Transmission Components and Operation
The Automatic Transmission Operation consists of many components, the main components being clutches, drums, planetary gear set, valve chest, and oil.
Gear changing in an automatic transmission is similar to the action taken while riding your bike, the difference being that a series of valves, electronic sensors and electronic control units “ECU” decide when to upshift or downshift to a larger or smaller gear.
The torque converter takes the place of a clutch in a manual transmission. It is a fluid filled unit that converts drive from the engine through a process called hydraulic coupling.
Internally, it is fitted with components called a lock up clutch, turbine, impellor and stator.
The impellor “connected to the engine” directs oil to the turbine “connected to the gearbox” which in turn re-directs the oil back to the impellor via the stator. As the engine revs increase, the continual flow of oil will increase and provide the required torque multiplication and oil supply which is required to drive the gearbox.
A conventional torque converter suffers with a small amount of slippage between 2-8%. To increase fuel economy, most modern torque converters are fitted with a component called a lock up clutch.
As the car approaches 40-50 mph, oil pressure is directed through the transmission input shaft to activate a piston; this locks the turbine to the impellor removing any slip within the torque converter.
Most modern cars use a multiple disc clutch operation, in this set up a series of friction plates are sandwiched between steel plates. The clutch plates are compressed together by the movement of a piston which is normally hydraulically operated.
The whole assembly is known as a clutch pack, when fluid pressure is applied to the clutch pack, the piston engages locking the assembly together providing drive to the driving wheels.
Planetary Gear Sets
Automatic transmissions contain many gears in various combinations. In an automatic transmission, however, the gears are never physically moved and are always engaged to the same gears. This is known as a planetary gear set.
The basic planetary gear set consists of a sun gear, a ring gear and two or more planet gears, all remaining in constant mesh. The planet gears are connected to each other through a common carrier which allows the gears to spin on shafts called “pinions” which are attached to the carrier.
One example of a way that this system can be used is by connecting the ring gear to the input shaft coming from the engine, connecting the planet carrier to the output shaft, and locking the sun gear so that it can’t move. In this scenario, when we turn the ring gear, the planets will “walk” along the sun gear (which is held stationary) causing the planet carrier to turn the output shaft in the same direction as the input shaft but at a slower speed causing gear reduction (similar to a car in first gear).
The valve body is the brain of the automatic transmission. It contains channels and passages that direct fluid to valves, the valves in turn activate the appropriate clutch pack to change to the appropriate gear for each driving situation. Each valve in the valve body has a specific purpose and is named for that function. For example, the 2-3 shift valve activates the 2nd gear to 3rd gear up-shift or the 3-2 shift timing valve which determines when a downshift should occur.
The manual shift valve being the most important valve is directly controlled via the gear lever and covers and uncovers various passages depending on what position the gear shift is placed in. When you place the gear lever into Drive, for instance, the manual valve directs fluid to the clutch pack(s) that activates 1st gear. It also monitors vehicle speed and throttle position so that it can determine the optimal time and the force required for the 1 – 2 shift.
Modern computer controlled transmissions will also have electrical solenoids that are mounted in the valve body, these direct fluids to the appropriate clutch packs and control more precisely control shift points.
The primary functions of automatic transmission hydraulic fluids are to lubricate internal components, reduce friction and wear and carry heat from the transmission to the oil cooler.
A transmission delivers power from the engine to the driving wheels changing automatically to provide the optimum power and efficiency for your driving style and speed.
The job of the transmission fluid is to provide drive through the torque converter, cool and lubricate the gears, bands, clutch plates and seals.
Over time the fluid degrades and its operational qualities diminish. Carrying out an automatic transmission service and changing the fluid at regular intervals helps to protect your gearbox and prevent premature wear, which can lead to expensive transmission failure.
The biggest enemy to transmission fluid is heat. Heat build-up is generally caused by constant friction between the clutch plates and gears, in time this causes important additives to oxidize and break down reducing the effectiveness of the fluid to lubricate and protect. Transmission oil coolers must be kept clean to allow removal of the heat from the lubricant.