driving in neutral to save fuel

DVardysShadow

prowla wrote: »

From the Highway Code:

Coasting. This term describes a vehicle travelling in
neutral or with the clutch pressed down. It can reduce
driver control because
• engine braking is eliminated
• vehicle speed downhill will increase quickly
• increased use of the footbrake can reduce its effectiveness
• steering response will be affected, particularly on bends and corners
• it may be more difficult to select the appropriate gear when needed.
The vehicle is also less likely to be heard by other road users.

That is one you really have to watch for. If brake shoes get hot, they can become almost ineffective.

cyclonebri1

Wig wrote: »

A lot of you keep saying that all modern cars have overrun, but none of you offer any proof of this.

I show you that my modern corsa manual says it is 'not a feature on all models' and none of you can explain this.



Sorry again, Wig inspite of what your manual says, overrun is not a feature, it's physics. Modern fuelling systems have used this to good effect, zeroing fuel usage in "no need" conditions. If you cannot accept this all your further points are invalid.

(Sorry for the delay but the system is !!!!!!ed at present)

adouglasmhor

Pew_Pew_Pew_Lasers! wrote: »

Of course fluids wear with age.

For the TC, tell me the difference between being in D at standstill, and D while driving. Because there is none.

More pressure and shearing force on the system while at higher revs.

Wig

cyclonebri1 wrote: »

Wig wrote: »

A lot of you keep saying that all modern cars have overrun, but none of you offer any proof of this.

I show you that my modern corsa manual says it is 'not a feature on all models' and none of you can explain this.



Sorry again, Wig inspite of what your manual says, overrun is not a feature, it's physics. Modern fuelling systems have used this to good effect, zeroing fuel usage in "no need" conditions. If you cannot accept this all your further points are invalid.

(Sorry for the delay but the system is !!!!!!ed at present)

A bit like saying all computers use a Windows OS, complete and utter buncum. Yes, ok you got me beat, just because you say so, Vauxhall must be wrong about the cars they designed and built. Because you say so, it must be programmed into every cars ECU, although you offer no evidence. Everyone used to think the world was flat you know.

hundredk

Wig wrote: »

"Certainly" but you cannot provide any proper evidence for it.
And yet my Corsa handbook 2003, says overrun is not a feature on all models.

No, I didn't research every model and look for overrun in the handbook. I made a comment regarding overrun based on my knowledge of how engines work. A difference between modern and older engines in that they all have an electronic brain that controls many engine functions including timing, fuel injection etc. It is one of the reasons why modern cars are more efficient and than their equivelant size older engines (I say older - ECU's have been in common use in cars since the 80's and, certainly the 90's and most certainly 2000 on). How often do you hear a modern engine "pink" - you don't because the ECU sorts it all out so fuel does not pre-ignite.

Wig wrote: »

A car in overrun is not going to get very far on a very slight gradient, the gears and the compression on the cylinders will slow it down - remember a car in overrun has no fuel, so it has no spark combustion to send the piston back down the cylinder.

A car with it's clutch in, however, will coast along a slight gradient a lot farther without engine assistance.

Overrun only works on a significant incline able to overcome the resistance of the engine compression cycle. Try it on a slight incline and you will see you slow down far too quickly for it to be of any use. If the car keeps going on a slight incline in "overrun" I would rather take that as evidence that the car does not have overrun because the idling engine must be the cause for the car not slowing down significantly.

This is an argument that no one can win without proper tests being run. There can only be opinions put forward by both sides - based on a few known facts.

I understand all of what you say but in practice the momentum of the vehicle carries the car more than suffcient distance on zero fuel to out do the same vehicle coasting. Remember when accelerating fron say 50-70, the car has momentum so it's not like pulling away from standstill which uses a lot of fuel. Overrun will work on any incline, even uphill - it just won't last as long because the car will slow quicker. On flatish or slight inclines (up or down), digital driving will be more efficient that coasting in neutral. I and other posters have tested this.

hundredk

ben501 wrote: »

141.3mpg = 50 kpl .

