Last night, everything came together and I was able to capture my commute home route at 4:00 AM (no other traffic) and get the fastest polling data from the Graham Miniscanner. This chart shows observed transaxle, electrical efficiency:
Conditions:
- Using cruise control to just past crest of overpass and shift into "N" for ballistics glide to flat: 1st is "useless overpass", 2nd is "Airport Road", and 3d is "Golf Road."
- 2003 Prius, NHW11 - modified with largest possible drive tires LRR Sumitomo T4s at 50 psi, low viscosity transaxle lubricant.
- temperature 77F, no wind, dry
- Start: 34.703460 -86.587693 598 ft., driving South
- Indicated 38 mph -> 40 mph true (GPS)
During the cruise segments on the flat, the engine is running and MG2 is generating the electrical power that passes back to MG1. Toyota calls this "energy recirculate" mode. In this mode, the electrical path efficiency (MG1 output, traction battery charge, and ~445W vehicle load is created by the MG2 electrical generation load.) This may seem low but the ICE rpm is retarded as it is working like an overdrive. Also, the electrical path is passing a fraction of the ICE power that reaches the wheels.
When reaching the upgrade of the overpass, the cruise control commands more power and this in turn switches MG1 to the generator and MG2 as the output load. Now the electrical path efficiency clusters around 70-90%. You can hear the engine spin-up to generate the climbing power needed.
At seconds 1,040, a traffic light turned red at 34.665647 -86.572284 612ft "Byrd Springs Road." When the light turned green, I accelerated and the electrical path efficiency clustered around 75-82%. Also notice that the ratio of ICE power taking the electrical path rapidly fell off as the car approached the target speed.
Clearly, there are two electrical power flow modes, normal and energy recirculate. Energy recirculate is less efficient but it is driving the engine to turn slower, effectively an overdrive. From the acceleration data, we can see the ratio of how much power takes on or the other path varies by speed and load.
Prius transaxle efficiency is not a simple, steady-state device and more studies are warranted. BTW, in this benchmark run, the vehicle indicated over 64 MPG (~68 MPG) not bad considering it seats five with a good sized trunk.
Bob Wilson