Stop/start technology
Discussion
Brewsters said:
Someone on here asks if it defaults to being on all time when you start up - yes it does on my 123d - not sure if you can change this through the on board computer - probably not if there is the button on the dash. As I said earlier, I think it's a load of b
ks, so always turn it off. You pays ya money and takes ya choices (not with BMW though!). BMW do not offer the option of NOT having it. It's like I'm putting my seatbelt on.... buckle up, turn OFF stop start tomfoolery and off I toodle. Each to their own eh?
ks, so always turn it off. You pays ya money and takes ya choices (not with BMW though!). BMW do not offer the option of NOT having it. It's like I'm putting my seatbelt on.... buckle up, turn OFF stop start tomfoolery and off I toodle. Each to their own eh? 
To undrestand how it saves fuel, you first need to undersatnd how stop/start systems work (And I can only tell you about the system I know about, so other manufacturer's may do something different)
For a start, the PCM (powertrain control module) doesn't just 'switch off' the engine, it applies additional load to the alternator to stop the engine in one of two positions (either cylinder 1 or cylinder 4 at TDC firing)it then remembers this stop position.
When the clutch pedal is depressed, the PCM flicks in the starter motor, just to get the engine moving.
As the engine was stopped at a point where it was about to fire, and some compression is still left in the cylinder, the PCM then injects a 4 m/s burst of fuel (about twice the amount used by an individual cylinder at idle) and fires the spark plug for that particular cylinder, meaning that the engine starts instantly, essentially starting within 180 degrees of crank rotation (During conventional engine starts, a modern engine doesn't run in normal operation for at least 720 degrees of crank rotation) -At the very worst, the engine will fire within 360 degrees of crank rotation, using a normal injection pulse for the next cylinder to fire
As for battery life, the system I know of runs two batteries of dissimilar capacity. This would normally cause a problem, as charging two different capacity batteries with the same charge current will damage the smaller battery - However, the PCM monitors the battery temperature and voltage of the smaller battery, and disconnects it from the charging system when fully charged or if the battery temp is too high (This is done using PCM controlled relays in the charge circuit)
So essentially, fuel is saved by using a different method of engine starting
As for lights, wipers, A/C etc, they all continue to operate normally (Although if A/C demand is high, the engine will not switch off) PAS on the vehicles which I know most about are all electronic or hydro electric syatems, so continue to work with the engine off
As for not restarting due to a flat battery, again, the PCM monitors the condition of both batteries, and cancels stop/start if either battery is low.
Stop/start syatems are also only designed to work on a fully warm engine, as the PCM's primary task after cold start is to get the catalyst up to full operational temperature in the shortest time possible (Stop/start would slow this process down)
For a start, the PCM (powertrain control module) doesn't just 'switch off' the engine, it applies additional load to the alternator to stop the engine in one of two positions (either cylinder 1 or cylinder 4 at TDC firing)it then remembers this stop position.
When the clutch pedal is depressed, the PCM flicks in the starter motor, just to get the engine moving.
As the engine was stopped at a point where it was about to fire, and some compression is still left in the cylinder, the PCM then injects a 4 m/s burst of fuel (about twice the amount used by an individual cylinder at idle) and fires the spark plug for that particular cylinder, meaning that the engine starts instantly, essentially starting within 180 degrees of crank rotation (During conventional engine starts, a modern engine doesn't run in normal operation for at least 720 degrees of crank rotation) -At the very worst, the engine will fire within 360 degrees of crank rotation, using a normal injection pulse for the next cylinder to fire
As for battery life, the system I know of runs two batteries of dissimilar capacity. This would normally cause a problem, as charging two different capacity batteries with the same charge current will damage the smaller battery - However, the PCM monitors the battery temperature and voltage of the smaller battery, and disconnects it from the charging system when fully charged or if the battery temp is too high (This is done using PCM controlled relays in the charge circuit)
So essentially, fuel is saved by using a different method of engine starting
As for lights, wipers, A/C etc, they all continue to operate normally (Although if A/C demand is high, the engine will not switch off) PAS on the vehicles which I know most about are all electronic or hydro electric syatems, so continue to work with the engine off
As for not restarting due to a flat battery, again, the PCM monitors the condition of both batteries, and cancels stop/start if either battery is low.
Stop/start syatems are also only designed to work on a fully warm engine, as the PCM's primary task after cold start is to get the catalyst up to full operational temperature in the shortest time possible (Stop/start would slow this process down)
My Fiancees 118d has it, and TBH I'm not a fan, I appreciate it saves fuel etc but I drive 50,000 -100,000 miles a year in my truck which ironically enough doesn't have it, and in the short distances I've covered in her car it just feels "wrong" when it starts and stops at lights etc.
