On the automobile quality complaint website, the top complaints are abnormal sound and failure of gearboxes. The main brands are Volkswagen and GAC Trumpchi, both of which use dry double-clutch gearboxes. So why do cars with dry double-clutch have many problems and poor driving experience?
Today, we will analyze the difference between AT automatic gearbox and DCT double clutch gearbox from the technical aspect, to see if double clutch is worth choosing.
How AT and DCT work
In the basic classification, AT and DCT are automatic gearboxes, that is, automatic shift, the driver does not need to step on the clutch pedal, just need to put the gear knob in the right position to automatically change gear.
In fact, DCT dual-clutch transmissions have more in common with manual transmissions than traditional AT automatic transmissions.
As the name suggests, a dual clutch transmission has two clutch assemblies and two input shafts. The two clutches are located on separate shafts that can be engaged and detached separately.
Like a manual transmission, the dry DCT has a synchronizer and a shift fork.
The difference is that the shift fork is not driven by the driver through the gearbox shifter, but is controlled by the gearbox or the powertrain electronic control module (ECM), which replaces the driver's brain with a computer.
Usually the odd-numbered gear (first, third, etc.) is on one input shaft, while the even-numbered gear is on the other shaft, and the two axles are sheathed together (red and green below).
Gearbox can be preselected at any time for the next gear and ready for shift up, so the biggest advantage of double clutch gearbox is fast shift speed.
The seven-speed Tremec TR-9070 DCT transmission used in the 2020 Ford Mustang Shelby, for example, can shift gears in 80 milliseconds.
The working principle of an AT automatic transmission is more complex than that of a dry dual clutch. The AT relies on three main systems to operate, including planetary gear sets, torque converters, and clutch sets with control system.
In addition to these systems, there is a gear driven oil pump that provides hydraulic pressure for the drive.
The torque converter belongs to the hydraulic coupler, the role is to engage and separate the transmission from the engine, to provide the function of clutch.
Torque converter has two groups of blades, one group is connected to the flexible connecting plate, the flexible connecting plate is fixed on the back of the engine crankcase with bolts, this group of blades is called the impeller.
The second is connected to the input shaft of the gearbox and is called the turbine.
When the converter rotates, the oil inside is pushed to the outer circumference by the impeller.
As the oil pressure around the circumference increases, the oil is pushed onto the blades of the turbine and begins to exert a rotating force.
The faster the engine speeds, the greater the oil pressure applied to the turbine, thus increasing the turbine's speed until it finally approximates the speed of the impeller.
It is a little complicated to understand directly, but if the impeller and turbine are understood as two fans placed opposite each other, one fan is powered on, the other is not powered on. When one fan starts to rotate, the wind blown out will slowly drive the other fan to rotate, and the last two fans will rotate at the same speed.
In addition to the torque converter, another major component of the AT automatic transmission is the planetary gear set.
The planetary gear set uses a ring of teeth, usually three planetary gears and a sun wheel at the center.
The planetary gear turns between the sun wheel and the ring gear.
The specific transmission ratio is determined by the diameter and number of teeth of the gear.
Since planetary gear sets have multiple components, as long as the rotation of one of them is restricted and the power input and output are changed, the transmission ratio will be changed;
When two or more planetary gear sets change together, multiple gears can be converted.
What changes the rotation of the planetary gear assembly parts is the clutch assembly, each consisting of several friction plates.
They are controlled by a hydraulic system.
The clutch automatically holds or opens the bracket of the input shaft, ring gear and planetary gear set to select the required transmission ratio.
Compared to the SCT, which can shift very fast, a traditional AT automatic transmission will slip before the torque converter locks and shift slowly.
At the same time the built-in hydraulic pump needs to consume a certain amount of engine power to drive.
As a result, the AT automatic transmission has a greater power loss.
And dry DCT is engine directly from the input shaft connected to the output shaft, the efficiency of high speed, but the dry dual clutch AT low speed, frequent automatic transmission will wear, easy to heat, cooling slowly when the efficiency is low, so there will be setbacks and problem, in terms of daily family car, the shifting speed and power nor the pursuit of high efficiency, durable and stable AT automatic-shift is a better choice.