Turning your engine off at traffic lights? - a900ss
I?ve often heard that to start a car?s engine requires more fuel than leaving it running at a set of traffic lights for a short while? Is this true?

If you have a warm engine, why does it need ?extra? fuel to start it?

If this is true, how long would you need to be waiting at the lights before it becomes more economical to turn the engine off and re-start when the lights go green?

For the purpose of this question, please ignore the fact that you will lose air con whilst the engine is turned off.
Turning your engine off at traffic lights? - Citroënian {P}
Citroen and VW have cars that turn off their engines when stopped in traffic and I understand the new new MINI is soon to have this feature as standard.

I guess they must beef up the mechanisms somewhere to cope with this, but the figures quoted for these suggest it's worthwhile in terms of fuel saved.

Turning your engine off at traffic lights? - Big Bad Dave
I would never turn my engine off at lights. I like to hold it on the foot brake, rev it while it's in drive and listen to the brakes groan and grind.
Turning your engine off at traffic lights? - DP
I would never turn my engine off at lights. I like to hold it on
the foot brake rev it while it's in drive and listen to the brakes groan
and grind.


Or if its a front driver, stick it in 1st, and give it full beans, sitting there frying the tyres with the handbrake on. Assuming the lights change before the tyres burst, just lazily roll the handbrake off, and let it slowly gather speed, leaving a nice anti-social smoke trail and two spectacular lines across the junction. If you've got something with a bit of grunt, see if you can get it into second and keep the revs up.

I have been a passenger in a friend's car when this was done. We were young and foolish, and he wanted to finish off the cheap remoulds the previous owner had fitted on the car, officer. When the smoke starts coming through the heater vents, it's unpleasant.

Cheers
DP

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04 Grand Scenic 1.9 dCi Dynamique
00 Mondeo 1.8TD LX
Turning your engine off at traffic lights? - tyro
I once asked this question and was told that the answer was "2 minutes".
Turning your engine off at traffic lights? - David Horn
I reckon it's closer to about 2 seconds! After all, when you start the car the engine doesn't rev hard, nor does it inject enormous amounts of fuel to get it running again.
Turning your engine off at traffic lights? - Altea Ego
2 seconds? Dont forget you have to put back several amps into the battery, & reover the inertia that you have lost in the engine by turning it off.
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TourVanMan TM < Ex RF >
Turning your engine off at traffic lights? - bathtub tom
I understood it was the fuel used caused by the load on the alternator replacing the charge in the battery that's taken out by the operation of the starter motor.
Turning your engine off at traffic lights? - Number_Cruncher
An ex-colleague did exactly this calc as part of his research into electric / fuel cell vehicles. He reckoned that at most traffic lights, it is worth shutting down, even accounting for the extra current required to re-charge the battery.

So, I think the answer lies somewhere between the 2 seconds and 2 minutes answers so far offered on this thread.

Number_Cruncher
Turning your engine off at traffic lights? - mfarrow
2 seconds? Dont forget you have to put back several amps into the battery &
reover the inertia that you have lost in the engine by turning it off.


The amp-hours you put back in the battery will equate to little more than the power consumed by keeping the engine ticking over.

The friction and heat loss saved will be far higher than the power used putting inertia back into the engine.

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Mike Farrow
Turning your engine off at traffic lights? - Altea Ego
Do you know the current surge required to start to turn over an engine? its quite consdierable
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TourVanMan TM < Ex RF >
Turning your engine off at traffic lights? - Stuartli
>>they must beef up the mechanisms somewhere to cope with this>>

The Golf launched in 1994 that cut the engine when stationary (it used the 64bhp standard diesel engine) had a heavy duty battery and starter motor.

It never caught on.

Not surprised at the time as I found the cutout was somewhat disconcerting when driving one in London on its Press launch.
- - - - - - - - - - - - - - - - - - - - -
What\'s for you won\'t pass you by
Turning your engine off at traffic lights? - Altea Ego
The cars last about 2 years before the whole idea starts to go wrong.
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TourVanMan TM < Ex RF >
Turning your engine off at traffic lights? - Cliff Pope
You normally use the starter motor once per journey. How long does a starter motor last? So how long will it last if you do say 5 restarts per journey?
Turning your engine off at traffic lights? - George Porge
How long will oxygen sensors and cats last being doused with unburnt pertol to excess? Its not exactly environmentally friendly either
Turning your engine off at traffic lights? - Number_Cruncher
Temporarily ignoring the complications of cat and starter motor life,

If you assume 12volts, 400 amps, and 4 seconds cranking are required to start an already warm car, and that car at idle would consume 0.6 litres of fuel with an LCV of 34.8MJ/l, per hour, starting the car costs about 3.3 seconds worth of idling.

