Shifting Auto-Trans to N when in traffic?

[Old Techo]

From Quora...


If you are driving a car with automatic transmission, when you stop at traffic or at a long red light, should you move to N or stay at D but put the brakes on?

This answer was taken from here: Use of Neutral Gear in Automatic Transmission

It was written in June 2019 by Mark User mark

It has been slightly edited.
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When the car is in N and you start the engine the hydraulic pump in the automatic transmission is not providing fluid pressure until the engine starts.

This fluid pressure is used to engage clutches in the transmission to engage first gear or reverse.

An automatic transmission does not have gears like a manual transmission which are physically meshed into one another or engaged.

The gears in an automatic transmission are always engaged or locked in but they are coupled to the drive shaft by multiple different clutches that are selected either by you or automatically. This is called the planetary gear and clutch system and it is very difficult to conceptualise.

In an automatic transmissions N is actually the same as P in that none of the clutches are engaged except in P there is a mechanical connection to an internal transmission brake. This brake has nothing to do with your wheel brakes and it comes on mechanically so the engine does not need to be running for it to work. But you should not rely on it alone when parking on steep hills.

In D when you’re at a stop the main forward clutches are engaged and the torque converter is providing pressure to rotate the forward gear but it does not rub or wear when it is doing this. Because of this resistance to free rotation in the transmission the engine begins to slow. In a modern car there is an idle control valve which the computer opens to increase air entering the engine so the engine does not stall. So there is more air entering and the computer senses this and adds a bit more fuel as well. So in D at the lights your car will use a little more fuel than if it was in N.

Also the wheel brakes are not rubbing but are stopped and they will not wear at all while holding the forward creep of the car when in D.

The forward creep is caused by the transmissions torque converter. There is nothing slipping or wearing to any significant extent in there. It’s all done hydraulically and nothing is wearing to worry about. BUT!!!! When you are in N and the engine is turning and the wheels are stopped then for this to occur the main forward clutch is disengaged and it is SLIGHTLY SLIPPING. So the clutch plates are wearing.

So you are actually wearing out the main wear component in an automatic transmission being the clutch plates if you shift from D to N at a stop. The only advantage is your saving a very small amount on fuel.

Personally I DO NOT and I have been told by an automatic transmission engineer that you should never shift the transmission from D to N at the lights. The reason being you are wearing your clutch plates in your transmissions. The oil also gets dirtier because of the friction and the dirtier transmission oil then contributes to other component failures.

Also it’s dangerous to be sitting there in N. You may need to drive away all of a sudden to avoid an accident but in N there is less chance of do this. You also may select the wrong setting like R when you wanted D.

Edit:  Modern transmissions will disengage the torque converter while at a stop, this has the same effect as shifting into N without the premature wear.

[spaceland]

If you really wish to save some fuel, switch your motor right off when you are expecting more than a brief stop like that normally experienced at traffic lights. I generally stop the motor and lower the windows if stopped at roadworks with traffic lights or when an opening bridge has opened and I am caught there.

[Old Techo]

Ditto PD

[nsgnomad]

If I am caught in a minor delay, I will sometimes shift into P rather than N. For a longer delay I will consider turning the engine off and opening the windows if it is warm.

[loub]

A lot of modern cars have a function that when you stop the engine turns off and starts when you move your foot of the brake.My missus's car has it.We both hate it and turn the function off.It spose to save fuel but I reckon it wears out your starting motor and battery prematurely and there expensive.

[Old Techo]

Peter,

My son's diesel Audi does that and I HATE it too

I drive it rarely, only to drop him off at the airport.

Yes, you can disable the feature, as I would, but his regular use is stop/go city traffic.

I agree about battery life but maybe that's offset by fuel savings.

I don't know but I suspect the starter motor and ring gear would be engineered for this technique and not wear prematurely.

[bazza44]

Wouldn't the engine use more fuel by cranking and starting again compared to idling for a traffic light.

Barry

[Old Techo]

I don't think so Baz.

The restart is instantaneous and I don't see why the engine CPU would need to supply more fuel, like say it would with a cold engine start.

[GerryP]

The wife's Mazda 3 has stop-start which we usually leave it to do its thing. When you get home and turn the car off the display tells you how many kilometers worth of fuel you have saved. We've had the car for about 4 years and it's showing We've saved almost 100km worth.
Mind you, the overall average fuel consumption is only showing 6.9 lph and don't drive it gently. 😀

[nsgnomad]

Gerry, 6.9 lph. What is that equivalent to as litre /100km?

[GerryP]

May 1, 2023 13:33:03 GMT 10 nsgnomad said:

Gerry, 6.9 lph. What is that equivalent to as litre /100km?

Sorry, that was meant to be litres per hundred.

So at 100km/h it would be correct both ways... 😉

[jr]

Is this really the case that the forward clutch is slightly slipping in neutral and not fully disengaged?

“BUT!!!! When you are in N and the engine is turning and the wheels are stopped then for this to occur the main forward clutch is disengaged and it is SLIGHTLY SLIPPING. So the clutch plates are wearing.”

I have always moved to neutral if I am stopped for some time as I thought that the torque converter is trying to drive when in ‘D’ so it is generating unnecessary heat.

When stopped and I shift from ‘D’ to ‘N’ the engine revs increase about 100RPM which would be due to the torque converter trying to drive the vehicle forward but now is just running free.

If the forward clutch is slightly slipping at stand still when in ‘N’ and in ‘D’ it does not then I will change my ways.


JR

[dieseltojo]

I have never gone into neutral, and I never sit on a hill with the toque converter  taking the load to stop the care going back wards either. I reckon it is better to brake when at a stop.

I never heard of the car getting put into neutral ever.  In fact I can't remember it being in the manufactures manual on how to drive an auto. But it might be a better way I don't know. It is at least an interesting concept alright.

[Frank and Brenda]

I always stay in D unless stopped at road works or longish delay then I'll go to park and even turn the engine off if a longer delay
I'll restart and go to D when I see the traffic about 50m ahead start to move

[jr]

I have been trying the stay in “D” at the traffic lights when stopped, but don’t like it unless I know I am only stopped for a very short time.

When stopped the vehicle wants to creep forward so the foot is on the brakes and those hot brake pads are parked in one spot on the disk which can create a hot spot and change the disk material characteristics in one spot. Only other option is to use the hand brake if it is a drum hand brake.

Has anyone seen any proof that the main forward clutch on the auto transmission does not fully disengage in 'N' other than the article in this thread?


JR