# Turning off car engine at stoplights **URL:** \ **Category:** Factual Questions\ **Created:** [June 17, 2008, 7:37pm UTC](https://boards.straightdope.com/t/turning-off-car-engine-at-stoplights/453247 "2008-06-17T19:37:01Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Mama\_Zappa](https://avatars.discourse-cdn.com/v4/letter/m/71e660/32.png) [@Mama\_Zappa](https://boards.straightdope.com/u/Mama_Zappa)\ **Post date:** [June 17, 2008, 7:37pm UTC](https://boards.straightdope.com/t/turning-off-car-engine-at-stoplights/453247/1 "2008-06-17T19:37:01Z")
There was an article on hypermiling in the Washington Post on Sunday. One of the points made there was to turn off your car’s engine when stopped at a long light. Now, I gather a slight bit of extra gas is required to restart the car, so if you’ve just turned it off for a second or two before restarting, you’ll wind up expending _more_ fuel. Any ideas on what the payoff period is for such a maneuver? There are certainly plenty of lights around here (DC metro area) where one’s redlight wait is likely to be a minute or more, but what if you only reasonably expect to have a 10-second wait? ---
**Author:** ![Throatwarbler\_Mangrove](https://avatars.discourse-cdn.com/v4/letter/t/8c91f0/32.png) [@Throatwarbler\_Mangrove](https://boards.straightdope.com/u/Throatwarbler_Mangrove)\ **Post date:** [June 17, 2008, 7:51pm UTC](https://boards.straightdope.com/t/turning-off-car-engine-at-stoplights/453247/2 "2008-06-17T19:51:28Z")
Couple of issues off the top of my head: 1. You start the car using the battery. Car batteries generally are not designed to have to start the car every 5 min, if you keep it up it is likely you will run out of battery juice at some point. 2. Cold starting a car is very hard on the engine, mainly because the moving parts in the engine are moving before the oil has had a chance to fully lubricate everything. Now you are not talking about cold starting a car but starting it every once in a while, which will make it somewhat less harmful, as a still war engine will still be somewhat lubricated, but it’s still not something I would want to do. You can see that on a car like a Prius, it is fairly simple to implement ways to avoid these issues, but not on a regular car. ---
**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [June 17, 2008, 8:12pm UTC](https://boards.straightdope.com/t/turning-off-car-engine-at-stoplights/453247/3 "2008-06-17T20:12:45Z")
We have discussed this before, and IMHO the payback period is about 45-60 seconds or so. This assumes that you are driving a modern fuel injected vehicle, the engine is warmed up, and the car is in a good state of tune. I would note that if you shut off the car at every stop light you will probably wind up with a partially depleted battery, as the engine will not run long enough to rechage it before you shut it off again. This is not a good thing. Also the AC stops when you turn off the key. ---
**Author:** ![Duckster](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/duckster/32/1244_2.png) [@Duckster](https://boards.straightdope.com/u/Duckster)\ **Post date:** [June 17, 2008, 8:31pm UTC](https://boards.straightdope.com/t/turning-off-car-engine-at-stoplights/453247/4 "2008-06-17T20:31:34Z")
Run a week-long experiment. Count all the traffic light stops you make during a typical week. Multiply by some number, say 60 seconds, to find out how much you have to idle doing nothing. Use that amount of time to reduce your overall weekly driving. You will achieve a similar amount of gas savings without the wear and tear on your battery and ignition system. Bet still, strive to reduce your overall driving for even greater savings. While the Post article may have good intentions, it appears the writer did not do some basic commonsense research. The writer probably isn’t a Doper nor has ever heard of the SDMB. ---
**Author:** ![jjimm](https://avatars.discourse-cdn.com/v4/letter/j/ba8739/32.png) [@jjimm](https://boards.straightdope.com/u/jjimm)\ **Post date:** [June 17, 2008, 8:41pm UTC](https://boards.straightdope.com/t/turning-off-car-engine-at-stoplights/453247/5 "2008-06-17T20:41:08Z")
[Related thread I started](http://boards.straightdope.com/sdmb/showthread.php?t=467397). ---
