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Discussion Starter #1
I have a question for the engineering geeks here:

Let's assume I am at the top of a mountain, and I've fully charged a Gen 1 Volt.

Then, as I go down the LONG hill, the car puts some of the regenerative braking energy into the battery.

How does the car brake when the battery is absolutely fully, totally and completely as charged as it can be?

Does the car simply use the brakes, or does it somehow spin the engine and make use of pumping losses to slow the car down?

I'm asking because my '07 Prius would, when the battery's too full and you're still going down hill, use the energy to spin the engine and make use of its pumping losses instead of using the brake pads.

I've looked at the Volt powertrain diagram, and it isn't obvious how the car could easily dump energy.
 

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What KNS says, but after that the Volt deploys the FED-BS (Flintstone Energy Dissipation-Braking System) which consists of two large foot shaped devices equipped with sandals having tire tread soles (anyone besides me remember those?).

OK, back to my adult beverage.

VIN # B0985
 

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What KNS says, but after that the Volt deploys the FED-BS (Flintstone Energy Dissipation-Braking System) which consists of two large foot shaped devices equipped with sandals having tire tread soles (anyone besides me remember those?).
I think I still have a pair somewhere in the back of my closet.
 

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Discussion Starter #5
It runs the two motors against each other. If that isn't enough it uses the brake pads. KNS
Running the two motors against each other would result in the energy being dumped as heat into the transmission fluid and out to that radiator. I'm not sure it has the capacity for all that energy.

Looking more closely at the power train diagram, I think the bigger motor could operate as a generator and dump the energy to the smaller motor clutched to the engine. An engine being spun with the throttle closed is a very effective way to dissipate energy without burning things up. This is effectively how my Prius does it.
 

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Discussion Starter #6
What KNS says, but after that the Volt deploys the FED-BS (Flintstone Energy Dissipation-Braking System) which consists of two large foot shaped devices equipped with sandals having tire tread soles (anyone besides me remember those?).

OK, back to my adult beverage.

VIN # B0985
IIRC, they were the Flintstone Reactive Energy Dissipators, or FRED for short.
 

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HAHA, But this has been asked before if you want a longer version and is a good question and one you need to think about for home wind systems.
 

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Maybe the 2017 Chevy Bolt EV will do it better: put out a parachte to create drag (as jet fighters do when landing), then at the bottom of the hill it will pull and repack the chute, and stow it quickly.
 

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From a practical standpoint, this rarely occurs, and when it does, only for a short period of time.

With the Volt's design, a "fully charged" battery still allows for some more capacity to be stored from regenerating, so you'd have to live on a pretty tall hill and not use any energy for this to really take place. It's possible, but it's not likely in the majority of scenarios.
 

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The Bolt will not be able to do the motor-to-motor - heat-to-ATF thing because it only has 1 motor.

The answer is: Don't charge to 100% if you are on top of a mountain. Even if it's free...:eek:

A BEV with no regen braking would be the worst case coming down the mountain. It would be totally friction braking.
 

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Discussion Starter #11 (Edited)
Using http://www.plugincars.com/sites/default/files/imagecache/fullsize_620w/voltec-propulsion.jpg as the diagram


If C1 and C3 are engaged, the "Traction Motor" can operate as a generator, changing kinetic energy into electricity. This is how normal regenerative braking works, only the power is dumped into the battery.

The power that the "Traction Motor" is creating can be dumped into the "Generator." The "Generator" can spin the engine, and dissipate the energy by using the engine to create pumping losses (spin the engine while the throttle plate is closed).

This dumps the energy into the atmosphere through the engine without wearing brakes out.

My '07 Prius operates in a similar manner - once the (far smaller) battery is fully charged, it spins the engine to dissipate energy. On both the Prius and the Volt, it looks like it is something that the powertrain control computer could easily be designed to handle.

I don't live at the top of a steep incline, but I could see that at least one Volt owner could. So, for me, it's just a brain teaser, but I bet GM has thought of this.

Gents, love to know your thoughts.
 

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Using http://www.plugincars.com/sites/default/files/imagecache/fullsize_620w/voltec-propulsion.jpg as the diagram


If C1 and C3 are engaged, the "Traction Motor" can operate as a generator, changing kinetic energy into electricity. This is how normal regenerative braking works, only the power is dumped into the battery.

