Soapbox on regenerative braking

Technical discussion on converting internal combustion to electric
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Soapbox on regenerative braking

Post by zeva » Mon, 15 Dec 2008, 03:45

There have been a few threads lately discussing the merits of regenerative braking (regen), but I believe many people overplay it's importance. Regen is only useful when you would otherwise be using the brakes. The two cases are (a) pulling up to a stop and (b) restricting speed down steep hills.

The vast majority of energy used in a typical driving cycle goes to pushing the vehicle through the air (aerodynamic drag) and the tires along the road (rolling resistance) - this energy can not be recovered. A typical small car uses about 4.4kW of energy continuously to maintain 60km/h for example*.

Kinetic energy is the energy of movement. When you accelerate in an EV, you are basically converting electrical energy into kinetic energy. Our example vehicle at 60km/h has about 140kJ (0.04kWh) of kinetic energy - i.e the same energy as is used for 30 seconds of cruising at 60km/h ( = half a kilometre). It is this energy with regen can very effectively recapture.

But that kinetic energy is not entirely forfeit in a vehicle without regen. Much of it actually gets "used" when you take your foot off the accelerator pedal and coast up to a stop, where instead of power from the batteries being used to push the vehicle along, it's using the vehicle's kinetic energy (which slowly reduces). So how much of the kinetic energy goes to drag and RR, and how much can go to regen?

The formula is 1/2mv^2, so energy is proportional to the square of the velocity. What this means is that if you wash off just 18km/h before applying the brakes (quite feasible?), you will already have put half of your kinetic energy to good use, pushing the car through the air instead of heating up your brakes.

The other time you use your brakes in a car (and hence regen is useful) is to maintain your speed downhill, i.e recovering your gravitational potential energy. Of course you'll be off the accelerator pedal completely while going downhill, so much of that potential energy is again being used to push the car through the air. How much of the energy is being wasted in the brakes, or would hence be recoverable with regen?

Let's take Kalamunda Hill as one of the worst-case scenarios around Perth. According to my Garmin it's 225m change in altitude over 4km, i.e average 6% grade. Coasting down a 6% grade at 75km/h (Kala Rd speed limit +5) provides about 12kW of power from potential energy! Aero drag + RR our example vehicle at 75km/h is 7kWh. The remaining 5kW could potentially be recaptured with regenerative braking.

How much will this recharge your battery pack? 4km at 75km/h takes 3.2 minutes. If your regen is recovering the extra 5kW with 100% efficiency, you'll end up with 0.26kWh more energy in your battery pack. 0.26kWh of electricity costs about 5 cents (or 1.5 cents off-peak).

So is regen useful? Of course, but let's keep it in perspective.

* Sample vehicle: 1000kg mass, Cd = 0.35, 2m^2 frontal area, Crr = 0.015. Some formulas are here.
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Soapbox on regenerative braking

Post by Electrocycle » Mon, 15 Dec 2008, 04:17

spot on!

Having talked to quite a few people about regen, and the expected gains in range - it's generally been deemed not worth the effort (on a DC system)
The lack of engine drag in an EV lets you coast very effectively, so with careful driving (as you would do when trying to save fuel) you can actually do a fair bit of driving without using any power!

A very efficient regen system would work well in hilly areas but it won't increase range massively in normal suburban driving.

Using regen instead of mechanical brakes most of the time is good for saving brake pads though :)
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Soapbox on regenerative braking

Post by acmotor » Mon, 15 Dec 2008, 05:22

Points of interest for the EV community before you get talked out of regen ...

Drive an EV with AC regen braking.
Drive a prius or insight with AC regen braking.
Check with the world of EV manufacturers to see if they use regen braking.
Reseach ultracaps and their application to regen power.
Contemplate where regen braking will eventually lead - almost total replacement of mechanical brakes ?


I've done these and I am far more optomistic than Hoops !

Regen is worth 10-20% increase in range in my EV.

Now regen is not essential for an EV at all.

I for one have never suggested it is.

