Torque converters: what are they and what do they do?

A cross-section of a torque converter (hydro-kinetic).

Cars with automatic transmissions need a way for the engine to turn at idle without also turning the transmission and wheels.

In a manual car, simply depress the clutch to decouple the engine and transmission.

But in an automatic, things aren’t that simple.

To that, we introduce the torque converter (pictured above), a type of device that lets the engine spin independently when it’s mated to an automatic transmission.

It doesn’t use any sort of gear arrangement, but rather a fluid coupling.

How does a torque converter work?

A torque converter consists of four main components:

The turbine
The stator
The pump
Transmission fluid

The torque converter is attached to the engine’s flywheel, and it turns at whatever speed the engine happens to be at that point in time.

The engine drives an impeller, which flings transmission fluid onto a turbine.

The turbine is connected to the transmission, which causes it to spin and hence propel your car forward.

At lower engine speeds, the turbine rotates slower compared to the impeller.

As engine speeds increase, so does the turbine.

The stator, meanwhile, lives in the centre of the torque converter. It acts as a sort of barrier to fling fluid back to the turbine instead of back to the pump, which increases the torque converter’s efficiency.

When your engine is idling, fluid is pumped back to the turbine at a very slow rate, which subsequently causes the transmission to turn very slowly.

However, with more throttle input, the pump (impeller) propels more fluid into the turbine, which then turns the transmission more quickly.

But this system does not have 100 percent efficiency, which means the turbine always spins slower than the impeller.

Some cars have a lockup clutch that locks both the engine and transmission together when at speed.

That eliminates the problem above and improves engine efficiency.