Working principle of automatic transmission

Working principle of automatic transmission

Although there are many models on the market and many types of automatic transmissions, the control and use methods are similar. A few years ago, the most common among domestic cars was the 4 front speed automatic transmission. Now many models are updated and equipped with 5 front speed automatic transmission. The Audi A4 is even equipped with 6 front speed automatic transmission.

The automatic transmission seems to be complicated. In fact, as long as we understand the mysteries of some of the simple parameters, when buying a car, the quality of the automatic transmission can be seen at a glance. The most important parameter of an automatic transmission is the number of gears. Anyone who has driven a car can understand this, and anyone is willing to drive a car with many gears. If there are more gears, the transmission and engine power will be more closely matched, and the performance of the engine can be better played. But looking at the number of gears is not enough. In fact, the number of gears in an automatic transmission is not the core of the technology, because simply increasing the planetary gear set can increase the gears. Like Mercedes-Benz, Volvo's commercial trucks, some even have more than 20 gears. The technical core of the automatic transmission is its control mechanism. Because of a good automatic transmission, its shift quality must be characterized by fast response speed and small shift shock. And all this needs to be achieved by designing and improving the control mechanism with excellent performance.

Automatic transmissions use different hydraulic multi-plate clutches and brakes to limit or connect certain gears in the planetary gear set to obtain different transmission ratios. Therefore, the quality of the shift is directly related to these clutches and brakes. Depending on the grade of the car, for cost reasons, the control mechanism of the automatic transmission of the economical car is usually designed to be very simple. As shown:

The picture above shows the most commonly used brake mechanism in automatic transmissions. It limits the movement of planetary gears through braking bands. The brake belt can quickly wrap the braked gear or shaft under the push of the lever, thereby generating a strong braking force to limit the movement of the planetary gear. The lever is directly pushed by the jack, and the power of the jack comes from the hydraulic pressure. Therefore, the braking of the planetary gear is completely determined by the hydraulic pressure. This type of brake belt design has a very simple structure and low cost, and is often used in automatic transmissions for economic vehicles. But because the brake band brakes are very abrupt and the braking force comes very violently, the shift shock is relatively large. This design is rarely used in high-end cars. The multi-plate clutch brake design is often used in high-end cars. As shown below:

The picture above is the automatic transmission of Audi A4. The part in the green round basket is the multi-plate clutch type planetary gear brake mechanism. The automatic gearbox adopting this design can obtain very good shift quality. The movement during the shift is very soft and the vibration and shift shock can hardly be felt, but the manufacturing and maintenance cost is very high.

Early automatic transmissions were usually mechanically controlled, with at most a small number of electronic systems to assist. The mechanical automatic transmission hydraulic oil circuit has a complicated structure, high cost, and poor durability, requires frequent maintenance, and the maintenance cost is surprisingly high. Modern automatic transmissions have basically adopted an electro-hydraulic integrated design. In fact, this is not the case with single transmissions. Many automated designs now use an electro-hydraulic integrated design. The so-called electro-hydraulic integration refers to the electronic way to control the hydraulic oil circuit. In this way, various complicated hydraulic control valves and control pipelines are omitted, and the hydraulic valves are directly replaced with solenoid valves. The biggest advantage of solenoid valve is that it is easy to arrange, high reliability and response speed. We can imagine that it is easier to lay out complex hydraulic circuits or wires? The answer is of course the latter. In addition to the above advantages, the electro-hydraulic integrated variable speed control has a great advantage in that the control method is more intelligent. Because the solenoid valve is directly connected to the driving computer, the computer can easily adjust the control method according to the various states of the car. Unlike pure hydraulic control, the control mode is fixed. Therefore, in many vehicles equipped with an electro-hydraulic integrated automatic transmission, there are economic mode, sports mode, and snow mode to choose from. In the economic mode, the computer-controlled transmission shifts at a low speed to achieve fuel-saving purposes; in the sports mode, the computer-controlled transmission shifts at a high speed to exert the power performance of the engine; in the snow mode, the computer-controlled automatic transmission directly uses 2 gears Start off to avoid runaway due to tire slippage. Therefore, this electro-hydraulic controlled automatic transmission feels very intelligent and very obedient. All these control modes only need to modify the computer program, and no hardware changes are required, so the cost is lower than the traditional automatic transmission, but the performance is higher.

Of course, when using an automatic transmission, there are many differences from the manual transmission. First of all, both automatic and manual transmissions have neutral gear (that is, N gear). But the N-speed of the automatic transmission is completely different from the N-speed of the manual transmission. After the manual transmission is engaged in the N gear, the synchronizer separates the power of the gear and the shaft to completely cut off the power transmission; after the automatic transmission is engaged in the N gear, the power is not separated, but all the clutches and brakes on the planetary gear are released The planetary gears all rotate, but do not transmit power (this is a characteristic of planetary gears). Therefore, after the automatic transmission is in N gear, it does not mean that the power of the engine is cut off, but that the power of the planetary gears is not transmitted. If the automatic transmission is hooked into the N-speed car during high-speed driving, the lubricating oil pressure will be reduced, the lubrication will not keep up and the planetary gear will rotate at a relatively high speed, so it is easy to burn the gear. Another point is not to switch the gear lever from D gear to N gear during a short stop, because the automatic transmission uses hydraulic pressure to separate the clutches and the brakes to achieve the gear shift, and the same is true for the neutral gear. Therefore, frequent switching to N gear will increase the working intensity and wear of each clutch and brake, reducing the service life of the automatic transmission. In fact, you can rest assured that engineers have considered the parking problem when designing the automatic transmission. In fact, short-term parking in D gear will not have a bad impact on the transmission at all. Although the car is stopped and the engine is still rotating, but with The weak energy at speed can be completely absorbed by the torque converter to achieve balance. Unless it is stopped for a long time in a high temperature environment, the oil temperature of the torque converter will increase. funcTIon ImgZoom (Id) // Re-set the image size to prevent the form from being broken {var w = $ (Id) .width; var m = 520; if (w <m) {return;} else {var h = $ (Id) .height; $ (Id) .height = parseInt (h * m / w); $ (Id) .width = m;}} window.onload = funcTIon () {var Imgs = $ ("content"). getElementsByTagName ( "img"); var i = 0; for (; i

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