How an Automatic Transmission Works | YourMechanic Advice

How an Automatic Transmission Works

An automatic transmission allows a vehicle engine to operate within a narrow speed range, just like a manual transmission does. As the engine achieves higher degrees of torque (torque is the rotational power of the engine), the gears in the transmission allow the engine to take full advantage of the torque produced while maintaining an appropriate speed.

How important is a transmission to the vehicle’s operation? Without a transmission, vehicles only have one gear, take forever to reach higher rates of speed, and quickly wear out the engine due to the constant high RPMs produced.

The Principle Behind an Automatic Transmission

The principle behind the automatic transmission relies on the use of sensors to determine the appropriate gear ratio to use, dependent in large part on the desired speed of the vehicle. The transmission connects to the engine at the bell housing where the torque converter converts the torque from the engine into motive power, and in some cases even amplifies that power. The torque converter in the transmission does this by transferring this power to the driveshaft via the planetary gearset and clutch plates, which then let the drive wheels in a vehicle to turn, providing forward movement, with different gear ratios needed for different speeds. Depending on the make and model, this includes rear-, front-, and all-wheel-drive vehicles.

If a vehicle only had one or two gears, getting up to higher rates of speed would be a problem because the engine only turns at a specific RPM depending on the gear. This means lower RPMs for lower gears, and thus a lower speed. If the highest gear was second, then the vehicle would take forever getting up to speed at the lower RPMs, gradually building higher and higher RPMs as the vehicle gained speed. Stress on the engine also becomes an issue when running at higher RPMs for longer periods of time.

By using specific gears that work in conjunction with each other, a vehicle gradually gains speed as it goes up through the higher gears. As the vehicle switches into the higher gears, the RPMs decrease, putting less stress on the engine. The various gears are represented by a gear ratio (which is the ratio of the gears in both size and the number of teeth they have). Smaller gears turn faster than larger gears, and each gear position (first to sixth in some cases) uses different gears of varying size and teeth number to achieve a smooth acceleration.

A transmission cooler is necessary when hauling heavy loads, because a heavier load puts added stress on an engine, causing it to run hotter and burn up the transmission fluid. The transmission cooler sits within the radiator where it transfers heat away from the transmission fluid. The fluid runs through tubes in the cooler to the coolant in the radiator, so the transmission does not run as hot and can handle heavier loads.

What the Torque Converter Does

The torque converter multiplies and transfers the torque created by the vehicle engine and transfers it via the gears in the transmission to the drive wheels at the end of the drive shaft. Some torque converters also act as a locking mechanism, binding the engine and transmission when running at similar speeds. This helps to keep the transmission from slipping, which results in a loss of efficiency.

The torque converter can take one of two forms. The first, the fluid coupling, uses at least a two-element drive to transfer the torque from the transmission to the drive shaft, but does not multiply the torque. Used as an alternative to a mechanical clutch, the fluid coupling transmits the rotational power of the engine to the wheels via the driveshaft. The other, the torque converter, utilizes a total of at least three elements, and sometimes more, to increase the torque output from the transmission. The converter uses a series of blades and reactor, or stator blades, to increase the torque, resulting in more horsepower. The stator, or static blades, serve the purpose of redirecting the transmission fluid before it gets to the pump, dramatically increasing the converter's efficiency.

The Inner Workings of the Planetary Gearset

Knowing how the automatic transmission parts work together can really put it all into perspective. When looking inside an automatic transmission, in addition to the various bands, plates, and the gear pump, the main component is the planetary gearset. This gearset consists of the sun gear, planet gears, planet gears' carrier, and the ring gear. Roughly the size of a cantaloupe, the planetary gearset creates the various gear ratios required by the transmission in order to attain the necessary speeds to travel forward while driving, as well as to go into reverse.

The various gear types work together, operating as the input or output of the particular gear ratio required at any given time. In some cases, the gears serve no purpose in a particular ratio and so remain stationary, with the bands within the transmission holding them out of the way until needed. Another type of gearset, the compound planetary gearset, incorporates two sets of sun and planetary gears, though only one ring gear. The purpose of this type of gearset is to provide torque in a smaller space, or to give a vehicle more overall power, such as in a heavy-duty truck.

