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IN THIS SERIES, TheJournal.ie takes a look at an urban myth, old wives’ tale, or something that your mammy told you years ago to see if there’s any truth in it.
This week’s topic, while not necessarily a myth quite yet, is something which may have caused a few arguments with a back-seat driver in the past.
It could be something that pops into your head whenever you’re stuck in traffic, moving at a snail’s pace, or when you think of saving a few pennies ahead of Budget 2014.
Is it worth switching the engine off while you wait to move another few inches? Or does the car use up more fuel restarting the engine than when idling?
(Image Credit: GERY3dotCOM/YouTube)
Most modern cars automatically switch the engine off if you are stationary, taking the matter out of your hands. They are designed to behave like this, and have special starter motors and batteries which can handle the strain.
For cars without this feature, repeatedly turning on and off the engine cause excess wear to the start motor. It also requires some fuel to restart the engine.
However, according to The AA, this method may actually be able to save a few dribbles of increasingly expensive diesel or petrol — and it will only be a dribble, as a car will only use between “half and three-quarters of a litre of fuel per hour” when idling.
Only when stopped for a minute
They said that you might save fuel by switching your engine on and off in traffic, and without damaging it, but only if you will be stationary “a minute or more”.
“This may give a marginal gain in fuel economy without undue wear on the starter system,” they said.
What about when you’re stuck at traffic lights? That gives another opportunity to save a little bit more. The AA say to avoid coming to a halt in the first place, like this:
“Look well ahead and slow early if lights are red – if you’re still rolling when the lights turn green then you will save a little fuel as it takes more energy to get the car moving from stationary.”
Hat-tip to TheJournal.ie’s Paul “You definitely use more petrol when you restart the engine!” Hosford who suggested this week’s topic.
Is there a myth you’d like debunked? Email nicky@thejournal.ie
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To actually debunk a myth like this you have to do some sort of controlled scientific experiment. Just asking someone from the AA what they “think” isn’t good enough.
Hi Paddy. The AA seem to know a thing or two about cars, so I would say their answer to a myth like this would be quite robust. Thanks for your feedback.
Mythbusters also confirmed this to be true. With tests in a controlled environment.
The best way to save fuel is to turn off the engine and sneak up to the car in front with a 3ft hose pipe – siphon out the fuel and put into a watering can and then when you get home fill up your own fuel tank again. One caveat – make sure the guy behind you has not read this comment
I agree. This is the opinion of someone at the AA. An opinion is not necessarily factual so no myth has been debunked here.
I think it is grossly irresponsible and unfair to ask as recovering alcoholic their opinion on petrol usage. Their time at their meetings could be put to far better use than this
You could also try pulling up behind the apropriate bus, attaching your car with a short tow rope, and turn off the engine.
This way, you can listen to the radio while being driven home for free.
Be careful of passengers crossing behind the bus and use a taxi sign to avoid getting a ticket for driving in the bus lane.
Its debatable if switching off the engine saves fuel, It would depend how long the car is stationary, When the car restarts the engine turns with raw fuel being expelled without being burnt or only partially burnt this is measured as HCPPM or Hydrocarbons parts per million from the exhaust.You can see this measurement on an nct report. if the car is not burning fuel efficiently as you can guess its not good. Also after a restart the car then have extra load on the charging system to replaced the battery power used while at stand still this too increases fuel consumption. This is really used just to help calculate Co2 emissions to make the new car look more environmentally friendly, With 35 years in the motor trade my answer is its depends.
If the AA haven’t carried out, or commissioned scientific testing, then anything they have to say on the matter is purely conjectural. Also, I totally disagree with their advice to slow down ahead of a red light. This is a factor in causing increased traffic congestion at peak times. Drivers must be mindful of road users behind them and to make progress in traffic, without exceeding the speed limit of course.
The best way of saving fuel (and therefore money) is to have no car in the first place. But easier said then done, especially for the people living in the countryside
Doesn’t seem to stop the taxi meter rising when you’re stuck in traffic…
Just do fill ups. Who gives a shit
Couldn’t be arsed to read the whole article, a simple YES or NO could have answered it for me
But time enough to leave a comment? Ah Nigel.
:-)
Don’t worry Nicky, I’ve read a lot of your articles and I enjoy them :-)
Doubt it if Nicky is worried about you
I’m worried about YOU, HamSam.
Nicky, I think it really is time you contacted Mythbusters on this one. Seriously.
Cheers, knew you weren’t worried snout Nigel ;-)
** about
While the AA’s advice to roll slowly up to red traffic lights makes sense in principle, could it not in practice be dangerous, as one would tend to accelerate immediately the light turns green. This eliminates the few seconds delay before driving off that happens if one is fully stopped, and runs the risk of collision with that light jumper, be it driver or pedestrian. All for a dribble of fuel…
If people want to save fuel get of your hole and walk to the corner shop 150 yards down the road instead of driving like I do. And let your kids walk to school too – just like you yourself did.
