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from http://www.carbibles.com/transmission_bible.html

Also has great discussion about Sequential manuals etc....

Thanks to Mark @ Maserati of Minneapolis for the original ref to this site

Torque Converters

Just like a manual gearbox, an automatic gearbox needs a method of decoupling the constantly-spinning engine from the gearbox components. To do this it uses a torque converter which is a viscous fluid coupling (because it's full of hydraulic fluid). A torque converter consists of three basic elements. The impeller, the turbine and the stator. The impeller is attached to the torque converter housing which itself is attached to the engine flywheel. The impeller is basically a centrifugal pump. As the flywheel spins, so does the impeller and the vanes take the fluid from the central part of the torque converter and fling it to the outside creating a pumping action. The fluid then circulates around the outer edge of the torque converter and back into the turbine. The turbine is basically the opposite of the impeller - it's like a ships's propeller in that the fluid passing through it causes it to spin. The turbine is connected to the input shaft of the gearbox via a splined shaft so as the turbine spins, so does the input shaft to the gearbox. The fluid passes through the turbine from the outside towards the inside. Finally, as the fluid reaches the central core, it passes through the stator which is designed to help redirect the flow into the inner vanes of the impeller. (Without the stator, the whole system would be a lot less efficient) With this mechanism, the fluid is constantly being circulated. In the image below I've rendered the various parts of an example torque converter taken apart so you can see the internal construction.


When the engine is idling, the fluid is pumping around without a lot of force and the amount of torque on the turbine is minimal. As you accelerate, the impeller speeds up and creates larger forces on the turbine which in turn spins more quickly and with more torque. Because it's connected to the input shaft of the gearbox, this feeds more rotational speed and torque into the gearbox and the car starts to move forwards. It's because of this viscous liquid coupling that automatic gearboxes have a certain amount of 'slop' in them - the engine can rev up and down without the car actually changing speed too much. It's also the reason automatics are less fuel efficient because the torque converter uses up energy from the engine simply in its design by spinning the hydraulic fluid. In the image below I've rendered a cutaway of an assembled torque converter. The shaky yellow arrow is my attempt to show the basic circulation path of the fluid inside as it is pumped from the impeller (red) through the turbine (blue) and back through the stator (green).


For sportier vehicles or those with specialised needs, some torque converters include a hydraulic clutch. Once the car is moving and in top gear, the clutch engages and locks the turbine to the impeller. Once that happens, the whole torque converter spins as one and the viscous coupling becomes redundant - effectively the gearbox now behaves like a manual because the engine flywheel is connected directly to the gearbox input shaft. By locking all the components together, it makes the car as fuel efficient as a manual when in top gear because the energy that was being used up in the viscous coupling is no longer required. It also means instantaneous throttle response - you push the accelerator and the car accelerates instantly just as with a manual.

But why is it called a torque converter? Very simply, because it has the ability to multiply the torque from the engine 2 or 3 times in certain conditions. Basically, from a standing start, when the engine is spinning far faster than the gearbox, the whole design allows the torque from the flywheel to be multiplied. As the car gets up to speed, the multiplication factor drops until it becomes 1x once everything is in motion and the impeller and turbine are moving at almost the same speed.

Doing it yourself. In true Blue Peter fashion, you can demonstrate the principle behind a torque converter at home. Get a large bucket or bowl and a cordless drill with a paint-stirrer. Fill the bucket with water and put some bits of paper around the outside of the bucket, floating on the water. Stuff the paint stirrer in the middle and pull the trigger on the drill. To start with, the paint stirrer is spinning way faster than the water in the bucket, and the bits of paper will barely be moving. As the water in the bowl begins to speed up its circulation, the bits of paper will being circulating the bucket at speed. Eventually the water in the bowl will be circulating at almost the same speed as the paint stirrer is turning. (At this point your wife/husband will probably also be complaining that it's going all over the kitchen/bathroom - you've been warned) It's that "almost" that shows the inefficiency in a torque converter - the fluid can never spin at exactly the same speed and thus it can never impart the exact same torque and motion into the turbine. Now imagine that the bucket or bowl has vanes around the inside of it. As the water is circulating, it's going to be applying force to those vanes and given a slippery enough surface, your bucket or bowl will eventually start to spin. Voila. The drill and the paint stirrer are the input from the engine and the spinning bucket or salad bowl is the output to the gearbox.
The other way to do this is to take two desk fans and turn one on and point it at the other. Eventually the second fan will start to spin because of the air being forced past it by the first fan. This uses the same principle but with moving air instead of water and it's nowhere near as much fun to watch
 
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