Quote:
Originally Posted by bwilson4web
It is a sophisticated (i.e., technically challenging) EPA technique that takes a car to at least 70 mph and measures how long it takes to reach 15 mph, SAE J1263: - Standard day temperature, ~60 F.
- Very level, +/- a few feet, straight at least a mile track.
- Wind and other climate limits.
- 16 bi-directional runs.
From the roll-down metrics, you can develop three drag coefficients: - A - lbs per mph squared
- B - lbs per mph
- C - lbs
With these three coefficients, you can plot the drag force as a function of speed or more useful, the drag power needed as a function of speed. Combine that with the drivetrain efficiency, you can make a curve showing the MPG or kWh per mile as a function of speed.
My technique is to use at least three benchmarks to derive the quadratic equation. Two have to be on either side of the local minimum or maximum, and the third out into the high-speed region. Then use a quadratic solver to derive the A, B, and C coefficients. This is much easier and less complicated than the SAE J1263 procedure.
Using this procedure, we can test changes to improve a car's performance, either acceleration and/or range.
Bob Wilson |
It doesn't sound rigorous enough.The SAE Handbook lists the procedure for coastdown testing.The technical and instrumentation requirements are so rigorous as to be,in my opinion,beyond the reach of individuals like us.
Without the polar moment of inertia for every rotating mass on and in the car,it's impossible to accomplish the data reduction necessary to reveal the coefficients.I've tried it and gave up,ending up paying a qualified 3rd party to test it for a fee at a test facility.$500 in 1991 dollars.