efficiency of regenerative braking?

High.

How much kinetic energy is recovered?

Almost all. Losses are in heating only, brakes heat a lot,=20 cables, motors and batteries very little.

Isn't regenerative braking just the DC traction motors running backwards?

An alternator that worked over a broad rpm range touted 70% efficiency in a recent patent. It would be interesting if they could beat that.

Bret Cahill

No.

Graham

How is the electricity being regenerated? By another component, say, an alternator or DC generator mounted on the same shaft as the traction motor?

Alternators generally aren't all that efficient off the design point rpm.

Bret Cahill

According to Wikipedia, it's just a bit over 30%.

Some other sources appear to quote higher values, but I think the figures you find will depend on whether you're looking solely at the efficiency of the motor during regenerative braking, the amount of energy that ends up the batteries, or the amount of *recoverable* energy that ends up in the batteries -- the figures for each will all be different. It isn't clear to me which the Wikipedia article refers to.

Simply put, yes. In the current hybrid system common to several manufactures, there are two motors in the transaxle. The larger motor is designated for providing the majority of motive torque. The smaller motor is used as a "starter" as it is attached directly to the engine output, and also functions to react to the larger motor to effectively create infinitely variable output gearing ("shifting"). To get to the point, both motors are either energized or tapped by the HV ECU to create torque or recharge the HV battery, respectively.

The motors don't run "backwards" but are used in reverse current flow to charge the HV battery via an ac/dc converter that is managed by the HV ECU. Regeneration occurs whenever the various ECUs communicate to the HV ECU that charging is the correct strategy, be it during braking, extended battery-only operation, startup, or whatever else I'm forgetting.

Toyota MDT in MO

This will save about 20-25 per cent of that energy, making a real contribution towards tackling CO2 emissions."

tom

[1]

The answer depends on the details of the specific application.

Figure the motor/generator itself is 90% to 95% efficient. That would mean that you would lose 5 to 10% putting the energy into storage, then another 5 to 10% converting it back to mechanical energy. If the battery pack is used to store the energy, it has internal resistance so some of the energy is lost as heat. Batteries are not very efficient for rapid charge and discharge cycles. As a guess let's say you get

60% of what you put in back out, with the remainder going to heat. So you could be looking at 90%*90%*60%=48.6%

controler, bringing it down to just under 40%. There could be a bit more lost if a DC to DC voltage converter is used.

Using capacitors eliminates the battery losses, but they mean added complexity, cost and weight.

My guestimate would be that a good capacitor system should be able to recover 70%, a good battery system 50% and any decent system at least

30%.

Something to keep in mind is the energy of a moving vehicle is proportional to the square of its velocity. So a car doing 30 mph with a 50% recovery system could get back up to about 21 mph on the recovered energy. Shows how it can make a big difference in stop and go city driving.

Bruce

As others here have pointed out, the motors don't run backwards, the electrical current does. When DC electric motors run they also act as generators. The voltage produced by the generator, sometimes called the "back EMF" is proportional to the rpm. As the rpm goes up this back EMF goes up and limits how high the rpm will go with no load on the motor.

One way to make the motor charge the battery is to use a motor with field windings and control the back EMF by controling the field current. This same idea is used for the "voltage regulator" in many automotive alternators.

Actually I think the most efficient systems in use today are varialbe frequency AC systems. Many of the same ideas work and some are easier to impliment with AC rather than DC.

Bruce

It's called motor-generator action.

Traction motors aren't alternators. What gave you that idea about the rpm anyway ?

Graham

You know how to brake at just one rpm?

Bret Cahill

Well, you should keep in mind that the amount of kinetic energy present at any moment is a very small fraction of the amount of energy used overall. If you take a car with a mass of 1000kg going at 15m/s (about right for city driving) its kinetic energy is a tad over 110 kJ, the equivalent of 2.5g of gas. That's thermal, of course, factoring in efficiencies it may be the equivalent of 7-8g of gas (in terms of delivered mechanical energy). And now you recoup about 30% of this. Unless you spend all your driving accelerating like hell from the light, then slamming on the breaks before next light, I doubt you'll ever recoup more than a small fraction of what the system costs. But, it makes a good selling point:-)

Some difference, hardly big.

Mati Meron | "When you argue with a fool, snipped-for-privacy@cars.uchicago.edu | chances are he is doing just the same"

Is it 90 - 95% efficient acting as a DC generator over a broad range of rpms?

Bret Cahill

What's the efficiency of the motor-generator in generator mode?

Is it the same at any rpm?

Bret Cahill

Energy lost to friction braking is probably more than wind and rolling resistance combined for a commuter train.

Still, it seems they could do a little better than 20 - 25% considering economies of scale.

Bret Cahill

What *are* you talking about ?

Have you gone mad ?

Graham

Probably in the same region as its efficiency as a motor but I have to say I'm not totally sure.

The same principle applies.

Graham

That's why super-caps are used to store the regenerated energy. No such loss.

Graham

Where do you get this 30% from ?

Graham