Idling or turning off the engine? 3

[pietro82]

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

 

[3DDave]

Seems like a good research project. Have you any parameters? For example, in emergency vehicles, they are kept idling because that way they cannot fail to start, which could happen if they were shut off. They can also provide heating and cooling to the occupants. But without knowing why one is concerned it's tough to make a suggestion.

As a practical matter, with fuel injection, the amount of fuel to recharge the battery after starting is really small compared to the overall use in a motor vehicle, which I presume is also a parameter - the actual use for the motor.

 

[VEBill]

Years ago I looked into this question. I was amazed to discover that my car (at the time) required a significant fraction of the fuel flow just idling, as compared to driving gently on the highway. For example, if a car is 8L/100km on the highway at 100 kmh, that's 8L per hour. For my example (at the time), the idling fuel consumption was as much as several L per hour.

So I wouldn't leave my car running to "warm up" (except in the most extreme exceptions). But I'm not going to manually turn it off at red lights. I would consider switching off in traffic jams.

Diesels are generally much better, requiring very little flow to idle.

Many modern cars have Start/Stop systems, because they can save fuel. They're annoying, but can save fuel and reduced CO2 emissions.

 

[pietro82]

Thanks to both of you for the answer.

@3DDave
I know that many vehicles have to idle. I am thinking to the sleep trucks, that idle for 6 hours per day because people sleep overnight. But, my question is vehicle independent. In a website, I have found that if the stop is longer than 10seconds is more convenient to turn the engine off. However, there is no reference, so I do not have any clue to assess the data reliability.

 

[3DDave]

Plus you have wear on the starter switch, the starter relay, and the starter motor to account for; possibly made up for by using far more expensive components if frequent starting is desired.

Your question is very vehicle dependent as there's no universal law that governs the tradeoffs.

 

[Hoxton]

I guess you are in the USA?

I am in Europe, currently, and a lot of cars have auto stop start of engines at standstill. So if the battery is charged, and you stop the car at a junction, then the engine stops, and starts again when the clutch is depressed.

I suppose this is because the emissions limits mist are in grams per kilometer, on a given duty cycle, and stopping the engine gives an emissions benefit.

 

[VEBill]

"...stopping the engine gives an emissions benefit."

CO2 perhaps.

But a hot catalytic converter reduces traditional air pollutants to extremely low levels.

As opposed to letting it cool off and then re-starting while cold. Which can amount to a lot of the air pollution.

 

[malbeare]

My brother has a newer ford with this feature but the human factor . he has the option of disabling that function so he does as his words it is so annoying and in 40 degree day the air conditioning isnt happy for him .

A tidy mind not intelligent as it ignors the random opportunities of total chaos. Thats my excuse anyway
Malbeare

http://www.sixstroke.com

 

[GregLocock]

I've got a new Escape that has Autostop. It also tells me it uses 1.4 litres per hour when idling. So in hot weather I can save 3 cents for every minute the engine is off (woohoo) and the a/c is not running. Except of course when the engine starts again the a/c has to come in with a mighty roar to get the temperature back down. So I'd imagine the fuel savings are rather less than 3c/minute in reality. I think you can guess where I'm going with this.



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[CWB1]

I cant say I've had issues with a/c in any of the various vehicles I've had with start-stop, usually the vehicle restarts before the air blows warm regardless of driver inputs.

As mentioned, starting an engine requires relatively little low-cost electricity when compared to burning fuel at idle. It also puts out fewer emissions and isnt as hard on the engine. Win, win, win.

 

[BigClive]

As is well known - SI engines with load control by throttle plate suffer badly from maximum pumping losses at idle - they use a surprising amount of fuel at idle.

I have seen testing of an engine with continuously variable valve opening duration with the idling controlled mostly by LIVC and the fuel saving at idle was a bit startling - like about 40%.

There is (or was) a video of it being demonstrated on the internet somewhere.

 

[malbeare]

CWB1,
here in australia temperature of 40+ and tarmac at 70+ you dont want the Ac to cut out for 3 minutes at the stop lights


A tidy mind not intelligent as it ignors the random opportunities of total chaos. Thats my excuse anyway
Malbeare

http://www.sixstroke.com

 

[Deividas]

Very interesting video about this topic

https://youtu.be/dFImHhNwbJo

 

[malbeare]

Link the ozy view ?

A tidy mind not intelligent as it ignors the random opportunities of total chaos. Thats my excuse anyway
Malbeare

http://www.sixstroke.com

 

[romke]

Whether start/stop driving is efficient will depend on the driving conditions. Originally start/stop systems were designed to prevent pollution when waiting for a traffic light in buildup areas. That it also can reduce fuel consumption and CO2 emissions now is a added bonus.

If the car is equipped with a start/stop system then most likely the starter motor is designed in such a way that it can live with the far more frequent starts. A car without a start/stop system may well experience premature starter motor failure, because the starter motor will be operated far more then the designer anticipated.

I drive a Lexus hybrid that very frequently changes between engine and motor drive. The problem mentioned with AC in warm climates is overcome because the AC compressor is electrically driven, also the powersteering is electric. With a non-hybrid car in warm climates the temporarily stop of the AC may be a disadvantage - depending on the length of the stop.

At the moment some manufacturers experience problems with excessive camshaft wear on start/stop vehicles that are frequently operated in cold weather. A thinner oil might help prevent that to a certain extend. Both the carindustry and oil formulators are investigating what can and needs to be done about it. That might well result in future engine oil specifications that include a specific test to check what happens when operating the start/stop system under sustained driving in low temperatures in such a way that the oil will not reach the foreseen operating temperature. The problem with driving under those conditions is that the thermostatically controlled cooling system may reach the operating temperature quite fast but that the oil may need more then 20 minutes to reach a decent operating temperature - without the driver noticing because very few vehicles are equipped with a oil temperature gauge.

 

[pietro82]

@Deividas,

That is what I was looking for! Data reported on a scientific paper. Thanks

 

[dgallup]

UPS, one of the biggest fleets of delivery vehicles in the US has started shutting off all their diesel vehicles every time they stop to make a delivery. I'm sure they studied the economics down to last detail. Gasoline engines obviously take much less time to make shutting down cost effective.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[CWB1]

malbeare, I would have to ask around to confirm but believe there is commonly an algorithm to control restart based on cabin temperature for vehicles that don't have an electric a/c compressor.

 

[malbeare]

thanks CWB1.
I think it would be good to combat the wear factor in engines with stop start that there should be a small electric oil pump to keep the oil flow whilst the engine is stopped . and also electric water pump for a similar purpose together with electric fan . maybe an accumulator for the oil that slowly keeps small oil flow for 3 minutes

A tidy mind not intelligent as it ignors the random opportunities of total chaos. Thats my excuse anyway
Malbeare

http://www.sixstroke.com

 

[SomptingGuy]

I'm cynical about the stop/start on my car. Not convinced that the money saved in fuel not wasted is worth more than the cost of replacing the bits that will fail prematurely as a result of the additional cycling. Or the anxiety it gives me when I'm surrounded by other cars at a junction and my engine stops. I've had starter motors fail on me in the past, but I've usually been somewhere safe, like at home or at my destination.


Steve