driving in neutral to save fuel

hundredk

Wig wrote: »

You're assuming the car has overrun (see post #46)

Assuming it does have overrun you're ignoring the fact that you coast further and faster freewheeling than you do on overrun, it is entirely possible that the extra fuel you use when using overrun means that overall you use less when freewheeling.

Yes, the statement does rely on the premise that the car has overrun. All modern cars will have this feature, certainly those made in the last 10 years or so and probably a lot earlier. Car manufactuerers know that the cumulative effect of overrun is an extra few mpg and it helps lower emmissions. Easy to add to ECU program which all modern cars have.

Overrun freewheeling will always beat neutral freewheeling on the same road in the same conditions.

iltisman

I reckon you freewheel twice the distance in neutral and if that gets you to the top of the next hill no fuel burn is needed to get you over the hill and on to more freewheeling.
Whats all this nonsense about no brakes with no engine, you just go back 30 years to the era when brake servos were a rare luxury and braking hard bruised the top of your head.

missile

hothothot wrote: »

thanks for the humerous and helpful replies, I will not be doing the same! - and will let him know before he causes a crash, although I suspect he been doing this for a long time.

Don't be surprised when he tells you "if you don't like my driving get out and walk" :rotfl:

ben501

hundredk wrote: »

The display max's out at 141.3mpg ( a strange figure but probably a technical reason for it)

141.3mpg = 50 kpl .

Wig

hundredk wrote: »

Yes, the statement does rely on the premise that the car has overrun. All modern cars will have this feature, certainly those made in the last 10 years or so and probably a lot earlier. Car manufactuerers know that the cumulative effect of overrun is an extra few mpg and it helps lower emmissions. Easy to add to ECU program which all modern cars have.

Overrun freewheeling will always beat neutral freewheeling on the same road in the same conditions.

"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.

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.

Pew_Pew_Pew_Lasers!

bungle4x4 wrote: »

ummm wrong. if sat at idle for a period, take the car out of gear, it saves wear on the torque converter, saves fuel and extends the life of your gearbox.

Utter rubbish. The torque converter is a fluid-filled coupling and does not wear.

For an automatic gearbox, there is absolutely no difference between being sat still at idle, and driving. None. And don't start about ATF temperature, that's why cars with autoboxes have an ATF radiator.

adouglasmhor

Pew_Pew_Pew_Lasers! wrote: »

Utter rubbish. The torque converter is a fluid-filled coupling and does not wear.

Even fluids wear/become degraded with time or load. The edges of turbines and stators can become damaged, the vanes misaligned, the connections can be loosened. All these can be classified as wear. I suggest you are the one taling "utter rubbish".

Pew_Pew_Pew_Lasers!

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.

cyclonebri1

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.

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.

Wig, "overrun" as you put it, is not a feature of any car. It is the condition where the car is driving the engine as against the norm where the engine drives the car. IE the car is in such a gear, or decelarating at such a rate that the engine is braking the car. No (fuel injected) cars fuel under these conditions circumstaces. It's all programmed via the ecu. This ensures that provided that the engine speed is over a certain limit, the throttle is shut, and the road speed is above a certain limit, NO fuel is supplied. When the revs drop, the throttle opens or the road speed drops too low, the engine kicks back in as quick as you or me could control it.

I'm sorry but you clearly don't understand the issue.

Wig

cyclonebri1 wrote: »

Wig, "overrun" as you put it, is not a feature of any car. It is the condition where the car is driving the engine as against the norm where the engine drives the car. IE the car is in such a gear, or decelarating at such a rate that the engine is braking the car. No (fuel injected) cars fuel under these conditions circumstaces. It's all programmed via the ecu. This ensures that provided that the engine speed is over a certain limit, the throttle is shut, and the road speed is above a certain limit, NO fuel is supplied. When the revs drop, the throttle opens or the road speed drops too low, the engine kicks back in as quick as you or me could control it.

I'm sorry but you clearly don't understand the issue.

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.