Drivetrain measurements
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Having owned a Prius for six years, I'm still learning and a recent question had me go back and take more measurements:
A modified Prius was driven from 'Big Ed's Pizzeria' to 'North Alabama Computers' in Huntsville AL while recording these data elements:
So this is what the 5 minute, ~300 second, trip looked like: http://hiwaay.net/~bzwilson/prius/tran_010.jpg Initially, I had to wait in the parking lot for traffic to clear: http://hiwaay.net/~bzwilson/prius/tran_020.jpg 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: http://hiwaay.net/~bzwilson/prius/tran_030.jpg 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: http://hiwaay.net/~bzwilson/prius/tran_040.jpg 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: http://hiwaay.net/~bzwilson/prius/tran_050.jpg 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 / inputBob Wilson |
I don't think the question was about the transaxle as a whole, as there are numerous gear losses not being represented here, but it was in relation to the relative electrical power going through mg1 and mg2 and the battery. It was not clear if each motor and the battery had their own current sensors even, because you cannot infer one and then use that inference to measure efficiency (otherwise they will all be 100% efficient).
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For now, I'm after the 'low hanging fruit,' power generated and consumed ... the energy flow. The data is reasonably smoothed and will meet my interest. After all, it is measured in the field versus Appendix B of the Oak Ridge report, a lab bench test. I'm also thinking about how to measure wheel torque and rpm via another channel. There are new strain gauges that make this entirely practical and affordable. I could either mount on a drive shaft, the wheel hub, possibly a lug-nut, or even tire strain sensor. Then there are the off-the-shelf solutions. Good, fast, cheap, pick two. Bob Wilson |
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Would you mind going through these questions in order? I will assume anything other than "yes" is a long winded "no". 1. Is there an actual torque sensor on mg1? Or is it "inferred" by the ECU and current sensors or??? 2. Is there an actual torque sensor on mg2? Or is it "inferred" by the ECU and current sensors or??? 3. Is there an actual rpm sensor on mg1? Or is it "inferred" by the ECU and voltage sensors or??? 4. Is there an actual rpm sensor on mg2? Or is it "inferred" by the ECU and voltage sensors or??? 5. And I assume traction battery voltage and amps comes from actual voltage and current sensors on the battery. But if you could confirm that to please. Thanks, Dave. |
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Bob Wilson |
I've take the earlier data and used an averaging technique to go from point-to-point per sample to samples with points and the averaged values from earlier and later samples. Not quite a straight-line approximation, it is close enough to make the data more easily understood:
http://hiwaay.net/~bzwilson/prius/tran_100.jpg Starting: http://hiwaay.net/~bzwilson/prius/tran_110.jpg The interesting aspects are:
Acceleration: http://hiwaay.net/~bzwilson/prius/tran_120.jpg The main observations:
Cruse: http://hiwaay.net/~bzwilson/prius/tran_130.jpg Dealing with traffic and overpasses, the data looks pretty noisy but:
Stopping: http://hiwaay.net/~bzwilson/prius/tran_140.jpg Here we see classical, regenerative braking and sampling and possibly a new, previously undocumented MG1 mode:
Initial testing with a faster polling rate looks to be OK but I haven't recorded any data, yet. Also, converting point-samples into straight-line approximations is the way to go but it is not trivial. I've ordered a GPS receiver that should be here next week. Then I can start a series of benchmarks to accurately measure Prius transaxle energy flows. Bob Wilson |
I ask the questions just because it will be terribly hard to draw any conclusions without knowing precisely what you are measuring, especially regarding efficiency. Are there any sanity checks you can do on the readings? i.e. measure the cars weight/aero/rolling resistance and see if actual acceleration matches predicted based on all the torque readings and number and configuration of loaded gears?
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In fact, why don't you make a list of things you want calibrated. Will that include the GPS data too? I look forward to a post experiment, calibration exercise . . . just hold on to that 'rope of hope' and I'll get right on it. In the meanwhile, I've captured a fresh set of data using the higher sampling rate. Thanks to a happy accident, I retraced the same path as the earlier data and remembered I have a straight-line, approximation program to normalize the data. Bob Wilson |
I assume you are doing this in public to build some peer consensus. I am not sure you know precisely what you are measuring if looking at the output from the ECU, and it is critical to understand that if you are trying to measure transaxle efficiency. I have seen a lot of ECUs report bogus or computed data, so there isn't a lot of point in the data analysis without first figuring out what you are looking at, hence why I am asking for particulars about the system in question. I appreciate it is a bother to double check what the ECU says but there is good reason for it.
For example you did not mention current sensors on mg1, so is the ECU inferring the current there? and if so, it is not possible to use that reading to determine the efficiency of anything relating to mg1. Also since it appears to use current to derive torque, we don't know if it is "electrical" torque, or shaft torque after converting for losses, or how accurate the current to torque conversion reading is. Could be a 30% discrepancy there depending how the ECU programmer interpreted the "requirements" if any. Also, did you mention publishing data? Assuming the data is correct (or even if it isnt), it would be simpler to process things like "positive area under the blue line vs negative" with the actual values. But I do need to understand how things like what the ECU reports as torque relate to the real world before I will accept ECU funged data. Just like I wouldn't accept airflow as an approximation for fuel flow where precision is concerned. I have no other purpose here accept to ensure the claims are valid, and taking measuring shortcuts is rightfully going to draw some criticisms. I'm glad you are willing to do it, but I will pressure you to do it "right" before "publishing" any conclusions. |
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Bob Wilson |
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