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.