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.
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.