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  1. 1. Discovery Guides Automotive Transmissions: Efficiently Transferring Power from Engine to Wheels By Chao-Hsu YaoIntroductionBefore the steam engine was invented, all of thephysically demanding jobs like construction, agri-culture, shipping, and even traveling, were done bystrong animals or human beings themselves. Theinvention of the steam engine prompted the Indus-trial Revolution, at which time human beingsstarted using automated machines to reduce humanwork load and increase job efficiency. In 1705Thomas Newcomen invented the first version of thesteam engine, which is also called atmospheric en-gine [2]. Figure 1 shows the animation of how theNewcomen steam engine works. From this design,water (blue) is boiled and vaporized into steam(pink), which pushes the closed right valve (red)open (green). The steam pushes the piston to moveup, which causes the pressure inside the cylinder todecrease. Gravity will push the water from the Figure 1 Newcomen Steam Engine. Image is fromupper tank to open the left valve, and splash the “Newcomen Steam Engine,” Answers Corporation.water into the cylinder to cool steam. The steam http://www.answers.com/topic/newcomen-steam- engineinside the cylinder therefore is condensed, whichturns the cylinder vacuum and sucks back the pis-ton. The descending piston shuts two valves andfinishes one cycle.The Newcomen Steam Engine was only used to pump water out of mines at that time. In 1769,James Watt improved the function of the steam engine and made it practical in the real world [1],which is why most people still think Watt invented the steam engine.James Watt‟s steam engine is designed so that water goes into a high temperature boiler, isboiled and vaporized, and turns into high pressure steam. This steam pushes the piston, generat-ing a forward and backward motion (see http://static.howstuffworks.com/flash/steam-engine.swffor an animated picture) [3]. Because the combustion room is located outside the engine, thesteam engine is also called the external combustion engine.©2008 ProQuest Released January 2008 1
  2. 2. Yao: Automotive TransmissionsAccording to the physics rule of motion, when an object is in static status it needs a larger forceto overcome friction. When the object starts moving, the needed driving force becomes smallerand smaller, and the speed becomes faster and faster. Therefore, to move the piston in a steamengine from static position, very high pressure must be generated to push the piston. When thepiston starts moving, the pressure decreases, because it is released from the exhaust by themovement of the piston, before it can be compressed into high pressure air. At low speed, theengine creates high pressure steam to push the piston, while at high speed, the steam pressurebecomes low. That‟s why the old steam powered locomotives start very slowly, but still canreach a very high speed.The steam engine is very efficient at generating power based on the physics rule of motion; how-ever, it takes awhile before the machine can reach its highest efficiency. Another drawback isthat the steam engine occupies too much space. Therefore, scientists tried to develop an enginewith smaller size, but that can instantly generate the power needed. The internal combustion en-gine, which has been used for most machinery including vehicles, was invented. Several kinds ofinternal combustion engines have been widely used for vehicles, for example, in the two-strokecombustion cycle, four-stroke combustion cycle, and rotary engines. The first engine to use afour-stroke combustion cycle successfully was built in 1867 by N. A. Otto [9]. The design of theinternal combustion engine is much more complicated than the steam engine, however. All inter-nal combustion engines need to go through the following procedures to finish the combustioncycle: intake, compression, combustion, and exhaust. The shock wave filehttp://static.howstuffworks.com/flash/engine.swf [11] shows how the four-stroke internal com-bustion engine works. First, gasoline comes from „C‟ and moves the piston downward. Second,the piston moves upward and compresses the air. Third, the compressed air is fired and movesthe piston downward again. Finally, the fired air is exhausted through „L‟ and moves the pistonupward again. While fired once every two cycles for a four-stroke cycle internal combustion en-gine, a two-stroke combustion cycle internal engine is fired once per cycle, which can be seen onthe shock wave file http://static.howstuffworks.com/flash/two-stroke.swf [10]. The internal com-bustion design can instantly convert the power generated by the explosion of burning fuel intohigh pressure air to push the piston. Unlike the steam engine, for an internal combustion engineto move the piston faster and faster, more and more fuel is needed to generate higher pressure. Inother words, for an internal combustion engine, high pressure is needed to keep the piston run-ning at a high speed, while at low speed, only low pressure is necessary. This is just opposite tothe function of the steam engine.Even though it solves the dimension and slow start issues of the steam engine, the internal com-bustion engine generates another serious problem. When the piston is running at high speed, thepressure needed is also high, which violates the physics rule of motion. Running an engine atProQuest Discovery Guideshttp://www.csa.com/discoveryguides/discoveryguides-main.php Released January 2008 2
