Where as if I were to take it out of gear and allow the engine to idle I imagine I would get noticably better economy.>>
You will not be in full control of your vehicle.
If you wish to maintain the same speed downhill as on the flat, select the appropriate gear to do so - being out of gear could mean that one day you might find yourself relying only on your brakes to stop you hitting that solid stationary object.
If your engine stalled because you messed up trying to get back into gear then it could be an equally unhappy outcome..:-)
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I've read these threads a couple of times, but never come across the statement before, that fuel is cut-off from the engine on over-run! But because I'm "running-in" my new 2008 motor (1.6 Diesel HDI) I'm not "coasting" up to traffic lights if it's obvious they will be on red for some time, as I used to do & always have done in my other motors that I've owned in the past. I've never seen the point of racing up to a red light.
I'm a "Time served" Motor Mechanic, but have been out of the trade for a hulluva long time, now in Marine Engineering. So these engine management systems & plug in diagnostic fault finders etc. etc. are a no-brainer to me. So my question is, how does this fuel cut-off system work on over-run?
If neutral is selected whilst moving at any speed (for the sake of the discussion, forget the safety angle) the engine will go to tick-over, so fuel, has to be being injected otherwise we stall. But on over-run, I'm reading that fuel is not being injected. Which means, if I'm reading this correctly, that at any given speed, even m/way speeds, once the accelerator pedal is released, fuel stops flowing? I find that very hard to accept.
But if that is the case, at what speed does the fuel decide to flow again, when you consider that you could go on over-run from whatever speed, to a complete stop, but obviously the engine needs to maintain tick-over.
Really looking forward to the definitive answer on this one.
BeeJay
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As I stated earlier, my 1.6 petrol VW Bora will trickle along in first or second gear quite smoothly with my foot off the accelerator - it wouldn't do that without fuel getting through...:-)
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As I stated earlier my 1.6 petrol VW Bora will trickle along in first or second gear quite smoothly with my foot off the accelerator - it wouldn't do that without fuel getting through...:-)
Because you are'trickling along' you are not on the over run,so of course fuel is required.
Years ago a kit was available to fit carburettors with anti run on valves,i.e. a valve in the idle circuit which had to have 12 volts applied to it to remain open and allow fuel to flow.The kit used a vacuum sensor to determine when the engine was in the over run state and would interupt the 12 volt supply to the anti run on valve,thus saving fuel.
Modern cars do the same thing,possibly a bit more sophisticated.So,coasting in a modern car will use more fuel,coasting in a carburetted car will save fuel but loses some control.
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I tried coasting down a long (mile plus) gradient near my home with top gear engaged and my foot off the pedals and the trip computer read 99.99 mpg as the instant consumption figure.
I repeated the exercise in neutral, with the same entry speed of 40 mph and again the computer read 99.99 mpg.
The only difference was that my exit speed (hence momentum) was rather higher in neutral with no engine braking so do I conclude that to coast in neutral is more efficient? Or was the computer lying to me?
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It may have been. There may be no figure higher than 99.9mpg on the readout. The symbol for infinity might puzzle some drivers.
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In a very unscientific experiment carried out yesterday on a nearby hill, my Citroen allegedly does 471 mpg in neutral, and 999 mpg when in fifth gear but with no pressure on the accelerator. Going back up the hill flat out in second gear it does 11 mpg. All figures were provided by the trip computer.
Therefore I conclude that it is more efficient to go down the hill in fifth gear then neutral, though I can't quite comprehend why this should be the case.
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I tried coasting down a long (mile plus) gradient near my home with top gear engaged and my foot off the pedals
That's not coasting.
I repeated the exercise in neutral
That's coasting.
do I conclude that to coast in neutral is more efficient?
Runing down in gear should use no fuel at all, whereas coasting uses some fuel to maintain tickover.
