Fluid Coupler
Imagine two fans facing each other at a very close distance. If one fan was turned on, the blast of air would move the blades of the other fan. In this case the medium of power transfer is air. In a fluid coupling oil would be the medium. With the fans the power transfer would be inefficient because the area in which this is happening is not enclosed, while in a fluid coupling the area is enclosed. The fluid coupling is also more efficient than the fans because the two torus (each half of the fluid coupling) can be mounted extremely close to each other. If one were to put oil in a torus without it being enclosed by the other torus, spinning it would cause the oil to fly outward and upward. By putting in the other torus, that outward and upward motion hits the veins of the covering torus and causes it to spin. This is refereed to as rotary motion.
If the driven torus were to be placed above the driving one, the oil from the driving torus would hit the veins of the driven one. After striking the veins it travel up, over and back down into the driving torus. As the driving torus is rotating the oil will be thrown outward and upward against the veins of the driven torus. This motion is called vertex flow. When both the driven and driving torus are rotating at the same speed, rotary flow is constant and there will be little to no vertex flow.
The Torque Convertor
The torque converter is similar to a fluid coupling but there is one very important difference. A fluid coupling can transmit torque but not multiply it. The torque converter can multiply and transmit engine torque. The fact that it can multiply torque allows automatic transmissions to use less gears.
The torque converter also uses two torus' but in most cases they are refereed to as the impeller (also known as the pump or driver) and the turbine (driven).
Vanes
The converter vanes (blades) are curved to make the flow of oil less turbulent. The impeller blades are curved the opposite way of the turbine blades.
Stator
A stator is placed in between the impeller and turbine to help multiply torque. It intercepts the oil the turbine throws and redirects it so it will enter the impeller with minimal turbulence. As the impeller spins it throws oil into the turbine blades but instead of the fluid being thrown back to the impeller vanes (like in a simple fluid coupler), the oil first passes through the stator.
The stator is mounted on the input shaft and is allowed to spin only in the direction of the impeller, because the stator has a built in one way clutch.
Torque Multiplication
As the pump discharges oil it send it to the turbine vanes. But the oil does not just turn the turbine and go back to the impeller. It passes through the stator, which continues to redirect the oil to the turbine causing it to turn faster and faster. By redirecting all this oil the torque has been multiplied. Once the turbine has reached the same speed as the impeller the torque can no longer be multiplied, torque can only be multiplied when there is a difference in speed between the turbine and impeller. The greater the difference the greater the torque multiplication.
Stall
As the engine speed gets faster and faster, the impeller also speeds up, causing the oil in the converter to travel much faster which makes the turbine spin much faster. The stator makes all that oil that is coming back from the turbine enter the whirling motion of the impeller. The maximum torque multiplication is delivered when the pump has reached its max speed and the turbine is at stall (standing still). At this point the converter become efficient as a clutch in a manual transmission. You would not want it to be efficient at idle because if you come to a stop and its still acting like a clutch then the car would shut off
Lockup
Some converters include a lockup clutch which improves efficiency by making the impeller turn at almost the same speed as the engine. The downside of a lockup torque converter is that it makes for a much rougher engagement.
The Torque Convertor Acts Like A Clutch?
If the torque converter can act as a clutch, what would happen if the clutch were left engaged when a manual transmission was in gear and the car came to a complete stop? The car would turn off. This is the same thing that would happen with an auto transmission if the torque converter were to be left transferring power while at a stop. Since the converter is not very efficient at low RPMs, there is no power transfer from engine to transmission while the car is at a complete stop and in gear. So essentially at that point it would be like if the car automatically pushed a clutch pedal.
Although the converter is not very efficient in the low RPMs, it is extremely efficient when the engine is at a road speed. At road speed, the converter transfers power in a 1:1 ratio.
