What is an automatic transmission and how does it work? (2023)

Regarding the automatic transmission:

An automatic transmission is a type of transmissiontransmissionwhich can automatically change gears while the vehicle is in motion. The manufacturers also just call it Auto or AT, the automatic manual transmission. It is also known as an "n-speed" automatic, where "n" is the number of forward gears. It is a type of automotive transmission that frees the driver from shifting gears manually or independently. There is no clutch pedal on vehicles with an automatic transmission. The driver can only control the car using the accelerator and brake.

How does an automatic transmission work?

The most popular type of automatic transmission in automobiles is the hydraulic type. However, commercial vehicles and heavy industrial vehicles use similar but larger equipment. The automatic transmission uses a fluid clutch instead of a friction clutch. It drives gear changes by hydraulically locking and unlocking the planetary gears.

What is an automatic transmission and how does it work? (1)

Therefore, an automatic transmission has a defined set of gears. It usually comes with a parking brake that locks the transmission output shaft. This prevents the vehicle from moving forward or backward when parked. Some vehicles have limited speed ranges or stable engine speeds, such as forklifts and lawnmowers. These vehicles use only a torque converter to deliver variable engine speed to the wheels.

Hydramatic Transmission:

It's a kind of fully automatic transmission. Hydra-Matic drive combines four-speed automatic transmission forward and reverse. Also, it uses a fluid flywheel such as a fluid coupling or torque converter for this purpose. The gear drive consists of three spur planetary gear sets connected in series with constant meshing. Two sets of planetary gears provide four gear ratios forward while the other set is used for reverse. The fluid flywheel dampens the effects of automatic gear changes and reduces engine torque reactions.

What is an automatic transmission and how does it work? (2)

Components of an automatic transmission system:

The main type of automatic transmission works hydraulically. It uses a fluid coupling or torque converter and planetary gear set to provide a variety of gear ratios. Hydraulic automatic transmissions consist of three main components. They are - torque converters, planetary gears and hydraulic controls.

Torque converter:

The torque converter is a fluid coupling type. Hydraulically connects/disconnects the engine to the transmission. Also, the torque converter replaces the friction clutch in a manual transmission. It connects and disconnects engine power with planetary gears. Thus, the vehicle can stop without the engine stalling. In addition, it offers variable torque multiplication at low engine speeds. Thus, it increases the runaway acceleration. A fluid coupling works well when the impeller and turbine rotate at the same speed. However, it is very inefficient at initial acceleration where rotation rates are quite different.

The torque converter is constructed in a similar way to a hydraulic clutch. It increases the torque on the transmission input shaft at low engine speeds. It drops to the engine torque value at a given engine speed as it idles. The torque converter consists of an engine drive impeller and a turbine, which is converted into the transmission input shaft. It also has a small impeller called the stator between the impeller and the turbine.

How does the torque converter work?

Thus, it maximizes torque multiplication with the stator modifying the fluid flow depending on the impeller and the relative speeds of the turbine. The stator itself does not rotate. However, manufacturers shape their vanes so that the turbine rotates at low speed when the impeller rotates at high speed. The fluid flow hits the turbine blades to multiply the applied torque. This causes the turbine to spin faster when the vehicle accelerates. However, the torque multiplication decreases as the relative speeds become equal. Once the impeller and turbine are spinning within 10% of each other's speed, the stator stops working and the torque converter acts as a simple fluid coupling.

planetary gear

The planetary gear set consists of planetary gear sets. It also has clutches and bands that offer different gear ratios. They change the speed of the output shaft depending on how the driver engages the planetary gear. The transmission uses one of two types of clutches or bands to effect gear changes. Also, it keeps a specific element of the planetary gear set stationary while allowing another element to rotate. This is how it transmits torque and offers underdrive or overdrive ratios. A valve body actuates these clutches while the automatic transmission's production program controls their sequence.

Primarily, the automatic transmission uses a roller clutch for routine up/downshifts. It works like a ratchet. It only transmits torque in one direction while coasting or coasting in the other. The advantage of this type of clutch is that it eliminates the possibility of applying clutch disengagement to two planetary gears at the same time. It absorbs the load on the drive train when the clutch is engaged and releases automatically when the next gear clutch takes over the torque transmission.

The bands become active for manually selected gears such as low gear or reverse and act on the circumference of the planetary drum. Bands are not applied when Gearbox/Gearbox/Overdrive range is selected. Instead, clutches transmit torque. However, some cases use bands for braking.