So I generally turn it off,
However, if I had more opportunity to drive her car more often and for a longer period of time I have no doubts that I would eventually get used to it and in turn it would then feel "normal"
I do have my worries about these hi tech systems that even low end regular cars are now coming fitted as standard with, in a few years time I can forsee some big bills for rectifying these when they become faulty.
So I generally turn it off,
However, if I had more opportunity to drive her car more often and for a longer period of time I have no doubts that I would eventually get used to it and in turn it would then feel "normal"
I do have my worries about these hi tech systems that even low end regular cars are now coming fitted as standard with, in a few years time I can forsee some big bills for rectifying these when they become faulty.
Deva Link said:
kambites said:
The starter motor is just the same as usual though, as far as I know.
I think that's right - except for the system Mercedes use. I had a B Class courtesy car recently and was amazed at how seamlessly stop/start worked, especially the lack of starter motor noise. So I looked it up:"On the B-Class the stop-start system works almost imperceptibly. As you depress the clutch you find the engine is running even before you realise it with no conventional start-up sound.
And it's all down to the starter-generator which Mercedes fit rather than just relying on the conventional starter motor.
Connected to the crankshaft via the belt drive the starter-generator allows the engine to start much faster and far more quietly than with the conventional starter making it very easy to live with."
I regularly drive BMWs and MINIs and I am truly disappointed with their effort at stop/start. It can be done so much better with proven and existing technology, even the Citroen C3 stop/start used a better setup than the BMWs have.
Dracoro said:
collateral said:
When it's cold and you end up in stop-start traffic isn't it possible the car wouldn't get up to temperature? Probably worse on a diesel too
I think it only operates when warm.martin vr6 said:
I do have my worries about these hi tech systems that even low end regular cars are now coming fitted as standard with, in a few years time I can forsee some big bills for rectifying these when they become faulty.
I think people are a bit wary of latest-and-greatest after the diesel dual-mass fly debacle
Brake to halt. Engine dies. Press P button. Remove foot from brake (super-bright brake lights in a Prius and don't want to look like more of a tt than need be). Engine fires up if needed to power ancillaries, dies when no longer needed. Lights go green/traffic queue moves. Foot momentarily on brake, slot into D. Drive off, usually without engine, which fires up shortly afterwards.
Damn clever, works seamlessly, does mega mpg in these conditions. That is all.
t than need be). Engine fires up if needed to power ancillaries, dies when no longer needed. Lights go green/traffic queue moves. Foot momentarily on brake, slot into D. Drive off, usually without engine, which fires up shortly afterwards.Damn clever, works seamlessly, does mega mpg in these conditions. That is all.
As I said in my earlier post, the engine will not stop if the demand on the A/C is high (think, requesting 16 degrees from the A/C, with an outside temp of 28 degrees)
In some cases, where the demand is not quite so high (say a request of 18 degs from A/C, with outside temp of 22 degs) the car will stop as normal, but if more A/C is required, the car will restart.....
This is also the same the other way round, so high internal temp request, but very cold outside temp.
The Stop/start systems don't operate in very cold ambient conditions
There are also multiple failsafes for the system, one of the most common is auto restart if teh car starts to move. This is to ensure that there is sufficient vacuum available for the brake servo.
In some cases, where the demand is not quite so high (say a request of 18 degs from A/C, with outside temp of 22 degs) the car will stop as normal, but if more A/C is required, the car will restart.....
This is also the same the other way round, so high internal temp request, but very cold outside temp.
The Stop/start systems don't operate in very cold ambient conditions
There are also multiple failsafes for the system, one of the most common is auto restart if teh car starts to move. This is to ensure that there is sufficient vacuum available for the brake servo.
There seems to be quite a few negative remarks on here from people who haven't really looked into how the systems work in any depth and assume the system is stupid. Its not perfect but as an example on the BMW the stop start will only kick in if the car is in neutral with the clutch up and...
The car is pretty much stationary
The engine has warmed up
The outside temperature is above ~3c
When the air con is running, the outside temp is below a certain level or the windscreen mists up (dont ask me how it knows this!)
The battery is in good condition
There's not excessive drain on the battery from wipers/lights/heater etc.
You've not just come out of reverse
Once stopped, the engine will then automatically start if:
You dip the clutch obviously
If the battery level drops significantly
If the brake vacuum level gets too low
If the air con was on and the windscreen mists up
If the car starts rolling (ie if you stop on a hill and then start rolling, it will start again by the time you're doing about 2mph)
If the cabin starts getting significantly hotter and the air con is on.
