The Torque Converter

Not to be confused with a torque converter, also used in transmissions.

As we all know, there’s something called a clutch that separates the engine from the wheels, enabling us to stop the car, whilst not stalling the engine. What is less well known is that automatics can’t actually use a clutch as such and need a very different mechanism to perform the same job. And this extraordinarily ingenious device is called- yes well done, you guessed it- a Torque Converter.

Technically speaking, it’s a ‘fluid coupling’ and is brilliant in its simplicity, using the properties of a liquid, oil in this case, to transmit the engine’s rotational energy to the drive shaft in a manner that can be controlled. If you have ever tested an egg to see whether it is raw or boiled by spinning it on a hard surface, stopping it and seeing whether it continues to rotate when you let it go again, you have already grasped the fundamentals of the fluid dynamics at play in a torque converter.

There are essentially four main components, the pump, the turbine, the stator and the transmission fluid.

The housing of the converter is attached to the flywheel of the ‘prime mover’ or engine and so rotates with that at all times. The pump inside operates centrifugally, ie as it spins, it throws the fluid to the outside. In doing so, it creates a vacuum in the middle, which draws the fluid in a cycle back to the centre. Here, the fluid hits the blades of the turbine, which in turn is connected to the transmission, causing it to rotate, thus basically making the car move.

By making the interior blades directional, you can see how the movement of the fluid would make the turbine move in the desired direction.

With all this highly pressurised liquid squirting about, it is important that it doesn’t re-enter the mechanism and hit the pump adversely, drastically decreasing the efficiency of the converter and this is where the stator comes in, deflecting and redirecting the fluid with a very aggressively angled blade, before it can interfere with the pumping mechanism and reversing the direction of the fluid flow before it meets the pump, greatly increasing the efficiency of the conversion.

What is interesting in this fluid equation is that as the car moves off and picks up speed, there comes a moment when the pump and the turbine will be moving at almost the same speed, although the pump will always rotate slightly faster. At this point the stator is no longer needed as the fluid will return from the turbine moving in the same direction as the pump. However, the stator itself has its own internal clutch, preventing it from rotating the wrong way and hindering the process.

The simple and efficient design makes well-built torque converters one of the more trustworthy elements of car construction, requiring little maintenance and with very few complex moving parts to go wrong. Talk to any long-term Merc owner and they will wax lyrical about how good their ‘automatic clutch’ is.

Under duress or with a converter under-engineered for the purpose it is put to, they can of course go wrong and in spectacular fashion: over heating, stator clutch seizure or even breakage, blade deformation and ballooning of the casing. These last two are potentially the most dangerous, as the converters housing will be compromised and bits of metal and hot oil are duly redistributed widely around the immediate vicinity. Choice.

Most importantly though, like phones, you can also get them in any number of pretty colours.