What Is Automatic Transmission and How Does it Work?

A “transmission” is simply the name given to the device that joins the engine to the wheels. It’s designed for cars and changes the size of the gear according to the power output by the engine. In simple terms, it converts the power of the engine to the wheels.

A transmission consists of gears that are used to transfer the spin rotation motion of one shaft to another shaft. Transmissions can be either automatic transmissions, or manual transmissions.

What Is An Automatic Transmission?

An “automatic transmission” is one that doesn’t require the driver to switch the gears manually. Automatic car transmissions differ from conventional manual transmissions. The cars both have manual transmissions and automatic transmissions, although there are a few key aspects that are distinct between automatic and manual transmissions.

Automatic transmissions use hydraulic fluid to engage the proper gear and maintain the proper RPM. Automatic transmissions are used mostly in modern vehicles.

Manual transmissions are still widely used, as they allow better fuel economy and better control, but most people will choose an automatic over a manual purely for convenience.

Automatic Transmission Diagram

Parts of an Automatic Transmission

There are many parts that comprise an automatic transmission. Each automatic transmission has these eight parts, and they are all fundamental to the proper operation of your transmission.

But you have to know more than just the names of each part to understand how they and the other parts move your vehicle.

#1. The torque converter.

Automatic transmission vehicles do not have a clutch, they have a torque converter. The torque converter does the same job as a clutch.

You cannot shift gears if your torque converter is still engaged with the engine – if you did, you would snap the gears! This is why, if you suspect that you have a bad torque converter, you must act on it NOW!

A torque converter disconnects the engine so the engine can continue to run while the car is in gear, like when you pull in a manual transmission some gear and depress the clutch to disconnect it from the engine to shift.

The craziest thing about torque converters is that they do not connect an engine to a transmission with a physical connection, like with nuts, bolts, or gears. They connect and disconnect through transmission fluid.

The internal workings of the torque converter can be a bit complex, but it is much like how you can power a fan by blowing air from another fan. When you disconnect the first fan, you can stop the second without destroying anything.

#2. Oil Pump.

The oil pump in an automatic transmission is just like any other oil pump. It takes the transmission fluid from the transmission pan, then pumps it to the valve body, and the valve body directs the oil to all the other components as necessary.

The transmission pump is not a very complicated component. It is an important part of the system, and it is one of the few components in a transmission that sometimes fail.

#3. Planetary Gear Set.

Unlike a manual transmission that has a set of multiple gears that the gear stick is inserted into the gearbox, an automatic transmission typically has one or more planetary gear sets. 

Planetary gears have three levels of gears. The main gear is the sun gear, and this is in the center of the gear carrier. The second level is the planetary gears. Typically, there are three of these gears, and they mesh with the sun gear and the ring gear. 

The ring gear surrounds all of the planetary gears with one ring. This ring connects the entire system and brings it together. 

The whole system works by locking down two of the three gear sets at a time. And since each gear set is a different size wrt the gear ratios, this allows for a whole range of gear ratios by merely locking the other gears in place. 

It is an incredibly efficient system, and allows for the transmission to have several different gear options over the same setup and without the setup of a manual transmission.

#4. Clutch Packs.

While a manual transmission has a shifter and you select the gear you want to engage, an automatic transmission is a little more complicated. Instead of using a manual shifter to get your output to the desired ratio with an automatic, your transmission has a clutch pack.

A clutch pack has multiple plates compressed together. When the valve body sends oil pressure to the clutch pack, it locks a certain number of clutch packs together in order to give you your desired output ratio.

Depending on the amount of oil pressure, it will engage different quantities of discs, which will give your hub a different gear ratio.

#5. Output Shaft.

After your transmission has completed all its internal magic, the transmission then transfers power to the driveshaft, which transfers power to the hubs. But it all begins with the output shaft of your transmission, which is what drives the drive shaft.

Generally, the output shaft isn’t a very complicated component. One end is a gear, which connects to the transmission, and the other end is a splined shaft that connects uniquely to the driveshaft via a yoke-type universal joint.

6. Brake Band.

The only intended use for brake bands is to hold planetary gears temporarily. It allows the engine RPMs to line up with the gear ratio before the band is released, allowing for smoother shifts.

Brake bands function much like the drum brakes you have seen on your vehicle, with a piston pushing the band in a circle around the drum. The more the piston pushes, the more brake force is applied.

#7. Oil Pan.

Just like your engine has an oil pan, your transmission has one as well. It does exactly what you think it would do.

It holds the excess transmission fluid (oil) for the oil pump to pull from when needed, and it typically has a transmission fluid filter attached to it, which can be either internal or external.

It is probably the simplest component in your transmission, and is one of the components that is most likely to need repairs.

The gasket that sits between the transmission pan and transmission housing will wear out over time, and if you suspect any leaking, that gasket will need to be replaced.

#8. Valve Body.

You have an engine control unit (ECU) in your engine, and your automatic transmission has a valve body. The valve body is the component that directs all of the pressure from the oil pump to all of your transmission components to make the desired effect happen.

From the torque converter to the clutch packs, no component gets transmission fluid without the valve body telling them if or when they should get the fluid.

In an automatic transmission the valve body is controlled by three sensors, who ultimately works in conjunction with the transmission control module (TCM) to control the sending of the fluids. The valve body is a complex component of your transmission, but it is the most important component.

How does an Automatic Transmission Work?

Just because you have a simple idea of all the parts of an automatic transmission doesn’t mean you know how it works start to finish, and that is perfectly normal.

Transmissions are complicated technical parts, and unless someone has taken you through how they work, feeling or understanding the cohesive nature of it all can be difficult. From the minute you place your car in drive the transmission starts working.

First, the valve body receives a signal from the TCM that the car wants to move, and that it wants 1st gear. The valve body orders the correct amount of transmission fluid to the torque converter and clutch packs, and wakes everything up.

To do that, however, the oil pump needs to activate, moving oil from the transmission oil pan to the valve body, which disperses it to various needed components.

Then, the planetary gear set locks in the ring gear, 1st gear, and allows power to reach the output shaft and helps it move.

At this point in the function, the brake band has not been deployed; you won’t use it until you pass 1st gear, and it will try and shift smoothly.

When the transmission is going to shift gears, the valve body will divert fluid away from the torque converter so that the transmission can engage the correct planetary gear without a lot of force snapping things.

Your brake band will engage, if it has to, once the torque converter re-engages and allows everything to accelerate back up to speed. It works the same way in the other shift direction, the valve body sends a different signal when you are going down a gear.

Automatic Transmission Gears

Automatic transmission gears include:

When you shift your vehicle into Drive mode, engage the total number of drive gears. This allows the transmission to shift through it’s total gear range if needed.

Most transmissions have 6 gears, but others may still be four or five-speed automatic transmissions which can still be found in older vehicles and entry-level compact vehicles.

Third gear blocks the transmission in third gear and also limits the transmission to first second and third gear ratios. It uses power and traction; therefore, you could go uphill or downhill or could have the weight of a boat, RV or serial keeping torque while moving.

When the engine reaches specified revolutions per minute (RPM), in most cases the vehicle will lock into third gear automatically in order to prevent damage to the engine.

Second gear blocks the transmission in second gear and limits the transmission to either second gear ratios or first and second gear ratios. It can be used for uphill or downhill riding under less than ideal conditions like ice, snow, or unfavorable weather.

First gear is used when you want to lock the transmission into first gear. In some vehicles, the vehicle automatically turns this gear off to protect the engine for speeds over a specified limit. Like second and third gears, first gear will just cost to use if you’re towing, going downhill or uphill, and going downhill or uphill with slick conditions.

What are the types of automatic transmission?

Some of the most commonly used gearboxes in this category include:

• Automated Manual Transmission (AMT),
• Continuously Variable Transmission (CVT),
• Dual-Clutch Transmission (DCT) and
• Intelligent Manual Transmission (iMT)

#1. Automated Manual Transmission – AMT.

Automated Manual Transmission or AMT is an automatic transmission that relies on an actuator and an onboard computer to decide which gear to shift at which engine speed. With AMTs there is a selector stick which you will need to use to select the drive mode.

Once engaged, just sit back, relax, and drive, because the AMT will push the clutch and shift the gears for you. The AMT is quite smooth; however, you can expect a small jerk whenever it shifts automatically on upshifts and downshifts.

The AMT is a fairly affordable transmission system and it is available on different models like Tata Nexon, Maruti Suzuki Wagon R, Hyundai Grand i10 Nios to name just a few.

Pros

• Comfort in Driving – Changing the gear is automatic, without your input.
• Efficiency – Because the onboard computer can accurately predict the exact time to shift gears as per the engine speed, you are able to get good economy.
• Manual Mode – Some AMTs will also have a manual mode so that when desired you will always have the option of manual if you find that driving is becoming a bit easy. Unlike a manual transmission, you simply need to nudge the gear lever up and down to upshift or downshift, all without using a clutch.

#2. Continuously Variable Transmission – CVT.

Continuously Variable Transmission (CVT) is a additional type of automatic transmission. CVT is an automatic transmission that can continuously change gear ratios depending on the speed of the vehicle.

CVT uses two cone-shaped pulleys with a belt between them. As the two pulleys move apart and together the belt will change its diameters allowing the gear ratios to keep changing depending on the needed speed, weight of car, incline and decline.

CVT is a very efficient transmission because it tries to maintain constant engine revolutions per minute as the vehicle speeds vary.

Advantages

• CVT is very efficient because trying to maintain a constant engine speed, fuel efficiency will be increased.
• Smooth Ride Quality – With constant engine speed, the ride quality with CVT is smooth and pleasant.

#3. Dual Clutch Transmission/Direct Shift Gearbox – DCT/DSG.

A Direct Shift Gearbox or Dual Clutch Transmission has two clutches that use gears automatically. The odd gears 1, 3 5 and the even gears 2 4 6 are operated by separate clutches.

The gear set up allows no power loss unlike AMT and manual transmission as the gears are engaged with minimal power loss and waiting to be used as soon as allowed by one gear rev.

A DSG/DCT Transmission offers a smooth performance and efficiency with a high degree of accuracy, and is used in high end and expensive cars.

Advantages:

• Faster Shifting – With the set up of the gears and the dual clutches a DSG transmission has the ability to shift gears superfast while remaining smooth and direct.
• Better Mileage – As a result of the superfast shifting of gears and accuracy of the up shifting and down shifting the mileage is better.

#4. Intelligent Manual Transmission – iMT.

Hyundai’s Intelligent Manual Transmission is the latest addition to their line of automatic transmission. iMT is not your ordinary automatic and it does require a manual shifting of gears without the need of a clutch.

When the gear stick is moved, an actuator engages the clutch based on feedback from sensors. Similar to a manual transmission, the driver is in charge of the gear shifts and fuel economy.

Advantages

• Efficient – There is no compromise on efficiency with clutch control being automatic.
• Low cost – iMT is less expensive than most automatic transmissions available on the market.

Advantages of Automatic Transmission

• Easy to Operate. Automatic transmission is easier to operate than manual transmission! Without any gear shifting to do, the driver can concentrate on the road, which is a tremendous benefit to anyone who prefers not to be engaged in the driving process to a greater extent. This makes automatic the perfect option.
• Convenience in Traffic. If you’re ever in stop and go traffic, a manual transmission is a drag! You have to keep changing to the right gear for every change in motion from the stop to the go. An automatic transmission does the stop and goes for you gradually without you having to think about it.
• Reduced Learning Curve. Overall, it is easier to learn as a new driver to utilize a model of automatic transmission over manual (gearbox) transmissions. This is a substantial benefit for new drivers or drivers who prefer a simpler way of learning.
• A Greater Increase in Technology. A lot of the automatic transmissions today are quite sophisticated with features such as adaptive shift patterns, hill start assists, and sport mode. Overall, automatic transmission tends to provide a better driving experience and seems to afford an increased level of customizability.
• Resale Value. Automatic transmission is quite popular to almost a standard in most markets, therefore there may be a ratio to upsurge the resale value of the vehicle. This is going to increase your resale value especially in an area where automatic is more popular than manual.

Disadvantages of Automatic Transmission

• Cost and Maintenance. Automatic transmissions tend to be more expensive than their manual counterparts, both in the cost of manufacture and service. While the difference is not as convincing as it once was, there are instances where maintenance could be costly, therefore potentially increasing costs over the lifetime of the vehicle.
• Fuel Economy. Normally, manual transmissions have always provided better fuel economy over automatics. A lot has changed in automotive technology, and the speed with which the automatics are adapting, they have closed the gap. But still, many drivers are typically attracted to manual transmissions due to the better fuel economy.
• Less Control for Enthusiasts. Most of the time, driving enthusiasts want a manual because the control of the car is much more exciting and engaging than automatic transmissions, because most of the time they do not engage, therefore leaving less involvement with their vehicle of enjoyment.
• Towing Capacity. In certain situations, manual transmissions can provide better towing capabilities. If towing heavy loads is a regular part of your driving conditions, a manual may be a better fit.
• Upfront Cost. Vehicles with automatic transmissions appear to have a higher upfront cost than manual transmission vehicles. This may not be beneficial if you have a tight budget and do not have to deal with additional costs.

FAQs.