Idling versus Shutting Off the Car - Fuelly Forums

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Old 02-11-2006, 08:33 AM   #1
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Idling versus Shutting Off the Car

I was perusing wikipedia for some neat fuel economy ideas, and most of it was the usual junk, but I noticed they said to shut off if you'll be idling more than ten seconds. Does this make sense? I spend a lot of time idling and it'd be nice to shut off and see what kinda difference it makes.

PS: I added gassavers to the links list at the bottom, which is jawsome, mehbe we should start posting up wikipedia with stuff and referencing gassavers to mehbe attract some more attention from the "common folk."
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Old 02-11-2006, 08:51 AM   #2
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Re: Idling versus Shutting Off the Car

Quote:
Originally Posted by SVOboy
I was perusing wikipedia for some neat fuel economy ideas, and most of it was the usual junk, but I noticed they said to shut off if you'll be idling more than ten seconds. Does this make sense? I spend a lot of time idling and it'd be nice to shut off and see what kinda difference it makes.

PS: I added gassavers to the links list at the bottom, which is jawsome, mehbe we should start posting up wikipedia with stuff and referencing gassavers to mehbe attract some more attention from the "common folk."
It makes perfect sense to shut off the engine. When you are idling, you are getting 0 mpg. Although, you would have to keep in mind that doing this TOO much could result in premature starter death. Also, great idea about posting links to gassavers on wikipedia.
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Old 02-11-2006, 09:23 AM   #3
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Re: Idling versus Shutting Off the Car

Quote:
Originally Posted by SVOboy
I was perusing wikipedia for some neat fuel economy ideas, and most of it was the usual junk, but I noticed they said to shut off if you'll be idling more than ten seconds. Does this make sense? I spend a lot of time idling and it'd be nice to shut off and see what kinda difference it makes.
I have been shutting down on long downhill coasts and stop lights. I restart only at the stoplights hovever because at the end of a long coast, I drop it into 4th and let out the clutch.

One possible drawback of shutting off the engine is the initial rich condition during startup. It always takes several seconds for the idle to return to normal.

When I get the SuperMID, I can measure fuel useage at idle. Let's say that it is 1/4 gallon per hour. Then ask yourself, during your average tank, how many hours are spent idling. Multiply hours by gallons per hour and you will get gallons per tank used up by idling.

Sooo, 0.25 gal/hr at idle * 2hrs idling = 1/2 gallon per tank due to idling. If your average fill is 10 gallons at 500 miles, that 50 MPG figure will increase to 52.6 MPG if you eliminate the idling.
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Old 02-11-2006, 09:49 AM   #4
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never mind that but also

Never mind that you save a little gas by shutting it off but also you have to start again with no oil pressure and a hot engine so the rods are hot and coverered with little oil - you cycle the battery a little and the starter - you had brake lights on while stopped and maybe the radio and headlights heater etc next the alternator has to recharge all over again once you start back up - your cat cooled off a little and the exhost manifold maybe the radiator fan kicked in etc. Not for 10 seconds of stopping at a light. In a hybrid they factor all this in to the systems in the car and provide battery power to pumps etc to make everything continue to operate when the ice is off. Now if you wanted to go a little crazy then run all the belt driven things off a single high efficiency motor so you could turn off the ice including the vacuum pump for the brakes and the water pump to continue to circulate the water in the heater in the winter and prevent hot spots from forming in the engine as it cools a little. Better yet think about a starter alternator like in the hybrids and you will have the starter wear and tear taken care of with a drive connected brushless motor that can charge and start the engine. If I could figure out how to modify the valve timing on my xB I could configure the valves to reduce pumping losses and shut down the injectors and drive the engine with an electric motor directly right through the drive train.
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Old 02-11-2006, 10:31 AM   #5
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Not 10 sec, 30 sec

I agree that 10 sec is too short. I use 30 sec as my threshold. But my goal is to not have to stop at all. If you study alternate routes, ones that avoid traffic and stoplights, you will see huge benefits. After awhile you will begin to be able to time the lights such that you hit green lights most of the time. On my 25 mile commute I have 15 lights each way. Most of the time I can time 13 of the 15 lights. The remaining two are so long, 1 - 2 minutes, that I shut down as I approach the light. So including the initial start, I average 6 starts with the starter a day. That works out to one start every 8 miles.

That being said, when I find out what my idle consumption is, I will revisit the 30 second threshold. Four lights per day for a total of six minutes over ten round trip commutes per tank is equal to one hour of idling. At 1/4 gallon per tank, that translates to a 2.6% hit due to idling.
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Old 02-11-2006, 12:29 PM   #6
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idle

Can you lower your idle really low to reduce fuel use even lower if you have to idle? The Geo would get warmed up and idle so low you could hear and feel each cylinder firing. My xB is having trouble getting below 800rpm I think the added GP-7 has it reving a little extra to keep the emissions clean. I notice it when I push the clutch in to coast that it affects the MPG reading on the SG and if it idles low the mpg goes up a lot more.

