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An internal combustion engine produces torque, power, and optimum performance only within a limited speed range. For this reason the transmission, which converts the engine’s power to momentum which drives the wheels, must be equipped with several gear ratios that must be engaged one after the other as the speed of the car increases.

The automatic transmission on the other hand provides much more comfort for the driver, who no longer has to operate the clutch and gear stick.
This is why there’s no clutch pedal in automatic cars. You just need to use accelerator and brake pedal. The transmission system in fact senses when it is necessary to change gear and does it automatically. Driving an automatic car also allows you more time to focus on the road ahead, so, this might be the safer option for many drivers.

The semiautomatic transmission is the simplest type and consists of a normal manual gearbox controlled by actuators which in turn are driven by the control unit. Depending on the speed of the vehicle, these actuators determine when it should shift gears, and changes them using internal oil pressure. Generally with this kind of transmission it is also possible to manually select the gear using a lever, or using paddle shifters usually attached at the back of the steering wheel.
The main advantage of this type of automatic transmission is the low production cost compared to other types of automatic gearboxes.

The most popular automatic transmission is the one with epicyclic gear train, which is also very common in the US.
This type of transmission is able to achieve smooth gear shifts, but unfortunately, it has a lowyield as well as higher purchase and maintenance costs compare to other types of automatic transmission.

The epicyclic gear train differs from the others because it uses a planetary gear combination and a torque converter instead of gear couplings and clutch.

In this planetary gearset arrangement, we can notice that there are two inputs and one output. If we vary these two input speeds we can achieve a different output speed.

By adding another set, we can notice that the carrier (which is the output of the second set) is connected to the internal ring gear which is only one of the inputs of the first set.

This coupling forms a simple automatic twospeed transmission.
Two clutch packs are arranged on the internal ring gears.
When the clutch pack of the first set is pressed, it stops the movement of its gear ring (which is the first input) but not that of the sun gear (which is the second input).
In this way, the sun gear moves the planet gears and the carrier, and we will get the first gear.

By releasing the clutch pack of the first set and pressing the second set, the carrier (which is the output of the second set) will be able to rotate the ring gear of the first set giving a greater speed at the exit of the first set. And we will get the second gear.

More sets form the other gears, and if they were activated all together they form further ratios. So with just 3 sets we can get 6 gears and one reverse gear.

Shifting is almost instantaneous, this allows to decrease the loss of acceleration between one gear and another.
Instead of the clutch disc, the automatic transmission has a torque converter which automatically disconnects the engine from the gearbox when the vehicle is stationary and reconnects the two parts during acceleration.
It is composed of a turbine immersed in a fluid to gradually transmit the torque.
Unfortunately this device is not very efficient at low speed.

The dualclutch transmission is another type of automatic gearbox, it is common among sports cars as it is extremely fast and eliminates the loss of power between one gear and another.
The only disadvantage is its higher cost.
As for the manual gearbox, there are pairs of gears always meshed with each other, with the free gear of each pair equipped with a synchronizer.

Dualclutch transmission is therefore very similar to the manual one, with the difference that the gears are mounted on two different coaxial shafts: one for the even gears and one for the odd gears, connected to two different clutches.

When we ‘re driving in first gear, the second gear sleeve and synchronizer are already engaged. Then the first clutch is released, and at the same time the second clutch is...