Automatic transmission


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Automatic transmission

  1. 1. What is..? Automatic gear box that can change gear ratios automatically as the vehicle moves  Freeing the driver from having to shift gears manually  So there is no clutch pedal in an automatic transmission car 
  2. 2. The main components     Planetary gear sets that provide the various forward gear ratios as well as reverse Hydraulic system which uses a special transmission fluid sent under pressure by an Oil pump through the Valve body to control the Clutches and the Bands in order to control the planetary gear sets Torque converter which acts like a clutch to allow the vehicle to come to a stop in gear while the engine is still running Governor and Modulator or Throttle valve that monitor speed and throttle position in order to determine when to shift
  3. 3. Planetary gear set
  4. 4. Planetary gear set in Auto Transmission
  5. 5. Planetary gear set in Auto Transmission contd..     If both the clutch pack and the band were released, the system would be in neutral. Turning the input shaft turns the planet gears against the sun gear, but since nothing is holding the sun gear, it will just spin free and have no effect on the output shaft The clutch pack is used, in this instance, to lock the planet carrier with the sun gear forcing both to turn at the same speed To place the unit in first gear, the band is applied to hold the sun gear from moving  To shift from first to high gear, the band is released and the clutch is applied causing the output shaft to turn at the same speed as the input shaft
  6. 6. Hydraulic system - Clutches     Consists of alternating discs that fit inside a clutch drum Half of the discs are steel and have splines that fit into groves on the inside of the drum  The other half have a friction material bonded to their surface and have splines on the inside edge that fit groves on the outer surface of the adjoining hub  A piston inside the drum that is activated by oil pressure at the appropriate time to squeeze the clutch pack together so that the two components become locked and turn as one
  7. 7. Hydraulic system - Bands    Is a steel strap with friction material bonded to the inside surface One end of the band is anchored against the transmission case while the other end is connected to a servo At the appropriate time hydraulic oil is sent to the servo under pressure to tighten the band around the drum to stop the drum from turning 
  8. 8. Torque Converter - Principle
  9. 9. Torque Converter
  10. 10. Torque Converter contd..
  11. 11. Governor and Modulator or Throttle valve      The Governor is connected to the output shaft and regulates hydraulic pressure based on vehicle speed. It accomplishes this using centrifugal force The more load you place on the engine, the longer the transmission will hold a gear before shifting to the next one The Throttle Cable simply monitors the position of the gas pedal through a cable that runs from the gas pedal to the throttle valve in the valve body The Vacuum Modulator monitors engine vacuum by a rubber vacuum hose which is connected to the engine. High vacuum is produced when the engine is under light load and diminishing down to zero when the engine is under a heavy load.  When an engine is under a light load, high vacuum acts on the modulator which moves the throttle valve in one direction to allow the transmission to shift early and soft.  As the engine load increases, vacuum is diminished which moves the valve in the other direction causing the transmission to shift later and more firmly.
  12. 12. Semi-automatic or clutchless manual transmission   uses electronic sensors, processors and actuators to do gear shifts on the driver’s command This removes the need for a clutch pedal ,since the clutch itself is actuated by electronic equipment which can synchronise the timing and torque required to make gear shifts quick and smooth
  13. 13. Semi-automatic transmission contd..     The gear lever moves forward and backward to shift into higher and lower gears Hall effect sensors sense the direction of requested shift, and this input, together with a sensor in the gear box which senses the current speed and gear selected, feeds into a CPU. This determines the optimal timing and torque required for a smooth clutch engagement CPU powers a hydro-mechanical unit to either engage or disengage the clutch, which is kept in close synchronization with the gear-shifting action the driver has started The system lies can react much faster and more precisely than a human, and takes advantage of the precision of electronic signals to allow a complete clutch operation without the intervention of the driver