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 Front-wheel-drive vehicles 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 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 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 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 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 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 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 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

Hydraulic System and Fluid Pump 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 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 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