Thread: The Grey Box of EVIL.. (aka autotransmission)

Sorry gents - just realised I may have hijacked the thread, I'll move it elsewhere

The Torque converter... (the good bit.)

all these pics are thanks to howdoesitwork.com

While its not technically part of the auto transmission Its the heart of it....and before you say WTF allow me to explain...

If you removed the torque converter and installed a normal clutch the automatic gear changing part of the auto would still work and the gears would change but it would be rougher than a 4BDI running on 3 cylnders... the pumps would still run and make the box work but there would be no slip to help cushion the changes nor torque multiplication to give the box a wider useable range of ratios.

so a fluid coupling was added.. think of the 2 fans I explained earlier and your bang on the money, put the fans less than 1mm apart and fill em with oil and thats it the first fluid coupling drive... but things can always be improved and now we have the torque converter.

So whats in em..... heres a cut away... It doesnt show all the critical parts but it gives the general layout.
torque-cutaway.jpg

The front oil pump (fills the TC and makes everything work) would attach to the little black spigot over the stator shaft. and The Torus ring or Donut is missing from this simplified view.

At this point in time we will assume that the magical elfs have filled the TC for us and filling it with oil or cooling it are things we just dont need to do.

So the impeller or pump....
torque-pump.jpg
If you take the hollow looking ring out of the middle (the torus ring) it looks the same as every high preformance impeller type pump you care to name, from your engines water pump to a brigs and stratton 5hp fire pump. The principles simple, you spin it while its immersed in fluid and centrifical force makes the oil move outwards at a rate of knots.

The torus ring does a couple of things for us the most important being the directing of oil in the right directions...(put that thought on the back burner and keep an eye on it) it also helps speed the flow of the oil through the vanes.

Ok so weve got the oil moving lets do something with it...lets push it into a turbine. which looks like this.
torque-turbine.jpg
now when we sandwich both these together the direction of rotation marries up... to proove it print the pictures on a single piece of A4 with the arrows on the outside (impeller on the right turbine on the left) and fold the page in half. Surprisingly doing this also helps understand the oil flow.

so to recap... the oil flows from the center of the impeller, outwards towards the edge of the impeller around the torus then into the outside of the turbine then around the next half of the torus to the center of the turbine to flow back into the impeller to be moved outwards to the edge of the impeller..... you get the idea...

In and of itself that would get drive.. but it wouldnt be all that efficient. the oil coming out of the turbine would be moving in the opposite direction to the impeller.. this would reduce the available driving effort... so lets turn it around and make it move in the same direction as the impeller... with.. a stator

torque-stator.jpg

this thing simply turns the oil 90 degrees and pushes it into the impeller helping the impeller turn while the turbine is moving slower than the impeller... and this is where we get out torque multiplication...

when you stomp on the loud pedal the impeller moves the oil, and quite quickly.. and the faster the oil comes out of the impeller the faster the oil is moving through the stopped tubine the faster it hits the stator, the faster it hits the stator the faster it pushes the impeller around the faster it pushes the impeller around the quicker the impeller goes... Cool isnt it. well not really it gets hot a hell doing it.

as the turbine speeds up the oil flow through the vanes of both the impeller and turbine slows down so the stator becomes redundant this is where its one way clutch comes in. When the oil stops pushing against the stator and begins to pull against it the one way clutch lets it turn in the direction of rotation of the whole TC.

Once we've got the turbine and impeller truning at the same speed we dont really need the inefficiency of the fluid coupling so a lockup clutch engages and makes the 2 rotate as one.

Now we can worry about oil... Oil is provided by the oil pump that is attached to the impeller in the gearbox, as soon as the engine is turning the impeller is turning so the pump is pumping and its first priority is to get oil into the TC.

Once the TC is full it will begin to drive whenever the engine is turning....

Now lets talk about pressure......IF Im creating vacume to move a fluid the maximum pressure differential i can generate is 14.7psi, not a hell of a lot..If im pushing a fluid I can generate as much pressure as I can make prior to something failing.

this in part is why you need to bring the TC up to speed before any engine braking will occour...once the oil is moving around the TC the stator will redirect it into the impeller... same concept as above.. BUT you Must get it started and keep the oil flow high enough to keep it happening.

Even then this will not always work and what you will be resorting to is overworking the brakes by making them hold back the weight of the vehicle AND the driving force of the engine. The driving force of the engine will stop the wheels from locking up.

The Ideal solution would be the ability to have the T/CLC engage at any time when Turbine speed exceeds Impeller speed And in theory this can be done, quite easily in a totally electronic box (they might just change my alliegence) and theoretically possable in an old school hyrdaulic brained box.

now Stall speed, Ive touched on this breifly.... now in detail

Stall speed is the maximum speed that is attained by the engine when the turbine is stopped from turning.

the converter is generally rated for an amount of power and a stall speed.... If you put a baby TC (low power and low stall) onto a stinkin great V(lots) and step on the loud pedal the stall speed will be higher than rated and you'll kill it.

alternatively if you put a gargantuan TC (huge power and super high stall) on a litle engine it will never reach rated stall and just rob you of what little power you had...

as a general rule, lower stalls are for smoother application of power and typically do not have as much multiplication to them (1.5-2.5:1) and higher stalls are for sudden takes offs, like a drag racer and have much higher multiplications (3+:1)

And thats about it for the night....stay tuned for our next exciting adventure where we
will delve into simple planetry gearing and why they are so robust and suited to use in the grey box of evil... And in related news we will be touching on the brain.