AUTOMATIC AUTOMATIC TRANSMISSIONTRANSMISSION

What is Automatic What is Automatic TransmissionTransmission??

Changes gear ratios Changes gear ratios automatically as the vehicle automatically as the vehicle accelerate or deaccelerateaccelerate or deaccelerateFreeing the driver from Freeing the driver from having to shift gears manuallyhaving to shift gears manuallySo there is no clutch pedal in So there is no clutch pedal in an automatic transmission car an automatic transmission car

Gear-Shift ModeGear-Shift Mode

Automatic Transmission Automatic Transmission OperationsOperations

MostMost A/T have 3 or four forward speed, PARK, NEUTRAL, and A/T have 3 or four forward speed, PARK, NEUTRAL, and REVERSEREVERSEItIt starts the vehicle in 1 starts the vehicle in 1stst gear. Then it shifts to 2 gear. Then it shifts to 2ndnd, 3, 3rdrd & 4 & 4thth without assistance from driver . This happen automatically as without assistance from driver . This happen automatically as vehicle speed increases & the engine load or throttle opening vehicle speed increases & the engine load or throttle opening decreases.decreases.ToTo slow and stop the vehicle the driver only needs to release slow and stop the vehicle the driver only needs to release accelerator pedal or apply brake.accelerator pedal or apply brake.TheThe A/T disengages the torque converter clutch & automatically A/T disengages the torque converter clutch & automatically to downshifts until 1to downshifts until 1stst gear and when the vehicle stops. gear and when the vehicle stops.

The main componentsThe main components Planetary gear setsPlanetary gear sets that provide the various that provide the various

forward gear ratios as well as reverseforward gear ratios as well as reverseTorque converterTorque converter which acts like a clutch to which acts like a clutch to

transmit torque while the engine is still transmit torque while the engine is still running running

Hydraulic systemHydraulic system which uses a special which uses a special transmission fluid sent under pressure by transmission fluid sent under pressure by an Oil pump to control the an Oil pump to control the ClutchesClutches and theand the planetary gear sets planetary gear sets

GovernorGovernor and and ModulatorModulator or or Throttle valveThrottle valve that monitor speed and throttle position in that monitor speed and throttle position in order to determine when to shift order to determine when to shift

Planetary gear set•In Neutral, Motion of input shaft turns the Planet gears against the sun gear, it will just spin free and have no effect on the output shaft•In Gear Clutch pack , in this case, lock the output shaft with the sun gear forcing both to turn at the same speed place the unit in.

Torque ConverterTorque Converter - - PrinciplePrinciple• Large nut shaped device that is mounted between the engine and the transmission • It uses oil or transmission fluid to transmit power.

THE HYDRAULIC SYSTEM• Complex maze of tubes that sends fluid under under

pressure to all parts of the transmission and torque converter.

• Transmission fluid serves in shift control, general lubrication and transmission cooling so to keeps the transmission at normal operating temperature

• OIL PUMP• VALVE BODY• SHIFT VALVES• BANDS

Oil Pump(Gear Pump)Oil Pump(Gear Pump)

• Produce all the oil pressure that is required in the transmission

• The oil is then send under pressure to the valve body and the rest of the components, as required.

Valve BodyValve Body

• The brain of the automatic transmission

• Contains a maze of channels and passages that direct hydraulic fluid to the numerous valves

• These valves activate the appropriate clutch pack or band servo to smoothly shift to the appropriate gear for each driving situation.

Shift Valves

• Supply hydraulic pressure to

the clutches and bands to engage each gear

• Determines when to shift from

one gear to the next

GOVERNERGOVERNER Connected to the output Connected to the output

shaft and regulates shaft and regulates hydraulic pressure based on hydraulic pressure based on vehicle speedvehicle speed

COMPUTER CONTROLLSCOMPUTER CONTROLLS

• Detect such things as throttle position, vehicle Detect such things as throttle position, vehicle speed, engine speed, engine load, etc. speed, engine speed, engine load, etc.

• Downshifting automatically when going Downshifting automatically when going downhill.downhill.

• Up shift when braking on a slippery surface.Up shift when braking on a slippery surface.

Comparison with manual Comparison with manual transmission transmission

•Smooth OperationSmooth Operation•Ease of control Ease of control •Infinite number of gearing available Infinite number of gearing available •Minimum power losses during gear shifting Minimum power losses during gear shifting

Higher cost, lower fuel efficiency Higher cost, lower fuel efficiency Complicated designComplicated designPossibility of oil leakage Possibility of oil leakage

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