Heavy-Duty Aerodynamic Testing for CO ₂ Certification: A Methodology Comparison

Aerodynamic drag testing is a key component of the CO₂ certification schemes for heavy-duty vehicles around the world. This paper presents and compares the regulatory approaches for measuring the drag coefficient of heavy-duty vehicles in Europe, which uses a constant-speed test, and in the United States and Canada, which use a coastdown test. Two European trucks and one North American truck were tested using the constant-speed and coastdown methods. When corrected to zero yaw angle, a difference of up to 12% was observed in the measured drag coefficients from the US coastdown procedure and the EU constant-speed test. The differences in the measured drag coefficient can be attributed, among others, to the assumptions in the speed-dependence of the tire rolling resistance and axle spin losses, the data post-processing required by each methodology, unaccounted frictional losses in the transmission, the behavior of the automated manual transmission during the coastdown run, and the yaw angle correction.