You just need to know it exists and it IS worthwhile.
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Soapbox on regenerative braking

Post by acmotor » Mon, 15 Dec 2008, 05:40

BTW Hoops related his regen example back to 1.5c of off peak electricity.

Think of it another way, 2km more range for 4km regen down a hill !
10-20% improvement in range comes up very quickly given the thousands of traffic lights in a City !
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Soapbox on regenerative braking

Post by woody » Mon, 15 Dec 2008, 07:45

The extra range 10-20% regen gives could be bought with an extra 10-20% on your battery pack, i.e. $1500-$3000 each time you buy lithiums :-)
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Soapbox on regenerative braking

Post by Taffy » Tue, 16 Dec 2008, 16:01

Also need to think about weight transfer when you brake, as you stop most of that weight goes on to the front of the car and rear lifts off. So if you have a rear wheel drive you loose even more of that regen, this was one of the primary reason we didnt put the regen into our race car this year. We ran twin front brakes and a single rear to give you some idea how much weight transfer plays a part, this is a race car but you get the idea.

There are plans for upgrades to regen next year but we found once you have speed its possible to hold it with minimal power, though we didnt think about how twisty (Lots of brake use) the track was!

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Post by Rob M » Tue, 16 Dec 2008, 18:23

I might have mentioned this before but I'll post it again anyway.
I drive 26 km to work each day and 26 Km return and can opt for the freeway or the streets with traffic lights. Using data from the E-meter, I use 5.2 Kwh each way going the streets and 5.5 Kwh the freeway. The freeway is an extra 1.2 Km.
When going the street route, I stop on average 12 times at the traffic lights and use the brakes for about 5 seconds each stop. Hard to be precise here. This works out to be 60 seconds of braking for the journey. At 200 amps (30Kw), which is on the high side for rear wheels only, that calculates out at .5 Kwh of energy. Assuming 100% efficiency that is about 10% of the energy I would use.
In practise, 100 amps would be closer to the mark and 80% efficiency would be more likely. This would reduce the energy recovered to about 0.2Kwh or 4%.
4% of 120 km is 5km. I would never cut it that fine and the extra comfort from being able to travel 125 km would be not worthwhile
In my case it would be difficult to justify regenerative braking especially since I use the freeway when I get out of bed early enough to beat the traffic..
If I lived in hilly terrain or drove a bus that stopped every 500 metres it would be a totally different scenario.
So its horses for courses.
I believe that car manufacturers emphasise regenerative braking because is a good marketing tool.   

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Soapbox on regenerative braking

Post by acmotor » Tue, 16 Dec 2008, 23:23

Taffy,
Agreed, well understood the front / rear issue. There have been threads on this forum and others on just those issues.

Remember that many high performance vehicles go for AWD and the options for controlling front / rear drive and braking are already (well ?) established.
Electric drives will revolutionise this, in particular as we head (maybe) toward hub motors.

To me, it would be a massive advantage in your FSAE car, if you could regen under your hard braking to maybe ultracaps. Even if this means that you had to go AWD (although the FSAE class may prohibit that).
In any case, you will always be able to run regen to the equivalent of engine braking (quite significant compared with an ICE doing 10,000RPM I would have thought ?) without changing the vehicle dynamics.
Keeping in mind that regen from series DC is doable but not cleaver.


On the more domestic front, I for one drive with minimal hard braking (sorry striker, not much fun I know ! Image ) so for me, front or rear wheel drive make little difference in the braking associated with regen in normal driving. As already mentioned, if you are serious, then it is not a FWD / RWD question when AWD is out there ! I don't wish to start anything there, but I hope you get my point.


My impression of the effectiveness of regen in normal driving is very much linked to two factors...

