A Comprehensive Analysis of Automatic Transmission (AT) Technology

Among the numerous complex systems in an automobile, the automatic transmission (Automatic Transmission, abbreviated as AT) is undoubtedly a key technology with great innovation and practicality. Its emergence has completely transformed people’s driving experience, making driving more relaxed and convenient. This article will delve into the technical details of the automatic transmission, including its working principle, structural composition, development history, and future trends.

The working principle of the automatic transmission is based on factors such as the engine speed, engine torque, vehicle speed, throttle opening, and power load of the vehicle to automatically shift up or down gears, without the need for the driver to operate the clutch and shift lever to manually shift gears. Its core is to achieve speed change and torque conversion through a series of precise mechanical and hydraulic systems.

The torque converter is one of the most distinctive components in the automatic transmission. It is composed of components such as the pump impeller, turbine, and stator. It can directly receive power from the engine and transmit the torque to the subsequent gear system. Its working principle is similar to that of two fans placed opposite each other. When one fan is powered on and rotates, the generated airflow will drive the other fan to rotate. In the torque converter, the pump impeller is connected to the engine. When the engine runs, the pump impeller rotates accordingly. The oil in the pump impeller is thrown out under the action of centrifugal force and rushes towards the turbine, driving the turbine to rotate, thereby transmitting the engine’s power to the turbine.

By adding a stator between the pump impeller and the turbine, the stator generates a reaction force to create a speed difference between the pump impeller and the turbine, thus achieving speed change and torque conversion. When the vehicle starts or is driving at a low speed, the turbine speed is relatively low, and the speed difference with the pump impeller is large. The reaction force of the stator on the oil is large, which can increase the torque output and help the vehicle start and accelerate smoothly. When the vehicle is driving at a high speed, the turbine speed approaches the pump impeller speed, the reaction force of the stator on the oil decreases, and it may even rotate with the oil. At this time, the torque converter mainly plays the role of transmitting power and reduces energy loss.

In addition, modern torque converters are usually equipped with a lock-up clutch. After the vehicle speed reaches a certain level, the lock-up clutch will directly connect the pump impeller and the turbine, allowing the engine power to be directly transmitted to the transmission, avoiding energy loss during the hydraulic transmission process, improving the transmission efficiency, and reducing fuel consumption.

The planetary gear system is another important component of the automatic transmission. It achieves different gear ratios through different gear combinations, thereby achieving the purpose of speed change. The planetary gear system is composed of components such as the sun gear, planetary gears, and ring gear, and its structure is similar to the planets in the solar system revolving around the sun.

The three components of the sun gear, planet carrier, and ring gear in the planetary gear system can rotate around the same transmission shaft center. Any one of these components can be locked, and either one of the other two components can be the driving element, and the remaining one is the driven element. By controlling the clutches and brakes, different components can be fixed or rotated to achieve different gear ratio combinations. For example, when the sun gear is the driving element, the ring gear is fixed, and the planet carrier is the driven element, the output speed is low, and the gear ratio is large, which is suitable for situations where the vehicle needs a large torque during starting or climbing. When the sun gear is fixed, the planet carrier is the driving element, and the ring gear is the driven element, the output speed is high, and the gear ratio is small, which is suitable for situations where the vehicle needs a small torque during high-speed driving.

The hydraulic control system is a key system in the automatic transmission for controlling and adjusting the gear ratio. It is mainly composed of a hydraulic pump, a hydraulic valve body, oil circuits, and a control unit. The hydraulic pump is used to provide hydraulic energy, pressurize the automatic transmission fluid (ATF), and deliver it to each hydraulic component. The hydraulic valve body is the core of the entire hydraulic control system. Through a series of valves and oil circuits, according to the vehicle’s driving status and the driver’s operation instructions, it precisely controls the flow direction and pressure of the hydraulic oil, thereby realizing the control of the clutches and brakes in the planetary gear system and completing the gear shift operation.

