Using "N" while travelling downhill to save fuel?

[sowlerthjh]

I have a W124 300D

Is it a good idea to save fuel by moving the automatic gearbox to "N" when driving downhill?

The engine idles, so it must be using less fuel.

 

[television]

Sorry bad idea, most of our cars do not have a rear pump, and that can cause the fluid to over heat. It should not be done, going down hill with your foot off the throtle, is not using fuel anyway.

Malcolm

 

[marnix]

Very bad idea and dangerous with that.
If you dont press the gas pedal no more fuel is going in the engine than on idle speed. If you have a fuel consumption display (computer), it will show 999 or --- MPG with the foot of the gas while driving down hill. You also keep control of the car and drive to the wheels in case you need it.
When teaching new drivers, they are told not to coast around corners or down hill; you are doing exactly that when selecting neutral while the car is moving.
Maybe a course with the IAM or similar would be to your advantage.

 

[cubicincubi]

hmm. not good.

if the gearbox is engaging the flywheel you will get relative crank rpm, rpm is not indicative of fuel consumption - throttle position is what you need to worry about.

it is rate of acceleration of rpm that chows the juice matey mate.

 

[mlc]

As already stated coasting in neutral is not saving fuel because you would be going downhill without using fuel anyway.
Engine braking provides stability that you lose when you select neutral and finally as you will remember from your highway code it is illegal to travel in neutral.

Mark.

 

[cubicincubi]

oh, also additional fatigue on brake components if not using engine load to brake.

 

[television]

You do not have engine braking until the car speed has reduced to lock up.

Malcolm

 

[djb]

N running

3 thoughts


1 when in gear & foot off and on engine overrun (ie rpm higher than idle)there is no fuel going to the engine, in any injection engine , petrol or other wise.

2 when you select a gear again usually auto boxes will go for the lowest, so there colud be an allmighty bunny hop along witk loads of wear to the plates & bands.

3 if done at speed there might be inadiquate lubrication in parts of the box that depend on fed pressure from the main gallery.

 

[tom7035]

I have a W124 300D

Is it a good idea to save fuel by moving the automatic gearbox to "N" when driving downhill?

The engine idles, so it must be using less fuel.

Why do you think it is that an automatic shouldn't be towed? Coasting in neutral is having the same effect.

 

[mlc]

Just re-read this and it struck me that actually you would use more fuel!

On the overrun we have all agreed that no fuel is being injected, so all the way down the hill is free. On the other hand in neutral the car will be ticking over, and using fuel to keep it running.

Mark.

 

[television]

Just re-read this and it struck me that actually you would use more fuel!

On the overrun we have all agreed that no fuel is being injected, so all the way down the hill is free. On the other hand in neutral the car will be ticking over, and using fuel to keep it running.

Mark.

It cant use more fuel than a manual in nuetral, Auto or manual in N are the same, engine at idle.

Malcolm

 

[cubicincubi]

Overrun

Overrun was a new term to me and I had never thought about it...

I found this statement during a google;

"Overrun conditions occur when the momentum of the vehicle causes the engine to turn faster than it would have been otherwise. The faster turning engine is trying to take in air that corresponds to the speed at which it is turning. The air intake is partially restricted by the throat valve. This results in a high vacuum that forms in the inlet manifold.

The most vacuum ever required by the vehicle is during idling conditions (± 50 kPa on most vehicles). During overrun conditions vacuum levels of up to -75 kPa are very common. With the throat valve at say 10% open a vacuum of minus -75kPa will cause 20% - 25% more air through the throat valve than a vacuum of -50kPa.

All of this is happening when the engine is not performing any work i.e. driving downhill, slowing down to stop or whenever you are reducing speed for whatever reason."

I understand that some engines are fitted with an overrun fuel cut off....

So am I right in my understanding that more fuel is used during overrun if the engine has no regulatory device?

This will be why I get pops and flames from my turbo car during shifting down... high levels of hydrocarbons will be igniting...

 

[television]

Overrun on an auto box does not happen with the car in D, lock up only takes place when 3rd (on newer boxes)or a lower gear has been selected and the speed of the car has reached a certain point. So under normal driving no lock up takes place = no overrun. If the car is in the lock up mode and the throtle is closed, the only fuel used is that for idle.

Many years ago, with the Borg warner 35 they were the last to be fitted with a rear pump, with a rear pump the car can be tow started.
I see that the rear pump is now an option on some of the S class MB's.

Over run is not a desirable thing, it causes the piston rings to viabrate,making small grooves in the bores.

