Effects of vehicle speed and engine load on emissions from in-use light-duty vehicles

Air pollution is a significant health and environmental concern on many temporal and spatial scales. Combustion, in particular in motor vehicle engines, is a major contributor to many air quality problems. This dissertation focuses on measuring pollutant emissions from light-duty vehicles in an on-road setting in order to provide knowledge of the effects of vehicle speed and engine load on those emissions. A fuel-based approach to measuring emissions is combined with instantaneous descriptions of vehicle speed and engine load.; The pollutants studied are carbon monoxide (CO), oxides of nitrogen (NO _x), non-methane organic compounds (NMOC), ammonia (NH₃), and carbon dioxide (CO₂). Significant changes in the emission factors of these pollutants were observed in a California highway tunnel due both to changing roadway grade and vehicle speeds. The traffic direction through the 4.1% roadway grade tunnel is changed twice daily to accommodate the dominant commute direction, with downhill driving speeds typically 80–100 km h−1 and uphill driving speeds typically 60–90 km h−1. Below a certain engine load threshold, the CO emission factor is approximately constant, and increases dramatically above the threshold. The NO_x emission factor increases with load across the entire range of operating conditions observed. Distance-normalized emissions of both CO and NO_x show greater variability with changing driving conditions than fuel-normalized emissions. In contrast, fuel-normalized emissions of NMOC are greatest during low-load downhill driving, while distance-normalized NMOC emissions are approximately the same for both downhill and uphill driving. Both the emission factor for NH₃ and the NH₃ to NO _x ratio were observed to increase with increasing vehicle speed and engine load. Fuel consumption in this highway tunnel investigation was measured to be ∼3.6 L/100 km for downhill driving and to range between 14.2 and 16.4 L/100 km for uphill driving with lower values of fuel consumption observed at higher speeds.; Emissions have been measured at the same tunnel sampling site under similar driving conditions during summers 1994–1997, 1999, and 2001. Reductions of 49 ± 4% for NO_x, 62 ± 5% for CO, and 67 ± 7% for NMOC emission factors were observed over this 7-year period. While these reductions include contributions from both gasoline reformulation and new vehicle emission controls improvements, it appears that improvements to emission controls on vehicles combined with turnover in the vehicle fleet are responsible for most of these reductions.