For example, when the vehicle needs to shift up, the control unit will send a signal to open the corresponding valve in the hydraulic valve body. The hydraulic oil enters the specific clutch cylinder, pushing the clutch plates to engage, and at the same time, separating another clutch plate, thus achieving the gear shift and reaching the purpose of shifting up. Conversely, when the vehicle needs to shift down, the hydraulic valve body will perform the opposite operation to achieve this.

The automatic transmission is mainly composed of a hydraulic transmission mechanism, a speed change mechanism, an electro-hydraulic system, and a control system.

The hydraulic transmission mechanism is mainly composed of three main components: the rotatable pump impeller and turbine, and the stationary stator. Each working wheel is precisely manufactured from aluminum alloy or stamped and welded from steel plates. The pump impeller is integrated with the torque converter housing and is fixed with bolts on the flange or flywheel at the rear end of the engine crankshaft. The turbine is connected to other components of the transmission through the power output shaft. The stator is fixed on the fixed housing of the transmission through the stator shaft. After the three working wheels are assembled, they form an annular body with a circular cross-section. The pump impeller, stator, and turbine are essential basic working components for the torque converter to convert energy, transmit power, and change torque.

The planetary gear speed change mechanism is one of the core components of the automatic transmission. It is mainly composed of a planetary gear set, clutches, and brakes. The planetary gear speed change mechanism achieves different gear ratios by changing the position and combination of the planetary gears, thereby meeting different driving requirements.

The electro-hydraulic control system is mainly responsible for transmitting hydraulic energy. It is composed of a hydraulic pump, a hydraulic valve body, oil circuits, and a control unit. The hydraulic pump is used to provide hydraulic energy, the hydraulic valve body is used to control the flow direction and pressure of the hydraulic oil, the oil circuits are used to transmit the hydraulic oil to each component, and the control unit is used to receive the vehicle’s driving signals and control the operation of the hydraulic control system.

The control system is composed of electronic parts such as a control chip, sensors, and valves, and it controls and monitors the automatic transmission. Specifically, the electronic control system instantly detects the vehicle’s speed and driving status through sensors, and adjusts the hydraulic transmission system as needed, so that the clutch automatically switches to the corresponding gear for speed increase, achieving the gear shift operation of the transmission.

The development history of the automatic transmission is full of innovation and breakthroughs. From its initial prototype to the highly intelligent state today, each stage has witnessed the progress of automotive technology.

In 1904, the Sturtevant “horseless carriage gearbox” was considered the first true automatic transmission. But the truly milestone event was in 1939, when General Motors (GM) in the United States launched the four-speed hydraulic automatic transmission named “Hydra-Matic” and equipped it on vehicles in 1940. This was the first mass-produced automatic transmission. It adopted a hydraulically controlled planetary gearbox, which could automatically shift gears according to the vehicle’s driving speed, greatly simplifying the driving operation. In the following decades, the automatic transmission technology continued to develop. With the rise of electronic technology, the electronic control system was gradually applied to the automatic transmission, making the gear shift more accurate and rapid, and also improving fuel economy and driving comfort. For example, after 1965, the number of gears of the automatic transmission began to increase, gradually developing from the initial 4-speed to 6-speed, 8-speed, and even 10-speed. More gears can enable the engine to operate efficiently within a wider speed range, further enhancing the vehicle’s performance.

• Simple Operation: One of the greatest advantages of the automatic transmission is its simple operation. The driver does not need to frequently step on the clutch and shift gears, but only needs to control the accelerator and brake, which greatly reduces the driving difficulty, especially suitable for driving in urban congested road conditions.
• High Driving Comfort: The automatic transmission can automatically select the most suitable gear according to the vehicle’s driving status. The gear shift process is smooth, reducing the sense of jerking, and providing a more comfortable riding experience for passengers.
• High Driving Comfort: The automatic transmission can automatically select the most suitable gear according to the vehicle’s driving status. The gear shift process is smooth, reducing the sense of jerking, and providing a more comfortable riding experience for passengers.
• Integration with Advanced Technologies: With the development of automotive technology, the automatic transmission is more likely to be integrated with other advanced technologies, such as autonomous driving assistance systems and engine start-stop technology, further enhancing the overall performance and intelligence level of the vehicle.

