Capture more regen from front wheel or awd cars.

bhzmark · Aug 8, 2013

A big remaining inefficiency in the model s is the limited amount of regen you can capture from the rear wheels before they lock up. Since the strongest braking effect comes from the front wheels the strongest regen would come from the front wheels as well. They really need to do a awd or front wheel drive car to capture more of the braking energy as regen.

Converting all that kinetic energy into brake dust and heat from applying the regular brakes on top of the regen when need to stop quickly is a waste.

montgom626 · Aug 8, 2013

My model S could double it's regen on dry pavement without any problems. Stability control will solve any traction issues when the ground is slippery as it does now.

Kaivball · Aug 8, 2013

It tops out at 60kw right now.

Under acceleration you use easily 120-180 and no traction control coming on.

I would think the same is possible for regen.

dhrivnak · Aug 8, 2013

But in all testing the gain is small very small.

gregincal · Aug 9, 2013

I would think the limiting factor would be how quickly the battery could be supplied with power, not the wheels. I agree that the regen could easily be a lot stronger without worrying about the read wheel drive. However, it's currently strong enough for 98% of my stops. I rarely use the brakes except for emergency stops.

ElSupreme · Aug 10, 2013

While you are correct that the front tires have more absolute braking, do to weight transfer, when stopping you are neglecting a few things.

The first thing is why do you need to lock the wheels with regen? The Model S today decelerates quick enough that I rarely need to stop quicker. Only when cut off, or getting nipped by a turning light are about the only times I use the friction brakes. With the obvious exception of low speeds.

The second thing you need to consider is whether or not the car could capture more energy. There are a few limitations that I am aware of, but not what the actual limits are. The first is the charge you can dump into the battery cells. Right now superchargers are putting in 90kW, and will bump up to 120kW. But this is only when the battery SoC is below ~60%. I am not sure the pack could handle much more than 90kW or so. You also have the limit of power the motors can produce. And that limit decreases with speed. So even at low speeds you will still have to use friction brakes. That or switch to permanent magnet motors.

brianman · Aug 10, 2013

Should we expect a more even (front/back) distribution of tire wear with AWD -- both from acceleration and regen scenarios?

ElSupreme · Aug 10, 2013

brianman said:
Should we expect a more even (front/back) distribution of tire wear with AWD -- both from acceleration and regen scenarios?

Yes you would probably get very even tire wear with this sort of setup. Not any real gain on tire life though. And the backs would most likely still wear faster.

I am not sure how easy it would be to capture energy, via regen, with two motors at the same time. I don't think combining that power would be that easy. I am not an electrical engineer (but did take power electronics in college). But I would think that would be a challenge. But I could see you being limited to either RWD regen, or FWD regen. Again these challenges get way easier with permanent magnet motors, which I am glad Tesla does not use.

Another challenge I would see with FWD regen is stability. Sure when pointed straight your fronts have more threshold grip for braking. But when you turn the steering wheel you lose a lot of that grip to changing direction. I would be concerned about the driver lifting off the accelerator while in a tight corner, and locking the front wheels. Where dragging the rears is much less difficult of a problem to solve

bhzmark · Aug 10, 2013

ElSupreme said:
why do you need to lock the wheels with regen?

No one said that you need to lock the wheels with regen.

I said "model s is limited in the amount of regen you can capture from the rear wheels before they lock up."

In other words, you can't regen from wheels that are locked. Regen applies braking forces to the wheels. stronger regen --> stronger braking. eventually the regen is limited by how much traction the wheels have because at some point they will lock up. The rear wheels will lock up well before the front wheels do when braking. Thus the amount of regen from the rear wheels is limited by the traction from the rear wheels.

You can capture more regen from the front wheels because the dynamic braking force from the front wheels before locking up is more than 3 times that at the rear wheels for most cars, and still probably at least twice as much for the low COG MS.

ElSupreme · Aug 10, 2013

bhzmark said:
No one said that you need to lock the wheels with regen.

I said "model s is limited in the amount of regen you can capture from the rear wheels before they lock up."

Right so there is no advantage to moving regen to the front (in terms of quantity) unless you want to be able to lock the rear wheels with regen. Otherwise the rears could handle the amount of power desired to brake.

bhzmark said:
In other words, you can't regen from wheels that are locked. Regen applies braking forces to the wheels. stronger regen --> stronger braking. eventually the regen is limited by how much traction the wheels have because at some point they will lock up. The rear wheels will lock up well before the front wheels do when braking. Thus the amount of regen from the rear wheels is limited by the traction from the rear wheels.

You can capture more regen from the front wheels because the dynamic braking force from the front wheels before locking up is more than 3 times that at the rear wheels for most cars, and still probably at least twice as much for the low COG MS.

