A torque converter is a device which performs a function similar to that of the gearbox, that is, to increase the torque while reducing the engine speed. The gearbox provides only a small number of gear ratio but, the torque converter provides a continuous variation of gear ratio from the lowest to the highest. A torque converter is designed such a way to obtain a mechanical advantage or gear ratio by hydraulic means in the same manner as gears do by mechanical means. It provides a higher gear ratio starting from rest and eventually decreases that ratio as the vehicle gains speed.
The construction of torque converter is similar to a fluid coupling, the only difference is that it has an additional stationary member called the reaction member or stator. Thus, a torque converter consists of three principle members instead of two, these are:
- Impeller or driving member or pump, which is connected to the engine.
- Driven member or rotor or turbine, which is connected to the propeller shaft.
- The stator or Stationary member or reaction member, which is fixed to the frame. It is that member which makes it possible to obtain a variation of torque between input and output shafts. The fluid coupling does not have this member and cannot produce any change of torque. the stator is mounted on a freewheeling mechanism or one-way clutch so that it can turn only in one direction. A stationary shaît, called the reaction shaft, extends from the geared transmission in back of the converter through the pump to support the stator. The driven member (turbine) faces the stator and is splined to the input shaft which rotates inside the reaction shaft and connects curve blades to project most of the oil into a hollow cylinder which is parallel to the axis of rotation. The turbine is driven by the stream of oil sent to it by the pump. The oil enters the blades near the rim and passes through the blades. The entire assembly is contained and connected to the engine flywheel.
Torque Converter cross-sectional view
When the engine is running idle, the pump produces a slow circulation of oil, as the engine drives it. This light circulation of oil is not sufficient to move the turbine. When the throttle is opened, the engine speed increases and the pump moves faster projecting the oil toward the turbine. But still, the oil is having less force which is not sufficient to turn the turbine. Therefore, the fluid returns back to the pump, with almost no loss of energy, except that due to friction. The entire effect of the redirection of flow is to increase pump output. The pump output under these conditions is the output velocity that it is creating because of the engine torque, plus the input velocity of the redirected oil.
A built-up output velocity of two to three times the output velocity due to the engine alone is achieved when the turbine is stationary. This leads to the turning force of the turbine to three times that of the engine. The increased torque of the turbine enables it to rotate and thus drives the input shaft, which in turn transmits the power to the geared section of the transmission.
Whereas the fluid coupling transmits the same torque as given to it by the engine, the torque converter increases the torque in a ratio of about 2: 1 to 3: 1. Thus, it serves the same purpose as that of a gearbox but in a better way. In a gearbox, the torque variation is only in a finite number of steps but in a torque Converter, the torque variation is continuous. The efficiency of a torque converter is high only within narrow speed limits.
The figure shows a simplified diagram of a single-stage three-element torque converter. The three elements-impeller, stator, and turbine are clearly shown. Other designs have more elements and additional stages to improve efficiency over a wide range.
TORQUE CONVERTER AS A FLUID COUPLING
A torque converter is a type of fluid flywheel that uses a fluid to transmit torque from one shaft to another. Because a torque converter has a stator, which is not in the fluid coupling, it increases the torque ratio. At a higher speed, the torque converter works as a fluid flywheel, giving the gear ratio 1:1.
|Torque Converter Impeller|
When the vehicle speed increases, the need for mechanical advantages decreases and the gear ratio of the torque converter gradually changes to that of a fluid coupling. It is because of the turbine speed gradually approaches the pump speed. This reduces the vortex flow so that less fluid is sent back to the pump by the help of turbine and stator. When the turbine speed reaches a level where the oil flow to the stator is no longer reflected, the stator begins to move with the rotating oil. This is the stage of fluid coupling where the gear ratio becomes 1:1.
This action depends on the throttle opening and vehicle load. At low throttle and steady load, the gear ratio may reach 1: 1 at low speed. The torque converter automatically provides the required gear ratio as the need arises. Such situations like hill climbing or rapid acceleration produce the change. In descending steep grades, the torque converter is as effective as a fluid coupling in transmitting torque to produce engine braking.