|
For anyone wondering....
In top gear, engine rpm varies little for speed changes. So for example, in the Insight, a few km/h difference in speed may be only 40-50 rpm.
In throttle-stop testing, the engine is at a constant throttle. The air/fuel ratio is constant. Ignition timing varies little - and in fact in the Insight with this variation in engine speed and manifold vacuum, is constant. Volumetric efficiency varies inconsequentially. Internal frictional losses in the engine vary inconsequentially.
As is then obvious, the torque output of the engine is very nearly constant over this very small change in rpm. This means the push backwards on the road by the wheels is very nearly constant.
The power output of the engine varies a little (very little, but a little) but that's why when we do the maths to calculate change in drag, we use the square rule (for force) not the cube rule (for power).
It's not like this was all just dreamed up in some random way: it was thought-through very carefully, discussed with some top experts for their thoughts, and then tested to see if a deliberate change in drag gave the expected test results. (Which it did.)
As with any car testing, you can do it really badly and get completely unreliable results (eg testing in low gear, on a peaky turbo engine at revs where it comes onto boost, on two days with completely different weather, and with different run-up speeds to the test section) but you can also do it and get excellent, repeatable results - and vastly better than normal coastdowns.
|