Regen ... which is more efficient?

If you're looking for max regen and conservation of energy...you're probably on the right track. Again, I don't know your commute so if max regen is an issue for you then have at it. For me, I don't mind missing a few kw because it keeps me in the 4-6 bar range on my commute.

 

Close approximation: Slow down using a constant brake application until you get to 7 MPH (when the friction brakes come on regardless), then stop as quickly as possible. The hard part is judging the correct constant brake application for a given situation.

 

I'm missing something in this whole conversation, and I'd appreciate a little education from the folks who've spent so much time on this question (and others like it).

I don't see any discussion that involves a non-glide coast.

Is there so much energy loss from the spinning ICE that it is better to physically brake quickly in order to get to below 42 mph than it would be to simply coast it down? (Again, by coast I do not mean a feathered-pedal glide, but simply an "all feet off pedals" coast.)

This is not such an uncommon situation. I really do find it necessary to stop at each and every stop sign -- even some red lights are not avoidable!

 

So have I misunderstood this all along? Is there no regen going on when in a coast? In other words, must the brake pedal be depressed before regen starts?

Whew! This certainly will change the way I drive, if I've had this wrong all along.

 

A 2 kW loss from ice spin is a lot -- equal to about 10 amps.

I'll just restrict myself to the relatively obvious: if you know you are going to to have to brake, get it over with early so that ice spin is saved.

 

I'll say this...regen braking, while incredible, is overrated in the overall operation of the vehicle.

Imagine you are in a non-hybrid vehicle that is rated at 50 mpg (sometime in the future). The last thing you would think about is how to improve braking to a stop. The focus is on improving mpg.

I understand this thread is about capturing the most energy possible from regen braking but it's just not necessary to the overall operation of the vehicle unless you're in a desperate battery situation.

Use the B brake or just brake normally. Until the Prius can recapture energy from brakes to heat a home or save gas by running the heater in the car it's a moot point.

 

Thank you for your posts and for directing us to the focus on maximizing or at least improving MPGs. Still, it seems there are two ways this whole discussion could be of benefit. The first is to grab more energy as opposed to less through the same event. The benefit is that the energy "grabbed" therein is not acquired from gasolined burned in the ICE. (Theoretically, then, it should allow more power at the next acceleration to come from the battery as opposed to the gas tank.) The second is that we can find out whether it is more valuable to preserve the brake pads by gliding, coasting, or braking gradually, as opposed to harsh braking.

Yes, I understand this may be a small portion of the event, but is it really a non-issue? In a non-hybrid car it is simply a non-issue, but that doesn't seem to be the focus in this thread. Just as clearly, I do not know as much about this topic as the other folks offering comments, so I welcome the opportunity to learn quite a bit more through any further responses.

I suppose this addresses, somewhat, my second point raised above. I don't think it is a moot point that I may not have to have the rotors turned, or perhaps even the brake pad replaced, until I've reached 100,000 miles. Yes, that is not part of the question of regen, but it does hit some of the value of the brake system.

 

Nope, all the energy "grabbed" from using the brakes was originally produced by burning fuel in the ICE. This is even the case when you are coasting downhill, because you originally had to burn fuel to get to the top of the hill to begin with.

When you accelerate from a stop, you are burning fuel and converting it into momentum of the vehicle. When you are using the brakes, you are converting the momentum into electrical energy (regen braking) with significant conversion losses and/or heat (friction braking) with 100% loss.

Regen braking is better than friction braking, but no braking is more efficient with respect to FE.

 

Dogfriend -- I don't want to pick a fight here, but I'm not a dolt.

The whole point of my post is that we are dealing with a hybrid car that has the ability to recapture energy to be used later. Without the regen, there would be no recapture.

Yes, basically all of the energy "grabbed" was produced, at some point, by burning fuel in the ICE in that car. In a car with a regen system, some of that energy can be recaptured for later reuse. In a car without regen, all of that energy is dissipated in sound, heat & (perhaps!) light. The energy I'm talking about is that delta. It is (for lack of a better term but in order to "conserve energy") a subset of the sound, heat & light from the non-regen system.

That is the point of my question.

