What is a transmission in a car?

The power generated by the engine flows through the transmission before it goes into the drive wheels. The basic role of the transmission is to regulate the speed and torque to the drive wheels under different driving conditions. For instance, if the vehicle climbs a steep hill, it needs more torque, as it requires more force to do so. By reducing the speed of the transmission, it is possible to attain higher torque for the same power input. On the other hand, if a car doesn't need much torque, it can increase the transmission speed.

Manual Transmission

One of the most interesting components in the manual transmission car is the clutch pedal. The clutch is a mechanism to disconnect the power flow between the engine and the transmission. During normal driving, the engine power is transmitted to the transmission system as the engine and the transmission are connected. But when the driver presses the clutch pedal, the power flow to the transmission will be discontinued, thus stopping the vehicle. During this time, the driver can change the gear and then release the clutch to drive on.   

Automatic Transmission

In an automatic transmission car, there isn’t a clutch pedal. Instead, the torque converter in the automatic transmission system does the work of the clutch pedal. The torque converter can efficiently disconnect the gearset and engine when the vehicle is not driving. The torque converter is filled with pressurized fluid to speed up or slow down the vehicle, which is called hydraulic control. The oil pressure spins the turbine and sends power through the transmission. When the engine is not running, the transmission has no oil pressure, and thus the vehicle cannot move.     

Gear Selector in an Automatic Transmission Car

P - Park

In Park, the transmission locks the output shaft and then the wheels. It prevents the car from driving forward or backward.

R - Reverse

In Reverse, it will cause the sun gear to rotate reversely, leading to the output shaft rotating in the opposite direction.

N - Neutral

In Neutral, the transmission is in a position that no gear is selected. If the brake pedal is released, the vehicle will start to move, but it cannot accelerate.

D - Drive

In Drive, the transmission increases the torque going to the wheels for the vehicle to accelerate. A torque converter in automatic transmission senses when the car accelerates and shifts it to a higher gear.