How Does an Automatic Gearbox Work? (HAT, DSG, CVT) MILTA Technology

How Does an Automatic Gearbox Work? (HAT, DSG, CVT)

Automatic gearboxes are like a great orchestra. And just like in an orchestra, all the musicians or in this case, gearbox components, have to work properly and in a synchronic rhythm. Due to the fact that automatic gearboxes have been installed in cars for quite some time now, engineers perfected the art of automatic gear shifting and they even developed advanced and different versions of automatic gearboxes such as the DSG and CVT.

Learning how they work will help you understand why it is crucial to keep your automatic gearbox serviced and taken care of by professionals.

The “Standard” or Hydraulic Automatic Gearbox

Also known as: “Tiptronic”, “Geartronic”, “Torque-Converter Automatic”

This is the most common and the oldest type of automatic gearbox. The technology of hydraulic automatic gearboxes has stayed the same for the last 30+ years but engineers never stopped building on top of that basic principle. This is why the hydraulic automatic gearboxes remain known for their reliability and durability. And they are not just a piece of reliable old technology, you can still find them in luxury modern cars such as the Mercedes S-class which sports a 9-speed hydraulic automatic gearbox. They are still highly common and relevant.

How the Hydraulic Automatic Gearbox works:

The entire process of automatic shifting in a hydraulic automatic gearbox is highly complicated and involves many engineering principles and technologies. Below is the 5 step depiction of what happens as we press on the throttle.

  • The engine output shaft is coupled to the transmission by a part called a torque converter. The torque converter uses a turbine and an impeller. The turbine spins with the engine, while the impeller connects to the transmission shaft.
  • As the engine rotates, centrifugal force directs transmission fluid through the turbine blades which is then pushed onto the impeller that transfers power to the transmission.
  • To handle the difference in speed between the turbine and the impeller, the torque converter uses a stator that directs the transmission fluid and allows the torque converter to operate at a wide range of torque and gear ratios.
  • To shift higher or in reverse, the automatic transmission uses multiple sets of “planetary gear sets”. They are called the planetary gear sets because they consist of a central gear (“the sun”) and several smaller gears that are placed around it (“the planets).
  • By combining multiple planetary gear sets (depends on the number of gears) and the data gathered from the gearbox control unit and the gear selector, the gearbox then decides when it is the time to shift up, down and which gear or gear ratio to engage.

To understand this process in more detail and with better presentation, we recommend you check out 3D animated guides on Youtube.

Direct-shift or Double-clutch Automatic Gearbox (DSG)

Also known as: “Dual-clutch automatic”, “dual-clutch transmission (DCT), PDK (Porsche), PowerShift (Ford), S Tronic (Audi)

All double-clutch transmission work in the same way, despite the different commercial names from different manufacturers (PDK, DSG, DCT, EDC etc.) A double-clutch transmission consists of two sub-gearboxes and two clutches, hence the name double-clutch. If we are being technical, it is a direct-shift transmission and not a classic automatic transmission like the one we described before. A double-clutch transmission is in essence, an automated manual transmission.

A 6 or 7-speed direct-shift transmission is divided into two areas, two sub-gearboxes. One gearbox shifts the uneven gears (1,3,5 and 7) and the other gearbox comes into play when shifting into odd gears (2,4 and 6). The advantage of this principle is the fact that as you are driving around, the next gear is already engaged which means that gear changes are quick and there’s is almost no loss in traction. Both sub-gearboxes work on a common transmission output shaft.

How a double-clutch or direct-shift automatic gearboxes work:

To understand this principle, we will try and present the operation of a double-clutch transmission on a real-world example.

Let’s say you are driving around the town in the 4th gear. As you are driving around, the 5th gear is already engaged in the other sub-gearbox. The second clutch remains open and prevents the 5th gear to come in contact with the output shaft.

As you hit the open road and accelerate, the clutch of the 4th gear opens and in 1/100 of a second the second clutch controlling the 5th gear closes and fully engages the 5th gear. At the same time, the sub-gearbox responsible for odd gears already prepares and engages the 6th gear.

While you are driving around, the central element or the mechatronics of a double-clutch gearbox uses a combination of the data provided by control electronics, oil pressure pump and actuating mechanics to predict your next move and prepare the right gear for every speed and situation.

CVT Automatic Gearbox

Also known as: “e-CVT”, “Xtronic”, “Audi Multitronic”, “Mercedes Autotronic”, “Toyota Multidrive”

CVT stands for Continuously Variable Transmission. This means that while accelerating, you don’t jump from gear to gear, you simply glide between an infinite number of gears. There are no gear changes and physical gears. Even though CVT automatic gearboxes are not as common in Europe as they are in Japan, big European many car manufacturers like Audi and Mercedes have developed and implemented their versions. The idea behind a CVT transmission is to bridge the gaps between the individual gears. That makes sense because internal combustion engines only achieve their highest efficiency in a very narrow speed and rev range. The CVT transmission varies the ratio and thus optimizes the acceleration process.

How a CVT transmission works:

The CVT principle is rather simple. Imagine a bicycle drive with flexible chain wheels – both the front and the rear can be either smaller or larger. A chain or a link belt transfers the power from the input to the output shaft. This is made possible by two funnel-shaped pulleys on each shaft. These conical pulleys have the ability to move away from one another. The chain or a belt simply glides between the pulleys as they move. As a consequence, different gear ratios are achieved.

The pulleys on the other shaft move in opposite directions. When the pulleys open on the input shaft, they close on the output shaft and vice versa. This constellation has the same effect as a small gear on shaft one and a large gear on shaft two, i.e. a small gear. Thanks to the cooperation of the conical pulleys, the chain is always well tensioned. The exact position of the chain is controlled by the electronics in a fully variable manner – this is why the CVT transmission is step-less.


While there are also other variations of automatic transmissions, such as the “Automated manual transmission (AMT)”, the most common are the ones we described above. Each of them has its own advantages and disadvantages but they all have something in common. If you want them to work and last, you need to take care of them. Most of the problems that arise with modern automatic gearboxes are commonly connected to a lack of maintenance or improper use. There are still manufacturing problems, but most of them are quickly discovered and solved under warranties. It is up to you to keep your automatic gearbox running for years to come.

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