Firstly, high rolling resistance and aerodynamic drag can well mask the energy avaiable for regen under normal driving conditions.
Having just driven the latest model prius over 9,000 km in both city and country, I have now a good feel for the advantage of intelligent regen, and just how much it can do for us. Sorry to keep mentioning hybrid, but it does experiment with many of the future EV issues.
The prius has quite low cd and rr and other drive train losses. It also has AC drive (my favourite, for good reason, and I will plug for it all I like !Image). AC drive offers the lowest cruising loss (no brush drag) and best regen. Statement of fact - not conjecture.
All these goodies together result in the prius providing city stop start or hilly driving at less than 1/3 the country (freeway) fuel consumption. (2-4 L/100k city c.f. 5-8 at maintained speed)
This hybrid fuel input corresponds reasonably to kWh energy consumed in a BEV. There are even some magic numbers thrown around out there 18.7532 maybe.Image
It could be said the a prius would make a good EV donor !Image
Not to be critical, but Rob's numbers are for a DC EV. Perhaps if a BEV sees no energy consumption difference, city stop/start to country driving then it actually needs regen or lower cd/rr ? This is, after all, a thread on regen braking.
I am certain the spreadsheet gurus can run with this one.
My EV has horrible cd/rr but still sees regen advantage quite well... I'll post up some more logged data.


Secondly, the ability to extract kinetic energy rapidly (and store it) and also extract ke at low / near zero speed, as in the last 20kmph pull up to the lights is important in regen. Storing this energy may well be part of toyota's option to stay with NiHH batteries in terms of recharge times. (I know there may be political reasons as well).
Ultra caps seeming to be the best option for these short regen bursts.

Quite simlpy, if as much of the energy as possible is stored when stopping a vehicle from 60kmph in city driving and returned on accelerating, then city driving (as in the prius) becomes far more energy efficient. This can be seen as green / less batteries / less battery stress / less mechanical brake wear / better traction and braking control and if your drive system already supports regen then go for it.(read > AC)
As Rob says though, regen may not be worth the trouble / expense in your particular application or drive type and the EV still works works quite happily. Bear this in mind with your EV planning.      Image   
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Soapbox on regenerative braking

Post by zeva » Wed, 17 Dec 2008, 01:27

As a corollary to my earlier post, I also believe we are hugely under-playing the importance of vehicle efficiency, which can have a far greater impact on your range than regenerative braking. Factors in particular are aerodynamic drag, rolling resistance, vehicle mass, and frontal area. Your average vehicle in a normal driving cycle will lose around the same energy to rolling resistance as aerodynamic drag.

Let's say regenerative braking would offer most of us about 10% more range. A good start. What else might help us get longer range?

Reducing aerodynamic drag: A Prius has a Cd of 0.26. My MX5 has a Cd of 0.39 - much worse! Assuming equal frontal area, the Prius loses 33% less energy to aero drag (at any speed). At 100km/h, this is about 3.5kW less power needed! (Any my MX5 is far from the worst example of vehicle aerodynamics..)

Reducing rolling resistance: Modern low rolling resistance (LRR) tires have a coefficient of rolling resistance (Crr) around 0.008 (reference). Normal car tires are around 0.02, i.e you can better than HALVE your rolling resistance with the right tires. This alone will offer you around 20% more range in a normal driving cycle.

Vehicle mass: Rolling resistance is proportional to vehicle mass, so for a given set of tires, a 2-tonne vehicle will lose TWICE as much energy to rolling resistance as a 1-tonne vehicle. At 60km/h, this alone would shorten your range by about 25%. (Yet another good argument for choosing lithium batteries?)

Frontal area: Vehicle drag is proportional to frontal area, so a vehicle with 20% smaller frontal area will lose 20% less energy to drag.. and gain about 10% more range.

And transmission losses: Simply going without a gearbox can gain you 10% more range due to lower transmission losses.. (all other things being equal)

So in conclusion: If you need long range, the most important factor may be a wise choice of donor vehicle. If it has just a few of the above working in its favour, you can stand to more than double your range from a given battery pack. There are also a whole lot of things you can do to your existing vehicle to gain additional range, such as LRR tires and aerodynamic improvements (under-body panelling is one example).
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Soapbox on regenerative braking

Post by acmotor » Wed, 17 Dec 2008, 04:58

off topic Hoops ? Image

Add emotor efficiency and it would be a reasonable summary. Image

But you would need to spend more time on defining "rolling resistance" as this is a lot more than just tyres. In most calculations it includes the "resistance to rolling" by bearings, diff, gearbox, and emotor as they are all connected and consume power at all times.

