In order to drive a car, you need a transmission. And while cars come either with a manual or an automatic transmission, in today’s world (at least here in America), you’re far more likely to buy an automatic transmission than a manual.
What’s the difference? In simplest terms, with a manual, you control the movement of the gears. You shift from first to second to third and so on based on your needs and desires.
An automatic transmission handles everything for you.
In reality, it’s a little more complicated than that.
Why a vehicle needs a transmission
In order to understand why a vehicle needs a transmission, it’s important to discuss how a car engine works first. The engine is designed to create power. To move the car, the power generated by the engine must be transferred to the wheels. That’s what a drivetrain does, of which the transmission is a part of.
An engine only spins at a certain level of speed. If it spins too low, you won’t be able to get the car moving. Too quickly and the engine could self-destruct.
That’s why you need something to further the control of the power produced within the engine. That’s the transmission’s job.
The transmission makes sure your engine spins at the optimal rate while sending power to your wheels at the right amount to move your car. It sits between the engine and powertrain, and operates as a control box for moving your car.
With a manual transmission, you accomplish this by moving the gears yourself into place. You connect different sized gears with one another depending on how fast you want to go, and how fast the engine needs to perform. You control the engagement of the gears by pressing in the clutch and shifting the gears into place.
With an automatic transmission, this process is done for you. Through engineering, the gears are engaged at the appropriate time to make your car go no matter what speed you desire.
The parts of an automatic transmission
In order to fully understand the automatic transmission, you’ll have to start with the casing. It resembles a bell, which is why you’ll sometimes hear it referred to as a bell casing. It’s typically made of aluminum, and is designed to protect all of the moving parts of the transmission.
When you start your car, it doesn’t suddenly take off in a forward or backward lurch. That’s because of the torque converter. The power from the engine to the transmission remains disconnected until you tell it to engage. With a manual, you engage the clutch and put it into gear. With an automatic, you use a torque converter. With hydraulic fluid, it varies the vehicle’s ability to go and stop by producing the exact amount of power necessary to accomplish the goal. It spins independently from the transmission and engages as needed. If you ever feel erratic behavior while driving at different speeds, it’s a torque converter malfunction.
As your vehicle picks up speed, it needs additional gears to move to in order to accomplish its goal. With a manual transmission, the gear shift changes the gear ratios. With an automatic transmission, it uses a planetary gear designed to do the process for you. In includes:
- A sun gear – it sits in the center of the gear set
- Planet gears – three or four smaller gears that surround the sun gear. They are mounted to their own shaft and are designed to orbit the sun gear
- Ring gear – this is the outer gear and contains teeth to mesh with each of the planetary gears
A planetary gear set can contain a reverse drive and up to five levels of forward drive.
A pump is located between the torque converter and planetary gear set. It’s what draws transmission fluid in and creates pressure for the torque converter and transmission. Automatic transmissions rely heavily on this fluid for the process to work. Think of it as the heart of the transmission system.
Bands and clutches
Bands and clutches are used to help the gears rotate, engage, or disengage. They are made from metal lined with organic friction material, used to hold the rings and gears stationary or tight, depending on what is called for.
With today’s modern technology, more of this process is computerized than ever before. Sensors control the different functions in the transmission to help control the speed of the engine and wheels in order to decide what gear to use.
The valve body is the control center of the transmission system. It’s the part that regulates incoming transmission fluid and uses it to run the valves and pistons. It determines what gear ratio by sending fluid in the proper quantities to the appropriate places within the transmission.
Transmission fluid is crucial to the overall process. It has both lubricating and cooling properties so that your transmission stays in good working condition and won’t overheat. It’s made from a variety of synthetic liquids and oils, and includes things like detergents, rust preventatives, and lubricants necessary to keep your automatic transmission working the way it should.
How the automatic transmission works
That’s a lot of working parts to ensure your automatic transmission works the way it should. It takes a special combination of mechanical, electrical engineering to give you a car safe enough to drive in all kinds of conditions.
In an easy-to-understand way, the automatic transmission works like this.
The engine starts and sends power to the torque converter, and into the pump.
The pump engages and begins sending transmission fluid into the system. A vortex of power is created inside the torque converter.
This engages the central shaft within the transmission, starts the spinning of the shaft and turbine, sending power to the first gear in the planetary gear set.
Depending on which part of the gear system is engaged, it will allow the gear to move or remain stationary. It sets the gear ratio and powers the transmission into action.
That, in a nutshell, is how the automatic transmission system works. Knowing how it works gives you a better understanding of how your vehicle works. And that makes you better at watching for things to go wrong, as well as a better understanding of the work that needs to be done when you bring your car in for service.
Our goal isn’t just to repair your car. We want to ensure you understand the mechanics of how your vehicle works too. It will make you a better driver, and a better owner.
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