Previously I have noted that sometimes the MFD will show the motor as not being on when I can hear it starting and running: Motor runs but does not show up on MFD? | PriusChat I have also previously noted that sometimes when the car stops the wheels on the display are shown turning for quite a while while the real ones are not - as the car is not moving. Today we went to a toxic waste drop off. This involved crawling around a parking lot, moving one car length at a time, for 40 minutes or so, following a previous 40 minutes doing the same thing on a city street just to get into the event. Naturally this drained the HV pack down to two pink bars. (Annoyingly the car isn't bright enough to realize what is going on and let the ICE run long enough to charge up the pack. Instead at that charge point it starts the ICE for every move, then shuts it right down again.) Anyway, while it was in this state, I noticed that at most stops the display was showing power going from the pack to the electric motor. Put it into park and all the arrows instantly went away. Put it back into drive and they returned. Press harder on the brake, much harder than I normally would to remain still on flat ground, and once again the arrows disappear. My hypothesis is that with brake pressure that is light the car tries to get the car to crawl by sending power from the pack to the electric motors, but the physical brakes hold. Press harder on the brakes and then it decides that the driver doesn't want to move and doesn't send power. If this is correct then the current should show up in Techstream. I felt that the amount of brake pressure needed to cancel the power transfer shown in the display was excessive, corresponding more to what I might use while holding the car on a significant grade, and overkill for keeping it still on the flat. (Not just this car, any car.) It crosses my mind that this might be something which needs to be calibrated, and it is out of calibration. Either because the car needs to go through a relearning procedure of some kind, or because the brake position sensor isn't sending the right information. Note, even though it was a fairly hot day (around 85F outside at the time) neither the AC nor the fan were on, we had all the windows down instead.
That is what it does. Whether the pedal pressure threshold is too high in yours is a question I can't answer, and seems maybe like a matter of personal taste. I like making very gentle stops, so usually my brake pedal pressure is quite light at the moment the car comes to a stop. Then, once it's no longer moving, I press harder and hold, so the drive torque cuts off. It's long-ingrained habit by now.
Seems counter intuitive to me. The car came to a stop with 3 psi on the brake pedal but once stopped (so no more KE to burn off) it now needs 6 psi. Sort of the backwards Bizarro world version of "an object in motion tends to stay in motion". A traditional automatic transmission ICE vehicle will crawl at a couple of miles per hour whenever the motor is on if the brake is not applied (or the car not aimed up a steep hill) because when in gear even idle transmits a little power through the torque converter. On an EV, or a hybrid in EV mode like the Prius under these conditions, why would the car try to turn the wheels at all when there is pressure on the brakes and none on the accelerator? It could be set to crawl at 0.5 mph (or similar) with no pressure on either pedal if in drive, and work up from there with accelerator pressure. That's 1/6th normal walking speed which is very slow - I don't think I can regulate velocity on this or any other car well at speeds less than that. Looks like Mazda's i-Stop works a lot like the Prius though - it won't turn off the motor when there is light pressure on the brake even if the car is fully stopped.
Because the design decision Toyota made early on was for it to feel a lot like that "traditional automatic transmission ICE vehicle" so it wouldn't be unfamiliar to people. That traditional vehicle won't just crawl "whenever the motor is on if the brake is not applied"—it starts crawling as soon as your brake pedal pressure is light enough for that engine-idle torque to overcome it. If the Prius waited for you to be fully off the brake before applying any torque, that would feel different and weird. The main difference is, the engine idle torque in the traditional vehicle never cuts off: even while you're sitting there with the brake pressed hard enough to remain stationary, the transmission is still applying torque, meaning engine power being burned off as heated transmission fluid in the torque converter the whole time. At least in the Prius that waste of energy ceases as soon as the car decides you're holding the brake hard enough that no "crawl" simulation is needed.