Rolling resistance is typically constant with speed (although in practice, changing slightly with speed e.g. by oil drag in diff / gear selected etc.)
Note for clarity. "rolling resistance" is separate to transmission loss which is a power in / power out factor.

Talk about an over statement - LRR tyres giving 20% greater range !



Perhaps though, prospective EV convertors can break these efficiency items into categories in another way as well.

Those things they can do something about, and those they can't

Can't do MUCH about. (once donor vehicle decided upon)
... total rolling resistance
... frontal area and aerodynamic drag

Can do something about.
... gearbox or not (possibly)
... battery weight / kWh
... AC or DC efficiency
... regen braking or not

This puts a different complexion on the matter ! Image

Remember all these things need to pass inspection as well.

Hoop's conclusion that there is a lot you can do to an existing vehicle is true, just rather optomistic. Check it all out before you count the savings.
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Post by woody » Wed, 17 Dec 2008, 06:48

acmotor wrote: Talk about an over statement - LRR tyres giving 20% greater range !
In my (infamous?) spreadsheet I've got rolling resistance as 0.015 including tyres (12) + brakes and steering (3) - numbers I got from Uwe's EV calculator webpage. If I drop to 0.011, eg tyres with 2/3 the rolling resistance (0.008), it drops my energy consumption by 11 Wh/kilometre, which is about 15% at 60kph, nothing to be sneezed at.

I talked to Peter in Sydney with the Holden Combo at the last meeting, 2 points of interest - he's upgraded his 6.7inch DC motor to 8 inch DC, which he's very happy with, and, he's running his tyres at 60PSI for more range. 60PSI! On Sydney roads!
acmotor wrote: Can't do MUCH about. (once donor vehicle decided upon)
... total rolling resistance
I think you can do a fair bit. Keeping your tyres pumped up is a great start. 60PSI is extreme I think.
acmotor wrote: ... frontal area and aerodynamic drag
More effective if your car is fairly unaerodynamic to start with (e.g. Ute, convertible, 4wd) you can do a few things ranging from the simple to extreme like:
wind up the windows
drive slower
put the roof up/hardtop on
Moon Discs
Wheel Spats
block off your radiator opening
get a tonneau or sloping canopy for your ute tray.
underbody streamlining
chop job (lower the roof = lower frontal area)

This guy reduced his frontal area dramatically with a few mates in just 3 days.
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Post by Electrocycle » Wed, 17 Dec 2008, 15:49

I don't think we can use the Prius' regen braking as a direct comparison with regen on a BEV, because the prius is using regen to average out the load on the IC engine, letting it run at a more constant lower power output while the electric motor adds boost when needed, and absorbs the excess when it's not.
It is still the same principle, but I think the effect is magnified.
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Post by Goombi » Wed, 17 Dec 2008, 18:57

While regen is engaged how does it effect changing gears?

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Post by woody » Wed, 17 Dec 2008, 22:59

Goombi wrote: While regen is engaged how does it effect changing gears?
Hmm, good question. I think in AC land you'd need some smarts to engage "free-wheeling" mode when changing gears.

It would only be a little bit harder to get the AC drive to match the motor speed with the right speed for the gear which you're selecting, so that synchro wasn't stressed.

When you are up-changing, you want the motor to slow so maybe it would help with gear changing.

I don't know of any cars in Oz which have both regen and gears....

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Post by bga » Thu, 18 Dec 2008, 03:16

Freewheeling:
An inhibit input on the drive is needed to tell the controller to stop driving or braking so that the shaft speed is preserved. A switch on the clutch pedal would suffice to sense this.

I've been musing on the idea of an auto-sync so that when the clutch is pressed, the motor adjusts it's speed to match the next likely gear ratio to be selected. This would need a tailshaft sensor or one of the ABS sensors so that the controller can compute the correct motor speed. With some 'simple' servo software, this should generate fast crunchless/clutchless gear changes very time.