If you could usefully use the extra speed gained by coasting then it may well be more efficient as the amount of fule used to maintain tickover would be minscule. However coasting is bad driving practice and is of course illegal.
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>>
However coasting is bad driving practice and is of course illegal.
Unless of course you are driving a car that has a free-wheel device, in which case it is legal, saves fuel, and is presumably therefore good driving practice.
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'So there!' one is tempted to add.
It's none of anyone else's business if a competent driver chooses to freewheel. It isn't necessarily dangerous and it can be economical.
Not a good idea to turn the engine off in cars with assisted brakes and steering though.
:o}
Edited by Lud on 06/07/2008 at 17:36
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I recall my Dad's Rover 90 had optional freewheel,I think there was a large knurled wheel under the dash to switch between engine braking and freewheel.Having tried it in freewheel mode once he decided it was downright dangerous,a conclusion Rover came to,as the option was soon deleted.
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>>Having tried it in freewheel mode once he decided it was downright dangerous
Thank you.
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Saab used to have the feature too. Works best on empty roads though. Wouldn't be much use on today's mimser-infested A roads.
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I had Rover 16,75 with frewheel. I garee it was very danderous as the loss of control on downroads was quite noticeable.. and especially those heavy cars with drum brakes down steep hills> Brake fade is not nice and is very smelly.
As a result I never coast - ever...
The prospect of the engine stopping and losing all servo assistance to the brakes and power sterring is too awful to contemplate.
(As anyone who has had to stop a car if the servo fails will know).
Partiicularly true in diesels with engine driven vacuum pumps.
Fuel injection shutoff on overrun was introduced in the early 1970s with BMW if I recall correctly.
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I tried coasting down a long (mile plus) gradient near my home with top gear engaged and my foot off the pedals
REPLY That's not coasting.
You are right of course.
>>However coasting is bad driving practice and is of course illegal.
Bad practice perhaps, but is it really illegal?
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Really looking forward to the definitive answer on this one.
First EFi car I had back in 1990 was a 1988 Rover 216 Vitesse with the 1.6 O-Series engine (ALA montego). The Fuel was cut off on overrun triggered, by the zero signal from the Throttle potentiometer, providing the engine rpm was above 1200. When the rpm dropped back down to 1200, the fuel injection cut back in. So it was more economical to actually change down a gear at low speeds when taking the foot off the throttle, so that the rpm was over 1200.
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>>Really looking forward to the definitive answer on this one.
It's impossible to be definitive, as each engine management system is programmed differently.
For example figures, overrun fuel cut off might work anywhere above, say, 1400 - 1500 rpm, with fuelling being switched back on again as the engine drops below, say, 1100 rpm. Rather than the exact figures, which vary from car to car, the main point of note is that there is a difference between the fuel cut off speed, and the fuel cut on speed to prevent the engine dithering if you happen to be at that set point.
As far as no fuel on the overrun being a difficult concept, it isn't as though the engine can immediately stop when fuel is cut off - especially in a high gear, the inertia of the car will continue to turn the engine, and if the cut off/on sequence is well programmed, the driver will feel no jerks or bangs.
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Coasting - take out of gear, no point in sitting there with the clutch depressed.
Loss of control - I dont see how coasting causes a loss of control?
Coasting with the engine off - dangerous - steering locks, servo assistance etc
Efficiency - depends on the conditions. If you will be coming to a stop then leave in gear and allow engine braking to help stop the car. General consenus is that the fuel will be cut off to the engine. If you are not coming to a stop then the extra momentum gained down an incline will allow you to travel further, so in certain circumstances coasting is much more efficent.
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General consenus is that the fuel will be cut off to the engine.
Hmmmm! Had a word with the service manager at my local Citroen dealership this morning, & asked the question.
His reply was in favour of coasting against over-run, if approaching a red light etc.
When I asked about the fuel being cut off when applying an over-run situation, he said that there is a slight reduction in fuel supplied to the engine, but that only occurs at relatively higher speeds than you would expect in town driving when the acc pedal is released. At town driving speeds is just doesn't occur.
