When it comes to automobile gearbox, the key word "which is more advanced" is always mentioned. However, for a certain model, there is no more advanced concept, only the difference in user experience and quality. (This article does not focus on a certain brand, but only discusses the characteristics of two types of "optional gearboxes")
Discrete cosine transform; Distal convoluted tubule; Dual clutch automatic transmission
Double? Clutch? Transmission is defined as an automatic gearbox with two sets of clutches (double clutches for short). This gearbox was tested and promoted by a German brand after 2000, and the China automobile market became its "test field". So this machine is regarded as a "new gearbox", but the fact is that the dual clutch has a history of 80 years.
The dual clutch technology was invented by the Frenchman Aodlphe kégresse (Adolf Garch), and the expected application model is Citroen Traciton. However, this machine was not put into practical use until the Germans applied for a patent in powershift the following year, and the first batch of trial-produced vehicles were trucks. Therefore, Audi cars have been promoting this way, but they are more or less boastful. (The picture below shows the DCT designed by Garch, which has the characteristics of the current dual-clutch gearbox. )
DCT dual clutch gearbox is not a subversive automatic gearbox, and its prototype has two evolutionary processes: manual? Transmission manual gearbox, AMT electronically controlled mechanical automatic gearbox; On the basis of the former, the latter realizes the automatic control of the clutch and shift structure through the electric control system of the motor, so as to simplify the manual operation to the automatic operation.
However, the shift principle of AMT is the same as that of MT, and the shift action is slow, so there will still be a large degree of speed decrease during the shift, and there will be a short engine braking after the shift, which will cause frustration. So what can be done to solve this problem and improve driving quality?
Improving the shift speed is fundamental! The frustration of AMT is due to the slow shift speed and engine braking. In order to improve the speed, the two power input shafts must be controlled by two groups of clutches respectively, and each shaft is provided with odd and even forward gears respectively.
When shifting gears, the two groups of clutches are synchronized and semi-linked, making the expected separation and combination actions, and realizing the instantaneous separation and combination with the target gear. In this way, the shift speed can be greatly improved to achieve the purpose of smoothness, and at the same time, the power loss in the transmission process can be reduced. This is the characteristic of dual clutch gearbox, which is more advanced than AMT or even AT in theory, but this machine also has obvious shortcomings.
"Dry&; The difference between wet clutches is huge! Because the double clutch control mechanism is equal to "semi-linkage frequency ×2", the high temperature caused by semi-linkage during gear shifting is exaggerated, and the friction coefficient of clutch friction plate will decrease in high temperature environment, resulting in problems such as increased transmission loss, gear shifting error, serious friction plate wear, and even inability to drive due to high temperature power failure.
Therefore, the durability of the early dry friction clutch was very poor, and it could only be used on the track without traffic jams (used by Porsche racing cars in the 1980s). Ordinary scooters can only be matched with "wet dual-clutch gearbox", because this structure will be lubricated with gearbox oil to protect the clutch from heat dissipation, and it has high durability and stability. So DCT is optional, but it must be wet.
Continuous variable transmission
Continuous? Variable? Transmission is defined as an automatic gearbox with stepless speed change. This machine is generally regarded as "invented by the Japanese", but Japanese car companies just missed it. CVT originated in Europe-Dutch van Domingz was its inventor. (1960s)
The so-called "continuously variable transmission" and AT/AMT/DCT gearbox are different definitions. These machines use different gear combinations to achieve different transmission ratios (forward gears). In the process of shifting gears, it is necessary to cut off the power input of the engine so that the gears can be separated and combined after deceleration. This concept can be understood as "interruption, connection and transmission". However, CVT does not need to cut off power when shifting gears. Refer to the shift structure characteristics of CVT in the figure below.
CVT is clamped by two groups of conical wheels controlled by hydraulic pressure, and transmission is realized through friction between them. Shift is to change the included angle of bevel gear and the transmission angle of belt synchronously, so as to increase the torque and change the speed. The change of power input during gear shifting does not affect the fit and angle change of the bevel belt, so it is necessary to shift gears without cutting off power, which is the basis of stepless speed change.
However, the disadvantage of this structure is very prominent, because the friction coefficient between the belt and the bevel gear is very low, and the torque output by the engine will exceed the friction limit if it is slightly larger, resulting in slippage between the two. Therefore, CVT is only suitable for small-displacement vehicles, while European cars generally pursue performance, so CVT originating in Europe has not been recognized.
Steel belt and steel chain upgrade! The friction coefficient between belt and helical gear is low. By using the resistance between metal chain or steel belt and helical gear, greater friction can be obtained under high pressure.
The answer is indeed yes. At present, the CVT that can bear the maximum input torque has reached 450n·m, but what is this? Among passenger cars, 450n·m is a performance car, but it has just started in commercial vehicles. Therefore, no commercial vehicle will choose this low-grade transmission, and the fact that the option of CVT for mass production vehicles is generally an ordinary scooter with peak torque ≤ 300 N m; You should know that excellent 1.5T engines will have this level, and the mainstream 2.0T also has a high water level of 360 ~ 400 N m. ..
Key points: wear, frustration and cold start protection are the absolute shortcomings of CVT! Even if the tapered steel belt structure matches the low-performance engine without slipping, the normal rolling friction is still worn. For example, car tires never slip when driving lightly. Will they wear to the limit in a few years? The essence of friction is the result of molecular collision on the surface of matter. Friction not only produces heat energy, but also wears away. So the durability of CVT is very poor. Most brands will be seriously worn out after hundreds of thousands of kilometers of use, and a few brands may have problems after tens of thousands of kilometers.
Because the steel belt of the cone wheel is worn, there must be enough lubrication to ensure that the durability is barely qualified; However, the fluidity of gearbox oil is very poor when it is started at low temperature, so it can't effectively lubricate the steel strip at this time. The result can only be local heating with a heater after cold start, the time is as short as one or two minutes, and the time may be close to ten minutes; However, during cold start, centralized fuel injection will cause insufficient combustion, lead to carbon deposition and obvious increase in fuel consumption, so CVT is contrary to fuel saving and emission reduction.
As for setbacks, I don't need to go into details. The CVT upshift process is really smooth, but the bevel steel belt can't "clutch" when decelerating. Therefore, there will be obvious engine braking when slowing down, which is equivalent to frustration; When the speed is reduced to about 30km/h, the disappearance of resistance will bring another setback. Therefore, the user experience of CVT is actually not as good as AT/DCT with ≥6 gears. This kind of machine is generally only accepted by Japanese cars and entry-level cars to illustrate the level of product positioning.
Summary: DCT &；; CVT has no "advanced" concept, the difference is that DCT/AMT can be used in various models (including commercial vehicles), while CVT can only be used in ordinary entry-level scooters, that's all.
(DCT was invented in the 1940s, but it didn't become popular until 60 years lATer, because at gearbox was invented by American car companies in the same year.
This article comes from car home, the author of the car manufacturer, and does not represent car home's position.