As family cars become more and more popular in China, Chinese people are becoming more and more aware of the importance of gearboxes to a car. As one of the three major parts of the car, the gearbox undertakes the functions of changing speed, twisting and distributing engine power, which has a great impact on the dynamics and driving texture of the car. The gearbox commonly used in automobiles can be roughly divided into manual (MT), hydraulic machinery (AT), stepless (CVT), dual clutch (DCT), etc. Everyone often quarrels over which gearbox is good. Today Lao Hou from the perspective of the gearbox gear switching process to give you an analysis, which gearbox is better.
First, let's look at the shift process of the manual transmission (MT). The manual transmission is the simplest type of gearbox, its main structure is two shafts, on which two sets of gears with different numbers of teeth are installed. By manually controlling the meshing of different gears, the gearbox ratio is changed to achieve gear switching. For example, the first gear is 11-tooth active gear driving 48-tooth passive gear rotation, the transmission ratio is 4.36; the second gear is 16-tooth active gear driving 41-tooth passive gear rotation, the transmission ratio is 2.56; and so on, the higher the gear, the larger the active gear, the smaller the passive gear, the smaller the transmission ratio, the general five-speed transmission five gearbox is the direct gear with the transmission ratio equal to 1, the six-speed transmission five gear is the direct gear with the transmission ratio equal to 1, and the sixth gear is the speed gear with a transmission ratio less than 1.
The most important thing in the manual transmission shift process is "synchronization". In the process of gear shifting, there is a forced detachment and meshing of the gear, and the two gears are to mesh, the speed must be equal, this process of making the two gears to be meshed equal speed is called "synchronization". There are two ways to synchronize, one is to step on the accelerator pedal to control the engine speed, so that the active gear and the passive gear speed are equal, but this method requires the driver to have extremely high operating skills, the entire shift process only two shift points, it is difficult to grasp; the other method is forced synchronization, that is, the use of synchronizers to force the two gears to be engaged to be equal, which is now the method used by most manual transmissions. The technology and quality of the synchronizer directly determine the feel and smoothness of the gear shift. For example, Volkswagen's MQ200 manual gearbox adopts a unique synchronous ring structure, the gear to be engaged is synchronized quickly, the gear switching process is fast and smooth, and the suction sense is strong.
In addition, during the shifting process of the manual transmission, it is also necessary to step on the clutch pedal and disconnect the connection between the engine and the gearbox, so that the active gear is in a free state, which is more conducive to the synchronization of the gear. In this process, the clutch is in a semi-combined state, and the amount of wear is large, so the clutch should also be replaced regularly. During the entire gear change, due to the forced synchronization and meshing of the gears, it is inevitably accompanied by mechanical wear, so the manual transmission has a certain service life. During the shifting process, due to the sudden change in the transmission gear ratio, the pedal the accelerator pedal, the degree of clutch combination, and the speed should be coordinated to adapt to the change of engine load, which requires higher driving technology, labor intensity is also large, and it is used less and less in passenger cars.
The full name of the AT gearbox is "hydraulic mechanical transmission", and its main structure can be roughly divided into three parts: hydraulic torque converter, planetary gear transmission mechanism, and hydraulic control mechanism. The current AT gearbox is basically electronically controlled, using electronic systems to control the hydraulic control mechanism, so it is also called "electronically controlled hydraulic mechanical gearbox".
The torque converter is responsible for transmitting the power of the engine to the planetary gear transmission mechanism, and the hydraulic control mechanism can change the transmission ratio of the planetary gear transmission mechanism by controlling the combination and separation of the clutch and brake. A planetary gear set is composed of three parts: a solar wheel, a planetary gear, and a ring of gear, each of which is connected to a clutch or brake. We use any one of them as an active wheel, and then fix a gear, and then we can output power from the other gear. In theory, a planetary gear set can be composed of six different power transmission methods, with six transmission ratios. We combine several planetary gear sets to form a gearbox suitable for car use. For example, a normal six-speed AT transmission with three or four planetary gear sets in it.
The shifting process of the AT gearbox is actually the process of arranging and combining these planetary gear sets. The gearbox electronic control unit calculates the gear position that the gearbox should be attached to according to the parameters such as vehicle speed and engine load, and then sends a control signal to the transmission electromechanical unit, the solenoid valve in the transmission electromechanical unit cuts off or connects the corresponding oil circuit, controls the combination and separation of the clutch and brake on the corresponding planetary gear group, and can form different transmission ratios to achieve gear switching.
