smart alternators

[JHZR2]

I went to a comparison driving event at a local BMW dealer, and drove a 2011 535i. It has the smart alternator - clutched, water cooled. It was kind of neat. Keeps load off the engine, and then adds load upon deceleration. Apparently this gives up to a 3 MPG bump in some driving.

It was kind of weird though... It would engage anytime that I let off the throttle, or so it seems. But there must be power electronics between the alternator and battery, because once I hit the brake enough, something would engage that would really slow the car fast.

It felt kind of like a car that is equipped with the emergency brake assist function, where when the brake is hit hard enough and it thinks it is a panic stop, it helps engage the brakes harder.

And depending upon the exact position of the brake pedal (pressure) it would either engage this clampdown or not. I believe this was the alternator, per the explanation the BMW rep gave to me. Now maybe there was an additional fancy brake assist, but that is not how it was explained.

Anyone else drive something like this? Seems pretty neat so long as the battery can take the high-rate recharge and the clutches aren't a big deal. I wouldn't think they would be, given how reliable AC compressor clutches seem to be... Plus in high load situations, the clutch will remain engaged, so Id think it wont go as much as one would initially think.

Is this in reality, similar to the GM "mild hybrid" system?

 

[Pablo]

The principle seems simple enough, but to actually be braking the momentum as much as you state even after the normal friction brakes must be a bit hard on the mechanicals. Belt life? What is the max rated output of the alternator? Is this a 12V system?

 

[JHZR2]

12V system, was told that it is a 200A alternator. That is 2.8kWe, which is not a huge amount...

Im not sure what I encountered was entirely the alternator, but perhaps rather some fancy brake assist...

It felt kind of like when you "engine brake" by dumping a MT into a lower gear, but based upon the brake position, it could be switched on and off.

 

[MrMeeks]

Looks like they are using 'dynamic braking' to assist the normal braking system.This could anger those working in the brake pad factory (somewhere) and cause rioting.

 

[ComfyShorts]

Quote:
The new 5 Series is equipped with BMW’s Brake Energy Regeneration system. This system controls the times at which the alternator charges the battery. The brakes charge the vehicle when it’s decelerating or braking; otherwise, the alternator freewheels, drawing virtually no power from the engine. An electronically controlled clutch, somewhat similar to that used with the air-conditioning compressor, is added to engage and disengage the alternator.


And it feels like you're down shifting a MT because it shuts off the injectors during braking.

 

[tonycarguy]

I'd be concerned about belt life because the belt is acting as a conduit for the brakes here.

My other concern is battery life. There's an ideal amount of current you should use while charging a lead-acid battery. Too much current shortens the battery's lifespan. This alternator sounds too powerful to be good for the battery.

 

[The_Jeff]

Was it an automatic? An AT needs to change the valving for engine braking. 2nd gear in Drive does not perform the engine braking you get in 2nd gear/manual. For the alt to grab higher RPMs you'd need some trans help.

I don't see why they bother with a clutch; PCM control of the field circuit should have virtually the same effect. Oh, wait, because they're German.

So would the lights flicker with this setup? Blower fan waver in pitch? Sounds dreadful.

 

[SteveSRT8]

The BMW Alt system simply de-clutches the alt during acceleration.

It does not assist braking. And 200 amps is not produced all the time, only as needed to operate the increasingly power hungry accessories we all demand.

Field circuit control only matches to load, as long as the car is running there is some load present.

I seriously doubt this system could save 3 mpg, but maybe 1.

 

[The_Jeff]

I found this about a 1994 Civic VX:

Quote:

The 94 shop manual doesn't list any difference between the VX and other trim levels. However it does say the ECU tells the alternator to switch from 14.5 to 12.5v if you have the starter switch on or all the following are true:

Load below 10A
Speed below 40 MPH
Engine speed below 3600 RPM
Coolant temp above 140 F
A/C off
Intake air temp above 65 F
Brake switch off
Fuel cut-off not active

Even if it did this more often, it wouldn't be that big of a deal: 12.5v won't drain the battery. It just wouldn't recharge it.


