Automatic transmission

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Car gears shifter
Once a car with automatic transmission is in drive, the gears shift automatically
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What is an automatic transmission and how does it work?

Most of us recognize the term automatic transmission and know if our cars are equipped with an automatic or standard mechanism, but how many of us really know how an automatic transmission works? Read on to learn the ins and outs of what makes your automatic car run.

What is an Automatic Transmission?

An automatic transmission is a gearbox that allows a car to shift gears while in motion without the driver having to change gears manually. The majority of cars made in America come equipped with automatic transmissions; in Europe, most automobiles have standard transmissions.

The transmission is connected to the back of the engine and sends power from the engine to the drive wheels. A car's engine runs its best at a certain RPM (revolutions per minute) and it is the transmission's job to use various gears to send power to the wheels while keeping the engine running within its optimum RPM range.

The Main Components of an Automatic Transmission   
  • Planetary Gear Sets mechanical systems that provide the forward and reverse gear ratios
  • Hydraulic System uses a transmission fluid sent under pressure by an oil pump through the valve body to control the clutches and the bands in order to run the planetary gear sets
  • Seals and Gaskets used to keep the oil in the proper location
  • Torque Converter works like a clutch to allow the vehicle to come to a stop while in gear with the engine still running
  • Governor/Modulator to monitor speed and throttle position to determine when a shift is necessary
  • Computer to control shift points


In addition to the various forward gears (1-5 in most cars, although some sports vehicles may have 6), the transmission also has a neutral position which disconnects the engine from the drive wheels; a reverse gear to allow the wheels to turn in the opposite direction and the car to back up; and a park gear in which a latch mechanism is inserted into a slot in the output shaft to lock the drive wheels in place.  

Cars with an automatic transmission do not have clutches or gear shifts. Once the car is in drive, the gears shift automatically. The transmission is responsible for the vehicle's torque; the various gears allow the car to move at different speeds. The key difference between a manual transmission and an automatic is that a manual transmission locks and unlocks various gears to the output shaft to achieve different gear ratios and an automatic transmission uses a planetary gearset to produce all of the gear ratios.

Types of Automatic Transmissions

There are two types of automatic transmissions, depending on if the car is front wheel or rear wheel.

On a rear wheel vehicle, the transmission is usually mounted to the back of the engine. A drive shaft connects the rear of the transmission to the final drive, located in the rear axle, and is used to send power to the rear wheels. Power moves from the engine through the torque converter and then through the transmission and drive shaft until it reaches the final drive, where it is divided and sent to the two back wheels.

On a front wheel drive, the transmission is normally combined with the final drive to form what is called the transaxle. The engine on a front wheel drive is usually placed sideways with the transaxle tucked under it on the side of the engine that faces the rear of the car. Front axles are connected to the transaxle and provide power to the front wheels. Power flows from the engine, through the torque converter, to a large chain that sends the power through a 180-degree turn to the transmission that is alongside the engine. Then the power is sent through the transmission to the final drive where it is split and sent to the two front wheels.

The modern automatic transmission is a complicated mechanism that contains electrical systems, computer controls, mechanical systems and hydraulic systems all of which all work together to effectively run a vehicle.

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