Idling or turning the engine off?

• Automotive
serbring
Hi all,

I am aware that idling is fuel inefficient and a waste of fuel, but I guess that it is not always convenient to turn the engine off, since the turning on requires more energy than idling. So, I think that for long periods, turning the engine off is more fuel efficient than idling, but not for very short periods. Is there any study or paper that reports when it is more efficient to idle than turning off the engine?
Thank you so much.

Best regards,

Pietro

Staff Emeritus
Not an answer, but did you know that some cars shut off and restart automatically at every red light?

russ_watters
serbring
Mentor
...since the turning on requires more energy than idling.
Careful; That statement conflagurates power and energy and is therefore not necessarily true; how much energy you use by idling (and save by shutting off the engine) depends on how long it idles.

I think you will find that starting a car that is already warm does not require much energy, and so the shutdown doesn't need to be very long to save energy.
Is there any study or paper that reports when it is more efficient to idle than turning off the engine?

https://www.edmunds.com/car-reviews/features/do-stop-start-systems-really-save-fuel.html

There are others you can find...

I will add this; the misleading part of the old shop guy's claim you often hear that starting a car uses more energy than idling for X minutes (I've seen 20 via google) is that it isn't starting that burns a lot of fuel, it's warming up. When warming-up, the engine runs rich and maintains fuel flow even when coasting. So shutting down the engine doesn't change that and if anything will reduce the energy consumption from warm-up by making sure the car is only doing its warm-up mode when the car is moving.

I have a telemetry monitor on my car, and the higher fuel flow during warm-up is very noticeable.

Last edited:
Nik_2213, BillTre and berkeman
Gold Member
Not an answer, but did you know that some cars shut off and restart automatically at every red light?
Is that some of the hybrids, or do some of the non-electric do that also now?

Mentor
Is that some of the hybrids, or do some of the non-electric do that also now?
Non-electric - and it's quite a lot of them.

256bits
Gold Member
Is there any study or paper that reports when it is more efficient to idle than turning off the engine?
How could it be more efficient to let the engine idle than turning it off? When it is off, the fuel consumption is zero.

The real challenge that needed to be addressed with stop-start systems was the starting system itself (as explained in the link found in post #4).

That reminds me of when people used to say that it was better to leave a light on that switch it off. It is hard to be more efficient than no energy consumption.

russ_watters
serbring
How could it be more efficient to let the engine idle than turning it off? When it is off, the fuel consumption is zero.

Turnng off lead to a zero l/h consumption, but the restarting lead to an addition fuel and electrical energy than just keeping an idling engine.

I have found this paper that is rather intestering
https://saemobilus.sae.org/content/2004-01-1896

Gold Member
restarting lead to an addition fuel and electrical energy than just keeping an idling engine.
The starting process takes about ½ second. Even if you took twice the amount of fuel during the entire time (Note that a «rich» mixture is only about 15% richer than the stoichiometric ratio; Note also that the average rpm during the starting process should be half the idling rpm), it would still be worth it compare to letting the engine idle more than 1 second.

In addition, taking the energy from an electric starter to turn the engine instead of fuel won't change much the amount of energy spent when recharging the battery with fuel power. That is basically how the Chevrolet Volt works; Use electricity to power the vehicle and charge the battery with a combustion engine.

russ_watters
Mentor
I have found this paper that is rather intestering
https://saemobilus.sae.org/content/2004-01-1896
That link didn't work for me. Could you try to repost please.

And please put some thought into what @jack action said -- really think about the fuel and electrical requirements of starting an engine. There is no good reason why the fuel requirement during that half second should be more than the steady-state idle fuel consumption. And the electrical requirement is just what is required to accelerate the engine up to idle speed.

Gold Member
It shows quite a range of results, extremes of 14% gain in lab tests and 0.7% loss in road tests.
But in road tests where there were no gains, it was estimated that it was because the idle-stop (IS) system did NOT engage, not because starting uses more fuel than idling:
4.4.2.1 Fleet fuel economy performance p. 19 said:
There are several possible reasons for this result, including the heavy usage of A/C that is required in the testing environment. A more likely reason is that because the vehicles were tested in a fleet, the difference in drivers and routes overwhelmed the effects of the IS system. The Volkswagen and Mazda vehicles are manual (while the Smart vehicle is automated manual), and the IS system will not engage unless certain conditions are met, namely that the vehicle is in neutral and the clutch is engaged. If the drivers left the cars in gear or left the clutch disengaged while stopped, the IS systems would not cause the engine to stop, and the FE benefits would not be realized. The drivers were instructed on how the IS systems operated, but it cannot be determined if the instructions were followed.

russ_watters
essenmein
Is that some of the hybrids, or do some of the non-electric do that also now?

Technically start stop systems fall under the "micro hybrid" category, and depending on how they do it could have some actual hybrid capability. Some simpler systems just use the existing starter motor so no regeneration there, but most are P0 configuration, ie connected to the serpentine belt of the engine. These co opt the alternator to be both the starter motor and generator, and they are getting some decent torque boost/regeneration capability in addition to the start stop function, esp 48V systems.

FYI the restarts are pretty fast, the systems will typically position at least one barrel to TDC and then crank+squirt fuel+ignition and its sub 0.1sec re start (warm engine only lol).

256bits
The only cars with this feature I have driven myself were rentals. I was curious as to how the systems hold up over time; I have had older cars that take a lot more than 1/2 seconds cranking to start (even when warm). My other concern was failure to rapidly restart leading to being hit from behind by impatient / inattentive drivers.

