How Does an Automatic Transmission Work? If you have ever driven an automatic car, you will certainly notice two distinct differences between an automatic vehicle and one with a manual transmission.
Differences Between Automatic and Manual Transmissions
In an automatic transmission, you will not find a clutch pedal or a gear shift lever (1, 2, 3, 4, etc.). You only need to move the gear selector to the D (drive) position, and everything else is automatic.
Both automatic transmissions (with a torque converter) and manual transmissions (with a dry friction clutch) serve similar functions, but they operate based on entirely different principles. Upon deeper exploration, we will find that automatic transmissions perform remarkably complex tasks.
In this article, we will delve into automatic transmissions. We will start with the key component of the entire system: the planetary gear set. Then we will look at how the various components of the transmission are assembled, how they function, and finally discuss some complex issues related to controlling automatic transmissions.
Similar to manual transmissions, the primary task of automatic transmissions is to accept engine power within a certain speed range while providing a broader speed range at the output.
The transmission utilizes gears to effectively leverage engine torque, allowing the engine to provide the wheels with the most suitable speed range based on load conditions and driver preferences.
The main difference between automatic transmissions and manual ones is that manual transmissions change the engagement of gears to create different gear ratios between the primary shaft (connected to the engine) and the secondary shaft (connected to the drive axle). In contrast, the automatic transmission uses the planetary gear set to perform all these complex tasks.
How the Planetary Gear Set Works
When you look inside an automatic transmission, you will see a structured arrangement of separate components within logical spaces. Among these components, you will find:
– A planetary gear set.
– A belt brake used to lock the components of the planetary gear set.
– A set of three wet clutches that work in oil to lock the components of the transmission.
– A hydraulic system to control the clutches and belt brake.
– A large gear pump to circulate transmission fluid within the transmission.
The most crucial component of the system is the planetary gear set. The first task is to create gears with different ratios and then facilitate their operation. An automatic transmission consists of basic planetary gear sets that are integrated into a single unit within the transmission.
Any basic planetary gear set consists of three main parts:
– Sun Gear (S)
– Planet Gears and Planet Carrier (C)
– Ring Gear (R)
Input | Output | Locked Stationary | Calculation Formula | Gear Ratio | |
| A | Sun (S) | Planet Carrier (C) | Ring (R) | 1 + R/S | 3.4:1 |
| B | Planet Carrier (C) | Ring (R) | Sun (S) | 1 / (1 + S/R) | 0.71:1 |
| C | Sun (S) | Ring (R) | Planet Carrier (C) | -R/S | -2.4:1 |
Locking two of the three components together will lock the entire mechanism into a single unit (gear ratio is 1:1). Note that the first listed gear ratio (A) is a reduction ratio – the speed of the secondary shaft (output) is less than the speed of the primary shaft (input). The second (B) is an increase ratio – the speed of the secondary shaft is greater than the speed of the primary shaft. Finally, there is also a reduction ratio, but the direction of rotation of the primary shaft is opposite that of the secondary shaft, which means it is a reverse gear. You can check them according to the following simulation diagram:
This basic gear set can achieve different gear ratios without engaging or disengaging with any other gears. With two interconnected basic gear sets, we can achieve 4 forward speeds and one reverse speed. We will discuss the two interconnected basic gear sets later.
This interconnected automatic transmission is also a planetary gear set, known as a dual planetary gear set, structured similarly to a single planetary gear set but composed of two combined planetary gear sets. It has one ring gear that is always connected to the secondary shaft of the transmission, but it features two sun gears and two sets of planet gears.
Take a look at the following image:
In the image below: the planet gears are placed on a carrier. Looking closer: the planet gear on the right is lower than the gear on the left. The right gear does not mesh with the ring gear but meshes with the adjacent planet gear. Only the left planet gear meshes with the ring gear.
Next, look inside the planet carrier. The shorter gears mesh with the smaller sun gear. The longer gears mesh with the larger sun gear while simultaneously engaging the smaller planet gears.
