Automatic Transmission Fundamentals Chapter 73 2012 Delmar Cengage

Automatic Transmission Fundamentals Chapter 73 © 2012 Delmar, Cengage Learning

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 © 2012 Delmar, Cengage Learning

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 © 2012 Delmar, Cengage Learning

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 © 2012 Delmar, Cengage Learning

© 2012 Delmar, Cengage Learning

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 © 2012 Delmar, Cengage Learning

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 © 2012 Delmar, Cengage Learning

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 © 2012 Delmar, Cengage Learning

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 © 2012 Delmar, Cengage Learning

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 © 2012 Delmar, Cengage Learning

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 © 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 • Turn sun gear while holding ring gear • Holding sun gear while turning ring gear – Reverse • Use only the rear gearset © 2012 Delmar, Cengage Learning

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 © 2012 Delmar, Cengage Learning

Driving and Holding Devices and Clutches • Planetary gearset – One member held 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 © 2012 Delmar, Cengage Learning

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 © 2012 Delmar, Cengage Learning

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 © 2012 Delmar, Cengage Learning

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 © 2012 Delmar, Cengage Learning

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 © 2012 Delmar, Cengage Learning

Types of Pumps • Three types – Rotor type – Internal/external gear crescent type – Vane type © 2012 Delmar, Cengage Learning

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 © 2012 Delmar, Cengage Learning

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 PRNDD 2 L © 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 – Results when engine vacuum changes • Governor pressure – Results from increase in vehicle speed © 2012 Delmar, Cengage Learning

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 © 2012 Delmar, Cengage Learning

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 © 2012 Delmar, Cengage Learning

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 © 2012 Delmar, Cengage Learning

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 © 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 – 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 © 2012 Delmar, Cengage 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 © 2012 Delmar, Cengage Learning

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 © 2012 Delmar, Cengage Learning

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 © 2012 Delmar, Cengage Learning

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 © 2012 Delmar, Cengage Learning

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 © 2012 Delmar, Cengage Learning