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Old 05-04-2015, 08:25 PM
rchern rchern is offline
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Brake Energy Regeneration

According to the BMW online brochure"To supply a battery with electrical energy, the generator (alternator) is usually driven using engine power which requires fuel. With Brake Energy Regeneration, the alternator generates electricity only when you take your foot off the accelerator. Kinetic energy that was previously unharnessed is transformed into electrical energy which is then fed into the battery. In this way electricity is generated without consuming fuel." If it's Brake Energy regeneration but the alternator generates electricity when you take your foot off the accelerator what does the brake have to do with it? Is the alternator not running off the belt from the engine? If so if you're constantly using the transmission to downshift are you not charging the battery enough? Sorry if these are stupid questions but I'm not that knowledgeable on automotives.
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Old 05-04-2015, 11:45 PM
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My guess on this is - typically an alternator drags on the engine all the time. In the bmw it only kicks in and drags on the engine when you are OFF the throttle and basically coasting. This action by the alternator may act like a brake of sorts by slowing down the car when coasting. I'm guessing the 4 wheel brakes are not involved like it sounds. And I'm guessing when you're downshifting there are still times when you are OFF the throttle and its charging. And if not I'd guess it kicks in and charges no matter what if things get too low.

My best guess.
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Old 05-05-2015, 04:36 AM
John MS John MS is online now
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I suspect that the charging system connects the alternator when the battery reaches some level of discharge regardless of whether the car is under acceleration, cruising or braking. If BMW only engaged the alternator when decelerating it could have situations where the alternator was rarely engaged. Imagine an all day summer drive through the flat midwest with all energy consuming devices on.

Last edited by John MS; 05-05-2015 at 04:41 AM.
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Old 05-05-2015, 05:17 AM
F32Fleet F32Fleet is offline
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Originally Posted by Heel328 View Post
My guess on this is - typically an alternator drags on the engine all the time. In the bmw it only kicks in and drags on the engine when you are OFF the throttle and basically coasting. This action by the alternator may act like a brake of sorts by slowing down the car when coasting. I'm guessing the 4 wheel brakes are not involved like it sounds. And I'm guessing when you're downshifting there are still times when you are OFF the throttle and its charging. And if not I'd guess it kicks in and charges no matter what if things get too low.

My best guess.
Sounds about right.

It seems when coasting in comfort/sport mode the dash will display 'charging' and you can feel engine braking as it's still engaged with the transmission . In eco-pro, sailing is active so you only get charging when you tap or engage the brake which re-engages the transmission.
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Old 05-05-2015, 10:28 AM
rchern rchern is offline
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I googled BMW Brake Energy Regeneration System and found a film which for some reason I can't get the sound for but on BMW's website under BMW Insights it says " As soon as you brake or take your foot off the accelerator the kinetic energy is captured and fed to the battery. This reduces the amount of power the battery takes from the engine and hence lowers fuel consumption. When the driver presses the accelerator on the other hand the alternator is decoupled from the drivetrain. With fewer components drawing power from the drivetrain more of the engines output can go into accelerating the car." I wonder how the alternator is decoupled? I wish I could figure out why I can't get the sound.
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Old 05-05-2015, 11:02 AM
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Sounds about right.

It seems when coasting in comfort/sport mode the dash will display 'charging' and you can feel engine braking as it's still engaged with the transmission . In eco-pro, sailing is active so you only get charging when you tap or engage the brake which re-engages the transmission.
Perhaps for Autos, but I have a 6MT so...
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Old 05-05-2015, 11:16 AM
F32Fleet F32Fleet is offline
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Perhaps for Autos, but I have a 6MT so...
So what happens in eco-pro? You don't have sailing?
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Old 05-05-2015, 11:46 AM
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Not sure, but if I tap or engage the brake it's not going to be "re-engaging the transmission," since I control all that myself.

Only way my car coasts is if I push the clutch in or put it in neutral. Think "sailing" must be a term for Autos.
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Last edited by Heel328; 05-05-2015 at 11:49 AM.
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Old 05-07-2015, 02:36 PM
Funkee Funkee is offline
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Originally Posted by F32Fleet View Post
Sounds about right.

It seems when coasting in comfort/sport mode the dash will display 'charging' and you can feel engine braking as it's still engaged with the transmission . In eco-pro, sailing is active so you only get charging when you tap or engage the brake which re-engages the transmission.
I believe so. There is a long downhill road on my way to work. When I am coasting in Eco-pro on that stretch , the speedometer shows my car is accelerating. However, my car is slowing down when I drive in Comfort or Sport mode on the same stretch; which means the brake is engaged.
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Old 05-07-2015, 05:03 PM
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I believe so. There is a long downhill road on my way to work. When I am coasting in Eco-pro on that stretch , the speedometer shows my car is accelerating. However, my car is slowing down when I drive in Comfort or Sport mode on the same stretch; which means the brake is engaged.
Or the transmission is staying in gear and you're getting engine braking. Which seems much more likely to me.

