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Old 06-30-2008, 05:32 AM   #1
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Sitting at idle or turn off engine?

From the 201 Tips To Save Gas:

33. Don't start and stop needlessly
Don't start and stop the engine needlessly. Idling your engine for one minute consumes the same gas amount as when you start the engine, which for your information is a lot!

How much fuel does an average car enigne use at start up?

I got to thinking: if a car engine is idling at 6-800 prm sitting at a red light for 10 seconds, that's approx. 60-80 rpm that the engien turned while idling. Depending on the engine size, could that be similar amount of fuel used to start an engine?

On my 250cc scooter, I turn off the engine and coast to a traffic light when I can estimate that I will have to come to a complete stop and wait for a few seconds for the light to change. Since my scooter's carb and valves are adjusted that it would fire up the engine within one crank revolution, it's not such a big deal to turn off and restart the engine numerous times. But then again, my 250cc scooter uses less than 1/10 of the fuel of a 2.6 liter engine per engine revolution.
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Old 06-30-2008, 05:57 AM   #2
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'96 up cars break even at 10secs apparently, however some say emissions break even at 5 mins, older carbed cars have been reported as having 5-10min break-evens, 85ish-96 cars are reported to have 2 min break evens.
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Old 06-30-2008, 06:07 AM   #3
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Question

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Originally Posted by RoadWarrior View Post
'96 up cars break even at 10secs apparently, however some say emissions break even at 5 mins, older carbed cars have been reported as having 5-10min break-evens, 85ish-96 cars are reported to have 2 min break evens.
Can you be more specific about this "break even" business?
what breaks even with what? on what size engine? what rpm range?

5 minutes idling, that's over 3000-4000 rpm in the engine, that's quite a bit of fuel burned for sitting without motion, does a car engine require that much fuel at start up?
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Old 06-30-2008, 06:46 AM   #4
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RW's talking about the difference between the steady consumption at idle and the large amount of fuel required to start the engine. Starting does use more fuel than idling for a few seconds, and the break-even point is a question of just exactly how many seconds of idling uses the same amount of fuel as starting.

To start the engine, it needs a very rich squirt of fuel (choke on your lawmower, an actual squirt in my 1980 Buick, or just long injector openings on a modern car), and then it will idle fast for a few seconds even if it's already warm.
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Old 06-30-2008, 09:05 AM   #5
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I find turning off the engine for a few seconds to be useless waste of your starter, time, and possbily gas if you have car off shorter amount of time.. What I would like to know is if bump starting a manual tranny would be better, I think it would but that means I have to buy a new car... damn
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Old 06-30-2008, 09:23 AM   #6
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Quote:
Originally Posted by theholycow View Post
RW's talking about the difference between the steady consumption at idle and the large amount of fuel required to start the engine. Starting does use more fuel than idling for a few seconds, and the break-even point is a question of just exactly how many seconds of idling uses the same amount of fuel as starting.

To start the engine, it needs a very rich squirt of fuel (choke on your lawmower, an actual squirt in my 1980 Buick, or just long injector openings on a modern car), and then it will idle fast for a few seconds even if it's already warm.
Yup, more modern cars have a more controlled warm startup, measuring the gas, some older FI systems are known to just turn the injectors full on while cranking, changing to bank firing them rich at high idle for a minute or two, before passing back to closed loop.
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Old 06-30-2008, 09:53 AM   #7
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i've done better shutting off my car if i know i'm going to sit at a light for 30secs or more. u have to know your car and the stop lights in the area for it to work. its useless if your just going to sit there for anything less then that
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Old 06-30-2008, 11:16 AM   #8
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I shut off if it's going to be more than 10 seconds. Seems to work...

See this study: http://www.slate.com/id/2192187/ Tested - restarting uses as much fuel as 6 seconds of idling.
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Old 06-30-2008, 11:35 AM   #9
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I usually don't because it takes my car time to get back into lean burn after a shutdown.

But, you should pay more attention to Pale Melanesian because he is doubling his mileage.

It definitely increases wear and tear on the starting system.

My alternative is to maximise my DFCO and try to not sit at the light in the first place.

If you drive a repetitious route where you know when the lights are going to be green, try longer DFCO events to time your arrival at the light to when it's green, if that is possible in your situation.

Your objective is to arrive at the light after it has changed and traffic starts moving. DFCO is the same (as far as fuel use is concerned) as turning off the engine and restarting it when you cruise through the intersection without stopping at all.

This works with manual transmissions, but no where near as well with automatics.

regards
gary
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Old 06-30-2008, 11:39 AM   #10
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Good points there.

Hadn't though of the lean burn issue (how many times have I said that? apologies to the VX'ers). My "normal" engine goes into closed loop within a second after restarting.

Also, a warmed-up restart is almost instant, so there's not a huge amount of wear going on. Admittedly, there is more starter wear than not doing it, but it's minimal.

DFCO into a light is another good approach. Six of one, half dozen of the other.
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