Been experimenting with that. "Hard enough" is harder than I normally step on the brakes for any stopped car. It isn't "stand on the brakes like there is no power assist", but still more than I like. For me putting it into park, and then back to drive, is more comfortable. Have not yet had a chance to see how much current is involved. It may be the simplest thing would be to just not look at the display and not worry about it if the power usage is negligible.
That's a question that came up in another context, ➡here⬅. The context there was trying to pull the rear wheels over a 1-inch block, which definitely takes more torque than you get just having your foot off the go pedal. I had to push the pedal to 17.5% to do it (figuring my car was probably loaded to about half the rear axle weight rating; fully loaded would have taken more). The power usage with foot off the go pedal would be a lot less. Without PIDs for the MG2 winding voltages or currents, you have a couple indirect ways to get at the power involved. You can measure the MG2 torque (there is a PID for that) while the car tries to push forward against a block or wall with no foot on the go pedal, and then look at ORNL's MG2 torque vs. winding current graph (see that linked post, but bear in mind that was a gen 3 thread). The other way is to just observe the traction battery current and voltage: under that same condition, and then with the brake pressed hard enough to turn the crawl-simulation off. For each condition, multiply the volts and amps to get watts; subtract the brake-pressed watts from the brake-not-pressed watts, and what's left is the power being used to simulate crawl. Both of those methods are valid ways to the truth, so of course they should agree, but you might do a double take at first, because the watts you get by the second method will seem way way lower than what you get if you multiply the motor current from the first method by the motor's nameplate voltage (500 V for gen 2). The double take is because we think of these as high-voltage motors (650 V in gen 3, 500 V in gen 2), but they are also, in other conditions, very low voltage motors. How it works out in my gen 3: I just went out and chocked the rear wheels and read 2.4 amps traction battery current with no foot on any pedal and the car trying to crawl against the chocks. Pressing the brake hard enough dropped that current to 1.5 amps, so the "crawl simulation" current out of the battery was 0.9 amps. Comes to about 187 watts if the battery voltage was around 208 at the time. Using the winding resistances from the thread linked above, that works out to a winding current around 35 to 40 amps, and from the ORNL current vs. torque graph, that might mean about 30 Nm of "crawl" torque from MG2. "But wait!", you might think, "isn't 35 to 40 MG2 amps a lot of power? At 650 volts that'd be like 25 kilowatts!" But the motor isn't operating at 650 volts just then. It's operating at more like 4.7 to 5.3 volts at that moment, which is all it takes to make 35 to 40 amps flow through its windings when it isn't rotating and making back-EMF you have to overcome. Again, all those example numbers are from gen 3 'cause that's what I have, but for gen 2 shouldn't be a whole lot different. A "crawl" power of 187 watts is about ¼ HP. Whether that's enough to bother pressing harder with your brake foot or not, I'm in no position to tell you.
Under normal circumstances, where the stops are likely less than 25% of the "driving" time, probably not. In that toxic waste dropoff though, where it was something like 95% of the time, probably yes. Rounding it to an hour, that is .187 kWh and the pack is around 1.3 kWh, so around 14% of the capacity of the pack wasted. Honestly though, I think being able to tell the car to charge the HV pack all the way up, and then only use electric until it needed a recharge again, would have been more useful. Running an ICE for 2 seconds every 30 seconds must be really inefficient. One could emulate that by shifting into park and then stepping on the gas to force the motor to run, but I would be afraid of screwing up once, leaving it in gear, and ramming the car in front of me - an event which would clearly negate any possible benefits! This particular driving scenario is truly an edge case. Even in the heaviest LA stop and go traffic I usually travel at least multiple car lengths when the car finally moves, whereas here it was monotonously one car length at a time, for an hour. Also a truly stupid display of traffic management. This was in the Santa Anita race track parking lot, which was otherwise almost entirely empty. Tons of space: Google Maps They could easily have directed the cars into rows and had them wait there, then sent one row at a time over to the actual collection area.