  3. 3. Yao: Automotive Transmissionshigh speed with high pressure is not efficient, and also decreases the engine life. To solve thisproblem, the transmission system was invented.To transfer engine power efficiently, the gear ratio between the engine and wheels plays a veryimportant role. When we use a screwdriver, the portion we hold has a larger diameter, while theportion contacting with the screw has smaller diameter. This design makes users use less force tounscrew a screw while applying force on a larger diameter portion of the screw driver. There-fore, attaching a smaller gear to the engine side and connecting it to a larger gear to deliverpower to wheels helps overcome friction when moving a static vehicle. Figure 2 shows that thelarge gear of the wheels needs less force to drive it. However, it also shows that when the enginegear turns one circle, the wheel gear only turns about one half. The car won‟t run as fast as pos-sible. Figure 2Consider the following situation from Figure 3: the wheel gear has a smaller size, which needsmore force to move it while the car is static. It won‟t even be possible to move the car if the en-gine power is not large enough. However, when the engine gear turns 1 cycle, the wheel gearmay turn 2, which makes the car run faster. Figure 3Based on the physics rule of motion, after the object starts moving, the driving force needed be-comes smaller. Therefore, if the car can run on the large gear condition (Figure 2) when starting,but change to a small gear (Figure 3) when moving, that is, applying a large force when starting,but a small force when moving, this will makes the power transmission much more efficient.ProQuest Discovery Guideshttp://www.csa.com/discoveryguides/discoveryguides-main.php Released January 2008 3
  4. 4. Yao: Automotive TransmissionsDifferent Kinds of Transmission Systems Used for the AutomobileThe most common transmission systems that have been used for the automotive industry aremanual transmission, automatic transmission, semi-automatic transmission, and continuously-variable transmission (CVT).The first transmission inventedwas the manual transmissionsystem. The driver needs todisengage the clutch (see Fig-ure 4) to disconnect the powerfrom the engine first, select thetarget gear, and engage theclutch again to perform the Figure 4 The transmission system delivers the engine power to wheels. Image isgear change. This will chal- from “How Manual Transmissions Work,” HowStuffWorks, Inc. http://auto.howstuffworks.com/transmission.htmlenge a new driver. It alwaystakes time for a new driver toget used to this skill.An automatic transmission uses a fluid-coupling torque converter to replace the clutch to avoidengaging/disengaging clutch during gear change. A completed gear set, called planetary gears, isused to perform gear ratio change instead of selecting gear manually. With the invention of theautomatic transmission, a driver no longer needs to worry about gear selection during driving. Itmakes driving a car much easier, especially for a disabled or new driver. However, the indirectgear contact of the torque converter causes power loss during power transmission, and the com-plicated planetary gear structure makes the transmission heavy and easily broken.A semi-automatic transmission tries to combine the advantages of the manual and automatictransmission systems, but avoid their disadvantages. However, the complicated design of thesemi-automatic transmission is still under development, and the price is not cheap. It is only usedfor some luxury or sports cars currently.CVT has been used for low-powered machinery like scooters for a long time due to its highlyefficient gear change. However, it is a challenge to install it on high power machinery because ofthe strength of the driving belt. With the progress of materials technology, engineers have beensuccessfully installed it on automobiles, making the power transmission efficient.ProQuest Discovery Guideshttp://www.csa.com/discoveryguides/discoveryguides-main.php Released January 2008 4
