Help Forum

Help requested posted on 5th August 2016:

Engine idling and costs

Found this on a poster by Aberdeen City Council.
"It costs you money - every 10 minutes of vehicle idling costs at least one-tenth of a litre in wasted fuel."

Does anyone have figures to confirm this and how it can be verified? Trying to address vehicle emissions/air quality with some costs to the general public idling engines of private vehicles! Thank you in anticipation.

Peter Wilson

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Response posted on 7th August 2016 by:
Roy Brocklebank

E: roybrocklebank@talktalk.net
T:

Fuel at idling

I don't have any empirical data but last year we inadvertently got caught up in the holiday rush for the south of France. We were planning a 400 mile autoroute journey from Zeebrugge to Germany. Normally my diesel will do about 6.8 l/100km on this journey it was 10.1 l/100km and that over the whole journey not the 2 hours getting through the Lille interchange.

Parts of the journey were at normal high speeds and in Lille I often switched off my engine.

To go back to the OP, they say that in one hour of idling you will use an additional 0.6 litres of fuel. If driving at 60 mph for one hour I use 6 litres or perhaps 9 and a lower speed, then you are looking at just 10-15% greater consumption from idling. My empirical figures suggest this a gross underestimate.


Response posted on 7th August 2016 by:
Roy Brocklebank

E: roybrocklebank@talktalk.net
T:

Costs Fall

The other problem on idling fuel consumption is where costs fall. If a council could eliminate stop-start traffic flows, avoid often lengthy one-way systems, and provide pedestrian bridges and walkways, this would reduce consumption and pollution. Costs fall on the council, ie rate payers.

If, OTOH, the council retain traditional calming and control measures, impose lengthy weighting times on motorists then extra fuel is burnt, there is more pollution, and costs fall on the motorist.

Encouraging motorists to switch off rather than idling, or using increasingly common stop start devices can impose additional costs on replacement starter motors and batteries.

I fitted a stop-start device 40 years ago. It worked but with relatively light traffic on the A96 my stop periods were very short. Frequently the engine would stop at precisely the moment I wished to go :(


Response posted on 7th August 2016 by:
Martin evans

E: martin@verodrive.co.uk
T:

Idling fuel

On cars with an mpg gauge, when you are stopped they often show consumption in litres or gallons per hour. It's a simple matter to calculate 10 mins worth.


Response posted on 8th August 2016 by:
Roy Brocklebank

E: roybrocklebank@talktalk.net
T:

10 minutes worth

Martin, not quite that simple. It is an extra 10%. All 10 minutes idling will show is 10 minutes worth of consumption. How you then calculate that this is an extra 10% is another matter.

Take the same vehicle and drive it for 10 minutes ensuring no idling, then you have a base line. Except that that 10 minutes is valid only for a particular speed and conditions at the time, is slowing and accelerating.

You would really need controlled tests with a prescribed set of conditions and ideally more than one vehicle.


Response posted on 8th August 2016 by:
Katherine Duncan

E: kathduncan@aberdeencity.gov.uk
T:

Engine idling and costs

Hi Peter,

The leaflet was produced back in 2011/12. I wasn't involved but a colleague was saying he thinks that the figures came from Canadian research if that's any help.

http://www.nrcan.gc.ca/energy/efficiency/communities-infrastructure/transportation/idling/4459


Response posted on 8th August 2016 by:
Roy Brocklebank

E: roybrocklebank@talktalk.net
T:

Per litre

I looked at Kath's link. What jumps out is that the tenth of a litre of wasted fuel is actually:

1/10th litre per litre of engine size.

ie 1/5th for a 2 litre
0.3l for a 3 litre etc.

Referring to my experience in Lille, and assuming idling for 2 hours, that would be 3.6l on my 2l diesel. My experience was the extra burn was far higher than that.


Response posted on 10th August 2016 by:
Neil Greig

E: neil.greig@iam.org.uk
T:

Engine idling and costs

The amount of fuel used by a car which is idling is dependent on the engine, fuel and technology. Stop/start systems eliminate the problem for all practical purposes (provided people don't turn the stop-start system off).

