I know some countries enforce having cars automatically shut down the engine when stopping at a traffic light.

  • How much resources are really saved by this process?
  • Does the car need to be designed differently to make this effective? For example, do they make the ignition system different?
  • Can we do the same manually (just remember to shut down when stopping, and start again when the light turns green) and get similar benefits?
  • Cars with auto start-stop will always have upgraded batteries (often marketed as 'start-stop' batteries) and a more powerful starter motor, often 'tandem solenoid' versions which cope better in the scenario where the engine has started to shut down but is still spinning, and the driver wishes to accelerate again. – John M Feb 13 '19 at 21:37

I’ve already touched on this topic on the sister SE site once or twice. To summarize what is told there (check for more detail), there would be several considerations as related to energy use and sustainability:

  • Idling in itself wastes fuel for no good reason, and that adds up. In US (country whose residents drive probably more than anyone else on Earth, to put it in perspective) the average cars idles 16 minutes a day (including warming up, waiting, and sitting in traffic). Even 2.0 L compact car would burn (at a rate of 1 L per hour) almost 100 L of fuel a year. Obviously, some idling is unavoidable, but even half of that saved and multiplied by the number of all the cars in the world would amount to significant impact.
  • Idling negatively affects the engine internals due to incomplete combustion (glazing of the combustion chamber, fouling of the spark plugs) which further decreases fuel efficiency for all other regimes of operation.
  • What’s more, excessive idling affects other components (burning up of the catalytic converter, increased corrosion of exhaust system, increased engine wear due to higher vibration etc.), whose premature wear and replacement lead to increased use of raw materials.

What one can do (and what I do) is to turn off engine during long traffic lights, railway crossings and so forth. Anything more than 10 to 15 seconds (very generously) would save fuel, because car that is already warm does not need that much effort to re-start, as it is at the proper operating temperature, and oil is distributed around the engine and at a high lubricity. Motor stays within the efficient range of temperatures for more than few minutes, and takes several hours to completely cool down (depending on the weather), so there is definitely no need to keep the car running during short errand (never mind someone could simply run off with the vehicle).

Any concerns about premature wear of the starting infrastructure are moot considering savings on fuel and other engine components (one could afford to replace starters every 3-4 years at the worst). See the linked posts for detailed arithmetic. And any energy losses that occur during starting, and keeping the electrical loads going during the time the engine is off are minuscule compared to energy used by petrol combustion while idling. See relevant question at Physics.SE.

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    @Flimzy, Average ICE efficiency outlays put maximum of 2 to 3 % for all accessory power take-off (including AC, electrical and electronics), which is order of magnitude less than friction and heat losses inside the motor itself. Never mind that when car idles it goes nowhere fast, so it wastes 100% of all energy input. – theUg Mar 22 '13 at 0:22
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    1) The starter is not included in the 2-3% for acceories, in the link you provided. 2) "It wastes 100% of all energy input" isn't accurate, at minimum, by considering the energy required to re-start the motor (and the loss associated with generating and storing that electric energy). It's also not true if running other accessories that depend directly on the engine (A/C seems the obvious example). And any accessories that can run off of the alternator rather than battery will benefit by an idle engine versus a stopped engine, due to efficiency lost in storing and retrieving electric energy. – Flimzy Mar 22 '13 at 21:13
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    Your answer provides a figure of 10-15 seconds, and maybe that's right (although it sure seems low to me). Can you provide a source for this figure? – Flimzy Mar 22 '13 at 21:24
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    @Flimzy, of course it takes energy to re-start and regenerate the battery. But it also more efficient to do it while car does useful work (that is, moving). – theUg Mar 23 '13 at 2:28
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    @Flimzy While the "100%" figure is easy to disprove, the conditions required to do so are far from typical driving conditions. This answer on the Physics SE site provides some helpful numbers. Based on some quick searches, it looks like the alternator for the example vehicle can push out up to 70A at the voltage required, so I will estimate (conservatively) that it takes 3-4x the time required to restart to replace the energy lost. That works out to about 9-12 seconds of engine-on time, so it might be a concern in stop-and-go traffic. – Evan Johnson Sep 3 '13 at 20:09

I have been using this method for years, on both my Toyota and my Ford Explorer. I save lots of gas, nothing has ever happened to my batteries, and have never had a mechanical problem. You need to use common sense. I typically shut it off when it's a long light and I'm in a long line, railroad crossings, and any other situation where it's going to be at least a minute wait. People seem to panic when shutting off their car in traffic, thinking some mechanical issue is going to happen, the battery will all of a sudden go bad, etc., what's the difference when shutting the car off at the mall? Then going to the grocery store and shutting it off there? The responders in this article make me laugh when they say they tried it and all of a sudden the battery went bad and there were mechanical problems. I'm pretty sure if the battery went bad in a parking lot somewhere or if a mechanical problem developed, that it was just normal wear and tear. Listen up folks, starting a car in the roadway is not different than starting it in your driveway or the parking lot of a grocery store. Use your head.

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    user1806, welcome to SE. The first few lines could be a helpful answer, but you might consider removing the snarkiness and superiority. The reason for peoples' concern is not that turning the engine off in traffic is intrinsicly different to turning it off in a car park (although the consequences of it not starting would be!), but simply because doing this regularly adds to the total number of engine starts per year. If there is an expected number of starts before faliure, then this approach could conceivably hasten faliure. – Flyto Nov 6 '14 at 9:31

I'm trying this out just now, car in question 1.2 carburetor astra mk 4/ astra g estate/caravan/station. Thus far I seem to be making a saving, (as long as I allow the engine to warm up a little) but it's entirely possible my general fuel conscious driving is making more of a difference than turning off the engine.