Swings & roundabouts, I say. ;)
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If taking your foot off the accelerator really does cut the fuel supply to nil, then surely that is like saying the engine has stopped running, ie firing? When it restarts again, why isn't there a jerk, as when bump-starting?
I could understand that the supply of fuel might be cut to an absolute minimum on overrun, but nil?
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the engine doesn't stop - it's still being turned over, hence no jerk. General thinking is that the fuel is only cut off when over 2k rpm, any lower and I guess the engine could stall - which would then cuase a jerk.
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"Never coast in neutral; idling uses more fuel than running in gear on a closed throttle."
from
tinyurl.com/5cemm9
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the engine doesn't stop - it's still being turned over hence no jerk. General thinking is that the fuel is only cut off when over 2k rpm any lower and I guess the engine could stall - which would then cuase a jerk.
I realise it is being turned over - so is an engine on the starter motor before it has fired, and so is a car being bump started.
My point was, it is turning over but it can't be firing because there is no fuel. So why is there no jerk when the fuel supply is restored and the engine starts firing again?
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I would guess its becuase of the higher rpms?
A starter motor wont crank an engine over at 2k rpm.
When bump starting a car you are normally applying throttle, so when the engine fires it has a bit more kick.
Imagine bump starting a car at say 20mph with your foot off the throttle. Other than the jerk from when you put it into gear, i would imagine it would be rather smooth.
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Totally unscientific I know but what with the recent fuel increases and all I changed my driving technique whereby I look for every opportunity to "coast" on the overrun in top gear, try to brake as little as possible and accelerate quite briskly to my required speed.
Typically on an A road I will accelerate to 60mph then coast in top to about 50, then accelerate again and so on. Done properly it can be achieved with an acceptable level of smoothnes. It is possible to do this to and from work where the traffic is light. I am not so inconsiderate as to do this when someone is following me, choosing instead to maintain a steady 55-60mph on the smallest possible throtle opening.
The result is that the economy on my 1.8 litre Laguna has increased from 30mpg to 35mpg. Not to be sneezed at.
Edited by stackman on 07/07/2008 at 17:40
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Excellent stackman. Spot on.
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So do you find then, stackman, that accelerating to 60, closing throttle to 50, then accelerating again to 60 is more fuel efficient than maintaining a steady 57ish? Do you have cruise control and do you use it when maintaining a constant speed?
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One of the ways to reduce any jerk upon the re-admission of fuel is to retard the ignition, thus reducing the engine's capability to produce torque. Then, after the fuel is switched on again, the ignition timing is advanced again over a few hundred engine revs.
Yes, the fuel is fully cut off, the injectors go completely dead. No fuel at all is injected on overrun fuel cut off.
If you want to see what's going on with your injectors (petrol engines only!), why not tempoarily wire an LED in, and run it up to the dashboard. Then, you'll be able to see when there's no fuel being injected, if your car doesn't have a computer.
Engines do feel rough when being cranked or bump started because they are running at slow speed, among other reasons, because they are vibrating at too low a frequency for the engine mountings to be effective vibration isolators - during overrun cut off, this isn't an issue.
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So do you find then stackman that accelerating to 60 closing throttle to 50 then accelerating again to 60 is more fuel efficient than maintaining a steady 57ish?
That's exactly how the extreme fuel economy guys (the one that do 2000MPG+ in torpedo shaped carts on bicycal tyres) drive.
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Except that they tune their engines very lean and rough, and turn them off altogether (or did in the old carburettor days) until they were doing 15 or 20 mph, then drive at full throttle in top gear to 50 or so, engine off, etc. etc.
Total nightmare.
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Judging by the figures on the instantaneous fuel consumption display then the dash and coast approach seems more economical.
Due to an electrical glitch my trip computer displays only in metric measurements so I work in litres per hundred kilometers !
