[esoulliard]

I generally try to use the auto stop/start feature anytime I'm sitting for more than 10 seconds or so. I'm guessing that there's a certain amount of extra gas used when restarting the engine. Does anyone know the break even time for how long the engine has to be off in order to save fuel?

[overcoil]

I'm not so sure restarting requires extra gas - or at least more gas then what some idle changes like A/C cycling might incur. Remember these cars use regenerstive brake, large batteries and upgraded starter.

[Zooks527]

Restarting the engine requires the starter to be engaged, which draws down the battery, which will then need to be recharged from somewhere. That somewhere is the engine, via burning gasoline, so it's certainly going to require some gasoline, energy recovery systems be damned. Quantifying that number becomes the trick.

An old number (very old, 20+ years or so) that I remember is ~30 seconds before the energy consumed in idling exceeds that required to turn the engine over from a full stop.

Past that, you're imposing some wear on the components in the system, decreasing the lives of those components (starter, solenoid, etc.), upgraded or not. Auto start / stop is at its heart a means to beat governmental MPG testing regimes, and will cost real-world consumers less money overall (in both gasoline usage, increased repair costs, and increased upfront part costs) in fairly limited circumstances.

[allthatisntnow]

I was always under the impression that one of the auto stop/start systems operation criteria was that you needed to have enough recovered energy stored from brake re-gen before it would engage. Components, such as the starter motor, are also significantly upgraded over vehicles without the system installed to handle the extra usage. Turning over the engine requires no extra fuel under the conditions this system works in. Remember that the engine is at full operating temperature when auto stop/start is working (it won't cut the engine when its still warming up) and the longer you are at a stop with the engine off the better economy results you will see. Also bear in mind that with ac climate control on, the engine will fire up again the moment the car begins to warm on the interior which will reduce its effectiveness.

[3.0L]

While appreciate BMW's attempt to squeeze out every last bit of MPG, I don't use the auto engine stop feature for two reasons:

1. I like the sound of my engine idling.
2. I can hold my breath to compensate.

[Zooks527]

Originally Posted by allthatisntnow

I was always under the impression that one of the auto stop/start systems operation criteria was that you needed to have enough recovered energy stored from brake re-gen before it would engage.

"Stored" where? The only place energy is stored when the engine is off is in the battery as electrical potential or in the fuel as chemical potential.

The system may not engage until the battery is fully charged, but the battery is going to be brought up to full charge once it falls away from it, and once you use some of its capacity to restart the engine, the charging system is going to top it off again.

As a side note, it's more correctly "braking regen", where they leave the transmission locked to spin the engine and alternator with the injectors off to charge the battery during deceleration. The more common use of "brake regen", using generation at the wheels to provide drag, is not implemented on the 2-series.

Originally Posted by allthatisntnow

Turning over the engine requires no extra fuel under the conditions this system works in.

See above. TANSTAAFL. If you're doing work (turning the engine over), that energy needs to come from somewhere. If it's the battery, fuel will be burned to replace it.

Originally Posted by allthatisntnow

Components, such as the starter motor, are also significantly upgraded over vehicles without the system installed to handle the extra usage.

And the cost to implement those upgrades isn't free, either when the car is new, or when the car is x years old and the starter needs to be replaced. Mechanical life cycles for the starter are real, significantly upgraded or not. If you don't use the starter as frequently, it will last longer and you'll avoid the (now higher than before) replacement cost.

[allthatisntnow]

Originally Posted by Zooks527

"Stored" where? The only place energy is stored when the engine is off is in the battery as electrical potential or in the fuel as chemical potential.

The system may not engage until the battery is fully charged, but the battery is going to be brought up to full charge once it falls away from it, and once you use some of its capacity to restart the engine, the charging system is going to top it off again.

As a side note, it's more correctly "braking regen", where they leave the transmission locked to spin the engine and alternator with the injectors off to charge the battery during deceleration. The more common use of "brake regen", using generation at the wheels to provide drag, is not implemented on the 2-series.



See above. TANSTAAFL. If you're doing work (turning the engine over), that energy needs to come from somewhere. If it's the battery, fuel will be burned to replace it.