Tim
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Old 05-07-2015, 06:16 PM
rchern rchern is offline
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In the thread 2016 3 Series Full Details go to tim330i (the 2nd thread)and click on his link. Scroll down to Improved manual and automatic transmissions heading. It says that in ECO PRO with the auto transmission when the driver lifts off the throttle on a downhill grade the engine decouples from the transmission.

Last edited by rchern; 05-07-2015 at 06:18 PM.
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Old 05-07-2015, 07:48 PM
kev314 kev314 is offline
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Speculation:

The generator is mechanically engaged or disengaged as needed through some kind of clutch. The energy savings results from the absence of generator drag on the engine in normal driving and when coasting in eco.

In eco coasting mode, a tap on the brake takes you out of coasting and engages the generator. This provides a bit of braking action. Kinetic energy that otherwise would have been dissipated as heat from the brakes instead goes back into the battery. Even without a tap on the brake, below a certain speed the transmission (and generator) reengage.

In eco or regular mode, regardless of other variables, if the battery falls below a set level then the generator reengages till the battery is restored to a target level.

I believe the bmw uses a generator, not an alternator. Don't know why that is. Generators were used up through the 1950's and then there was a great switchover to alternators. I believe generators have DC output while alternators naturally have AC output, to be subsequently converted to DC by appropriate rectifiers and control circuitry. Alternators provide more power at lower revs. In pre 1960's cars at idle you would often see the charge light flickering on and off, and hard to start cars due to discharged batteries were common.

OK, now that I've added my own misinformation I hope someone who knows what they are talking about will provide definitive clarification. The concept can't be that complicated, at least if the details are ignored.
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Old 05-08-2015, 05:35 AM
F32Fleet F32Fleet is offline
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Originally Posted by kev314 View Post
Speculation:

The generator is mechanically engaged or disengaged as needed through some kind of clutch. The energy savings results from the absence of generator drag on the engine in normal driving and when coasting in eco.

In eco coasting mode, a tap on the brake takes you out of coasting and engages the generator. This provides a bit of braking action. Kinetic energy that otherwise would have been dissipated as heat from the brakes instead goes back into the battery. Even without a tap on the brake, below a certain speed the transmission (and generator) reengage.

In eco or regular mode, regardless of other variables, if the battery falls below a set level then the generator reengages till the battery is restored to a target level.

I believe the bmw uses a generator, not an alternator. Don't know why that is. Generators were used up through the 1950's and then there was a great switchover to alternators. I believe generators have DC output while alternators naturally have AC output, to be subsequently converted to DC by appropriate rectifiers and control circuitry. Alternators provide more power at lower revs. In pre 1960's cars at idle you would often see the charge light flickering on and off, and hard to start cars due to discharged batteries were common.

OK, now that I've added my own misinformation I hope someone who knows what they are talking about will provide definitive clarification. The concept can't be that complicated, at least if the details are ignored.
Don't forget BMW literature says it's using kinetic energy to charge so in all likelihood it's getting it from your momentum via the transmission.

Just look at the efficiency gauge on the dash. When in comfort/sport mode it swings to charging when you're coasting (momentum is assisting with turning the engine). In Eco-pro it doesn't because your sailing but tap the brake and the transmission engages an the gauge will show some charging. Of course when you're braking in all modes you'll see charging.
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Old 05-08-2015, 05:38 AM
F32Fleet F32Fleet is offline
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Originally Posted by tim330i View Post
Or the transmission is staying in gear and you're getting engine braking. Which seems much more likely to me.

Tim
+1

Brake regeneration = engine braking.

I wonder if fuel gets cut off when charging. That would be kinda cool.
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Old 05-08-2015, 06:26 AM
kev314 kev314 is offline
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+1

Brake regeneration = engine braking.

I wonder if fuel gets cut off when charging. That would be kinda cool.
Brake regeneration = engine braking; don't think so.

I believe the fuel is cut off when you take your foot off the gas.

The physics are straight-forward. When in motion the car has kinetic energy = mv^2/2. Take your foot off the gas, the velocity starts to decrease because of energy losses to aerodynamic drag and, without coasting, engine compression which heats the block and ends up as outgoing thermal energy from the block and radiator.

Put on the brakes, then the velocity decreases further (decelerates, more accurately) due to brake friction with the kinetic energy converted into radiated thermal energy from the brake pads and disks.