Accelerating back to sixty sees the display read about 12l/100km, then down to zero while I "coast" but maintaining a steady 57mph gives a continual figure of 8-9 l/100km.
I would estimate that the ratio of accerating to coasting would be about 1:3 or 1:4.
I don't have cruise control, just use the lightest touch on the throttle pedal.
I must say that since adopting this technique I find it more satisfying than when in my youth I would try to get everywhere as quickly as possible. Interestingly journey times have not been affected. I do now have a searing resentment for any uphill gradients as this means I have to put my foot down !
I'm not sure who said it but I do recall a quote which says that accerating uses fuel, braking wastes it.
Edited by stackman on 08/07/2008 at 10:59
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I do now have a searing resentment for any uphill gradients as this means I have to put my foot down !
How do you feel about mimsers who brake on downhill stretches to keep their speed down to 45 on a NSL A road when you can see the hill opposite and no hazards, just enough traffic coming the other way to prevent an overtake?
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After reading this thread yesterday, I too tried a similar approach to stackman on my run to work this morning. The MPG on my 1.6 Seat Toledo (according to the trip computer) went from the usual disappointing 28 to 39! Incredible! I had been blaming the car for dreadful fuel economy (I only got 275 miles out of my last tank, from full to the first warning light coming on), but all along it's been my fault.
Edited by Alanovich on 08/07/2008 at 10:38
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Did a 420 mile round trip yesterday. Outbound 210 miles I caned it because I was late and left the AC on because I didn't want to look sweaty and dishevelled on arrival. Homebound I drove like a nun and with the AC off. Set the fuel trip before each leg. 46 mpg outbound 46.8 homebound. Hmmm.
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46 mpg outbound
I'd love to know what sort of car you have that you can "cane" (and with the a/c on) and still get 46MPG?
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I'm getting intrigued by this now. Suppose you're slowing to a halt over a long straight, and you've been delicate with the throttle so that there's no need to brake and you can just use engine braking to slow the car. You leave the car engaged in 5th gear and close the throttle. The car gradually slows, but at (say) 2000rpm fuel is injected again to prevent stalling. Is it then more fuel efficient to change down a gear to increase revs so that fuel is cut off again? Is the best way to stop by changing progressively down through the gears as the revs drop off?
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The answer to that has to be yes, but the difference would be miniscule, as changing down on a level road each time the revs dropped would bring the car to stop pretty quickly.
If you coasted, with the engine ticking over, or even left the car with 5th engaged until the juddering became unbearable, you'd use some fuel but the car would have travelled further.
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Report in today's Telegraph "Five ways to make your fuel go further":
!) "By building up your speed gradually and coasting towards a red light".
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one thing I've learnt form this thread is that driving like a nun will not improve your fuel enconomy. I guess nuns drive badly and should take up some the tips in the Telegraph, then they might be able to improve their mpg...
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I'm getting close to filling my new Berlingo 1.6 HDI Desire up. I've only done 510mls since getting it 30/05/08 when it was filled to full by the dealer, & the fuel gauge is just above 1/4. So still a bit to go before the warning light comes on (I know it's not wise to let the fuel drop so low, but I'm experimenting) So the tank holds 60ltrs (13.2galls)
@ 510mls, that's just short of 39mpg so far. I hope when I've completed the run-in & got maybe a couple of Ks on the clock that the figures should be around 50ish.
Had a few friendly words with the service manager today, after I'd checked the tyre pressures, which should be 2.3bar & found them all to be 2.8bar??? a difference of 8psi.
BeeJay
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Well for those interested, I finally filled up today at 571 mls. getting 53.87 ltrs in, which works out at 48.19mpg. That's after running on reserve for app. 8mls.
That's pretty amazing, considering that mileage was all urban, & on a tight engine to boot.
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Not bad, would love to see what it could do on a run with a worn-in engine.
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