Regenerative Brakes question... (silly?)

No it doesn't convert heat to electricity. It converts the Kinetic Energy of the car into electricity. It starts the motor/generator and that creates drag that slows the car and generates electricity.

 

Normal "friction" brakes convert kinetic energy to heat.

Prius substitutes the motor-generators resistance for the normal brakes when it can (generally over 7 mph in a non-emergency brake force application).

So, Prius produces electricity instead of heat, but not from heat.

 

No heat, except that which is wasted in the electrical process.

Tom

 

But to answer your question, yes, braking harder (or longer) will generate more electricity. There's a top limit, but it's fairly high before the friction brakes are engaged.

However, this electricity is not a free lunch, because you lose some energy in the conversion process, storing it in the battery and retrieving it, and using it in the electric motor to convert it back into kinetic energy for the wheels. That's why hypermilers actually try to use the battery as little as possible. It turns out it's better to use just the gas engine for acceleration/maintaining speed and then glide/coast to stops as much as possible (and use pulse-and-glide when the conditions are right).

 

This seems to be the usual line for explaining why hyper-milers do what they do, but I am not sure it's correct. Are there energy conversion losses? I suppose so but I don't think that they are very large. If charging and discharging the HV battery is so inefficient then the HV battery should be removed for BETTER mileage! I believe that all the factors of the Prius design are additive and the engine, MG(s) HV battery and other subsystems are able to work in almost perfect harmony to yield a better than average mpg. Unless, sabotaged by some non-caring owner by miuse of the design. I do believe the brake pedal can adjust the HV charge rate in a minute and controlled amount. The Prius designers knew what they were doing. According to our PC Techies the brakes are not really applied until 8mph or less. If using 100mph as a guide that means that from 100 mph to 8 mph, equals regeneration potential. Just stepping lightly on the pedal may be sufficient to induce regeneration with-out actually attempting to slow down or STOP ( God forbid). :cheer2:

 

andy, again, you only hurt your mpg by braking, period. regen is better than friction, but still inefficient. Brake when you have to. Additional braking is throwing money away. If you can go downhill all the time, great. If not, conserve kinetic energy. It is that simple.

 

Watch on the CHG part of the HSI screen.
When you push brake pedal harder, the CHG bar becomes longer.
If the CHG bar became 100%, the regenerating power does not increase any more, then the friction brake is used to help the braking power.
So, please keep the CHG bar less than 100% for better efficiency.

[email protected]

 

Braking uses the electric motor only as a generator. The ICE engine is not required to turn on at all because of the regen braking, but it can turn on for other reasons while braking.

B Mode forces the ICE engine to spin up as an air pump and wastes power by NOT storing it in the battery. Useful for when going down a long (long long decent) and normal 100% regen braking would fill the battery before the end. Then it will automatically switch to friction braking making heat and causing wear on the disks, also spin the engine to waste power (basically automatic B mode). Alternatively you can put it in B mode where some of that downhill energy is wasted in the engine and some captured. Time it correctly and you can be just under fully recharged at the bottom of hill, and not spinning the engine to waste power.

 

There are energy conversion losses, and they are significant. You are correct that removing the HV battery and other hybrid components would produce a car capable of even better mileage. Unfortunately this car would be undrivable under ordinary circumstances. The HV battery and other hybrid components allow for an otherwise efficient gasoline powered car to do such useful things as accelerate from a stop, pass other cars, and climb hills.

All cars need to start, stop, pass, and do other things detrimental to good mileage. The hybrid part of the Prius mitigates the effect of these actions, but it doesn't change the fact that these actions are bad for mileage. There is a reason that hypermilers try to not brake or otherwise use the HV battery.

Tom

 

I would expect this top no-friction braking limit to be fairly low, and variable with speed.

The GenIII battery limit is 27kW. I don't know how much more power is lost in the MG and inverter and other places before the regenerated power reaches the battery, but would guess total regen power at the input shaft to be in the 40-something-kw range.

Full force friction braking on a half-loaded Prius at 60 mph would start at about 400kW. For you folks traveling faster, make that 500+ kW.

