Overview

A torque converter is a type of fluid coupling fitted between an internal combustion engine and a gearbox to transmit power and, under certain conditions, multiply torque. Unlike a direct mechanical clutch, it transmits torque through a circulating hydraulic fluid, allowing the engine to continue running while the vehicle is stationary and smoothing power delivery during acceleration.

Main components and how it works

Internally a torque converter contains three primary elements: the pump (or impeller) driven by the engine, the turbine connected to the transmission input, and the stator positioned between them. The pump throws transmission fluid outward by centrifugal force; that moving fluid drives the turbine and returns through the stator, which redirects flow to increase effectiveness at low speeds. This redirection is the source of torque multiplication when turbine speed is significantly lower than pump speed.

Key characteristics

  • Torque multiplication: At low speeds the stator can boost output torque relative to input, aiding acceleration.
  • Slip and stall behavior: The converter allows controlled slip so the engine can run while the vehicle is stopped; "stall" refers to the maximum engine speed with the driven side held stationary.
  • Lock-up clutch: Many converters include a clutch that locks the turbine to the pump at cruising speeds to eliminate slip and improve fuel efficiency.

History and development

Torque converters evolved from early fluid couplings and were developed to improve drivability of vehicles with automatic gearboxes. They became widespread in mid-20th-century automatic transmissions, and have been refined over decades with better hydraulic design and the addition of lock-up clutches to reduce losses at highway speeds.

Uses and applications

Torque converters are common in automatic passenger cars and light trucks, and in many heavy-duty applications such as buses, construction machinery and some rail and marine drivetrains. Their ability to multiply torque and allow smooth low-speed operation makes them useful where strong, gradual starts are required. For more on the physics of torque multiplication see torque multiplication, and for information on transmissions and drivetrains see vehicle transmissions.

Distinctions and notable facts

Compared with purely mechanical clutches, torque converters provide smoother engagement and permit some speed difference between engine and gearbox, but they introduce hydraulic inefficiencies unless locked up. Modern designs balance acceptable losses with benefits in comfort and controllability, and remain a standard solution in many automatic drivetrains.