The oil in the torque converter transmits torque smoothly, above all during the starting-up process, and reinforces it until the engine and drivetrain are approximately synchronized. The more softly and smoothly start-up and gear changes are effected, the less favourable is efficiency.
Two ranges are distinguished in a torque converter:
– the conversion range, with speed differences worth mentioning between impeller and turbine wheel
– the coupling range, with very minor speed differences.
In distinction from the hydraulic coupling, although the oil still flows directly from the impeller to the turbine wheel, it returns indirectly via a third wheel (the guide wheel). Thus, the hydraulic coupling manages with two bucket wheels whereas the torque converter needs three. Altogether, the oil flow is turned around twice through the impeller – turbine wheel – guide wheel – impeller, against the direction of rotation and inwards in the turbine wheel, and again in the direction of rotation and outwards in the guide wheel. For this purpose, the guide wheel must stay fixed in the conversion range. Free-wheeling lets it accompany the turning motion uninterruptedly in the coupling range.
During the second deflection of the torque converter oil in the stationary guide wheel, there is a tailback and the torque delivered to the turbine wheel in the first deflection is reinforced. The greater the differences in rotational speed between the impeller and the turbine wheel, the stronger is the back pressure on the paddles of the turbine wheel and thus the torque reinforcement. The output torque can thus be up to 2.5 times the input torque, which is why vehicles with fully automatic transmission and a torque converter in the downstream planetary gearing frequently have one gear fewer.
The work of the torque converter is often associated with the generation of a great deal of heat, particularly in the lower conversion range. It is led off through an oil circulation circuit to a separate transmission fluid cooler. The oil reaches and leaves the torque converter through the shaft hole and/or between the shafts to the following automatic transmission. The pump that causes the oil to circulate is also incorporated there.
The torque is not completely transferred in the coupling range. This means a loss of power and efficiency, which can be balanced out by installing a torque converter lock-up clutch (photo). The friction lining of the latter is pressed to the left against the casing by the reversal of the torque converter oil stream. It thus connects the impeller and the turbine wheel. The result is that the torque converter lock-up clutch works like a single-plate oil-bath clutch. All of the oil pressure is used for pressing it closed. In a modern, electronically controlled fully automatic transmission, this pressure, and thus the slip, can still be varied. In this operating condition, no oil circulation takes place. Efficiency and power output are improved. The lock-up clutch now operates in all gears, even in the conversion range, mitigating the consumption disadvantages of the fully automatic transmission as against competing systems in the superior class range, e.g. continuously variable and sequential transmissions.
In principle, the torque converter transmits the engine torque hydraulically to the transmission input shaft. The pump and thus the complete torque converter housing is connected non-rotatably with the engine and the turbine and non-rotatably with the transmission input shaft via hub toothing. The whole torque converter is filled with transmission fluid. Paddle wheels that cause a circular oil flow between pump and turbine when there is a speed differential are fitted in the pump and the turbine. The oil is admitted by suction from the internal diameter of the pump and is pressed outwards by centrifugal force. The oil is then flung from the pump into the turbine and is diverted there by the turbine blades, as a result of which a torque is generated in the turbine and in the transmission input shaft .
During starting up, or when there are high speed differentials between the pump and the turbine, the oil flow is diverted in the turbine in such a way that the guide wheel would have to rotate backwards. Free-wheeling is however installed in the guide wheel, as a result of which the guide wheel is blocked via the stator shaft if it tries to rotate backwards. A guidewheel torque is thereby generated which, because of the torque equilibrium in the torque converter, increases the transmission input-shaft torque by a factor of up to 3 over the engine torque. The efficiency of the torque converter is thus particularly high, especially during start-up situations.
Account must be taken of the fact that the hydrodynamics of the torque converter can only transmit torque when there is a speed differential between the pump and the turbine. Thus, in continuous driving mode, when the rotational speeds of the pump and the turbine have matched up, a lock-up clutch hydraulically driven by the transmission takes effect. Slip is eliminated, no more power is lost due to the operation of the torque converter, and reduced fuel consumption is the consequence. Early locking-up and thus economic consumption are ensured by LuK using innovative dampers.
On the engine side, the torque converter is mounted on a flexible flywheel. The pump throat usually also serves to drive the oil-supply pump of the automatic transmission. To get the best possible performance, LuK systematically uses Computational Fluid Dynamics (CFD) simulation tools here to optimize flow guidance and reduce the vehicle’s consumption.