Manual Transmission
The basic four-speed manual transmission has an aluminum housing called the transmission case, an input shaft that is turned by the engine, and an output shaft that transmits power to the axle. Inside the transmission case, 11 gears—steel wheels of various sizes with deep grooves or teeth around their edges—are arranged on the shafts. The teeth of the different wheels mesh together, enabling one gear to cause the other to turn. Some of the gears are located on a third shaft called a countershaft.

The gearshift controls levers and selector forks that move the gears along the shafts and cause them to mesh. The gears rotate in a bath of heavyweight transmission oil.
A. How It Works
The torque, or twisting force, of the engine’s crankshaft is passed along by mechanical linkage to a rotating shaft that enters the transmission. A gear at the end of this input shaft meshes with a gear on the countershaft, causing the countershaft to rotate. Gears of different sizes attached to the countershaft drive other gears on the input and output shafts. The gearshift enables the driver to select different combinations of gears to determine how fast and with how much torque the output shaft turns. Because they are connected by gears and other devices, the rotating output shaft turns the driveshaft, axles, and wheels.
Virtually all modern manual transmissions have four or five forward speeds, and some have more. Heavy duty trucks require a significantly larger number of gears to gain the maximum amount of pulling power from the engine. Many have as many as 20 forward-speed gears and several reverse-speed gears.

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Automatic Transmission System   Automatic Transmission System
The automatic transmission is one of the key components of an automobile. Located just behind the engine, the transmission changes the speed and power ratios between the engine and the driving wheels of a vehicle.

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Full Size The basic elements of an automatic transmission are a gearbox and a torque converter, which uses fluid to transfer energy from the engine to the gearbox. An automatic transmission is designed to automatically adjust its gear ratios according to a vehicle’s acceleration and road speed, and the strain on the engine. The drive components are engaged and disengaged by changes in the pressure of a special oil, called transmission fluid, inside the transmission. Sensors, control modules, and valves direct the flow of transmission fluid.

A. Torque Converter
Engine power is transferred to the gearbox by a torque converter, which is filled with transmission fluid. The fluid transfers energy between two fanlike devices inside the converter. One of the devices, called an impeller, is attached to the engine and rotates when the crankshaft rotates. The other fanlike device, a turbine, faces the impeller and is connected to the input shaft leading to the gearbox. Another device, called a stator, helps direct the flow of fluid from the impeller to the blades of the turbine. When the impeller rotates, it pushes fluid against the turbine’s blades, causing the turbine and the gearbox’s input shaft to rotate.

Turbine speed lags behind impeller speed during accelleration, but the two eventually run the same speed. As vehicle speed increases, the amount of slip decreases as the transmission moves up through its gears, using fluid pressure drawn from the transmission fluid pump to adjust the gear ratio.

B. ‘Lock Up’ Converters
Many of the latest automatic transmissions feature electronic controls with lock up torque converters that electrically lock together so that no slippage occurs at highway speed. The result is improved fuel economy. This feature is part of the modern electronic engine management system that operates not only the action between the engine and the transmission, but also the engine’s internal functioning, exhaust-emission levels, and fuel economy.

C. Automatic Transmission Gearbox
The gearbox for an automatic transmission typically contains two groups or clusters of planetary gears. Each group consists of a sun gear in the center, planet gears that rotate around the sun gear, and a ring gear that rotates around the planet gears. Each gear turns on its own axis as it rotates around the sun gear. Clamping the two clusters of gears together in various combinations produces three forward gear ratios and a reverse gear.

D. Hydraulic Shifting
Gears are shifted in an automatic transmission by changes in hydraulic pressure, in this case, the pressure of a transmission oil created by the impeller. A governor valve routes pressurized oil to shift valves, which control the clutches and bands that shift the gears and hold them in place. The shift valves contain pistons that move in and out, depending on the amount of hydraulic pressure. As the vehicle’s speed changes, the governor valve changes hydraulic pressure at the appropriate shift valves to cause a gear shift.