The working principle of an automatic transmission

How does an automatic transmission work? If you have ever driven an automatic car, you will surely notice two significant differences between an automatic vehicle and a manual transmission vehicle.

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.). The only operation you need to perform is to shift the gear selector into position D (drive), and then everything else is automatic.

Both automatic transmissions (with torque converters) and manual transmissions (with dry friction clutches) serve the same function, but their operational principles are entirely different. Upon deeper investigation, we will find that automatic transmissions perform remarkably complex tasks.

In this article, we will explore automatic transmissions. We will start with the key component of the entire system: the planetary gear set. Then we’ll look at how the components of the transmission are assembled, how they operate, and finally discuss some complex issues related to the control of automatic transmissions.

Similar to manual transmissions, the primary duty of an automatic transmission is to receive engine power over a certain speed range while providing a broader output speed range.

The transmission uses gears to effectively exploit the torque of the engine and help the engine deliver the most suitable wheel speed according to load conditions and the driver’s preferences.

The main difference between automatic and manual transmissions is that the manual transmission changes the engagement of gears to create different gear ratios between the primary shaft (connected to the engine) and the secondary shaft (linked to the drive axles). In contrast, in an automatic transmission, the planetary gear set performs all these complex tasks.

Working Principle of the Planetary Gear Set

When looking inside an automatic transmission, you will see distinct components arranged in separate logical spaces. Among these, you will find:

– A planetary gear set.

– A band brake used to lock parts of the planetary gear set.

– A set of three wet clutches that operate in oil to lock parts of the transmission.

– A hydraulic system to control the clutches and band brake.

– A large gear pump to circulate transmission fluid within the gearbox.

The most crucial component of the system is the planetary gear set. The first task is to manufacture them with different gear ratios and then to help them operate effectively. An automatic transmission consists of basic planetary gear sets combined into a single unit within the transmission.

Any basic planetary gear set has three main components:

– Sun gear (S)

– Planet gears and planet carrier (C)

– Ring gear (R)

When two of the three components are locked together, the entire mechanism is locked as a single unit (with a gear ratio of 1:1). Note that the first gear ratio listed above (A) is a reduction ratio – the speed of the secondary shaft (output) is lower than that of the primary shaft (input). The second (B) is an increase ratio – the speed of the secondary shaft is greater than that of the primary shaft. Lastly, we have another reduction ratio, but the direction of the primary shaft is opposite to that of the secondary shaft, which means reverse gear. You can check them according to the following simulation diagram:

This basic planetary gear set can perform different gear ratios without engaging or disengaging with any other gears. With two connected basic planetary gear sets, we can achieve 4 forward speeds and one reverse speed. We will discuss the two connected planetary gear sets later.

This connected automatic transmission is also a planetary gear set, called a dual planetary gear set, structured similarly to a single planetary gear set but consists of two combined planetary gear sets. It has an outer ring gear that is always connected to the secondary shaft of the transmission but features two sun gears and two sets of planet gears.

Let’s take a look at the following image:

In the image below: the planet gears are mounted on a carrier. Upon closer examination, the right planet gear is positioned lower than the left. The right gear does not mesh with the outer ring gear but meshes with the adjacent planet gear. Only the left planet gear engages with the ring gear.

Next, look inside the planet gear carrier. The shorter gears mesh with the smaller sun gear. The longer gears engage with the larger sun gear while simultaneously meshing with the smaller planet gears.

The simulation diagram below shows how the assemblies are fitted together in a transmission:

Number 1

In number 1, the smaller sun gear rotates clockwise in conjunction with the turbine of the torque converter. The planetary gear tends to rotate counterclockwise but is held in place by a one-way clutch (allowing only clockwise rotation), transmitting motion to the secondary shaft through the ring gear. The sun gear has 30 teeth, and the 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 is the same as that of the primary shaft. The first planetary gear set meshes with the second gear set, and the second planetary gear set rotates the ring gear. This combination reverses the direction of motion. You can see that this will also cause the larger sun gear to rotate, but since the clutch is disengaging, the larger sun gear rotates freely in the opposite direction of the turbine (counterclockwise).

Number 2

This gearbox performs intricate operations to achieve the appropriate gear ratio for number 2. They function as two planetary gear sets connected in series on a common planetary gear carrier.