 

Exactly! The car was built for FE...not for excellence in braking! I'm not trying to upset anyone here...I understand the reason for the thread...but...all of the energy conserved by braking perfectly is really inconsequential to the performance of the vehicle. The car tells you this itself...it will burn off excess energy if you conserve too much.

Under normal driving, which is 99.9% of Prius drivers, you will never overly tax the battery...even in exreme EV mode for FE. Every time you start the vehicle it recharges itself...every time you hit the gas it recharges itself.

Again, if you are in an extreme battery situation and need to maximize regen braking...by all means use B brake down hills or drag the brakes to a complete stop. I think we answered the guys question here...drag it to a stop to maximize regen braking or find an actual scientific test that proves otherwise.

On another note, take a long hard look at ways to maximize mpg. There's a sticky on this site that will give you the basic techniques to do this. For more in depth hypermiling go to ecomodder.com if you are interested in modding the physical aspect of your vehicle (and I'm not talking about adding shiny things to your vehicle...I'm talking grill blocks, undertrays, engine block heaters, max psi, side mirror/antenna/rear wiper deletes). It's not illegal to improve your mpg!

 

I wholly agree that alot of people can benefit from a discussion on improving the way you brake. There may be people out there that are uniformed and think hammering the brakes is good because the MFD shows 4 little green cars on their display in a 5 minute segement...maybe they are driving to get as much regen as possible and they are wearing out their braking system...but it's not the best thing for FE or their car.

You're right...this is a great website where myths are disspelled...I've learned more from this site in a month than from driving my car...I respect your opinion.

My only point in bringing FE into the discussion is that braking in the Prius is not what is important...the ability to drive in EV mode is important. And if you drive far enough in EV mode and drain the battery 50 miles down the road (if you have God-like road conditions)...alright you got me...then perfect regen braking is important.

I only wanted to present the FE side of things.

 

My opinion is I would avoid the hard braking. Heat is not a good thing for batteries or electrical parts, I suspect hard braking provides a higher amp charge, and more heat on the electrical parts. Most batteries are better off with a slow charge vs a high amp rate fast charge. For this reason I prefer to do any braking as lightly as possible. Traffic allowing I prefer to look ahead and plan my stops so I can come in as close as possible to a rolling/no braking stop.

 

Brad,

I'm sure everyone has the right intention in their replies including me. Everyone's commute is different so it takes you to decide what works best for you. For me, braking lightly is the way to go...in any car...protect your investment. Bottom line...we all have a great vehicle...and it has regen braking which is even cooler.

I just noticed your from PA...where? I grew up in Towanda, PA and have friends and relatives scattered all over the Keystone State...good to hear from a fellow PA'er!

 

All too true. I have read many posts from readers and contributors who are excited to have gone great distances on battery power or to have racked up many green cars. Such discussions do not promote maximum fuel economy and, in fact, do harm to the cause of max FE, because whatever reduces momentum and speed only requires more energy be expended to reinstate such motion.

Still, I found it easy to accept the OP's questions as having come from the perspective of one who understood such points. The OP, and I, only reasonably inquire whether it is possible that there is a noticeably better way to maximize regenerization during a required braking/stopping event.

If it is slow, steady braking, then we have an answer. If it is different when driving at 42mph and above, then we have two valid answers.

1400

 

I'd think that a Prius that gets 'only' 100k miles on its first brake pads is being driven inefficiently. My household's non-hybrids -- my Honda and DW's Acura -- both achieved (barely) 100k miles on the original pads. The Subaru was a disappointment for getting only 90k miles.

 

How much braking power can the batteries absorb?

I believe that most cars can brake fairly close to a 'g' force of 1.00. For a 3200 pound car (e.g. Prius lightly loaded) traveling at 60 mph, this full force braking produces 382kw. But the 2G Prius battery can absorb only 21kw, or less than 6% of this. The rest must be dumped, wasted.

Put another way, the maximum braking rate for which the Prius regeneration could recapture all the braking power at 60 mph, is a very light 0.06g. This upper limit gets larger at lower speeds -- 0.11g at 30 mph, 0.17g at 20mph, 0.33g at 10 mph.

Beware that these figures are theoretical limits for perfect lossless machines and electronics. Prius is not yet this good.

 

I answered the question.