The construction of a torque converter is similar to that of the fluid flywheel, the only difference being that it has an additional stationary member called the stator or the reaction member and all the members have blades or vanes of specific shape.


The operation of the two, however, is not similar. 

Whereas the fluid flywheel transmits the same torque as given to it by the engine shaft, the torque converter increases the torque in a ratio of about 2:1 to 3:1. 

Thus it serves the purpose as that of a gearbox and that too in a better way. 

Whereas in the gearbox the torch variation is only in finite number of steps, in the case of torque converter torque output variation is continuous. 

However, the efficiency of a torque converter is high only within narrow limits of speed.

A single stage torque converter is shown in figure. 

It consists of three main parts (i) the impeller or the driving member which is connected to the engine, (ii) the turbine or the driven member which is connected to the road wheels through the transmission gears and the driveline, and (iii) the stator fixed to the frame through a freewheel. 

In addition, there is a transmission oil pump which keeps the converter full of oil under pressure pressure. 

Pressure is necessary to keep the converter full of oil when rotating. 

Due to rotation the centrifugal force pushes the oil in the outward direction and this tends to form air pockets near the centre of the converter. 

This phenomenon of forming air pocket due to low pressure is called Cavitation, to avoid which the converter pressure is kept between 200 to 1200 kPa.

When the engine is started, the impeller starts rotating. 

Initially, the oil from the impeller is pushed into the turbine because of the higher centrifugal force at the impeller, due to it being driven by the engine and the turbine being stationary. 

Thus the oil, having taken high kinetic energy from the engine through the impeller, hits the outer edge of the turbine. 

This flow of the high energy oil provides the force that tends to rotate the turbine. 

This force increases with the increase of engine speed. 

When it is great enough, the turbine starts rotating and the vehicle moves. 

The turbine blade angle is such that it changes the direction of the oil flow so that when it comes out of the turbine blade at the centre, its direction is effectively backward. 

If there were no stator and it were to enter the impeller directly, it will push the impeller in the opposite direction and will thus cause a loss of power. 

to avoid this dragging action on the impeller, the fluid from the turbine is made to strike a stationary member that is stator which changes its direction suitably, so that the oil leaving the stator strikes the impeller in the favourable direction that is in the same direction in which the impeller is turning. 

(The stator takes the reaction while doing so and is therefore called, a reactor also). 

Then the impeller throws the oil back into the turbine at the outer edge. 

This goes on continuously. 

Thus repeated pushing of the turbine blades causes the torque on the turbine to increase, the phenomenon being called torque multiplication. 

It must be remembered, however, that the helping action of the stator in deflecting the fluid in the favourable direction and subsequent torque multiplication occurs when the turbine speed (proportional to the vehicle speed) is less than the impeller speed (that is the engine speed). 

Thus the maximum torque multiplication occurs when the turbine is stationary and impeller is running fast at the engine speed. This is called stall. 

The maximum torque multiplication at stall is about 2.1 to 2.6. 

When the vehicle begins to move, the turbine speed starts to increase and the torque multiplication gradually reduces as the difference in the impeller and turbine speed decreases. 

Torque multiplication will become unity as the turbine speed becomes equal to the impeller speed. 

This is equivalent to direct gear.

When the turbine speed has increased and is nearly 85% to 90% of the impeller speed, the coupling point is reached and the oil leaves the turbine in the forward direction, hitting the back of the stator blade. 

The stator blades thus cause a hindrance in the flow of fluid from the turbine to impeller. 

To avoid this, stator is mounted on a freewheel clutch (also called one way clutch), which allows it to rotate in the direction of the turbine or impeller. 

It cannot, however, rotate in the opposite direction. 

With this arrangement, the stator rotates in the desired direction and does not cause hindrance to the fluid motion.

It is observed that efficiency of the torque converter is maximum within a very narrow speed range. 

Because of this the use of torque converter will involve heavy losses. 

Two methods of avoiding this which have been employed are :-

(i) By so arranging that the torque converter behaves as a fluid flywheel at higher speeds. This is achieved by mounting stator on a freewheel as described earlier.

(ii) By disconnecting the torque converter at high speeds and instead employing a direct drive. At low speeds, the torque converter is kept operative, because the car is running at low speeds only for very small time. This may be done in a number of ways, some of which are described below :-

(a) The double clutch connected to input shaft contains two friction plates A and B. Plate C can be moved to the left or to the right as required, by means of hydraulic pressure from the transmission hydraulic system. Friction plate A is connected directly to the output shaft, whereas plate B is connected with the impeller of the torque converter. The turbine (driven member) of the torque converter is further connected to the output shaft through a freewheel.

(b) Instead of a two way clutch, a centrifugal clutch may be used along with the conventional three-member torque converter. The centrifugal clutch consist of a number of sliding friction shoes arranged around the circumference of the damper assembly which consists of damper springs and a free wheel. With increase of the turbine speed, the frictions shoes slide outward due to centrifugal force till this come into contact with the cover. when this happens, power from the converter cover flows directly through the damper assembly (which dampens the torsional shocks), to the turbine shaft. The centrifugal clutch is so designed that there is some slip at higher loads. As a result of this, when the vehicle is under load, there is a split between the direct mechanical drive and the hydraulic drive through the torque converter.

(c) Another alternative is to incorporate a simple epicyclic gear set into the torque converter so as to provide varying degrees of direct mechanical drive under different operating conditions. Such a planetary gear set is sometimes called a 'splitter gear' because it splits or divides the engine torque between mechanical and hydraulic transmission in lower gears. In case of a typical splitter gear, in the second gear, the turbine supplies 40% of the torque hydraulically and 60% mechanically, whereas in third gear only 7% is transmitted hydraulically the rest 93% torque being transmitted mechanically.