Other things Ive noticed with it, some sensible, some annoying. If you put it in gear whilst the engine is off, it won't then start the engine when you press the clutch, I guess to prevent you from jumping forward with a partially depressed clutch. If you stall the car, the stop/start kicks in so to start the engine again you just dip the clutch. This is quicker in theory but the one time I've stalled the car, I had the stereo on so really couldnt hear the engine, and naturally dipped the clutch and pressed the starter button to start the engine not realising the engine was starting up as I dipped the clutch, so my press of the button then turned it off again! Much confusion ensued for a second or two trying to pull away with the engine off until I realised what had happened, luckily not in a position where me or the car was in any danger!
The car is pretty much stationary
The engine has warmed up
The outside temperature is above ~3c
When the air con is running, the outside temp is below a certain level or the windscreen mists up (dont ask me how it knows this!)
The battery is in good condition
There's not excessive drain on the battery from wipers/lights/heater etc.
You've not just come out of reverse
Once stopped, the engine will then automatically start if:
You dip the clutch obviously
If the battery level drops significantly
If the brake vacuum level gets too low
If the air con was on and the windscreen mists up
If the car starts rolling (ie if you stop on a hill and then start rolling, it will start again by the time you're doing about 2mph)
If the cabin starts getting significantly hotter and the air con is on.
Other things Ive noticed with it, some sensible, some annoying. If you put it in gear whilst the engine is off, it won't then start the engine when you press the clutch, I guess to prevent you from jumping forward with a partially depressed clutch. If you stall the car, the stop/start kicks in so to start the engine again you just dip the clutch. This is quicker in theory but the one time I've stalled the car, I had the stereo on so really couldnt hear the engine, and naturally dipped the clutch and pressed the starter button to start the engine not realising the engine was starting up as I dipped the clutch, so my press of the button then turned it off again! Much confusion ensued for a second or two trying to pull away with the engine off until I realised what had happened, luckily not in a position where me or the car was in any danger!
Good posts Superhoop.
Drove a test car a few times with stop/start and it was quite nerve-racking the first few times. First time I drove it I couldn't bring myself to bring the clutch back up when in traffic for fear of being the one to then block the road if it didn't work. Once I got over that though found it pretty good. The one thing that was really satisfying for some reason was the engine cranking :- something that was really good about the car instantly doing 800rpm instead of cranking away on the starter motor for a second. (Note that it was an Integrated system not a starter motor belt-driven one :- i.e. a bloody big motor on the end of the crankshaft that doubles as your alternator)
Also drove a Prius as some research to compare it and found it disconcerting have the engine noise seemingly independant of wheel speed etc. Seemed odd.
Probably fair to say though (even the 'lesser' manufacturers people seem to like to look down their noses at) that anything PH has thought of in the space of an hour or two the designers have probably thought of during the months and months and months of system design, development, testing, retesting, etc etc.
Drove a test car a few times with stop/start and it was quite nerve-racking the first few times. First time I drove it I couldn't bring myself to bring the clutch back up when in traffic for fear of being the one to then block the road if it didn't work. Once I got over that though found it pretty good. The one thing that was really satisfying for some reason was the engine cranking :- something that was really good about the car instantly doing 800rpm instead of cranking away on the starter motor for a second. (Note that it was an Integrated system not a starter motor belt-driven one :- i.e. a bloody big motor on the end of the crankshaft that doubles as your alternator)
Also drove a Prius as some research to compare it and found it disconcerting have the engine noise seemingly independant of wheel speed etc. Seemed odd.
Probably fair to say though (even the 'lesser' manufacturers people seem to like to look down their noses at) that anything PH has thought of in the space of an hour or two the designers have probably thought of during the months and months and months of system design, development, testing, retesting, etc etc.
Superhoop said:
To undrestand how it saves fuel, you first need to undersatnd how stop/start systems work (And I can only tell you about the system I know about, so other manufacturer's may do something different)
That was a very interesting post - thanks 
I have often wondered why these systems don't (or if any do) employ a clutched flywheel which disengages the clutch when the engine is stopped and then re-engage it to restart the engine.
SJobson said:
Has anyone been stuck in a traffic jam on a hot day with the engine stopped this way? What happens to the air-con; does it continue to run under battery power or does it stop?
Very few cars have electric aircon compressors and if they do they are atleast 167V (Honda Civic)systems as part of a hybrid. So no, i imagine most cars have an eco mode for the AC and normal/auto, where in Eco or something it will stop the engine at car static, but auto mode it will keep the engine running to maintain cabin temps.Certainly how my insight works anyway.
DangerousMike said:
Superhoop said:
To undrestand how it saves fuel, you first need to undersatnd how stop/start systems work (And I can only tell you about the system I know about, so other manufacturer's may do something different)
That was a very interesting post - thanks I have often wondered why these systems don't (or if any do) employ a clutched flywheel which disengages the clutch when the engine is stopped and then re-engage it to restart the engine.
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