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© 2012 Delmar, Cengage Learning Automatic Transmission Fundamentals Chapter 73

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Automatic Transmission FundamentalsIdentify 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
© 2012 Delmar, Cengage Learning
Front-wheel-drive vehicles
Most automatic transmissions use a torque converter
Some use a dual clutch arrangement
© 2012 Delmar, Cengage Learning
Torque converter, input shaft, transmission pump, valve body, planetary holding members, etc.
Methods of transmitting power
Fluid, friction, and gears
Planetary holding members use fluid and friction
Gears transmit power and change speed and torque
© 2012 Delmar, Cengage Learning
© 2012 Delmar, Cengage Learning
Flexplate and Torque Converter
Flexplate and torque converter
Torque converter
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
© 2012 Delmar, Cengage Learning
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
© 2012 Delmar, Cengage Learning
Torque converter stator
Redirects fluid flow
Direct fluid in a smooth pattern
© 2012 Delmar, Cengage Learning
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
© 2012 Delmar, Cengage Learning
Stall speed
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
© 2012 Delmar, Cengage Learning
Planetary gears
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
© 2012 Delmar, Cengage Learning
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
Reverse
© 2012 Delmar, Cengage Learning
Compound Planetary Operation (Simpson)
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
Fluid clutch
Bands and clutches operate when fluid pressure applied
Multiple disc clutches
Steels: held against one element of clutch pack
Friction discs are splined to corresponding part
© 2012 Delmar, Cengage Learning
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
© 2012 Delmar, Cengage Learning
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
© 2012 Delmar, Cengage Learning
Single or double wrap
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
© 2012 Delmar, Cengage Learning
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
Valves can be moved
Pressure regulator valve
Orifice restricts fluid flow
© 2012 Delmar, Cengage Learning
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
© 2012 Delmar, Cengage Learning
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
Governor pressure
© 2012 Delmar, Cengage Learning
Governor and Vacuum Modulator
Vacuum modulator valve
Controls throttle pressure
Has diaphragm and hose fitting attached to vacuum source at intake manifold
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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
© 2012 Delmar, Cengage Learning
ATF is oil
Cooling
Heat damages transmission fluid
© 2012 Delmar, Cengage Learning
Auxiliary cooler/heat exchanger
Resembles small radiator
Installed before the radiator cooler
Park pawl
Lever that locks transmission output shaft when shift lever is in park
© 2012 Delmar, Cengage Learning
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
Manually overriding the computer
Computer decides shift points based on power output from the engine
Adaptive learning
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
© 2012 Delmar, Cengage Learning
On/off variable force solenoids
Opening or closing a passage
Different types of solenoids are used
Depends on application
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)
© 2012 Delmar, Cengage Learning
Infinite driving ratios
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
© 2012 Delmar, Cengage Learning
Hybrid Planetary Transmission Operation
Toyota hybrid system
Some use a third electric motor on rear axle
Double regenerative braking