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Which method uses less gas?

Less gas to idle for 5 mins
3
14%
Less gas to restart the engine
17
77%
Basically the same
2
9%
User avatar
 #241598  by carlr
 
Howdy,

I once had a friend tell me that you use less gas to keep an engine idling for five minutes as opposed to restarting it.

Scenario:

You head to your local grocery store to pick up a case of beer and some frozen pizzas for a good night.
You park your car, keep it idling for 5 minutes while you get the stuff and then leave.

OR

You park your car, stop it, then restart it in 5 minutes when you leave.

Which uses less gas?
[Let's assume it's a nice breezy 70 degree day, so your windows are down and the heat and air are turned off (also, you live in a nice neighborhood, so the wheels won't be jacked whilst in the store)]

Are there any estimates to how much gas your engine uses for an ignition as opposed to a 5 minute idle?
I know this is mostly pointless, but semi-interesting for me - I added a poll to make it more fun.
User avatar
 #241599  by carlr
 
So, after some googling I found this information:

"According to a field experiment by the Florida section of the American Society of Mechanical Engineers, the researchers concluded that restarting a six-cylinder engine-with the air conditioner switched on-uses as much gas as idling the same car for just six seconds.

Idling is similarly wasteful in frigid temperatures. Contrary to popular belief, cold-weather drivers needn't warm up their cars for longer than 30 seconds. The best way to raise an engine's temperature to optimal levels is to drive it almost immediately after startup; according to a study by the Ontario Ministry of Transportation, a car driven for 12 minutes in 14-degree-Fahrenheit weather will achieve the same temperature as one that idles for 30 minutes. (However, it's best to avoid rapid acceleration during that 12-minute warm-up drive.)"

I guess that mostly answers it, what do you guys think about it?
 #241610  by Tarheel
 
The poll was for a 5 min. idle not 6 seconds. :?
User avatar
 #241617  by Bill Putney
 
tarheel wrote:The poll was for a 5 min. idle not 6 seconds. :?
But it answers the question. If it's sitting for more than 6 seconds, it would use less gas to turn it off rather than keep it running. (However, it doesn't sound like the test addressed extra fuel to replenish the battery's charge after a restart - so that probably would move the break-even point out a few more seconds.)
carlr wrote:...(However, it's best to avoid rapid acceleration during that 12-minute warm-up drive.)"...
Yes. I've read that the optimum lubrication properties of motor oil do not kick in until it gets up to something like 160-170°F.
User avatar
 #241624  by grayslater
 
If you know you're going to be idling more than 30 seconds, they say its more economical to restart the vehicle.
 #241628  by Tarheel
 
grayslater wrote:If you know you're going to be idling more than 30 seconds, they say its more economical to restart the vehicle.
That would be every traffic light and many stop signs. :roll:
User avatar
 #241629  by Bill Putney
 
tarheel wrote:
grayslater wrote:If you know you're going to be idling more than 30 seconds, they say its more economical to restart the vehicle.
That would be every traffic light and many stop signs. :roll:
...or a slow drivethru. :)
 #241632  by Tarheel
 
Not that I want one but this is where electric cars are best.
User avatar
 #241658  by 300maximilien
 
Bill Putney wrote:
tarheel wrote:
grayslater wrote:If you know you're going to be idling more than 30 seconds, they say its more economical to restart the vehicle.
That would be every traffic light and many stop signs. :roll:
...or a slow drivethru. :)
Thats probably why the Prius shuts off at lights??
User avatar
 #241678  by Opticon
 
The only scenario in which a vehicle startup uses more fuel than idling is when the engine is dead cold and the ECU stays in open loop until the o2 sensor is up to temp. So shutting a hot car down then restarting it 5 minutes later (still hot) will use less fuel than idling for 5 minutes.