Torque Converter - Operating Principles

Plain-English summary: what the system does

A torque converter is a fluid-based coupling used in automatic transmissions to transmit engine torque to the transmission while allowing the engine to continue running when the vehicle is stationary. It enables smooth vehicle launch, multiplies torque during acceleration, and permits slip between the engine and transmission to manage speed differences without a mechanical clutch.

How it Works - Step by Step

1. Engine-driven input
   The torque converter housing is bolted to the engine. As the engine rotates, it drives the impeller inside the converter.

2. Fluid acceleration
   The impeller contains curved vanes and is filled with transmission fluid. As it spins, centrifugal force accelerates the fluid outward and directs it toward the turbine.

3. Turbine rotation
   Fluid leaving the impeller strikes the turbine blades, causing the turbine to rotate. The turbine is connected to the transmission input shaft.

4. Slip between impeller and turbine
   The turbine rotates more slowly than the impeller, allowing controlled slip. This enables the vehicle to remain stationary while the engine runs and allows smooth acceleration.

5. Torque multiplication
   At low vehicle speeds, fluid exiting the turbine is redirected by the stator before re-entering the impeller. This redirection increases the effective force on the impeller, multiplying torque.

6. Acceleration phase
   As vehicle speed increases, the speed difference between the impeller and turbine reduces, and torque multiplication decreases.

7. Coupling phase
   At higher speeds, the turbine approaches impeller speed. The torque converter then behaves like a simple fluid coupling with minimal torque multiplication.

8. Lock-up operation
   In many operating conditions, a lock-up clutch mechanically connects the turbine to the housing, eliminating slip and improving efficiency.

Key Components Involved

• Impeller (pump)
  Driven by the engine and responsible for accelerating the transmission fluid.

• Turbine
  Receives energy from the fluid and transmits torque to the transmission input shaft.

• Stator
  Positioned between the impeller and turbine; redirects fluid flow to enable torque multiplication.

• One-way clutch (stator clutch)
  Allows the stator to lock during torque multiplication and freewheel during coupling.

• Transmission fluid
  Transfers energy between components and provides cooling and lubrication.

• Lock-up clutch
  Mechanically links the turbine to the housing under suitable conditions to reduce losses.

• Converter housing
  Encloses the assembly and connects directly to the engine.

Common Misconceptions

• “The torque converter is always slipping”
  Slip is present during launch and acceleration, but lock-up eliminates slip during steady driving.

• “Torque converters waste power”
  Modern designs minimise losses through efficient fluid control and lock-up clutches.

• “Torque multiplication occurs at all speeds”
  Torque multiplication only occurs when there is a significant speed difference between the impeller and turbine.

• “A torque converter replaces the gearbox”
  The torque converter transmits torque; gear ratios are still provided by the transmission.

Why This Matters