Automatic gearbox shift mechanism
An automatic transmission (AT) is a complex mechanical and hydraulic unit that controls the transfer of power from the engine to the drive wheels. The automatic transmission’s shift mechanism ensures that the appropriate gear is selected and engaged to optimize performance and fuel economy.
Main components of the automatic transmission switching mechanism:
* Hydraulic control unit: The central “brain” of the automatic transmission, which interprets input signals and issues shift commands.
* Gear selector: Allows the driver to select the automatic transmission mode (park, reverse, neutral, drive).
*Mode Switch: Changes shift modes (eg Eco, Sport, Manual).
* Clutch packs: Used to transfer power between shafts. Each packet corresponds to a specific transmission.
* Brake Bands: Used to hold the shafts and change down gears.
* Planetary gear sets: Sets of gears that provide different gear ratios.
* Drum: The central element of the planetary gear set that turns the other gears.
Working principle:
When the driver selects a gear through the selector, the hydraulic control unit sends a signal to the corresponding solenoids, which activate the hydraulic valves.
Pressurized hydraulic fluid enters the clutch packs or brake bands, pressing them against the drums or each other. This causes the gears to change.
Planetary gear sets and clutch packs are used in various combinations to create different gear ratios. For example, to move forward, the car starts in first gear (the lowest gear ratio) and then automatically shifts to higher gears as the engine speed and vehicle speed increase.
To achieve smooth shifting, a torque converter is used, which is a clutch filled with hydraulic fluid. The torque converter reduces vibration and shock when changing gears.
Automatic transmission limitations:
*Higher cost compared to manual transmission.
* Slight shock when shifting gears.
* Inability to tow heavy loads on some types of automatic transmission.