4WD and regen

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markrmarkr
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4WD and regen

Post by markrmarkr »

I've been wondering how 4WD goes with regen -vs- 2WD with regen. I keep hearing that the most efficient configuration is front wheel drive, and 4WD is the worst because of losses through multiple differentials, drive shafts etc.

But I'm wondering - With a 4WD and regen you should get almost double the regen that you get with 2WD.   The question is:

     Does the extra regen make up for the extra losses?

It shouldn't be too hard to test - a lot of 4WDs have a way of changing between 4WD and 2WD.   Does anyone have any real world experience?




Cheers

Last edited by markrmarkr on Thu, 20 May 2010, 06:02, edited 1 time in total.
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Post by Johny »

Hi Mark. Regen is a bit difficult to quantify as it depends a lot on driving style and terrain. On one extreme driving quietly on flat and level roads, regen would be almost any help at all. The vehicle could slow down just from it's own drag. On the other extreme, accelerating quickly and braking over and over again, then regen may contribute to range (and thereby 'reduce' losses). Likewise with hilly terrain.

The consensus is that regen may give you 0 to 15% back.
The losses in extra diffs on an AWD/4FD are continuous and unavoidable and I'm assuming about 7 to 10% over FWD or RWD.

The comments on getting 'double regen' on 4WD isn't quite right. If the vehicle is slowing rapidly and safely on a 2WD, then using all 4 wheels isn't going to increase regen into the batteries. On the other hand in poor road conditions the regen system would be a lot safer on a AWD/4WD.

It is an interesting design problem (in the control system) of how to reap maximum return from regen braking with a 2WD vehicle and have it safe to drive for anyone.

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4WD and regen

Post by markrmarkr »

Johny wrote:

It is an interesting design problem (in the control system) of how to reap maximum return from regen braking with a 2WD vehicle and have it safe to drive for anyone.


Yes I was thinking about that too. Using regen on a 2WD will effectively change the brake bias, where as on a 4WD this effect is reduced. Of course this all assumes that the weight distribution, moment of inertia etc weren't changed in the EV conversion - a marginal assumption.

I should probably be using the term AWD rather than 4WD as I'm thinking about normal use, rather than off road.

Another point is I understand regen doesn't really work at very low speeds. Does this mean it can have a sort of ABS type effect?
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Post by Johny »

markrmarkr wrote:I should probably be using the term AWD rather than 4WD as I'm thinking about normal use, rather than off road.
Agreed - I'll do that from now on as well.
markrmarkr wrote:Another point is I understand regen doesn't really work at very low speeds. Does this mean it can have a sort of ABS type effect?
We (forum folk) have discussed this one as well (I'll try to find the thread). The consensus was that while regen tends to drop out below 5-10kmh it doesn't help with ABS style operation at higher speeds. Excessive regen that causes the wheels to turn at a slower speed than the nominal road speed will still cause loss of direction control. ABS allows the wheels to free spin periodically to retain directional control.

This isn't the thread I was thinking of but may be helpful:
viewtopic.php?p=24133&t=1911&keywords=#p24133
Last edited by Johny on Thu, 20 May 2010, 06:02, edited 1 time in total.

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Post by Johny »

Found other relevant discussion of regen/ABS on this thread.

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Post by markrmarkr »

Jonny,
thanks for those links. I guess I shouldn't be surprised that all my thoughts have already been thought of before.

Back to the original topic AWD with regen vs 2WD with regen - you quote 0 - 15% gain, but I think that assumes conventional 2WD regen.

The way I understand most regen setups work is with two levels.

1. The first level comes in when you take your foot of the accelerator and simulates engine braking.

2. The second is a step increase in braking effort which comes in when your brake light goes on.

The engine braking effect would be the same for 2WD and AWD.

Most setups would be afraid to use too much braking with brake-light method because this would make the car too jumpy. So normally there wouldn't be much to gain here either (AWD vs 2WD), I'm hoping to use both the 1st and 2nd methods, BUT my commutes involve a lot of stop start driving and my controller (Curtis 1238) has an analogue input for braking effort.   So I'm hoping to have a 3rd regen method which measures brake pedal position (or something like that) and passes this to the controller.

Any ideas how to measure brake requirements?   I don't like the idea of having a separate leaver or pedal as this could lead to confusion.
Last edited by markrmarkr on Thu, 20 May 2010, 11:07, edited 1 time in total.
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Post by a4x4kiwi »

I am also planning an Impreza (search the forums for "Ampreza")

The gearbox inefficiencies are an unfortunate byproduct of AWD, and especially the way the Impreza transfer box works.
My main reason for AWD is because I would like to experience a more significant amount of regen...& I like Subarus. I have had 2 Liberties in the '90s. The Impreza weighs about 1200kg. A little heavier than I would like, but not too bad.

You can really only get a significant safe regen by applying it to all wheels, hence my wish for AWD.

