An Investigation of Road Load Effects on Fuel Economy and Nitric Oxide Emissions of Hybrid and Conventional Transit Buses

The fuel economy and exhaust emissions of transit buses are usually measured using a chassis dynamometer which simulates on-road vehicle load based on the mass of the vehicle, and road-load parameters including the coefficients of rolling resistance (CRR) and aerodynamic drag (CD). Although demonstrated as having significant effects on fuel consumption and exhaust emissions, it is difficult to get accurate rolling resistance and drag coefficients by limited coast down tests. Using PSAT as a platform, a model is developed to simulate the fuel consumption and exhaust emissions of transit buses including a conventional diesel bus and a series hybrid diesel electric bus. The validated model is used to investigate the impact of the variation in rolling resistance and drag coefficients on the variation of fuel economy and NOx emissions of transit buses operated over the Urban Dynamometer Driving Schedule (UDDS) and Washington Metropolitan Area Transit Authority (WMATA) intercity transit cycle. The effect of the variation in road load parameters on the changes in fuel consumption and NOx emissions are investigated by simulating the operation of transit buses with the values of rolling resistance and drag coefficients randomly selected from their normal distributions by specifying their coefficient of variation (COV). With a COV equal to 5%, 10%, 15% and 20%, the simulations are performed by randomly selecting the individual coefficient of rolling resistance, the coefficient of drag, and randomly selecting both parameters simultaneously. The results show that the fuel economy is more sensitive to variation in the road load parameters than the NOx emissions for both buses operated over both driving cycles. In comparison, the effect of road load parameters on NOx emissions is relatively small. The fuel economy of the hybrid bus is more sensitive to variation in road-load parameters than the conventional diesel bus. This is mainly due to the significant effect of road-load parameters on the amount of energy that can be captured during regenerative braking events, a significant feature of hybrid buses. This study shows that the coast down coefficients should be within 5% (10%) of their real value to have a ±2% (±5%) confidence in fuel economy and emission testing results, when the road load is the only factor varied