Automatic Transmission Fundamentals
Chapter 73
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
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
Automatic Transmission Parts and 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
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
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
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
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
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
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
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
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
Driving and Holding Devices and 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
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
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
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 servos 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
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
Types of Pumps
Three types
Rotor type Internal/external gear crescent type Vane type
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
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
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
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
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
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
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
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
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
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
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
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
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