  5. 5. Yao: Automotive TransmissionsManual TransmissionManual transmission is also referred to as stickshift transmission because you need to use thetransmission stick every time you change thegears. To perform the gear shift, the transmis-sion system must first be disengaged from theengine. After the target gear is selected, thetransmission and engine are engaged with eachother again to perform the power transmission.Figure 5 shows the components of the manualtransmission gearbox, and Figure 6 shows howthe clutch works. From Figure 5, the gear se-lector fork can be moved forward and back-ward to move the collar to engage it to the tar-get gears (blue). The gears (blue) are driven by Figure 5 Two-speed manual transmissionHowStuffWorks, is gearbox. Image from “How Manual Transmissions Work,”the engine, while the collar (purple) connects Inc. http://auto.howstuffworks.com/transmission.htmto the wheels. To engage the collar to the gear, the power from the engine must be temporarilyremoved so the gear will lose the driving force. Therefore, the gear and the collar can be engaged without causing grinding due to different spin speeds. However, the collar and gear still have different spinning speeds due to the gear ratio change when shifting. For the modern car, a mechanism called a synchronizer is added to the collar to syn- chronize the spinning speed between col- lar and gears to make the gears mesh smoothly. The synchronizer is made of frictional materials. When the collar tries to mesh with the gear, the synchronizer will touch the gear first and use frictionFigure 6 Clutch. Image is from Research Machines plc force to drive the gear to spin at the same speed as the collar. This will ensure thatthe collar is meshed into the gear very smoothly without grinding (see Figure 7). For some carswithout synchronizers, a driving skill called “double clutching” must be performed during gearshifting to avoid gear grinding.ProQuest Discovery Guideshttp://www.csa.com/discoveryguides/discoveryguides-main.php Released January 2008 5
  6. 6. Yao: Automotive Transmissions For a standard 5-speed manual transmission system, three dif- ferent gear selector forks are used. Figure 8 and Figure 9 show the side-view and top-view of the respective positions of three forks. This “H” shift pattern enables the driver to select fivc different gear ratios and a reverse gear. The clutch, used to temporarily disconnect power from the en- gine, is shown in Figure 6. When the thrust pad is pushed, it lifts the pressure pad so the driven pad (clutch plate) is disen- gaged from the flywheel. The flywheel, which is connected to the gears of the transmission system, will stop spinning. On the other hand, when the thrust pad is released, the spring pushes the driven pad back to engage the flywheel and drives it spin- ning. The whole procedure is shown on the following Shock- wave file: http://static.howstuffworks.com/flash/clutch- fig5.swf. The driven pad is made of frictional materials like brake pads,Figure 7 The synchronizer drives the which use friction to drive the flywheel spinning. Like thegear to spin at the same speed as thecollar to avoid grinding when meshing.Image is from “How Manual brake system, the pad will become thinner and thinner, andTransmissions Work,” HowStuffWorks,eventually it will need to be replaced. The life of the driven padInc. really depends on the driver‟s skill and how carefully the clutchhttp://auto.howstuffworks.com/transmission.htm is maintained. Like brake pads, if signs of wear occur, a proper adjustment is needed. Regular adjustment is the best way tokeep the clutch and brakes in good shape. Withthe progress of technology, a new mechanismwas invented called a self-adjusting clutch,which uses hydraulic pressure, instead of thetraditional school linkage, to push the thrust pad[4]. This idea came from the hydraulic brake,which was first invented in 1927 by Magura lo-cated in Bad Urach, Germany and used forBMW motorcycles. The company also inventedthe first “adjust-on-the-fly” clutch in 1968 [12].Regular adjustment will no longer be necessary,but transmission oil must still be frequently Figure 8 The Side-View of a Standard 5-Speed Manualchanged to enhance the transmission‟s life. Al- Transmission System. Image is from “How Manual Transmissions Work,” HowStuffWorks, Inc.most all of the manual transmission vehicles http://auto.howstuffworks.com/sequential-built later than 10 years ago were equipped with gearbox.htm/printableProQuest Discovery Guideshttp://www.csa.com/discoveryguides/discoveryguides-main.php Released January 2008 6
  7. 7. Yao: Automotive Transmissions a self-adjusting clutch, which is said to last longer than 120,000 miles without breaking down. Basically, the gear is shifted by depressing the clutch and releasing the gas, shifting the gear, and releasing the clutch and stepping on the gas. For some very old cars, the transmission collars aren‟t equipped with synchronizers. For some heavy-duty trucks, the syn- chronizers are removed in order to carry more heavy products. For some rally race cars, to reduce gear shift time, the synchronizers are removed so the driver can quickly move the stick into the target gear. To avoid gear grinding, a technique called “double clutching” is always used. To perform double clutching, instead of shifting to the target gear di- rectly, it is necessary to shift to neutral first, release the clutch, step