For regen...
1. is a given as previously mentioned, feels like engine braking.
2. brake pedel step increase. is also what I intend.
3. Increase regen proportionally with brake line pressure. (Need to consider how this will be effected by ABS operation) I have no problems putting a T in a brake line for a 0-5v pressure transducer.Like this

3. could also be used with regen in the top part of the brake travel and mechanical brakes in the lower part of the travel, but this might be a bit tricky mechanically. The simplest solution I have considered would be to use a potentiometer that looks like a hydraulic strut attached to the brake pedal. Like this

For a 2WD Front to Rear brake balance also plays an important part. in my Electrolux ute, and probably other cars as well, there is a proportioning valve that is activated by the ride height of the rear of the vehicle. The higher the rear, the less rear brake bias.

The important thing is that the vehicle wont loose traction in the wet on a roundabout in mode 1 or 2.

Realistically you are limited by what options the controller controller provides. From memory, Azure Dynamics supports this 3 step approach, even in the older UMOC range. The level of regen needs to be configurable for all 3 modes of operation as each vehicle will be different in the way it responds.

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Post by Johny »

For all the reasons outlined previously for 2WD vehicles I am going to limit regen to just higher than normal ICE breaking. There will be a "slow down faster" lever that the informed driver can use on dry straight line de-acceleration.
I need the vehicle to be able to be driven by anyone (wife, kids) so it has to behave pretty much normally. The spring-loaded lever seems like the safest, easiest option for me.

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Post by markrmarkr »


Mal,
I think I've met you at the last Sydney meeting. You had your hilux there, with a genset on the back.   You said it kept tripping out because of the switch mode charges you use.   Was that you?

I was going to contact you after Johny pointed me at your thread a couple of posts back, but I had no opportunity as I had to go home.

Are you getting an Azure controller? Which motor to go with it.

I've never been comfortable with the idea of mode 3 on a 2WD either. The bias instability really makes me nervous.   The bias issue didn't worry me so much for modes 1 and 2 because changing from ICE to EV will cause weight transfer issues anyway and because it's a fixed quantity you would be able to get used to it.
Last edited by markrmarkr on Thu, 20 May 2010, 17:56, edited 1 time in total.
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Post by markrmarkr »

Johny wrote: The spring-loaded lever seems like the safest, easiest option for me.


I've been thinking about that too and the reason I don't want to go that way is not because of other drivers, it's because I'm worried about me. I don't want to have a situation where I need a brake quickly but can't make up my mind which one to go for.

But on a 2WD setup it probably makes more sense because of the biasing issue I outlined in the previous post.

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Post by Johny »

markrmarkr wrote:
Johny wrote: The spring-loaded lever seems like the safest, easiest option for me.
I've been thinking about that too and the reason I don't want to go that way is not because of other drivers, it's because I'm worried about me. I don't want to have a situation where I need a brake quickly but can't make up my mind which one to go for.
I agree that it is a point for concern but I think you would just drive normally and if an easy situation arose, like slowing down at the end of the freeway off ramp - and you remembered, then you could just increase the regen for a few seconds.

I drive 3 different cars during the week so it's important that the EV drives like the other ones. If I forget to use the regen control - that's fine.

Edit: Typo
Last edited by Johny on Fri, 21 May 2010, 05:45, edited 1 time in total.

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Post by a4x4kiwi »

Hi Mark,
Yes, that's me, the one and the same.

Cheers, Mal.
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Post by a4x4kiwi »

... and no I don't have a motor and controller yet. I had my eye on an old Solectria AC50 and UMOC645 one in Singapore but it didn't work out. Still looking for that allusive bargain. If EVComponents goes under, I wonder if there will be goodies at a reduced price.
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Post by markrmarkr »

Wow Mal, your after performance.   And your keeping the gearbox too. (ed typo)

I was planning on using this but upping the voltage to 120V. It's a 9 inch AC motor (bolt-on equivalent to DC 9inch).   I think it should be ok with the Impreza.   What do you think?
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Post by markrmarkr »

Johny wrote: We (forum folk) have discussed this one as well (I'll try to find the thread). The consensus was that while regen tends to drop out below 5-10kmh it doesn't help with ABS style operation at higher speeds. Excessive regen that causes the wheels to turn at a slower speed than the nominal road speed will still cause loss of direction control. ABS allows the wheels to free spin periodically to retain directional control.


I have to say I'm not a big fan of ABS - having used it a few times. Don't get me wrong - it's better than nothing, but it's still quite easy to loose directional control.

They taught me in high school Engineering-Science (in 1977-1978) that there are two types of friction. Sliding or dynamic, and static.   This is a good approximation to reality but as with most things the truth is more gray than black and white. If you introduce partially wet surfaces, gravel, or hot sticky tires, all sorts of non-black and white effects are possible.

The bottom line is that if you loose traction but your wheels don't lock up, you are a lot better off than if they do. ABS may be better still, but perhaps not by much.

Just my 2c. :)

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Post by antiscab »

markrmarkr wrote:
I was planning on using this but upping the voltage to 120V. It's a 9 inch AC motor (bolt-on equivalent to DC 9inch).   I think it should be ok with the Impreza.   What do you think?


The voltage will be limited by the controller more than anything else (IIRC, curtis don't mkae AC controllers above ~96v).

btw, dont get that kit from evcomponents, they've since gone under.

