AT VS MT VS CVT VS DCT VS AMT (Easytronic) Transmission comparison, pros and cons
What is a gearbox (transmission) and why is it needed.
The gearbox is an integral part of any car with an internal combustion engine. The purpose of the gearbox is the transmission and conversion of torque from the engine to the wheels, as well as the implementation of power take-off for drives of other units and additional equipment. This process allows for optimal traction and vehicle speed, as well as reversing. Moreover, the box helps to disconnect the engine crankshaft from the drive wheels, which ensures that the car idles or stops completely.
It should be noted that gearboxes have become widespread not only in vehicles. Switch boxes are widely used in industrial mechanisms, machine tools in production.
Since the appearance of cars on the roads, manufacturers have improved not only engines, but also gearboxes. The development of this direction has led to the emergence of modern cars with different types of transmissions.
Types of transmissions
More than a century of automotive history has brought to the modern world not only environmentally friendly and powerful engines, but also advanced gearboxes. Today, four main types of gearboxes are installed on cars:
1. Manual gearbox
2. Automatic transmission
3. Robotic gearbox
4. Variable (stepless) gearbox
5. Dual-clutch transmission
Let’s take a closer look at each type of box.
The peculiarity of the operation of an internal combustion engine is that the working power develops only in a small speed range. For this reason, an additional mechanism is needed to change the torque.
The history of creation goes back more than a hundred years, and the invention belongs to Karl Benz. Structurally, the device of the first box was primitive and extremely simple. The box mechanism was implemented from a pair of pulleys of different diameters, which were located on the drive shaft, the pulleys were connected to the motor shaft with a belt. Depending on the driving conditions, the belt was moved from one pulley to another with the help of a specially provided lever. This made it possible to change the torque transmitted to the drive wheels. Such a simple mechanism has found application in the modern world, the gears on bicycles are switched according to the same principle.
Modern mechanical boxes have stepped much further from such a mechanism. Structurally, the box consists of a set of gears, and the gear change is carried out by engaging the gears with a lever.
Manual gearboxes can be equipped with a different number of steps. The most popular is the five-speed gearbox. In turn, mechanical gearboxes are divided into two-shaft and three-shaft gearboxes.
Two-shaft mechanical gearboxes are installed on cars equipped with front-wheel drive. Three-shaft gearboxes are installed on cars and trucks, which can be equipped with both front- and rear-wheel drive.
Advantages of manual transmission:
・Simple and reliable design
Easier driving in off-road conditions
Driving in economy mode
Low cost maintenance
Disadvantages of manual transmission:
Inconvenience of management in a complex urban mode
Automatic transmissions (Automatic, automatic transmission)
The idea of comfortable driving was born almost immediately with the advent of the car itself. Such comfort could provide automatic gear shifting. But this idea was not immediately realized. Only in 1947, cars with an automatic gearbox entered the series, and Buick cars began to be equipped with automatic transmissions.
Although in fact serial automatic transmissions appeared a little earlier. Automatic transmissions were equipped with city buses in Sweden as early as 1928.
It should be noted that three independent lines of development led to the emergence of a hydromechanical gearbox, which were later combined into its design. The automatic transmission was based on a torque converter, an invention of Professor Fettinger, for which he received a patent back in 1903. The other two elements are the planetary gear and the hydraulic control system.
A modern automatic transmission, unlike classical mechanics, works in different conditions and according to a different principle, although the main purpose is unchanged.
A torque converter, or torque converter, includes a pump, a turbine, and a stator. All parts of the torque converter are enclosed in a common housing. The torque converter is filled with special oil, the pump creates an oil flow inside the torque converter, which rotates the stator wheel and the turbine. Thereby transferring torque from the engine.