The power that the "Traction Motor" is creating can be dumped into the "Generator." The "Generator" can spin the engine, and dissipate the energy by using the engine to create pumping losses (spin the engine while the throttle plate is closed).

This dumps the energy into the atmosphere through the engine without wearing brakes out.

My '07 Prius operates in a similar manner - once the (far smaller) battery is fully charged, it spins the engine to dissipate energy. On both the Prius and the Volt, it looks like it is something that the powertrain control computer could easily be designed to handle.

I don't live at the top of a steep incline, but I could see that at least one Volt owner could. So, for me, it's just a brain teaser, but I bet GM has thought of this.

Gents, love to know your thoughts.
I don't think the Volt motors the ICE, at least not in Gen 1.
 

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Should this rare condition occurs it simply (and temporarily) disables regenerative braking and the base hydraulic brake system will take over all negative torque application and dissipate the energy as heat like any normal ICE car.
There can be no generation taking place in MG2 to create the negative torque forces without a connection to the battery through the inverter.
Since there's no place to store it, it must simply be lost to the atmosphere.
There's nothing fancy about it.
WOT
 

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Side note: once the downhill regeneration has charged the battery to the maximum allowed soc, the system, in effect, shifts from Charge Depleting to Charge Sustaining mode. Until that environment is changed (e.g., bottom of the hill is reached, heat/ac turned on to consume power) and the Volt returns to CD operations, the distance traveled in CS mode, by definition, should be recorded as Gas Miles.
 

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Discussion Starter #17
Should this rare condition occurs it simply (and temporarily) disables regenerative braking and the base hydraulic brake system will take over all negative torque application and dissipate the energy as heat like any normal ICE car.
There can be no generation taking place in MG2 to create the negative torque forces without a connection to the battery through the inverter.
Since there's no place to store it, it must simply be lost to the atmosphere.
There's nothing fancy about it.
WOT
We know that the traction motor can also be used as a generator - that's how regenerative braking works.
We also know that the generator can be used as a motor - that's how the Volt starts the engine. WE also know that the generator can be used as a motor when the car is moving at high speeds - both the traction motor and the generator are operated as motors.
I bet that the control computer can permit the traction motor to dump power in to the battery AND have the generator pull power out of the battery to spin the engine to burn off energy as pumping losses.

The hardware capabilities all appear to be there - the traction motor can be operated as a high power generator, the generator can be operated as a high power motor (when the car's in high speed all electric mode), and the computer controls the throttle plate and the injectors. I think the question becomes does the software use this technique to burn off extra energy?
 

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We know that the traction motor can also be used as a generator - that's how regenerative braking works.
We also know that the generator can be used as a motor - that's how the Volt starts the engine. WE also know that the generator can be used as a motor when the car is moving at high speeds - both the traction motor and the generator are operated as motors.
I bet that the control computer can permit the traction motor to dump power in to the battery AND have the generator pull power out of the battery to spin the engine to burn off energy as pumping losses.

The hardware capabilities all appear to be there - the traction motor can be operated as a high power generator, the generator can be operated as a high power motor (when the car's in high speed all electric mode), and the computer controls the throttle plate and the injectors. I think the question becomes does the software use this technique to burn off extra energy?
Sorry no. To what end would the car operate in this fashion?
There is zero benefit. If you cannot store the energy for future tractive power usage then there's simply no rationale.
Without a place of storage there will be no DC current flow and as such MGB will not be acting as a generator.

You somehow believe it's wise to create additional software driven complexity and unnecessary energy transfer and waste heat elsewhere in some imaginative way simply because they can?
Why would anyone think this was necessary?
No
I can assure you the Volt does not do this.
In the rare and short-lived potential for this to occur you will simply lose regen and the brakes operate conventionally until there is sufficient SOC headspace created.
That is all there is to it (and essentially all GM engineering info and service literature states as such)
WOT
 

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Sorry no. To what end would the car operate in this fashion?
There is zero benefit. If you cannot store the energy for future tractive power usage then there's simply no rationale.
Without a place of storage there will be no DC current flow and as such MGB will not be acting as a generator.

