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Engine brake in Automatic gearbox



 
 
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  #1  
Old August 26th 06, 02:05 PM posted to rec.autos.tech
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Posts: 3
Default Engine brake in Automatic gearbox

Hello.

In an Automatic gearbox, instead of a clutch, there's a torque
converter, right?
Now, I know that in manual gearbox, engine brake happens because the
engine is 100% connected to the wheels - so engine revs down = car
slows down.
However, in automatic gearbox, there's the torque converter - which is
generally two parts in which the engine part moves the wheel part, and
it's done by oil pressure. The question is - how can the engine slow
down the wheels - when there's no direct connection betw/ them, only an
oil pressure one?

thank you.

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  #3  
Old August 26th 06, 02:21 PM posted to rec.autos.tech
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Default Engine brake in Automatic gearbox

Well, it's not exactly the same, because you can, by increasing the
pressure, increase the speed of the wheels, but how can you, by
reducing the pressure, slow it down? won't it just "ignore" the change?
for example (I know it's not the same) - in a bicycle - you can
accelerate by the pedals, but you can't slow down by turning them
slower...

Mark Olson כתב:
> wrote:
>
> > In an Automatic gearbox, instead of a clutch, there's a torque
> > converter, right?
> > Now, I know that in manual gearbox, engine brake happens because the
> > engine is 100% connected to the wheels - so engine revs down = car
> > slows down.
> > However, in automatic gearbox, there's the torque converter - which is
> > generally two parts in which the engine part moves the wheel part, and
> > it's done by oil pressure. The question is - how can the engine slow
> > down the wheels - when there's no direct connection betw/ them, only an
> > oil pressure one?

>
> You might as well ask, how can an engine drive the transmission through
> the torque converter, since there's no direct mechanical connection
> between them?
>
> In other words, the torque converter works both ways.


  #4  
Old August 26th 06, 02:29 PM posted to rec.autos.tech
Mark Olson[_1_]
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Posts: 6
Default Engine brake in Automatic gearbox

wrote:
> Mark Olson ???:
>>
wrote:
>>
>>> In an Automatic gearbox, instead of a clutch, there's a torque
>>> converter, right?
>>> Now, I know that in manual gearbox, engine brake happens because the
>>> engine is 100% connected to the wheels - so engine revs down = car
>>> slows down.
>>> However, in automatic gearbox, there's the torque converter - which is
>>> generally two parts in which the engine part moves the wheel part, and
>>> it's done by oil pressure. The question is - how can the engine slow
>>> down the wheels - when there's no direct connection betw/ them, only an
>>> oil pressure one?

>> You might as well ask, how can an engine drive the transmission through
>> the torque converter, since there's no direct mechanical connection
>> between them?
>>
>> In other words, the torque converter works both ways.


> Well, it's not exactly the same, because you can, by increasing the
> pressure, increase the speed of the wheels, but how can you, by
> reducing the pressure, slow it down? won't it just "ignore" the change?
> for example (I know it's not the same) - in a bicycle - you can
> accelerate by the pedals, but you can't slow down by turning them
> slower...


No offense intended, but the problem is that you have a mistaken idea
about how a torque converter actually works. Perhaps this will help.

http://auto.howstuffworks.com/torque-converter.htm
  #6  
Old August 26th 06, 04:23 PM posted to rec.autos.tech
JustSayGo[_1_]
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Posts: 1
Default Engine brake in Automatic gearbox


You are correct a bicycle is not the same as a torque converter. Think
back to your tricycle for a more accurate illustration. Torque
converters do not continue to slip at higher RPM as they are so
designed to slip at low RPM. Hydraulic pressure locks the converter
input to output when it turns fast enough. This lock-up point is called
stall speed. Think about centrifugal force when you put water in a
bucket and crank it around in a circle with your arm. The real high
hydraulic pressure in a torque converter is generated by centrifugal
force of the fluid being driven to the outward edges of the turbine
fins inside of the converter by the same principle as the bucket of
water rather than oil pump pressure. As torque converter RPM increases,
hydraulic pressure increases. Torque converters are still hydraulically
locked by the rpm of the trans being driven by the wheels as the car
slows down. For the past 30 years most automatic transmissions have an
electronically controlled Torque Converter Clutch to take advantage by
eliminating slip under desirable conditions as the torque converter RPM
drops below the stall speed RPM.


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  #7  
Old August 26th 06, 07:05 PM posted to rec.autos.tech
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Posts: 3
Default Engine brake in Automatic gearbox

thank you all for the answers

 




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