So WHY does the regen have to cut out?

Evan,

I cant speak to the purely electrical/HSD reasons for the regen cut out.

However, I have read that the 8 MPH cut out/crossover helps keep the
little used brake disks/drums swept clean of rust for maximum
effectiveness/smoothness should it be needed at some point.

I suppose the same thing could be achieved with combined
regen/mechanical stopping below 8 MPH.

 

As am I. I'm thinking that since it isn't being covered by the media, nobody cares.

As for your original question, my guess is that Toyota hasn't figured out how to get a seemless transition. This sort of thing can't possibly be easy. Also, Telsa has the advantage of not having to deal with an engine. MG1 and MG2 have to work together to be able to run an engine while trying to slow down.

 

Perhaps it would be patent infringement? I think it is a good question too and I just don't think anyone really has a good reason. Although it has been mentioned that MG1/2 needs to be protected from a huge inrush.

 

I remember something along the lines that below 7mph there is not enough current for regen to be useful/effective. If I'm not mistaken I believe it was Hobbit that explained it and explained it in one of his technical session at Hybridfest.

 

As you slow the MG rotation rate drops. As that drops the output also drops. Critically, the voltage drops along with the current. When the voltage gets too low, you don't have enough to generate the output needed to actually -charge- the battery. Also, as the input voltage drops the inverter current can get pretty high if it is trying to maintain the output. So it will also dramatically effect the design of the inverter. Remember, the MGs run on 540 or so volts, while the battery is a 200 or so Volt device, so you have to design the inverter/charger to operate over a wide range of voltages in the MG side but a narrow range on the output side. I suspect the input range would be exceeded at low speeds/RPMs.

It is probably also true that when the output voltage of the MGs gets too low, you would experience some rather weird and possibly abrupt load changes on the system. Engineers try to control when those happen, hence the cutoff when -they- decree it should be.

The Tesla doesn't have an ICE to make the system more complex. For them, I suspect at low speeds they actually don't regenerate. They just don't specify a cutoff speed.

 

The first thing to understand is the voltage and power generated by MG2 is driven by the rpm. There is an excellent "REPORT ON TOYOTA/PRIUS MOTOR TORQUE CAPABILITY, TORQUE PROPERTY, NO-LOAD BACK EMF, AND MECHANICAL LOSSES", Hsu, J.S., Ayers, C.W., Commer, C.L., Wiles, R.H., Campbell, S.L., Lowe, K.T., and Michelhaugh, R.T., Oak Ridge National Laboratory, ORNL/TM-2004/185, Sept. 2004, that discusses MG2 motor performance. According to 'Fig. 13. Measured back emfs versus speed' it looks like ~1,500 rpm, the back EMF matches the traction battery voltage.

To dump energy into the traction battery, the voltage would have to be boosted up to traction battery voltage. Worse, the amount of energy saved is the product of the current and voltage. To keep the energy savings constant, the current from the motor has to increase and higher currents generate more waste heat ... we call it 'I-squared R loss.'

I've got to collect more data about the braking anomaly but I'll try to work up the energy numbers later today. But the simple answer is there is a point where the only way MG2 can continue to slow the car is to run it as a motor going in reverse.

Bob Wilson

 

I don't know, yet. I was only able to reproduce the problem Friday morning, the second time I was testing the X6-2 accelerometer.

Fortunately, some of the 2010 users have reported being able to reproduce the problem frequently and I'll be sending my X6-1A accelerometer out on Monday ... after doing another test this afternoon with both units. The streets are dry so I don't expect to see the anomaly ... but that is why experiments are important.

I've got some nice speed bumps in the neighborhood that I can hit at 30-40 mph. So I'll try them too.

Bob Wilson

 

From the Gen II talk years back it seemed to be an attempt to protect the transaxle by Toyota. Since MG2 and the drive wheels are directly connected to the PSD, if there were any large difference in wheel spin between left/right, the drive wheel reduction gear might somehow overspeed the ring gear and tear up the PSD. Nothing electrical about it. The Prius always readies the friction brakes when regen flips out, certainly when the traction control does it since the ABS ECU has to tell the TRAC ECU the speed difference. Toyota has probably done this in a very conservative manner to protect the PSD, and will now whittle down the margin of safety to protect the PSD in order to satisfy the new demographic of Prius drivers who can't grasp this car might behave a bit different than their 10 year old chevy.

Wayne

 

Bob, you need something that will affect one wheel, not both. I can induce this rather easy, the passenger side wheel seems best. Just take a decent bump or pothole at 25-30MPH with your foot on the brake pedal and the HSI indicating about half-full regen capacity.

Wayne

 

The higher speed regen shutoff when a bump is encountered can be explained. I just don't know if it's the -right- explanation.

Consider what would happen if you -didn't- shut off regen when the wheel hops. Regen adds a large drag to the wheels. The wheel would stop spinning very quickly, perhaps even before it regained the road contact. So it would reach the road at 0 RPM and would squeal or chirp (think aircraft landing). Now -THAT- would get people complaining! It would also jerk the steering wheel and car.

I'm not certain exactly -what- triggers the regen shutoff. As I've commented in other posts, it "feels" to me that it's a shock to the chassis, not delta wheel rotation rate. But that is just the feeling. I have no data. I also have not experienced in my GII the cutoff at higher speed, only at low speed approaching a stop. Oh, and on a gravel road on "washboard". Now -THAT- was interesting! BTW, that was not a Prius only experience. Most vehicles loose some control over washboard. Pesky physics in action.

 

I missed this thread when it was first posted; sorry for the delay.

As pointed out above, the low speed cutout is simply a matter of physics. The MG isn't spinning fast enough to do any good, so a transition to friction braking is necessary.

As for the higher speed transition, I believe this is done to avoid shock loading the HSD. Sudden loss of traction also implies suddenly regaining traction. It's the suddenly gaining traction that is worrisome. Suddenly reloading the HSD comes with three potential problems: 1) Mechanical shock load, 2) Mechanical overspeed, and 3) Electrical spikes.

Shock loading the HSD could break mechanical parts. The gearing on a Prius is more complex than on the Tesla, and is probably more delicate. Likewise the Tesla has a more powerful inverter and battery system which will most likely better stand up to electrical spikes caused by sudden changes in the MG (dv/dt issues). Finally, there is always the issue of over-spinning MG1. Very quick changes allow little time for the control system to respond, and inertial limits physical response times.

Tom

 

There are some answers here:

http://priuschat.com/forums/gen-iii...-question-about-regenerative-brake-prius.html

 

Making some speculation here:

Could it be a way for the HSD to avoid dealing with voltages/currents higher than what it can handle? Once the energy is converted from kinetic to electrical, it needs to be converted to chemical energy, that may, in some condition, not happen quickly enough. Wouldn't the inverter take a hit then? Having some kind of huge resistor to dissipate energy could be a solution; preventing such a condition by not using regenerative breaking might be an easier one?

 

May be the reason to cutoff regeneration could be to prevent shock or Jerk due to additional braking force provided by frictional brakes.

But then frictional brakes could simply be slowly engaged, unless brake pedal is fully depressed. There should be no reason to cut off regeneration.

I feel from the loss of braking issue from my own experience, that this does not happen when switching between Frictional and regeneration happens. But rather related to loss of traction for front wheels due to bump or slippery surface, since regenerative braking, unlike frictional braking, happens only with front wheels.

I had put my thoughts in another post here.

The http://priuschat.com/forums/gen-iii...easons-braking-loss-possible-way-resolve.html