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Automatic Transmission Fundamentals

Automatic Transmission Fundamentals

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Automatic Transmission Fundamentals

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  1. Automatic Transmission Fundamentals Chapter 73

  2. Objectives • Identify the basic parts of an automatic transmission • Describe the operation of the major sections of an automatic transmission • Explain how automatic transmissions shift gears • Understand how an electronic automatic transmission works

  3. Introduction • Automatic transmission • Shifts gears automatically • Does not require a manual clutch • Front-wheel-drive vehicles • Combine transmission with differential in transaxle • Most automatic transmissions use a torque converter • Some use a dual clutch arrangement

  4. Automatic Transmission Partsand Power Transmission • Automatic transmission consists of several parts • Torque converter, input shaft, transmission pump, valve body, planetary holding members, etc. • Methods of transmitting power • Fluid, friction, and gears • Torque converter transmits power using fluid • Planetary holding members use fluid and friction • Gears transmit power and change speed and torque

  5. Flexplate and Torque Converter • Flexplate and torque converter • Replace the flywheel • Flexplate is fastened to crankshaft • Torque converter • Allows vehicle to idle at a stop sign • Slips during initial acceleration to prevent stalling • Fluid coupling: compared to two fans • First fan (impeller): turns faster • Second fan (turbine): picks up energy and turns • Pump produces fluid flow to develop pressure

  6. Torque Multiplication and Torque Converter Operation • Torque converter • Increases torque • Torque is multiplied whenever the impeller spins faster than the turbine • Operation • Impeller rotates at idle speed: fluid is thrown from impeller toward turbine • Centrifugal force of rotating torque converter also throws fluid to outside of housing

  7. Torque Converter Stator and Operation • Torque converter stator • Makes torque increase possible • Stator between impeller and turbine • Redirects fluid flow • Split half-rings in centers of turbine and impeller blades • Direct fluid in a smooth pattern

  8. Stator Clutch Operation • Stator clutch • Locks in one direction and freewheels in other • Fluid strikes stator at a high angle: clutch locks • Speed of turbine catches speed of impeller: stator clutch freewheels • Converter • Becomes efficient at power transfer when engine reaches 2300 rpm • Turbine speed is 9/10 of impeller: no torque multiplication

  9. Stall Speed and Lock-Up Converters • Stall speed • Point of maximum torque multiplication • Lower stall speed converters: more efficient • Lock-up converter • Pressure plate behind turbine locks it to back of converter housing • Provides mechanical link between crankshaft and transmission input shaft • Fluid is directed to one side of pressure plate and is exhausted from the other

  10. Planetary Gears and Simple Planetary Gearset • Planetary gears • Change gear ratios by holding and turning different members • All gears are in constant mesh • Load is distributed over several gears • Simple planetary gearset • Has sun gear, planetary pinions, carrier, and a ring gear • Several types: compound, Simpson, Ravigneaux, and tandem

  11. Simple Planetary Operation • Basic gear rules • Two gears with external teeth in mesh rotate in opposite directions • Two gears in mesh, one with internal and one with external teeth, rotate in same direction • Forward gear reduction • Turn sun gear while holding ring gear • Holding sun gear while turning ring gear • Reverse • Use only the rear gearset

  12. Compound Planetary Operation (Simpson) • Double reverse: Simpson geartrain low-gear operation • Results in forward operation • Ravigneaux operation: two sun gears, two sets of pinions, and a ring gear • Large and small sun gears • Six planetary pinions: three long and three short • Lepelletier geartrain • Combines different planetary arrangements

  13. Driving and Holding Devicesand Clutches • Planetary gearset • One member held and another is driven • Fluid clutch • Holds rotating member to input shaft • Bands and clutches operate when fluid pressure applied • Multiple disc clutches • Used for holding or driving • Steels: held against one element of clutch pack • Friction discs are splined to corresponding part

  14. Clutch Operation • Hydraulic pressure not directed at the clutch: clutch releases • Friction discs and steels turn independently • Driving clutch engaged: fluid directed into clutch drum • Fluid pressure is applied to large piston on inside of drum • Piston is applied against discs to compress springs and lock clutch through pressure plate • Pressure is released: piston is pushed away

  15. One-Way Clutches • Characteristics • Holds part of planetary gearset from turning • Commonly used in drive low gear • Have inner and outer race and a set of springs and rollers • Sprag clutch: different-shaped locking device between inner and outer races • Mechanical diode: used in some torque converters and transmissions