Thanks Ben. Not so strange and quite logical when pointed out. Validates my comments too.

hundredk

Wig wrote: »

I absolutely do understand the issue. What did I write that made you think I don't?

A lot of you keep saying that all modern cars have overrun, but none of you offer any proof of this.

I show you that my modern corsa manual says it is 'not a feature on all models' and none of you can explain this.

A car on overrun has no fuel going to the engine (I said that)
The car is driving the engine (I said that too, just in different words that's why I said the car would slow down quicker). In driving the engine, the car will be meeting with a lot of resistance from the engine, because the pistons still have to go up and down and one of the strokes for each piston will be a compression stroke.

It is basic logic that will tell you a car in overrun will reduce speed much quicker than one which is freewheeling, (unless it has a freewheeling clutch but AFAIK there aren't any cars with these today) and hence, you have to apply throttle earlier once you reach your lower speed limit. If the hill is steep enough the car will keep going but really how many long steep hills are there that we encounter in general driving.

If the incline is not steep, as I said, and the car keeps going (longer than you would expect it to) when the conditions for overrun are met, without slowing significantly then I would argue that is indicative that the engine is running with fuel (like car B below) i.e. it does not have overrun as a feature, i.e. the engine idling on fuel is helping the car, because if the engine was not fuelled the compression strokes would slow it down a lot quicker.

Obviously when the car slows enough the anti stall technology will kick in, but I am not talking about that, I am talking about how long it takes to slow down enough before the anti stall kicks in.

3 different scenarios are
*Car A freewheeling from 60mph, (i.e clutch is in) Lets say it takes 30 seconds (random figure just to illustrate the point) to reach 40 mph - due to wind resistance, tyre resistance & drag.
*Car B without overrun as a feature from 60mph, lift off all pedals leave car in top gear, car slows to 40mph in 20 seconds Because the extra resistance of the gear driven wheels slows it down. (or more accurately the wheel driven gears) There is resistance from the engine/gearbox in this car, but the fuel being ignited means there is less resistance than car C below.
*Car C with overrun as a feature from 60mph lift off all pedals leave car in top gear, car slows to 40mph in 10 seconds because of resistance of gear driven wheels AND resistance of compression strokes in the engine.

none of these car have reached the slow revs required to allow anti stall technology to kick in.

The resistance, engine braking etc slowing the car down quicker than coasting in neutral is not in dispute. The degree to which this makes a difference is because in practice the fuel saving more than compensates for the extra resistance. The examples in bold alone are arbitary enough to skew your opinion but lets go with what you say.

Car A is consuming fuel for 30 secs, at the end of which amounts to X1 gallons and in this period travels Y1 yards. Car C uses no fuel for 10 secs but because it slows quicker, only travels Y2 yards. At Y2 yards it accelerates using fuel X2 until it covers a distance of Y1-Y2. X2 will be less than X1 and, at the point Y1 yards car C will also have greater speed than car A and be nearer to another cosating run.

Also, questionable that the fuel ignited in car B would make any difference since the wheels and drivetrain would be pushing the piston down with more force than the ignition so the power would effectively be following the piston and effectively wasted which is the reason why overrun is used in the first place.

Wig

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.

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.

iltisman

You are right wig just let them burn more gas.

cyclonebri1

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"
my evidence of my manual is the only evidence on this issue that we have either way.
*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.

Wig, motorsport was a hobby of mine. I had friends who worked in the tuning industry where it was routine practise to interogate the original manufacturers "program", log all the sites up, and modify as neccessary.
Also I've worked on loads of aftermarket programmable ecu's for track cars etc.
On everything that has had fuel injection as a basis for fueling as against carburation, there are no examples where there has been any reason not to shut of the fuelling under overrun conditions. 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.

Haynes have a manual, it's getting on a bit now, that details the varying methods of fuel injection as it came to be used on british, eurpean, jap and yank cars, interesting reading.

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;)