The effect of different road load implementation strategies on fuel economy of USPS step vans.

摘要

West Virginia University (WVU) is continuously improving and updating their testing quality, procedures and goals. As tests are conducted on the Transportable Heavy Duty Vehicle Emissions Testing Laboratory (THDVETL), real world implementation is crucial in order to compare and contrast vehicles within fleets, use of exhaust aftertreatment devices and hybridized vehicles. WVU implements road loads on the chassis dynamometer using a method described in 40 CFR §86.1229-85. The proposed method requires three variables from the vehicle: maximum height, maximum width and vehicle weight. The issue with this method arises due to the fact that it does not cover a wide range of heavy duty vehicle physical characteristics.An alternative form of implementing road loads is to conduct on-road coastdowns and use regression analysis to determine the vehicles? characteristics such as coefficient of drag and coefficient of rolling resistance. The coastdown procedure involves driving a vehicle to a speed and setting the transmission to neutral and letting the vehicle slow until it reaches a complete stop. Since there is no power being transmitted to the wheels, regression analysis of the speed versus time can be used to determine physical characteristics of the vehicle. Using the road load equation, which consists of four components (hill climbing load, inertial load, aerodynamic resistance and rolling resistance) one can implement real world power demand on the chassis dynamometer.20 tests were conducted using the FTP-75 test schedule and two USPS step vans with one being a hybridized version. Four test configurations were used for this study, loaded and unloaded for each of the two vehicles. The empirical method of road load implementation proved to be more suitable for this USPS step van compared to the theoretical method. The theoretical method assumes that the vehicle's aerodynamic drag is 0.735 compared to the empirical method's equates to 0.669. WVU uses eddy current dynamometers as a power absorption system to simulate aerodynamic drag and rolling resistance. The power absorption setting for the theoretical model was higher than that of empirical model, as expected. A noticeable fuel economy comparison for both vehicles arose due to more aggressive setting from the theoretical method compared to that of empirical method. The hybrid vehicle showed a 34.4% better fuel economy compared to the baseline vehicle using the empirical method. The theoretical method showed an improvement of 24.8% from the hybrid vehicle compared to the baseline vehicle. Comparing the amount of work done for each vehicle during the test cycles, the theoretical method showed a 2.29% difference between the two vehicles compared to the empirical method of 15.0%. This study proves that the theoretical model forces the hybrid vehicle to operate at higher loads where the full potential of the system is not used.