The purpose of a light vehicle transmission is to transmit engine torque to the driving wheels. It also has to provide for this torque to be increased or decreased to suit differing road and operating conditions.
In a manual transmission this is done by the driver, manually selecting gear ratios to suit.
In an automatic transmission, it is done by both the transmission control system, automatically selecting the gearing according to load and speed, and by the torque converter.
The torque converter is mounted on the engine in the same place as a manual clutch, and does the same job, transmitting engine torque to the input shaft of the transmission. It can also multiply torque according to driving conditions.
It’s a fluid coupling, with the fluid acting as the driving medium, since none of the converter components are physically connected to the others. It acts as an automatic clutch. At engine idle speeds, it allows the engine to operate while the vehicle is stationary and the transmission is in a drive range. In its simplest form, a single-stage torque converter has three elements: The Impeller, the Turbine and Stator.
All three have angled or curved vanes and are contained in a single casing. They are separated from each other by thrust bearings, but maintain a close relationship for efficient torque transmission.
The impeller has a large number of vanes attached to the converter case to form the driving member. The vanes rotate with the casing as the engine rotates. Each one has a slight curvature and is set radially in the case.
The turbine is similar in construction to the impeller, but with more vanes, and with a greater curvature. The direction of curvature of the turbine vanes is opposite to that of the impeller vanes. The turbine is free to rotate in the casing, and a central spline mates with a spline on the input shaft. When the fluid coming from the impeller rotates the turbine, the transmission input shaft also rotates.
Both the impeller and the turbine are fitted with a torus or guide ring. This helps to secure the vanes in position. It also reduces turbulence at the center to improve efficiency.
The stator has a small set of curved blades attached to a central hub and is positioned between the impeller and the turbine. The central hub is mounted on a one-way clutch, splined to a stator support shaft, and fixed to the transmission case. The one-way clutch allows the stator to rotate only in the same direction as the impeller. Trying to rotate the stator in the opposite direction locks the stator on the support shaft and holds it stationary.