And the cost to implement those upgrades isn't free, either when the car is new, or when the car is x years old and the starter needs to be replaced. Mechanical life cycles for the starter are real, significantly upgraded or not. If you don't use the starter as frequently, it will last longer and you'll avoid the (now higher than before) replacement cost.

Im not trying to say the system uses 0 fuel. Im trying to say that it ends up saving net fuel in a relatively short space of time. The "break even period" for a stop that the OP is on about will be a matter of seconds when stopped. The energy from using the alternator as a generator whilst going down hill, braking, coasting etc is stored in the battery to top it up after it is depleted. Only as a last resort will it hook up the alternator under acceleration conditions. A good chunk of the energy is gained back "free" meaning the "break even period" is reduced operating the system.

With regards to cost, my 2008 and 2011 euro spec mini coopers (one of which was diesel) both had the stop start system installed. i remember reading at the time that US minis did not have the system on the r56 due to the markets unwillingness to absorb the extra cost of such equipment. This was stated to be around an extra £130 per vehicle ($200 usd). Now i know thats the cost to the manufacturer, but its not an astronomical sum when you consider you'll end up saving more than that in fuel over the life of the vehicle. They are intended to last the equivalent of a regular starter in a car without the system (say 100000+ miles worth of stops and starts). I certainly wouldn't feel short changed if it packed in after that. In the vehicles I've had with the system, none have skipped a beat.

Anyway, back to OPs original question. If you're sitting for longer than 10 seconds with the engine shut off, you will most certainly be in the green on the fuel saving front. The scenario where you can be in stop and go traffic and have it cutting in and out frequently is detected and the car disables the system.

[AWheeler]

Not a direct answer to the original question, but interesting fodder for the discussion here: http://www.edmunds.com/bmw/3-series-...ings-test.html

[Pyewacket69]

Originally Posted by Zooks527

"Stored" where? The only place energy is stored when the engine is off is in the battery as electrical potential or in the fuel as chemical potential.

The system may not engage until the battery is fully charged, but the battery is going to be brought up to full charge once it falls away from it, and once you use some of its capacity to restart the engine, the charging system is going to top it off again.

As a side note, it's more correctly "braking regen", where they leave the transmission locked to spin the engine and alternator with the injectors off to charge the battery during deceleration. The more common use of "brake regen", using generation at the wheels to provide drag, is not implemented on the 2-series.

See above. TANSTAAFL. If you're doing work (turning the engine over), that energy needs to come from somewhere. If it's the battery, fuel will be burned to replace it.

And the cost to implement those upgrades isn't free, either when the car is new, or when the car is x years old and the starter needs to be replaced. Mechanical life cycles for the starter are real, significantly upgraded or not. If you don't use the starter as frequently, it will last longer and you'll avoid the (now higher than before) replacement cost.

I'm no fan of start/stop (I never use it on my 235).

Having said that, electrical energy from the battery can be produced from the alternator and stored in the battery in multiple ways.

Of course, the traditional way is to charge the battery is by constantly running the alternator and regulating the energy into the battery via a device such as a voltage regulator. Another way is by engaging the alternator when the accelerator is lifted, using the drag from the active alternator to slow the vehicle. That's exactly what the BMW i3 and i8 do as a regenerative source of power (using deceleration drag to generate electricity)...

Of course, sometimes this process doesn't work quite as well as the theory would suggest.

You may find the link below interesting, as it relates to this process....

http://www.roadandtrack.com/car-cult...ns-bmw-n63-v8/

The start/stop systems are widely used in Germany, and implemented the right way, do indeed save fuel.

[Zooks527]

Originally Posted by allthatisntnow

With regards to cost, my 2008 and 2011 euro spec mini coopers (one of which was diesel) both had the stop start system installed. i remember reading at the time that US minis did not have the system on the r56 due to the markets unwillingness to absorb the extra cost of such equipment. This was stated to be around an extra £130 per vehicle ($200 usd). Now i know thats the cost to the manufacturer, but its not an astronomical sum when you consider you'll end up saving more than that in fuel over the life of the vehicle. They are intended to last the equivalent of a regular starter in a car without the system (say 100000+ miles worth of stops and starts). I certainly wouldn't feel short changed if it packed in after that. In the vehicles I've had with the system, none have skipped a beat.