Just did a loop run and got 42.6 mpg daytime 32 degrees, the other night I got 41.8 mpg at 22 degrees, before that 42.6 mpg warmer daytime, seems the acetone has little effect other than bringing up the mileage and keeping it there although I did have to pull out fast today when cold and the tires just wanted to spin when I punched it. This xB is hard to hold onto the wheel and steer when you are pulling some G's in a turn.
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Old 02-12-2006, 02:40 PM   #7
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krousdb, did i read in one

krousdb, did i read in one of the hybrid forums that you've got an ignition kill switch on your del sol, seperate from the key switch? if so, what does it do for you?

i'm wondering about this because when i switch off the ignition in my car (e.g. at a long stop, or while coasting), my scangauge goes to sleep. since it takes about 20 seconds to "wake up" again after i resume using the engine, it skews my mileage data.

(ironically, the result of this is i'm actually using slightly more gas than i would be without the SG, because i leave the engine running sometimes when i could switch it off just to keep the current trip data accurate.)

i was wondering if i killed the engine by cutting the spark directly, rather than using the key, if the scangauge would continue to work...

--

on a slightly different topic, on my 89 accord, i regularly switched off for lights and coasting, and then usually turned the key back to "on" while waiting, or when i was coasting and planning to restart with the clutch.

turns out my alternator did not like this. i had intermittent charging problems that i was able to correlate to this practice of leaving the key "on" with the engine "off".

can anyone comment on this? i seem to recall advice from an old-timer years ago that it's not good to leave the key switch "on" when the engine is not running.

eventually the alternator died - and i have wondered if i killed it prematurely.
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Old 02-12-2006, 03:49 PM   #8
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Re: krousdb, did i read in one

Quote:
Originally Posted by MetroMPG
krousdb, did i read in one of the hybrid forums that you've got an ignition kill switch on your del sol, seperate from the key switch? if so, what does it do for you? i'm wondering about this because when i switch off the ignition in my car (e.g. at a long stop, or while coasting), my scangauge goes to sleep. since it takes about 20 seconds to "wake up" again after i resume using the engine, it skews my mileage data.
Yes, I did have the kill switch. I installed it because shutting down causes the speedo/odometer to stop while I waited for the engine to die and turn the ignition back on. The idea was two fold. I would capture all of the distance traveled instead of losing some to shtudown. Secondly, I wanted to save wear and tear on the ignition switch. Well the ignition kill worked pefectly, and the speedo continued to register. But the engine would not start, either with clutch or with starter until I shut off the ignition ant turned it back on again. Turns out that the ECU shuts off the fuel pump when it senses that the engine died. It could be some sort of safety thing. But it defeated the purpose of the kill switch. So I removed it.


Quote:
Originally Posted by MetroMPG
(ironically, the result of this is i'm actually using slightly more gas than i would be without the SG, because i leave the engine running sometimes when i could switch it off just to keep the current trip data accurate.)

i was wondering if i killed the engine by cutting the spark directly, rather than using the key, if the scangauge would continue to work...
My guess is that it would, but you may have the same fuel pump shutoff issue that I did. You could try supplying power to the scangauge directly from the battery. That way it wouldnt shut down at ignition off.

Quote:
Originally Posted by MetroMPG
on a slightly different topic, on my 89 accord, i regularly switched off for lights and coasting, and then usually turned the key back to "on" while waiting, or when i was coasting and planning to restart with the clutch.

turns out my alternator did not like this. i had intermittent charging problems that i was able to correlate to this practice of leaving the key "on" with the engine "off".

can anyone comment on this? i seem to recall advice from an old-timer years ago that it's not good to leave the key switch "on" when the engine is not running.

eventually the alternator died - and i have wondered if i killed it prematurely.
I have yet to experience ill effects from this. Wayne Gerdes has used this technique for years in his Ford Ranger and lately on his 2005 Accord Automatic. The Accord has a lifetime average in the high 40's. He has reported no ill effects either.
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Old 02-12-2006, 03:57 PM   #9
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Quote:Yes, I did have the

Quote:
Yes, I did have the kill switch.
how did you wire it? did you simply tie it in to the existing key-ignition circuit?
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Old 02-12-2006, 04:16 PM   #10
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I don't think all cars have

I don't think all cars have the fuel pump issue that was mentioned. I know someone else with a Saturn that spliced a switch into his injectors to shut off the engine so he wouldn't lose any mileage from waiting for the engine to die. I would like to try this sometime but haven't gotten around to it. As for the alternator problems, I haven't noticed any ill effects from coasting with the engine off.
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