This paper summarizes and compares the two main test methods for aerodynamic evaluations of heavy trucks in the United States and the European Union: coastdown testing and constant-speed testing, respectively. The methodologies were compared in terms of their instrumentation requirements, test-track requirements, testing effort, data analysis and use within simulation tools, precision, and accuracy. Both methods have relative advantages and deficiencies.
The ICCT conducted measurements on a U.S. tractor-trailer, an EU tractor-trailer, and an EU rigid truck in order to evaluate the two methodologies. For this study, all three vehicles were tested under the coastdown and constant procedures. The study found a difference of 9.2% for the U.S. tractor-trailer between the coastdown and constant-speed procedure. For the EU vehicles, the difference was 8.6% for the rigid truck and 12.1% for the tractor-trailer. The explanation of the divergence between the two procedures can be partly attributed to adjustment factors, speed-dependency of tire rolling resistance, speed-dependency of axle spin losses, and the differences in post-processing approach.
These findings suggest that the U.S. coastdown test results in a lower estimation of the CDA when compared to the EU constant-speed test, even when accounting for the quantifiable sources listed above. The available evidence also suggests that the constant-speed procedure has a higher repeatability and reproducibility than the coastdown procedure. However, increasing the number of coastdown tests reduces the measurement uncertainty.
Countries and regions interested in developing policies to reduce fuel consumption and CO2 emissions ideally would leverage the work already done by the United States and the European Union, adapting the existing simulation tools and component testing methodologies to their specific markets. Regardless of the chosen methodology, regulatory agencies should develop a post-processing tool which takes as input the raw data and outputs the aerodynamic drag value to be used in simulation, as was done by the European Union. This would eliminate the room for misinterpretation of the regulatory provisions and guarantee consistency in the analysis. Based on the observed differences between the methodologies, we recommend that if both are going to be allowed for aerodynamic certification, that a conversion factor is used to guarantee comparability of results, as is done in the United States.