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Post by markrmarkr »

antiscab wrote:   

The voltage will be limited by the controller more than anything else (IIRC, curtis don't mkae AC controllers above ~96v).

btw, dont get that kit from evcomponents, they've since gone under.

Matt


Apparently the Curtis 1238-7501 can be pushed to 120v with issues from 130V. I know of some people who are trying to use it at 120V. I'll be watching with interest to see how they go.

Pitty about evcomponents. You'd never know there was a problem from the web-site.
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Post by coulomb »

markrmarkr wrote: Pity about evcomponents. You'd never know there was a problem from the web-site.

That was true for quite a long time. Now (and for at least a few days) there is a message in the smallest font on the front page only "Due to a theft...".
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Post by Johny »

markrmarkr wrote:The bottom line is that if you loose traction but your wheels don't lock up, you are a lot better off than if they do.
That would be good. Image

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Post by antiscab »

markrmarkr wrote:
Apparently the Curtis 1238-7501 can be pushed to 120v with issues from 130V. I know of some people who are trying to use it at 120V. I'll be watching with interest to see how they go.


well a "96V" controller has to tbe able to deal with a 96v battery just off charge.
for a lead acid pack thats ~115v.
a bit of safety margin, and sure the controller could run at 120v.

the difference of course is that you can't run it with a 120v nominal battery.

it gets progressively harder to keep a controller happy, the closer you are to the voltage limits.

you could go to a 108v nominal battery for lead acid.
you would have to knock off the surface charge before powering up the controller.
at full charge, voltage would be ~120v.

in lithium, you could go to 35 cells, and set your chargers CV stage to 120v.

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Post by markrmarkr »

antiscab wrote:
you could go to a 108v nominal battery for lead acid.
you would have to knock off the surface charge before powering up the controller.
at full charge, voltage would be ~120v.

in lithium, you could go to 35 cells, and set your chargers CV stage to 120v.

Matt


I was planning to use 36 cells of SE charging to 3.4 each or 122.4 for the pack.
3.4 is pretty conservative for a charge voltage I know, but from what I can see there isn't much to be gained by going higher and the risk of overcharge becomes significantly more of an issue the higher you go.

This is all subject to change - I'm waiting to see how some other people go with a very similar setup.   If it doesn't work out for them at that voltage I'll just use one or two cells less and maybe increase the cell AH to 200. Or even rethink the whole approach.

Thanks for your suggestions.

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Post by antiscab »

having a couple of extra cells is *always* better than having too few and wanting to get a couple more.

go for it :D

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Post by Johny »

Mark, I urge a bit more caution that Matt. I can't imagine that Curtis would use components that were rated any higher than they had to be to meet the 96V requirement. Having to knock off the surface charge before connecting the controller would be an accident waiting to happen (to coin a cliché).

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Post by markrmarkr »

Johny wrote: Mark, I urge a bit more caution that Matt. I can't imagine that Curtis would use components that were rated any higher than they had to be to meet the 96V requirement. Having to knock off the surface charge before connecting the controller would be an accident waiting to happen (to coin a cliché).

Jonny,
Thanks for that. If it was just a case of 96v being the highest voltage the controller can take I'd agree with you.   Unfortunately it's not that simple.

The 1238 comes in range of variants. The one I'm after is the 1238-7501. I have never seen any sort of information from Curtis which specifically talks about the 7501 variant, but I have seen information from distributors which talks about it's rating to 108V eg thunderstruck-ev. The 96V limit (I think) is for the 1238-65XX and lower numbered variant.   If you read carefully they don't even mention the 7501. Curtis really needs to update their documentation.

I've been looking and waiting for someone to actually use one of these controllers.   Hopefully they will document their findings, and my questions (and a lot of other peoples questions too) will be answered. Until then I'm not in a hurry.   I plan to start early next year, so I can afford to wait.
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Post by 7circle »

markrmarkr wrote: .. The 96V limit (I think) is for the 1238-65XX and lower numbered variant.   If you read carefully they don't even mention the 7501. Curtis really needs to update their documentation.
Have checked out the Curtis website.
They have a section for the 1238-75. Maybe the model name variation makes it hard to find. (Edit on Data Sheets page )
Manual 1238-75
And Data Sheet 1238-75

But they only refer to volatge range of 72-96V. and 2min RMS Power Rating 62.3 kVA.

Being such a compact controller requiring heatsinking, it would be useful to know what size/rating DC capacitor bus is used.
If the controller was operating close to peak voltage supply extra DC bus capacitance may help protect against battery & cable circuit inductance overshoots.

Under regen the bus voltage may also be driven high and beyond Max causing damage to controller.

markrmakr wrote:I've been looking and waiting for someone to actually use one of these controllers.   Hopefully they will document their findings, and my questions (and a lot of other peoples questions too) will be answered. Until then I'm not in a hurry.   I plan to start early next year, so I can afford to wait.
I was impressed that thunderstuck site provides a spreadsheet of an analysed system. Power Curve AC-50 (?? 1238-7501)


Is this the info you need?
Last edited by 7circle on Sat, 29 May 2010, 13:02, edited 1 time in total.

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