Automatic transmission - how does it work?

[jimsunz]

I am a very practical person, and am familar with engines and manual
gearboxes/clutches. But automatic transmission is a new ball game to
me.

Can one of you guys explain how an automatic works. Are there clutch
plates to wear out? How does it know when to change gear?

ps. I managed to successful fit the kit cruise control on my 740
automatic, taking the pulses from 3 magnets and pickup coil on the
prop shaft.
Absolutely magic!on this my first automatic transmission car.
Delighted to find out that one can press the resume button at say
15-20 miles per hour and it goes up thro' the gears to the preselected
cruising speed.
I've lost count of how many cars this kit cruise control has been on,
and it is still in working order.

 

[Stewart Hargrave]

I am a very practical person, and am familar with engines and manual
gearboxes/clutches. But automatic transmission is a new ball game to
me.

Can one of you guys explain how an automatic works. Are there clutch
plates to wear out? How does it know when to change gear?

Start here:

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

Typically, an autobox will use an epicyclic gear set, sometimes called
planetary gear. But there have, over the years, been various other
arrangements. I'm not up on the lastest developements, but I would
imagine that computer control and 'fly-by-wire' engine management is
set to revolutionise (ahem!) transmission systems.

Most conventional autoboxes do have friction surfaces of some sort to
control the changes - old ones would have brake bands, but modern ones
have a type of clutch. These are used to control which parts of the
gear set are able to rotate. They differ from the conventional clutch
that you find on a car, in that they are immersed in oil, so wear is
minimal, to the extent that you can expect an autobox, with proper
care, to achieve 200k miles. Usually they outlast the car, but with
Volvo's reputation for high mileage this can become an issue.

The link between engine and gearbox is most often by a torque
converter, (also called a fluid flywheel, though in the strictest
sense this is not necessarily correct), which is detailed here:

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

One of the most unlikey arrangements I came across was by Renault,
probably around the late 50s or early 60s. They used a conventional
gearbox with some sort of rams to effect the gear changes (I can't
remember what controlled them). Instead of a clutch, they had a drum
filled with iron filings bolted to the flywheel. The output shaft to
the gearbox was finned, and the drive was engaged by passing a current
through a coil round the outside of the drum, to magnetise the iron
filings. I think history tells us that this can't have been a success.
--

Stewart Hargrave


For email, replace 'SpamOnlyToHere' with my name

 

[Stephen Henning]

The first ones made by Oldsmobile and were like a clutch where the
plates never touched. There was a viscous fluid that transfered force
between the plates. Then they went to hydraulic pump designs. The
newer ones all have several things in common. They need engine rpm
sensors of some sort and speed sensors of some sort. They can change
their ratios depending upon load and speed.
They need the ability to slip when stopped and to lock up at cruising
speed. Check the following for details:

http://www.familycar.com/transmission.htm

 

[Bonnet Lock]

In an earlier contribution to this discussion,

The first ones made by Oldsmobile and were like a clutch where the
plates never touched. There was a viscous fluid that transfered force
between the plates. Then they went to hydraulic pump designs. The
newer ones all have several things in common. They need engine rpm
sensors of some sort and speed sensors of some sort. They can change
their ratios depending upon load and speed.
They need the ability to slip when stopped and to lock up at cruising
speed. Check the following for details:

http://www.familycar.com/transmission.htm

Have a look at the service notes for a Borg-Warner 35 transmission, as
fitted to a Triumph 2000 about 30 years ago
http://www.200025pi.co.uk/lucas/greenbook/borgwarner/borgwarner.htm

It gives a good idea of the principle of operation.

More modern transmissions have more gears and electronic (rather than
hydraulic) control logic - but they still have a torque converter and
several cascaded sets of epicyclic gears. Some have lock-out clutches in
parallel with the torque converter to eliminate slip in the cruise
condition.

Gear change points are a function of both speed and throttle position - so
at high throttle openings, they hang on to the lower gears for longer - boy
racer style!

 

[Peter K L Milnes]

The Magnetic Clutch was also used by NSU in their Ro80 car. The three-speed
automatic transmission was engaged/disengaged by light pressure on the gear
knob which operated the electric current for the Magnetic Clutch which
allowed the transmission to work. The three ratios used were 0 to 40mph, 0
to 80mph and 0 to 120mph.

Cheers, Peter.