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automatic transmission

  1. 1. Automatic Transmission System 1. INTRODUCTIONAutomatic transmission system shifts the gears without assistance from thedriver. They start the car moving in first and then shift into higher gears as thecar speed increases and engine load decreases. The shifts are produced byhydraulic pressure acting through the transmission fluid.The control system takes into account the engine load and in general producechanges up when the engine load is light and changes down when the engineload is heavy.The main components that make up an automatic transmission include:• The Torque Converter: This acts like a clutch to allow the vehicle to come to a stop in gear while the engine is still running.• Planetary Gear Sets: They are the mechanical systems that provide the various forward gear ratios as well as reverse.• The Hydraulic System: It uses a special transmission fluid sent under pressure to control the transmission______________________________________________________________ 1
  2. 2. Automatic Transmission System 2. TORQUE CONVERTER2.1 INTRODUCTION:On automatic transmissions,the torque converter takes theplace of the clutch found onstandard shift vehicles. It is thereto allow the engine to continuerunning when the vehicle comesto a stop.2.2 CONSTRUCTION:A torque converter is a large doughnut shaped device that is mountedbetween the engine and the transmission. As shown in the fig.1, there are fourcomponents inside the very strong housing of the torque converter:• Impeller or pump (driving element)• Turbine (driven element)• Stator (reaction member)• The one way clutchThe housing of the torque converter is bolted to the flywheel of the engine, soit turns at whatever speed the engine is running at. The pump of the torqueconverter is attached to the housing, so it also turns at the same speed as theengine. The pump inside the torque converter is a type of centrifugal pump.The pump has many curved vanes, along with an inner ring, which formpassages for the fluid to flow through. The turbine is inside the housing and is______________________________________________________________ 2
  3. 3. Automatic Transmission Systemconnected directly to the input shaft of the transmission providing power tomove the vehicle. To get maximum force on the turbine vanes when themoving fluid strikes them, the vanes are curved to reverse the direction offlow. The stator is mounted on a one-way clutch so that it can spin freely inone direction but not in the other.2.3 WORKING:As the pump spins, fluid is flung to the outside due to centrifugal force. Asfluid is flung to the outside, a vacuum is created that draws more fluid in at thecenter. The fluid then enters the blades of the turbine. Since the blades of theturbine are curved, the fluid, which enters the turbine from the outside, has tochange direction before it exits the center of the turbine. It is this directionalchange that causes the turbine to spin. The fluid exits the turbine movingopposite the direction that the pump (and engine) is turning. This is shown infig. 2. If the fluid were allowed to hit the pump, it would slow the engine down,wasting power. The stator resides in the very center of the torque converter.Its job is to redirect the fluid returning from the turbine before it hits the pumpagain. The stator has a very aggressive blade design that almost completelyreverses the direction of the fluid. Because of the one-way clutch, the statorcannot spin with the fluid (it can spin only in the opposite direction), forcing thefluid to change direction as it hits the stator blades.2.4 FLUID COUPLING PHASE:As the speed of the turbine catches up with the pump, the fluid exit the turbinein the same direction as the pump is turning, so the stator is not needed. Atthese speeds, the fluid actually strikes the back sides of the stator blades,causing the stator to freewheel on its one-way clutch so it doesnt hinder thefluid moving through it. All three now elements begin to turn at approximatelythe same speed.______________________________________________________________ 3
