TORQUE CONVERTER

The torque converter is modified form of fluid flywheel. Fluid flywheel is used for the transmission of power, whereas torque converters are used to transmit the power with varied torque as per the requirement. In addition the driving member (impeller / pump) and driven member (turbine), it consists of a reaction member also (stator). In its simplest form it consists of an impeller connected to the crankshaft, turbine connected to output shaft and stator mounted on overrunning clutch on stationary component impeller is normally an integral part of the converter housing. (It is generally welded to the cover half during the manufacturing). 

Turbine and stator are enclosed within the welded housing. The stator incorporates a one-way clutch and mounts on a stationary support shaft that is grounded to the transmission case directly or indirectly through the transmission pump assembly. The impeller and turbine blades are designed with special features. The curved shape of the impeller blades in a backward direction gives added acceleration and energy to the oil as it leaves the impeller. The curved shape of the turbine vanes is designed to absorb as much energy as possible from the moving oil as it passes through the turbine. The vane curvature has two functions that give the turbine excellent torque absorbing capacity. It reduces shock losses due to the sudden change in oil direction between the impeller and turbine. It also takes advantage of the hydraulic principle that the more the direction of a moving fluid is diverted, the greater the fore that fluid exerts on the diverting surface. The fluid impact is absorbed along the full length of the vane surface as the fluid reverses itself. The stator is the third bladed member of the converter. During the torque phase, its function is to redirect the fluid flow as it leaves the turbine and reenters the impeller. This assists the impeller rotation and gives a thrust boost to the fluid discharge.

Converter starts operating when the impeller starts rotating, with the engine providing the required input. The impeller creates a centrifugal pumping head or vortex flow. At the same time, the fluid must follow the rotational inertia or the effort of the impeller. These two fluid forces combine to produce a resultant force in the form of an accelerated jet stream against the turbine vanes. The impeller and turbine attempts to act as an effective fluid coupling by featuring curved impeller and turbine vanes rather than a straight radial design. The turbine vanes reverse the fluid direction. The curved turbine vanes provide efficient energy transfer, but the reentry of the remaining fluid thrust back to the impeller, works against the impeller and crankshaft direction. Hence, it is necessary to introduce the stator element to make the converter work. The stator is employed between the turbine, outflow and impeller inflow to reverse the direction of the fluid and make it flow in the same direction as that of the impeller. Instead of the fluid opposing the impeller, the fluid energy now assists the impeller and crankshaft rotation. This results in boosting the rpm of’ the impeller. This allows the impeller to accelerate more and recycle the fluid with a greater thrust against the turbine vanes. The purpose of using the remaining fluid energy to drive the impeller is referred as regeneration gain. The stator is mounted on a one-way clutch. During the torque phase, the stator remains locked and at coupling speed it overruns.

The torque multiplication occurs when the turbine is turning at a slower speed than the impeller and the stator is stationary / reactionary. This is called torque phase, slip phase or stall phase. This sequence generates a boost in output torque. Recycling of the fluid permits more of the impeller input to be used in increasing the jet stream velocity and turning effort of the turbine. By helping the impeller to accelerate the fluid thrust against the turbine, the stator provides the basis for torque multiplication The curved turbine blades absorb the energy from the impeller discharge until the force of fluid is great enough to overcome the turbine resistance to motion. The converter torque is equal to the product of effective fluid force and working radius of the turbine (torque= force x lever arm radius). It is similar to the torque multiplication by gear reduction. The maximum throttle occurs with the engine at wide open throttle (WOT) and zero turbine speed. This is commonly referred as torque rating or stall torque of the converter. For best efficiency, engineering design of the three-element converter keeps the maximum torque ratings within a range of 2:1 to 2 5:1.

During the torque phase, vortex flow is the predominant force in the fluid. Therefore the fluid cycles like a continuous chain from the impeller to the turbine and back to the impeller through the locked stator. This action is continuous until the turbine speed is at nine-tenths of the impeller speed, at which the converter has achieved a speed ratio more than 90%. After a moment at stall the turbine and vehicle starts moving. Once the turbine starts, it becomes easier and easier for the fluid force to drive the turbine and vehicle. The turbine rpm actually starts to gain and approach impeller speed. As the turbine gains the speed, the turbine lever arm absorbs less and less of the fluid force and converter torque output gradually drops. 

The fluid thrust under vortex influence is trying to hit a moving target that is moving away from it faster and faster. Finally when the converter has reached a speed ratio of more than 90%, the converter enters the coupling phase of the operation. The stator is no longer needed and must freewheel with the rotary flow. The vortex effect on the fluid has dropped significantly and the rotary flow is now the main force. The rotating inertia of the fluid mass, impeller and the turbine form a hydraulic lock or bond. The converter is now in coupling phase and the torque ratio is 1:1. When the speed difference between the impeller and turbine is at its minimum, it is referred as coupling phase. It occurs when the torque converter is operating at its greatest hydraulic efficiency. At this point the stator freewheels and there is no torque multiplication.