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How much braking energy will be lost for regeneration because the Volt's rear wheels can't feed the generator?

I know that the front wheels do most of the work of braking since the weight transfers forward under braking. The question is whether linking the rear wheels to a generator would be worth the cost, weight, and complexity?

This question obviously could be tangled with the debate that has already occurred about wheel hub motors and unsprung weight.

Thanks for any insight you can offer.

Rick
 

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The electric motor that drives the front wheels *is* the generator. There is no advantage to having an additional generator on the rear wheels, in terms of energy recovered through regenerative braking. An additional rear motor would be nice because it would give you 4 wheel drive without an additional transfer shaft and center differential. But as long as the wheels don't slip, 2 wheel drive and 4 wheel drive are equivalent in terms of energy efficiency.
 

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The problem with regen braking as I understand it is that unless they can perfect some sort of ultra capacitor to store the burst of energy created by the generator during braking, most of it now is just wasted. Batteries cannot absorb large bursts of energy so most of the excess energy just goes to heat sinks and the mechanical brakes finish the job. Adding rear regen brakes at this point would seem just a waste of money and weight. However there are rumors that GM is looking at capacitors for this very purpose to use on Gen 2 E-Flex.
 

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It's probably too early to say but my understanding is that basically if electric system can dish it out then it can take it. Thus it can theoretical charge at the max power output rate with similar time constraints. Since more "weight" shifts to the rear wheels during acceleration and to the front during breaking, there is potentially more power during breaking than can be utilized in accelerating with the front wheels. Thus it wouldn't make much sense to only design for rear wheel regen, but as Joshua mentioned you could also get 4-wheel drive. This is the application I think we will definitely see in some future E-flex vehicles. Not only do you get 4-wheel drive cspability and rear regen, but you can also get better performance. EV drive trains will offer tremendous advantages as they develope.
 

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Since more "weight" shifts to the rear wheels during acceleration and to the front during breaking, there is potentially more power during breaking than can be utilized in accelerating with the front wheels. Thus it wouldn't make much sense to only design for rear wheel regen

Koz, I think you lost me here. I'm pretty sure you understand about kinetic energy but your sentence is confusing. No worries mine are often that way. ;) I just want readers to know that from physics it doesn't matter if you used your feet at any corner of the car to stop. If you stopped it then all that kinetic energy was dissipated by your feet. Yes, they would probably hurt.

If all the energy was not dissipated through your feet the car would continue to move. I think you understand that and are saying that if the electric motor was only attached to the rear wheels then you would be wasting the energy that was dissipated by the front brakes when you have to brake harder than the rear wheels can handle. If so, then I agree.

They have prototype electric cars that have no friction brakes at all. The regen can be applied (if the software is written well) in such a way that will make you think you have standard friction brakes but infact they are just electric motors that are running in regen mode. I also agree that this is the future - having four wheel-hub motors. Having full control (both during acceleration and deceleration) of all four wheels is just too beneficial to ignore, if only for the added safety.
 

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Let's see if I can explain myself better. The maximum motive power that a wheel can effectively delivery is determined force (weight) acting on the wheel and the static coefficient of friction between the tire and the road. Since some "weight" is shifted to the rear wheels during acceleration due to the ensuing moment, the effective maximum motive power is lower for the front wheels than the rear wheels. When breaking the physics is reversed, and the effective maximum breaking power of the front wheels is greater than the rear wheels. Basically, you can accelerate faster with the rear wheels versus the front, and you can stop faster with the front wheels versus the rear. Of course you can accelerate and decelerate fastest with all four. Not sure this explains what I was thinking much better than my previous effort.
 

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Yes, that adds to the understanding. I often here someone say, "Hey, if I put a generator at each wheel I can get more energy!" They do not realize that if I have only one generator on one wheel and stop the car with it that I get the same energy. It's the kinetic energy of the car. Period. You can't get four times the kinetic energy out of it. I know you understand this Koz I was just concerned that others would be confused with what you said.

The weight distribution and where the power is applied does make a difference if you are driving hard and are losing grip with your tires but if you use regen to stop your car (not using your brake pads) that kinetic energy (minus the efficiency of the system) will be directed to the battery, regardless of which tire(s) it's on. Note however that stopping a car with only rear brakes (like using your emergency brakes) will not give you the performance that front brakes can, due to what Koz was talking about.
 

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It's probably too early to say but my understanding is that basically if electric system can dish it out then it can take it. Thus it can theoretical charge at the max power output rate with similar time constraints.
So you believe that the batteries can handle charging at any voltage over any time period? If you're going down a steep hill, and stand on the brakes, the burst of high current is going to cook the batteries. It has to be regulated. Regulation means some energy gets wasted.
 

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So you believe that the batteries can handle charging at any voltage over any time period?
No, I don't. That's where the time constraints come in. I was saying that if the battery, power electronics, wiring, motor, transmission can deliver say 130KW for x amount of time, they can receive it for the same period of time. There are no hills that require even close to 130KW continuous, but there can be overwhelming situations as you mention. Regulation could come in the form of power resistors or simply the control of the friction brakes. Perhaps a combination of the two.
 

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Texas, yes power and energy. Two related but differing concepts.

For those I may have totally confused with my challenged writing skills, power is important to how fast you stop and energy is what it takes in total to get the job done.
 

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More thoughts.

During decelerating on level ground the weight is shifted to the front of the car. This is a plus for the Volt's front tire drive. Using the max torque of the Volt motor, with its mass, tire radius and assumed gear ratio gives a motor stopping force of 0.7 g. Thus a 40% larger motor is needed for optimal breaking with 1g tires.
 

