driving in neutral to save fuel

Wig

hundredk wrote: »

If you think about it, all it is doing is driving in such a way as to maximise the overrun, which we know saves fuel.

You don't know if it saves fuel under all circumstances, which is what I have said in this entire thread.

Cylonebri1 has covered the first point in #72

Not very well. Saying, "it is because I say it is" is not covering the point very well at all.

On the second, there are many variables such as momemtum, incline, weight of vehicle, friction through drivetrain etc, and therefore the best way is to test in practice. I and other posters have do this and found a noticable increase when this type of driving is adopted. Not a scientific test but it is indicative of what the results would be if one was caried out. Manufactuers program overrun to maximise efficiency and all digital driving does is take advantage by maximising the time spent on overrun.

No, not very scientific, and if I was to do the same, I dare say I would find my method works best for me.

Yes but if those 2 or 4 people were "pushing" a car at say a constant, 1 mph and then the car hit a downhill gradient sufficient that the cars own momentum could sustain say 2mph (ie the same condition as overrun), the effort of the people would have no effect on the car because the momentum exceeds their effort. In effect their force to push the car at 1mph is not required because it is already at or above this speed. Similrly, the piston already has more than sufficient force from the drivetrain.

But as you should be aware, because it has been said by me in this thread so many times, that I am talking of the road conditions where upon releasing the accelerator pedal the car starts to slow down. If the car slows down, the gradient is not sufficient to keep or increase the speed (obviously), under these conditions if there was some fuel assistance to the engine the required gradient to keep the speed would be lessened. Or to put it another way on the same gradient the car with fuel assistance however small, would not slow as quickly as the one with no fuel. Which is what I was saying.

hundredk

Wig wrote: »

Yes on that one it says so. But you see those little * marks all over the manual? those denote features not present on all models, on mine there is an * on the overrun article. It could just be a typo.

You have suggested several times that your manual may have a typo. I would suggest you are correct in this assumption, especially when you look at the quote taken from the linked document - no * marks on overrun.

Overrun
The fuel supply is automatically shut off
during overrun, e.g. when the vehicle is
being driven downhill or when braking. To
enable the overrun cut-off to take effect,
do not accelerate during overrun and, with
manual transmission, do not depress clutch
pedal. To prevent damage to the catalytic
converter, overrun cut-off is temporarily
deactivated when the catalytic converter
temperature is high.

Overrun
The fuel supply is automatically shut off
during overrun, e.g. when the vehicle is
being driven down long gradients or
when braking – see page 155.
To enable the overrun cut-off to come
into action and save fuel, do not
accelerate or depress clutch pedal
during overrun.

Wig

That's a different manual 2005 onwards I believe.

overrun cut-off is temporarily
deactivated when the catalytic converter
temperature is high.

So not as 'necessary' as cyclone would have us believe then. And I suspect it it is fed with a reduced amount of fuel, otherwise as darich pointed out how would it slow down

hundredk

Wig wrote: »

You don't know if it saves fuel under all circumstances, which is what I have said in this entire thread.


Not very well. Saying, "it is because I say it is" is not covering the point very well at all.

No, not very scientific, and if I was to do the same, I dare say I would find my method works best for me.

But as you should be aware, because it has been said by me in this thread so many times, that I am talking of the road conditions where upon releasing the accelerator pedal the car starts to slow down. If the car slows down, the gradient is not sufficient to keep or increase the speed (obviously), under these conditions if there was some fuel assistance to the engine the required gradient to keep the speed would be lessened. Or to put it another way on the same gradient the car with fuel assistance however small, would not slow as quickly as the one with no fuel. Which is what I was saying.

The most scientific way to test would be on something like a rolling road. The next best method (and arguably more realistic and accurate) is to test in real conditions since that is what the car will be driven in. I have carried out trials using the cars fuel computer on a reasonable length 20+ miles journey, reset the computer and drove digitally for the whole journey on a clear road. Done the same with freewheeeling. Carried out the same test several times with different cars with consistantly better mpg using digital driving method between 5 and 10%. If I get to drive a Corsa I'll carry out the same test but as I've said, not one car has produced a better result freewheeling.

Wig

You're right, not very scientific, not very scientific at all.

cyclonebri1

darich wrote: »

Isn't idling only around 800-900 revs though?

If I'm decelerating on a motorway slip road my revs are maybe as high as 4000.
If the ecu is only programmed to use fuel for idling (ie 800-900rpm) then it means that my car is using much less fuel than needed at those higher revs when i release the accelerator which is in essence overrun is it not?

If the engine is turning at 4000rpm but decelerating then it must be using less fuel in order to slow down. If the car had no overrun and continued to use fuel as Wig suggests, then how would the engine slow down? It quite simply must have less fuel going in to slow down otherwise it would continue at the same rate until the fuel ran out. Wig also suggests "the engine will just 'idle' as it always does, of course it will be running at a higher rev due to the momentum of the car, but that won't stop the sparks from igniting, the fuel going into the cylinders, and the throttle or idle valve allowing sufficient air into the intakes."

How can the car continue to use fuel at the higher revs if I'm not pressing my accelerator?
If it continues to spark and burn fuel at the speed the engine is turning, how does it decelerate without me pressing the brake?
Isn't it also a safety feature so that in the event of an accident the fuel supply is cut off when the accelerator is not being pressed?

Search google for "deceleration fuel cut off" and you'll find plenty of places stating that below a certain point in the revs (typically 1700-2000 depending on injection system) if decelerating, fuel is cut off to the engine.

Exactly:T:T:T

cyclonebri1

hundredk wrote: »

The most scientific way to test would be on something like a rolling road. The next best method (and arguably more realistic and accurate) is to test in real conditions since that is what the car will be driven in. I have carried out trials using the cars fuel computer on a reasonable length 20+ miles journey, reset the computer and drove digitally for the whole journey on a clear road. Done the same with freewheeeling. Carried out the same test several times with different cars with consistantly better mpg using digital driving method between 5 and 10%. If I get to drive a Corsa I'll carry out the same test but as I've said, not one car has produced a better result freewheeling.

Rolling road conditions are what I was talking about earlier, the only way we can get to see what the engine is doing apart from having the tuner strapped into the passenger seat with the laptop, and believe me I have done that, but as a driver.


The issue that I thing Wig mentioned in another post of the cat overheating is valid, it is a programmable feature on modern cars that overides the no fuel overrun rule to protect the delicacies of modern motors;)

Trying to put this back into a nutshell;
All modern cars have a system of not supplying any fuel unless there is good need

hundredk

cyclonebri1 wrote: »

All modern cars have a system of not supplying any fuel unless there is good need

Yes, I agree.

cyclonebri1 wrote: »

The issue that I thing Wig mentioned in another post of the cat overheating is valid, it is a programmable feature on modern cars that overides the no fuel overrun rule to protect the delicacies of modern motors;)

Agree with that too, although I read Wig's post as saying the cat will always overheat rather than in certain circumstances, ie prolongled overrun.

From an ECU programmers POV, I would say there is no such a thing as overrun overide, ie create an event with one rule and cancel with another. They would just programe overrun fuel cut off only if several conditions exist, ie no throttle, high revs and cat temp below x degrees (plus any others not discussed here). Same result, better logic.