Quote:
Originally Posted by jakobnev
Not even a hint of suspicion that the aerodynamic properties of the car are used to determine the resistance of the dyno rollers?  (among other things like mass) |
I think there is some misunderstanding coming from where this thread started. Gino Sovran of General Motors wrote a report showing how reducing aerodynamic drag would affect mpg figures obtained from the EPA testing procedures for a given car. The report makes the point that these changes would not be applicable to "real word" mpg. YMMV is all he meant.
The EPA tests are done on a programmable dynamometer. In order to have a dynamometer simulate the total road load of a vehicle, the dynamometer power absorber must be adjusted to reflect the road load characteristics of the vehicle. Most certification vehicles are tested using dynamometer power absorption values obtained according to the methods in the "EPA Recommended Practice for Determination of Vehicle Road Load." In this method the basic concept is to perform a series of road or track coastdowns with the vehicle. The coastdown measures the rolling resistance and aerodynamic drag in terms of varying deceleration. Coastdowns are then performed on the dynamometer at different power absorber settings, and the dynamometer power absorber adjustment is determined when the vehicle dynamometer coastdown time matches that of the road coastdown in a speed range of 55 to 45 mph. Since 1986 manufactures are allowed to use alternative methods to calculate road load, like using wind tunnel results to estimate aerodynamic drag (
see here).
Aerodynamic drag
is included in the dynamometer load. The procedure is far from ideal, and modern equipment could give more accurate results, yet the EPA mpg figures are very close to real world numbers for most cars.
-mort