The lack of a clutch also eliminates the need to pay attention to shifting manually and monitoring the tachometer to make the necessary shifts, which gives you more attention to focus on the task of driving. Many drivers also find it easier to control an automatic transmission at low speeds than a manual transmission.
The hydraulic automatic transmission creates a phenomenon called idle creep, which encourages the vehicle to move forward even when idling. For any feedback or correction requests please contact us at research caranddriver. New Cars. Buyer's Guide. Type keyword s to search. Today's Top Stories. Operation of an Automatic Transmission The most common type of automatic transmission uses hydraulic power to shift gears.
Gears of an Automatic Transmission The gears of an automatic transmission include the following: According to How a Car Works , when you shift your vehicle into drive, you engage all available forward gear ratios.
This means that the transmission can move between its full range of gears as needed. Six-speed automatic transmissions are the most common number of gears, but older cars and entry-level compact cars may still have either four or five automatic gears. Third gear either locks the transmission in third gear or limits it to the first, second, and third gear ratios.
This provides the power and traction needed to go either uphill or downhill or to tow a boat, RV, or trailer. The planetary gearset is the central player in the automatic gearbox system. This system of gears connects the central sun gear to the outer ring gear. There may be several gearsets working together to achieve the varying gear ratios. The brake bands can tighten to hold either the ring or sun gear, to allow them to spin or keep them stationary.
These are controlled, once again, by a hydraulic system. The clutches connect and release the planetary gears, activating the gearing required. The control of these clutches is delivered by a combination of mechanical, hydraulic and electrical functions. The central reactor converts this turning effort by redirecting the flow of fluid back to the impeller to give higher torque at low speeds. Once the engine speeds up and develops more power, the need for this torque amplification decreases and the reactor freewheels.
The torque converter then acts as a fluid flywheel, connecting engine to gearbox. The main components of a torque converter are shown in the diagram - the impeller, reactor or stator and turbine. The locking sequences of the epicyclic gear train are achieved by hydraulic pressure operating brake bands or multi-plate clutches.
The bands are tightened round the ring gear to prevent it turning, and the clutches are used to lock the sun gear and planets. The correct sequence of pressure build-up and release is controlled by a complex arrangement of hydraulic valves in conjunction with sensors that respond to engine load, road speed and throttle opening.
A mechanism linked to the throttle - known as a kickdown - is used to effect a change-down for rapid acceleration. When you press down the accelerator suddenly to its full extent, a lower gear is selected almost instantly. The clutches and bands are operated by pins and valves that are activated by pressurized transmission fluid.
By creating different gear ratios, the transmission takes the power from the engine and increases or reduces it on its way to the output shaft, which sends power toward the wheels.
In first gear, the engine is turning relatively slowly as the driver gradually pushes the throttle, so the transmission uses a low gear to multiply the torque going to the wheels to give them the power needed to accelerate.
When the transmission is put into Reverse, the small sun gear turns the outer ring gear backwards. For Park, a small toothed parking gear is firmly held by a small latch called a parking pawl, which prevents the output shaft from turning the wheels. On a front-wheel-drive vehicle, the differential gears are bundled into the transmission housing, and the entire unit is commonly called a transaxle. Sign up to receive Driving. A welcome email is on its way. If you don't see it, please check your junk folder.
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