The first stage of the planetary gear carrier uses a larger sun gear instead of the ring gear. Thus, 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 tight 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 for every one rotation of the smaller sun gear. In the second stage, the planetary gear carrier acts as the primary shaft for the second planetary gear set, the fixed larger sun gear acts as the sun gear, and the ring gear serves as the output, resulting in the following gear ratio:

1 / (1 + S/R) = 1 / (1 + 36/72) = 0.67:1

To obtain the total reduction in the second gear, we multiply the gear ratios of the two stages together: 2.2 x 0.67 = 1.41. This may seem illogical, but that is the reality.

Number 3

Most automatic transmissions exhibit a 1:1 ratio in gear 3. Remember that in the previous section, we learned that the 1:1 ratio is achieved by locking 2 of the 3 planetary gear components. This is quite straightforward, and all we need to do is lock the sun gears with the turbine.

If both sun gears rotate at the same speed, the planetary gears become locked as they can only rotate in the opposite direction. This leads to the ring gear being locked with the planetary gears, and they all rotate as a unit, creating a 1:1 gear ratio.

Increasing Speed
As mentioned earlier, increasing speed means the output speed is greater than the input speed. Speed increase is the opposite of the function of a reduction gearbox. In this drive 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 the clutch. The smaller sun gear runs freely on the shaft, while the larger sun gear is retained by the acceleration brake band. They are not connected to the turbine but directly to the torque converter casing. Looking back at our diagram, this time the planetary gear carrier will be the input, the fixed sun gear remains, and the ring gear is the output. The gear ratio is:

i = 1 / (1 + S/R) = 1 / (1 + 36/72) = 0.67:1

Thus, the secondary shaft will rotate approximately one time when the engine rotates two-thirds of a turn. If the engine speed is 2000 RPM, then the output speed of the gearbox is 3000 RPM. This allows the car to run at high speeds while the engine speed remains moderate and low.

Reverse Gear
Reverse gear is similar to number 1, except that the smaller sun gear, rotating with the torque converter’s turbine, 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 from the previous section, we have:

i = – R/S = 72/36 = 2.0:1

Thus, the gear ratio of reverse gear is slightly lower than that of number 1.

Gear Ratios

This transmission has 4 forward gears and one reverse gear. Let’s take a look at the table below:

After reading the information above, you might be surprised at how different inputs are engaged and disengaged. This is accomplished through a series of clutches and bands inside the transmission. In the next section, you will learn how they work.

Clutches and Bands

For automatic transmissions, when in acceleration mode, the shaft (connected to the torque converter and flywheel) is linked to the planetary gear set through a clutch. The smaller sun gears run smoothly on the shaft, while the larger sun gears are held back by the acceleration band. Nothing is connected to the turbine; only the primary shaft is in the torque converter.

To increase the transmission speed, many components need to be engaged and disengaged through clutches and bands. The planetary gear set connects to the torque converter via a clutch. The small sun gears release from the turbine so they can rotate freely. The larger sun gear is held back by a band, preventing it from rotating. Each gear shaft causes a series of such issues through clutch engagement and band application. Let’s take a look at a band within the transmission.

In the image above, you can see one of the bands inside the transmission casing. When the gears are removed, the metal lever is connected to the piston that activates the band.

In the image above, you can see two pistons that can drive the bands. Hydraulic pressure causes the pistons to push the bands, locking the gear assemblies with the transmission casing.

The clutches in the transmission are a complex assembly. This transmission has four friction disc clutches. Each friction disc clutch is driven by pistons inside the clutch via hydraulic pressure. Springs help the clutch discs separate when the hydraulic pressure decreases. Look at the image below to see the piston and drum of the clutch. Note the rubber seal of the piston, which is one of the components that needs to be replaced when servicing the transmission.

The next image shows the arrangement of friction discs and steel discs in order. The inner friction discs connect to one of the gears, while the outer steel discs are locked with the casing of the transmission. The friction discs need to be replaced when servicing the transmission.

The Complexity of Automatic Transmissions
Automatic transmissions in your vehicle must perform a myriad of complex tasks. It is difficult to know how many different actions occur while it operates. However, the key functions while it is working can be described through the following examples:

– If the car is in acceleration mode (only with a 4-speed transmission), the transmission automatically selects the gear engagement based on the vehicle speed and throttle position.

– If you gradually increase the throttle, the shifting will occur more gradually than if you press the accelerator hard.

– If you lift your foot off the throttle, the transmission will automatically shift to lower gears.

– If you shift the gear lever to a lower gear, the transmission will downshift unless the vehicle is moving too fast for that gear. If the vehicle speed is too high, it will wait for the speed to slow down before shifting to a lower gear.

– If you place the gear selector in 2, it will only shift up or down to 2, even if the vehicle comes to a complete stop, unless you move the gear lever.

Have you ever seen something like this? It is truly the brain of the automatic transmission, controlling all functions and more. The hydraulic oil passages lead to many different locations within the transmission. The channels cast into the metallic structure are the most efficient oil pathways; without them, how many pipes could connect the components in an 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, usually a gear-type pump. This pump is mounted directly on the transmission casing. It draws oil from a reservoir at the bottom of the transmission and pushes the 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 is connected to the casing of the torque converter, allowing it to rotate at the speed of the engine. The external gear is rotated by the internal gear through its teeth, drawing hydraulic oil from the reservoir to this side of the crescent-shaped space (as shown above), and pushing the oil to the other side into the hydraulic system.

The operation of the oil system is managed by an intelligent regulator that can inform the transmission whether the vehicle is moving fast or slow. It is connected to the output of the transmission, so the faster the vehicle moves, the faster the regulator rotates. Inside the regulator is a spring-loaded valve that can open depending on the speed of rotation; the faster the regulator turns, the larger the opening of the valve. The oil line from the pump to the regulator runs through the secondary shaft of the transmission.

The faster the vehicle runs, the wider the valve of the regulator opens, and the greater the pressure in the hydraulic system…

To shift gears appropriately, the automatic transmission must know the load level at which the engine is operating. 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 which gear is used, the valve will supply the appropriate oil line. For example, if the gear lever is in position 3, the valve will provide a circulation to prevent engaging other gears.

The shift valves supply hydraulic pressure to the clutch and band brakes to engage the gears. The hydraulic valve body of the transmission contains several shift valves. The shift valves determine when to shift from one gear to another. They are supplied with oil from the pump and push the oil flow to one of two circuits to control the gear required.

The shift valves will prevent shifting when the vehicle accelerates too quickly. If the vehicle accelerates gently, the shift will occur at a lower speed. Let’s see what happens when the vehicle accelerates gently.

As speed increases, the pressure from the regulator gradually rises. This pressure forces the shift valve to close the first gear circuit and open the second gear circuit.

When accelerating suddenly, the throttle valve provides higher pressure to the shift valve. This means that the pressure from the regulator needs to be higher (and the vehicle shifts faster) so the shift valve moves to engage the second gear.

Each shift valve handles a different pressure range, so as the vehicle moves faster, the shift from gear 2 to gear 3 will continue to operate, as the pressure from the regulator is sufficiently high to activate this valve.

Electronically controlled transmissions appear in some newer vehicles but still use hydraulic systems to control the clutch discs and band brakes. However, each hydraulic circuit is controlled by an electromagnetic coil. This makes the transmission more flexible while allowing better coordination of the control systems.

In this final section, we will explore some additional automatic control methods used by this transmission. Electronically controlled transmissions also incorporate more advanced control components. The additional features include controlling engine speed and throttle position, meaning the transmission controller also manages engine speed and monitors the ABS system; if the brake pedal is pressed, it switches to ABS mode.

Overview

Using the insights above and advanced control systems based on fuzzy logic—an artificial intelligence control method—electronic control systems can also perform the following tasks:

– Automatically downshift when the vehicle descends to self-regulate speed, taking advantage of 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 on winding and uneven roads.

What can be said about the following conditions: limiting upshifting when you are driving on winding and uneven roads? When you drive uphill through winding mountain roads. If you encounter straight sections, and you are in gear 2, how much do you need to accelerate? If you reach a curve and go downhill, you take your foot off the accelerator and press the brake. Most transmissions will upshift to gear 3 or the overdrive gear when you lift your foot off the accelerator. Then, when you accelerate out of the turn, they downshift again. But if you are driving a vehicle with a conventional manual transmission, you will definitely keep the vehicle in one gear throughout the journey. Some automatic transmissions with advanced control systems can recognize when you are continuously making sharp turns, and they will not continuously shift up and down.

Quang Hùng (translator)