POST TEST ANALYSIS
The best estimate, a 4% drag energy reduction, is ~40% of my end goal, reversing the ~10% battery capacity degradation. But I need to:
- Review the SAE paper for further drag energy reductions.
- Improve benchmark methodology
SAE PAPER REVIEW
Citation: Brandtm A.m Bergm H., Boizon, M., and Josefsson, L., "The Effects of Wheel Design on the Aerodynamic Drag of Passenger Vehicles," SAE Int. J. Advances & Curr. Prac. in Mobility 1(3):1279-1299, doi: 10.4271/2019-01-0662.
Starting with:
Coverage area dominates the wheel drag. The pizza pan covers fully the wheel area except for five lug nut access holes.
The lug nut holes are the remaining hub depth and have an effect:
The five holes around the center are needed to access the wheel mounting lug nuts. They are open but a smaller, cover could close them and make it flush. But there is one design issue not addressed.
The vehicle side of the wheel spokes are exposed and have a 'squirrel cage' drag effect. The inside of the spokes move air, another drag force. There are design approaches that could also eliminate hub depth.
BENCHMARK IMPROVEMENTS
I used a quadratic equation derived from three data points to generate the trend lines for the 48F, 60F, and 63F benchmarks. A second order quadratic equation is perfect for drag calculations but the EV reports energy units, kWh/mi. Energy typically is a 3d degree equation that requires one more data point.
An ideal, triple point benchmark would include the energy loss at 0 mph. This can be measured by blocking the wheels; putting it in "D" mode, and; measuring the energy loss over time. Do this at different temperatures and we can combine the other two points. This might also give insights to the cold weather effects.
FOLLOW UP- Long distance drives to give the Battery Management System (BMS) a chance to recalibrate the EV range with the new covers. This primary metric relates directly to the goal.
- Cold weather, static in "D" metrics of vehicle energy overhead. Three or more temperature points can provide a basis of estimate for a 3d degree, energy polynomial.
- Design a fully enclosed, spoke wheel cover system. This could eliminate the lug nut holes associated with a minor hub depth effect.
- Aerodynamic cleanup of front bumper cover.
Bob Wilson