driving in neutral to save fuel

Wig

cyclonebri1 wrote: »

Indeed it's quite the opposite, it's neccessary to prevent misfiring and damage to the cat as all air is shut of at this time too untill the cars comes out of it's overrun condition.

Disagree, overrun is in no way 'necessary', if you leave it switched off in the ECU programming, 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.

darich

Wig wrote: »

think of it like people pushing a car, the effort required by 2 people to push a car is more than if you had 4 people to push. It will be the same with the engine idling the car will roll further more easily due to the ignition in the cylinders, because it's like an extra pair of hands.

Strictly speaking this is is not entirely correct without further details.

2 people pushing a car can put the same effort in as 4...but the car will only move at half the speed....or travel half as far before they're exhausted.

To put it another way....1 person can move a car 10metres without putting a lot of effort into it...almost leaning on a car will start it moving....but 4 can do it a lot faster and really put their back into it.

I'm also intrigued about this arguement on whether a car uses fuel on overrun (if it's even fitted) but it occurs to me that in normal driving there is a decent percentage where I don't need to press the accelerator. This happens everytime I'm braking for a junction/roundabout/traffic lights or even simply coasting down a long incline and want to remain within the speed limit.
It also occurs to me that cars use their economy as a selling point and if the manufacturers can use the time where I'm not pressing the accelerator to stop the car using fuel, it would be a significant saving.
If both points above are true (and they are) then why wouldn't a car have overrun and stop using fuel when not pressing the accelerator?
Coasting in neutral is just bad driving plain and simple.

I also know that unburnt fuel passing through a catalytic converter destroys it. So doesn't that indicate that every car fitted with a catalytic converter (ie since 1991) has over run to prevent the converter being destroyed before you're home from the garage with your new car?

Unburnt fuel being dumped from an engine (effectively what happens if it continues to use fuel while decelerating) would result in backfiring, destroyed converters, and probably choked and sooted engines due to the the excessive fuel. And your economy would be poorer too because you're using plenty fuel you don't need to.

Why would manufacturers make a car do all the above when they can simply use overrun and everyone's happy?

Wig - how old is your Corsa and what model is it?

darich

Wig wrote: »

Disagree, overrun is in no way 'necessary', if you leave it switched off in the ECU programming, 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.

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.

Wig

darich wrote: »

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

yes, and that's why I put idle in quotation marks, because it's not really idling.

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?

I agree it would be overrun if the fuel supply is reduced or cut off. But I would say I am using the word overrun to describe what vauxhall themselves say that it is when fuel is cut off completely.

I will still not agree that in all cars, absoultely no fuel is going to the engine under these conditions, because my manual says it does not happen on all models. I will say that it is likely that fuel has been cut off, and I can't think of any reason why it wouldn't be, but it is possible that some fuel is still going in, in some cars, otherwise why would the manual say that (it's a 2003 Corsa manual covering all corsa models)? Maybe Vauxhall made an error, I think I might even write and ask them, and hope my letter gets answered by someone who knows.

Of course the car is still slowing down more rapidly than it would with the clutch in.


I'll reply to your other post now.

Wig

darich wrote: »

Strictly speaking this is is not entirely correct without further details.

2 people pushing a car can put the same effort in as 4...but the car will only move at half the speed....or travel half as far before they're exhausted.

If we say effort = force then 2 people putting the same effort in will push it the same speed as the 4 people, but they will only last half the distance. I think we can all agree on that. You're right that when I wrote
think of it like people pushing a car, the effort required by 2 people to push a car is more than if you had 4 people to push.
It wasn't very well worded, the combined effort will be the same, the effort required by each individual will be greater in a group of 2 than in a group of 4. So *if* the engine is getting *some* fuel this will help the car to travel further because it will help to overcome some of the compression forces.

I'm also intrigued about this arguement on whether a car uses fuel on overrun (if it's even fitted) but it occurs to me that in normal driving there is a decent percentage where I don't need to press the accelerator. This happens everytime I'm braking for a junction/roundabout/traffic lights or even simply coasting down a long incline and want to remain within the speed limit.
It also occurs to me that cars use their economy as a selling point and if the manufacturers can use the time where I'm not pressing the accelerator to stop the car using fuel, it would be a significant saving.
If both points above are true (and they are) then why wouldn't a car have overrun and stop using fuel when not pressing the accelerator?

You would think so, but all I'm saying is that I try not to believe something blindly, maybe they all do maybe not all of them do, that's all I'm saying. Maybe Vauxhall made a mistake when they printed my cars manual, maybe they didn't. Maybe Vauxhall knows something that all of us here do not know.

Coasting in neutral is just bad driving plain and simple.

I don't coast in neutral, I use the clutch, but I'm not one of the nanny state members who would villify someone who selects neutral, because I have done it and it worked fine for me, the reasons I stopped doing it included meshing the gears upon reselection, and not wanting to use the brakes too much (I prefer engine breaking) but there are very few hills in UK long/steep enough to require repeated brake use that would affect the efficiency of the brakes, on such a hill I think even someone who uses neutral a lot on normal roads will have enough common sense to put the car into a suitable gear.

