How does a car gear box function?

Answer 1

There are four types of transmissions used in automotive applications; Manual, Automatic, Semi-Automatic, and Constant Variable.

A manual transmission has an input shaft, connected to the engine, an output shaft connected to the drive shaft, and a counter, which holds the drive gears. Manual transmissions also use a clutch to disconnect the engine's power from the transmission, to make it easier to change gears. With the engine running and the transmission in neutral, the engine's power is rotating the input shaft and the counter shaft, which is turning the gears on the output shaft. The gears on the output shaft ride on bearings, so they "freewheel" in neutral, and non of the engine's power is transmitted to the wheels. When a gear is selected, the shift fork pushes a collar into the gear, which locks it to the output shaft and allows the engine to move the vehicle. Unless the gear box is in neutral, there is a direct link from the engine to the drive wheels, which is why vehicles with manual transmissions stall when stopped in gear.

An automatic transmission does not have an actual link between the engine and transmission. The engine's power is transmitted to the wheels through a torque converter. The torque converter has two main parts; an outer drum connected to the engine and an inner turbine connected to the transmission input shaft. When the engine is running, the out side of the torque converter spins, and causes the Transmission Fluid inside to build up pressure. The pressure of the spinning fluid forces the turbine to spin, which drives the internal components of the transmission. The spinning fluid allows slippage between the engine and drive train, and it is this slippage that allows an automatic to remain running when stopped in gear. For fuel economy purposes, some vehicles have a lock-up converter, which has a computer commanded solenoid, which engages a clutch in the converter and created a solid link between the engine and transmission.

Another difference between automatic transmissions and manual transmissions is automatics do use a counter shaft. Most automatic transmissions use two planetary gears (or epicyclic gearing) to obtain all of the gear ratios needed. A planetary gear set uses a central (sun) gear, which is rotated by several planetary gears, which are meshed into an outer ring gear. Different gear ratios can be obtained depending on which of the three is being held by the transmission or turned by the engine's power. Depending on the age of the transmission, the gears can be selected either hydraulically or electronically, but are usually engaged using hydraulic clutch packs (or a band for second gear), which clamp down on a component to make it turn with the engine or to hold it stationary. Much of the work done in an automatic is done by the valve body, which sends hydraulic pressure to different components.

Semi-Automatic transmissions have been around since the '40's and can be found today in some modern vehicles. The earlier transmission usually combined a manual gear box with a fluid clutch (a torque converter) and some used vacuum powered servos to "automatically" change gears. More current variations use a duel clutch set up, where electronic motors and sensors control the clutch engagement and disengagement when the drive changes gears. Some semi-automatic transmission can be used as either automatics or manual transmission, using a "paddle" shifter.

The Continuously Variable Transmission (CVT) is an oddball transmission, used sporadically by different manufactures, usually for European and Asian markets. It is popular in lawn tractors though. It basically works using a pulley system. The engine power comes into the input shaft, which dives one pulley, which drives a belt, which drives a second pulley, which drives the output shaft. The different gear ratios are produced by reducing the distance between the pulleys or the belt's pitch radius. By not having fixed gears, a CVT can obtain almost any gear ratio.