The planetary gear consists of several gears (they are called planetary or pinion gears) rotating around the central gear. The planetary gears are locked together with a carrier. In addition, the additional outer ring gear is internally meshed with the planetary gears. Satellites mounted on the carrier rotate around the central gear, the outer gear around the satellites. Gear ratios are achieved by fixing different parts relative to each other. To obtain a larger range of gear ratios, modern gearboxes use several planetary gears.
Hydraulics works in complete symbiosis with the rest of the automatic transmission and its work can be compared with the circulatory system. The fluid used as a working fluid, in addition to creating pressure in the system, also has a set of useful functions. Such as lubrication, heat dissipation and cleaning the inside of the automatic transmission from contamination.
Advantages of automatic transmission:
・Comfort and ease of use
Ability to change gears at full engine power
Smooth running during gear changes
Protection of engine parts from overload when choosing the wrong gear
Disadvantages of automatic transmission:
Cost and frequency of maintenance
Greater fuel consumption
Less vehicle dynamics
Robotic gearboxes (Robots)
The robotic gearbox is a logical continuation of the development of a manual gearbox. The robot is nothing more than a manual transmission, in which the clutch release and gear shifting are performed by two servo drives (actuators) controlled by an electronic unit. In fact, the robot has absorbed all the positive aspects of the manual transmission and the convenience of the machine.
The first prototype of the robot appeared in 1939, Adolf Kegress created a dual clutch transmission, but further development of this promising invention stopped for the next 40 years. This is due to the lack of funding for the project.
Robotic gearboxes did not get into the series very soon, but Porsche engineers decided to test the technology. Robots were introduced on the 956 and 962C models, the cars were intended for circuit racing. Unfortunately, a design flaw and a significant weight of the box did not allow the technology to go beyond the track.
The serial robotic box appeared only in 2003. Volkswagen dared to take this step by installing a preselective transmission on the sports version of the Golf 4 R32. The box was made by BorgWarner. To this day, VAG is actively promoting this type of box on its models.
The peculiarity of such a box lies in the design, namely the presence of two clutches. The principle of operation of such a box is that even gears are tied to one clutch, and odd gears to the second. During the movement, the torque is transmitted through one clutch, i.e. disk is closed. At the same time, the disk of the second clutch is open, but inside the box itself, the next gear has already been formed, and when the time comes to switch, the first disk simply opens, and the second one closes synchronously. This scheme of operation ensures smooth switching and the absence of jerks.
In turn, robotic boxes are divided into two types:
· With wet clutch – used on cars with a powerful engine, the torque of which exceeds 350 Nm.
Dry clutch – used on cars with low-power engines up to 250 Nm of torque.
Smooth shifting and running
Economical fuel consumption
Possibility to select the mode of operation of the transmission
Low reliability, both of the design itself and mechatronics
The cost of maintenance and repair
Sensitivity to heavy road conditions
CVT transmissions (CVTs)
Variable speed transmissions (CVT) are considered direct followers of classic hydromechanical gearboxes. There is a strong opinion that CVT boxes are the future, again, given the urban operation of cars. Particular emphasis on CVT transmissions is made by Japanese manufacturers such as Nissan and Subaru. The first CVT appeared in series on a DAF car in the 1950s. This car was not a truck, as many might think, but a small car.
Unfortunately, the design did not differ in particular reliability and long service life. Volvo, in turn, has been trying to develop the technology for many years, but it all ended with the curtailment of development. Japan gave an unexpected continuation of the history of the CVT.
The reason for the return and refinement of the CVT was the need to adapt automatic transmissions to operating conditions in urban traffic jams. The work of shifting gears on an automatic transmission is directly tied to engine speed. A classic automatic in city traffic jams, at a short distance and at low speed, began to shift gears from first to second, when this was completely unnecessary. In another case, moving “coasting”, the automatic transmission kept the gear without going down, for a long time waiting for the driver’s command to accelerate. This behavior of the box gave a large load on its own components, which led to increased fuel consumption, increased wear and early failure. All this led to intensive refinement of the automatic transmission, but the result was a fundamentally new type of gearbox – CVT.