The simulation diagram below shows how the assemblies are arranged within a transmission:
Gear Ratio 1
In Gear Ratio 1, the smaller sun gear rotates clockwise along with the turbine of the torque converter. The planet gear tends to rotate in the opposite direction but is restrained by a one-way clutch (allowing only clockwise rotation), while the outer ring gear transmits motion to the secondary shaft. The sun gear has 30 teeth and the outer ring gear has 72 teeth, resulting in the following gear ratio:
i = – R/S = – 72/30 = – 2.4/1
Since the rotation is in the opposite direction at a ratio of 2.4:1, this means that the secondary shaft rotates in the opposite direction to the primary shaft. However, the direction of the secondary shaft aligns with that of the primary shaft. The first planetary gear set meshes with the second planetary gear set, causing the second planetary gear set to rotate the ring gear. This combination reverses the direction of motion. You might also notice that this will cause the larger sun gear to rotate, but since the clutch is disengaged, the larger sun gear rotates smoothly in the opposite direction of the turbine (counterclockwise).
Gear Ratio 2
This gearbox performs a truly intricate task to achieve the appropriate gear ratio for Gear Ratio 2. It operates like two planetary gear sets connected in series on a common planetary gear carrier.
The first stage of the planetary gear carrier utilizes the larger sun gear to replace the ring gear. Therefore, the first stage consists of the sun gear (the smaller sun gear), the planetary gear carrier, and the ring gear (the larger sun gear).
The primary shaft is the shaft of the smaller sun gear, and the ring gear (the larger sun gear) is held firmly by a brake band, while the secondary shaft is the planetary gear carrier. For this stage, with the sun gear as the primary shaft, the planetary gear carrier as the secondary shaft, and the ring gear fixed, we have the following formula:
1 + R/S = 1 + 36/30 = 2.2:1
The planetary gear carrier rotates 2.2 times when the smaller sun gear rotates once. In the second stage, the planetary gear carrier acts as the primary shaft for the second planetary gear set, the larger sun gear (fixed) acts as the sun gear, and the ring gear functions as the output, resulting in the gear ratio:
1 / (1 + S/R) = 1 / (1 + 36/72) = 0.67:1
To obtain the total reduction in Gear Ratio 2, we multiply the gear ratios of the two stages together. 2.2 x 0.67 = 1.41. This may seem counterintuitive, but indeed, it is the case.
Gear Ratio 3
Most automatic transmissions have a gear ratio of 1:1 in Gear Ratio 3. Remember that in the previous section, we noted that the 1:1 ratio occurs when 2 of the 3 components of the planetary gear are locked together. This is quite simple, and all we need to do is lock the sun gears with the turbine.
If both sun gears rotate at the same speed, the planet gears will be locked since they can only rotate in the opposite direction. This results in the ring gear being locked with the planet gears, causing everything to rotate as a unit and producing a gear ratio of 1:1.
Acceleration
As mentioned earlier, acceleration refers to an output speed greater than the input speed. Increasing speed is the opposite of the nature of a reduction gearbox. In this transmission mode, the system engages two planetary gear sets into a working unit. In the acceleration mode, the shaft is connected to the planetary gear carrier through a clutch. The smaller sun gear runs freely on the shaft, while the larger sun gear is held back by the acceleration brake band. They are not connected to the turbine but are directly linked to the torque converter casing. Looking back at our diagram, this time the planetary gear carrier will be the input, the fixed sun gear will be the output, and the ring gear will serve as the output. The gear ratio is:
i = 1 / (1 + S/R) = 1 / (1 + 36/72) = 0.67:1
Thus, the secondary shaft will rotate about one turn when the engine turns two-thirds of a turn. If the engine speed is 2000 RPM, the output speed of the gearbox is 3000 RPM. This allows the vehicle to operate at high speeds while keeping the engine speed at a moderate and low level.
Reverse Gear
The reverse gear is similar to Gear Ratio 1, except that the smaller sun gear rotating with the turbine of the torque converter is replaced by the larger sun gear driven by the turbine, while the smaller sun gear rotates freely in the opposite direction. The planetary gear carrier is held back by the reverse brake band. According to the equation in the previous section, we have:
i = – R/S = 72/36 = 2.0:1
Thus, the gear ratio of the reverse gear is slightly less than that of Gear Ratio 1.
