Can one of you very smart blokes tell me this. (It puzzles me).

Is the fuel used to start automatically greater than would be used by sitting and idling. Or does it vary with how long it is between stop and start.  OR in other words does stop start REALLY save fuel or just make you feel good about saving the air quality (while wrecking your starter motor and your battery).

Just for info I really like stop start (but hate its lousy performance when everything is cold) but don't like the feeling I might have been "had".

Could have been researched by some students one day.

Best wishes. Thank you very much.

I turn the stop / start off every time I start a cold engine and might turn it back on when the engine oil is above 75 C.

But only if I will be stopped for say at traffic lights.

Most of my journeys are NOT on urban traffic heavy roads though.

1.2 TSI petrol, last tank was 60MPG, original battery was tested last Saturday and is still very good after 6 years and 64K miles.

 

Thanks, AG Falco

 

look here for your answer:- 

 

 

 

When I had my BMW 123D many, many years ago, I read somewhere (think it was in BMW blurb) that the stop/start was worth the effort if the car was stationary for more than 4 seconds. 

It is not wrecking air quality having the engine stopped when not needed and restarting when it is needed so it will be helping.

There are now different systems in vehicles to have it done more efficiently and it has not been wrecking starters and when good batteries have been used by manufacturers it has not wrecked batteries.  There are mild hybrid systems where the car moves off using electric and not the ICE firing up when stationary.

Where there has been euro / penny /cent / dollar pinching with starter batteries there have been issues.

@HopeImRight

What car /engine do you drive?

I run a PHEV and it's almost impossible to tell when the petrol engine fires anyway, but I usually let the car decide which is its preferred method, my M3 however, I turn the stop start off.

 

I think with the 123D I had, BMW considered the amount of energy that would be needed, to replenish the energy used from the battery in restarting the car, had the afore mentioned 4 second break even rule. That was a car that decoupled ancillary devices under acceleration, had some early form of brake regeneration and closed its grill vents for areo/thermal efficiency reason as well as stop/start. Quite ahead of it's time for a car that was first introduced nearly 15 years ago. 

I've bumped up the voltage at which stop/start is permitted (mainly to stop it knocking off my hardwired dashcam at an inopportune moment) and it was a surprise the other day when the car actually had the engine turn off, as stop/start now works so infrequently. (Probably because of the parking mode on the dashcam.)

57 minutes ago, EnterName said:

I've bumped up the voltage at which stop/start is permitted (mainly to stop it knocking off my hardwired dashcam at an inopportune moment) and it was a surprise the other day when the car actually had the engine turn off, as stop/start now works so infrequently. (Probably because of the parking mode on the dashcam.)

The ignition stays on when stop/ start cuts the engine, can’t see why it would cause any problems with dashcam.

1 hour ago, Kenny R said:

The ignition stays on when stop/ start cuts the engine, can’t see why it would cause any problems with dashcam.

The dashcam is hardwired and has a voltage detection that switches the dashcam off when the voltage drops below a certain level.

So if at a junction, and the car has just started, the voltage dip during starting could fall below the "switch off" voltage, and switch the camera off just as the numpty behind drives into the back of me.

It's not a big problem, and I can tweak it to suit, but right now I'm happy enough with what it's doing.

TBH I've stopped worrying about it, the difference is probably tiny and not worh fretting about. Nevre had nor have I ever heard of any problems with stater motors etc by this.

 

I turn it off when I'm in busy moving traffic since I don't want the hesistation of the engine having to restart if I'm trying to dart out.