Why Dont Auto's Stall

[jpskiller]

my friend has just asked me why auto's dont stall, ie when at lights and gear is in 'D' and have brake pressed, soon as release brake you start moving.

I cannot explain why this is, is there a simple explanation

cheers

 

[Howard]

They just don't ...















Seriously , i expect one of the more technical members will be along with an explanation soon ....

 

[smartbrabus]

http://auto.howstuffworks.com/automatic-transmission.htm
http://auto.howstuffworks.com/torque-converter.htm

 

[BTB 500]

Because the engine isn't rigidly connected to the transmission like it is in a manual.

 

[stats007]

It's all down to the torque converter.

 

[PaulE230]

Is it best to keep a auto in Neutral at the traffic lights or to hold your foot on the brake pedal ?

 

[nimmat]

Mine does when I activate SBC - Hold :bannana:

 

[Bobby Dazzler]

Right in my simple mind it works like this:

The torque converter links the engine to the transmission. The torque produced by the engine is connected to a propellor like device. There's a second propellor like device connected to the transmission, and sealed between the two is the transmission fluid. This is the Torque Converter.

When the engine side propellor spins it moves the fluid which in turn drives the propellor connected to the transmission. The more the engine revs, then the more torque it produces, and the more torque is transferred through the fluid to the transmission, and on to the wheels.

When idling there's relatively little torque produced - even in Drive there is little torque being transmitted to the wheels, and so the footbrake is enough to stop the car moving. Release the footbrake and the small amount of torque is enough to move the car slowly (ie creep).

As there's no direct connection between the engine and the transmission, the engine can continue to run and doesn't stall.

Clever stuff, and my description doesn't do it justice, but hopefully is makes the point.

 

[KillerHERTZ]

Is it best to keep a auto in Neutral at the traffic lights or to hold your foot on the brake pedal ?

I'd say nether, best in 'P' - not that I do

 

[Bobby Dazzler]

Is it best to keep a auto in Neutral at the traffic lights or to hold your foot on the brake pedal ?

Keep in D with your foot on the brake assuming it's just a normal sequence of traffic lights.

The torque converter is designed to deal with this, and doesn't place any significant load on the transmission or it's components.

If you switch to N then you effectively introduce two unnecessary changes - D to N and N to D - which and increases wear on the transmission.

It won't cause damage but over time will wear the transmission more than having left it in D.

Of course, the brakes have to resist the torque being transmitted to the wheels, but as is often said, it's cheaper and easier to replace your pads than it is to rebuild a transmission.

 

[Bobby Dazzler]

Don't put it in P when in a queue for a McDonalds Drivethru though. Our Omega got stuck in P whilst in the drivethru lane, with very high kerbs either side. Not a popular man that night!!

 

[Gollom]

my friend has just asked me why auto's dont stall, ie when at lights and gear is in 'D' and have brake pressed, soon as release brake you start moving.

I cannot explain why this is, is there a simple explanation

cheers

We all know that there is a little man in the engine really who sorts it all



P.S. Is he called Ian Walker?

 

[purplegoddess]

We all know that there is a little man in the engine really who sorts it all



P.S. Is he called Ian Walker?

the engine or the little man?

 

[television]

Its not only wear and tear on the box when moving to P at the lights.

The whole chain from prop to rear axle and half shafts take up the play when you move from P to D, in D it all stays under tension, much better all round.

I have been in some cars where the driver sticks it into P at lights, the lights go green and he slips it into D and accelerates before the slack has been taken up in the drive chain and it make a hell of a bang. When a car clonks badly by going to R and then R you know that this is the way that it has been driven

 

[PaulE230]

Keep in D with your foot on the brake assuming it's just a normal sequence of traffic lights.

The torque converter is designed to deal with this, and doesn't place any significant load on the transmission or it's components.

If you switch to N then you effectively introduce two unnecessary changes - D to N and N to D - which and increases wear on the transmission.

It won't cause damage but over time will wear the transmission more than having left it in D.

Of course, the brakes have to resist the torque being transmitted to the wheels, but as is often said, it's cheaper and easier to replace your pads than it is to rebuild a transmission.

Is fuel consumption effected by keeping it in D ?

 

[television]

Is fuel consumption effected by keeping it in D ?

No not really,its so small if there is any change, possibly on the older 201's 123 and 126 and early 124's as they use an idle relay that increases the idle when in gear by 50 RPM and when the Air con came on

 

[DieselE]

Of course, the brakes have to resist the torque being transmitted to the wheels, but as is often said, it's cheaper and easier to replace your pads than it is to rebuild a transmission.

The actual wear on the brake pads is miniscule. Whilst you are stationary there is no wear at all. The only wear is that tiny fraction of a second when the car starts to creep after you start releasing the brakes and before the pads fully clear the disk. The technical expression for the amount of wear is "f***k all".

Remaining in Drive causes less stress to the transmission than selecting Neutral or Park and then re-selecting Drive when ready to move off.

Hopefully, we will not have too many posts about the energy burnt by the brake lights when holding the car stationary whilst still in Drive.

 

[television]

Hopefully, we will not have too many posts about the energy burnt by the brake lights when holding the car stationary whilst still in Drive.

But you are ignoring the wear on the drivers shoe.

 

[MisterMicawber]

Hopefully, we will not have too many posts about the energy burnt by the brake lights when holding the car stationary whilst still in Drive.

I'd never thought about the extra energy used by brake lights on auto cars! But I've often wondered why car makers don't set their cars to turn off the brake lights if the car has been stationary for, say, 10 seconds. I have only owned autos for many years so I'm a great supporter of them. But high-level brake lights can sometimes be very bright on the car in front when it's dark and you're sitting in a queue of traffic which isn't moving. It would be nice if the brake lights didn't stay on.

Maybe there's a regulation that says you must keep the brake light on while the brake pedal is pressed. Any ideas why the light shouldn't go off after 10 seconds?

 

[BTB 500]

Any ideas why the light shouldn't go off after 10 seconds?

I guess there'd be scope for this to malfunction or fail such that the brake lights didn't go on at all, which would be worse.

I had an old Granada auto where the brake light lenses were discoloured and slightly melted ... I guess because it had been driven in heavy traffic a lot of the time.