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Old 05-28-2008, 09:41 PM   #1
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Engine Off at Stop Lights

I'm starting to experiment with turning off the engine at stop lights. I only plan on doing it at major intersections where I know the car would normally be idling for a while. When I see a light turn yellow, I'll kill it and then coast in neutral. My main concern is that starting the car will use the same amount or more fuel starting than it would idling. Does anyone have some solid numbers on how long the car would need to be off before I start seeing benefits?

Thanks to all you knowledgeable folks
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Old 05-29-2008, 04:29 AM   #2
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I dont usually kill the engine, but my situation may be different.

Most of the lights here are timed and you can avoid a complete stop.

If you know you are going to have to stop (or at least slow down) downshift to keep your revs above 1000 RPM, try to make it to the light after it has changed and you dont have to stop completely.

With your foot off the gas and the car in gear you have no fuel delivery which is the same as shutting off the engine, except you dont have to restart it with the starter, or risk restart enrichment.

Your starter will last a lot longer.

In cases where you have no choice remember you are using a very small amount of fuel idling.

If you drive the same routes daily, try both ways and see which works better.
In my daily route there are two intersections where if I get caught in the worst case I have to wait 1 min 45 secs for the lights to cycle. Then I will turn the engine off.


regards
gary
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Old 05-29-2008, 05:05 AM   #3
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Quote:
Originally Posted by R.I.D.E. View Post
With your foot off the gas and the car in gear you have no fuel delivery which is the same as shutting off the engine, except you dont have to restart it with the starter, or risk restart enrichment.
Gary, I hate to disagree, but if your engine is running, it is using fuel. If there was no fuel being delivered, it would shut off.

Fuel enrichment for a warm restart is negligible. Starter wear is a toss up. I've had starters last 200,000 miles. I've also had them last 30,000 miles.

Standard procedure for me is to kill the engine if the car is not moving. If I'm not moving, I don't need gas. Simple.
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Old 05-29-2008, 05:48 AM   #4
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You're using no to very little gas at some points when you are slowing down in gear to the light,because the power to turn the engine is coming from the wheels rather than the engine turning the wheels If you stop the engine is going to use gas to idle.Slowing down in gear is better if you are not going to stop.
If you are going to stop it's best to switch the car off. You could run the battery down if the lights are close together and you're using headlights or ( God forbid) the A/C. Brake lights use a lot of electricity too.
Starter wear is not an issue. I've been switching off at light for months with no problem.
You can save gas if only off for 30seconds but not worth the trouble; It takes very little gas to start the engine. I lost 3mpg on current trip sitting at a light after driving 1.5miles.It's probably the easiest way to save gas.
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Old 05-29-2008, 05:52 AM   #5
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I only kill the engine at lights that I know have a long pattern, otherwise I neutral idle.
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Old 05-29-2008, 05:55 AM   #6
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Originally Posted by OdieTurbo View Post
Fuel enrichment for a warm restart is negligible.
On OBD-II motors and later it might be... There's some earlier motors though that turn all the injectors on until the ECU realises the motor is running, then it fires 2 at a time at high idle until it reads that the motor is up to temperature, and then it kicks into closed loop and starts metering fuel properly. This can take 30 seconds to go through even on a warm engine. So 10 second break even point might be true for OBD-II and newer, but when it's an older motor that's using like 12x as much fuel for 5 seconds, 6x as much fuel for 25 seconds, you can begin to see where it's thought the breakeven point might be 2-5 mins. I'm going on a fuel timing table and discussion I saw for the non-turbo 6 cyl Mitsubishi Eclipse.
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Old 05-29-2008, 06:57 AM   #7
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Quote:
Originally Posted by R.I.D.E. View Post
With your foot off the gas and the car in gear you have no fuel delivery which is the same as shutting off the engine
That's called DFCO (Deceleration Fuel Cut Off), and its behavior differs between vehicles. Are you sure that the OP's vehicle will do it at low speeds and hold it all the way down to 1000rpm? I've been measuring actual fuel rate and can see in realtime when I'm in DFCO, and behavior varies vastly between different vehicles, as well as being a little less predictable than you say.

Depending on whether or not DFCO will be achieved, it may be better to slow in neutral and just try to time it so you'll keep as much speed as possible when the light turns green.

Quote:
Originally Posted by OdieTurbo View Post
Gary, I hate to disagree, but if your engine is running, it is using fuel. If there was no fuel being delivered, it would shut off.
Common misconception. It's hard to believe that DFCO exists the first time you hear about it; I thought the same thing. In fact, fuel gets completely cut in both my 2008 VW Rabbit and my 2002 GMC Sierra under certain conditions, and the engine does not stall or have any problems.

Take a look at my thread "DIY fuel flow/injector gauge results" for details on exactly how DFCO behaves in those two vehicles.
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Old 05-29-2008, 07:29 AM   #8
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DFCO is standard on most modern engines. Both of mine do it. My civic does it above 1200 rpm.

If you're actually stopped, turn it off. No need to waste the fuel. If you know the light's going to turn in 10 seconds, don't. Be reasonable about it.

Of course, you'll save even more if the engine is off BEFORE you arrive at the light.

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Wayne at CleanMPG drives an automatic and P&G's it extensively. I'm talking about 10+ P&G cycles per mile. Being an auto, that means every single P&G cycle includes a key restart. His starter lasted about 95,000 miles. That might be a little premature, but it's not bad.
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Old 05-29-2008, 07:39 AM   #9
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random thought about pre OBDII cars... my protege fires 2 injectors at a time. seems fairly wasteful, since one is on the intake stroke and the other is on the power stroke. all it does is blow fuel onto the intake valve, and when wrapped out under WOT, that's a lot of wasted fuel. but that's not idle. Even at idle it still wastes some. small savings add up over time.
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Old 05-29-2008, 07:46 AM   #10
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Here's a good article on the subject. http://www.slate.com/id/2192187/
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