The most amazing thing is that the first CVT was invented by Leonardo da Vinci in 1490. In the drawings of the inventor, one can see a scheme of parallel cones and a belt thrown between them, capable of moving across the axis of rotation of the cones, which made it possible to change the gear ratio of the pair.
The CVT or CVT is a continuously variable transmission. The main parts of the CVT box are a torque converter and two sliding pulleys, plus a belt connecting them (pulleys). The cross section of the belt has a trapezoidal shape. The principle of operation is as follows – the shifting halves of the drive pulley push the belt outward, which leads to an increase in the radius of the pulley along which the belt runs, this action increases the gear ratio. When a reduction in gear ratio is required, the driven pulley moves apart, the belt moves to a smaller radius. The torque converter in this design provides starting off, after which it is blocked. The pulleys are controlled electronically.
Advantages of CVT (Variator) :
Gear shifting is imperceptible, without jerks
Economical fuel consumption
Cons of the CVT (Variator) :
Incompatible with powerful motors
The cost of maintenance and repair
A large number of sensors affecting the operation of the CVT
Sensitivity to heavy road conditions, towing
What is a DCT or Dual-Clutch Transmission?
If you’ve done any research on current cars, you’ve probably seen references to dual-clutch transmissions (DCTs). And you might well have asked yourself, “What is a DCT? What is a dual-clutch transmission?” If you are young enough, you might also ask yourself, “What is a clutch?” To explain all that, let’s go back in time for a minute.
There was an era when learning to drive meant learning to master the intricacies of changing gears manually while manipulating the clutch pedal, the accelerator, and the gear selector lever all at once. Though the accelerator (gas pedal) and gear selector (shifter) are familiar to virtually all current drivers, the clutch pedal might not be.
In a car with a manual transmission, the clutch disengages the engine from the transmission for a few moments so the driver can switch from one gear to another. The driver pushes the clutch pedal in to operate the clutch. When pulling away from a stop in a manual transmission-equipped car, your feet do a little dance as you engage first gear or reverse, let out the clutch pedal, and add the proper amount of acceleration with the accelerator. The procedure takes a degree of skill that some find challenging to master.
Thus, to make cars easier to drive, inventors and tinkerers have sought ways to automate the process so the machine accomplishes what the driver used to perform.
Today, several different types of automatic transmissions do the gear-changing for us. Conventional automatics use torque converters to facilitate the gear changes. Continuously variable transmissions (CVT) use belts or chains and pulleys to manipulate the gear ratios. This article’s subject – the dual-clutch transmission (DCT) – internally works similar to a manual transmission but doesn’t require the driver to operate a clutch pedal
What is a DCT or Dual Clutch Transmission?
As we said earlier, in automotive terms, a clutch disengages the engine’s power-providing crankshaft from the transmission, so the driver can switch from one gear to another without causing potential binding of the machinery.
A dual-clutch transmission has – this should come as no surprise – two clutches. Why? Because a DCT is essentially two transmissions working in concert, each of which requires a clutch for the same reason a manual transmission requires a clutch.
Further, dual-clutch transmissions do not require the driver to manually change from one gear to another using the shift lever. Instead, the gear selection process is automated so that a DCT can act as an automatic transmission. But as we’ll explain, it does not select the gears in the same way that a conventional automatic transmission with a torque converter performs that task.
The magic of the dual-clutch transmission is that it can change from one gear to another very quickly. This speedy shifting is possible because while a gear is engaged in one of the two internal transmissions in the DCT, the next gear is pre-selected in the second of the two internal transmissions and ready for immediate engagement.
Thus, the transition from one gear to the next takes mere milliseconds, making gear changes with a DCT much quicker than with a traditional manual transmission or a conventional automatic transmission. This is especially helpful in performance and racing cars, where split seconds can be the difference between winning and losing.
