Although they are frequently reliable, automatic transmissions need to be serviced regularly to keep them in good working order. Commonly, mechanics will check the level of the transmission fluid that is held in the system to ensure it is always properly lubricated. They will also make sure it does not make any grinding or excessive movements which may indicate that an internal component needs to be repositioned or replaced. That said, how do automatic transmissions work?
To begin with, an automatic transmission system relies on gears just like conventional, manual transmissions. The major difference is that the gears will be engaged by hand in a manual gearbox. When it comes to automatics, sensors are used to determine the use of gears. Lowering revs mean that the gearbox kicks down while rising revs will result in a higher gear being chosen. Automatic transmissions shift gears by using internal, pressurised oils to push the gears into place with one another once the sensors have determined which gear should be used next.
Gear Disengagement and Re-Engagement
For an automatic transmission system to shift from one gear to the next and maintain the desired level of revs, the current gear that is being used will have to be disengaged. With a manual transmission system, a clutch will be deployed so that the gears are no longer in physical contact with one another. With an automatic gearbox, a torque converter carries out the equivalent process. Basically, this is a bladed mechanism which spins fluid in a closed-loop connected to both the car's crankshaft and the transmission system's input shaft from the engine.
When there is a differential between the two spinning speeds of these shafts, the blades of the torque converter either speed up or slow down which effectively disengages the current gear and re-engages it with another. Some automatic transmissions use a locking mechanism to temporarily connect the crankshaft and input shaft at high revs, something that helps to maintain fuel efficiency at cruising speeds. This disengages when either the accelerator is pushed down hard or the brake is applied.
Most automatic transmission systems have so-called planetary gears. Unlike a manual system, there is a central gear and the other ones are arranged in an orbit-like fashion around it, hence the name. When different combinations of gears are selected, so new gear ratios can be created to cope with various driving conditions. With this approach, the transmission system is able to derive power from the car's engine and manipulate how much is transmitted to the output shaft.