on the gas when down-shifting or re- lease the gas when up-shifting so the gear can reach the same spin speed as the collar, depress the clutch again and release the gas, shift to the target gear, andFigure 9 The Top-View of The Standard 5-SpeedManual Transmission System. Image, from “How finally release the clutch and step on the gas [5]. ThisManual Transmissions Work,” HowStuffWorks, Inc. link of a video clip from youtube,http://auto.howstuffworks.com/sequential-gearbox.htm/printable http://www.youtube.com/watch?v=p--jE1cfhj0, shows how to do double clutching when driving a 1960 GMC bus.Most modern manual transmission cars have been equipped with synchronizers so doubleclutching is no longer necessary. However, you still can use this technique when driving to en-hance the synchronizers and clutch pad‟s lives.ProQuest Discovery Guideshttp://www.csa.com/discoveryguides/discoveryguides-main.php Released January 2008 7
  8. 8. Yao: Automotive TransmissionsAutomatic Transmission Driving a manual transmission vehicle is not an easy job for a beginner, and it takes time for a new driver to attain the skill. Improper driving always causes the car to stall, and could damage the transmission system. For some disabled people who are not able to use both legs, driving a manual transmission car is impossible. In 1941, Chrysler introduced the first automatic transmission system, which included a fluid coupling between en- gine and clutch. The gear set is the same as those in a manual transmis-Figure 10 Planetary Gear Set. Image is from “The Transmission Bible,”Carbible.com, http://www.carbibles.com/transmission_bible.html sion box; however, a vacuum cylinder or a hydraulic cylinder is used to per- form automatic gear shifting. The clutch selects the gear range only but isn‟t used when driving. The first range (or low range) contains the 1st and 2nd gears, while the second range (or high range) contains 3rd and 4th gears. To move the car, the clutch and brake must be depressed and a gear range must be selected (low, high, or reverse.) After the gear range is selected, the clutch can be released. To move the car, the brake is released and the gas is stepped on. The gear is changedFigure 11 Compound Planetary Gear Set. Image is from “The Transmission automatically (between 1 and 2 orBible,” Carbible.com, http://www.carbibles.com/transmission_bible.html between 3 and 4) during driving [6]. The automatic system for currentautomobiles uses a planetary gear set instead of the traditional manual transmission gear set. Theplanetary gear set contains four parts: sun gear, planet gears, planet carrier, and ring gear (seeFigure 10.) Based on this planetary set design, sun gear, planet carrier, and ring gear spin centri-ProQuest Discovery Guideshttp://www.csa.com/discoveryguides/discoveryguides-main.php Released January 2008 8
  9. 9. Yao: Automotive Transmissionsfugally. By locking one of them, the planetary set can generate three different gear ratios, in-cluding one reverse gear, without engaging and disengaging the gear set. The shockwave filehttp://static.howstuffworks.com/flash/automatic-transmission-planetary.swf shows how thismechanism works when you click on the left buttons. Normally an automatic transmission sys-tem has two planetary gear sets with different sizes of sun gears with their planet gears inter-meshed (see Figure 11.) Only one planet carrier is used to connect both sets of planet gears. Thisis called a compound planetary gear set. This design can generate four different gear ratios andone reverse gear. The inputs, outputs, and gear ratios are summarized in Table 1 [7]. Gear Input Output Fixed Gear Ratio 1st 30-tooth Sun 72-tooth Ring Planet Carrier 2.4:1 30-tooth Sun Planet Carrier 36-tooth Ring 2.2:1 2nd Planet Carrier 72-tooth Ring 36-tooth Sun 0.67:1 Total 2nd 1.47:1 30- and 36- 3rd 72-tooth Ring 1.0:1 tooth Suns Over Drive or 4th Planet Carrier 72-tooth Ring 36-tooth Sun 0.67:1 Reverse 36-tooth Sun 72-tooth Ring Planet Carrier -2.0:1 Table 1In order to lock the gears to perform gear ratio change, a band and clutches are used. The band isconnected to an actuator piston by way of a lever link (see Figure 12). The piston pushes thelever link to force the band to lock the gear.ProQuest Discovery Guideshttp://www.csa.com/discoveryguides/discoveryguides-main.php Released January 2008 9
