Comparison of CNG and Gasoline Vehicle Exhaust Emissions: Mass and Composition - The Auto/Oil Air Quality Improvement Research Program 952507

Exhaust emissions of three vehicles fueled with compressed natural gas (CNG) were compared with emissions of three counterpart gasoline vehicles. The natural gas vehicles were tested on four CNG fuels covering a wide range of pipeline natural gas compositions. The gasoline vehicles were tested on AQIRP Industry Average gasoline and a reformulated gasoline meeting California 1996 regulatory requirements.

Nonmethane hydrocarbon (NMHC) and toxic air pollutant emissions of the CNG vehicles were about one-tenth those of their counterpart gasoline vehicles, while methane emissions were about ten times those of the gasoline vehicles. Carbon monoxide (CO) and nitrogen oxides (NO_x) emissions were more variable among the three vehicle pairs. CO emissions ranged from 20 to 80% lower with CNG than with gasoline, and NO_x ranged from 80% lower with CNG to equivalent to gasoline.

Comparing the four CNG fuels, methane increased about 40%, NMHC decreased about 40%, and reactivity-weighted emissions decreased about 15% when CNG fuel methane content increased from 91 to 98% of fuel hydrocarbons (86 to 97% of total gas composition). There were no statistically significant effects on CO, NO_x or toxic air pollutants.

Fuel economy expressed as miles per million BTUs was about 20% lower for the CNG vehicles than for their gasoline counterparts. Reactivity-weighted emissions (RWE), calculated to estimate differences in potential ozone forming characteristics of emissions, were about 90% lower for the three CNG vehicles than for their gasoline counterparts when the calculation was based on NMHC emissions alone, and about 80% lower when exhaust methane and CO were included. The fuel effects on RWE parallel the effects on NMHC mass.

Reformulated test gasoline C₂ representing California Phase 2 gasoline, tested in the gasoline vehicles, had consistently lower NMHC, CO, and NO_x and toxics emissions than AQIRP Industry Average gasoline. The average decreases in the three emissions were 24%, 21%, 12% and 36%, respectively. These results are consistent with other AQIRP data for these fuels.