Or else read this. http://www.theguardian.com/money/2010/may/01/save-800-pounds-year-petrol
How to save £800 a year in fuel when driving. Works too.
Lol
I get more fun out of revving the hell out of it in traffic and picturing Eamonn Ryan crying like the baby jebus
Buy a car wit peddles will save you lots fuel
The self stop/start idea is an emission saver and not a fuel saver. It lowers the emissions by not burning the fuel . A catalytic converter on a car is not operating at idle, it needs the temperatures of driving revs to burn off the harmfull stuff.
This start/stop option is what gives some cars the lower road tax bracket that may not have otherwise got down to.
Money saved there right away !!!!!!
There’s only one way to answer this for certain. You guys need to call in the Mythbusters on this one.
Who ya gonna call!!???
Take the bus. It’s saves a lot of your fuel
Or steal someones else’s car.
That would be naughty.
Syphon the fuel from a bus.
You’ll save a few cents in fuel, but you’ll fork it all back out again plus a lot more for a new starter motor which will die prematurely.
Freewheel downhill where traffic is light and road is safe, change gears lightly, avoid braking, keep RPM around 2000 when cruising, heat your fuel before starting (turn the key to engage and back off several times – especially on cold mornings), allow car to idle for 20 seconds before driving off after first start, allow car to idle for another 20 seconds before turning engine off, keep your speed to 55pmh… all this will gain around 10 miles per gallon on diesels and 5 or 6 mpg on petrol.
* avoid unnecessary braking!
Freewheeling down a hill is lunacy . You will only have idle vacuum on your brakes and if required may not be enough to stop you if the unexpected happens.
Freewheel down along straight and try it, you’ll notice your brakes not as good as normal.
First bit of sense spoken in the freewheel discussion. Absolutely true Paul…
Plus rolling down a hill in neutral actually uses more fuel than if u were gear. Engine uses more fuel to keep itself ticking over.
No it does not… how can an engine idling at 500rpm use more fuel than at 2000rpm labouring?
Can ya explain that a bit more please? If a engine is in gear going down hill, the revs are still high, surely that’s burning fuel too? So which burns more, an idling engine or a revved up engine?
http://www.mpgtips.co.uk/faq/if-i-freewheelcoast-downhill-in-neutral-will-i-save-fuel/
Depends on whether or not you drive an old or a new car actually
Google deceleration cut off. Never free wheel.
The engine idling needs fuel to keep running so coasting needs fuel. A car rolling down a hill in gear will use no fuel as the engine uses the drive of the vehicle to keep running using no fuel…
What keeps the engine running in gear going downhill Graham? fresh air?
Gravity
The car rolls down the hill thanks to gravity. The fact that it is moving downhill means the wheels are driving the engine. Think of it as rolling down a hill on a bicycle with no freewheel. The pedals would turn even though you are not pedaling
No smart-arse,the fact that the car is in gear means that the engine has to keep turning over at that speed therefore using no fuel to keep running…
Going downhill the wheels turn the engine. The engine is doing no work. No fuel is used in a modern car. The instantanious fuel readout reads 0 fuel used like this.
An engine in gear going down a hill will slow the car down, this is the main way trucks us their engine to slow down the heavy load.
The only way besides idle fuel that the engine uses fuel is when the driver pushes the throttle pedal.
The pedal is the supplier.
A lot of misinformation being thrown around today on this subject.
Yes newer cars switch off when rolling in gear and the engine switches back on automatically usually around 1200 rpm or so depending on the car. An older car’s engine doesn’t switch off but still uses less fuel rolling in gear than idling in neutral. When rolling in gear it’s the wheel’s driving the engine that put’s the rpm up not extra fuel going into the system. You get extra fuel in the engine when you put your foot on the accelerator. There is still a small amount of fuel that goes in when rolling in gear but less than idling. Hope this helped.
Nothing it’s switched off. Starts up again when it drops below 1200 rpm. Generally only newer cars tho. Say about 5 years maybe more
Free wheeling does not save fuel. Keeping your car above 1100rpm & in gear with the accelerator fully off shuts off the fuel being injected into the car so using your engine to slow down is good for mpg & avoid unnecessary braking. Try to keep your car rolling at all times especially approaching traffic lights. Tyre pressure is a big help roof boxes or extras on your.car like wide wheels are a big draw back. If traffic is heavy & you time on your hands take a break & let the traffic die down.
Anti stall and idle settings will always make sure the engine has at least idle fuel mix. There is never a no fuel input unless engine stopped.
Paul fuel is completely shut off on overlap if the engine is about 1100 rpm for cars & 700 rpm for lgvs & in gear & the accelerator potentiometer is fully off. Their is no anti stall on most vehicles.
@Mjhint
You’re correct the injectors close on the engine over run,
This is only possible as revs come down with the TPS is in the closed position in a downhill run.