  10. 10. Yao: Automotive Transmissions The clutch lock is much more complicated than the band lock. A set of clutches is shown in Figure 13. It is constructed with a clutch housing (drum), clutch plates (pres- sure plates), and clutch discs (friction plates). From different auto makers, several differ- ent types of automatic transmission systems were designed and developed. Figure 14 shows one of the simple automatic trans- mission system designs. From this design, the sun gear (yellow) is connected to a drum (yellow), which can be locked by a band (red). The ring gear (blue) is directly con- nected to the input shaft (blue), which trans- fers power from the engine. The planet car- rier (green) is connected to the output shaft (green), which transfers power into the wheels. Based on this design, when in neu- tral, both band and clutch sets are released. Turning the ring gear can only drive planet gears but not the planet carrier, which stays static if the car is not moving. The planetFigure 12 The structure of the actuator piston, lever link, and band gears drive the sun gear to spin freely. Insystem. Images are from “The Transmission Bible,” Carbible.com,http://www.carbibles.com/transmission_bible.html and Martin L. this situation, the input shaft is not able toCulpepper, “2.000 How and Why Machines Work [3-3-3], transfer power to the output shaft. Whenhttp://pcsl.mit.edu/2.000/start.htmlshifting to 1st gear, the band locks the sun gear bylocking the drum. The ring gear drives the planet car-rier to spin. In this situation, the ring gear (input shaft)spins faster than the planet carrier (output shaft). Toshift to higher gear, the band is released and the clutchis engaged to force the sun gear and planet carrier(output shaft) to spin at the same speed. The inputshaft will also spin at the same speed as the outputshaft, which makes the car run faster than in 1st gear. Figure 13 Clutch Packs. Image is from CharlesUsing a compound planetary gear set generates more Ofria, “A Short Course on Automatic Transmission,” The Family Car Web Magazine,gear ratios with a special gear ratio, over-drive gear, http://www.familycar.com/ProQuest Discovery Guideshttp://www.csa.com/discoveryguides/discoveryguides-main.php Released January 2008 10
  11. 11. Yao: Automotive Transmissionswhose gear ratio is small than 1. This will make the gear shift more smooth [8]. How can the band and clutches perform the lock function to select the right gear auto- matically? Both the band and clutch piston are pressurized by the hydraulic system (see Figure 15). The part connecting the band or clutches to the hydrau- lic system is called the shift valve, while theFigure 14 Planetary Gear Sets with Band and Clutches. Image is from Charles Ofria, “A Short one connecting the hy-Course on Automatic Transmission,” The Family Car Web Magazine,http://www.familycar.com/ draulic system to the output shaft is calledthe governor (see Figure 16). The governor is a centrifugal sensor with a spring loaded valve.The faster the governor spins, the more the valve opens. The more the valve opens, the more thefluid goes through and the higher the pressure applied on the shift valve. Therefore, each bandand clutch can be pushed to lock thegear based on a specific spin speed de-tected by the governor from the outputshaft. To make the hydraulic systemwork efficiently, a complex maze ofpassages was designed to replace alarge number of tubes (see Figure 17).For modern cars, an electronic con-trolled (computer controlled) solenoidpack is used to detect throttle position,vehicle speed, engine speed, engineload, brake pedal position, etc., and toautomatically choose the best gear for a Figure 15 Hydraulic system. Image is from HowStuffWorks, Inc.,moving vehicle. http://auto.howstuffworks.comProQuest Discovery Guideshttp://www.csa.com/discoveryguides/discoveryguides-main.php Released January 2008 11
  12. 12. Yao: Automotive Transmissions Figure 16 Governor. Image is from HowStuffWorks, Inc., http://auto.howstuffworks.com Figure 17 Hydraulic System with Maze Structure. Image is from Charles Ofria, “A Short Course on Automatic Transmission,” The Family Car Web Magazine, http://www.familycar.com/ProQuest Discovery Guideshttp://www.csa.com/discoveryguides/discoveryguides-main.php Released January 2008 12
  13. 13. Yao: Automotive TransmissionsUnlike a manual transmission system, automatic transmission doesn‟t use a clutch to disconnectpower from the engine temporarily when shifting gears. Instead, a device called a torque con- verter was invented to prevent power from being tem- porarily disconnected from the engine and also to pre- vent the vehicle from stalling when the transmission is in gear. Consider two fans facing each other: when one of them is turned on and starts spinning, the other one will also start spinning at a lower speed (see Fig- ure 18). That‟s because the first fan moves the air to drive the second fan to spin. This is the same idea as the torque converter of an automatic transmission system, except that it uses fluid instead of air as the Figure 18 Image is from Charles Ofria, “A Short transportation media. Course on Automatic Transmission,” The Family Car Web Magazine, http://www.familycar.com/ A torque converter consists of four parts, a pump (im- peller), turbine, stator, and transmission fluid. Thepump is connected to the engine and transfers engine power to the transmission fluid. The fluidcauses the turbine, which is connected to the input shaft, to spin. The stator is used to redirectfluid returning from the turbine before it hits the pump, again to increase the efficiency. In thisdesign, even