Does starting up use more fuel than when idle?

[jupiter2step]

When I learnt to drive 20 odd years ago, my instructor told me never to turn off the engine when stationery in traffic or stopping momentarily as the engine used a lot less fuel when idle than it did when starting off. I still follow this advice, but I do wonder if there is any truth in it?

 

[Bellow]

The current fad for StopStart systems says otherwise. Modern electronics making the difference.

How long would you let it idle though, before deeming it wasteful?

 

[@MARK]

The whole stop start thing is not about economy. Indeed it would not surprise me if the stop start does indeed use more fuel.

Stop start is driven by the need for lower emissions. Obviously a car sat with it's engine not running has zero emissions thus impacting the average.

A lot of people seem to have missed that there has been a shift from fuel economy to emissions. I put this behind what we are seeing with some of the new diesel engines having much worse MPG than the older ones even though they claim to be greener.

 

[StevenN]

Just to add to the problem.
Is it not bad practice to switch off a hot turbo charged car without giving it time to idle and cool the turbo down a bit first?
Just thinking as if you are pootling down the mway at your usual cruising speed, at your exit junction you hit standing traffic. Would a modern engine cut the power and cook the turbo?

 

[markjay]

The issue with idling is that the distance covered is zero, so miles divided by gallon gives... zero. This is very bad for average consumption figures.

If you took the long route but kept moving, or the short route with many traffic lights, the longer route may consume more fuel but still return superior mpg...

As a car owner, you want to save fuel, but manufacturers care more about average consumption figures than actual saving.

 

[st13phil]

Would a modern engine cut the power and cook the turbo?

Based on my experience of a MINI Cooper S with Stop/Start the answer would be no.

There were a multitude of criteria used by the engine management system to decide whether to a) stop the engine in the first place, and b) decide to start it again even if the driver didn't do anything to trigger the start, such as dipping the clutch.

 

[robtt]

Remember, a few years ago, some tests were done on this subject and, at that time, if you were stopped for more than ten seconds then turning off engine would save you fuel. I now have stop start and must admit I wonder if it saves fuel when stopping engine for a few seconds only.

 

[Bellow]

Stop start is driven by the need for lower emissions. Obviously a car sat with it's engine not running has zero emissions thus impacting the average.

And the emissions are lower because no fuel is being burned. Worse fuel economy then?

 

[Dieselman]

Stop start is driven by the need for lower emissions.

A lot of people seem to have missed that there has been a shift from fuel economy to emissions.

The two are inextricably linked, burning fuel creates Co2 output in a linear relationship.

 

[st4]

The two are inextricably linked, burning fuel creates Co2 output in a linear relationship.

There is also a drive to reduce particulate matter, and NOx which do not go hand in hand with fuel economy and Co2.

As ever, its a balancing act.

Stop start may well have its place, but the mechanical wear on the engine and oil not circulating etc when the engine is off, then on, must take its toll and affect the total life span of the car.

 

[Peter DLM]

The handbook for the Smart says that the Stop/Start feature starts saving fuel after 10 seconds after the engine stops. I turn the feature off for the most part, if I know that my next stop will be at least 10 seconds then I'll flick it on again.

Yes, I'm that sad, and even sadder is that I'll count the seconds out to reassure me that I'm achieving the saving. It hurts me if I have to move off after only 8 seconds.

 

[davidjpowell]

How does stop start work with an auto? Manual it seems sensible to put it in N, auto, I would normally rarely bother. Do you need to make the conscious decision in an auto?

 

[Peter DLM]

How does stop start work with an auto? Manual it seems sensible to put it in N, auto, I would normally rarely bother. Do you need to make the conscious decision in an auto?

Nope. It takes care of itself. Engine restarts the moment you take your foot off the brakes. Clever stuff.

 

[Deleted member 6183]

Can't listen to the radio with the engine off, battery doesn't like it.

 

[Peter DLM]

Can't listen to the radio with the engine off, battery doesn't like it.

Not on any car I've used recently

Everything keeps on working. Even in the Smart.

 

[Deleted member 6183]

Not on any car I've used recently

Everything keeps on working. Even in the Smart.

It's a well documented issue with E60 Beemers, something to do with IBS battery lead apparently. Sounds about right, car has IBS, owner is an irritable BS too.

 

[John]

Just to add to the problem.
Is it not bad practice to switch off a hot turbo charged car without giving it time to idle and cool the turbo down a bit first?
Just thinking as if you are pootling down the mway at your usual cruising speed, at your exit junction you hit standing traffic. Would a modern engine cut the power and cook the turbo?

My understanding was that this applied to old ceramic turbos that I believe you got in stuff like 300ZXs and Renault 5 GT turbos etc.

 

[Dieselman]

The problem was due to using mineral oil, not synthetic.

Synthetic oil cannot caramelise onto the turbo shaft.

 

[Peter DLM]

It's a well documented issue with E60 Beemers, something to do with IBS battery lead apparently. Sounds about right, car has IBS, owner is an irritable BS too.

Ah, I'm referring to MBs.

 

[st4]

The problem was due to using mineral oil, not synthetic.

Synthetic oil cannot caramelise onto the turbo shaft.

I still let me turbo cool after a run, 30 secs or so. Never had a turbo let go yet, had most else but not a turbo.