Effect of truck operating weight on heavy-duty diesel emissions

Heavy-duty diesel vehicles are substantial contributors of oxides of nitrogen (NO(x)) and particulate matter (PM) while carbon monoxide and hydrocarbon (HC) emissions from diesel vehicles receive less attention. Truck emissions inventories have traditionally employed average fuel economy and engine efficiency factors to translate certification into distance-specific (g/mi) data, so that inventories do not take into account the real effects of truck operating weight on emissions. The objective of this research was to examine weight corrections for class 7 and 8 vehicles (over 26 000 lb (11 793 kg) gross vehicle weight) from a theoretical point of view and to present a collection of original data on the topic. It was found by combining an empirical equation with theoretical truck loads that the NO(x) emissions increased by approximately 54% for a doubling of test weight. Emissions data were gathered from specific tests performed using different test weights and using various test schedules, which can consist of cycles or routes. It was found experimentally that NO(x) emissions have a nearly linear correlation with vehicle weight and did not vary much from vehicle to vehicle. NO(x) emissions were also found to be insensitive to transient operation in the test schedule. The observed trends correlate well with the theory presented, and hence, the NO(x) emissions can be predicted reasonably accurately using the theory. If NO(x) data were considered in fuel-specific (g/gal) units, they did not vary with the test weight. HC emissions were found to be insensitive to the vehicle weight. CO and PM emissions were found to be a strong function of weight during transient operation. Under transient operation, the CO emissions value increased by 36% for an increase in test weight from 42 000 (19 051 kg) to 56 000 lb (25 401 kg). However, CO and PM were found to be insensitive to the vehicle weight during nearly steady-state operation.