when the vehicle is in gear but not moving (the turbine is forced to stop), the pumpcan still keep spinning without causing the engine to stall. When the vehicle speed is slow, the turbine is always spinning more slowly than the pump. However, when the vehicle moves at high speed, the turbine can spin at almost the same speed as the pump. Therefore, for mod- ern cars, a “lock up” will occur between pump and turbine when the vehicle is at high speed in fourth gear, which makes the torque converter work more efficiently. Tiptronic transmission is a special type ofFigure 19 The Structure of a Torque Converter. Image is from automatic transmission with a computer“What is a Torque Converter?” Pacific Driveline,http://www.pacificdriveline.com/torque.htm controlled automatic shift. The driver can switch the transmission to manual mode,which lets her shift the gear at her wish sequentially up (+) or down (-) (see Figure 20) withoutdisengaging the clutch. This works just like a manual transmission; however, it still uses a torqueconverter to transfer power from the engine. Unfortunately, this is less efficient than a manualtransmission.ProQuest Discovery Guideshttp://www.csa.com/discoveryguides/discoveryguides-main.php Released January 2008 13
  14. 14. Yao: Automotive Transmissions Figure 20 Tiptronic transmission. Image is from “Test Drive: 2002 Audi A4 1.8T,” Canadian Driver, http://www.canadiandriver.com/articles/pw/02a4cvt.htmContinuously-Variable Transmission (CVT)The continuously-variable transmission is also an automatic transmission system, which changesthe diameters of input shaft and output shaft directly, instead of going through several gears toperform gear ratio change. This design can generate an infinite number of possible gear ratios. Unlike the complicated planetary automatic transmis- sion system, a CVT only has three major parts; a drive pulley connected to the input shaft, a driven pulley connected to the output shaft, and a belt. Figure 21 shows the structure of the most common pulley based CVT systems. If two cones of the pulley fall apart, the diameter of the pulley becomes small; while they are close, the diameter of the pulley is large. Because the length of the driving belt is fixed, when the diameter of the drive pulley becomes small, the diameter of the driven pulley must become large by closing two cones of the pulley together, and vice versa. Based on this Figure 21 Pulley Based CVT. Image is from “How infinite number of gear ratios design, it is said that CVT Works?” HowStuffWorks Inc., CVT is the most efficient transmission system in the http://auto.howstuffworks.com/cvt2.htm world.Another popular CVT is Nissan Extroid toroidal CVT. It uses two discs instead of pulleys andtwo rollers (wheels) instead of a belt, compared with a pulley based CVT. Figure 22 shows thatthe two rollers spin around the horizontal axis, and also tilt in or out around the vertical axis.When the two rollers tilt to the upper disc in Figure 22, they touch the upper disc with a largerProQuest Discovery Guideshttp://www.csa.com/discoveryguides/discoveryguides-main.php Released January 2008 14
  15. 15. Yao: Automotive Transmissions diameter, while touching the bottom disc with a smaller diameter. This works according to the same idea as pulley based CVT, by generating an infinite number of gear ratios. Even though the pulley based CVT is currently the most common CVT in the world, the belt is under extremely large stress between pulleys when they are spinning. Therefore, in the past the CVT could only be used for some low torque en- gines like lawn mowers or snow blowers. It wasFigure 22 Nissan Extroid toroidal CVT. Image is from also used for some small engine vehicles under“How CVT Works?” HowStuffWorks Inc.,http://auto.howstuffworks.com/cvt2.htm 1,500 cc. With the progress of material technol- ogy, Nissan has been able to put a pulley basedCVT into a 3,500 V6 Altima, which dramatically improves the fuel efficiency for a large enginevehicle.Because CVT is basically designed to performautomatic transmission without manually engag-ing/disengaging the clutch, it generates another is-sue: How to prevent the engine from stalling whenthe car is not moving? CVT has several clutch de-signs to achieve this goal. Figure 23 is a CVTclutch assembly normally used for a scooter. TheCVT clutch for an automobile is much more com-plicated but the idea is similar. The CVT clutch has Figure 23 CVT Clutch Assembly for 152QMI & 157QMJtwo parts, a clutch disc with frictional materials at- Engine. Image is from “CVT Transmission Parts fortached on the side, and a clutch cover (drum). The Qingqi, Taishan & Geely Scooters,” ScootUSA.com, http://www.scootusa.com/CVT-transmission.htmcover encircles the disc, with the frictional materialsbarely touching the inside wall. The cover is connected to the drive pulley, while the disc is con-nected to the engine. When the engine is in idle, the clutch disc spins slowly, and the frictionalpad doesn‟t contact with the inside wall of the cover. At high speed, the centrifugal force causesthe frictional pad to move away from its spinning axis to touch the inside wall of the cover, anddrives the cover to spin. That‟s why for most of CVT vehicles, when the gear is shifted to “D”,the car is still not moving when the engine is in idle, which is different from an automatic trans-mission with a torque converter.Semi-automatic TransmissionA semi-automatic transmission is a very advanced system, which still uses a clutch to performthe gear shift instead of a torque converter. Unlike the manual transmission, the computer doesProQuest Discovery Guideshttp://www.csa.com/discoveryguides/discoveryguides-main.php Released January 2008 15