  4. 4. Automatic Transmission System2.5 TORQUE CONVERTER EFFICIENCY:It is seen that the efficiency of the torque converter is reasonably good at onlynarrow range of turbine speeds. The fall-off of efficiency at low speed end ofthe range can be tolerated because those speeds are used for short periods.But the fall-off of efficiency at high speeds cannot be tolerated and must becircumvented. The efficiency can be increased, by substituting a direct drivefor the torque converter at higher speeds.2.6 THE LOCKUP IN TORQUE CONVERTER (DIRECT DRIVE):Because the only connection between two sides of a torque converter is afluid connection, there is always a little slippage, running from about 2-8%. Toincrease efficiency and mileage, most modern automatic transmissions alsohave something called a lockup clutch.It works like this. As the two speed of the car reaches 50 to 60 kph, the highlypressurized transmission fluid is channeled through the transmission shaftand activates a clutch piston. This metal pin locks the turbine to the pump, ineffect bypassing the torque converter and giving a direct drive. It remains thisway until the vehicle slows below 50 kph, at which point the clutch pistondisengages and the torque converter kicks in again.______________________________________________________________ 4
  5. 5. Automatic Transmission System 3. SIMPLE PLANETARY GEAR SET fig. 3The basic planetary gear set as shown in fig. 3 consists of a sun gear, a ringgear and two or more planet gears, all remaining in constant mesh. Theplanet gears are connected to each other through a common carrier. Each ofthese three components can be the input, the output or can be heldstationary. Choosing which piece plays which role determines the gear ratiofor the gear set. Following table shows the different gear ratios possible: Input Output Stationary Gear Ratio Sun (S) Planet Carrier (C) Ring (R) 1 + R/S Planet Carrier (C) Ring (R) Sun (S) 1 / (1 + S/R) Sun (S) Ring (R) Planet Carrier (C) -R/S Table 1 4. COMPOUND PLANETARY GEAR SET______________________________________________________________ 5
  6. 6. Automatic Transmission System4.1 INTRODUCTION:The compound planetary gear set looks like a simple planetary gear set butactually behaves like two planetary gear sets combined. It has one ring gearthat is always the output of the transmission, but it has two sun gears and twosets of planets.4.2 CONSTRUCTION:Fig. 4 shows the exploded view of the compound planetary gear set. The fig.5 shows the planets in the planet carrier. The planet on the right sits lowerthan the planet on the left. The planet on the right does not engage the ringgear, it engages the other planet. Only the planet on the left engages the ringgear. The shorter gears are engaged only by the smaller sun gear. The longerplanets are engaged by the bigger sun gear and by the smaller planets.In such gear sets, the sum of number of teeth on sun gear and ring geardivided by the number of planets must be a whole number. Otherwise certaincombination of tooth numbers cannot be assembled because of need of equalspacing on the planets.4.3 GEAR RATIOS:Consider a planetary gear set with ring gear having 72 teeth the smaller sungear having 30 teeth and the larger sun gear having 36 teeth.First Gear:______________________________________________________________ 6
  7. 7. Automatic Transmission SystemIn first gear, the smaller sun gear is driven clockwise by the turbine in thetorque converter. The planet carrier tries to spin counterclockwise, but is heldstill by the one-way clutch (which only allows rotation in the clockwisedirection) and the ring gear turns the output. Referring to table 1, the gearratio is: -R/S = - 72/30 = -2.4:1So the rotation is negative 2.4:1. But the output direction is really the same asthe input direction. This is due to the two sets of planets. The first set ofplanets engages the second set, and the second set turns the ring gear; thiscombination reverses the direction. This would also cause the bigger sun gearto spin; but because that clutch is released, the bigger sun gear is free to spinin the opposite direction of the turbine (counterclockwise).Second Gear:This acts like two planetary gear sets connected to each other with a commonplanet carrier. The first stage of the planet carrier actually uses the larger sungear as the ring gear. So the first stage consists of the sun (the smaller sungear), the planet carrier, and the ring (the larger sun gear). The input is thesmall sun gear; the ring gear (large sun gear) is held stationary by the band,and the output is the planet carrier. For this stage, with the sun as input,planet carrier as output, and the ring gear fixed, referring to table 1 the gearratio is: 1 + R/S = 1 + 36/30 = 2.2:1The planet carrier turns 2.2 times for each rotation of the sun gear. At thesecond stage, the planet carrier acts as the input for the second planetarygear set, the larger sun gear (which is held stationary) acts as the sun, andthe ring gear acts as the output, so referring to table 1, the gear ratio is: 1 / (1 + S/R) = 1 / (1 + 36/72) = 0.67:1To get the overall reduction for second gear, we multiply the first stage by thesecond, 2.2 x 0.67, to get a 1.47:1 reduction.______________________________________________________________ 7
