Idling vs. shutting the engine down: how many seconds make a difference?
by Sebastian Blanco on May 18th 2009 at 12:45PM 
It's easy to calculate that a car with the engine running but not moving is getting zero mpg. New technologies allow some cars to take control of the on/off switch from the driver when stopped. Whatever their name - stop-start, micro hybrid - automatic systems that shut down a vehicle's engine when the car is at rest help boost efficiency. Automakers - especially in Europe - are happily introducing the technology to vehicles like the Fiat 500 to the Audi A4 and A5 to Ford vehicles that will get the new 1.6L EcoBoost four cylinder engine to every Mercedes. But here's a question: just how much fuel does idling waste? While the exact amount is determined by the size of your engine, there is a specific amount of time that most sources say should be the idling limit. How long is that time? Take your best guess in the poll below and then follow us after the jump for the answer.
[Source: Vanderbilt Lawyer, Green Car Advisor. Image from Canada's Idle-Free Zone campaign]
A recent survey by the Vanderbilt University Climate Change Research Network found that, for the whole of the U.S., drivers who let their cars and light trucks idle (at places like drive-thrus, bank lines, driveways, etc.) "account for 17 billion pounds of carbon dioxide emissions each year." In fact, idling makes up about 1.6 percent of America's mobile and stationary pollution, says Green Car Advisor.As for the poll, the correct answer is ten seconds. That's what we found that over on the Vanderbilt page as well as on Treehugger and a bunch of other places.
Reader Comments (Page 1 of 1)
tom engineer 2:15PM (5/18/2009)
more practically, auto-trans users can save gas by placing transmission in Neutral (or P) while idling. This reduces idling fuel consumption by ~20-30% since torque converter no longer puts a load on the engine, w/o risking not being able to start the car, delaying others, or having starter/battery issues down the line. Enjoy.
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BoneHeadOtto 9:37AM (5/19/2009)
or you can drive a manual and have these gas savings all the time, even while braking.
What i want is a car where i can control the start stop. Let me put my manual in neutral and let the clutch out as a means to stop the engine. Then if i hit the brakes, or depress the clutch, the engine should turn back on. In the hands of a decent driver this would save a good amount of gas. And it could be done with no safety risk with a tiny amount of engineering effort
Ghen 4:16PM (5/18/2009)
Good point, I wonder if this study takes into account extra wear and tear on a vehicle starting it back up 30 seconds later. Not just the starter/battery either but all the oil has drained to the bottom of the engine by now as well.
Rick 2:16PM (5/18/2009)
Sorry, but it's too damn hot in Florida to shut down when idling. I need the A/C. I'll try to make it up somewhere else more practical and less intrusive.
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Bip-D-Bo 3:04PM (5/18/2009)
These cars would need AC compressors with electric motors. Otherwise, I wouldn't buy it.
Rick 2:17PM (5/18/2009)
^There you go. I'll put it in neutral from now on.
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Scatter 2:17PM (5/18/2009)
What about the lifetime of the starter motor? If you switch off the motor every time you stop for 10s and you do that, say, 10 times each journey it'll be much more likely to fail prematurely. The stop/start hybrids have beefed up starter motors with higher cycle lives.
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Bip-D-Bo 3:01PM (5/18/2009)
A much more important factor is the life of the engine. Most of the wear on your engine happens during starting. The oil pressure is zero when starting. More importantly, when the engine components change temperature frequently, they are subjected to cyclic fatigue and loose strength. Every component of an engine would need to be re-engineered to assure that longevity was not sacrificed. This re-engineering would result in lower compression, more material, etc, which would result in a larger, heavier engine with less power and less efficiency. Engines also don't work as well or as efficiently as soon as they are sarted because they are not at optimal temperature or lubrication. Also, how would the start/stop system know how long the the engine is to be off? Another point is that the start stop feature only really has a benefit for city drivers. If city drivers want to be green, they should get hybrids, which have higher city mpgs than highway mpgs.
s10 8:24PM (5/18/2009)
The starter motors in hybrids have no problems, so there is no reason a simple start-stop system would be any worse. It is a small electric motor anyway.. not much can go wrong with that.
