An Empirical Approach in Determining the Effect of Road Grade on Fuel Consumption from Transit Buses 2010-01-1950

Transit buses contribute a meager amount to the U.S. criteria pollutant and greenhouse gas (GHG) inventory, but they attract a lot of attention from the public and from local government, due to their nature of operation. Transit bus fleets are often employed for the introduction of advanced heavy-duty vehicle technology and the formulation of new performance models. Emissions and fuel consumption data, gained using a chassis dynamometer, are often used to evaluate performance of these buses. However, the effect of road grade on fuel consumption and emissions most often is not accounted for in chassis dynamometer characterization. Grade effect on transit buses' fuel consumption was investigated using the road-load equation. It was observed that two parameters, including the type of terrain that buses traverse and the percentage of grade for that terrain, needed to be determined for this investigation. It was assumed that all bus routes started and ended at the same point so that no average grade could be ascertained. The authors addressed this issue by assigning a sinusoidally varying road grade with a specified maximum positive or negative grade, and this was applied over the duration of the route, but was not aligned specifically with activity within the route. The grade effect depended on the ratio of the power demanded with grade to the power demanded without grade, provided the vehicle operated at a fixed weight and consumed auxiliary power at a uniform rate during its operation. This analysis was then extended to a number of conventional North American drive cycles to examine the effect of road grade on fuel consumption for varying grades on synthesized rolling and mountainous terrains. Results showed that the effect of road grade on fuel consumption was also affected by bus operating speed. When a road grade analysis is performed and taken into consideration in conjunction with chassis data, it is likely to provide a better insight into the real-world emissions and fuel consumption from transit buses.