**Author:** ![Myglaren](https://avatars.discourse-cdn.com/v4/letter/m/f1d935/32.png) [@Myglaren](https://boards.straightdope.com/u/Myglaren)\ **Post date:** [June 17, 2008, 9:13pm UTC](https://boards.straightdope.com/t/turning-off-car-engine-at-stoplights/453247/6 "2008-06-17T21:13:59Z")
The latest BMW’s turn off the engine at lights or other stops in traffic. I recall them doing this some years ago. Whether they have done this in the interim period I couldn’t say but I am sure my daughters and my son in law’s BMW 525’s don’t have this option. It would be interesting to see if there is any discernible improvement in economy using this system. ---
**Author:** ![Throatwarbler\_Mangrove](https://avatars.discourse-cdn.com/v4/letter/t/8c91f0/32.png) [@Throatwarbler\_Mangrove](https://boards.straightdope.com/u/Throatwarbler_Mangrove)\ **Post date:** [June 17, 2008, 9:27pm UTC](https://boards.straightdope.com/t/turning-off-car-engine-at-stoplights/453247/7 "2008-06-17T21:27:57Z")
> [@](#): > > The latest BMW’s turn off the engine at lights or other stops in traffic. I’m almost certain they don’t. You must be thinking of their “regenerative braking” system, by which the alternator is only powered during braking. This is done to improve acceleration and horsepower, although it may also have some minor benefit for fuel economy. ---
**Author:** ![Gary\_T](https://avatars.discourse-cdn.com/v4/letter/g/3ec8ea/32.png) [@Gary\_T](https://boards.straightdope.com/u/Gary_T)\ **Post date:** [June 17, 2008, 9:27pm UTC](https://boards.straightdope.com/t/turning-off-car-engine-at-stoplights/453247/8 "2008-06-17T21:27:59Z")
One thing to keep in mind is that this technique requires using the starter more, perhaps a lot more for some driving/stoplight patterns. While money can be saved from using less fuel, at least some it will eventually be spent replacing the starter sooner than would have been needed otherwise. ---
**Author:** ![ZenBeam](https://avatars.discourse-cdn.com/v4/letter/z/3ab097/32.png) [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)\ **Post date:** [June 17, 2008, 11:58pm UTC](https://boards.straightdope.com/t/turning-off-car-engine-at-stoplights/453247/9 "2008-06-17T23:58:42Z")
I used to do this at a particularly long light when I lived in California. One thing not yet mentioned is that the cranking time for an engine that’s only been off a minute or two is very short, much less than starting cold. I barely had to turn the key, presumably because the engine is hot and still well lubricated. ---
**Author:** ![Sunspace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sunspace/32/1250_2.png) [@Sunspace](https://boards.straightdope.com/u/Sunspace)\ **Post date:** [June 18, 2008, 12:31am UTC](https://boards.straightdope.com/t/turning-off-car-engine-at-stoplights/453247/10 "2008-06-18T00:31:20Z")
A number of cars have this feature built in; it’s called ‘idle-stop’. I think at the very least they have to beef up the battery and starter motor. I got a lift to Toronto once in a Honda Insight that did this; its ‘starter motor’ also provided power assist during acceleration as a ‘mild’ hybrid. ---
**Author:** ![PaulParkhead](https://avatars.discourse-cdn.com/v4/letter/p/7ea924/32.png) [@PaulParkhead](https://boards.straightdope.com/u/PaulParkhead)\ **Post date:** [June 18, 2008, 12:41am UTC](https://boards.straightdope.com/t/turning-off-car-engine-at-stoplights/453247/11 "2008-06-18T00:41:13Z")
[QUOTE=Sunspace] A number of cars have this feature built in; it’s called ‘idle-stop’. I think at the very least they have to beef up the battery and starter motor. I got a lift to Toronto once in a Honda Insight that did this; its ‘starter motor’ also provided power assist during acceleration as a ‘mild’ hybrid. [/QUOTE] Yup - I recall a Volkswagen back in the 1980s that did this. Shifting to neutral caused the engine to stop (once it had warmed up). It would restart when you moved the gear lever back into 1st. It didn’t seem to catch on, though, so I had assumed the idea was, um, a non-starter. ---
