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How does the Volt dump kinetic excess kinetic energy?

17K views 27 replies 14 participants last post by  canehdian 
#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.
 
#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.
 
#3 ·
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
 
#4 ·
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.
 
#7 ·
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.
 
#8 ·
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.
 
#9 ·
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.
 
#10 ·
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.
 
#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.
 
#13 · (Edited)
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
 
#17 ·
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?
 
#14 ·
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.
 
#21 · (Edited)
I didn't meant to confuse things by talking about 2 issues... it's just that OP was asking about possible methods to handle extra kinetic energy, and he posited the ICE-compression method based on his experience with the Prius. But it was implied he would want to know of any technique / mode the Volt has to do this, other than resorting to just hydraulic brakes, so I added the links I found on the alleged "two motors fighting" dissipation that has been discussed before.

Again, you're the technical expert so I'm certainly inclined to believe you, but what you are saying above really doesn't sound the same as what the Chevy Rep said... you're talking about normal regen in 2-motor operation. He seemed to be discussing some sort of special case (from the 2nd quote in my post above: You have come across a feature of the vehicle which protects the battery from overcharging and With a full battery, the volt can descend Pike's Peak without issue with a combination of friction brakes and the electric motors)

Later, he even says (post#25):

The emissions from a spinning, cold engine without fuel come from small particles of engine oil passing by the piston rings into the combustion chamber and out the exhaust. Once the piston rings warm up with normal combustion, then the oil no longer gets past the piston rings. The Volt chooses not to spin a cold engine for regen protection because we can use both electric motors to resist one another while descending a hill, when needed. Our competitors do not have that ability today in their products.

I'm just trying to understand this all... using 2 motors resisting each other for regen protection sure sounds like a completely different thing than you mentioned... is he just mis-representing the two-motor regen you explained?
 
#22 ·
I'm the OP, and I sure appreciate the insight you've provided. I know that the Prius does this as it's mentioned in the documentation on the car. There is a rather prominant "B" mode that acts like low gear on a volt. If you're going down a hill, the battery charges and then, once it's full, you can hear and feel the engine being spun. It makes sense from a "don't burn out your brakes" point of view and the engine makes a convenient engine brake.

I think that the Volt could do this. And, yes, it would take far more effort to put the Volt in this mode because its battery is much larger than the Prius' battery.

At higher speeds when both the Volt traction motor and the generator are clutched to the wheels, the only way to get some engine braking would be to clutch the engine to the wheels too. This could provide some braking, adjustable only by controlling the throttle plate of the (unfueled) engine.

At lower speeds, when only the traction motor is clutched to the wheels, the generator could be clutched to the engine. The engine and the generator could then dissipate energy, being adjusted both by the throttle plate and the generator's driven speed.

Based on what's been said in this thread, GM has decided not to do this though. It makes some sense - arguably it is a rare event during normal use of the car. And, the brakes look to be sized big enough to handle this without requiring engine braking.

Interesting little car!
 
#27 ·
Interesting topic,, I noticed during long trip how after using the ICE , the coolant in the electronics and battery are as warm as the ICE coolant reservoir , spite most of the time the car has used the ICE alone ,with no much battery charging or discharging...I assume the hydraulic fluid of engine A gets warm when the ICE is in use,in part by direct contact metal to metal from the heat generated by the internal combustion engine itself, since when the car goes pure electric, no heat at all ,spite using both generator A & B.
 
#28 ·
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