For example, the first planetary gear sun wheel input power, planetary gear fixed, ring gear output power to the second planetary gear set, the transmission ratio is 8.1:1; the second planetary gear planetary frame is input, the ring of gear is fixed, the sun wheel output power to the third planetary gear set, the transmission ratio is 1:2.6; the third planetary gear ring and planetary frame are fixed, the sun wheel output power, the transmission ratio is 1. So the total gear ratio is
（8.1:1）×（1:2.6）×1=3.1
That is, the gear ratio of this gear is 3.1:1.
In the same way, if the hydraulic control mechanism changes the input and output gears of the planetary gears, the transmission ratio changes accordingly. Multiple planetary gear sets can be combined to combine many different gear ratios. We selected the meaningful parts of it to form a multi-gear AT transmission. Interestingly, there are now some multi-gear AT transmissions, in fact, do not need to increase the mechanical parts, as long as you change the control logic, you can increase the number of gears.
In the shift process of the AT gearbox, there is no meshing and disengagement of mechanical gears, only the hydraulic control mechanism is in action, so the gear switching process is very gentle and smooth. After the gear switch is completed, in order to improve the transmission efficiency, the locking clutch will be used in the torque converter, and the engine and the gearbox will be completely locked, and the two will output power as a whole, and the gearbox at this time is a rigid gear transmission mechanism. When it is necessary to switch gears, the lock clutch is separated, and the torque converter absorbs the shock and vibration caused by the sudden change of gearbox and engine load, reduces the impact of gear shifting, and realizes smooth gear switching.
The basic structure of the CVT gearbox is similar to that of the AT gearbox, except that the planetary gear shift mechanism is replaced by a steel belt and taper wheel. The control logic is also similar, the gearbox electronic control unit calculates the transmission ratio of the gearbox according to the parameters such as speed and engine load, and then sends a control signal to the transmission electromechanical unit, the solenoid valve in the gearbox electromechanical unit controls the on-off of the corresponding oil circuit, controls the diameter of the active cone wheel and the passive cone wheel, you can change the transmission ratio and achieve the change of gear.
In theory, a CVT gearbox can have an infinite number of gear ratios, and the steel belt can stay anywhere on the taper wheel to transmit power. However, in practice, in order to reduce the complexity of the control logic, it is usually necessary to select several fixed transmission ratios and let the steel belt stay in these positions. For example, the common simulated ten-speed CVT gearbox is to select ten points on the cone wheel, and then according to the parameters such as speed and engine load, let the steel belt stay in one of these ten points to transmit power. However, in the process of changing the position of the steel belt (such as from 1 point to 2 points), the transmission ratio is continuously changed, and there will be no mutation. Therefore, the CVT transmission shift process is extremely smooth.
Finally, let's talk about the gear change process of the dual-clutch transmission. The dual-clutch gearbox is similar in structure to the manual gearbox, and the meshing process of its internal gears is basically the same as that of the manual gearbox. Only the dual-clutch transmission combines the clutch and the gearbox together, and the separation and combination of the clutch, the disengagement and meshing of the internal gears of the transmission, are all completed by the electronic control system, and no manual control is required.
A dual-clutch transmission can be seen as two manual transmissions combined in series. Two sets of clutches are used to drive two sets of gears. When one set of gears transmits power, the other set of gears has been hung into the corresponding gear standby, and the shifting process is actually only the separation and combination of clutches, so the gear change speed is very fast. In order not to interrupt the power output during the shift, there will be a brief combination of the two clutches at the same time. At this time, both clutches are in a semi-clutch slip state, and the speed ratio of the two is called "slip rate".
The control logic of the dual-clutch transmission is not very easy to compile, and the wear of the clutch plate will cause the clearance to change frequently, and the fluctuation and abnormal sound are the biggest problems. Especially at low speed, due to frequent switching between low-speed gears, the clutch is in a semi-clutch state for a long time, and the heat of the clutch cannot be dissipated in time, which will lead to ablation of the clutch, which in turn will lead to the setback of gear shifting, jitter and so on. When driving at medium and high speeds, the engine is rigidly connected with the gearbox, the transmission efficiency is very high, and the power transmission is direct, and the performance is very good.
Finally, to sum up, the manual transmission shift process is completely controlled by manual, accompanied by mechanical wear; the AT and CVT gearbox shift process is actually the action of the hydraulic control mechanism, there is no change in mechanical connection, there is almost no mechanical wear; the dual-clutch gearbox shift process is similar to the manual gearbox, but from manual to electronic control. Friends, which gearbox do you think is the best and most suitable for home use? Everyone is welcome to pay attention to the discussion in the comments section.