Of course there are nanny members who villify me for coasting with the clutch down. But seriously I don't care, I've done it for years, nothing untoward has ever happened, when I want to slow down I just release the clutch and apply the brakes aswell if needed. My hands are always on the steering wheel. It's no big deal.

I also know that unburnt fuel passing through a catalytic converter destroys it. So doesn't that indicate that every car fitted with a catalytic converter (ie since 1991) has over run to prevent the converter being destroyed before you're home from the garage with your new car?

No I don't agree, If the engine is being fuelled the fuel is going to be burnt so it won't reach the cat.

Unburnt fuel being dumped from an engine (effectively what happens if it continues to use fuel while decelerating)

No, because the fuel would still be burnt by the sprk plugs.

Why would manufacturers make a car do all the above when they can simply use overrun and everyone's happy?

I have no idea, but maybe the manufacturers do. I'll ask them..... that should make everyone happy.

darich

According to this website fuel cut off during over run seems to be standard on all vauxhall models.
Click the model you're interested in and then read the "Operation, Safety and Maintenance" brochure. Search within the brochure for "overrun".

Vauxhall states that fuel is cut off during deceleration on corsa, astra and vectra which is the three I checked.

Maybe Vauxhall has corrected their earlier mistake or earlier models didn't have the feature. It seems that they all have it now though.

darich

Wig wrote: »

I don't coast in neutral, I use the clutch, but I'm not one of the nanny state members who would villify someone who selects neutral, because I have done it and it worked fine for me, the reasons I stopped doing it included meshing the gears upon reselection, and not wanting to use the brakes too much (I prefer engine breaking) but there are very few hills in UK long/steep enough to require repeated brake use that would affect the efficiency of the brakes, on such a hill I think even someone who uses neutral a lot on normal roads will have enough common sense to put the car into a suitable gear.

Of course there are nanny members who villify me for coasting with the clutch down. But seriously I don't care, I've done it for years, nothing untoward has ever happened, when I want to slow down I just release the clutch and apply the brakes aswell if needed. My hands are always on the steering wheel. It's no big deal.

You've done it for years and nothing untoward has happened....that doesn't mean it's a good thing.
You also said you've had to replace the clutch on your last 4 cars....I don't press the clutch on downhill slopes and I've never replaced a clutch....even when my cars have exceeded 100k miles.
I was never taught to "coast" down a hill by pressing the clutch....and I doubt anyone is today.

But you're driving a Corsa and the site I linked to above shows that Vauxhalls will use more fuel by idling, you're costing yourself money by pressing the clutch on downhill slopes.

Wig

darich wrote: »

According to this website fuel cut off during over run seems to be standard on all vauxhall models.
Click the model you're interested in and then read the "Operation, Safety and Maintenance" brochure. Search within the brochure for "overrun".

Vauxhall states that fuel is cut off during deceleration on corsa, astra and vectra which is the three I checked.

Maybe Vauxhall has corrected their earlier mistake or earlier models didn't have the feature. It seems that they all have it now though.

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.

hundredk

cyclonebri1 wrote: »

I won't get into an argument of whether coasting is more cost effective than allowing the natural overun of the car to do it's work, as Ive often wondered that myself but no way can it be proven. I do know however which way gives more control and is actually the correct way of driving;)

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.

Wig wrote: »

hundredk,

As I said, as no-one can provide any evidence on either point i.e.

• That all modern cars have this feature. And I don't accept "it is because we say it is"
  the evidence of car manuals saying it exists on some vehicles and doesn't exist on some vehicles is the only evidence we have.
• The comparable fuel useage of freewheeling Vs Overrun on any given stretch of road

Then we are all only giving opinion and none of us knows who is right, not if we are honest with each other.

Cylonebri1 has covered the first point in #72

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.

Wig wrote: »

Oh & BTW I disagree about the fuel ignition in car B not making any difference, even if what you say was true, think of it like people pushing a car, the effort required by 2 people to push a car is more than if you had 4 people to push. It will be the same with the engine idling the car will roll further more easily due to the ignition in the cylinders, because it's like an extra pair of hands.

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.

Wig

darich wrote: »

You've done it for years and nothing untoward has happened....that doesn't mean it's a good thing.
You also said you've had to replace the clutch on your last 4 cars....I don't press the clutch on downhill slopes and I've never replaced a clutch....even when my cars have exceeded 100k miles.

I've already explained those clutches, 2 of them were on taxis, clutches do get changed on taxis I think you will agree.
One of them was a car bought at auction with 80K on the clock and which was previously a driving instructors car. I changed the clutch at about 85K
They cost £80 and £100 in labour to change (the original point in mentioning them).
I actually can't remember any other clutch I have changed, I must have lied when I said 4 - or wrongly decided as it was not very important 3 or 4 it wasn't worth thinking about too closely to come up with the exact correct number, at the time I didn't think I would be cross examined on it later.

I was never taught to "coast" down a hill by pressing the clutch....and I doubt anyone is today.

And your point is? I wasn't taught either, it's just something I do sometimes, because I'm completely in control of my car, it doesn't wear the clutch out and in the right circumstances it will IMHO save money.

you're costing yourself money by pressing the clutch on downhill slopes.

As neither of us can prove it either way, we can only each of us do what we feel is best. I think one thing you think the opposite. That's fine by me, hope it is also fine by you.