Automatic gearbox and torque converter (still) belong together.
The basic functions are the same as those of the hydro dynamic clutch. The kinetic energy of the engine is transferred by an impeller paddle-wheel to the oil flow and then to a different shaped turbine paddle-wheel, thus to the gearbox. In this case, we're dealing almost exclusively, with the automatic gearbox.
Apart from the impeller- and turbine-wheel an idler is now also necessary.
With the torque converter one distinguishes between two areas: - the converter area - the clutch area. The converter area is connected, with appreciable RPM differences, between the impeller and the turbine wheel. In the clutch area, all the parts rotate as a unit. As in the hydraulic clutch, the oil flows directly from the pump- onto the turbine wheel. The flow-back is different, this takes place indirectly through a third wheel (idler). Thus, the hydraulic clutch manages with only two paddle-wheels, the torque converter needs three.
Altogether, in it's path, impeller wheel - turbine wheel - idler - impeller-wheel, the oil-flow is reversed twice, in the turbine-wheel inwards and against the rotation direction, and in the idler, once again, but in the rotation direction and outwards. To do this, the idler in the converter area must be prevented from turning backwards. It is supported against the transmission housing by a freewheel, which, in the clutch area, ensures an interference-free co-rotation.
Flow-reversing causes a build-up -> torque boost
The build-up is caused through the second reversing of the converter oil-flow in the stationary idler, thus, the torque delivered to the turbine wheel is increased. The greater the RPM difference between the impeller- and the turbine wheel is, the greater the build-up to the paddles of the turbine wheel is, thus the increase in torque. Whereby, the output torque can be up to 2,5 times higher than the initial torque, one reason why vehicles with an automatic transmission and torque converter often have one gear less in the subsequent planetary gearbox.
Torque converter with it's own cooling system
Particularly when pulling off, the work of the torque converter is often associated with a great deal of heat-development. The heat is led off by an oil circuit to a separate gearbox oil cooler found either directly on the gearbox or in the vicinity of the engine cooler. The oil reaches, or leaves the converter through drillings, either in- or between the shafts, to the subsequent automatic gearbox. This is where the pump is installed, which causes the oil circulation.
Lock-up clutch: reduces the impeller-/turbine wheel slip
The torque is not completely transferred in the clutch area. This means a loss of performance and efficiency, which can be compensated for by installing a converter lock-up clutch. Through the reversal of the oil flow, it's friction pad is pressed to the left against the casing. Thereby, the impeller- and turbine wheel are united. In this case, the converter lock-up clutch above, functions like a single-disc oil-bath clutch. To facilitate this, the entire oil pressure is used. This, and with it the slip, can, in modern, electronically controlled automatics, be varied. In this operative state, an oil circulation does not take place. Efficiency and delivered performance improve. To reduce consumption drawbacks that the automatic has, compared with the pressing competition in the luxury class from, e.g., the continuously variable transmission and the sequential gearbox, the converter lock-up clutch now operates on all gears and even in the converter area. 11/11