*related though another matter... Coasting down hills is probably saving more fuel than shutting off at traffic lights, and though not very safe certainly avoids premature starter wear by virtue of leaving the ignition on and bump starting when I reach the bottom of the hill.

  • Re the second paragraph: I'm not wishing to tell the answerer what to do or not to do, but for others, bear in mind that if coasting down hills with the engine off your braking will be severely impaired after the first use. Also, don't even think about this if you have power steering!!! – Flyto Feb 27 '14 at 8:59
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    Modern fuel-injected cars shut off fuel delivery in over-run conditions (when RPM is high relative to idle, and throttle is released), so if you are going down hill with no throttle and with gear engaged, you would actually not use any petrol. – theUg Mar 4 '14 at 14:12

I tried this method on my old 323 manual early last year for about 3 months with no issues but then that car went in for a normal service and I was forced to borrow my mother's smaller Jazz auto for a few days. It broke down the 2nd day of this use, the battery had gone dead. This car had a good history and the battery wasn't that old. In fact the battery was able to be recharged and is still in use today.

My uncle who's a mechanic said this method was bad for the car (no specifics). I also stopped using this method with my old 323 manual as I wanted to sell it soon in good working order (although it did gain an unknown motor issue about this time that fast forwarded my selling it, a mechanic bought it). I won't using this method with my new auto. I can't risk breaking down on my way to work again and I don't want to damage my nice new i30. I would like to be convinced otherwise if you have good evidence, I want to save fuel and reduce my carbon footprint.

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    To be fair, you do not provide good evidence to the contrary a as well. ;) – theUg Mar 4 '14 at 14:15

Nobody who turns off their engine ever mentions the fact that restarting their engine causes a slight delay in getting away and must result in increased congestion. This will increase the fuel consumption of everyone in the queue who should have got away but didn't and those who had to wait a few moments longer.

  • Welcome to Sustainability.SE! If you are new to Stack Exchange we recommend taking the tour first to get familiar with how everything works. – THelper Jul 23 '16 at 17:24
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    Welcome to sustainability.SE! This doesn't really answer the question, since it specifically asks about changes in technology to automatically shut engines off at traffic lights: your answer addresses only the second part of the question, in the case that drivers are manually shutting off their engines. Simple technologies, like the redlight countdown timers which are prevalent in India, can address this issue.. – LShaver Jul 23 '16 at 17:40
  • I have heard this before but I think it is around 0.5 seconds at worse, probably the same amount of time it would take to put the car in first gear and find the biting point...the amber light should allow for that :-) – atreeon Oct 8 '19 at 13:43

No, you won't save any fuel. You should remember that it takes a lot of energy to start the engine. This intially comes from the battery. However as soon as the engine starts this energy must be replenished by the engine and this is more than is required to crank the engine (because the battery is only 70% efficient).

Also all electrical loads are supplied by the battery when the engine is stopped. When the engine is running these loads are from the alternator (less losses). When you start your engine, you have to drive something like 5-8 miles to recharge. So if you are stopped at traffic lights for 5 minutes you have to add this to the 5 mile equivalent. So in reality it can cost more fuel! People are wrong thinking they can get something for nothing, like the high MPG of hybrid cars where the diesel or petrol engine is driving an alternator, loss 70%. rectifier, controller and motor loss 30%, instead of the engine driving the wheels directly.

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    Welcome to Sustainable Living! Do you have any references for your claim that it takes 5-8 miles to recharge the energy you lost when starting? – THelper Apr 16 '14 at 19:39
  • Having to drive x km to recharge does not necessarily cancel out savings; one can only conclude that if one has the figures. In the daytime without air-conditioning I suspect that the electricity consumption is minimal. – PJTraill Nov 21 '18 at 20:33

I tried this for three months (turning off the engine at traffic lights). The result is that my car battery stopped working and there was smoke coming from under the bonnet. I then had to arrange to have the car towed to a garage. I was told that I would need a new starter motor at a cost of £150. It certainly doesn't represent a saving.

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    If shutting off the engine at traffic lights actually caused this problem, please provide evidence. Otherwise it could just as easily have been a faulty part or lack of maintenance. – Evan Johnson Sep 5 '13 at 15:27
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    It would also be valuable to describe what kind (make/model/year) of car this was (and maybe the battery age, too). Anecdotal evidence is difficult to go off of, especially if the anecdote is incomplete. It may also depend on your driving habits. If you're only in stop and go city traffic, and the car never gets a chance to recharge at high rpms on the highway, I could see that being harder on the battery. – Nate Sep 6 '13 at 4:36
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    Yea, Anecdotally, I do it all the time, on my moto, and in my cars. Done it for years — nothing broke so far. Certainly, not in three months. – theUg Sep 6 '13 at 15:42

I have been a great fan with this technology, however after some thought, I find that there are some drawbacks with this tech.

The part that made this work is a special starter that starts faster. So I assume it draws more power than normsl starter. So, I think this will eventually kill a battery faster unless thr battery is more powerful, which in turn becomes more expensive... meh...

Then there's the problem when starter reliability fails. It would cost more to replace this powerful starter.. meh..

Then there is doubt on the actual savings once you take in the recharging cost of fuel to power this start stop. Conventional cars only charge after the start of a car once in it's journey.. how much fuel used? I'm not sure.. but if you keep restarting the engine surly it will need to recharge more.. so, probably the savings wont be as much...

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    Welcome to Sustainable Living! As you can read in the theUg's answer estimates are that turing your engine off becomes more efficient if you stop for more than 10 seconds. If you have references to other numbers, please share them. – THelper Jul 1 '14 at 6:13

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