**Author:** ![Throatwarbler\_Mangrove](https://avatars.discourse-cdn.com/v4/letter/t/8c91f0/32.png) [@Throatwarbler\_Mangrove](https://boards.straightdope.com/u/Throatwarbler_Mangrove)\ **Post date:** [June 18, 2008, 12:55am UTC](https://boards.straightdope.com/t/turning-off-car-engine-at-stoplights/453247/12 "2008-06-18T00:55:57Z")
> [@](#): > > I got a lift to Toronto once in a Honda Insight that did this; its ‘starter motor’ also provided power assist during acceleration as a ‘mild’ hybrid. The Honda Insight is a FULL FLEDGED hybrid. More so than the Prius. > **[Honda Insight](https://en.wikipedia.org/wiki/Honda_Insight)** > > The Honda Insight (ホンダ・インサイト, Honda Insaito) is a hybrid electric car which was manufactured and marketed by Honda. Its first generation is a two-door, two-seat liftback (1999–2006) and its second generation is a four-door, five-seat liftback (2009–2014). In its third generation, it became a four-door sedan (2018–2022). Its fourth generation, it became a four-door crossover SUV (2026–present) e:N2-based. It was Honda's first model with Integrated Motor Assist system and the most fuel effici Ho... What other non-hybrid car have you seen with this “feature”? This is certainly news to me, and I thought I knew cars reasonably well. ---
**Author:** ![Balthisar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/balthisar/32/2825_2.png) [@Balthisar](https://boards.straightdope.com/u/Balthisar)\ **Post date:** [June 18, 2008, 12:58am UTC](https://boards.straightdope.com/t/turning-off-car-engine-at-stoplights/453247/13 "2008-06-18T00:58:28Z")
[QUOTE=PaulParkhead] Yup - I recall a Volkswagen back in the 1980s that did this. Shifting to neutral caused the engine to stop (once it had warmed up). It would restart when you moved the gear lever back into 1st. [/QUOTE] I can only ever imagine this idea working on hybrids or manuals. We’re in the USA; hybrids are still a tiny segment of every vehicle on the road, and for that matter, so are manuals. In my big, fast V8 automatic, I’d be upset at the hesitation it takes when I step on the gas pedal. I supposed in a manual, at least I’d have control. Throw it in neutral, it stops. Put it in gear, it starts. Leave it in gear when stopped with my foot on the clutch, and the engine stays running. Actually, I like that idea very, very much. I wish it weren’t so hard to get a full-sized car (a real, EPA full-sized car, not “rental company” full-sized) with a manual transmission in the USA. I really loved the giant Opel I drove a few months back. ---
**Author:** ![Sunspace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sunspace/32/1250_2.png) [@Sunspace](https://boards.straightdope.com/u/Sunspace)\ **Post date:** [June 18, 2008, 1:29am UTC](https://boards.straightdope.com/t/turning-off-car-engine-at-stoplights/453247/14 "2008-06-18T01:29:52Z")
[QUOTE=Throatwarbler Mangrove] The Honda Insight is a FULL FLEDGED hybrid. More so than the Prius. > **[Honda Insight](https://en.wikipedia.org/wiki/Honda_Insight)** > > The Honda Insight (ホンダ・インサイト, Honda Insaito) is a hybrid electric car which was manufactured and marketed by Honda. Its first generation is a two-door, two-seat liftback (1999–2006) and its second generation is a four-door, five-seat liftback (2009–2014). In its third generation, it became a four-door sedan (2018–2022). Its fourth generation, it became a four-door crossover SUV (2026–present) e:N2-based. It was Honda's first model with Integrated Motor Assist system and the most fuel effici Ho... What other non-hybrid car have you seen with this “feature”? This is certainly news to me, and I thought I knew cars reasonably well. [/QUOTE] ‘Mild’ means it can’t run on electrics alone, as the Prius can. Am I wrong? Edit: nope. > [@The Wikipedia article](#): > > The Insight is not considered a “full” hybrid vehicle because it cannot run on the electric motor alone, whereas its competitor, the Prius, can be operated solely on the electric motor. A feature shared by the two hybrids (and now appearing in others) is the ability to automatically turn off the engine when the vehicle is at a stop (and restart it upon movement). Since it is more powerful (10 kW) than most starters of conventional cars, the Insight’s electric motor can start the engine nearly instantaneously. ---
**Author:** ![Sunspace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sunspace/32/1250_2.png) [@Sunspace](https://boards.straightdope.com/u/Sunspace)\ **Post date:** [June 18, 2008, 1:37am UTC](https://boards.straightdope.com/t/turning-off-car-engine-at-stoplights/453247/15 "2008-06-18T01:37:22Z")
And here’s a [non-hybrid BMW with idle-stop](http://car-reviews.automobile.com/news/hybrid-style-idle-stop-technology-added-to-bmw-1-series/2770/). ---
