# Car idling - effect on pollution **URL:** \ **Category:** Factual Questions\ **Created:** [July 24, 2003, 4:12am UTC](https://boards.straightdope.com/t/car-idling-effect-on-pollution/190550 "2003-07-24T04:12:17Z")\ **Posts on this page:** 10\ **Page:** 1
**Author:** ![KarlGauss](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/karlgauss/32/3713_2.png) [@KarlGauss](https://boards.straightdope.com/u/KarlGauss)\ **Post date:** [July 24, 2003, 4:12am UTC](https://boards.straightdope.com/t/car-idling-effect-on-pollution/190550/1 "2003-07-24T04:12:17Z")
Is there a difference in the amount of pollution produced by a car that idles for an hour as opposed to a car travelling at, say, 30 or 60 mph for an hour? How about restarting your engine? If so, what would you substitute for the XX’s in the sentences below: “Letting your car idle for an hour is equivalent to driving for XX hours at XX mph.” “Re-starting your engine produces as much pollution as letting it idle for XX hours.” Thanks for your help. Depending on your answers, my next GQ question may be on certain aspects of divorce law! 😃 ---
**Author:** ![SlickRoenick](https://avatars.discourse-cdn.com/v4/letter/s/e68b1a/32.png) [@SlickRoenick](https://boards.straightdope.com/u/SlickRoenick)\ **Post date:** [July 24, 2003, 4:41am UTC](https://boards.straightdope.com/t/car-idling-effect-on-pollution/190550/2 "2003-07-24T04:41:20Z")
It wouldn’t necessarily have to do with XX hours at XX mph but rather the RPM that the vehicle is on. If you can do 30 mph at 5000 rpm in first gear it would be the same as doing 120 mph at 5000 rpm in 4th gear. At least that is how I interpret what you are asking. ---
**Author:** ![Jeff41](https://avatars.discourse-cdn.com/v4/letter/j/e99b99/32.png) [@Jeff41](https://boards.straightdope.com/u/Jeff41)\ **Post date:** [July 24, 2003, 4:53am UTC](https://boards.straightdope.com/t/car-idling-effect-on-pollution/190550/3 "2003-07-24T04:53:08Z")
That’s not entirely correct, Slick. At 120 mph, the air resistance will be 16 times greater than at 30 mph (if I remember my physics). Friction will be greater too. It will take a lot more power, and therefore more fuel to move a car at the higher speed. Fuel consumption relies not only on engine speed, but also air temperature, throttle position, manifold air pressure (or, some cars use a mass air flow sensor instead of a MAP), engine temperature, and possibly some other variables I’m forgetting about. ---
**Author:** ![KarlGauss](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/karlgauss/32/3713_2.png) [@KarlGauss](https://boards.straightdope.com/u/KarlGauss)\ **Post date:** [July 24, 2003, 11:17am UTC](https://boards.straightdope.com/t/car-idling-effect-on-pollution/190550/4 "2003-07-24T11:17:58Z")
OK, let me rephrase to ask the bottom-line question: In terms of pollution, is much good done by turning off your engine when you’re not moving, rather than letting it idle? Example - if I arrive five minutes early at the spot where I am picking someone up, is it of any significant benefit to turn off my engine as opposed to letting it idle while I wait? ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [July 24, 2003, 11:32am UTC](https://boards.straightdope.com/t/car-idling-effect-on-pollution/190550/5 "2003-07-24T11:32:12Z")
> [@](#): > > \*Originally posted by Jeff41 \* > **That’s not entirely correct, Slick. At 120 mph, the air resistance will be 16 times greater than at 30 mph (if I remember my physics).** I think **SlickRoenick** meant that fuel consumption rate (gallons per hour) is proportional to engine RPM. That seems to make sense to me - the piston sucks up the same amount of fuel/air mixture per cycle regardless of load on the engine. You don’t _need_ to rev the engine to 5000 rpm whem driving at 30 mph, but if you do anyway, you’ll be using the same amount of fuel per minute as when driving 120 mph at 5000 rpm. Anyway, of course a stopped engine consumes less fuel than one at idle. If the above assumption is true, and assuming idle speed is 800 rpm and it takes 1600 rpm to maintain 30 mph, idling for 1 hour takes as much fuel as driving half an hour at 30 mph. ---
**Author:** ![zut](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/zut/32/2875_2.png) [@zut](https://boards.straightdope.com/u/zut)\ **Post date:** [July 24, 2003, 1:21pm UTC](https://boards.straightdope.com/t/car-idling-effect-on-pollution/190550/6 "2003-07-24T13:21:45Z")
> [@](#): > > \*Originally posted by SlickRoenick \* > It wouldn’t necessarily have to do with XX hours at XX mph but rather the RPM that the vehicle is on. If you can do 30 mph at 5000 rpm in first gear it would be the same as doing 120 mph at 5000 rpm in 4th gear. > [@](#): > > \*Originally posted by scr4 \* > I think **SlickRoenick** meant that fuel consumption rate (gallons per hour) is proportional to engine RPM. That seems to make sense to me - the piston sucks up the same amount of fuel/air mixture per cycle regardless of load on the engine. No! Not true at all. Fuel consumption is roughly proportional to power, which is engine speed _times load_ (or times torque, if you prefer). That should be intuitively obvious: if you stomp on the gas (without downshifting), the engine speed does not change abruptly, but fuel consumption does (because you’re accelerating the car, which increases the load). This is complicated, of course, by the fact that engine