Coupling system:

Each unit includes a multi-plate clutch to lock the gears in the direct drive. The front unit consists of two steel plates or discs keyed into the sun gear drum to form a direct drive unit. It also has three or more composition discs spaced alternately between the steel discs. These are inserted into the hub of the planetary cage and form the driven part of the clutch. The rear clutch is constructed in a similar way. Except that it includes more clutch discs. In addition, steel plates or discs are attached to the internal gear drum. Splined steel and composite discs on intermediate shaft hub. Just like the drum, the clutch discs are also circular.

What is an automatic transmission and how does it work? (3)

When you engage either clutch, an annular piston forces the two sets of clutch discs into contact. This causes them to rotate together as a single locked unit. The oil pressure provided by the controller actuates the piston and acts as a pressure plate in a mechanical clutch. Depressing the clutch locks the sun gear and planetary cage to the front unit. However, the clutch locks the internal gear drum on the intermediate shaft hub on the rear unit. Two passages through the gearbox supply the clutches with hydraulic pressures to actuate the annular pistons. They lead from the valve control to the oil delivery sleeve.

Manufacturers of automatic transmission systems:

BorgWarner,Cummins, zZFare among the world's leading manufacturers of automatic transmissions.

Common automatic transmission modes:

P – P stands for parking. This selection allows you to mechanically lock the transmission output shaft. Thus, it restricts vehicle movement in any direction. However, the non-driven wheels of the vehicle that are still free can rotate. The driven wheels can also rotate individually due to theDifferentialAction. Therefore, you must always use your handBrake(parking brake) as it locks the wheels and prevents them from moving.

R – R stands for reverse. It shifts the automatic transmission into reverse and allows the vehicle to roll backwards. To select reverse on most transmissions, you must come to a complete stop, press the lock button, and select reverse.

N – N means neutral or no gear (N). It separates all gear trains within the transmission. It effectively separates the transmission from the drive wheels, allowing the vehicle to spin freely under its own weight and gain momentum.

What is an automatic transmission and how does it work? (4)

Go on:

D – D stands for driving mode. This position allows the automatic transmission to use the full range of available forward gear ratios. It allows the vehicle to move forward and accelerate through its gear range.

[D] - The [D] stands for Overdrive. Some transmissions use this mode to activate automatic modeOverdrive. In these transmissions, Drive (D) blocks the automatic overdrive. As a result, OD (Overdrive) intervenes at around 35-45 mph (56-72 km/h) at constant speeds or at low throttle. However, under heavy acceleration or below 56-72 km/h (35-45 mph), the transmission will automatically downshift.

L/1 - L means low gear. This mode blocks the automatic transmission only in first gear. In older vehicles, it does not shift to a different gear range. Some vehicles will automatically shift out of first gear in this mode to prevent engine damage when a certain RPM range is reached.

Depending on the manufacturer and model, there are also other modes. These include:

D5 – Cars with a five-speed automatic transmission usually use this mode for on-road driving. It uses all five forward gears.

D4 - Cars with a four or five speed automatic transmission use only the first four gears. It is mainly used for stop-and-go traffic such as city driving.

D3 or 3 - Cars with a four-speed automatic transmission use only the first three gears. However, people mainly use it for stop-and-go traffic like city driving.

D2 and D1 - Older Ford cars used these modes. D1 uses all three gears, while in D2 the car starts in second gear and shifts to third.

S or sports

S usually means sport mode. It works the same as “D” mode. However, upshifts occur at a much higher engine speed. Maximizes engine performance and improves vehicle performance, especially when accelerating. This mode also downshifts at a much higher rpm than “D” mode, maximizing engine braking. However, this mode results in lower fuel economy.

Some modern vehicles also offer a few more modes, such as:

+ −, e M

M stands for manual gear mode selection on certain automatic transmission vehicles. However, some cars do not have an 'M.' Instead of this; they have '+' and '-'. This sets it apart from the other shift modes. As with a semi-automatic transmission, the driver can upshift and downshift at will by flipping the shift lever. You can engage this mode via a position selector, or actually shift by tilting the steering wheel paddles.

Winter (F)

Some models also offer a winter mode. When leaving a parking position, it shifts into second gear instead of first. Reduces traction loss due to wheel slip on snow or ice.

Brake (B)

B represents the braking mode that some models, including electric cars, offer. You can use it to slow down or maintain speed downhill without using the vehicle's brakes. In non-hybrid vehicles, B mode selects a lower gear to increase engine braking. In electric mode, mode B increases the level ofregenerative brakingwhen releasing the accelerator pedal.

See the automatic transmission in action here:

Leia:What driving modes are there? >>

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