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Old 04-04-2008, 07:40 AM   #1
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shut off engine if stopped > 7 sec?

Hi, I'm new here...

I know I've read a few times that some people say to shut off the engine if you are going to sit at a stop light for more than 7 seconds. Does someone know where this information comes from? I thought I read that time was chosen because it saved gas and covered the extra wear you put on a starter.

Anyway, I was thinking that it didn't sound right to me so I wanted to do my own calculations. I'm assuming 0.2g/hr idle useage and $3.20 /gal.
so for 7 seconds shut off time I get:

(0.2g/hr)*(3.2$/g)*(hr/3600s)*7s= $0.00124444
10 sec = 0.001778
20 sec = 0.003556

Now how long does a starter last?
If you start your car 6 times a day for 10 years that is 21,900 starts.
What if the starter would last for 50,000 more starts? How much do you save?

Save at 7 sec = $62.22
10 sec = $88.89
20 sec = $177.78

Or look at it another way how many starts does a starter need to last to pay for its replacement? A 2005 honda accord starter is $125. Install is probably at least $125. So we have to recover $250 over the life of the starter.

@ 7 second = $250/$0.00124444 = 200893 starts
@ 10 second = 140625 starts
@ 20 seconds = 70313 starts

From my experience starters don't last that long (200,000 extra starts) I can see them lasting 70000 starts (30 years at 6 starts / day).

So "my" conclusion is to let the car idle unless you are going to sit at a light for 20 second or more.

What do you think?
(I did my calculations in a spreadsheet with little rounding so the math may be a little different if you round)
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Old 04-04-2008, 08:48 AM   #2
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Probably pretty good calculations. But you'd want to increase the cost of the starter in the future. If the cost of replacement increases 10% every year, the cost would be doubled in a little over seven years.

Nice job!
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Old 04-04-2008, 08:56 AM   #3
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Is there any extra gas used to start the vehicle?
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Old 04-04-2008, 09:05 AM   #4
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I'm sure there probably is, but I don't think it would be alot when the engine is warm. With the other assumptions, I didn't think the restart gas would be a big factor. On the other hand, if it runs rich for a minute or two after restart, it would be... I doubt that it works that way. I'll have to check the scangauge and see if it is open or closed loop mode after hot restart..
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Old 04-04-2008, 10:10 AM   #5
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Idling vs Restarting

I have been wondering this very same thing.

My main concerns are fuel consumption during idle, extra fuel to start and run the car for the first few seconds, extra pollution upon restart and wear to the battery and starter. I add the battery in because I am mostly driving at night and must have the lights on during this coast and sitting at a light for safety.

Searching for this information I found this from the Car Talk Guys. They (sort of in their joking manner) recommend only shutting off for much longer idling (3-5 min), but not for the short 30-60 second stops due to wear and tear on the starter and battery.

DanQ I would like to see when/if open-loop occurs upon restart of a warm engine. Of course every car would vary.
In a discussion there was a good explaination about the open vs closed loop which was informative for those of us (me) that didn't know exactly what that meant.
"When a car starts up (and it's cold), the engine management software operates in whats called open-loop. In this mode, the software uses pre-programmed values to determine how much fuel is needed, how much spark advance to use, what the idle speed is, and more. These values are designed to insure that your car starts reliably, minimizes damage to the engine and allows it to be drivable immediately. They are not chosen to be fuel efficient.

Once the engine has warmed up, the engine moves to closed-loop operation. By now, the O2 sensor is working and the car adjusts the fuel/air ratio based on how much oxygen remains in the exhaust after combustion. The knock sensors also come into play, allowing the engine to advance the spark to the bleeding edge of knocking, meaning more power and efficiency.

If the car is already warm when it is started, it may move very quickly to closed-loop. However, some open-loop operation still happens, the length of which can vary."
So their conclusion is that this really depends on the car and it's software.

My thought is that "some open-loop operation still happens" after start up in most cars. This could be small, but considering the amount of fuel you are saving there is a trade off. 7s= $0.00124444 gal or even 20 sec = 0.003556 could be more than that small amount of extra fuel savings during restart/first seconds of running rich. Especially in older model cars. A new hybrid car that shuts down may have better programing to compensate for this since it is designed to shut off and restart more often.

Another comment puts that time of fuel savings between 30 sec and 2 minutes and indicates that there is more emissions upon start up.
"From what I understand, 30 seconds to 2 minutes is the cross-over point for fuel consumption. Older cars take more gas to start than newer ones, so turn off your late-model cars earlier.

The trade-off is for increased tailpipe emissions. After 30 s to a minute the catalytic converter cools off. On restart, the car produces higher levels of NOX and CO until the converter reheats.

To summarize: turning off saves fuel but creates more smog. Idling burns more fuel, but keeps the air cleaner."
So my thought is that it you can save fuel if you are going to idle for around a minute and you might make more smog. Depending upon how often that occurs in your daily driving may require more replacement of the starter and battery.
I have been shutting off only at longer lights or coasts with lights at the end of them.
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Old 04-04-2008, 10:50 AM   #6
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SD26 makes a good point too. That brings me to conclude 40 seconds is now a better guideline....

razmataz: I think you are correct about emissions being worse doing frequent shutoffs. I guess it depends on what you are trying to optimize. If you just want to save gas, shutting off at lights will do that. If you want to optimize the cost of driving, I think you need to shut off at only the longest lights.

This analysis tells me that you don't want to shut off engine while coasting unless you can do it for a suitable length of time (unless you can bump restart).
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Old 04-04-2008, 12:44 PM   #7
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Quote:
Originally Posted by razmataz View Post
.... indicates that there is more emissions upon start up.
Good post.

I will add to that that the EPA, etc. has really cracked down on starting emissions. That's one of the reasons why lighter weight oils and fuel injection had to be developed, and why even lawn and garden equipment like lawn mowers don't have manual chokes but have primer bulbs: starting emissions is a factor in getting your certification.

I think it's probably not as bad on cars made in the past ten years, if not longer.
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Old 04-04-2008, 07:17 PM   #8
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I think the math on this is extremely vehicle dependent. Something that gobbles 0.4 gph obviously would pay back in half the time, and then there is the issue of the cost of the starter vs. labor to install it.

My Tercel for example could be fitted with a remanufactured starter for about $65 plus 1/2 hour of my labor, vs. my Escort which would run about $110 plus about 1 hour. A Motorcraft starter for a 1990 F250 however can be picked up for about $25. I doubt labor would exceed an hour there either.
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Old 04-04-2008, 10:43 PM   #9
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depends on car mostly but for me, i notice that when i start my car no matter what temp it starts in a second of crankign but the rpm's go about 500 more than idle for a few seconds, meanign the gas i just saved by shutting it off, now just got burned up by the priming of the fuel lines, and the higher idle rpm...

so i only shut my engine off if a train is comming(which is quite rare around here)cuz those usualyl can last a few minutes and when the traffic lights get tripped by the RR crossing lights the traffic lights are even longer
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Old 04-07-2008, 11:38 AM   #10
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FYI, I checked on my car. Open loop on a warm restart lasts between 2 and 5 seconds. It varies because of the sample rate of the scangauge.
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