Physical intuition behind torque converter A torque converter (also here) is a device used in some cars. It uses several "fans" coupled through a liquid (transmission fluid) in order to perform the function of a clutch, but more importantly it acts as a liquid gear in the sense that it multiplies the torque going from the engine to the wheels. 
Is there an intuitive way to explain what is happening in the liquid? In particular, is it possible to explain the torque multiplication effect without resorting to numerical analysis?
 A: A torque converter contains a propeller that is spun by the engine and another close by which is connected to the rest of the transmission. these two propellers face each other in a chamber filled with oil, so that when the engine-driven prop spins, it pumps oil through the second prop and causes it to spin as well. 
Between these two props is a disc with blades in it which catches the oil after it has passed through the second prop, and diverts its flow back around to the engine-driven prop which then picks up that return oil and pumps it into the second prop again. 
This disc is cleverly designed so the return oil flow strikes the engine-driven prop blades at such an angle that the energy contained in the return flow adds to the energy being pumped into the flow by the engine-driven prop, in an amount proportional to the difference in rotating speed (called the slippage) between the engine-driven propeller and the second prop.
Recovering the energy in the return flow multiplies the torque force exerted on the second prop by as much as ~3X in the high-slip limit where the second propeller is standing still and the engine is running at speed (which is called the stall torque condition)- as they would be when you put your foot on the gas from a standstill.
In the limit where both props are running at roughly the same speed (the no-slip condition), the torque multiplication factor falls to ~1X. 
The action of that extra disc of blades is the key element in getting torque multiplication during the slip condition and is the hardest thing to grasp in the operation of a torque converter!
A: Check this video below that perfectly describes the operation of torque converters.
Fluid Coupling: Principles of Operation (1953)
Torque converters (TC) are defined by a torque ration, which is in turn defined by the turbine design withing the converter. There are two turbines, as described in the video. If you make a TC out of two identical turbines you will get 1:1 ratio. The torque remains the same. This is very similar to the operation of the transformer.
However, if you change the design of the output turbine so that is increases or decreases the torque (this is done by changing the number and/or shape of turbine blades) you will get different torque ratios too.
One turbine propels the fluid and serves as a pump. The other turbine is driven by the moving fluid. In this way, two turbines in TC are coupled by the liquid. BY changing the design of secondary turbine you can make changes in the value of the output torque. Read Euler's pump and turbine equation.
A: To me, this is by far the most understandable video that explains this mechanism:
https://www.youtube.com/watch?v=bRcDvCj_JPs&feature=emb_rel_pause
To summarize, the torque multiplication is the result of the reactor that helps the pump increase the oil pressure. When the difference of speed between the pump and the turbine is high, the reactor derivates the oil flow in the same direction as the pump. The energy unused by the turbine is thus given back to the pump, which increases the torque. When the turbine speed and the pump speed are near, the reactor starts spinning, and the torque multiplication ceases.