From a fuel efficiency point of view, it seems clear that if you know you are going to be stopped for, say 30 seconds or more *and* you don't need to get away fast - i.e., you have some forewarning of when you will need to set off, it's probably worth shutting down.

On modern cars, I don't think starter motor life is a big issue - they certainly aren't as much of a fast moving item as they used to be.

I don't know how one would avoid damaging the cat - perhaps by not injecting fuel for a cycle or two after cranking starts?

Number_Cruncher

Turning your engine off at traffic lights? - Xileno {P}
One's air con won't work. Silly idea.
Turning your engine off at traffic lights? - ForumNeedsModerating
In addition to all the above 'per car' calculations, if all or most people had this facility or did it manually, how much longer would queues take to clear? If it was longer, which you could well imagine in a city or subeuban traffic queue, this would make journeys longer & probably more costly overall in terms of fuel and/or journey times.

Look at this very effect on 'phantom' queues often seen on motorways due to the inertial effects of cars braking.
More math needed I think before this is considered cost efective.
Turning your engine off at traffic lights? - L'escargot
Stationary traffic is bad enough as it is with most drivers not even anticipating putting their car into gear before the car in front has started to move.
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L\'escargot.
Turning your engine off at traffic lights? - Dynamic Dave
One's air con won't work. Silly idea.


The climate control in my new Vectra still works for a little while without the engine running. Mind you, along a similar line to BBD's comment, I prefer to leave the engine running and my foot on the brakes to annoy the person behind me with my high level LED brake light.
Turning your engine off at traffic lights? - L'escargot
I just can't imagine what the effect would be all the unburnt fuel coming out of everyone's exhaust pipe every time they restarted their engine at the lights.
--
L\'escargot.
Turning your engine off at traffic lights? - Lud
I used to turn my engine off at the lights and restart it, although Skoda starters were far from everlasting. I didn't do it to save petrol or the environment. I did it because the engine cooling fan had lunched itself and I hadn't yet put another one that worked in the car. That way the overheating could be delayed.

All you rich guys and company car drivers have little idea of true motoring and the terrific fun it can be.... :o)
Turning your engine off at traffic lights? - Cliff Pope
Neither rich nor company car driver, but I have experienced bodgernomics in the past - eg patching up the body shell of a decrepit Austin A30 using metal sheet and pop rivits pinched from the university engineering department.

In my experience turning off the engine is the surest way to make it overheat. While it is running the water pump is circulating water through the radiator, and the car heater makes quite a good alternative to the proper fan. Turn off the engine and the cylinder head, already hot, goes on boiling the now stationary water.
Turning your engine off at traffic lights? - mfarrow
In my experience turning off the engine is the surest way to make it overheat.


You should get some sort of thermo-siphoning effect. Granted it won't help an overheating engine, but as soon as the engine's cut out the heat input has ceased, so it won't get any hotter.

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Mike Farrow
Turning your engine off at traffic lights? - Cliff Pope
>> as the engine's cut out the heat input has ceased so it
won't get any hotter.



I don't think that is strictly true. Obviously no more heat is being generated in the sense that no more fuel is being burnt, but there is a considerable store of heat already held in the metal block and cylinder head, which cannot dissipate once the circulation is cut off. The metal surrounding the combustion chamber will be a lot hotter than 100 degrees.

Try the experiment of observing the temperature gauge just before you turn off the engine. (I am assuming that the sensor is in the head, not the radiator hose. ) Wait 5 minutes, then turn the ignition on again. Often the gauge has shot up into the red during the waiting time, dropping only if the engine is restarted or a longer cooling time allowed.
Turning your engine off at traffic lights? - Lud
Often the gauge has shot up
into the red during the waiting time dropping only if the engine is restarted or
a longer cooling time allowed.


But that is a result of heat sink and convection raising the temperature locally. The overall temperature of the whole engine and cooling system will have decreased. On restarting the engine, the temperature gauge quickly reverts to normal, slightly lower than it would if the engine had been left running...
Turning your engine off at traffic lights? - Cliff Pope
But that is a result of heat sink and convection raising the temperature locally. The
overall temperature of the whole engine and cooling system will have decreased. On restarting the
engine the temperature gauge quickly reverts to normal slightly lower than it would if the
engine had been left running...


