In a Summary, a torque converter is a fluid coupling that sends torque from the engine to the transmission. It is planted between the engine and transmission, bolted directly to a ‘flex plate’ that is spun by the crankshaft.

Internal combustion engines generates power by burning fuel that forces the pistons to turn the crankshaft located at the bottom of the engine. This rotational force is transferred to the transmission by the fluid pressure inside the torque converter.

Inside of the torque converter cover is a series of propeller-like blades known as the pump. This assembly turns in sync with the engine crankshaft, forcing transmission fluid onto another blade assembly that is   called the impeller. This second set of blades is attached to the transmission input shaft. The amount of hydraulic pressure that it creates inside the transmission dictates the gear and ultimately, the speed of the car.

The impeller’s speed is modulated by the engine side of this hydrodynamic circuit ( speed of the pump blades). When the vehicle is stationary, or the driver matches the brakes, the impeller will slow considerably, while the pump continues to spin. This allows the torque converter to operate like the clutch in a manual transmission – it allows the engine to continue running while the vehicle is at a complete stop.

immediately the transmission fluid has been hurled onto the impeller blades, it has to return to the pump in order to maintain the cycle going. Since the fluid is now flowing in a different direction than the pump, it has to be reversed to avoid slowing down (thus stalling) the engine.

To archieve this, a third finned wheel refered to as the stator is located between the two turbines on the transmission pump shaft. Its blades are precisely positioned so that when the transmission fluid touches them, it reverses direction and gets channeled back to the pump. When the vehicle stops, its built-in one-way clutch causes it to stop spinning, thus breaking the hydrodynamic circuit.

Once the car starts to accelerate from a stop, the stator is once more free to spin. In the split second that the transmission fluid hits the back of the now-released stator, it starts to spin the transmission pump, and briefly increases the torque coming from the engine side of the circuit. This makes the transmission pump to force more fluid in the transmission, resulting in movement.

Once the vehicle is driving, the stator’s one-way clutch makes it to start spinning in the same direction as the other turbines, reversing the fluid flow and completing the hydrodynamic circuit.

when all of the transmission gears have been shifted through and the vehicle has reached cruising speed, the lockup clutch engages, connecting the front cover of the torque converter (also known as the pump) to the impeller. This causes all of the turbines to work together in a direct drive/overdrive situation.