Gear Ratios
This gearbox contains 4 forward gears and one reverse gear. Let’s take a look at the table below:
Gear | Input | Output | Fixed Part | Gear Ratio |
| 1 | 30-Tooth Sun Gear | 72-Tooth Ring Gear | Planet Gear Ratio | 2.4:1 |
| 2 | 30-Tooth Sun Gear | Planet Gear Ratio | 36-Tooth Ring Gear | 2.2:1 |
| Planet Gear Ratio | 72-Tooth Ring Gear | 36-Tooth Sun Gear | 0.67:1 | |
| Whole Number 2 | 1.47:1 | |||
| 3 | 30 and 36-Tooth Sun Gears | 72-Tooth Ring Gear | 1.0:1 | |
| Direct Transmission | Planet Gear Ratio | 72-Tooth Ring Gear | 30-Tooth Sun Gear | 0.67:1 |
| Reverse | 30-Tooth Sun Gear | 72-Tooth Ring Gear | Planet Gear Ratio | -2.0:1 |
After reading the above information, you will surely be amazed at how different inputs are engaged and disengaged. This is accomplished by a series of disc clutches and band brakes inside the transmission. In the next section, you will learn how they work.
Clutches and Band Brakes
For automatic transmissions, when accelerating, the shaft (connected to the torque converter and flywheel) is connected to the planetary gear set via the clutch. The smaller sun gears run smoothly on the shaft, while the larger sun gears are held back by the band brake. Nothing connects to the turbine, only the primary shaft in the torque converter.
To increase the speed of the transmission, many components need to be engaged and disengaged thanks to clutches and bands. The planetary gear set connects to the torque converter via a clutch. The smaller sun gears disengage from the turbine so they can spin freely. The larger sun gears are held back by a band brake, preventing them from rotating. Each gear shaft causes a series of such issues through the engagement and disengagement of clutches and band brakes. Let’s take a look at a band brake within the transmission.
This transmission has two bands. The band brakes in the transmission are made of steel and wrap around most of the outer ring of the planetary drive, holding them tightly against the transmission case. They operate thanks to hydraulic cylinders within the transmission.
In the image above, you can see one of the band brakes inside the transmission casing. When the gear is removed, the metal lever connected to the piston activates the band brake.
In the image above, you can see two pistons that can operate the bands. Hydraulic pressure causes the pistons to push the bands, locking the gear assemblies with the transmission case.
The clutches in the transmission are a complex assembly. This transmission features four friction disc clutches. Each friction disc clutch is operated by pistons within the clutch using hydraulic pressure. Springs help the clutch discs separate when the hydraulic pressure decreases. In the image below, you will see the piston and the clutch drum. Note the rubber seal around the piston, which is one of the components that need to be replaced when servicing the transmission.
The next image shows how the friction discs and steel discs are arranged. The friction discs inside are connected to one of the gears while the steel discs outside are locked with the transmission casing. The friction discs need to be replaced when servicing the transmission.
The Complexity of Automatic Transmissions
Your car’s automatic transmission needs to perform a multitude of complex tasks. It is difficult to know how many different operations occur while it is working. However, the main characteristics of its operation can be described through the following examples:
– If the car is in acceleration mode (with only a 4-speed transmission), the transmission will automatically select the gear engagement based on the vehicle speed and throttle position.
– If you gently press the accelerator, the shifting will occur more gradually than if you press the gas pedal firmly.
– If you lift your foot off the gas pedal, the transmission will automatically shift to lower gears.
– If you shift the gear lever to a lower gear, the transmission will shift to that lower gear unless the vehicle is moving too fast for that gear. If the vehicle’s speed is too high, it will wait for the speed to decrease appropriately before shifting down.
– If you set the gear selector to 2, it will only shift down or up to second gear, even if the vehicle comes to a complete stop, unless you move the gear lever.
Have you ever seen anything like this? It is truly the brain of the automatic transmission, controlling all functions and more. The hydraulic oil channels lead to many different positions within the transmission. The channels cast into the metal structure are the most efficient oil passages; without them, a multitude of pipes could connect the components within the automatic transmission. First, let’s discuss the main components of the hydraulic system, and then we will see how they work together.