  16. 16. Yao: Automotive Transmissionsall of the clutch disengaging, gear shifting, and clutch engaging. This not only makes the gearshifting faster than manual transmission, but also prevents the vehicle from stalling when the caris stationary. Like a tiptronic transmission, a semi-automatic transmission can also be switchedto manual mode to perform gear shifting at the drivers‟ wish. The two most common semi-auto-matic transmissions are direct shift transmission (aka dual-clutch transmission) and electrohy-draulic manual transmission (aka sequential transmission). The direct shift transmission was de- signed and developed by the Audi and Volkswagen auto groups. Figure 24 shows the structure of the direct shift transmission. Like a conventional manual transmission system, it uses a collar, syn- chronizer, and gear set to perform gear shift. The clutch set is like that inside the automatic planetary gear transmission system, which controls the gear ratio change. Unlike the conventional manual transmission system, there are two differ- ent gear/collar sets, with each connected to two different input/output shafts. One set contains odd (1st, 3rd, 5th…) gears, while the other contains even (2nd, 4th, 6th…) gears. It is just like two conven- tional manual transmission gear boxes inFigure 24 6-Speed Direct-Shift Gear Box. Image is from “How Dual-clutch Transmissions Work?” HowStuffWorks Inc., one. To automatically shift from 1st gearhttp://auto.howstuffworks.com/dual-clutch-transmission.htm to 2nd gear, first the computer detectsthat the spinning speed of the input shaft is too high, and engages the 2nd gear‟s collar to the 2ndgear. The clutch then disengages from 1st gear‟s input shaft, and engages the 2nd gear‟s inputshaft. Controlled by computer, the gear shift becomes extremely fast compared with a conven-tional manual transmission. Using direct contact of the clutch instead of fluid coupling also im-proves power transmission efficiency. Another advanced technology used for direct shift trans-mission allows it to perform “double clutching” by shifting the gear to neutral first, adjusting thespinning speed of the input shaft, and then shifting to the next gear. This makes gear shiftingvery smooth.ProQuest Discovery Guideshttp://www.csa.com/discoveryguides/discoveryguides-main.php Released January 2008 16
  17. 17. Yao: Automotive Transmissions Another famous semi-automatic trans- mission is an electrohydraulic manual transmission (aka sequential transmis- sion). The gear set is almost the same as the conventional transmission sys- tem, except that the shifting of the se- lector is not an „H‟ pattern. Instead, all selector forks are connected to a drum (see Figure 25). The drum has several grooves, and each has a ball sliding in it. Each fork hooks up to a ball and can be moved forward and backward when the drum is turning (see Figure 26). Based on the pattern of the grooves on Figure 25 Electrohydraulic Manual Transmission. Image is from “The the drum, by turning the drum, each Transmission Bible,” Carbible.com, http://www.carbibles.com/transmission_bible.html fork can move forward and backward in turn, which makes gear selection se-quential. Therefore, it is impossible for an electrohydraulic manual transmission to perform agear shift from 1st to 3rd or 4th to 2nd. The shifting must be sequential, like1st2nd3rd4th, or 4th3rd2nd1st.Comparison of Transmission SystemsThe traditional automatic transmission systemis much more complicated than the manualtransmission system. It occupies more space,so normally a small car can easily be designedup to 5-speed manually, but only up to 3- or 4-speed automatically. The automatic transmis-sion system is also heavier than the manualtransmission system, which wastes moregasoline. The fluid torque converter also losessome power transferred from the engine due toindirect gear contact. The worst part of the Figureis from “How Sequential Gearboxes Work?” Image 26 Collar and Fork Move When Drum is Turning.automatic transmission is that it is easily bro- HowStuffWorks Inc., http://auto.howstuffworks.com/sequential-gearbox.htmken due to its complicated design, and whenthere is a problem it is very costly to fix.However, an automatic transmission system is easy for a new driver to learn to use quickly, be-cause engaging/disengaging the clutch by foot is not necessary, and the car will never stall dur-ing gear shifting. The CVT uses solid coupling instead of fluid coupling, which not only solvesthe power loss problem but also prevents the clutch from engaging/disengaging to perform thegear ratio change. However, the CVT is also heavy, and the strength of the driving belt is theProQuest Discovery Guideshttp://www.csa.com/discoveryguides/discoveryguides-main.php Released January 2008 17
