What is Fluid Coupling Or Fluid Flywheel?


The fluid coupling is one name for liquid coupling to transmit the turning effect from the engine to the clutch. It is located between the clutch and the crankshaft. Because the liquid coupling is always a major part of the engine flywheel assembly, it is sometimes called a fluid flywheel.

Working Principle Of Fluid Coupling

The working principle of a liquid coupling is shown in the below figure with the help of two fans. When one fan is turned on and faced toward the other, the air stream causes the second fan to turn, even though it is not switched on. The 1st fan is the driving member or the impeller and the 2nd fan is the driven member or the runner. Here the air is the medium of power transmission. This is a simple fluid coupling, air serving as the fluid.

Fluid Coupling
Fluid Flywheel


The figure shows the simple construction of a liquid coupling. It consists of, in general, two half-dough nut-shaped shells equipped with interior fins that radiate from the hubs. One shell is mounted on the crankshaft and it is called an impeller or driving member. The other shell is mounted on the clutch shaft and is called a runner or driven member.

The two shells are very close with their ends facing each other and enclosed in a housing so that they can be turned without touching each other. It is a complete assembly of a Fluid Flywheel or liquid coupling. The liquid (oil) is filled in the housing.

When the crankshaft starts, the oil trapped in the impeller fins threw towards the runner fins and causes it to move. ln this way the liquid transmits engine force to the clutch plate without any metal to metal contact. The lag of the runner behind the impeller is called ‘slip’. It depends upon the engine speed and load.

 The slip is maximum with the vehicle at rest (runner stationery) and throttle open, to cause the impeller to start circulating the oil. The oil under this process moves in two general directions at the same time. It rotates at right angles to the shaft(rotary flow) and also circulates between the impeller and runner(vortex flow) as shown in Figure.

The vortex flow is at right angles to the rotary flow and is produced by oil trapped in the fins of the impeller. The oil flies out against the curved interior due to the çentiñngl force. The rotary flow of oil starts the movement of the runner when it reaches sufficient force and volume.

Fluid Flywheel
Fluid Flywheel

The vortex flow is maximum when the slip is 100% and decreases as the runner speed approaches that of the impeller. As a result, the efficiency of the liquid coupling increases rapidly from zero at rest to nearly 99% at higher speeds. In an ideal liquid coupling, the runner would attain the same speed as the impeller, so as to receive all the power imparted to the coupling by the engine.

In commercial designs, the runner speed becomes almost equal to the of the impeller only under the best operating condition, when the efficiency of the coupling is highest. The liquid coupling or Fluid Flywheel is not suited for use with an ordinary gearbox. It is generally used in conjunction witłh epicyclic gears to provide a semi or fully automatic gearbox.

The liquid coupling is always a major part of the engine flywheel assembly and hence it is also known as a fluid flywheel. Another design of the fluid flywheel is shown in Figure. It consists of a split housing which is rotated by the engine crankshaft. Inside the housing, there is a driven rotor which is connected to the gearbox by a shaft. The flywheel housing is distributed into a number of cells in the form of radial vanes. These cells correspond to similar openings in the driven rotor.

Fluid Coupling
Fluid Flywheel

As the driving member rotates, the fluid flows outwards due to the centrifugal force and circulates from the flywheel to the drive rotor. Because the fluid is also moved around by the driving member, the fluid tends to rotate the driven member. Thus, the torque is transmitted from the crankshaft to the gearbox.

The fluid flywheel requires less attention than friction clutches Also, it needs no adjustment. The fluid drive is taken up smoothly, the torsional vibration of the engine crankshaft and the gearbox are damped out, the hydraulic fluid absorbs transmission shocks when braking and going down of a hill and the clutch pedal is eliminated.

For clear understanding watch the animation video of fluid flywheel below:

Animation video of a fluid flywheel


  1. A liquid coupling, when used with a conventional clutch and transmission enables the driver to use the clutch and gears with less skill and fatigue than with an all-mechanical linkage.
  2. If the driver does not de-clutch or shift to neutral before coming to a stop, the Fluid coupling will slip sufficiently to prevent stalling the engine.
  3. Selection of Improper gear or Unskilled clutch engagement will not produce the chattering and bucking.
  4. Any sudden load is cushioned and absorbed by the liquid coupling so that the dynamic stresses on the gear teeth of the transmission and rear axle are greatly reduced.
  5. The vehicle may be started in high gear when equipped with a liquid coupling because the coupling slips sufficiently to enable the engine to speed up.
  6. Liquid coupling at low speeds is not as efficient as mechanical ones. It reduces engine braking when slowing down and during high speed to start the vehicle.

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Creative Blogger And Automobile Engineer.

One thought on “What is Fluid Coupling Or Fluid Flywheel?

  • October 9, 2019 at 8:39 am

    Many thanks discussing this particular document and rendering it public


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