The transmission is one of the main components of a vehicle's drivetrain. It is a mechanism designed to alter the speed and torque from the engine, capable of fixing or changing the gear ratio between the output and input shafts. It enables the vehicle to reverse and idle stop, also known as the gearbox. The transmission consists of a gear-shifting mechanism and a control mechanism, with some vehicles also featuring a power take-off mechanism. Most transmission mechanisms use ordinary gear trains, while some employ planetary gear systems. Ordinary gear transmission mechanisms typically utilize sliding gears and synchronizers. The gearbox is usually located in the vehicle's engine compartment, connected to the engine, or sometimes beneath the central gear lever in the front row of the car. Opening the front hood generally does not reveal the gearbox; the vehicle must be lifted, and the protective panel on the front chassis opened to get a clear view. The position of the gearbox varies depending on the vehicle model and drivetrain configuration. In actual vehicle usage, scenarios such as starting, idling, low or high-speed driving, acceleration, deceleration, climbing, and reversing require the vehicle's driving force and speed to vary over a wide range. However, the widely used piston engines have a relatively limited range of output torque and speed variations. To adapt to constantly changing driving conditions and ensure the engine operates under favorable conditions (higher power, lower fuel consumption), the transmission is incorporated into the drivetrain system. The gearbox contains many gears, which are the core components of the transmission. A combination of several gears is referred to as a gear set. The gearbox also includes three shafts: the input shaft, output shaft, and intermediate shaft. The vehicle transmission alters the gear ratio to modify the torque from the engine crankshaft, meeting the varying demands for driving wheel traction and speed under different conditions such as starting, accelerating, driving, and overcoming road obstacles. At low speeds, gear sets with larger ratios are engaged, while at high speeds, gear sets with smaller ratios are used. The speed of the gears is inversely proportional to the number of teeth—the more teeth a gear has, the slower its rotation. By combining gears of different sizes, various gear ratios can be achieved. Vehicle transmissions are categorized into manual and automatic types. Automatic transmissions include AT (automatic transmission), DCT (dual-clutch transmission), CVT (continuously variable transmission), and AMT (automated manual transmission). AMT Transmission: This is the earliest type of automatic transmission, structurally similar to a manual transmission but equipped with an additional control mechanism for gear shifting and clutch operation. AT Transmission: Also known as a torque converter transmission, this is the most mature and widely used transmission type. AT transmissions offer smoother shifting and greater durability. DCT Transmission: Also called a dual-clutch transmission, this is currently the most popular transmission type. Its structure differs significantly from manual transmissions, featuring two clutches that separately control odd and even gears. DCT transmissions provide high transmission efficiency and rapid gear shifts. CVT Transmission: Known as a continuously variable transmission, CVTs are lightweight and compact. Vehicles equipped with CVTs typically achieve better fuel efficiency. The internal control mechanisms of automatic transmissions are highly precise, with minimal clearance between components. Most automatic transmissions require oil changes every two years or 40,000–60,000 kilometers. Under normal operating conditions, transmission oil temperatures reach around 120°C, necessitating high-quality and clean oil. Over time, transmission oil can form sludge, increasing wear on friction plates and other components, affecting system oil pressure and power transmission. Sludge in dirty oil can hinder valve movement, impairing oil pressure control and causing transmission abnormalities. Manual transmissions primarily consist of gears and shafts, achieving speed and torque variations through different gear combinations. Automatic transmissions, on the other hand, comprise a torque converter, planetary gears, a hydraulic torque conversion system, and a hydraulic control system, utilizing hydraulic power and gear combinations to vary speed and torque. Structure of a Vehicle Transmission: The basic design includes a housing, transmission components, and control mechanisms. The housing serves as the base, supporting all transmission components and containing lubricating oil. It features precisely bored holes for bearing installation. Since transmissions endure variable loads, the housing must be sufficiently rigid, with reinforced inner walls and a complex shape, typically cast from gray iron (commonly HT200). For ease of assembly, transmission and control components are often split, with the cover bolted to the housing and securely positioned. The housing includes ports for oil filling, draining, and level inspection, as well as provisions for heat dissipation.
