GM Volt Forum banner

How does the Volt dump kinetic excess kinetic energy?

16156 27
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.
21 - 28 of 28 Posts

·
Registered
Joined
·
1,400 Posts
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?
 

·
Registered
Joined
·
296 Posts
Discussion Starter · #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!
 

·
Registered
Joined
·
3,764 Posts
It must at least have some capability to do so, because that's how the engine is started. There isn't a traditional starter in the Volt.
That doesn't mean the system motors the engine to provide braking down a hill. The motor generator starts the engine and generate electricity either from the engine or regeneration. Doesn't automatically mean it motors the engine for braking.
 

·
Registered
Joined
·
296 Posts
Discussion Starter · #24 ·
That doesn't mean the system motors the engine to provide braking down a hill. The motor generator starts the engine and generate electricity either from the engine or regeneration. Doesn't automatically mean it motors the engine for braking.
It means that the hardware capability for spinning the generator is in place. It can be used to start the engine, and it is used in all electric mode at high speeds (to help reduce the speed of the traction motor).

You're right - having the capability for using the generator to spin the engine does not necessarily mean that the control computer is designed to do so. But, it at least has the hardware capability to use electricity to spin the engine.
 

·
Registered
Joined
·
1,623 Posts
It means that the hardware capability for spinning the generator is in place. It can be used to start the engine, and it is used in all electric mode at high speeds (to help reduce the speed of the traction motor).

You're right - having the capability for using the generator to spin the engine does not necessarily mean that the control computer is designed to do so. But, it at least has the hardware capability to use electricity to spin the engine.
Not wanting to have oil particles get into the cylinders of a cold spinning engine - as mentioned in a previous post - might be one reason not to do it.

KNS
 

·
Registered
Joined
·
296 Posts
Discussion Starter · #26 ·
Not wanting to have oil particles get into the cylinders of a cold spinning engine - as mentioned in a previous post - might be one reason not to do it.

KNS
That would make sense. A Prius pretty much always uses its engine, so the engine will be warm, and therefore (I assume) not as likely to get oil into the cylinders.
 

·
Registered
Joined
·
552 Posts
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.
 

·
Registered
Joined
·
3,504 Posts
21 - 28 of 28 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top