A fluid flywheel, also known as a fluid coupling or hydraulic coupling, works by using hydraulic fluid to transmit rotational power from one shaft (connected to the engine) to another shaft (connected to the gearbox or driven component) through the kinetic energy of the fluid, acting as a flexible coupling.
Understanding the Fluid Coupling Principle
Unlike a traditional friction clutch that mechanically locks two shafts together, a fluid flywheel uses liquid to transfer power. This allows the engine to spin while the output shaft remains stationary or spins at a different speed, providing smooth power transfer without the need for a mechanical clutch pedal.
Key Components
A fluid flywheel typically consists of three main parts enclosed within a sealed casing:
- Impeller (Pump): Connected to the input shaft (usually from the engine crankshaft), this component has radial blades that spin with the engine.
- Turbine (Runner): Connected to the output shaft (leading to the gearbox), this component also has radial blades facing the impeller.
- Casing: A sealed housing filled with a working fluid, typically oil. The casing is often bolted to the impeller, forming a single rotating unit on the input side.
The Principle of Operation
Here's a step-by-step breakdown of how it operates:
- Engine Rotation: The engine rotates the input shaft, which drives the impeller.
- Fluid Movement: The spinning impeller acts like a centrifugal pump, pushing the fluid outwards from its center towards its circumference.
- Energy Transfer: As the fluid is flung outwards by the impeller, it flows across the gap into the blades of the turbine. The kinetic energy of the moving fluid stream exerts a torque on the turbine blades.
- Turbine Rotation: This torque causes the turbine to rotate, driving the output shaft connected to the gearbox.
- Fluid Circulation: After passing through the turbine, the fluid flows back towards the center of the coupling and is drawn back into the impeller, creating a continuous circulatory flow or vortex pattern known as the "toroidal flow."
This continuous circulation of fluid between the impeller and turbine is what facilitates the transmission of torque.
Flexible Coupling and Power Transmission
A key characteristic of the fluid flywheel is its ability to act as a flexible coupling between the engine and gearbox. This means:
- At engine tick-over (idling speed), the impeller rotates slowly. The fluid flow isn't strong enough to generate significant torque on the turbine, so power transmission is essentially zero, allowing the vehicle to remain stationary without engaging a clutch.
- As the engine speed increases, the impeller spins faster, increasing the speed and volume of the fluid flow. This generates greater torque on the turbine, transmitting more power.
- Power transmission efficiency increases with engine speed. As per the reference:
- Power transmission is approximately 90% at 1/3rd full speed (around 1,500 revs/min).
- Power transmission reaches up to 98% at full power.
What is Slip?
Fluid flywheels operate with a degree of "slip." Slip is the difference in rotational speed between the impeller and the turbine.
- Slip is necessary for torque transmission; if the impeller and turbine spun at the exact same speed (zero slip), there would be no relative motion of fluid between the blades, and no torque would be transferred.
- High slip occurs at low engine speeds (like idling), resulting in minimal torque transmission.
- As engine speed increases, the turbine speed catches up, and slip decreases, leading to higher efficiency and power transmission. The difference in speed creates the required fluid flow to transfer torque.
Advantages
Using a fluid flywheel offers several benefits:
- Smooth Operation: Provides very smooth take-off and gear changes as power is transmitted gradually through the fluid.
- Vibration Damping: Helps absorb engine vibrations before they reach the transmission.
- Overload Protection: Can absorb sudden shocks and protect the engine and transmission from damage during harsh acceleration or shifts.
- Simplified Driving: Eliminates the need for a clutch pedal in early automatic or semi-automatic transmission systems.
In summary, the fluid flywheel utilizes the kinetic energy of circulating fluid within a sealed casing to form a dynamic, flexible link that transmits power from the engine to the transmission, allowing for smooth operation and shock absorption.
