A torque converter converts torque primarily by using hydraulic fluid to transmit power from the engine to the transmission, and more importantly, by multiplying that torque when there is a significant speed difference between the engine and the transmission input.
At its core, a torque converter is a type of fluid coupling, but with added components designed for torque multiplication. Here's a breakdown of the process:
Key Components
Understanding the main parts helps clarify how it works:
- Impeller (Pump): Connected directly to the engine's flywheel and crankshaft.
- Turbine: Connected to the transmission's input shaft.
- Stator: Located between the impeller and turbine, mounted on a one-way clutch.
The Process of Torque Conversion
The conversion of torque happens through the circulation of transmission fluid within these components:
- Fluid Pumping (from Impeller): The torque converter casing is connected to the flywheel, spinning at the same rate as the crankshaft. The impeller or centrifugal pump, which spins with the engine, effectively flings the transmission fluid outwards due to centrifugal force.
- Fluid Impact (to Turbine): This high-velocity fluid is directed towards the blades or fins of the turbine. The transmission fluid impacts the fins of the turbine that in turn spins or transmits the torque into the transmission. This is how power is transferred hydraulically.
- Fluid Return and Stator Action: After passing through the turbine, the fluid flows back towards the center of the converter. In a simple fluid coupling, this returning fluid would impede the impeller. However, in a torque converter, the fluid encounters the stator. The stator redirects the flow of the returning fluid, changing its direction.
- Torque Multiplication (Stall & Low Speed): This redirection by the stator is crucial. By redirecting the fluid flow against the direction the impeller is spinning, it adds to the force driving the impeller. This creates a reaction force on the stator (hence the one-way clutch allowing it to hold still when needed). This redirected fluid flow boosts the force applied to the impeller, resulting in torque multiplication at the output (turbine) relative to the input (impeller), especially when the turbine is spinning much slower than the impeller (like when starting from a stop).
- Coupling Phase: As the turbine speed approaches the impeller speed, the fluid flow becomes smoother, and the stator's role diminishes. The torque converter then acts more like a simple fluid coupling, transmitting torque without multiplication. A lock-up clutch in modern converters often engages here to create a direct mechanical link for efficiency.
Practical Application
This mechanism is fundamental to automatic transmissions, allowing vehicles to:
- Start smoothly from a stop.
- Provide significant torque multiplication during acceleration.
- Allow the engine to idle while the vehicle is stopped, as the fluid coupling allows for slippage.
In essence, the torque converter uses the dynamic forces of flowing hydraulic fluid and a strategic internal component (the stator) to not just transfer power but also to multiply torque under specific operating conditions.