Reading a work sheet from a Cessna 210 aircraft fiteed with new engines, it warns against using the prop to slow the plane down until X amount of flying time has been reached.

Malcolm

 

[Wezzel]

just take a look at the rev counter next time you are going down a hill (after driving in top gear) with your foot off the gas. The revs drop to somewhere around the 1000 mark.

It's not worth the risk of putting it into neutral.

 

[mlc]

Sorry bad idea, most of our cars do not have a rear pump, and that can cause the fluid to over heat. It should not be done, going down hill with your foot off the throtle, is not using fuel anyway.

Malcolm

Quote:
Originally Posted by mlc
Just re-read this and it struck me that actually you would use more fuel!

On the overrun we have all agreed that no fuel is being injected, so all the way down the hill is free. On the other hand in neutral the car will be ticking over, and using fuel to keep it running.

Mark.

It cant use more fuel than a manual in nuetral, Auto or manual in N are the same, engine at idle.

Malcolm
__________________

Malcom,

How can I argue with such a learned chap as you, but your second post doesn't make any sense, where did the reference to a manual come from? Your statement is of course correct but not related to the discussion.

As you correctly said in your earlier post on the overun in gear no fuel is used, in neutral it is. This will be the case in both manual or auto and means that our friend would be using more fuel by selecting neutral.

Mark.

 

[television]

Mark it all comes down to relativity. the very first thread was " Is it a good idea to save fuel by moving the automatic selector to N when driving down hill" Now, my brain tells me that many people in a manual car do this so that they can coast free of charge a little further. And this is relative to the first post. Now,No one is talking about turning the engine off, so therefore it will be at idle.

Now your Quote is
Quote:
Originally Posted by mlc
Just re-read this and it struck me that actually you would use more fuel!

On the overrun we have all agreed that no fuel is being injected, so all the way down the hill is free. On the other hand in neutral the car will be ticking over, and using fuel to keep it running.
Your first line Mark, Just re read this and it struck me that actually you would use more Fuel!.
More fuel relative to what mark, How the hell can the car use more fuel going down hill in N at idle, than going down hill with your foot off the throtle. No it cant, in fact it is the same.
I have been in the service industry for 55 years, with a top rating in what I have handled, you learn to read in between the lines as to what a person is really saying, you don't shoot people down because they cant express themselves, there are flaws in many parts of this thread, but I choose not to be clever, so I ignore them, since nothing but ill feeling could be gained.

I congratulate you on your third attempt at slagging me off, God, you must be horrible to live with.
I wish you better luck with your forth attempt.

Malcolm

 

[mlc]

Malcolm,

Sorry if you think this was personal, because it wasn't.

The original question was can I save fuel by going down hill in neutral in my automatic car. Several people (and I think that includes you) point out that in drive the car will not be using any fuel under these circumstances. My original comment said that coasting down hill was illegal and dangerous, it was only on rereading the thread and looking at other peoples answers that it struck me that it was also using more fuel.

If he can go down hill in drive and using NO fuel or in neutral with the engine using fuel to maintain tickover then its not rocket science to see which is the most economic.

Sadly my work / home balance isn't good enough for me to spend my life looking for the inperfections in other peoples postings on here, perhaps I can put that on my wishlist for retirement

O sorry just remembered, I have already promised to spend every waking hour of my retirement with the perfect MrsC, damm!

 

[television]

Malcolm,

Sorry if you think this was personal, because it wasn't.


If he can go down hill in drive and using NO fuel or in neutral with the engine using fuel to maintain tickover then its not rocket science to see which is the most economic.

!

My last word on this subject, when going down hill, in D and your foot is off the throtle, the amount of fuel being used is that for idle or tickover, and that is the same as going down the hill in N, so it is the same, and not more.

Malcolm

 

[mlc]

Sorry bad idea, most of our cars do not have a rear pump, and that can cause the fluid to over heat. It should not be done, going down hill with your foot off the throtle, is not using fuel anyway.

Malcolm

Sorry that you cant respond Malcolm. This is item number 2 from this thread where you quite correctly state that in gear and on the overrun NO fuel is used.

QED.

 

[marnix]

Come on people, play nicelly.
This forum is about helping each other, not scoring points of each other.

It is not possible to use less fuel than on tick over or with your foot of the gas pedal. In both cases it uses the same amount. Since the amount of fuel getting in the injectors is set by the idle valve, even with your foot off the gas pedal that amount of fuel is still going in the engine.
Now please stop bickering like two old women.
Thanks