  8. 8. Automatic Transmission SystemThird Gear:Most automatic transmissions have a 1:1 ratio in third gear. All we have to dois engage the clutches that lock each of the sun gears to the turbine. If bothsun gears turn in the same direction, the planet gears lockup because theycan only spin in opposite directions. This locks the ring gear to the planetsand causes everything to spin as a unit, producing a 1:1 ratio.Overdrive:By definition, an overdrive has a faster output speed than input speed. Its aspeed increase. When overdrive is engaged, a shaft that is attached to thehousing of the torque converter (which is bolted to the flywheel of the engine)is connected by clutch to the planet carrier. The small sun gear freewheels,and the larger sun gear is held by the overdrive band. Nothing is connected tothe turbine; the only input comes from the converter housing. With the planetcarrier for input, the sun gear fixed and the ring gear for output, referring totable 1 the gear ratio is: 1 / (1 + S/R) = 1 / (1 + 36/72) = 0.67:1Reverse:Reverse is very similar to first gear, except that instead of the small sun gearbeing driven by the torque converter turbine, the bigger sun gear is driven,and the small one freewheels in the opposite direction. The planet carrier isheld by the reverse band to the housing. So, referring to table 1, the gear ratiois: -R/S = -72/36 = -2.0:1So the ratio in reverse is a little less than first gear. 5. HYDRAULIC SYSTEM______________________________________________________________ 8
  9. 9. Automatic Transmission System5.1 INTRODUCTION:The hydraulic system provides the pressurized fluid to operate an automatictransmission.Major components of the hydraulic system include the bands, clutches and oilpump. Other major components are the governor, throttle valve, modulatorand the valve body.5.2 FUNCTIONS OF THE HYDRAULIC SYSTEM:The effective operation of an automatic transmission relies upon a hydrauliccontrol system to actuate the gear changes relative to vehicle’s road speedand acceleration pedal demands with engine delivering power.The system performs the following functions, • Supplies fluid to the torque converter. • Directs pressurized fluid to the bands and clutches. • Lubricates the internal parts.5.3 TRANSMISSION FLUID:Transmission fluid serves a number of purposes including: • Shift control. • General lubrication • Transmission cooling.______________________________________________________________ 9
  10. 10. Automatic Transmission SystemUnlike the engine, which uses oil primarily for lubrication, every aspect of atransmissions functions is dependant on a constant supply of fluid underpressure. This is not unlike the human circulatory system where even a fewminutes of operation when there is a lack of pressure can be harmful or evenfatal to the life of the transmission. A typical transmission has an average often litres of fluid between the transmission, torque converter, and cooler tank.In fact, most of the components of a transmission are constantly submergedin fluid including the clutch packs and bands. The friction surfaces on theseparts are designed to operate properly only when they are submerged in oil.5.4 COOLING THE TRANSMISSION FLUID:In order to keep the transmission at normal operating temperature, a portionof the fluid is sent through one of two steel tubes to a special chamber thatis submerged in the radiator. Fluid passing through this chamber is cooledand then returned to the transmission through the other steel tube.______________________________________________________________ 10
  11. 11. Automatic Transmission System 6. PARTS OF THE HYDRAULIC SYSTEM6.1 INTRODUCTION:For the change of gears, lots of things have to be connected anddisconnected. The clutches connect different members to be driven and thebands hold the required member stationary. The hydraulic system controlswhich clutches and bands are energized at any given moment. The hydraulicsystem receives information from the governor and throttle cable or vacuummodulator.6.2 CLUTCHES:A clutch consists of alternatingdisks that fit inside a clutch drum.As shown in fig. 6, half of thedisks are steel and have splinesthat lock on the inside of thedrum. The other half have afriction material bonded to theirsurface and have splines on theinside edge that lock onto one ofthe gears. There is a piston insidethe drum that is activated by oil pressure at the appropriate time to squeezethe clutch pack together so that the two components become locked and turnas one.______________________________________________________________ 11
