A Drag Coefficient for Test Cycle Application 2018-01-0742
The drag coefficient at zero yaw angle is the single parameter usually used to define the aerodynamic drag characteristics of a passenger car. However this is usually the minimum drag condition and will, for example, lead to an underestimate of the effect of aerodynamic drag on fuel consumption because the important influence of the natural wind has been excluded. An alternative measure of aerodynamic drag should take into account the effect of non-zero yaw angles and a variant of wind-averaged-drag is suggested as the best option. A wind- averaged-drag coefficient is usually derived for a particular vehicle speed using a representative wind speed distribution. In the particular case where the road speed distribution is specified, as for a drive cycle to determine fuel economy, a relevant drag coefficient can be derived by using a weighted road speed. This approach has been used to determine an effective drag coefficient for a range of cars using the proposed test cycle for the WLTP, (Worldwide Harmonised Light Vehicle Test Procedure). A terrain related wind profile, to give different mean wind velocities acting on the car, was applied to the various phases of the drive cycle and an overall drag coefficient was then derived from the work done over the full cycle. This method has been updated using more detailed drag data at small yaw angles and, in this paper, is also applied to the EPA drive cycle. This cycle averaged drag coefficient is shown to be very similar to that obtained with the test cycle for WLTP and, in both cases, is significantly higher than the nominal zero yaw drag coefficient. The shape factors and add- on components which influence the drag rise at yaw are considered.