How Does a Dual-Clutch Transmission Work?
A dual-clutch gearbox has two clutches but no clutch pedal. With a traditional manual transmission, clutch engagement requires the driver to use a clutch pedal. With a DCT, electronics control clutch engagement, issuing commands to the transmission’s internal hydraulics.
Each of the two clutches is associated with one or the other of the two internal transmissions. Most often, one of those transmissions houses the odd-numbered gears, with the other controlling the even-numbered gears. (One of the transmissions also houses the reverse gear for backing up.)
While driving a car with a DCT, depending on the selected gear, one of the internal transmissions is engaged with the engine and powering the car while the other is disengaged from the engine but with the next gear pre-selected for use. The change between gears is swift and occurs without interrupting the torque flow through the driveline.
Two Modes — Automatic and Manual
In the typical DCT, when driving automatic mode, the transmission will pre-select the next gear and then perform all the actions necessary to change between gears. The DCT will actuate and release the two clutches as necessary to upshift during acceleration and downshift during deceleration. Gear changes are sequential, which means in numerical order, up and down. However, a DCT can progress through its full range of gears quickly because of the dual-clutch gear pre-selection process.
Beyond automatic operation, a modern DCT can also operate as a clutch-less manual. That means that the driver can select the next gear manually, usually using levers (paddle shifters) on the steering wheel. The driver does not need to use a clutch pedal or actuate a clutch in any way. The transmission’s electronics and hydraulics automatically engage both internal clutches. Manual shifting enables very rapid up-and-down gear changes and gives a driver complete control over transmission behavior.
How Does a DCT Differ From Other Automatic Transmissions?
Conventional automatic transmissions use a torque converter to transfer the engine’s power to the transmission and onward to the drive wheels. Essentially a hydraulic torque transfer device, a torque converter enables power delivery but can also decouple power from the transmission when the engine is operating at very low engine speeds. This allows drivers to stop and sit idle without shifting the car out of gear. In a DCT, the two clutches perform the coupling and uncoupling action.
Continuously variable transmissions (CVTs) are another type of automatic transmission, and they are growing increasingly popular. A CVT uses a moving pulley system in conjunction with a belt or chain to adjust the gear ratio across a wide range. A CVT transmission does not involve the discrete changing of gears from one to another as in a conventional automatic or a DCT. Instead, it can vary the drive ratio based on power requirements.
CVTs typically offer excellent fuel efficiency, but they have characteristics that some drivers find objectionable. To combat this, automakers commonly program specific CVT operation ratios, making acceleration feel and sound more like that of a conventional automatic transmission. Some even offer paddle shifters to give the driver control over those pre-programmed ratios, such as in modern Subaru models.
In contrast to a CVT, a dual-clutch transmission changes gears like a conventional automatic transmission, but it does it more quickly and with much less power loss in the transition. A DCT also feels and sounds more satisfying to the ears of a driving enthusiast.
Why Some People Dislike Dual-Clutch Transmissions
Like CVTs, DCTs typically deliver good to excellent fuel economy versus other types of transmissions, but they do have a few downsides.
- Some drivers might find the quick shifts with some DCTs to be abrupt and uncomfortable
- When shifting between reverse and drive, a DCT can sometimes feel hesitant
- A similar hesitation is sometimes evident during initial acceleration from a stop
- Dual-clutch transmissions are typically not as adept at creeping forward at extremely low speeds when at a stoplight or inching forward into a parking place
For these reasons, many car makers prefer to use traditional torque-converter automatic transmissions or CVTs in their vehicles instead of DCTs. But for performance vehicles, dual-clutch transmissions remain a popular choice.
We examined the main types of gearboxes. We identified the main pros and cons of each type. But it is impossible to give an unambiguous answer which unit will be the best. Each is good in its range of tasks, and the choice of the unit that the car will be equipped with, given the range of tasks, already falls on the shoulders of the car designers and the consumer.