Yes, you are quite right, I am only pursuing this to be cussed. But if the engine is overheating you can get localised boiling even though the whole system is cool. Remember in the old old days with no antifreeze, and cars boiling over yet with frozen radiators?
Turning your engine off at traffic lights? - Altea Ego
Heat soak is a genuine problem. If a local temperature is held down by some cooling influence, and is designed to work at that cooled temperature, when the cooling influence is taken away, heat will radiate/convect/conduct to it from a hotter area. Possiby putting out of design spec for temperature.
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TourVanMan TM < Ex RF >
Turning your engine off at traffic lights? - Lud
Don't tell me about juggling, DH. Once it's getting really hot you have to stop and give it a rest anyway or risk damage. Of course I am talking about turning it off well before it overheats, and having of course the car heater turned on full with the fan going. Once an engine is warm, idling at the lights pumps heat into it quicker than anything. Turning it off delays the moment when it overheats, or threatens to.

I have to say though that all that sort of thing is tiresome and bad for the blood pressure. Cars whose main components are functioning correctly are best.
Turning your engine off at traffic lights? - Lud
David Horn and Cliff Pope, I apologise for mixing you up. Happens sometimes.
Turning your engine off at traffic lights? - billy25
Is this self-shutting down thingy "REALLY" a "built-in design feature"? - cos I know a "friend who's pulled all his hair out and spent hour after hour trying to work out why his wont "idle" at tick-over for more than a couple of mins!!. If this Feature is really true, I dont think i dare tell him!!
Turning your engine off at traffic lights? - BazzaBear {P}
All you rich guys and company car drivers have little idea of true motoring and
the terrific fun it can be.... :o)


Ah, you're reminding me of the fun of keeping my first car running. I certainly wouldn't dare turn it off at lights. Once it got past a certain heat (and my fan didn't work either as it happens) the starter motor would overheat and no longer work, so turning off was a big no-no. Added to this, it was supposed to have a manual choke, but it was nowhere to be seen, so when it was cold I used to have to approach traffic lights without letting the revs reduce, otherwise it would die.
Being no kind of proponent of heel-and-toe this tended to involve a late and violent braking manoevre to a very slow speed, so my foot was off the accelerator for the minimum possible time, and also so the wheel speed was keeping the engine going, followed by clutch in and accelerator pressed and the last few mph bled off with the handbrake.
I did get around to fitting a (sort of) working fan and clutch after a while, but never got the starter motor properly sorted out. I became an expert at bump starting, I could get that engine firing up with the bare minimum of velocity.
Turning your engine off at traffic lights? - milkyjoe
>> One's air con won't work. Silly idea.
The climate control in my new Vectra still works for a little while without the
engine running. Mind you along a similar line to BBD's comment I prefer to leave
the engine running and my foot on the brakes to annoy the person behind me
with my high level LED brake light.


sitting at the lights with your foot on the brake pedal is highly dangerous, a shunt from the rear and your foot is off the brake whether you like it or not (newtons third law), consequence three pensioners and a mother and toddler in a buggy smeared all over the tarmac and the underside of your car, basic common sense really use the blooming hand brake!!!!
Turning your engine off at traffic lights? - Dynamic Dave
basic common sense really use the blooming hand brake!!!!


Apart from having to reload my fishing rod because someone has taken the bait.....

It would mean that I would fail to burn the retina's out of the motorist behind me with my bright LEDs if I used the handbrake ;o)
Turning your engine off at traffic lights? - wemyss
Why three pensioners ? Why not three 25 year olds.
This is blatant ageism (if there is such a word) Where are the moderators when they are needed ?..
wemyss
Turning your engine off at traffic lights? - BazzaBear {P}
Why not three 25 year olds.


they're all in a stolen car, high on drugs, driving dangerously in YOUR neighbourhood.
Turning your engine off at traffic lights? - Lud
I imagine you mean the pensioners Bazza? You would if you lived round here.
Turning your engine off at traffic lights? - BazzaBear {P}
Hell's Grannies?
Turning your engine off at traffic lights? - Marko
Why not just jump the lights? Sorted.
Turning your engine off at traffic lights? - Robin Reliant
I could forsee some spectacular hold ups if engines were made to shut down after a short period of idling. Can you imagine a line of twenty cars waiting at the lights and all having to restart when they changed? There would be bound to be an empty whirring somewhere down the line where someones motor refused to fire.