What Bip-D-Bo is saying is even less of an issue. Of course the engine will run when it needs to get on temperature and all, just like for a hybrid it is the software ans sensors that will make sure it works as it should. Once the engine is on temperature, in normal (almost all) situations, there is no need to reheat or "re-lube" the engine after a short stop. So there is no extra wear on the engine.
James 2:50PM (5/18/2009)
I've been doing this with my Miata for years and have yet to replace the starter motor. Even so, I'd rather give that money to the guy rebuilding starters than Shell or BP. Consider it your small donation to homeland security if you like.
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Chris M 3:08PM (5/18/2009)
The old style starter motors had limited power and torque, they needed to be geared up considerably to be able to get the engine up to starting speed, which is below idling speed. This meant additional fuel was needed to get up to idle speed, and also, the starter had to be disconnected by a solenoid, to avoid over-revving the starter motor at higher engine speeds. So, for cars with old fashioned starters, yes, 10 seconds of idle fuel use would be equal to the fuel used for starting.
Hybrids, and even "start/stop micro-hybrids" have much more powerful high torque starter motors, they don't need to be geared down and can remain connected at full engine speeds. No more troublesome solenoid! Even better, the hybrid starter motor can rev the engine up to full idle speed before applying spark and fuel, that saves gas. So, for hybrids the fuel used for starting would only equal a second or two of idle time.
As for air conditioning, most hybrids have electric air conditioning and don't need to run the engine to keep things cool. The hybrids that still power the A/C from an engine belt will automatically start the engine only when needed to keep things cool.
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John Pelletier 3:19PM (5/18/2009)
i knew the 10 second rule, but for older cars eg from early 90's i think before 2000 usually it is a 30 second timer due to the harder requirements when starting. doing that and some neutral coasting, esp up to lightss i can eek out on average about 10mpg from the base 25 of my 1993 honda accord. there is only a split second delay in our 2005 civic hybrid with the stop-start, so clearly its starter is much more robust.
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Martin 7:30PM (5/19/2009)
When you coast in neutral you're actually wasting gas because your engine has to use gas to idle.
Better to coast in a high gear if you have a manual to minimize engine braking.
H4MM3R 11:13PM (5/18/2009)
engine idling equals fuel spent
engine off equals zero fuel spent
restarting a warm engine uses very very little fuel with todays engines being EFI or DI
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Peter Buxton 2:17PM (5/20/2009)
First off, as exhibited nicely by your marvelous commenters, I would smear Repu--sorry, Rethuglicans. Nothing says "I'm smart!" like getting someone defensive and angry about the idea you're trying to promote.
Tell corporations they must make this technology affordable, easily manufactured and serviced. Fine them 40% of their cash flow... if they *succeed*.
Make sure the technologies you promote have, not simply an Achilles heel, but grotesquely complete, real deficiencies so that the despised IC engine is, fact, clearly superior.
Finally, as part of a broader Oil Strategy:
Outlaw sugar cane grown by darker-skinned races, which can produce alcohol at a 9:1 ratio, in favor of white bread corn, at a dismal 2:1. Besides, alcohol is just an oil extender, which is definitely not the Hollywood ending you would want for a villain such as "Big Oil."
Ignore that we will be importing oil for a long time, because the "Strategic Oil Reserve," with 90 days of oil, is in fact the Tactical Reserve. The Strategic Reserve is under American coastal waters. We will never touch that oil until WWIII.
Stridently demand America's total energy independence while insisting the US needs UN permission to blow its nose (preferably at the same time to highten the cognitive dissonance).
Admit to no one that even Iran imports gasoline. Ignore anyone who tells you that the all countries need trade. (Even hard-liners as paleo-con Pat Buchanan, the old America Firsters and Ronulans never displayed such selectively savage xenophobia.)
Do not talk about reducing the U.S. corporate tax in favor of a 3% consumption tax, which would apply to everybody and everything sold in the U.S. This would encourage exports to make up for the money "lost" paying for oil.
Finally, send a fat ex-political hack (and secret fan of extraordinary rendition) around the planet on a private jet to promote Green Technology. Discover later his home burns crude oil in open pits for heat and light. Try to gloss over the use of (CO2-spewing, manufactured) concrete to insulate said home after outcry.
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