Quote:
Originally Posted by JulianEdgar
There is possibly a way of doing this with electronic throttle.
- You will need two people, flat and straight road, decent speed update OBD data.
- Drive car at constant speed eg 100 km/h. You will probably be able to do that more accurately than the cruise control, but see which is better.
- Passenger reads actual throttle blade position (or total engine load if such a figure is available).
- Figure needs to stabilise eg less than 1 per cent variation as you hold speed constant.
- Repeat test and make sure figures are very close.
- Open all windows. Go back over same stretch of road with all windows open.
- Maintain same speed.
- Read what actual throttle blade position (or total load number) now is.
If the blade opening or load number has distinctly increased, then you have a base reading for what causes a major (eg typically 12 per cent) change in drag.
If that is the case, you have a go / no go indicator for aero mods - so long as the change with the aero modification is above the variation in the readings at a constant speed.
However, do not apply any maths to attempt to work out percentage changes - too hard when throttle blade position / load number is probably not linear with power (or torque).
(And I wouldn't do it with the Prius yet, in case electric power is also changing.)
Let us know how you go! |
Another way of doing this would be to use MAP readings.
If the engine speed at 100 km/hr is say 3000 rpm then first obtain the MAP reading under no load conditions at 3000 rpm.
This is your zero power MAP or MAP_0.
To make the maths easy lets say this reading was 4.5 psi and at WOT the reading is 14.5 PSI.
If the baseline run gives a reading of 8.5 PSI and the run with aero mods gives 8.0 PSI then clearly the aero mods have reduced the drag.
However if we make the assumption that the change in engine torque output is proportional to MAP for small delta P then we can calculate the approx change in road load.
reduction in road load = 100 * (8.5 - 8.0) / (8.5 - 4.5) = 12.5 %
One caveat here is that road load includes drag due to rolling resistance thus the above result is not a direct measure of the aero drag reduction.
In actual fact the true drag reduction would be greater than the road load reduction.
Having said all that your method should be first choice if at all possible due to its simplicity and the fact that the rolling resistance terms cancel out in the equations.