Perhaps a switch on the gear knob would be better than a clutch pedal, the clutch can then be ignored most of the time.

Cheers

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Post by acmotor » Thu, 18 Dec 2008, 03:19

what is a clutch ? Image
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Post by acmotor » Tue, 30 Dec 2008, 02:04

Electrocycle,
I can recommend a drive in a prius. It is a dirty hybrid however it has many design features not to be sniffed at. (it would make a good EV donor vehicle !)
The regen braking does help smooth out the load and recover energy that would normally be lost in braking. But that is what regen is all about.

I've gone out to pump my tyres up to 80 psi.... that should give me 30% more range, and if I fill them with helium..... ! Image
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Post by acmotor » Tue, 30 Dec 2008, 02:14

Woody,
You didn't like my can & can't list ?
It was designed to help people think about the donor vehicle.

I didn't think it was too far off the mark. I did say MUCH.
Some things are locked in once you select. Like rust.

After all, there is little I can do to the red suzi to turn it into an MX5 ! Image
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Post by woody » Tue, 30 Dec 2008, 02:42

On the contrary, I did like it, I just thought that Rolling resistance was in the wrong category ;-)

I'd replace it with: size, weight, and passive safety / crumple zones

I.E. A S merc of any era is going to be big and heavy and safe, a VW bug is going to be the opposite :-)

Rust you can do something about: just ask Mal what we did today :-)
The Cortina has one bad rust spot, but there are so many other good points about the car that it's worth fixing for us. If your looking for a common car, then look hard for rust and don't buy it if there's any holes :-)

PS: can you post your Danfoss settings?

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Post by woody » Tue, 30 Dec 2008, 02:46

Re: MX5 - it's already a convertible you're most of the way there already :-)
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Post by acmotor » Tue, 30 Dec 2008, 04:31

Woody,
Danfoss settings..
viewtopic.php?p=7637&t=615#p7637
Image
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Post by Johny » Sat, 25 Apr 2009, 20:27

This post is to do with accelerator response with regen. braking.
From an extract on the Circuit EV that Dodge in the USA are attempting to get ready:

"...and Chrysler still has some bugs to work out with its EV platform. Right now the regenerative braking bites right away, instead of letting the car coast, requiring constant use of the accelerator to keep moving, but Chrysler is working on it. There are still big questions on price and availability, but..."

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Post by Martin » Sun, 26 Apr 2009, 03:26

I have to agree with acmotor, drive a prius and enjoy the braking without brakes...

Preferably regen comes on with first 1/2 of the brake pedal, not when the accelerator is backed off.

The ac controller apparently is suitable for regen, I'm hoping in a brushless dc set up - the motor produces a higher voltage than the batteries are delivering, so regen occurs. (Using a smaller pulse width to take the return current)
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Post by Hemonster » Fri, 29 May 2009, 02:32

Wanted to run this past the forum, see what you'll thought. What if I set regen to be equivalent to my car's current engine braking force (by experiment), but this level of regen can be reduced down to zero by pressing the clutch pedal in.

The cluth itself will not be connected to the gearbox as the motor is driving direct into the gear box. The result of this is that when the clutch pedal is fully depressed the motor will freewheel and you can change gears - effectively you have an electrical clutch system.

If you wanted to glide either push the clutch in or shift to neutral, otherwise the car will slow down at the max regen rate allowed (also set to be within acceptable limits for the battery pack).

This I believe would mimic how a standard manual car works, and you can independantly control the regen force and resistive braking force just as before, and have your gliding ability as before.

A micro switch on the accelerator pedal would disengage regen all together so that you can't accelerate and regen at the same time.

Thoughts?

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Post by Goombi » Fri, 29 May 2009, 02:52

The way i understand regen operation is while you press accelerator down the power is sent to the motor when you start removing the foot off the accelerator the regen engages and starts braking thus sending charge to batteries. Its the zero0- zero0 zone-- You can physically and effectively increase the braking-Charging regen with a programmer
Regen should be on par with ICE brake horsepower.

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