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No, I don't. That's where the time constraints come in. I was saying that if the battery, power electronics, wiring, motor, transmission can deliver say 130KW for x amount of time, they can receive it for the same period of time.
I get the equal energy idea. However I'm saying that there are optimal charging rates for all batteries. Once again, it's the battery that is the weak link. To illustrate what I mean; think about how long it takes and how much highway is used to eccellerate from 0-60 mph as fast as possible. Now think of how long in terms of time and distance it takes to go 60-0 in braking as fast as possible. Most modern cars can do it in 120ft or less. In your model, the energy would all neatly go back in the battery minus a little waste, but sadly, I don't believe this is the case with the batteries we have now. After the optimal charging rate is exceeded, the excess energy is just diverted and dissipated.

Adding a rear motor/generator might be beneficial for AWD, but as to recouping energy, at this time in battery and capacitor technology, I think it would be of no use because the single generator we have now is more than enough. In the future, we will no doubt be getting much more of this energy back.
 

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avoid using mechanical breaks

hopefully the designers will make the breaking system versatile enough to avoid using mechanical breaking if possible. I agree with DaV8or that the motor on the front will be sufficient to max out the battery recharge capabilities.
 

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Control applications

ABS requires separate mechanical breaks for modulation, which is needed for delicate balancing during cornering, banking, tire slip from snow ice, and so forth.
 

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ABS requires separate mechanical breaks for modulation, which is needed for delicate balancing during cornering, banking, tire slip from snow ice, and so forth.
I would guess that ABS would be greatly improved or simplified with front and rear wheel generation.
 

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hopefully the designers will make the breaking system versatile enough to avoid using mechanical breaking if possible. I agree with DaV8or that the motor on the front will be sufficient to max out the battery recharge capabilities.
Mechanical brakes are a must. When the car is at a stand still only mechanical brakes will keep the car from rolling forward or backward without using up power. They will also be needed for the parking/emergency brake function. The good news is that the size and weight of the mechanical brakes can be reduced and the maintenance intervals can be extended.
 

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I get the equal energy idea. However I'm saying that there are optimal charging rates for all batteries. Once again, it's the battery that is the weak link. To illustrate what I mean; think about how long it takes and how much highway is used to eccellerate from 0-60 mph as fast as possible. Now think of how long in terms of time and distance it takes to go 60-0 in braking as fast as possible. Most modern cars can do it in 120ft or less. In your model, the energy would all neatly go back in the battery minus a little waste, but sadly, I don't believe this is the case with the batteries we have now. After the optimal charging rate is exceeded, the excess energy is just diverted and dissipated.

Adding a rear motor/generator might be beneficial for AWD, but as to recouping energy, at this time in battery and capacitor technology, I think it would be of no use because the single generator we have now is more than enough. In the future, we will no doubt be getting much more of this energy back.
I think we're right. There is optimal charge rates but that will rarely be achieved with regen, unless regen power to battery limit is set very low. Cars can stop in very short distances, but requires a tremendous amount of stopping power (>130KW) and breaking on all four wheels. My "model" did not assume or state that the batteries could charge at all possible levels, rather I stated that it could charge at the similar rates at which it can discharge. My admittedly limited knowledge about batteries is that they suffer from internal resistance (Ir). Higher power batteries tend to have less Ir. The battery's characteristics are defined by it's chemistry and Ir. Ir for charging is similar to discharging. So if the battery is capable of supplying 130KW for 8.5s, then should be capable of receiving ~130KW for 8.5s. You are correct in that this will not be enough to stop the car in 120ft. Another interesting possibility is that the motor can be (not necessarily is) the limiting factor and not the battery. If they are sized optimally for accelleration then the motor can be sized for about 10% lower power than the battery due to power electronics losses. In this case the max power returned to the battery from regen will be 20% less than max power delivered.

I agree that the main benefit of a second motor will be AWD for traction as well as performance. If you always brake conservatively enough, then basically you can achieve nearly 100% regen with only the front wheels. Unfortunately reality will intervene sometimes and you will need more braking power. It would be nice to have a visual indicator in the Volt showing when friction breaks are applied that can help "teach" us to maximize our regen braking.
 

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ABS requires separate mechanical breaks for modulation, which is needed for delicate balancing during cornering, banking, tire slip from snow ice, and so forth.
I disagree with this statement. ABS will be much better under the full control of the AC motor controller. No friction brake system will be needed to satisfy this function. I do agree that a small mechanical brake (emergency/parking break) will be needed but not for ABS.
 

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if possible means if safe

Mechanical brakes are a must. When the car is at a stand still only mechanical brakes will keep the car from rolling forward or backward without using up power. They will also be needed for the parking/emergency brake function. The good news is that the size and weight of the mechanical brakes can be reduced and the maintenance intervals can be extended.
I guess I misspoke when I said "if possible". What I meant was that I would like it if the breaking system is versatile enough not to use mechanical breaks if the desired deceleration is within the limits of the regeneration system. I agree that a mechanical system is needed, for your above stated reasons, and the concern of coming to a complete stop on a hill.
 

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I guess I misspoke when I said "if possible". What I meant was that I would like it if the breaking system is versatile enough not to use mechanical breaks if the desired deceleration is within the limits of the regeneration system. I agree that a mechanical system is needed, for your above stated reasons, and the concern of coming to a complete stop on a hill.
I'm sure they will. The GM designers seemed to be attuned to regenerative braking and how cool it is. My guess is that it will be used where ever practical.
 
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