An automatic transmission (sometimes abbreviated AT) is a multi-speed transmission used in motor vehicles that does not require any input from the driver to change forward gears under normal driving conditions.

A car's automatic transmission/gearbox

1. AUTOMATIC TRANSMISSIONS

2. Transmissions The automatic transmission is the gearbox that needs no drivers' effort to change or trigger gear change. The automatic transmission exists in two types which are stepped gear ratio gearbox and continuous variable transmission (CVT).

3. Stepped gear ratio transmission It changes gear in steps The steps of change in gear ratio depending on the vehicle's speed in relation to that of the engine. The gear ratio change is managed by the electronic control module The stepped ratio gearbox is the commonly used transmission in modern cars. The gearbox is connected to the engine through a fluid clutch (fluid coupling)

4. Stepped gear ratio transmission It is a planetary gear set since it form a combination like that of the solar system The planetary gear sets have the Sun gear placed at the centre Planet gears that rotate about their own axes and revolve around the sun gear The planet gears are attached to the gear holder called planet carrier Ring gear encloses the planet gears.

5. Operation of the stepped gear transmission The meshed gears rotation directions

6. Operation of the stepped gear transmission The three gear sets of the transmission operate in pairs of gears. One gear set must be locked, and the other two form the gear ratio of the transmission One will be input, and the other must be output The transmission can select the input and output gear set by means of wet clutch plates.

7. Operation of the stepped gear transmission Neutral gear that is no output required to the drive wheel

8. Operation of the stepped gear transmission First gear (speed reduction 1)  

9. Operation of the stepped gear transmission Second gear

10. Operation of the stepped gear transmission Third gear ratio

11. Operation of the stepped gear transmission Fourth gear

12. Operation of the stepped gear transmission Reverse gear

13. Continuously Variable Transmission (CVT) The CVT is the pulley and belt system that transmits power from the engine to the drive wheels The system consists of drive and driven pulleys. The pulleys are connected together by the belt. The pulleys have variable belt grooves that expand and contract by the action of mechanical, electrical or hydraulic means. The CVTs transmit power in a continuously varied gear ratio.

14. Operation of a CVT During vehicle start, the drive pulley diameter is lower than the driven wheel. Then, the CVT remains at gear reduction ratios until the vehicle accelerates to equalise the speed of engine. During acceleration, both diameters of drive and driven pulleys vary continuously.

15. Operation of a CVT At speed equalisation point, the diameter of the pulleys are equal. Overdrive speed zone, the drive pulley gets smaller than the driven pulley. A reverse clutch engages leading to the forward drive clutch disengagement