  12. 12. Automatic Transmission System6.3 BANDS:A band is a steel strap with friction materialbonded to the inside surface. Fig. 7 shows theband and its servo. One end of the band isanchored against the transmission case whilethe other end is connected to a servo. At theappropriate time hydraulic oil is sent to theservo under pressure to tighten the bandaround the drum to stop it from turning, thuslocking that part of the gear train to the casing.6.4 OIL PUMP:The automatic transmission systems use a gear pump. The gear pump isresponsible for producing all the oil pressure that is required in thetransmission. The oil pump is mounted to the front of the transmission caseand is directly connected to a flange on the torque converter housing. Sincethe torque converter housing is directly connected to the engine crankshaft,the pump will produce pressure whenever the engine is running as long asthere is a sufficient amount of transmission fluid available. The oil enters thepump through a filter that is located at the bottom of the transmission oil panand travels up a pickup tube directly to the oil pump. The oil is then sent,under pressure to the pressure regulator, the valve body and the rest of thecomponents, as required.______________________________________________________________ 12
  13. 13. Automatic Transmission System6.5 PRESSURE REGULATOR:The pump’s output pressure will increase roughly in proportion to the engine’sspeed. However, the pressure necessary to actuate the various valves and toenergise the clutch and band servo pistons will vary under different workconditions. Therefore the fluid pressure generated by the pump, is unlikely tosuit the many operating requirements. To overcome these difficulties, apressure regulating valve is used which automatically adjusts the pump’soutput pressure to match the working requirements at any one time. Thepressure regulating valve is normally a spring-loaded spool type valve.As pump pressure builds up with rising engine speed, line pressure isconveyed to the rear face of the plunger and will progressively move theplunger forward against a control spring, causing the exhaust port to beuncovered, which feeds back to the pump intake. Hence as the pump outputpressure tends to rise, more fluid is passed back to the suction intake of thepump, thus regulating the fluid pressure. To enable the line pressure to bevaried to suit the operating conditions, a throttle pressure is introduced to thespring end of the plunger, which opposes the line pressure.6.6 GOVERNOR:The governor tells the transmission how fast the car is moving. The governoris connected to the output shaft and regulates hydraulic pressure based onvehicle speed. It accomplishes this using centrifugal force to spin a pair ofweights against pull-back springs. As the weights pull further out against thesprings, more oil pressure is allowed past the governor to act on the shiftvalves that are in the valve body which then signal the appropriate shifts.______________________________________________________________ 13
  14. 14. Automatic Transmission System6.7 THROTTLE CABLE AND VACUUM MODULATOR:Vehicle speed is not the only thing that controls when a transmission shouldshift, the load that the engine is under is also important. The more loads youplace on the engine, the longer the transmission will hold a gear beforeshifting to the next one.The throttle valve and modulator serve the purpose of monitoring engine load.A transmission will use one or the other but generally not both of thesedevices. Each works in a different way to monitor engine load.The Throttle Cable simply monitors the position of the accelerator pedalthrough a cable that runs from the gas pedal to the throttle valve in the valvebody. The further the gas pedal is pressed, the more pressure is put on thethrottle valve.Engine vacuum reacts very accurately to engine load with high vacuumproduced when the engine is under light load and diminishing down to zerovacuum when the engine is under a heavy load. The vacuum modulator isattached to the outside of the transmission case and has a shaft which passesthrough the case and attaches to the throttle valve in the valve body.______________________________________________________________ 14