You somehow believe it's wise to create additional software driven complexity and unnecessary energy transfer and waste heat elsewhere in some imaginative way simply because they can?
Why would anyone think this was necessary?

WOT
WOP, you are definitely the expert here, not me, but I have heard this before on these forums several times, that the motors will somehow engage each other on a sustained downhill with a full SoC. I've read people here say they experienced some weird noises when they operated the car in this situation, which they reported as 'proof' of this weird mode. The fact that the Prius does this (according to OP, I did not fact-check this) and the drive trains are similar on the two cars... well, I'm confused. I think there is some value in such a mode... the average ICE car allows engine braking on sustained downhills, which can prevent brakes from overheating and failing. True, it would likely be rare for a Volt... you might have to climb Mt. Washington with a full battery in Hold mode to set up the scenario. I'll see if I can find those old posts where people claim to have experienced this, just to make sure I'm not dreaming...

Edit: Ok, it was discussed here, see post #12 by saghost (another very knowledgeable forum member):

The Volt holds a little headroom over the top of a full charge to allow it to do regen on a "full" battery. Once it fills that, the car starts dissipating the excess as heat by spinning the motors against each other, losing power in all the conversions (wheels-MG B, MG B-DC bus, DC Bus-MG A, MG-A-Wheels.) Instead of heating up your brake pads, this heats the power electronics coolant and the transmission fluid, both of which have nice big radiators on the front of the car to transfer it into the environment.

Not the same as what OP was describing (using ICE to compress air w/o combustion), but still it's something... was Walter mistaken? Then post #24 says that someone in that scenario had their engine come on (w/ no gas usage) for a few miles. I don't know what to think...

Edit2: This post is saying Walter was right. There is DashDAQ data showing it, and Chevy Customer service even verifies this in post #18:

You have come across a feature of the vehicle which protects the battery from overcharging. Under certain circumstances, the electric motors will resist one another to provide braking in addition to the friction brakes on the vehicle. In order to meet emission requirements, the Volt does not spin the engine, but uses clutch 2 in the drive unit to link both motors. When the vehicle is at low speeds, clutch 2 requires that the resultant planetary gearset speeds increase to compensate. You will hear the electric motors at higher speeds, which is certainly a change from their normally silent operation.

The Volt was validated using the steepest, longest descent in the nation, Pike's Peak. With a full battery, the volt can descend Pike's Peak without issue with a combination of friction brakes and the electric motors. This is part of the Voltec propulsion system which has many more delighting features waiting for you to discover.
 

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WOP, you are definitely the expert here, not me, but I have heard this before on these forums several times, that the motors will somehow engage each other on a sustained downhill with a full SoC. I've read people here say they experienced some weird noises when they operated the car in this situation, which they reported as 'proof' of this weird mode. The fact that the Prius does this (according to OP, I did not fact-check this) and the drive trains are similar on the two cars... well, I'm confused. I think there is some value in such a mode... the average ICE car allows engine braking on sustained downhills, which can prevent brakes from overheating and failing. True, it would likely be rare for a Volt... you might have to climb Mt. Washington with a full battery in Hold mode to set up the scenario. I'll see if I can find those old posts where people claim to have experienced this, just to make sure I'm not dreaming...

Edit: Ok, it was discussed here, see post #12 by saghost (another very knowledgeable forum member):

The Volt holds a little headroom over the top of a full charge to allow it to do regen on a "full" battery. Once it fills that, the car starts dissipating the excess as heat by spinning the motors against each other, losing power in all the conversions (wheels-MG B, MG B-DC bus, DC Bus-MG A, MG-A-Wheels.) Instead of heating up your brake pads, this heats the power electronics coolant and the transmission fluid, both of which have nice big radiators on the front of the car to transfer it into the environment.

Not the same as what OP was describing (using ICE to compress air w/o combustion), but still it's something... was Walter mistaken? Then post #24 says that someone in that scenario had their engine come on (w/ no gas usage) for a few miles. I don't know what to think...

