That doesn’t sound like the best way to phrase the question, but anyhoo- I was once told (by someone with very little automotive knowledge) that if you are desperately low on gas, the last thing you should do is shut it off, because of the amount it takes to restart it (that is, if you’ll need to restart before getting to some gas). Does anyone know if this is accurate, and if so, what a rough estimate for the amount you could travel at average speed with the amount you use during starting up?
With pretty much every car being fuel injected these days it really doesn’t take much gas at all to start a car.
The old rule of thumb was that if your parked in your car somewhere you should just leave it running because it will use more gas than it will save if you shut it off and restart it.
The new rule is that if your going to be sitting in you car for more than 5-10 minutes you should shut it off to save on gas.
5- 10 minutes? I think it’s more like 20 seconds…
Fuel injection systems will fatten up the injection time (make it longer) during starting, and the system will run in open loop (not paying attention to the oxygen sensor feedback system) until certain parameters are met. Also if the engine is allowed to cool off, more fuel will be required during cold start and warm up.
So the question becomes, how long are you leaving the engine off, and was the engine fully warm when you shut it off?
If you are talking about a fully warmed up engine, and shut off for for less than say 15 minutes. The difference would be minimal. Say 30 seconds to 1 minute of idling. If however you are talking about going for gas tonight with a fully warmed up car, or parking the car overnight in sub zero weather and then trying to make it to the station in the morning. Under those conditions I think you might find your ass in Reebok mode the next morning.
On preview: crazyjoe I agree. 5- 10 minutes is what used to be the rule of thumb on old carburetor cars.
Probably true but I don’t think you use that much gas while idling. And it would seem to be a pain to have everyone stoping and starting their cars in a drive-thru whenever they were sitting for more than 20 seconds.
You don’t use that much gas idling, but you use a hell of a lot more of it idling for 10 minutes than you do restarting a modern car.
True. Aslo, while the engine is running under normal load the rotating crankshaft keeps a small viscoelastic layer of oil that physically seperates them from the bearing surfaces, minimizing bearing wear. But when the engine stops, the shaft rests directly on the bearings, and when you start up there is a small amount of shearing stress that results in bearing wear. So while shutting off your engine might save a little bit of fuel (although the typical modern passenger engine at idle sips very slowly) but it shortens engine lifetime and reduces mechanical efficiency as bearings wear (which causes other parts to wear prematurely). As a general rule of thumb I’d say that it is better to leave an engine idling for five minutes than to turn it off, but for a longer wait I’d just turn off the car.
Stranger
No actual cite on this, but I was told that on my last car (a Mazda RX-8), it took a large amount of fuel (say nearly a litre) to start the engine from cold, thanks to the Euro IV emissions standard which means that the catalytic converter has to be up to operating temperature within (I think) one minute. Basically to get this to happen, the engine would run incredibly rich and at a fast idle for the first minute or two. That car drank gallons of fuel anyway (20mpg on a good day, part of the reason I got rid of it – bear in mind that a US gallon of fuel costs over £3.50 here, more than $7 at current exchange rates) so I can well believe it…
Twenty seconds is about right. I posted some figures in an old thread about idling; depending on who you listen to, starting your car takes as much fuel as 10 seconds to thirty seconds of idling.
Idling also uses somewhere between 1/5 to 1/10 of the fuel you use at 60mph. A bit of calculation shows that starting your car uses somewhere in the neighborhood of 1sec - 6secs worth of fuel at 60 mph (i.e., 1/60 - 1/10 of a mile travelled), so that would be approximately what the correspondance of that the OP asked for
How does one measure fuel use in seconds?
What do you mean?
If you’re asking exactly how one would measure the amount of fuel used in a few seconds, there are a lot of ways. The most direct would involve a dyno test cell and a fuel flowmeter. On an automobile in your driveway, the easiest way I can think of is to fill up the tank, run the car at a constant rate for a few hours, and refill the tank. Divide fuel amount as necessary.
If you’re asking why fuel use is measured in the unit “seconds,” it’s not. It’s measured in (fuel quantity)/second, or total quantity in X seconds.
Or ae you asking something else?
I have heard not to shut off the car if your alternator is not working because you will use much more electric power from the battery then keeping it running.
As for shutting off the car for gas savings, you have to remember that it’s not just the amount of fuel required to start the engine, but the extra load on the alternator that has to replace the power taken from the battery - this extra load takes a while to replace and cost gas.