Examining the Gears

While the engine operates, the transmission responds to whatever gear the driver puts it in at the time. In Park or Neutral, the transmission does not engage since vehicles do not need torque while the vehicle is not in motion. Most vehicles have a variety of drive gears useful when moving forward, ranging from first through fourth gear.

High-performance vehicles tend to have even more gears, even up to six, depending on the make and model. The lower the gear, the lower the speed. Some vehicles, especially medium-sized to heavy-duty trucks, utilize an overdrive to help maintain higher speeds while also keeping a better fuel efficiency.

Last of all, vehicles use the reverse gear to back up. The reverse gear uses one of the smaller gears to engage a larger planetary gear, as opposed to the other way around when driving forward.

How the Transmission Uses Clutches and Bands

Additionally, an automatic transmission uses clutches and bands to help it achieve the various gear ratios necessary, including for the overdrive. Clutches come into play when connecting the parts of the planetary gears to each other, while bands help hold the gears stationary so that they do not rotate when not needed. The bands, controlled by hydraulic pistons within the transmission, lock into place parts of the gear train. Hydraulic cylinders and pistons also control the clutches, making them engage the gears needed for a particular gear ratio and speed.

The clutch plates sit within the clutch drum in the transmission and alternate with steel plates in between. The clutch plates, in the form of discs, bite into the steel plates through the use of a special coating. Instead of damaging the plates, the discs gradually grip them, slowly applying power that then transfers to the drive wheels of the vehicle.

The clutch discs and steel plates represent a common area where slippage occurs. Eventually, this slippage leads to metal shavings entering into the rest of the transmission and eventual transmission failure. A mechanic will inspect the transmission if a vehicle suffers from transmission slippage problems.

Hydraulic Pumps, Valves, and the Governor

But where does the “real” power come from in an automatic transmission? The real power lies in the hydraulics built into the casing of the transmission, including the pump, various valves, and the governor. The pump draws the transmission fluid from the sump located at the bottom of the transmission, feeding it to the hydraulic system to actuate the clutches and bands contained within it. In addition, the inner gear of the pump connects to the outside housing of the torque converter. This allows it to spin at the same speed as the vehicle’s engine. The outer pump gear turns in accordance with the inner gear, allowing the pump to draw the fluid up from the sump on one side while feeding it into the hydraulic system on the other side.

The governor regulates the transmission by letting it know the speed of the vehicle. The governor, which contains a spring-loaded valve, opens up more the faster the vehicle travels. This allows the hydraulics of the transmission to let in more fluid at higher speeds. An automatic transmission uses one of two kinds of kind of devices, a manual valve or vacuum modulator, to determine how hard the engine is working, increasing pressure as necessary and prohibiting the use of certain gears depending on the gear ratio in use.

By properly maintaining the transmission, vehicle owners can expect it to last for the life of the vehicle. A highly durable system, an automatic transmission utilizes many different parts, including a torque converter, the planetary gears, and the clutch drum to provide power to the vehicle’s drive wheels, keeping it at the speed desired.

When you have problems with an automatic transmission, seek the help of a mechanic to maintain the fluid levels, inspect it for damage, and repair or replace it if necessary.

Common Issues and Symptoms of Automatic Transmission Problems

Some of the more common problems associated with a faulty transmission include:

  • A lack of response, or hesitation, when placing the vehicle in gear. This usually indicates slippage within the transmission.
  • The transmission makes a variety of weird whines, clunks, and hums. Have a mechanic check your vehicle when it makes these kinds of noises to determine where the problem lies.
  • Leaking fluid indicates bigger problems, and you should have a mechanic address this problem as soon as possible. Transmission fluid does not burn off like motor oil. Having a mechanic check the fluid levels on a regular basis can help fix a potential problem before it gets started.
  • A burning smell, especially from the transmission area can indicate a very low fluid level. The transmission fluid keeps the gears and parts in a transmission from overheating.
  • A Check Engine Light can also indicate a problem with an automatic transmission. Have a mechanic run a diagnostic to find the exact problem.

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