What is a Fluid Drive or Liquid Coupling in Automobiles?

Fluid drive is the technology where the engine and transmission power will be in a single line with the help of the liquid. These technologies are used in the mid 20’s vehicles. these technologies are used in old vehicles where the automatic transmission system depends on the fluid drive or the liquid coupling. In this article, I will discuss the fluid drive’s working principle and the constructional details.


Fluid Drive

“Fluid drive” in automobiles typically refers to a type of automatic transmission technology that was used in some vehicles in the mid-20th century. This technology is closely associated with the Chrysler Corporation, which used the term “Fluid Drive” as a brand name for their version of this transmission system.

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

Working Principle of Fluid Drive


source- makeagif.com

The working principle of liquid coupling is shown in Fig 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 first man is the driving member or the impeller and the second fan is the driven member or the runner. Air is the fluid used as the medium of power transmission. This is a simple fluid coupling, with air serving as the fluid.

Read more – Compensating Devices in Carburetor | All Methods.

When the engine is running, the impeller is rotated by the engine’s crankshaft. The motion of the impeller causes the hydraulic fluid to circulate within the coupling. When you accelerate or decelerate, the fluid drive smoothly adjusts the power transfer, allowing for seamless speed changes without the need for manual gear shifts.

The fluid drive effectively multiplies the engine’s torque to the transmission, which helps move the vehicle. This is particularly useful in low-speed, high-torque situations, such as when starting from a stop or climbing a hill.

Construction of a Liquid Coupling

This FIg 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 is called an impeller or driving member. The other shell is mounted on the driven shaft and is called the 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 liquid coupling. The liquid(oil) is filled in the housing.

Read more – How does a Pneumatic Governor work?

The space between the impeller and the turbine is known as the working chamber. It is filled with hydraulic fluid, which is typically an oil-based substance. This fluid is responsible for transmitting power and allowing speed differences between the input and output shafts.

When the crankshaft turns, the oil trapped in the impeller fins impinges on the runner fins and causes it to move. In this way, the liquid transmits engine force to the clutch plate without any metal-to-metal contact.

What is the Vortex Flow in the Fluid Drive


The lag of the runner behind the impeller is known as Slip. It depends upon the engine speed and load. The slip is maximum with the vehicle at rest(runner stationary) and the throttle open, to cause the impeller to start circulating the oil.

The oil under these conditions moves in two general directions at the same time. It rotates at right angles to the shaft(rotary flow) and circulates between the impeller and runner(vortex flow) as shown in Fig. 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 centrifugal force. The rotary flow starts the movement of the runner when it attains sufficient force and volume.

The vortex flow is the maximum when the slip is 100% and decreases as the runner’s 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, to receive all the power imparted to the coupling by the engine. In commercial designs, the runner’s speed becomes almost equal to that of the impeller only under the best operating conditions, when the efficiency of the coupling is highest.

The liquid coupling is not suited for use with an ordinary gearbox. It is generally used in conjunction with epicyclic gears to provide a semi or fully-automatic gearbox.

Fluid Flywheel in Fluid Drive


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 Fig. above. It consists of a split housing that is rotated by the engine crankshaft.

Read more – How to Convert Gasoline Engine to Natural Gas?

Inside the housing is a driven rotor which is connected by a shaft to the gear box. The flywheel housing is divided into several cells using radial vanes. These cells correspond to similar openings in the driven rotor. As the driving motor rotates, the fluid flows outwards due to centrifugal force and circulates from the flywheel to the driven rotor. Because the fluid is also carried out around by the driving member, the fluid tends to rotate the driven motor. Thus, the torque is transmitted from the crankshaft to the gearbox shaft.

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

Advantages of a Liquid Coupling

The advantages of a liquid coupling are –

  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 fails to de-clutch or shift to neutral before coming to a stop, the coupling will slip sufficiently to prevent stalling the engine.
  3. Unskilled clutch engagement or selection of improper gear will not produce chattering and bucking.
  4. Any sudden load is cushioned and absorbed by the 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 once. It reduces engine braking when slowing down and requires high speed to start the vehicle.


Liquid coupling or the liquid drive is the option that allows the automatic transmission system to operate with the help of the fluid. These systems have not been used in recent days because of the lag and the discomfort of performance. Thanks for reading this article. If you like the article then please do share it.

Spread the love

1 thought on “What is a Fluid Drive or Liquid Coupling in Automobiles?”

Leave a comment