  18. 18. Yao: Automotive Transmissionscritical issue which determines the CVT‟s performance. The infinite number of gear ratios makesgear change smooth, and the result is better fuel efficiency. However, some drivers who enjoyquick acceleration may not like the CVT‟s slow and smooth acceleration.Two different semi-automatic transmission systems are the most advanced transmission systemsused in race cars and luxury cars. They use computers to perform gear shift faster than conven-tional manual transmission, and also with better fuel economy. However, the direct shift trans-mission has too many machine parts, and also a complex clutch set, which makes the gear boxheavy, and easily broken. The high repair cost is also an issue. The electrohydraulic manualtransmission has a simpler gear structure with a rotating drum to perform gear ratio change. Thedrum can be rotated manually, or by computer control. This makes the gear shift extremely fast,and due to its simple structure, the gear box is not heavy, and only occupies a small space. Thatis why it‟s also used for manual transmission on motorcycles. However, if you need to make asudden stop at high speed, it‟s impossible to shift from 6th gear directly to 1st gear. You willneed to go through all gears from 5th to 2nd. Without a clutch design, the gear shift is notsmooth. That‟s why it is only used for some high speed race cars like Toyota MR2 and Ferrari,which only consider speed but not comfort.The Future Development of Automotive Transmission SystemsIt is likely that, with the progress of new material technology, the CVT will gradually replace theconventional automatic transmission due to its high fuel efficiency and smooth gear shift. Thetechnology of semi-automatic transmission systems will also be improved to perform smoothgear shift and extend the cars‟ lifetime, without losing fast acceleration and fuel efficiency. Thetorque converter with fluid coupling may be improved, or may no longer be used for cars in thefuture due to its low-efficiency power transfer. The gear shift of the manual transmission will becontrolled by computer instead of engaging/disengaging the clutch and moving the gear selectorby hand. For some modern cars, using push-buttons behind the steering wheel, instead of a con-ventional shift lever or stick, also saves gear shift time when shifting manually. An adaptivetransmission control (ATC) has also been invented by using a computer to recognize and memo-rize different drivers‟ styles, and determining the best shifting timing for different drivers. Asmentioned in the Introduction, a transmission system is needed for a vehicle due to the internalcombustion engine‟s property of running at high pressure at high speed but low pressure at lowspeed. If someday an engine with different properties is invented, the transmission system mayno longer be necessary, but can still get the vehicle to reach its maximum speed in a couple ofseconds.ProQuest Discovery Guideshttp://www.csa.com/discoveryguides/discoveryguides-main.php Released January 2008 18
  19. 19. Yao: Automotive TransmissionsReferences 1. “Steam Engine Invented,” Antique Farming. http://www.antiquefarming.com/steamengine.html 2. “How Steam Engines Work?” HowStuffWorks, Inc. http://www.howstuffworks.com/steam.htm 3. “Steam Engine Operation, “ HowStuffWorks, Inc. http://static.howstuffworks.com/flash/steam-engine.swf 4. “Self-Adjusting Clutch Mechanism,” PatentStorm LLC. http://www.patentstorm.us/patents/5456345-fulltext.html 5. “Double Clutch,” Wikipedia. http://en.wikipedia.org/wiki/Double_clutch 6. “Semi-Automatic Transmission,” Wikipedia Foundation, Inc. http://en.wikipedia.org/wiki/Semi-automatic_transmission 7. “How Automatic Transmission Works?” HowStuffWorks, Inc. http://auto.howstuffworks.com/automatic-transmission3.htm 8. Charles Ofria, “A Short Course on Automatic Transmission,” The Family Car Web. Magazine, http://www.familycar.com/ 9. “Internal-Combustion Engine,” Answers Corporation http://www.answers.com/internal%20combustion%20engine 10. “How Two-Stroke Engines Work?” HowStuffWorks, Inc. http://science.howstuffworks.com/two-stroke2.htm 11. “How Car Engines Work?” HowStuffWorks, Inc. http://auto.howstuffworks.com/engine1.htm 12. “Magura Motorcycle Components,” Sand Wizards Racing. http://www.sandwizards.com/Magura_review.htmProQuest Discovery Guideshttp://www.csa.com/discoveryguides/discoveryguides-main.php Released January 2008 19

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