This paper will explore the effect that non-constant function CD (as observed during wind tunnel testing) would have on the coastdown derived drag coefficient and other regulatory drive cycles. It is common in wind tunnel testing to observe road vehicle drag coefficients that vary with speed. These varying CD values as a function of velocity will be expressed as CD(V) in this paper.
Wind tunnel testing for product development is generally conducted at 110 km/h (68.3 mph) which are similar speeds and typical of the United States (US), European, and Asian highway speeds. Reported values of CD are generally gathered at these speeds. However, coastdown testing by definition takes place over a large range of speeds mostly lower than the wind tunnel test speeds. This paper will explore the effect that six typical functions of CD(V) have on the coastdown derived CD. One of the six functions is a constant, to represent a wind tunnel reported CD.
The process we used to study these effects is based on a combination of the Test Cycle Averaged Drag Coefficient methodology developed by Howell, Forbes, Passmore and Windsor [References 3 & 4] and Ingram’s “Wind Averaged Drag Coefficient” Reference 10. This process establishes a drag coefficient that is “Averaged” over the course of a particular test cycle with varying speeds. The “Test Cycle Averaged Drag Coefficient” methodology provides an excellent foundation for determining the effect that several functions of CD(V) have on the coastdown derived drag coefficient. However, since we are not applying the effects of terrain or shear, but are applying wind effects, we will refer to the derived values as CDV¯ similar to the CD¯ employed by Ingram in Reference 10. It is important to note that Ingram assumed the vehicle speed was constant in his analyses. For comparison, and in addition to the coastdown “test”, we will evaluate the effects that the six CD(V) functions have on the CDV¯ values derived from the following certification test cycles: Environmental Protection Agency (EPA) Federal Test Procedure (FTP), EPA Highway, EPA Combined FTP and Highway, and World Harmonized Light-Duty Vehicles Test Cycle (WLTC) to determine the extent that they are affected and how well the 110 km/h wind tunnel CD correlates with them.