Automatic transmission system
||Automatic transmission system
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Automatic transmission system shifts the gears without assistance from the driver. They start the car moving in first and then shift into higher gears as the car speed increases and engine load decreases. The shifts are produced by hydraulic pressure acting through the transmission fluid.
The control system takes into account the engine load and in general produce changes up when the engine load is light and changes down when the engine load is heavy.
On automatic transmissions, the torque converter takes the place of the clutch found on standard shift vehicles. It is there to allow the engine to continue running when the vehicle comes to a stop.
A torque converter is a large doughnut shaped device that is mounted between the engine and the transmission. As shown in the fig.1, there are four components inside the very strong housing of the torque converter
FLUID COUPLING PHASE:
As the speed of the turbine catches up with the pump, the fluid exit the turbine in the same direction as the pump is turning, so the stator is not needed. At these speeds, the fluid actually strikes the back sides of the stator blades, causing the stator to freewheel on its one-way clutch so it doesn't hinder the fluid moving through it. All three now elements begin to turn at approximately the same speed.
TORQUE CONVERTER EFFICIENCY:
It is seen that the efficiency of the torque converter is reasonably good at only narrow range of turbine speeds. The fall-off of efficiency at low speed end of the range can be tolerated because those speeds are used for short periods. But the fall-off of efficiency at high speeds cannot be tolerated and must be circumvented. The efficiency can be increased, by substituting a direct drive for the torque converter at higher speeds.
THE LOCKUP IN TORQUE CONVERTER (DIRECT DRIVE):
Because the only connection between two sides of a torque converter is a fluid connection, there is always a little slippage, running from about 2-8%. To increase efficiency and mileage, most modern automatic transmissions also have something called a lockup clutch.
COMPOUND PLANETARY GEAR SET
The compound planetary gear set looks like a simple planetary gear set but actually behaves like two planetary gear sets combined. It has one ring gear that is always the output of the transmission, but it has two sun gears and two sets of planets
The hydraulic system provides the pressurized fluid to operate an automatic transmission.Major components of the hydraulic system include the bands, clutches and oil pump. Other major components are the governor, throttle valve, modulator and the valve body.
The pump’s output pressure will increase roughly in proportion to the engine’s speed. However, the pressure necessary to actuate the various valves and to energise the clutch and band servo pistons will vary under different work conditions. Therefore the fluid pressure generated by the pump, is unlikely to suit the many operating requirements. To overcome these difficulties, a pressure regulating valve is used which automatically adjusts the pump’s output pressure to match the working requirements at any one time. The pressure regulating valve is normally a spring-loaded spool type valve.
The valve body is the brain of the automatic transmission. It contains a maze of channels and passages that direct hydraulic fluid to the numerous valves which then activate the appropriate clutch pack or band servo to smoothly shift to the appropriate gear for each driving situation. Each of the many valves in the valve body has a specific purpose and is named for that function. For example the 1-2 shift valve activates the 1st gear to 2nd gear up-shift.
THE MANUAL VALVE:
The manual valve is directly connected to the gear shift handle and covers and uncovers various passages depending on what position the gear shift is placed in. If the gear shift lever is moved in the 1st gear or 2nd gear position, the up-shift and down-shift are inhibited by the manual valve.
Shift valves, as shown in fig. 8 supply hydraulic pressure to the clutches and bands to engage each gear. The valve body of the transmission contains several shift valves. The shift valve determines when to shift from one gear to the next. The shift valve is pressurized with fluid from the governor on one side, and the throttle valve on the other. The shift valve will delay a shift if the car is accelerating quickly. If the car accelerates gently, the shift will occur at a lower speed.
1. It minimizes driver fatigue, especially in heavy traffic by eliminating the need to operate the clutch pedal and gear lever for starting from rest and changing gear.
2. It contributes to safer driving because the concentration of the driver is not disturbed by the need to change gear; also, both hands can remain on the steering wheel.
3. Progress can be smoother under normal driving conditions, because gear changes will occur at the theoretically correct moment in terms of road speed and throttle opening.
1. Cars with automatic transmission are costlier than those having manual transmission.
2. Fuel economy of cars with automatic transmission is not very good.
• The automatic transmission with its torque converter and planetary gear set, can successfully replace the clutch and the manual transmission gear box.
• The planetary gear set gives the required gear ratios and the hydraulic system controls the planetary gear set.
• Automatic transmission system shifts the gears automatically, depending upon both, speed of vehicle and load on the engine.