 

ok, silly question, but I have heard of the no braking to improve mileage before, i heard about pulse and glide, but where i get a little mixed up, is when you say do not use the battery when possible. when you pulse and glide, aren't you gliding on the battery????

at least, how i have been doing it, is i try to get to speed, then release the gas pedal until the gas used goes to zero, and then press the gas pedal again until the bar is just before the middle on the ECO screen. That way i am able to keep speed, however, i was under the impression that i was using the battery. am i doing it wrong?

 

For true pulse and glide you use the momentum of the car, not the energy in the battery. If you run on the battery you are going to lose mileage to conversion losses.

In theory, here is how the *ideal* pulse and glide works (note that it works just as well for non-hybrid vehicles):

1) Accelerate to speed using the engine at its most efficient setting.

2) Disconnect and power down the engine.

3) Coast to a lower speed.

4) Start the engine and repeat.

It's really a very simple idea: Extract energy from the engine at its most efficient power level, store the energy in the car as momentum, and finally, extract the kinetic energy stored as momentum to keep the car moving through the glide.

The reason this works is because all cars are overpowered for straight and level driving. Car engines have to be larger than necessary for straight and level cruising because cars need to accelerate and climb hills. There is also the marketing advantages of a big engine. This means that all normal cars cruise down the road on an engine running at less than optimal levels. Pulse and Glide reduces the power output of the engine by pulse width modulation at the most efficient setting instead of throttling.

Tom

 

One additional note: I do stop trying to deliberately regenerate when the HV is solid green. ( since I have owned it )

 

For good mileage, you should never deliberately try to regenerate. You should try to avoid braking.

Tom

 

Lets go back to the often posted question of the inefficiency of converting kinetic energy of car motion into electricity, storing it, then re-using it. Otherwise known as "regeneration".

Warning - I don't have figures for the Prius components so I have to "guess". If you can find them somewhere, please post the corrections.

From my many years in electronics and power:
Electric motor/generators vary in efficiency mostly based on size. The Prius unit is actually fairly large (20 HP) and so fairly efficient - I would guess in the 85-90% area. I'll use 87%.
Battery charging of NiMH batteries is at best 80% efficient. At worst 60%. I would guess the Prius unit is in the 80% area. Drawing the electrical energy from the battery is usually a bit more efficient - lets use 90%.

So, starting with 100% of some work number we reduce it to 87% by converting the kinetic energy into electricity.
Then we loose some of that charging the battery - 87 * 80% = 69.6.
Now we want to use that energy.
So we loose a little more withdrawing it from the battery - 69.6 * 90% = 62.6.
Then we loose a little more in the motor, converting it to mechanical energy - 62.6 * 87% = 54.5.

The generally accepted number is 50% efficiency from kinetic energy through the electrical system back to kinetic energy. So my "guess'" are pretty close, I suppose. The difference is probably accountable by the inverter, which is probably running in the 95-98% efficiency range (all the DC power must be converted back and forth into three phase AC and the voltage changed).

And, yes, I'm amazed it came out so close to the quoted 50% number (this is the first time I've bothered to actually do the calculation).

 

Regeneration is a wonderful thing. It's important to keep in mind that regeneration only recaptures waste energy. If you don't waste the energy in the first place, there is nothing to regenerate.

In a conventional system the momentum or kinetic energy is wasted as heat via the brakes. The Prius just lets us get SOME of that energy back.

Improving your driving habits to minimize the amount of energy regenerated will allow the greatest economy. I just consider the regenerated energy to be a bonus!

 

Hello Tom: We are getting to be ole friends now. I must beg to differ with you on one very important aspect. " A body in motion tends to stay in motion" This sounds familiar to most of us. Putting the hyper-milers aside, as they are playing a different game that is not pertinent to everyday life. No matter what one does the vehicle slows down, except, on a down hill roll. In order to continue at the same speed OR to slow down less gradually one must press on the GO pedal. Then the order of precedence is HV Battery, then engine. IF, the HV only shows a minimum of bars then, there is NO additional energy to draw from. ( Energy could have been saved but was not!)This is exactly why the Toy designers produced the Synergy concept. I basically have been saving energy in cars for about 55 years when the opportunity presented itself. ( by using neutral ) Everyone has of course noticed that using CC causes continuance of energy transfer, the Synergy system is seeking to maintain that optimum level of design. Controlled use of the brake pedal is simply duplicating the CC, except the CC attempts to maintain one speed and manual speed control allows a greater span, perhaps a 20 mph span.
:rockon:

 

Exactly! :rockon: