What is an Automatic Transmission, and How Does it Work?

Because we use them every day, the operation of a car seems simple, but a car is a very complex machine. One of the most complex systems in the typical vehicle is the automatic transmission. This ingenious system performs a function—changing gears—that the driver would otherwise have to do. An automatic transmission executes gear changes automatically, so many drivers don't have a good sense of what this component is and what it does. But understanding how an automatic transmission works can make you a better driver and give you a better appreciation of what your car can do.

GM 10-Speed Automatic Transmission

Why Automobiles Need Transmissions

You don't need to be a technological genius to understand why a car needs an engine. It provides the motive force to move the vehicle. And it is also pretty evident that it is critical to get the motive force from the engine to the wheels to drive the car. The system that does that is called the transmission. But there's more to it than a simple mechanical hookup between the engine and drive wheels.

An internal combustion engine creates power and torque in a very narrow operating range. This necessitates that the car has gears that enable the engine to operate in its range as the vehicle accelerates, reaches a constant cruising speed, and decelerates. Without that, a car's speed potential would be extremely limited.

In an internal combustion engine car with a manual transmission, the driver changes the gears. A reasonably straightforward mechanical arrangement enables the driver to use a shift lever to choose the individual gear from among a selection of gears appropriate for the immediate driving situation. Manual shifting between gears requires the frequent disconnection of the engine from and reconnection to the driving mechanism—the propellor shaft and half-shafts that go to the drive wheels—during any drive. The mechanical clutch is the means of disconnection and reconnection. It is foot-operated using the car's clutch pedal. Changing gears requires dexterity and footwork. Though doing it well can be fun, it also takes effort.

From Manual to Automatic Transmissions

It didn't take long for automotive pioneers to begin tinkering with the idea of automating the gear-changing process. Doing so would simplify driving and open the freedom and convenience of personal transportation to those lacking the skill or strength to change gears manually using the typical process. But it wasn't until the 1930s that General Motors engineers developed a practical system of making automatic gear changes. That system was what we now call the automatic transmission.

Though the design has been refined over the years—most recently by the addition of electronic controls—the original engineering concept of the 1930s remains the basis of the automatic transmissions that change gears in today's cars, trucks, vans, and SUVs.

Key Components of Automatic Transmissions

An automatic transmission does something that is simple to describe in words. It changes gears without direction from the driver, aside from choosing Drive, Reverse, Neutral, and Park. But getting that to happen involves many components that must work in harmony in a variety of conditions over and over and over again. Here are the key elements:

Torque Converter

A torque converter was a fundamental breakthrough leading to the development of a practical automatic transmission. 

A torque converter essentially performs the function that a mechanical clutch does in a manual transmission. It enables the engine connection to and disconnection from the driveline as needed to facilitate the changing of gears. 

The use of fluid dynamics can transfer torque from the engine when needed by the car to move forward, but it will essentially soak up that torque when moving forward is not desired. 

This capability enables you to leave your car in Drive while you have your foot firmly on the brake when stopped at a stoplight. When you release the brake, the torque converter enables the car to creep forward.

Planetary Gears

If your car had a torque converter and a single forward gear, your forward speed would be severely limited by the engine's operating range and torque production ability. That is why gear-changing is necessary. 

An automatic transmission uses a different type of gearset than a manual transmission vehicle. It uses a set of planetary gears to create several gear ratios as the automatic transmission's hydraulics controls them.

Brake Bands and Clutches

Brake bands and clutches actuate the changing of gears in the planetary gearset. Brake bands tighten to hold a particular gear stationary or loosen to enable that gear to spin. And it is the combination of stationary and spinning gears in the gearset that produces each individual gear ratio.

The clutches in the transmission have a similar function, helping to determine the particular gear ratio the planetary gearset is creating at that moment. To go from one gear ratio to another involves the tightening and loosening of clutches and bands. This is directed by hydraulics, electronics, or a combination of both.

How an Automatic Transmissions Works

When you drive a vehicle with an automatic transmission, putting the car in Drive activates the system. By pushing on the accelerator pedal, the engine's crankshaft begins rotating more rapidly and this, in turn, leads to more pressure within the torque converter. As the car moves forward, the transmission's hydraulics and electronics sense its speed in relation to engine speed, and the transmission changes gears automatically in response.

In the classic automatic transmission, hydraulics within the transmission sense those speed changes. Those hydraulics then activate other hydraulics to change gears in the planetary gearset. Today, electronics supplement that sensing process, and the automatic transmission works in concert with the car's other systems.

So, the next time you drive your car, you should be aware that millions of calculations go into the gear that your transmission chose at that moment. Your car is doing a lot of work every second to make driving much easier for you.