If the fuel savings is really 14% that would save a user $200 to$300 a year (in the US) and more elsewhere with much higher gasoline costs. Probably worthwhile to pay for the extra parts and complication. But if it is only a few percent, then I'd rather do without it.

russ_watters
Ketch22
It will take a little bit for me to track the studies down the last few that I have looked at showed that a gasoline engine broke even for energy at a 7 second shutdown and that Diesels broke even at near 10 seconds. There is some justifiable concerns with reduced starter motor life when city driving leads to significant changes in duty cycle.

I will chase down the studies I have looked at (and not filed where I thought) and get some links up soon.

Mentor
It will take a little bit for me to track the studies down the last few that I have looked at showed that a gasoline engine broke even for energy at a 7 second shutdown and that Diesels broke even at near 10 seconds...

I will chase down the studies I have looked at (and not filed where I thought) and get some links up soon.
I would be very interested in seeing the studies; that's a surprisingly long time and would likely be enough to invalidate their use.

Ketch22
Hello again,
There are two studies I located quickly one was published in the Journal of Energy policy in August of 2009. I could not find a good web link but it is visible and downloadable at : https://bit.ly/2wGkrcp
There is also a study by SAE (this is the same one previously referenced) . Their's is available online at http://bit.ly/PzNRzZ
I have not located the others i have looked at however while searching online I also found a good youtube video that actually references these two and does a pretty good job in his presentation.

More possibly later.

Staff Emeritus
There is also a study by SAE (this is the same one previously referenced) . Their's is available online at http://bit.ly/PzNRzZ

Also, we don't like URL shorteners like bit.ly here on PF. We like to see where the link goes before clicking, because of the obvious security risks.

Nik_2213
Nik_2213
As my wife's 'gasoline' car's engine, when cosy-warm, would turn itself off when car halted for more than a few seconds, initially I found this VERY disconcerting. Yes, I still remember that a 'stall' on driving test was usually a 'fail'...
D'uh...
But, halted in traffic, queued for junctions or road-works, among lots of other modern cars doing likewise, the air was significantly quieter and cleaner, with far less fumes being sucked into the aircon...
And, on foot, waiting to cross junctions, I didn't feel that oxygen masks were about to un-spool from above me...

Some of our local buses now do this --Replacement cycles are LONG-- and boarding them at canopied bus station, previously a 'smoke house', is a much less unpleasant experience.

ps: Agreed on the 'no shortened links, please'. My paranoid Norton's distrusts such, for good reason...

russ_watters
Homework Helper
Gold Member
More disconcerting is that the car can turn the engine back on when it detects the cars climate control needs the air conditioning on or that the battery is getting low.

anorlunda and Nik_2213
Mentor
Hello again,
There are two studies I located quickly one was published in the Journal of Energy policy in August of 2009. I could not find a good web link but it is visible and downloadable at : https://bit.ly/2wGkrcp
There is also a study by SAE (this is the same one previously referenced) . Their's is available online at http://bit.ly/PzNRzZ
I have not located the others i have looked at however while searching online I also found a good youtube video that actually references these two and does a pretty good job in his presentation.

More possibly later.

The video is a good summary, but the study results are mixed together. The Energy Policy study is here:
https://pdfs.semanticscholar.org/6bba/b0a43ebaf351f512843d2bd0d4afc3b2c4e1.pdf

It discusses only peoples' beliefs (misconceptions) about idling, not the issue of how much gas it takes to re-start a car. The title is "Costly Myths:..." @jack action it is *exactly* the same as the CFL light myth. It's just as silly and I have no idea what could possibly make people believe something so wrong/illogical.

I will say I did do some counting while driving home from work tonight. The 7 seconds still sounds wrong, but I suppose if the average cycle time of a stoplight is 1 minute, the average time stopped is 30 seconds, so it should still help. But driving patterns will matter a lot. For example, if you are one of those people who stops short and then nudges forward for the next minute [I hate you], that would cause the system to start and stop repeatedly and waste energy. So you have to coast to a full and un-interrupted stop to maximize its use.

Mentor
The only cars with this feature I have driven myself were rentals.
Me too - on vacation in Europe 5 years ago. I've seen a few in luxury cars too, and they are slowly trickling down. When I drove it, it really didn't bother me at all. I'm acutely aware of my idling time and would like my next car to have one. I suspect age plays a role here...
As my wife's 'gasoline' car's engine, when cosy-warm, would turn itself off when car halted for more than a few seconds, initially I found this VERY disconcerting. Yes, I still remember that a 'stall' on driving test was usually a 'fail'...
D'uh...
Yeah - I've read that these systems are very unpopular in the USA and I suspect that's part of the reason why...though conversely standard transmissions are more common in Europe (where these systems are more common) and getting pretty rare in the US. I'm 43 and 3 of my first 4 cars were standards, so I get the disconcerted feeling, but it's something people can get over. Driving a hybrid gives the same weird feeling.

Ketch22
Russ_Waters, Thank you for that. If I stay away from url shortening I cannot bring either to a direct download link. As Russ stated the Energy journal study is about peoples actions and perceptions. It can be searched in Volume 37, Issue 8, pages 2881 -2888 from August of 2009. If interested one can pay their own fees. The SAE research actually deals with the fuel consumption. It is in study # 2004-01-1896. One would also need to pay their own fees. I am sure that in the work cited portions of both additional research can be found some of which is in foreign journals which do not have the same protections as US papers.
I do my own Engine control programming on one of my vehicles. When I look at the injector timing on the fuel map 7 seconds at first glance is pretty close. When in a fully heated state the unloaded idle speed is maintained with a small fuel input. This would be balanced by the slightly larger fuel used in initial start enrichment and the lower initial idle speed that I would attribute to alternator load to recharge the battery from the start. I have not actually crunched numbers to see if the fuel flow actually does create a savings. I know that I have the smallest (read as lightest) starter that will function and so resist turning it off unless I need to.