Have heard that before,RF, but have to admit I just don't understand why it is more economical...assuming that it is of course.
At tickover of say 7-800 RPM the engine is consuming X amount of fuel per second, even the lightest feathering of the accelerator will increase the RPM and the RPM is only increased because you are feeding the engine more fuel. That tickover fuel per second rate *has* to increase, does it not?.
The other scenario would be, in gear, accelerator completely closed. I have heard mention that the ECU actually cuts off all fuel going to the engine so it will infact use less fuel than tick-over, but as was mentioned before, in gear coasting can slow the car so much so that you would need some throttle to maintain a steady speed. What would be interesting to know is how much fuel that uses compared to tickover.
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Coasting risks loss of power steering, if the engine cuts out! ! Rare, may be, but pretty nasty when it happens. So, if you Do do it, watch the instrument panel.
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About 20 years ago I bought a Saab 96 with a freewheel. What a lethal feature. I couldn't resist trying it out. The contrast between descending a local 1/2 mile 1:12 hill in 3rd gear, and freewheeling down it, was shocking! After I had crested the rise at the top of the hill it felt like driving off a cliff compared with the usual sensation - a bit like the sudden plunge off the top of the climb on a roller coaster. I was grateful to get to the bottom with brakes stll working and as soon as possible pulled out the T-handle under the dash, never to push it in again!
Best car I ever had in the snow though.
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Was it the V4 Ford engine or the 3cylinder 2 stroke? I had the 2 stroke version and the engine braking was minimal so the freewheel feature made more sense. If I recall, the brakes on it were huge drums.
I thought that I read in a fuel injection book that, on the over-run, fuel was cut off to the injectors until the revs fell to about 1800, then they were cut back in again.
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The Saab was the V4 Ford engine. I think you're right, the freewheel feature was a carry over from the 2 strokes - I always thought that it was to protect the engine, 2 strokes having no crankcase oil so no minimal lubrication on the overrun with the throttle shut.
I'm pretty sure mine had discs on the front. Probably why I survived the hill descent! A few years before that I had all but run out of brakes down the same hill in a Morris Minor that I shared with my brother; he had been given some very old brake shoes found in the back of the parts department at the garage where he worked, and they seemed to be made of cardboard. Suddenly all those "Engage Low Gear Now" signs started to make sense! Once the cardboard shoes had been properly cooked they seemed to lose their goodness permanently and we just kept putting new ones on until we ran out.
Later, long before anybody thought much about changing brake fluid, brother boiled his down this particular hill in a Viva GT - a car whose performance exceeded the capability of its underpinnings by a considerable margin. He lived to tell the tale but has been an avid fluid changer ever since.
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I reckon it's quite true that this effect happens where it uses more fuel to coast than it does to stay in gear.
My car has a dial to readout the instant fuel consumption, there are often situations where when in neutral it hovers over the 50mpg part of the display, when put in gear it suddenly shoots round off the scale, somewhere up near 100mpg I think.
Can't give a coherant explanation myself, although I think I understand it. I'm sure somewill will explain shortly.
Blue
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I'm no expert but I experianced the same when sitting at lights in neutral. Trip computer reports x mpg. When coasting in gear it reports much higher mpg.
I suspect at tickover the system needs to inject fuel into the engine, but when you are in gear the forward motion of the wheels connected to the gear box turns the pistons for "free".
My car seems to be faster in gear, no accellerator, down a hill than some others. Dont know if they are off the throttle or not but I often catch them up and have to apply the brakes!
(Sorry for spelling errors)
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sitting at the lights stopped you are doing zero mpg whatever gear you are in, engine on or engine off, if the trip computer reports anything else it is wrong or an average over a journey or distance
coasting when moving should be a better mpg that this
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I recall many years ago a friend telling me he had been stopped by the police because his Morris 1000 used to smoke a bit a tickover and the Police had follwed down a long hill and could see he was on tickover. The cautioned him under some road traffic offense bu he never heard any more about it. My own view is, do not do it, it is not best practice and in an emergency when you come off the throttle and hit the brakes the vacuum for the servo shoots up to its max, from tick over it is not. Also on bends the cars handling is effected adversly, crash investigators check what gear a car was in after an accident. Regards Peter
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A lot of automatics have very little engine braking when left in 'D' - so it's almost like coasting,
Cheers, SS
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with GM autoboxes, using the Sport button on descents effectively holds in a lower gear - this is very useful.