When the generator engages there is additional drag, causing addition deceleration. Only instead of the conversion from kinetic energy to heat there is conversion to stored energy in the battery.

The great unknown, at least to me, is exactly when and under what conditions the mighty BMW control system decides to engage the generator.

Last edited by kev314; 05-08-2015 at 06:30 AM.
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Old 05-08-2015, 07:09 AM
F32Fleet F32Fleet is offline
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Originally Posted by kev314 View Post
Brake regeneration = engine braking; don't think so.

I believe the fuel is cut off when you take your foot off the gas.

The physics are straight-forward. When in motion the car has kinetic energy = mv^2/2. Take your foot off the gas, the velocity starts to decrease because of energy losses to aerodynamic drag and, without coasting, engine compression which heats the block and ends up as outgoing thermal energy from the block and radiator.

Put on the brakes, then the velocity decreases further (decelerates, more accurately) due to brake friction with the kinetic energy converted into radiated thermal energy from the brake pads and disks.

When the generator engages there is additional drag, causing addition deceleration. Only instead of the conversion from kinetic energy to heat there is conversion to stored energy in the battery.

The great unknown, at least to me, is exactly when and under what conditions the mighty BMW control system decides to engage the generator.
Don't forget the official literature calls the alternator 'the generator' and states it's deactivated when accelerating.

"..the generator is activated only when you take your foot from the accelerator or apply the brake. The kinetic energy that would otherwise go to waste is now used efficiently.........when you apply the accelerator, the generator is deactivated..."

So, if the vehicle isn't using your momentum via the transmission to turn the engine thereby the alternator, why doesn't it charge when sailing in EcoPro? The transmission is decoupled at this point so charging via engine wouldn't slow the car down.


FWIW BMW did attempt to convert excess thermal energy (exhaust) into electricity but abandoned the project a few years ago. Turbo steamer I think was the name.

Last edited by F32Fleet; 05-08-2015 at 07:58 AM.
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Old 05-08-2015, 11:04 AM
kev314 kev314 is offline
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Originally Posted by F32Fleet View Post
Don't forget the official literature calls the alternator 'the generator' and states it's deactivated when accelerating.

"..the generator is activated only when you take your foot from the accelerator or apply the brake. The kinetic energy that would otherwise go to waste is now used efficiently.........when you apply the accelerator, the generator is deactivated..."

So, if the vehicle isn't using your momentum via the transmission to turn the engine thereby the alternator, why doesn't it charge when sailing in EcoPro? The transmission is decoupled at this point so charging via engine wouldn't slow the car down.


FWIW BMW did attempt to convert excess thermal energy (exhaust) into electricity but abandoned the project a few years ago. Turbo steamer I think was the name.
The generator is turned on or off by a clutch which (I guess) is activated electrically from the computer. However the generator is always spinning since it's connected to the engine via the serpentine belt. But the clutch determines whether it is spinning freely or generating electricity and therefore imposing a load on the engine.

In coasting mode you are correct that if the generator is engaged it would not slow the car. However, it would require extra gasoline into the engine. The electrical energy out of the generator has to come from somewhere - either reduction in speed or "directly" from the engine by burning additional gasoline.

The idea is to use the generator only when you need to slow down. So that instead of wasting the kinetic energy as heat out of the brake, at least some of it ends up back in the battery.

I see at least one problem in my understanding (or lack thereof): what keeps the engine turning when in coasting/sailing mode? Must be some remaining connection to the transmission (most likely), or a little bit of gasoline fed into the engine (less likely, IMHO).
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Old 05-09-2015, 01:39 PM
John MS John MS is online now
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Originally Posted by kev314 View Post
The generator is turned on or off by a clutch which (I guess) is activated electrically from the computer. However the generator is always spinning since it's connected to the engine via the serpentine belt. But the clutch determines whether it is spinning freely or generating electricity and therefore imposing a load on the engine.

In coasting mode you are correct that if the generator is engaged it would not slow the car. However, it would require extra gasoline into the engine. The electrical energy out of the generator has to come from somewhere - either reduction in speed or "directly" from the engine by burning additional gasoline.

The idea is to use the generator only when you need to slow down. So that instead of wasting the kinetic energy as heat out of the brake, at least some of it ends up back in the battery.

I see at least one problem in my understanding (or lack thereof): what keeps the engine turning when in coasting/sailing mode? Must be some remaining connection to the transmission (most likely), or a little bit of gasoline fed into the engine (less likely, IMHO).
I would be surprised if BMW uses an electric clutch switch the alternator like the one used on the ac compressor. The switching has to be electronic and the system would also monitor battery condition, engage the alternator when needed and be able to selectively shut down accessories to maintain a minimum battery charge level. Here's a good summary of what happens when not decelerating or braking from a BMW Tech Drive article.