efficiency varies at different combinations of speed and load, so fuel consumption is not exactly proportional to power. but it’s not simply proportional to speed. At idle, the engine load is zero. Or, actually, all the energy is being used up inside the engine overcoming friction and so forth. So the fuel consumption at idle is much lower than when the vehicle is moving. How much lower? Embarassingly, I don’t really know, but I did find a couple things that might help. First: > [@](#): > > _From [this Canadian government site](http://oee.nrcan.gc.ca/idling/issues_idling/benefits_action.cfm):_ > Idling a vehicle for 10 minutes a day uses an average of 100 litres of gas a year. 10 min/day = 3650 min/yr. This is 0.027 liters/min = 0.0072 gal/min. At 60 mph, a typical passenger car would use ~0.033 - 0.05 gal/ min (20-30 mpg), so as far as _fuel usage_ goes (not _pollution_), letting your car idle for one hour is equivalent to driving for 7-12 min at 60 mph. This is admittedly a little tenuous. Also, regarding idling versus turning off the engine: > [@](#): > > _From [the same site](http://oee.nrcan.gc.ca/idling/issues_idling/myths.cfm):_ > More than 10 seconds of idling uses more fuel than restarting the engine. > [@](#): > > _From [the EPA (a pdf)](http://www.epa.gov/otaq/consumer/18-youdo.pdf):_ > IDLING: You will save gas by turning the engine off and restarting it again if you expect to idle for more than 30 seconds. You will also prevent pollution by avoiding long idles. Try parking your car and going into restaurants, banks, and the like instead of idling in drive-up lanes. Ten seconds, thirty seconds, whatever. At least you get a feel for the comparison (as far as fuel usage goes) between idling and restarting. Finally, the other piece of the puzzle is the comparison of _pollution_ between idling and driving. Half of the puzzle is [in this pdf](http://www.epa.gov/otaq/consumer/f98014.pdf) giving average g/hr for emissions (229-371 g/hr of CO, 4.72-6.16 g/hr NOx for passenger cars, summer vs. winter). The other half is [in this pdf](http://www.epa.gov/otaq/consumer/f00013.pdf) giving average g/mile (20.9 g/mile CO, 1.39 g/mile NOx for passenger cars). Doing the calc says that idling for 1 hr is equivalent to travelling 11-17 miles (CO) or 3.3-4.4 miles (NOx). Assuming the average speed is, say, 45 mph (just a WAG, I’m afraid), that means that letting your car idle for an hour is equivalent to driving between 5 minutes and 23 minutes at 45 mph. I realize this post is kind of scattershot, but perhaps someone else can add a little more info and clean up the conclusions. ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [July 24, 2003, 1:59pm UTC](https://boards.straightdope.com/t/car-idling-effect-on-pollution/190550/7 "2003-07-24T13:59:32Z")
> [@](#): > > \*Originally posted by zut \* > \*\*Fuel consumption is roughly proportional to power, which is engine speed _times load_ (or times torque, if you prefer). That should be intuitively obvious: if you stomp on the gas (without downshifting), the engine speed does not change abruptly, but fuel consumption does (because you’re accelerating the car, which increases the load). \*\* OK, that sounds convincing. But the piston sucks up a constant amount of fuel/gas mixture per cycle, right? So what happens when you step on the gas? Does the gas mixture get more concentrated (richer)? Sorry for the minor hijack but I think it’s relevant to answering the OP. ---
**Author:** ![zut](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/zut/32/2875_2.png) [@zut](https://boards.straightdope.com/u/zut)\ **Post date:** [July 24, 2003, 3:37pm UTC](https://boards.straightdope.com/t/car-idling-effect-on-pollution/190550/8 "2003-07-24T15:37:01Z")
> [@](#): > > OK, that sounds convincing. But the piston sucks up a constant amount of fuel/gas mixture per cycle, right? Not right. The piston traverses a constant stroke, but the engine has a throttle valve that controls air intake. When the throttle is partially closed, the amount of air allowed to enter the intake system is restricted, so less air in the cylinder. [When you step on the gas](http://auto.howstuffworks.com/fuel-injection2.htm), you open the throttle valve, letting in more air. The engine controller senses this, and increases fuel quantity to match. ---
**Author:** ![Phage](https://avatars.discourse-cdn.com/v4/letter/p/a4c791/32.png) [@Phage](https://boards.straightdope.com/u/Phage)\ **Post date:** [July 24, 2003, 4:00pm UTC](https://boards.straightdope.com/t/car-idling-effect-on-pollution/190550/9 "2003-07-24T16:00:52Z")
Going back to the OP, I am curious as to how these figures fit into the big picture. How much fuel is consumed in starting the engine and how much time at idle does that equal? Which produces more wear, starting your car or letting it idle for the period required to consume that amount of gas? Would that wear equal an increase in pollution through the required manufacturing of new parts? ---
**Author:** ![KarlGauss](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/karlgauss/32/3713_2.png) [@KarlGauss](https://boards.straightdope.com/u/KarlGauss)\ **Post date:** [July 25, 2003, 4:08pm UTC](https://boards.straightdope.com/t/car-idling-effect-on-pollution/190550/10 "2003-07-25T16:08:09Z")
Sorry for the late response. Thanks to all who responded, especially **zut** for some impressive number crunching. Very helpful.