Edit2: This post is saying Walter was right. There is DashDAQ data showing it, and Chevy Customer service even verifies this in post #18:

You have come across a feature of the vehicle which protects the battery from overcharging. Under certain circumstances, the electric motors will resist one another to provide braking in addition to the friction brakes on the vehicle. In order to meet emission requirements, the Volt does not spin the engine, but uses clutch 2 in the drive unit to link both motors. When the vehicle is at low speeds, clutch 2 requires that the resultant planetary gearset speeds increase to compensate. You will hear the electric motors at higher speeds, which is certainly a change from their normally silent operation.

The Volt was validated using the steepest, longest descent in the nation, Pike's Peak. With a full battery, the volt can descend Pike's Peak without issue with a combination of friction brakes and the electric motors. This is part of the Voltec propulsion system which has many more delighting features waiting for you to discover.
Sorry but you are now talking about 2 different things.
This discussion is based on a battery with a full SOC that immediately gets exposed to a period of deceleration that traditionally would place the car into regen BUT now with no place to put any regen energy.
The OP was discussing the option of connecting all the powertrain elements using the clutches in order to use the non-functioning ICE as a mechanical brake and absorbing any of the energy as frictional heat in the ICE and/or some sort of electrical dissipation absorbed by the power electronics and drive axle cooling systems.

IN THIS EXACT SCENARIO the Volt simply doesn't do this and instead just disables the regen routines and lets the brakes function as a normal car would. It is obviously a very short duration event. This operation is easily viewable and verifiable using GDS2 and in fact there's a Volt diagnostic routine (for a DTC) where this is done to confirm these routines function properly.

What saghost and our first chev customer service rep Adam are actually describing is the methodology used for regenerative braking functionality whenever the drive unit is in "combined" 2-motor modes. The operational map is shown below and as you can see decisions as to when this occurs is based on road speed and "desired" axle torque. But the reality is THIS map is not showing the negative torque aspects of this behavior.

But if the car was already in a combined mode (in either CD or CS) then BOTH motors are utilized during the regen cycle with the planetary gearset between them. In that the carrier (connected to the road wheels via the final drive) is now the input and the sun and motor B negative torque the reactionary member (akin to holding the sun with a clutch but in this case it's actually more like resistance torque created by the generation) and the ring gear (still connected to MGA via the clutch) becomes the output and driving MGA permitting it too to generate. (*NOTE*There are also a few other "off map" conditions where this may be applied but the scenario above is not one of them)

It's important to point out at this juncture that the 3-phase AC waveforms created by MGA and MGB are NOT synchronous, (out of phase) at different amplitudes, and technically not even at the same frequencies due to the differences in mechanical pole design AND simple rotational speed differences due to the mechanical gearing. So there's really no way to directly couple these electrically somehow as is being suggested. The power inverter module simply collects the 3-phase AC from each source, rectifies, and pumps the DC current into the battery. If it can't do that for ANY reason then the generators are shunted within the PIM and no generating activity or torque will take place and for that reason neither will regen braking occur.

But assuming there's available SOC headspace this combo creates the desired negative torque in proportion to drivers braking input and the energy produced is put in the battery. But this mechanical arrangement will not create regen current if there's no place to put it. It can't and the battery DC connection and currently flow is critical and in direct proportion to the amount of negative regen torque being created. It's HOW we create the variable levels of regen. So there's no way for that energy to be created let alone dissipated and let alone the impossibility of 60,000 watts of instantaneous power to be successfully dissipated by a relatively low volume cooling system and simple airflow.

As such the hydraulic brakes look after all deceleration duties when there's no available connection to the battery. The exact same thing occurs if the diagnostic executive "pulled the plug" on the battery while driving using the main contactors. Yes you still have an MGB rotor mechanically connected to the roadway, but without an available connection to the battery (through the PIM) there can be no generation taking place and no regenerative braking torque created by MGB. (or MGA for that matter)

I can see how easily this behavior can be misconstrued and incorrect assumptions made especially when using something like the DashDAQ.. Sorry but the DashDAQ is just a toy. It cannot simultaneously display all of the data parameters (PIDs) necessary to correctly analyze and assess these conditions and behaviors. If you want to see what's actually happening you're going to need to obtain an MDI and GDS2.

HTH
WOT
 

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