Anyone who has ever had a tempramental starter at anytime in their lives (all of us?) would dread the idea.
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Turning your engine off at traffic lights? - Pugugly {P}
The Honda IMAs we have as work pool cars do this, stop at light, engine stops, press clutch in, engine fires up, horrid if you're not used to it.....there is a way of overriding it - perhaps with a stick
Turning your engine off at traffic lights? - JamesH
No reason why more cars shouldn't cut their engine automatically as per recent BMWs:
- A modern engine fires up in next to no time when warm (the 4 seconds estimated earlier seems too long)
- No air con isn't really an issue as the time spent waiting at lights isn't enough to significantly alter the ambient temp inside the car. No reason why the blower can't stay on.
- The electronics controlling the stop-start could suspend it if the engine isn't starting instantly or the alternator is past its best. The electronics could get around the air con issue by keeping the engine running when there's still a large temperature differential.

I haven't experienced a one of these setups myself, but I could imagine it would be disconcerting if it operated in a continuously stop-start traffic jam.

I do keep the engine off when stopped for a while. The main time is when taking stuff to the council recycling centre. In their wisdom, they closed the old spacious place and opened a new little site that can't cope with visitor numbers, so all the cars spew fumes into the atmosphere while queueing, offsetting a lot of environmental benefit of the recycling. However, the approach road is downhill so the inching forward can be done with the engine off, restarting occasionally to restore easier braking.

Turning your engine off at traffic lights? - Number_Cruncher
>>(the 4 seconds estimated earlier seems too long)

I agree - it was a deliberate excessive assumption - even with this extreme amount of cranking, you still only use 3.3 seconds worth of fuel.

If this is all handled electronically, with the engine cranking itself automatically when either the clutch is pressed on a manual, or the throttle is pressed on an auto, there wouldn't be any extra delay in queues.

I think this type of system should be encouraged, for both emission and noise pollution reasons, however, there's the usual luddite (sorry Lud!!) nonsense which opposes it - the types who would still be happy with starting handles, acetylene lamps and grease nipples.

Number_Cruncher


Turning your engine off at traffic lights? - Cliff Pope
Ok N.C., we have temporarily ignored the complication of accelerated starter motor wear.

Suppose a starter motor normally lasts 200,000 miles. If your journey to work involves stopping at only 5 sets of lights, that will multiply the rate of wear 5 times, so the motor will last only 40,000 miles. That would mean a new starter motor every other year.
Commuters with 10 sets of lights to cross would wear out a motor every year.


There will be a plea in the Technical section soon: - Why is my car eating starter motors?
Turning your engine off at traffic lights? - George Porge
NC, would you like to live next to a set of trafic lights with your stop / start system? I'm sure you'd appreciate the neat petrol fumes supplying the aromatic setting whilst enjoying your evening meal.

Cars are no more economical today than 30 years ago with increased weight due to safety features and idiotic gadgets people insist they need.

Back to basics, build lighter and lean burn is the way forward for improved economy
Turning your engine off at traffic lights? - Number_Cruncher
>>Why is my car eating starter motors?

More than likely caused by DMF failure!

;-)

Probably, the form and layout of the starter would change - you might even use an electric starter for first thing, and a sprung device when the engine's hot. You might even use a freewheeling flywheel, storing energy in its rotational momentum with a clutch to the crank.

Number_Cruncher
Turning your engine off at traffic lights? - Altea Ego
No air con isn't really an issue as the time spent waiting at lights isn't enough to significantly alter the ambient temp inside the car. No reason why the blower can't stay on.

Obviously not a big city driver then JamesH? one could be stationary (as in not moving) for 15 minutes at times. Your failed aircon becomes a real pain when its 28c and smoggy.

(yes i know all the smog is caused by the engines all being on)


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TourVanMan TM < Ex RF >
Turning your engine off at traffic lights? - JamesH
Obviously not a big city driver then JamesH? one could be stationary (as in not
moving) for 15 minutes at times. Your failed aircon becomes a real pain when its
28c and smoggy.


I'm glad I don't drive much in cities! My point was meant to be that in more frequent stop-start/traffic light conditions, short periods with the engine off may make negligible difference to the interior temperature of the car.

See third point above of my post though. It does rely on some form of climate control in the car, but if there is too much of a temperature difference between desired and actual then the ECU can keep the engine running or restart it if already off.
Turning your engine off at traffic lights? - PoloGirl
Never mind the air con and the fuel... why would you want to annoy everyone else in the queue by not being ready to go the second that light changes to green?