My y2k saturn alt is run off the PCM (BCM?) and not the traditional "idiot light through ignition on" field control they used since the 1960s. Would just need some computer logic to save MPG, and cars are starting to look like millions of lines of code anyway. I daresay anyone with an LED battery light has a computer running the alt "behind the curtain."

Pick an alternator off the shelf and spin it, the pulley and all the guts will spin freely as there's no load on it. No need for a clutch IMO. The water cooling, maybe, for 200 Amps. The CVPI alt gets a huge fanful of air from the rad fan and doesn't need water cooling for similar output.

 

[JHZR2]

I dont get the point of that Honda setup... the difference between 14.5V and 12.5V is just the amount of current (and likely heat losses). The load is constant (more or less) so not sure of the benefit.

I too would be worried about the batery life, though Im not sure the size of the battery. Cycles are far more shallow on a group 49 than on some dinky battery for the same typical load... And it seems that the controls dont allow that deep of a cycle.

The 535 was an auto. It could well have been a dynamic braking system or something like that. If so, I still like the idea of the smart alternator, but not the dynamic braking which I think will actually cause accidents due to the car acting differently than one would anticipate.

 

[SteveSRT8]

yeah, I guess the germans are really stupid?

How much cooling can you get from 200 degree radiator air? That might actually add heat to the alternator.

X 2 on the cars looking like a million lines of code.

Where do I buy the 'no frills' model?

 

[brianl703]

GM does something similar on the 2005 Pontiac Grand Prix, they lower the output voltage from the alternator when the battery is charged..and have a current sensor on the feed to the battery so the ECM knows when the battery is charged.

 

[JHZR2]

OK, well I can kind of understand when you are fully charged, I guess. Going too far overvoltage will boil the electrolyte (which can be a good thing to some extent), and you dont want that.

Generally you will charge at 2.25Vpc or so until the draw is 50mA or something around that. Then you can cut it back significantly.

But to support any real load at a lower voltage versus a higher one - if the load is the same, the higher voltage output will be more efficient because youre moving less current, thus less I^2R losses, right?

 

[Shannow]

yep

 

[brianl703]

Originally Posted By: JHZR2

Generally you will charge at 2.25Vpc or so until the draw is 50mA or something around that. Then you can cut it back significantly.


The battery on my '06 Saab 93 never drops to 50mA at 13.5V. It pulls about 300mA. It didn't used to do that. I think it's on it's way out, but it still passes a conductance test.

Quote:
But to support any real load at a lower voltage versus a higher one - if the load is the same, the higher voltage output will be more efficient because youre moving less current, thus less I^2R losses, right?


Most loads found in a car will consume less power when fed less voltage. Exceptions include those that use switching power supplies.

 

[XS650]

Originally Posted By: eljefino


I don't see why they bother with a clutch; PCM control of the field circuit should have virtually the same effect. Oh, wait, because they're German.

Jefe, windage losses on automotive alternators are higher than you might expect. I don't have any numbers, just remember it from going though a design problem years ago.

It still indicates Gemanness on their part though.

 

[Steve S]

All that just to get you to work and back. Fine german engineering. The more pipes there are the easier it is to clog up the pipes. so you save 10 gallons of gas over the life of the car and it costs $6000.00 to save $50.00.

 

[ekpolk]

Originally Posted By: SteveSRT8
...
Where do I buy the 'no frills' model?


About twenty-five years ago.

 

[mechtech2]

Ford has had this for a while, at least in some Police cars.
I kinda WANT full power on acceleration, for the coils and fuel pump. Plus it's a lot more crud to break.

 

[CBR.worm]

Bmw, Audi and VW have had a clutch on some of their alternators for a while, I had to pay $733 to replace my wifes alternator clutch/pulley with 63k on the clock.

On hers it would allow the alternator to rev with the engines power strokes, but freewheel on the compression strokes. Also if you would shift at REDLINE you would hear the alternator spin down slowly after the shift.

Hers does not have regenerative braking, although it did break a lot.