Using Transmission Data to Isolate Individual Losses in Coastdown Road Load Coefficients 2020-01-1064
As part of the U.S. Environmental Protection Agency’s continuing assessment of advanced light-duty automotive technologies in support of regulatory and compliance programs, multiple transmissions have been benchmarked to determine their efficiency during operation. The benchmarking included a modified “coastdown test,” with the transmission on an engine dynamometer, to measure the transmission output drag as a function of speed while in neutral. The transmission drag data can be represented in quadratic form, similar to that used for vehicle coastdown test results, as F0 + F1V + F2V2, where V is the vehicle velocity. When evaluating the transmission test data, the relationships among the three coefficients found to be highly predictable, and in some cases the magnitude of the coefficients were quite large. Additionally, for some tested transmissions the deviation between the quadratic regression and the measured drag at individual velocities can be significant.
To evaluate the effect of transmission losses in vehicle coastdown tests, the coastdown and dynamometer coefficients were pulled from the EPA’s published “Data on Cars used for Testing Fuel Economy” for an entire model year. The same relationships seen among transmission coefficients were observed in the vehicle coefficients contained in these data. These relationships can be used to estimate and eliminate the transmission and drivetrain losses from the coastdown values. The remaining losses can be divided to extract more accurate estimations of aerodynamic losses and rolling losses of all vehicles. This process can be applied fleet-wide, using only the reported coastdown and dynamometer test coefficients to separate and estimate losses from individual sources. The resulting data can be used to independently evaluate the effects of reducing each separate loss, without the need for detailed information on each vehicle in the fleet.