Using Transmission Data to Isolate Individual Losses in Coastdown Road Load Coefficients

As part of the U.S. Environmental Protection Agency's (EPA's) continuing assessment of advanced light-duty automotive technologies in support of regulatory and compliance programs, the National Vehicle Fuels and Emissions Laboratory has benchmarked multiple transmissions to determine their efficiency during operation. The benchmarking included a modified "coastdown test," which measures transmission output drag as a function of speed while in neutral. The transmission drag data can be represented as a second-order expression, like that used for vehicle coastdown test results, as F ₀ + F ₁ V + F ₂ V ², where V is the vehicle velocity. When represented in this fashion, the relationships among the three coefficients were found to be highly predictable. The magnitude of these coefficients can be quite large, and 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 target and dynamometer set 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. Therefore, the vehicle coefficients can be used directly to estimate the transmission and drivetrain losses and eliminate them from the coastdown values. With transmission losses eliminated, the remaining losses can be divided to extract more accurate estimations of aerodynamic losses and rolling losses. This process can be applied fleet-wide, using only the reported coastdown and dynamometer test coefficients to estimate the losses from individual sources. The resulting data can then be used to independently evaluate the effects of reducing each separate loss, without the need for detailed information on each vehicle in the fleet.