• High Cost: The structure of the automatic transmission is complex, and the manufacturing precision requirements are high. It contains a large number of precision mechanical components and electronic components. Therefore, its manufacturing cost is relatively high, which also leads to the fact that vehicles equipped with automatic transmissions are usually more expensive than manual transmission models.
• Difficult Maintenance: Due to the technical complexity of the automatic transmission, once a fault occurs, it is difficult to repair. Professional technicians and equipment are required for detection and repair, and the maintenance cost is also high.
• Low Efficiency in the Early Stage: In the past, the automatic transmission had relatively low transmission efficiency, especially when the torque converter was not locked, there was a certain amount of energy loss, resulting in lower fuel economy than the manual transmission. However, with the continuous progress of technology, modern automatic transmissions have significantly improved their transmission efficiency through optimized design and the adoption of advanced control strategies.

Looking ahead, the automatic transmission technology will continue to develop in the direction of high efficiency, intelligence, and environmental protection.

In order to further improve fuel economy and reduce emissions, the automatic transmission will continuously optimize its internal structure and reduce energy loss. For example, using more advanced materials and manufacturing processes to reduce the friction coefficient of gears and bearings; optimizing the design of the torque converter to expand the lock-up range, enabling the engine to directly drive the vehicle under more working conditions; developing new types of transmission mechanisms, such as multi-plate wet clutches and electronically controlled mechanical shift mechanisms, to improve transmission efficiency.

With the rapid development of artificial intelligence and sensor technology, the automatic transmission will have stronger intelligence and adaptive control capabilities. By real-time monitoring of the vehicle’s driving status, the driver’s driving habits, and road conditions information, the automatic transmission can automatically adjust the gear shift strategy to achieve more accurate and intelligent gear shift operations. For example, according to the driver’s acceleration intention, the automatic transmission can downshift in advance to provide stronger power output; under different road conditions, such as climbing, downhill, and cornering, the automatic transmission can automatically select the most suitable gear to ensure the vehicle’s driving stability and safety.

With the increasing global attention to environmental protection and sustainable development, the new energy vehicle market has rapidly emerged. As an important part of the vehicle’s transmission system, the automatic transmission will also actively integrate with new energy technologies. In hybrid vehicles, the automatic transmission needs to be efficiently matched with power sources such as the engine and the motor to achieve seamless switching between different power modes and improve the vehicle’s power performance and fuel economy. In pure electric vehicles, although a traditional multi-stage speed change mechanism is not required, the automatic transmission can achieve single-speed or two-speed transmission through optimized design, improving the motor’s working efficiency and extending the vehicle’s driving range.

In order to save interior space and improve the vehicle’s layout flexibility, the automatic transmission will develop in the direction of integration and miniaturization. Future automatic transmissions may integrate more functional modules together, such as the oil pump, valve body, and electronic control unit, reducing the number of components and the system complexity. At the same time, by adopting a compact structure design and new materials, the volume and weight of the automatic transmission can be reduced, further improving the vehicle’s performance and fuel economy.

As an important part of automotive technology, the automatic transmission has made great progress in the past few decades. It not only brings drivers a convenient and comfortable driving experience but also promotes the technological progress of the automotive industry. With the continuous advancement of technology, the automatic transmission will continue to innovate and develop, bringing more possibilities for future automotive travel. Whether in the field of traditional fuel vehicles or new energy vehicles, the automatic transmission will play an indispensable and important role and become a key force in promoting the automotive industry to develop in the direction of high efficiency, intelligence, and environmental protection.