Or, you can turn AS/S off, have the uprated starter last the life of the vehicle instead of going toe-up after 100k, and put the now-unnecessary $500 replacement cost against a whole lot of gasoline. Again, NFL.

Originally Posted by Pyewacket69

You may find the link below interesting, as it relates to this process....

http://www.roadandtrack.com/car-cult...ns-bmw-n63-v8/

From that link:
"But who'd have thought adding turbos in search of better fuel economy would accidentally increase battery consumption?"

Uh, anyone who understands lead-acid chemistry?

In all honesty, my first thought when I saw the "we only charge the battery under certain conditions" language in the manual was "Great. They don't always top the battery off before shutdown. That's not good." Add the increased battery replacement cost to the starter cost. The amount of gasoline you can buy with the savings isn't going down.

[CalCarNut]

Said it before..AS/S...good for cars that burn a lot of gas (M3/M4/M5, Hellcats,etc)..doubt it does much for a turbo 2L. Useless wear and tear on certain cars. Mine is ALWAYS off.

[Pyewacket69]

Originally Posted by Zooks527

Or, you can turn AS/S off, have the uprated starter last the life of the vehicle instead of going toe-up after 100k, and put the now-unnecessary $500 replacement cost against a whole lot of gasoline. Again, NFL.

From that link:
"But who'd have thought adding turbos in search of better fuel economy would accidentally increase battery consumption?"

Uh, anyone who understands lead-acid chemistry?

In all honesty, my first thought when I saw the "we only charge the battery under certain conditions" language in the manual was "Great. They don't always top the battery off before shutdown. That's not good." Add the increased battery replacement cost to the starter cost. The amount of gasoline you can buy with the savings isn't going down.

I have pretty much the same view on start/stop in general. Personally, i didn't buy a M235 with the goal of maximizing my gas mileage, but I can see why automakers install it... So they can increase the fleet fuel economy to meet tightening EPA standards. Unfortunately, that increased MPG isn't free, as you have noted. I'm fairly confident that when these folks that own vehicles included in the link in my previous posting aren't gonna be very happy replacing their battery at each oil change. Even at $4.00/gallon, it takes a lot of increased MPG to compensate for the increased costs of battery replacement.

IMO, start/stop technology can probably be beneficial to small displacement, lightweight economy cars (think Basic Corolla, etc.), but from a practical point of view, it seems worthless to the car buyer in cars like M235's.

[dtcoff]

Engines like the N55 use direct fuel injection and therefore use no more fuel to start, so it does actually save fuel.

As far as the potential for wearing out batteries and starter motors, I've had to replace these in earlier cars that didn't have AS/S, so why do we worry about something that may or not happen and then blame it on AS/S. Where is the evidence?

It makes much more sense in countries other than US, where we pay a lot more for fuel too.

[Zooks527]

Originally Posted by dtcoff

Engines like the N55 use direct fuel injection and therefore use no more fuel to start, so it does actually save fuel.

Assuming you ignore friction, inertia, and rotational momentum, sure. In the real world, not so much.

Originally Posted by dtcoff

As far as the potential for wearing out batteries and starter motors, I've had to replace these in earlier cars that didn't have AS/S, so why do we worry about something that may or not happen and then blame it on AS/S. Where is the evidence?

http://www.roadandtrack.com/car-cult...ns-bmw-n63-v8/

"The system's draw is more than the batteries can deal with, so BMW is now swapping out the old 90-Ah AGMs for 105-Ah units—and hoping they'll survive for at least 10,000 miles.

The simple solution would be to reprogram the engine computers to keep the battery's state of charge at a higher level. But in modern cars, everything affects something else, often in the most unlikely of ways. Charging the battery more often would affect fuel economy, which would require BMW to recertify the cars with the EPA. The revised mpg numbers would inevitably be lower than the ones advertised. Hello, class-action lawsuit.

