Now here is a problem I'm facing:
- How to separate transaxle losses from tire losses?
All power from the engine passes through the transaxle before reaching the tires. A Prius transaxle has two active elements, MG1 and MG2, which have (I**2)R losses. But we also know there is stirring and friction losses. Unfortunately, I can't use temperature change to quantify the stirring and friction losses because of MG1 and MG2. I do have an indirect measurement of friction losses from the oil test results. But the real challenge is how to separate tire rolling resistance from the transaxle effects.
Now I could replicate the work of Battelle Labs in their transaxle testing but that is involved. In particular, I have to 'lock up' the PSD (power split device) to get power to flow through the transaxle but then I'm not measuring PSD losses. This is not a trivial problem.
We can get a usable rolling friction metric by using a roll-down test. The tire and friction losses should be fairly independent of speed. But viscosity losses are exponential with speed and these are the ones that have tickled my curiosity.
Bob Wilson