Well your 3 times is entirely made up. It is highly dependent on weight bias. A 911 with ~70% of its weight on the rear axle will have much more rear brake balance, versus say a FWD small car where you probably have ~65% on the fronts.

But again this only matters if you want more regen that would lock the rears.

And you lose LOTS of braking force when you turn the front wheels. Which means you would possibly only have greater regen when traveling straight. And would have stability problems when turning, and letting off the accelerator.

deonb · Aug 10, 2013

ElSupreme said:
The second thing you need to consider is whether or not the car could capture more energy. There are a few limitations that I am aware of, but not what the actual limits are. The first is the charge you can dump into the battery cells. Right now superchargers are putting in 90kW, and will bump up to 120kW. But this is only when the battery SoC is below ~60%. I am not sure the pack could handle much more than 90kW or so. You also have the limit of power the motors can produce. And that limit decreases with speed. So even at low speeds you will still have to use friction brakes. That or switch to permanent magnet motors.

That high regen rate would only need to apply for a few seconds. Tesla can add a 30lbs SuperCapacitor in there to capture around 80 watt/hours (100kW for 3 seconds) and gradually offload it to the battery.

I would definitely be willing to pay $2500 or so more for a stronger regen option, provided it has hill-hold (which can be done in Software).

bhzmark · Oct 12, 2014

deonb said:
That high regen rate would only need to apply for a few seconds. Tesla can add a 30lbs SuperCapacitor in there to capture around 80 watt/hours (100kW for 3 seconds) and gradually offload it to the battery.

I would definitely be willing to pay $2500 or so more for a stronger regen option, provided it has hill-hold (which can be done in Software).

Anyone have any details on how the D cars use regen from the front wheels?

Does the autopilot apply braking forces from regen that exceed the moderate amount of regen braking from accel lift off?

Cosmacelf · Oct 12, 2014

I'd suggest starting your own thread for this question. Tacking on a new question to a thread over a year old is very confusing as most people won't realize that all of the discussion was done over a year ago...

jerry33 · Oct 13, 2014

My thought is that there will be somewhat less energy captured by regen with the dual drive system due to the additional reduction gear and it's associated friction. This will be most noticeable in cold weather/short trips as cold lubricant adds a surprising amount of friction.

Red Sage · Oct 21, 2014

Remember the amount of regen allowed on rear wheel drive cars is purposely limited to a 'safe' range that can be readily controlled and accepted by the battery. Having the additional friction of the front reduction gear allows for a smoother delivery of that energy to the battery. The mass of the vehicle in motion is what contributes most to the regeneration process. By taking the full stress of this slowing action during deceleration off the rear axle, and allowing some to be regenerated through the front tires, you also reduce tire wear. Overall, you are able to reach the maximum level of regen sooner, and more smoothly, at the same time, improving overall efficiency.

green1 · Oct 21, 2014

There's a good reason that most vehicles do most of their braking with the front wheels, With AWD we now have that option for regen as well, that should help the handling while braking some, especially in slippery conditions.

bhzmark · Oct 23, 2014

Red Sage said:
Remember the amount of regen allowed on rear wheel drive cars is purposely limited to a 'safe' range that can be readily controlled and accepted by the battery. Having the additional friction of the front reduction gear allows for a smoother delivery of that energy to the battery. The mass of the vehicle in motion is what contributes most to the regeneration process. By taking the full stress of this slowing action during deceleration off the rear axle, and allowing some to be regenerated through the front tires, you also reduce tire wear. Overall, you are able to reach the maximum level of regen sooner, and more smoothly, at the same time, improving overall efficiency.

green1 said:
There's a good reason that most vehicles do most of their braking with the front wheels, With AWD we now have that option for regen as well, that should help the handling while braking some, especially in slippery conditions.

Both excellent points. I hope they are now capturing add'l regen from the front wheels.

Certainly the car should capture it from the accel-lift off regen. But that amount, as you say, should be limited by two factors:

A) the amount of decel/braking force that is safe and a good driving experience from merely lifting off the accel. We wouldn't want the car to come to a screeching halt when someone lifted their foot off the accel. I've heard others mention only a .5g decel as the limit on this. I don't know what it is but it is likely that it is an amount, that at least some of the times it is less than what the battery could accept and what the motors could generate at full regen capacity.

B) the second limit would be what the motors could generate at full regen capacity, not limited by driver experience or an artificially capped limit.

If A< B, then the new elec brakes could allow the capture of more regen by using the brake pedal as a driver input to trigger add'l braking force beyond the capped A limit.

That would be a great and more efficient design. Perhaps that is what we have now.

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Capture more regen from front wheel or awd cars.

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