Regenerative braking

It's super simple. Imagine any old electric generating turbine... say wind or water-powered. The force of the wind or water pushes against great magnetic resistance, and by turning the turbine against resistance, electricity is born.

Now imagine you have a turbine in your car, attached to your wheels. When you coast or brake, the turbine engages, and the weight/speed of the car spin it to generate a current. At the same time, the resistance of the turbine happens to slow down your car because it's pretty strong.

Some energy is still lost as heat through various inefficiencies, but some of it makes it all the way back to the battery, where it can be used later. And on top of it all, you used little or no FRICTION (i.e. brake pads) to slow down, so you'll very rarely have to have those serviced or replaced.

 

Regenerative braking electronically switches the motor/generator to generate mode, and dumps the electrical energy generated into the "traction battery". You feel this as normal "braking", stronger than you would actually think it would be. So you convert the vehicle kinetic energy into electrical energy and not just to heat as a "friction brake" would.

Some things to keep in mind:
a) Regenerative braking works down to about 7 MPH/11 km/hr, at which point the Prius switches fully to "friction brakes".
bee) The motor/generator is perhaps 80% efficient in converting the kinetic energy into electrical energy (some of those losses are also in the electronics).
c) The battery is perhaps 70% efficient in storing the energy applied to it.
d) The above inefficiencies are also there when you try to reuse the recovered energy, so overall, you can use perhaps 50% of the energy normally lost through friction braking.
e) Regenerative braking occurs when you are gentle on the brake pedal. It is bypassed if you slam on the brakes or press very hard on the brake pedal.
f) My experience so far is, if you are gentle on the brake pedal you will probably not use much if any friction braking until you slow below the above minimum speed.

"bee" to stop the silly system from substituting an emoticon for "B".

 

Well heck I'll give it a shot.

Regenerative braking is one of many technologies used in hybrid's (Prius included) design to increase efficency.

In a conventional car braking converts the forward momentum to heat via the friction of the brake pads or shoes. A complete waste of that energy.

A hybrid uses the electric motor-generator as a brake, where we are able to recapture some of that energy and store it in the battery pack. We can then reuse that energy to aid acceleration, using that same motor-generator.

Hybrids still have conventional brakes. With much of the braking done by the motor-generator, the conventional brakes last a long time. They just don't get used as much.

Neat stuff, I love efficency.

 

<div class='quotetop'>QUOTE(LaurenceUK @ Oct 2 2007, 10:55 AM) [snapback]520335[/snapback]</div>

Ok, here's my stab at a not-too-technical definition and answering your "why" question.

Definition:
Energy is used to make the car go.
Something has to happen to that energy in order to make the car stop.
With "traditional" friction brakes that energy is converted to heat and then it is gone to the atmosphere, you can't re-use it.
With "regenerative" braking, that energy is converted to electricity and stored in the battery so you can re-use it later.

"Why"?

Why is it important?
For the same reason "traditional" friction brakes are important. It slows the car down.

Why is it useful?
Cars need electricity for many things (radio, lights, motors, locks, etc.) Since the only energy added to the car is gasoline, all that electricity comes from burning gasoline. In a non-hybrid car, an alternator adds a load to the engine to generate the electricity that is needed and stored in a 12V battery. This load increases the amount of gasoline burned. In the Prius, the large electric motors can be used as generators. When needed they can add a load to the engine to generate the electricity similar to an alternator, but they can also add a load to the wheels while slowing the car to generate the electricity. This re-use of the otherwise wasted energy that accelerated the car reduces the amount of gasoline required to provide the needed electricity. It is just one of several features of the Prius that improve it's fuel efficiency.

Notice that "regenerative braking" is not "important" to a Prius functioning, and the Prius would still have really good fuel efficiency even without regenerative braking. It's just that with regenerative braking the fuel efficiency is even better, and the friction brake pads last longer. Conceptually a crude, inefficient, and less smooth analogy in a non-hybrid car could be accomplished if the engine could be turned off when you pressed the brake pedal, and the alternator could be disconnected from the engine and connected to the wheels. In the Prius, since the electric motors are already connected to the wheels among other reasons, it is relatively easy to accomplish.

 

Power comes out of the car to make it go.

'Go' turns back into power to make it stop.



Regenerative 'breaking' would be much cooler, but then people would drive even more dangerously.

 

Regen braking slows down a car with magnetic field (in the electric motor) rather than rubbing two plates. The advantage of regen brake is that electricity is made and captured rather than loosing as heat and generate dust.

 

Perhaps surprisingly most of the Prius' fuel economy comes from the very clever drive train, which makes nearly optimal use of the internal combustion engine by combining its output with that of two electric motor/generators. Regenerative braking contributes relatively little to the fuel economy but it comes for free as a side effect of the drive design.

 

You can say that again!

 

I'll second that.

 

I don't have the equipment to do the measurements (Canview) so my numbers are guesstimates, based on my many years of experience with batteries and motors. I won't comment on whether it's 50% overall or 30%, just that it recovers energy that would otherwise be lost.

There was a short article in Scientific American suggesting about 20% of the fuel economy gain was from regen braking, but again, it appeared the numbers they were throwing around were estimates.

Bob Wilson may be able to shine some light on this, but I hesitate to ask, as we will be buried in graphs if he does.

 

Here is a question... We all know that regen breaking takes physical energy and converts it to electricity via MG1/MG2.... The question is how does "60amp" braking differ in terms of how much energy is captured compared to the 10amp regen-drag with the foot off the brake or light braking?

does the car normally send most of the energy from mg2 into mg1 or something during drag or something?

 

<div class='quotetop'>QUOTE(Bob64 @ Oct 3 2007, 12:06 PM) [snapback]520639[/snapback]</div>

No. Sending power from MG2 to MG1 wouldn't produce the braking drag, except for a little bit of heating. All of the regen braking works the same way, whether it's the light drag to simulate engine drag, or 60 Amps of foot stomping. Higher amperage rates will produce more heating in the wires and battery, and consequentially reduce the overall efficiency.

Tom

 

I just watched a show on TV about trains and it seems the idea is similar. Trains brake in a similar way. If they didn't have that, they'd destroy their brakes in no time.

P.S. There is no such thing as regenerative brEaking. There is only destructive brEaking. There is however, regenerative brAking ;-)

 

<div class='quotetop'>QUOTE(PriusOwner004 @ Oct 3 2007, 02:19 PM) [snapback]520722[/snapback]</div>

On diesel-electric locomotives it is call dynamic braking. With dynamic braking there is no regeneration - all of the generated electricity is burned off as heat. It helps save the brakes, but offers no efficiency improvement.

Tom

 

You know those toy cars that you can drag against a flat surface and store up energy, put 'em down and they'll zip along by themselves? Sort of the same concept--without huge thumbprints on your windows...

 

<div class='quotetop'>QUOTE(qbee42 @ Oct 3 2007, 11:28 AM) [snapback]520730[/snapback]</div>

Trivia: Those big fans you see on the roof of a locomotive aren't there to provide engine cooling. They cool the resistance grid that is used during dynamic braking.

 

<div class='quotetop'>QUOTE(qbee42 @ Oct 3 2007, 01:28 PM) [snapback]520730[/snapback]</div>

Ok, I know I'm being a bit too specific here, but unless I'm misunderstanding how it works, with dynamic braking there is regeneration there just isn't any storage or re-use. Electricity is generated and then used to accelerate the locomotive. Then the kinetic energy is converted back into electricity for deceleration (regeneration of electricity). Then rather than storing it and re-using it to accelerate the locomotive again, it is converted to heat and dumped into the atmosphere.