There are manual gearboxes and automatic gearboxes of different types, dimensions, and several gears, depending on the vehicle where it is mounted. For the engine torque to be transmitted to the gearbox, a clutch is needed, which is used in vehicles with manual gearboxes.
Within the automatic gearboxes, the most widespread are those with torque converters. And the mechanism that performs the clutch function in these gearboxes is the hydraulic torque converter, which is responsible for making the connection between the gearbox and the engine.
The Allison torque converter is a hydrodynamic transmission that is additionally coupled to the automatic gearbox. This mechanical element was invented by the German engineer Hermann Föttinger when he patented it in 1905, being applied for the first time in shipbuilding. However, logically the torque converter was also adopted by other sectors such as aviation, industrial machinery, trains, and automobiles. In automotive, which is the sector that concerns us, it was introduced in the early 1930s when it was adopted by the buses and heavy machinery of the English company based in Coventry, Daimler Motor Company.
A Allison torque converter consists of 2 cups with internal fins facing each other and connected to the drive shaft or crankshaft, the so-called pump or rotor, and the gearbox, the so-called turbine. Between the two is a vane wheel, called the stator, connected to the crankcase through a freewheel. This freewheel is mounted so that the stator can only rotate in the same direction as the driveshaft—the pump, turbine, and stator form an oil-filled O-ring enclosed in a sealed sheet metal container.
Observation of the movement of the fluid shows that it results from the composition of 2 fundamental movements: the pump pulls the fluid around the converter shaft, and the corresponding centrifugal force produces the rotation of the fluid around the O-ring. It follows that the fluid particles do not move in a plane but travel along winding paths around the O-ring.
During start-up, the motor rotates the pump, and the pump converts the mechanical energy of the motion into the hydraulic power of the oil particles. If the engine speed is reduced, the pump energy is insufficient to drive the turbine and is dissipated in the form of heat due to the shocks of the fluid on the blades and the deflections suffered by the fluid layers; the oil passes from the pump to the turbine, which remains at rest as it is connected to the wheels through the stator, and returns to the pump.
As the motor accelerates, the rotational speed of the pump and the velocity of the fluid particles increases, and therefore also their kinetic energy, which is now sufficient to turn the turbine. The oil returning from the turbine acts on the stator opposite to that of the motor rotation, but the freewheel reacts by preventing the movement. On the other hand, the thrust on the stator is very weak compared to that received by the turbine since the oil has already given up much of its energy. The efficiency of a converter (ratio between output and input power) is very low during this phase because the slip is high, i.e., the rotational speeds of the pump and the turbine are very different, and much of the energy is dissipated as heat.