I thought it was a good read. I am in skinflint mode at the moment coming up to Tuesdays budget so it’s good to hear what can and can’t save you money on the things we use daily.
There was once a guy I know who decided to save fuel by turning of the engine when he was going downhill. He did this one day on a hill with a slight bend and when he went to steer around the bend the steering wheel locked and he went straight into the ditch on the side of the road!!!!!!!!!!!
Slowing down before reaching the traffic lights will cause the lights to stay red for longer on most occasions. They won’t turn green until you have driven over or come in range of the sensors. So I wonder do you end up using more fuel because your journey time is lengthened by longer red lights each time? I think it’s definitely a job for myth busters.
Whether it does or it doesn’t, it’s better for the environment and its less noisy, so you can really enjoy Radio Nova, Highway To Hell.
We could save even more if drivers move off immediately when the green light comes on. But no it takes reetArds 20 seconds handbrake down, clutch in, 1st gear, stall. Handbrake wasnt diwn all the way. Stop using handbrake every time you pull to a stop. It’s wrong!
Differrent voice, Different quote…….NONSENSE !
The AA say “roll up to the lights instead of coming to a stop”
That practice would cause mayhem on the roads – totally unsafe and stupid.
How about …1) Walk, …2) Buy a Micra, …3) Get your mates to push your car along.
A micra is one of the more expensive models of car to run because of it’s small engine.
Why is that? Because of the cheaper insurance costs or the cheap spare parts or the lower tax or the light weight design maybe the cheap purchase cost of the car? Only time I can see that it’s not so efficient is on motorway doing the speed limit but all of the above make up for that
Pitch a tent at the top of a hill.
That way, you can roll to work and you’ll only have to drive home.
No more stupid a comment than some of the others here.
Note to Nicky.
When I read:
Debunked: Does turning off your engine save fuel….?
I thought – “Ok – so it doesn’t” and went to the next article – only returning when there was nothing else to read.
If it had said..
Debunked?: Turning off your engine saves you fuel…etc.
Curiosity would’ve had me straight in.
Either way car owners will get screwed over as usual in the budget, car tax, fuel duty, carbon tax, vrt etc will all increase, and we will sit in front of our computers moaning about it, but will still vote for the same robbing f**kers come the next election. When in god’s name did we become such a subservient lot in this country??
According to Nicky AA said; “They said that you might save fuel by switching your engine on and off in traffic”. “might” and “may” being conditional terms mean AA admit that they can’t give a definitive answer in the given scenario. “Debunked” in the title is however an incorrect choice of word, but it does grab a reader’s attention, nice one on that Nicky ;-) Then again we should give out to you over misleading titles :-/
Even Mythbusters “may” have a problem with this one. Every car is different depending on engine, fuel used, lifetime maintenance, design and components. What may be true of one car, may not be the same of another.
For starters, an engine is less efficient when it’s below its normal operational temperature. That’s why start-stop technology needs about 5 mins (far from an exact figure) of driving before it engages. Coupled to start-stop traffic flow, how long will the car be sitting before moving off and how much will the engine temperature drop? Hard to tell usually when coming to a stop. Idling isn’t the most effective method of warming up an engine either. How much engine wear is contributed by switching off and restarting on an already badly worn and poorly maintained engine, further reducing engine compression in the long run of things? What are the chances that an individual in traffic with an engine switched off accelerates harder to catch up with the flow of traffic that suddenly starts to move again and in the process uses extra fuel from harder acceleration than if the engine were already ticking over? How much idling does the car do throughout its lifetime- residues tend to build up with excess idling rather than being “blown out” usually reducing fuel economy.
The variables between engines, environment and conditions, driver habits make the focus of the title a very grey area.
When I was living in Germany the local town put in indicators in the traffic lights to show how long until they turned green. The idea being turn off the engine at red and restart when the traffic light showed it was about to turn green. They were scrapped as it was shown more damage was done to the environment by engines restarting rather then idling.
They should introduce amber light when red is going to green like everywhere in europe
Can’t understand why there isn’t such a light in Ireland.
The Toyota Prius stops automatically BUT because it can then drive for a few minutes at low speeds on its extra battery you do save fuel. I drove a Prius from north London to Holyhead, then Dublin to Cork on £50 worth of fuel. I was doing all the fuel savings things just to see what it would cost. VRT on a say 2009 Prius is €500, car tax about €100/year. Very high spec: 1.8L engine, automatic, cruise control, air con, good radio/CD system, roomy etc..
Get the Bus!
Read above, The bus is out of fuel. Rolly ‘n me took it.
And some thick tied his micra to the back of the other one.
Yeah Ghostbusters are great.
Ah no! Liam O’Broin, you’re way ahead of me!
Time to call in Mythbusters (unless they’ve done this topic already). I feel a programme coming on!
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