Effects of Average Driving Cycle Speed on Lean-Burn Natural Gas Bus Emissions and Fuel Economy 2007-01-0054
Although diesel engines still power most of the heavy-duty transit buses in the United States, many major cities are also operating fleets where a significant percentage of buses is powered by lean-burn natural gas engines. Emissions from these buses are often expressed in distance-specific units of grams per mile (g/mile) or grams per kilometer (g/km), but the driving cycle or route employed during emissions measurement has a strong influence on the reported results. A driving cycle that demands less energy per unit distance than others results in higher fuel economy and lower distance-specific oxides of nitrogen emissions. In addition to energy per unit distance, the degree to which the driving cycle is transient in nature can also affect emissions. Speed correction factors for heavy-duty diesel vehicle oxides of nitrogen emissions have been utilized to account for differences in driving cycles and show that distance-specific emissions have a non-linear relationship with average driving cycle speed. There have been high distance-specific values at low average speeds, the lowest values at moderate to high speeds, and a modest increase at very high speeds. The emissions performance and fuel economy from three 2005 model year, 40 foot (13 meter) buses powered by John Deere 280 hp (208 kW), 8.1 liter, six cylinder natural gas engines were characterized by the West Virginia University Transportable Emissions Testing Laboratory (Translab). Two of the buses were operated through six cycles at half passenger load while the third bus was operated through seventeen cycles, including an idle cycle. A comparison matrix for all three buses contained three common driving cycles, each utilized at three different passenger loading conditions (no load, half-load, and full-load). Average speeds for cycles ranged from 3.57 mph (5.71 km/h) to 43.6 mph (69.8 km/h). Emissions from these three buses were similar when operated on the same cycle at the same weight. Distance-specific emissions of carbon dioxide, oxides of nitrogen and total hydrocarbons were all similar, with the highest values resulting from the cycle with the lowest average speed (New York Bus Cycle) and the lowest values resulting from the driving cycle with the highest average speed (Commuter Phase of SAE J1376). Scatter of data points around the best-fit line (power regression) was examined to identify factors other than average speed that affect the emissions. Data on effects of test weight did not yield a clear trend.