For those without stop/start systems, the fuel wasted when idling is simplest to work out for petrol engines because they have to have air and fuel in the ratio of about 15 parts of air to one part of petrol, otherwise the petrol won't burn properly.
When we talk about a 1 litre or 2 litre engine, we're referring to the amount of air it draws in for every cycle of combustion that's every two engine revolutions for a four stroke engine. However, we control the engine power by restricting the amount of air being drawn in (by something called the throttle; it is literally throttling the engine starving it of air). When a petrol engine is idling, the throttle only allows the engine to draw in about a quarter of its maximum capacity. A 1 litre engine idling at 700 revs per minute has 350 working cycles per minute, so it draws in 350/4, or 87.5 litres of air per minute. Now, the 15:1 ratio is by weight (mass). 87.5 litres of air weigh 107 grams (at sea level); 107 grams of air will need 107/15 grams of petrol which is about 7 grams; 7 grams of petrol is about one hundredth of a litre.
So, a 1 litre petrol engine idling at 700 rpm will burn roughly a hundredth of a litre of petrol every minute, which means that in ten minutes it will burn a tenth of a litre, confirming the figure you have seen reported. The size of the engine is important a bigger engine draws in more air and so burns more petrol, but as a good approximation, if you divide the engine size by 10, that's the amount of fuel it will burn if it is idling for 10 minutes, so a 2 litre engine will burn 2 tenths of a litre and so on.
Diesel engines work differently, and you can't work it out in the same way, but burning a tenth of the engine capacity in fuel every ten minutes should be a fairly realistic approximation for them as well

Tim Shallcross

Head of Technical Policy and Advice
IAM Roadsmart

Phone: switchboard 020 8996 9600 direct 01267 267631 mobile 07968 701905


Response posted on 11th August 2016 by:
Martin Evans

E: martin@verodrive.co.uk
T:

Idling fuel

Kath...apologies if we are talking at cross purposes. I assumed by 'idling' the vehicle is stationery (in traffic) and the alternative is to switch off. In that scenario, the fuel used idling is ALL extra as the vehicle isn't moving.

Strictly speaking there is a small amount of extra fuel used to restart the engine and that would need to be taken into account for accuracy.


Response posted on 11th August 2016 by:
Dave Harris

E: dthonwheels@gmail.com
T:

Engine Idling

Hello

Try Chichester District Council who considered local regulations some 10 years ago.

Regards


Response posted on 11th August 2016 by:
Peter Wilson

E: pwilson@westminster.gov.uk
T:

Engine idling

Thank you to all who replied or contacted me directly.
I should have stated that the concern is parked vehicles ie outside schools and small delivery vans leaving the engine running. We have legislation to fine for this but are using a softly softly approach initially by asking drivers to switch off and want to give our staff and those parked an idea of what it costs them in terms of cash as weel as the air quality issue.
That said all your responses are very welcome and consistant with what we now know.All I now need is a calculator.
Thank you once again.


Response posted on 11th August 2016 by:
Edward Handley

E: edward@wrrsconsultancy.co.uk
T:

Engine Idling

No precise data, but I was given a lift in a Mazda 6 which had the stop-start system and a counter on the instrument panel that recorded cumulatively how long the engine had been switched off by the system. The car had done around 15000 mile and the counter recorded 16.5 hours. I was quite surprised by this, as 16.5 hours of ticking over represents and lot of fuel, and a lot of unnecessary pollution. I suspect that for an "average" motorist a saving of over 1 hour ticking over per 1000 miles driven would not be far off the mark. I am sure Mazda and some of the other manufacturers would have more precise data and even if there is no counter on the panel the data could be extracted from the engine management system.


Response posted on 14th August 2016 by:
andy garden

E: blueacorns@gmail.com
T:

Engine Idling

The OP is looking at the domestic driver who is currently paying about 1.10 per litre of fuel which would equate to about 1 0pence per 0.1litre
If the OP example of 10 minutes idling uses about 0.1 litre, then the simple comment would be a cost of about one pence of fuel per minute idling or about 60 pence per hour.
As for the emissions, one would have to be careful as to engine type , hot/cold before quoting figures. We know idling is not a good thing but for the average driver with a non-stop/start engine, who also need to keep the climate control operating to avoid fatigue etc, it remains a difficult concept to create a change of driving style. Certainly not a reason for the average motorist / company to make an early change of vehicle to one with stop/start technology.


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