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As others have stated, keeping the car in gear, on the over-run, shuts off the fuel completely on, as far as I know, all modern petrol cars. I'm not sure if diesels are the same, but see no reason why not.
On my previous petrol 406's (all 3 of them) the instantaneous fuel economy reading would go to 999MPG when running down hills in gear on no throttle. Put it in neutral and it would drop to around 100MPG (depending on road speed).
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Re the effect on steering response. I think this is the physics side of things coming into play. When driving round a bend it makes the car feel far more positive to apply gentle accelleration to drive a course round the corner, traction and the suspension responds. If you coasted round in neutral I'm sure the car would not feel as controlled. Applies much more to motorcycles, I remember when I learned a few years ago the instructor stressed that to go properly round a corner you should enter it at a lower speed then gently apply throttle whilst looking 'through' the corner to your exit, the bike follows naturally and takes a positive line. If you come into a bend too fast, stamp on the brakes then go round on a closed or trailing throttle the bike tends to sit up or drift round the corner. Bikers refer to this inferior method as 'threppeny bit' i.e. making a 50p shape mess of the line by adjusting the line mid bend. Takes a few years of experience to get it just right.
Also you can't see what's round most bends, eg a tractor, so always best to err on slow in, fast out. I digress, however I'm sure the same applies to four wheeled vehicles but in a less obvious manner. When I drive I try to use some biking methods, eg looking for the apex and line in a bend then gently accellerating though, the car feels much more positive.
Going back to the highway code recommendation about lack of steering response they must be pointing to driven wheels making a difference to the handling and positive line a car takes than a coasting object. There will be a formula somewhere that demonstrates all this, probably involving gyroscopes...
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Please delete this and last post Mods. Last one didnt cancel as I thought it had.Talking on the phone while replying.Big mistake.Got it wrong sorry
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Steve
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Most modern, computer controlled, cars use no fuel on the over run, undoubtably less then at idle. I've tried coasting on my regular run to wrok and found that it actually uses more fuel.
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Most modern, computer controlled, cars use no fuel on the over run, undoubtably less then at idle.
Very true, I think the speed of cut-off varies depending on the car, but I heard figures of around 2000 rpm.
PS, on a carburetted car, does the fuel consumption actually decrease that noticeably through coasting, or do the various restrictions in the idle circuit keep air/fuel mass flow relatively the same?
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Testing for the presence or absence of overrun fuel cut-off is a quick test which allows you to see if the throttle plate / switch / potentiometer / base idle is set up right. You plug an oscilloscope into the injector leads, give the car a good rev, and check to see that the injection pulse disappears until the engine slows down again.
Another way is to gradually close off the idle bypass pipe. As the idle integrator winds in more opening of the by-pass valve, clamp the pipe tighter. Then when the valve is fully open, and the engine management is struggling to speed up the idle, suddenly release the pipe. The engine revs up, until it reaches overun cut-off speed. As the throttle is shut, the fuel is cut off. Then, it slows until the speed where the fuel is switched on again. On older cars, with poor integrator anti-wind up strategies, the engine could be left yo-yo ing for 15 seconds or so. More modern ECUs aren't so easily caught out!
Number_Cruncher
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>Very true, I think the speed of cut-off varies depending on the >car, but I heard figures of around 2000 rpm.
In my experience anything over idle on the over run = no fuel use (acording to the on board computer) Mind you, I've run diesels for 20 years.
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Always understood this was bad practice and have seen nothing above to change this view.
Come on, the saving can't be that much! From my experience with the car computer, would have thought that lifting off the throttle would be just as effective and safer.
PS: Do you delay switching your lights on at dusk to save the battery?
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On my daily queue to work i often knock the car into neutral and coast slowly to a standstill from about 20mph. It saves me constantly holding the clutch in for a few seconds before crawling along a bit further - i just coast along in neutral then select first or second when needed, which i find a much more comfortable style of driving in these conditions. I don't see anything wrong with doing this?
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