  16. Bands and Accumulator • Steel straps with friction lining on inside • Single or double wrap • Double wrap bands are used for low and reverse • Servo operates a band • Fluid pressure is directed into the servo’s cylinder to apply the band • During shifts: some parts held and others driven • Shuddering or damage result if two components applied at same time • Accumulator has piston and reservoir that must fill before pressure applied to driving device

  17. Hydraulic System and Fluid Pump • Hydraulic system • Makes fluid pressure that transmits power through the torque converter • Fluid pump does several things • Creates hydraulic pressure • Lubricates transmission parts • Fills torque converter • Circulates fluid throughout transmission • Pressure operates valves

  18. Types of Pumps • Three types • Rotor type • Internal/external gear crescent type • Vane type

  19. Transmission Valves and Pressure Regulator • Spool valves • Lands and valleys control fluid flow • Valves can be moved • Done by spring, lever or rod, or hydraulic pressure • Pressure regulator valve • Determines pressure in the system • Orifice restricts fluid flow • Also reduces pressure of moving fluid

  20. Hydraulic Valve Body • Valve body • Senses engine load and adjusts shift points and fluid pressure • Usually bolted to bottom of transmission inside of the pan • Spacer plate fits between transmission and valve body • Manual control valve is attached to shift lever • Shift quadrant tells the gear the transmission is in • Shift order always PRNDL or PRNDD2L

  21. Transmission Automatic Shift Selection • Transmission selects correct gear range based on engine load and vehicle speed • Upshift: transmission shifts to higher gear • Downshift: transmission shifts to lower gear • Throttle pressure • Results when engine vacuum changes • Governor pressure • Results from increase in vehicle speed

  22. Governor and Vacuum Modulator • Governor • Located on output shaft • Variable-pressure relief valve • Pressure is no greater than line pressure • Vacuum modulator valve • Controls throttle pressure • Has diaphragm and hose fitting attached to vacuum source at intake manifold

  23. Kickdown Valve • Either manually operated by cable or electrically operated solenoid • Causes throttle pressure to go to highest point • Spring loaded: extends from the valve body side • Contacts lever that applies it from outside the transmission • Bushings are made of bronze alloy or steel with soft bearing surface • Thrust washers control end play • Snap rings maintain part position on a shaft

  24. Automatic Transmission Fluid and Automatic Transmission Cooling • ATF is oil • Specially formulated for automatic transmissions • Cooling • Transmission develops heat during operation • Heat damages transmission fluid • Most transmissions have a fluid cooler • Results of radiator heat exchanger leaks: • Engine running: ATF migrates into radiator • Engine off: coolant migrates into transmission

  25. Auxiliary Cooler/Heat Exchanger and Park Pawl • Auxiliary cooler/heat exchanger • Added to motor homes and vehicles that pull trailers • Resembles small radiator • Hooked into cooler line in series • Installed before the radiator cooler • Park pawl • Lever that locks transmission output shaft when shift lever is in park

  26. Electronic Automatic Transmissions and Operation • Shifts controlled by computer using engine load, vehicle speed, and other inputs • More precise control • Less expensive solenoids • Electronic transmission shifting • Driver shifts gears without assistance of a clutch • Manually overriding the computer • Computer decides shift points based on power output from the engine • Adaptive learning

  27. Electronic Torque Converter Control • Torque converter clutch • Computer controlled • Comes on after engine is warm • Typical speed required for lockup is about 40 mph • Engages if brake switch closed and throttle position sensor signal does not show a closed throttle

  28. Electronic Pressure Control • Electronic pressure control (EPC) types • On/off variable force solenoids • Pulse width modulated • Variable force solenoids • Electronic modulators • Pulse width modulation slides back and forth • Opening or closing a passage • Different types of solenoids are used • Depends on application

  29. Transmission Shift Control • Electronic shift control • Line pressure only • No governor or modulator pressure • Forward gears are controlled by computer • Reverse only works when solenoids are off • Honda/Acura and Saturn use multiple-disc hydraulic clutches and shift solenoids • Gearing similar to manual transmission • Dual clutch transmissions (DCTs) • Being used by several manufacturers

  30. Continuously Variable Transmission • Characteristics • Similar operation to variable-speed drill press • Infinite driving ratios • Increases fuel economy in the range of 25% • Engine can be run with constant rpm • Engine does not accelerate through each gear • Do not handle torque as well • Torque travels between steel cones and a steel chain • Special lubricant changes phase to a gassy solid

  31. Hybrid Planetary Transmission Operation • Hybrid planetary transaxle • Three inputs and one output • Toyota hybrid system • Two motor/generators • Hybrid motor/generators • Operate as motors when powering vehicle • Generate electricity to recharge battery pack • Some use a third electric motor on rear axle • Double regenerative braking