Category: Transmission system

construction and working of a torque convertor in an automobile | torque convertor a type of fluid coupling

construction and working of a torque convertor in an automobile

Torque converter an Alternate to Gear Box

Torque converter is a type of fluid coupling that uses a fluid to transmit turning effort from the one shaft to another. It is a device which performs a function similar to that of a gear box, namely, to increase the torque while reducing the speed. A gear box provides only a small number of fixed ratios, but the torque converter provides a continuous variation of ratio from the lowest to the highest.The torque convertor is somewhat similar to a fluid fly wheel, but differs from it in one aspect, namely it has three principal components instead of only two.


Components of a Torque converter

(i) The driving element or impeller or pump which is connected to the engine.

(ii) The driven element or rotor or turbine which is connected to the propeller shaft and

(iii) The fixed element or reaction member or stator which is fixed to the frame.

The fixed element is responsible to cause a change of torque between input and output shafts. The fluid flywheel does not have any fixed member, as such cannot produce any change of torque.

How does a Torque Converter Works

The three element torque converter can be seen in the picture. In the torque converter, torque multiplication take place in the following way. The oil leaving vanes hits the stator vanes. This oil is redirected into the pump vanes in a direction which helps the pump operation. This oil is again thrown into the turbine by the pump. In fact, this is a continuous process. The fluid pushing repeatedly on the turbine vanes increases the torque on the turbine. In many designs of torque converters, the torque almost doubled. However, as the speed of the turbine approaches the speed of the pump, the increase in torque falls of gradually till it becomes 1:1. At this stage, the oil starts striking the rear faces of the vanes in the stator, thus making the stator to turn. Now the stator does not take part in torque converter action. Under these conditions, the torque converter simply acts as a fluid fly-wheel.


Working Principle of a torque converter

The over running clutch (freewheeling device) used for mounting the stator. The spring loaded rollers in the freewheel allow the stator to freewheel when the oil starts striking the rear of the stator vanes. On the other hand, when the oil strikes the front of the stator vanes, it attempts to rotate the stator in the opposite direction. But this action locks up the freewheeling device and thus holds the stator stationary. Under such condition, the stator acts as a reaction member, and directs the oil leaving the turbine trailing edges in the proper direction before it enters the pump vanes.

fluid flywheel of an automobile | construction and working of fluid coupling of an automobile

Fluid flywheel or fluid coupling

A liquid coupling is used to transmit engine turning effort (torque) to a clutch and transmission. The coupling is always a major part of the engine flywheel assembly. As such it is sometime called a fluid flywheel.

Construction of flywheel

The fluid flywheel details can be seen in the picture. It consists of two half dough nut shaped shells equipped with interior fins. The fins radiate from the hub, and thereby form radial passages. The areas of these passages, perpendicular to their centre line, are kept constant by a suitable design. Since the circumferential width of the opening close to the hub is less than that at the periphery, the radial size of the opening, close to the hub is made greater than that at the periphery.


One of the shells is fixed to the crankshaft of the engine and the other to the clutch/gearbox shaft. The two shells are mounted very close, with their open ends facing each other, so that they can be turned independently without touching. Housing surrounds both units to make a closed assembly. About 80 percent of the interior of the assembly is filled with oil.

Working of fluid flywheel

The driving unit, called impeller, is linked to the engine crankshaft. When the engine throttle is opened, the oil in the impeller starts moving. Due to the force of the rotating, trapped oil impinges on the fins of the driven unit called runner and causes it to move. In this way, the moving liquid transmits the engine power to the clutch driving plat or to any other unit to which the runner is attached. This happens without any metal contact.

In the actual units, the runner speed becomes almost equal to that of the impeller only under the best operating conditions, when the efficiency of liquid coupling is highest. But usually the runner speed is less than that of the impeller. The (speed) lag of the runner behind the impeller is known as slip. This (speed) slip varies with many factors such as engine speed, vehicle speed and engine and vehicle load.


Flywheel Torque

The slip is greatest with the vehicle at rest (ie runner stationary), and the engine throttle being opened to cause the impeller to start circulating the oil. Under these conditions, the oil moves in two general directions at the same time. It rotates at right angles to the shafts, i.e., undergoes rotary flow. The oil also circulates between the impeller and runner, i.e., undergoes vortex flow. When the rotary flow attains sufficient force and volume, it causes the movement of the runner.

The vortex flow is at right angles to the rotary flow. The vortex flow is produced by the oil trapped in the fins of the impeller. The oil flies out against the curved interior, because of centrifugal force. The centrifugal force directs the oil across to the runner, thereby returning it to the impeller in the region of the hub.

The vortex flow is maximum when the slip is 100 percent (runner stationary), and decreases as the runner speed approaches that of the impeller. This results from the centrifugal force produced by the oil in the runner, which moves out and opposes the vortex flow. At cruising speeds, there is little or no vortex flow because the centrifugal forces produced in the impeller and runner are almost equal. As such, the efficiency of coupling increases rapidly from zero at rest to nearly 99 percent at higher speeds.

Fluid Flywheels of an Automobile

The torque or turning effort delivered to the runner through the liquid is equal to the torque applied to the impeller by the engine. But the power received by the runner is always less than that furnished by the engine. The power losses in the coupling appear as heat, which is dissipated as the assembly revolves.


Advantages of fluid flywheel

An ordinary friction clutch would be damaged by prolonged slipping, with increased fuel consumption. But by prolonged slipping, the fluid flywheel will not suffer any mechanical damage. Although it may become so hot as to burn one’s hand if one touched it.

When a liquid coupling is used with a conventional clutch and transmission, it enables the driver to use the clutch and gears with less skill and fatigue than with an all mechanical linkage. Unskillful clutch engagement or selection of the improper gear will not produce any chattering and bucking. Any sudden load is cushioned and absorbed by the coupling so that dynamic stresses on the gear teeth of the transmission and rear (drive) axle are greatly reduced.

Liquid coupling at low speeds are not as efficient as mechanical clutch. As such it reduces engine braking when slowing down the vehicle speed, particularly during coming down a hilly track. Further, it requires higher speeds to start a vehicle by pushing or towing it.