It's bad enough waiting for numpties to engage first gear and move, never mind having to wait for them to start the car!
Turning your engine off at traffic lights? - Welliesorter
What's this about turning the engine off and even using the handbrake? I thought you were supposed to leave it in first gear and gradually roll forward over the stop line while the lights are red. When the lights change, you're not moving off from a standstill, but just increasing the rate of acceleration. Surely the drivers round here (especially the bus and van drivers) can't all be wrong! ;-)
Turning your engine off at traffic lights? - Cliff Pope
Probably, the form and layout of the starter would change - you might even use an electric starter for first thing, and a sprung device when the engine's hot. You might even use a freewheeling flywheel, storing energy in its rotational momentum with a clutch to the crank.

Number_Cruncher



The engine itself is a large freewheeling flywheel. What about decompressing n-1 cylinders, switching off their injectors, and idling the engine very slowly in standby mode on one cylinder? Depressing the clutch would switch the non-running cylinders back into operation without the need for restarting.
Turning your engine off at traffic lights? - KMO
Doesn't half seem weird listening to people discussing the feasibility something my Prius has been doing for the past few years. Engine turns off while stopped, and indeed it usually remains off while creeping along in a jam. Much silence and bliss all round. Except for all the noise you can hear from the people idling their diesel tractors around you. Bit odd when the only vibration you feel through the steering wheel is coming from the car next to you.

The climate control problem is solved by using an electric A/C compressor, rather than an antique belt-mounted-to-the-engine job. So cold air still available while stationary. Also the coolant system has an electric pump that's used when the engine's off.

And there's no launch delay - the move off from stationary is on electric power, with the engine joining in somewhere between 5 and 15mph. The engine start is done by a balancing act between the two motor-generators; it gets spun up to 1000rpm or so before fuel is injected, so it's basically seamless. The engine stop also normally happens as soon as you start decelerating, rather than once you've stopped, which makes it less noticeable. All much more sophisticated than the Civic IMA.

Turning the engine off also helps the catalytic converter performance. It stays up to temperature much better with the engine off than with it idling and pushing relatively cool exhaust gas through it.
Turning your engine off at traffic lights? - Number_Cruncher
>>decompressing n-1 cylinders...

Yes, that might work - there might need to be some complication to avoid pumping a load of fresh air over the cat from the non-firing cylinders to avoid excessive NOx.

Number_Cruncher
Turning your engine off at traffic lights? - Cliff Pope
May be if decompressed the cylinders would not actually be pumping at all?
Turning your engine off at traffic lights? - Muggy
"...Suppose a starter motor normally lasts 200,000 miles. If your journey to work involves stopping at only 5 sets of lights, that will multiply the rate of wear 5 times, so the motor will last only 40,000 miles. That would mean a new starter motor every other year.
Commuters with 10 sets of lights to cross would wear out a motor every year..."


On a point of order:

Is it not more a case of how many starts it performs than how many miles it covers?

100,000 miles on trips of 1 mile each = 100,000 starts.

100,000 miles on trips of 100 miles each = 1,000 starts.

Yes, stopping the engine and starting again five times on a journey will use the starter motor five times more than keeping it going, but I don't see how the number of miles affects it?
Turning your engine off at traffic lights? - Cliff Pope
Yes stopping the engine and starting again five times on a journey will use the
starter motor five times more than keeping it going but I don't see how the
number of miles affects it?


No, it doesn't, of course. I was just trying to get a handle on how long a starter motor is currently expected to last, as a base line. Whatever it is, it would wear out 5 times faster if you make 5 starts per journey rather than one.
My point was that at present a starter motor's life stretches off into infinity - it is not a normal consideration. But if we are expected to accelerate the wear by 5 or perhaps ten times, it would become a regular replacement item, and would need to be factored into the environmental or financial equation.
Turning your engine off at traffic lights? - L'escargot
Wemyss, look at it my way. I'd sooner be in the spotlight (for whatever reason) than have people ignore me and pretend I didn't exist!
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L\'escargot.
Turning your engine off at traffic lights? - Aprilia
Does no one remember the Golf that used to do this in the early 1990's ( I think it was called the 'Ecomatic'). It had two batteries, and (IIRC) and oversize starter motor and electric power PAS and water pump. The motor not only turned off when stationary, but also if you were coasting. I think there was a similar Citroen (or maybe Pug) at one time as well.
Turning your engine off at traffic lights? - sierraman
It would mean that I would fail to burn the retina's out of the motorist behind me with my bright LEDs if I used the handbrake ;o)


Apply the handbrake and then put your foot on the brake pedal.In fact you could then flash the lights on and off for extra annoyance.
 

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