emissions were higher for the two fuels with the highest Wobbe numbers for the Honda under some test conditions, including the cold-start phases of the driving cycles, but did not show significant fuel differences for the Crown Victoria. Changing fuel composition impacted NOx emissions showed only limited fuel effects for the two vehicles.