Statistical Analysis of Emissions and Deterioration Rates for In-Use, High-Mileage CNG and Gasoline Vehicles

This study compares emissions test results among seven compressed natural gas and seven standard gasoline 1996 Ford Crown Victoria taxi cabs. The study applies a mixed-effects analysis of covariance (ANCOVA) model on data from three rounds of high-mileage emissions tests performed at target odometer readings of 60,000, 90,000, and 120,000 miles (i.e., 96,540, 144,810, and 193,080 km). The ANCOVA model describes emissions deterioration within each vehicle sample, while also accounting for individual vehicle variability within the sample. The model was used to determine whether there are statistically significant differences between each fuel's emissions profiles and deterioration rates over the mileage range studied. Results are reported for non-methane hydrocarbons, carbon monoxide, oxides of nitrogen, carbon dioxide, formaldehyde, and fuel economy.