Having owned a Prius for six years, I'm still learning and a recent question had me go back and take more measurements:
- What is the Prius transaxle efficiency?
Fortunately, it is possible to measure it and this is a look at some of the preliminary data.
A modified Prius was driven from 'Big Ed's Pizzeria' to 'North Alabama Computers' in Huntsville AL while recording these data elements:
- MG1 torque (Nm)
- MG2 torque (Nm)
- MG1 rpm
- MG2 rpm
- Traction battery volts
- Traction battery amps
Then using a few, key relationships, intrinsic to the transaxle axle, we can calculate:
Those relationships are:
- ICE rpm = (MG1_rpm + (MG2_rpm * 2.6)) / 3.6
- ICE torque = MG1_Nm / 0.28
- mph (Indicated) = MG2_rpm / 57
- power (W) = Nm * 2 * pi() * rpm / 60
But sampling is slow, ~0.50 seconds per sample, in this file. I will try a faster sampling rate which will help reduce the 'outliers.'
So this is what the 5 minute, ~300 second, trip looked like:
Initially, I had to wait in the parking lot for traffic to clear:
The initial engine start shows a peak of ~8kW to spin it up. Reverse is only via the traction battery and MG2. Then the Prius tries to use EV mode as much as possible until the catalytic converters reach operating temperature. Once stopped, the car recharges the traction battery and then shuts-down the ICE. Vehicle power is provided by the traction battery.
Then I accelerated to the 50 mph posted, speed limit:
The most efficient acceleration is one that does not draw on traction battery power. This is easily monitored in the cabin energy flow screen. Between seconds 80-85, the data points suggest MG1 ~= 9kW and MG2 ~=8kW giving roughly 88% efficiency. However, faster sampling and the actual numbers will give a more accurate reading and interpolating the traction battery load will also add to the energy picture.
Once at speed, cruise control handled the various overpasses:
The Parkway has a number of overpasses and this resulted in the traction battery doing a lot of swings from charge to discharge. This is not a flat route suitable for steady-state metrics plus I had to deal with lunch-time traffic.
Finally, I decellerated and came to a stop at North Alabama Computers:
With a lot of regenerative energy charging the traction battery, there was no need to run the engine. It was functionally an electric car as I exited, changed lanes, turned into the parking lot and found a parking place.
Sad to say, the Prius transaxle goes through many state changes that occur faster than the sampling interval. This results in some of the calculated values having 'impossible' or 'outliers' simply because we don't multichannel data recording. But these can be minimized by long runs in steady state conditions along with 'outlier' detection. Regardless, even this preliminary data provides insights to Prius transaxle efficiency.
Sad to say, my GPS mouse is missing the USB-to-PS2 adapter and the one I got at North Alabama Computers did not work out. I've ordered another GPS mouse and when it arrives, I'll be able to add true speed and altitude for kinetic and potential energy. The ultimate goal:
efficiency = output / input
Bob Wilson