So BMW can't actually fix the battery problem, it can only mask it. "

Originally Posted by dtcoff

It makes much more sense in countries other than US, where we pay a lot more for fuel too.

"We tax ourselves stupid, so you need to wear out components on your car faster to make up for it."

[allthatisntnow]

The mazda system doesn't even use the starter motor to restart. Just the direct injection of fuel.

"While conventional idling stop systems rely on a starter motor to restart the engine, Mazda's i-stop restarts the engine through combustion; fuel is directly injected into a cylinder while the engine is stopped and ignited to generate downward piston force. The result is a quick and quiet engine re-start compared to other systems and a significant saving in fuel.
To restart the engine by combustion, the compression-stroke and expansion-stroke pistons need to be stopped at exactly the correct positions to create the right balance of air volumes. Mazda's i-stop ensures precise control over the piston positions during engine shutdown. With all the pistons stopped at the optimum positions, the system then identifies the initial cylinder for fuel injection. It injects fuel and ignites it to restart the engine. Even at extremely low rpm, cylinders are identified for sequential ignition, making the engine quickly pick up to idling speed."

Now thats the way forward.

[esoulliard]

I got a response back from a BMW Genius about this and thought I'd share:

"The Auto Start/Stop feature saves fuel regardless of how long you're stopped. It only takes about a quarter crank to have the engine start back up meaning no extra fuel is being used. Basically, the longer you're stopped, the longer the engine isn't idling, and the more fuel your M235i isn't consuming.

There isn't an exact amount of time vs how much fuel is being used to start up the engine but it's very minimal due to the short start-up process."

[3.0L]

Originally Posted by allthatisntnow

The mazda system doesn't even use the starter motor to restart. Just the direct injection of fuel.

"While conventional idling stop systems rely on a starter motor to restart the engine, Mazda's i-stop restarts the engine through combustion; fuel is directly injected into a cylinder while the engine is stopped and ignited to generate downward piston force. The result is a quick and quiet engine re-start compared to other systems and a significant saving in fuel.
To restart the engine by combustion, the compression-stroke and expansion-stroke pistons need to be stopped at exactly the correct positions to create the right balance of air volumes. Mazda's i-stop ensures precise control over the piston positions during engine shutdown. With all the pistons stopped at the optimum positions, the system then identifies the initial cylinder for fuel injection. It injects fuel and ignites it to restart the engine. Even at extremely low rpm, cylinders are identified for sequential ignition, making the engine quickly pick up to idling speed."

Now thats the way forward.

Now that is impressive.

[jtzako]

I have a habit of not even using it. I'm sure they have calculated that it saves enough fuel (even after factoring in recharging batteries due to radio, lights, starter etc) or they wouldnt have gone through the expense of putting it into the cars. (we also have regen braking to help reduce alternator usage)

1) Where I am, if it is sunny, you cant have the AC go off for long or the car will rapidly heat up (high altitude UV heating) causing the car to turn itself back on again. The same can happen with the heater in winter on cold days.

2) I cant imagine frequent stop/start is good for the engine. Even at 'a quarter crank' per restart you are still running your starter significantly more than normal which could lead to a costly replacement in the future. Also, fluids likely begin to settle back into their reservoirs when you stop and the engine turns off, this means less lubrication for at least a few seconds after restart.

[Sportstick]

Originally Posted by jtzako

I have a habit of not even using it. I'm sure they have calculated that it saves enough fuel (even after factoring in recharging batteries due to radio, lights, starter etc) or they wouldnt have gone through the expense of putting it into the cars. (we also have regen braking to help reduce alternator usage)

1) Where I am, if it is sunny, you cant have the AC go off for long or the car will rapidly heat up (high altitude UV heating) causing the car to turn itself back on again. The same can happen with the heater in winter on cold days.

2) I cant imagine frequent stop/start is good for the engine. Even at 'a quarter crank' per restart you are still running your starter significantly more than normal which could lead to a costly replacement in the future. Also, fluids likely begin to settle back into their reservoirs when you stop and the engine turns off, this means less lubrication for at least a few seconds after restart.

+1. I turned it off on day 1 and it's stayed that way running in Comfort or Sport/Sport+.