  15. 15. Automatic Transmission System 7. VALVE BODY7.1 INTRODUCTION:The valve body is the brain of the automatic transmission. It contains a mazeof channels and passages that direct hydraulic fluid to the numerous valveswhich then activate the appropriate clutch pack or band servo to smoothlyshift to the appropriate gear for each driving situation. Each of the manyvalves in the valve body has a specific purpose and is named for that function.For example the 1-2 shift valve activates the 1 st gear to 2nd gear up-shift.7.2 THE MANUAL VALVE:The manual valve is directly connected to the gear shift handle and coversand uncovers various passages depending on what position the gear shift isplaced in. If the gear shift lever is moved in the 1 st gear or 2nd gear position,the up-shift and down-shift are inhibited by the manual valve.7.3 SHIFT VALVES:Shift valves, as shown in fig. 8 supply hydraulic pressure to the clutches andbands to engage each gear. The valve body of the transmission containsseveral shift valves. The shift valve determines when to shift from one gear tothe next. The shift valve is pressurized with fluid from the governor on oneside, and the throttle valve on the other. The shift valve will delay a shift if thecar is accelerating quickly. If the car accelerates gently, the shift will occur at alower speed.______________________________________________________________ 15
  16. 16. Automatic Transmission SystemWhen the car accelerates gently, as car speed increases, the pressure fromthe governor builds. This forces the shift valve over until the first gear circuit isclosed, and the second gear circuit opens. Since the car is accelerating atlight throttle, the throttle valve does not apply much pressure against the shiftvalve.When the car accelerates quickly, the throttle valve applies more pressureagainst the shift valve. This means that the pressure from the governor has tobe higher (and therefore the vehicle speed has to be faster) before the shiftvalve moves over far enough to engage second gear.Each shift valve responds to a particular pressure range; so when the car isgoing faster, the 2-to-3 shift valve will take over, because the pressure fromthe governor is high enough to trigger that valve.______________________________________________________________ 16
  17. 17. Automatic Transmission System 8. ADVANTAGES AND DISADVANTAGES8.1 ADVANTAGES:1. It minimizes driver fatigue, especially in heavy traffic by eliminating the need to operate the clutch pedal and gear lever for starting from rest and changing gear.2. It contributes to safer driving because the concentration of the driver is not disturbed by the need to change gear; also, both hands can remain on the steering wheel.3. Progress can be smoother under normal driving conditions, because gear changes will occur at the theoretically correct moment in terms of road speed and throttle opening.8.2 DISADVANTAGES:1. Cars with automatic transmission are costlier than those having manual transmission.2. Fuel economy of cars with automatic transmission is not very good.______________________________________________________________ 17
  18. 18. Automatic Transmission System 9. CONCLUSION • The automatic transmission with its torque converter and planetary gear set, can successfully replace the clutch and the manual transmission gear box. • The planetary gear set gives the required gear ratios and the hydraulic system controls the planetary gear set. • Automatic transmission system shifts the gears automatically, depending upon both, speed of vehicle and load on the engine. • However in cases like, descending hills, when it is desirable to employ a lower gear, though the load on the engine maybe nil or the engine maybe acting as a brake, the human element has to be retained in control.______________________________________________________________ 18
  19. 19. Automatic Transmission System REFERENCES 1. William H. Crouse & Donald L. Anglin, “Automotive Automatic Transmissions”, Tata McGraw Hill Publishing Co., Sixth Edition, 1996. 2. John Fenton, “Handbook of Automotive Powertrains and Chassis Design”, Professional Engineering Publishing Ltd., First Published 1998. 3. Heinz Heisler, “Advanced Vehicle Technology”, Butterworth- Heinemann Publishers, Second Edition 2002. 4. 5. 6. www.edmunds.com______________________________________________________________ 19
  20. 20. Automatic Transmission System REFERENCES 1. William H. Crouse & Donald L. Anglin, “Automotive Automatic Transmissions”, Tata McGraw Hill Publishing Co., Sixth Edition, 1996. 2. John Fenton, “Handbook of Automotive Powertrains and Chassis Design”, Professional Engineering Publishing Ltd., First Published 1998. 3. Heinz Heisler, “Advanced Vehicle Technology”, Butterworth- Heinemann Publishers, Second Edition 2002. 4. 5. 6. www.edmunds.com______________________________________________________________ 19