Stop/Start tech: How long does it need to be off?

[jjr1]

How long does the engine need to be off for before you start to save fuel? Assuming the car cuts off and starts again very quickly, that must be using more fuel than if it had stayed on all the time?

So is there a time at which you start to save fuel?

[kambites]

Depends on the system and the engine, but from what I've seen playing with OBD data-logging it seems to typically be something between about two seconds and ten. I think most modern cars are towards the lower end of that spectrum; the very latest systems which stop the engine just after TDC and simply inject/spark to restart, it's probably less than 1 second.

Edited by kambites on Sunday 8th March 19:46

[helidan]

kambites said:

Depends on the system and the engine, but from what I've seen playing with OBD data-logging it seems to typically be something between about two seconds and ten. I think most modern cars are towards the lower end of that spectrum; the very latest systems which stop the engine just after TDC and simply inject/spark to restart, it's probably less than 1 second.

Edited by kambites on Sunday 8th March 19:46

Well, I didn't know that. I thought the starter motor was always used. Obviously this would need a direct injection system.

[kambites]

helidan said:

Well, I didn't know that. I thought the starter motor was always used. Obviously this would need a direct injection system.

Yes but most modern engines are DI, especially on the sorts of cars which have stop/start.

[helidan]

At just past TDC, just after the engine has stopped there would be 'x' amount to pressure in the cylinder. How long would that pressure remain sufficient to permit an "inject/spark" restart? Just curious!

[jjr1]

kambites said:

Depends on the system and the engine, but from what I've seen playing with OBD data-logging it seems to typically be something between about two seconds and ten. I think most modern cars are towards the lower end of that spectrum; the very latest systems which stop the engine just after TDC and simply inject/spark to restart, it's probably less than 1 second.

Edited by kambites on Sunday 8th March 19:46

My interest is from my Mercedes C205 diesel engine if that makes any difference?

[kambites]

helidan said:

At just past TDC, just after the engine has stopped there would be 'x' amount to pressure in the cylinder. How long would that pressure remain sufficient to permit an "inject/spark" restart? Just curious!

I don't know really, I'd be amazed if it's more than about 20 seconds but I suppose by that point it's worth using the starter motor anyway. I'm guessing a petrol would last longer than a diesel.

Edited by kambites on Sunday 8th March 20:11

[Rich_W]

kambites said:

Depends on the system and the engine, but from what I've seen playing with OBD data-logging it seems to typically be something between about two seconds and ten. I think most modern cars are towards the lower end of that spectrum; the very latest systems which stop the engine just after TDC and simply inject/spark to restart, it's probably less than 1 second.

Edited by kambites on Sunday 8th March 19:46

Every Start/Stop system I've experienced. You can hear the starter motor. It works far more reliably than trying to turn the engine over from rest to 1000rpm in half a second. Which I don't think is even feasible!

[kambites]

Rich_W said:

Every Start/Stop system I've experienced. You can hear the starter motor. It works far more reliably than trying to turn the engine over from rest to 1000rpm in half a second. Which I don't think is even feasible!

Yeah, I've never driven one which doesn't use the starter either; just read about them.

Mazda's system apparently does it.

[Jasandjules]

I wondered this too but I know that mine won't do the "Stop/Start" unless the engine has been on for a while in the first place.

[rigga]

Stopping just after tdc and injecting some fuel and sparking would work great on a single cylinder, on a multi cylinder engine you're fighting against other cylinders, one at least would be on compression, causing resistance, so might not be that efficient, starter motor used would seem easier.

[Krikkit]

Don't forget with the starter-free system you don't actually need a full compression burn, just expand a cylinder full of warm gas to get the crank turning. With VVT you could then gradually phase the cams back in as the speed increases back up to normal idle.

[Superhoop]

kambites said:

Rich_W said:

Every Start/Stop system I've experienced. You can hear the starter motor. It works far more reliably than trying to turn the engine over from rest to 1000rpm in half a second. Which I don't think is even feasible!

Yeah, I've never driven one which doesn't use the starter either; just read about them.

Mazda's system apparently does it.

It still uses the starter motor to promote initial engine rotation though

[kambites]

Superhoop said:

It still uses the starter motor to promote initial engine rotation though

Ah doss it? I thought the primary reason for the system was to avoid the NVH issues associated with engaging the starter motor.

[Jonwm]

Bit off the main topic but as a few seem to know a fair bit about the systems I thought I'd ask.... Why does the system not work if you've just reversed?? I've had 4 years of start stop and this has always been an unsolved bug of mine :-). I've enjoyed reading this I thought they all just used the starter like when you first crank over.

[McWigglebum4th]

Dead end tech

As hybrids get more and more common start stop traffic will be a trail of silent cars wafting along without the infernal din of car engines

Well apart from fking buses they will still sound like to skeletons have a fight in a tin bath

[jjr1]

So the general consensus is if the car is off for a couple of seconds I am actually saving fuel?

[PositronicRay]

Jonwm said:

Bit off the main topic but as a few seem to know a fair bit about the systems I thought I'd ask.... Why does the system not work if you've just reversed??

It assumes you're manoeuvring, a bit irritating if the engine keeps cutting out.

[Jonwm]

PositronicRay said:

It assumes you're manoeuvring, a bit irritating if the engine keeps cutting out.

Thanks, never even crossed my mind!

[kambites]

jjr1 said:

So the general consensus is if the car is off for a couple of seconds I am actually saving fuel?

Every car will be slightly different, but something like that yes. I suppose it might be slightly longer for a diesel because the higher compression ratio makes them a bit harder to turn over.

Edited by kambites on Monday 9th March 08:24