The automatic transmission has a hydraulic pump, typically a gear pump. This pump is mounted directly on the transmission casing. It draws oil from the reservoir at the bottom of the transmission and pushes oil into the hydraulic system. It also supplies oil for the cooling system of the transmission and the torque converter.
The internal gear of the pump connects with the casing of the torque converter, allowing it to rotate at the speed of the engine. The external gear is driven by the internal gear through its teeth, with hydraulic oil being drawn up from the reservoir on one side of the crescent-shaped space (as shown above) and pushing the oil to the other side, which then rises to the hydraulic system.
The oil system is managed by an intelligent controller that can inform the transmission about how fast the vehicle is moving. It is connected to the output of the transmission, so the faster the vehicle moves, the quicker the controller spins. Inside the controller is a spring-loaded valve that can open depending on the rotation speed of the controller; the faster the controller spins, the larger the valve opens. The oil line from the pump to the controller runs through the transmission’s secondary shaft.
The faster the vehicle accelerates, the more the controller’s valve opens, increasing the hydraulic system’s pressure…
To shift gears appropriately, an automatic transmission must know the engine’s load level. There are two ways to determine this. Some vehicles have a cable connecting the engine’s throttle to the throttle valve in the transmission. The deeper you press the accelerator pedal, the greater the pressure applied to the throttle valve. Other vehicles use a vacuum valve to supply pressure to the throttle valve.
The position control valve for gear selection indicates the current gear position. Depending on the gear being used, the valve will supply oil through the appropriate line. For example, if the gear lever is in position 3, the valve will create a circulation to prevent shifting to another gear.
The shifting valves provide hydraulic pressure to the clutch and the band brakes to engage the gears. The hydraulic valve body of the transmission contains several shifting valves. These valves determine when to switch from one gear to another. They are supplied with oil from the pump and direct the oil flow to one of two circuits to control the necessary gear.
The shifting valves will prevent gear changes when the vehicle accelerates too quickly. If the vehicle accelerates smoothly, the gear change will occur at a lower speed. Let’s see what happens when the vehicle accelerates gently.
As speed increases, the pressure from the controller rises gradually. This force forces the shifting valve to close the first gear circuit and open the second gear circuit.
When the accelerator is pressed suddenly, the throttle valve provides greater pressure to the shifting valve. This means that the pressure from the controller must be higher (and the vehicle shifts gears faster), causing the shifting valve to move to engage the second gear.
Each shifting valve handles a different range of pressure, so as the vehicle moves faster, the shift from gear 2 to gear 3 will occur because the pressure from the controller is sufficiently high to activate this valve.
Electronically controlled transmissions that appear in some newer vehicles still use hydraulic systems to control the clutch discs and band brakes. However, each hydraulic circuit is managed by an electromagnetic coil. This makes the transmission more flexible while allowing better coordination of the control systems.
In this final section, we explore some additional automatic control methods used by this transmission. Electronic-controlled transmissions further enhance control features. The enhancements include monitoring engine speed and throttle position, meaning the transmission controller also regulates engine speed and oversees the ABS system, switching to ABS mode if the brake pedal is pressed.
Overview
Utilizing the above insights and advanced control systems based on fuzzy logic – a method of control using artificial intelligence, electronic control systems can also perform the following tasks:
– Automatically downshift when the vehicle goes downhill to self-regulate speed, utilizing engine braking and minimizing brake wear.
– Upshift when braking and when wheel slip occurs to reduce brake torque using the engine.
– Prevent upshifting when the vehicle is driving on winding and uneven roads.
What about the following situations: restricting upshifts when you’re driving on winding and uneven roads? When driving uphill on a winding road. If you encounter straight sections, and the gear is in position 2, how much do you need to press the accelerator? If you reach a turn and downhill, you release your foot from the accelerator and apply the brake. Most transmissions will upshift to gear 3 or overdrive when you lift your foot off the accelerator. Then, when you accelerate out of the turn, they will downshift. However, if you are driving a vehicle with a conventional manual transmission, you will certainly maintain the same gear throughout the journey. Some automatic transmissions with advanced control systems can recognize when you are continuously making sharp turns and will not execute continuous upshifts and downshifts.
Quang Hùng (translator)