A BMW equipped with IAC will only charge at idle, cruise, or during acceleration when the bat- tery charge level falls below 80%. This requires the addition of a system that can constantly estimate the battery’s “State of Charge” (SOC)
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Old 12-08-2019, 12:47 AM
Nimefax Nimefax is offline
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Originally Posted by kev314 View Post
The generator is turned on or off by a clutch which (I guess) is activated electrically from the computer. However the generator is always spinning since it's connected to the engine via the serpentine belt. But the clutch determines whether it is spinning freely or generating electricity and therefore imposing a load on the engine.

In coasting mode you are correct that if the generator is engaged it would not slow the car. However, it would require extra gasoline into the engine. The electrical energy out of the generator has to come from somewhere - either reduction in speed or "directly" from the engine by burning additional gasoline.

The idea is to use the generator only when you need to slow down. So that instead of wasting the kinetic energy as heat out of the brake, at least some of it ends up back in the battery.

I see at least one problem in my understanding (or lack thereof): what keeps the engine turning when in coasting/sailing mode? Must be some remaining connection to the transmission (most likely), or a little bit of gasoline fed into the engine (less likely, IMHO).
I would be surprised if BMW uses an electric clutch switch the alternator like the one used on the ac compressor. The switching has to be electronic and the system would also monitor battery condition, engage the alternator when needed and be able to selectively shut down accessories to maintain a minimum battery charge level. Here's a good summary of what happens when not decelerating or braking from a BMW Tech Drive article.

A BMW equipped with IAC will only charge at idle, cruise, or during acceleration when the bat- tery charge level falls below 80%. This requires the addition of a system that can constantly estimate the battery's "State of Charge" (SOC)
I reprogrammed my F20 with the Carly app so it always starts in ECO Pro. Will this kill my battery as it will only charge when below 80% or when cruising or braking?
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Old 12-08-2019, 01:23 AM
Autoputzer Autoputzer is online now
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If the car's in gear and your foot is not on the gas, the alternator will engage and charge the battery. Like compression braking (in gear), this adds some deceleration force. If you have a manual transmission and it's in neutral with your foot off the gas, or if you have an automatic and are in EcoPro with your foot off the gas, you are coasting or "sailing" in neutral. The engine is only running at idle speed, and it's using gasoline to do that. So, it does not charge the battery unless the battery needs charging.

I'm not sure if there's a clutch on the alternator. My guess is that there's not one. The mechanical drag of an alternator varies with how much current it is producing. So, if they electronically limit the current that would also limit the mechanical drag it puts on the engine and therefore on the car.

The "sailing" feature came in 2014. To take full advantage of it you have to plan your stops. If you see a red light and stopped traffic ahead, letting your foot off the gas and coasting up to them will greatly improve fuel economy. BMW's have high-efficiency wheel bearings and can coast an exceptionally long distance in neutral. From 55 MPH, I can coast for about 0.7 miles in neutral if there's no traffic behind me. If there's traffic behind me, it's usually some dumbass redneck in a jacked up pick-up truck, he's too stupid to plan ahead 30 seconds and drive at the same time, and he's going to be pissed off. There's a big bridge that separates eastern Bubba County from western Bubba county. When I come down the bridge, I coast when possible. But, one side has a traffic light near the bottom of the bridge. If it's red, I keep or put the car in gear and save my brakes.

I drive Frau Putzer's X3 in EcoPro. When I need to stop and want to use compression braking, I shift the transmission from D into M/S. That defeats the "sailing" logic, even in EcoPro, and puts the transmission is "Sport" automatic mode. If I need additional compression braking, I manually downshift with the "-" trigger on the steering wheel, being careful to not over-rev the engine. That puts the transmission in manual mode. EcoPro takes away my tachometer, which is annoying. I limit myself to only downshifting as far as "M3." After stopping, I have to remember to put the gearshift back in "D" (Sport automatic mode). Otherwise, the transmission will stay in manual mode (M1) when I start off again.

Diesel-electric locomotives have electric motors that also can act as a generators. They use their generator/motor for braking. But, for this to be effective the generator/motor has to be producing current. So, they have giant heating coils on the top of the locomotive. These heating coils draw a lot of current, therefore producing a lot of mechanical drag on the generator/motor.

Last edited by Autoputzer; 12-08-2019 at 01:26 AM.
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Old 12-08-2019, 01:11 PM
John MS John MS is online now
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I reprogrammed my F20 with the Carly app so it always starts in ECO Pro. Will this kill my battery as it will only charge when below 80% or when cruising or braking?
Shouldn't kill the battery.
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