**Author:** ![Throatwarbler\_Mangrove](https://avatars.discourse-cdn.com/v4/letter/t/8c91f0/32.png) [@Throatwarbler\_Mangrove](https://boards.straightdope.com/u/Throatwarbler_Mangrove)\ **Post date:** [June 18, 2008, 1:50am UTC](https://boards.straightdope.com/t/turning-off-car-engine-at-stoplights/453247/16 "2008-06-18T01:50:58Z")
OK, so it’s a feature available on 2007+ BMW 1 series with manual transmissions and 4 cylinder engines only. I guess that would explain why I’ve never seen it. :dubious: ---
**Author:** ![Magiver](https://avatars.discourse-cdn.com/v4/letter/m/4491bb/32.png) [@Magiver](https://boards.straightdope.com/u/Magiver)\ **Post date:** [June 18, 2008, 5:20am UTC](https://boards.straightdope.com/t/turning-off-car-engine-at-stoplights/453247/17 "2008-06-18T05:20:24Z")
It’s possible to do this without the starter and car companies have experimented with it. Not sure how reliable it would be. It requires a direct injected motor and a fully indexed crank. The computer injects fuel in the cylinder on the power stroke and then ignites it in sequence. There a lot of companies coming out with start/stop engines (Citroen, BMW, Chrysler etc…) and they probably all use high efficiency starters. ---
**Author:** ![Sublight](https://avatars.discourse-cdn.com/v4/letter/s/439d5e/32.png) [@Sublight](https://boards.straightdope.com/u/Sublight)\ **Post date:** [June 18, 2008, 8:21am UTC](https://boards.straightdope.com/t/turning-off-car-engine-at-stoplights/453247/18 "2008-06-18T08:21:19Z")
By coincidence, the Japan Times had an interesting article relating to this yesterday. [_What the Heck Is That?_: Idling drivers, by ALICE GORDENKER](http://search.japantimes.co.jp/cgi-bin/ek20080617wh.html) Unfortunately, Ms. Gordenker doesn’t provide any cites for her assertions, but here are a few snippets: > [@Alice Gordenker](#): > > So why do drivers do it? …often it’s because of outdated notions, including that cars need to warm up before being driven; that restarting the car uses up more gas than letting it idle; and that repeatedly restarting your car is hard on the starter and battery. None of these myths is true. > > In terms of gasoline consumption, experts have determined that you’ll save on gas by switching off the engine during even very brief stops, leading to the surprising “5-second” rule: If it looks like you’ll be idling for more than 5 seconds, turn off the engine and restart when you are ready to roll. > > [after a 3700km road trans-Japan experiment]…the idling-stop cars guzzled less gas — 5.8 percent less overall, with a 13.4 percent saving in city driving and a 3.4 percent reduction in rural areas. But what’s really fascinating is how much time the cars spent stopped: 24 percent of total driving time, and a whopping 47 percent on urban roads! ---
**Author:** ![Gary\_T](https://avatars.discourse-cdn.com/v4/letter/g/3ec8ea/32.png) [@Gary\_T](https://boards.straightdope.com/u/Gary_T)\ **Post date:** [June 18, 2008, 10:09am UTC](https://boards.straightdope.com/t/turning-off-car-engine-at-stoplights/453247/19 "2008-06-18T10:09:17Z")
**Alice Gordenker, in the Japan Times** : “[It is not true that] repeatedly restarting your car is hard on the starter…” Easy to say, but I seriously doubt she has any evidence to support this contention. While I wouldn’t choose the term “hard” on the starter, it’s simply not possible to use a starter, say, 16 times during a round-trip commute instead of 2 times and not cause more wear on it. Not that it’s necessarily 8 times more wear, but it’s more wear. ---
**Author:** ![Pushkin](https://avatars.discourse-cdn.com/v4/letter/p/ee59a6/32.png) [@Pushkin](https://boards.straightdope.com/u/Pushkin)\ **Post date:** [June 18, 2008, 10:38am UTC](https://boards.straightdope.com/t/turning-off-car-engine-at-stoplights/453247/20 "2008-06-18T10:38:02Z")
[QUOTE=Throatwarbler Mangrove] OK, so it’s a feature available on 2007+ BMW 1 series with manual transmissions and 4 cylinder engines only. I guess that would explain why I’ve never seen it. :dubious: [/QUOTE] Well, I know little about cars, but BMW have had Donald Sutherland touting their wares on TV for a couple of weeks now, passionately thanking stop signs for letting Beemer drivers save a little money on petrol. [Next page](https://boards.straightdope.com/t/turning-off-car-engine-at-stoplights/453247.md?page=2)