Automotive Transmissions
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Automotive Transmissions: Efficiently Transferring Power from Engine to Wheels By Chao-Hsu Yao Introduction Before the steam engine was invented, all of the physically demanding jobs like construction, agriculture, shipping, and even traveling, were done by strong animals or human beings themselves. The invention of the steam engine prompted the Industrial Revolution, at which time human beings started using automated machines to reduce human work load and increase job efficiency. In 1705 Thomas Newcomen invented the first version of the steam engine, which is also called atmospheric engine [2]. Figure 1 shows the animation of how the Newcomen 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 move up, which causes the pressure inside the cylinder to decrease. Gravity will push the water from the upper tank to open the left valve, and splash the water into the cylinder to cool steam. The steam inside the cylinder therefore is condensed, which turns the cylinder vacuum and sucks back the piston. The descending piston shuts two valves and finishes one cycle.
Figure 1 Newcomen Steam Engine. Image is from “Newcomen Steam Engine,” Answers Corporation. http://www.answers.com/topic/newcomen-steamengine
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, is boiled and vaporized, and turns into high pressure steam. This steam pushes the piston, generating a forward and backward motion (see http://static.howstuffworks.com/flash/steam-engine.swf for an animated picture) [3]. Because the combustion room is located outside the engine, the steam engine is also called the external combustion engine.
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According to the physics rule of motion, when an object is in static status it needs a larger force to overcome friction. When the object starts moving, the needed driving force becomes smaller and smaller, and the speed becomes faster and faster. Therefore, to move the piston in a steam engine from static position, very high pressure must be generated to push the piston. When the piston starts moving, the pressure decreases, because it is released from the exhaust by the movement of the piston, before it can be compressed into high pressure air. At low speed, the engine creates high pressure steam to push the piston, while at high speed, the steam pressure becomes low. That‟s why the old steam powered locomotives start very slowly, but still can reach a very high speed. The steam engine is very efficient at generating power based on the physics rule of motion; however, it takes awhile before the machine can reach its highest efficiency. Another drawback is that the steam engine occupies too much space. Therefore, scientists tried to develop an engine with smaller size, but that can instantly generate the power needed. The internal combustion engine, which has been used for most machinery including vehicles, was invented. Several kinds of internal combustion engines have been widely used for vehicles, for example, in the two-stroke combustion cycle, four-stroke combustion cycle, and rotary engines. The first engine to use a four-stroke combustion cycle successfully was built in 1867 by N. A. Otto [9]. The design of the internal combustion engine is much more complicated than the steam engine, however. All internal combustion engines need to go through the following procedures to finish the combustion cycle: intake, compression, combustion, and exhaust. The shock wave file http://static.howstuffworks.com/flash/engine.swf [11] shows how the four-stroke internal combustion 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 moves the piston downward again. Finally, the fired air is exhausted through „L‟ and moves the piston upward again. While fired once every two cycles for a four-stroke cycle internal combustion engine, a two-stroke combustion cycle internal engine is fired once per cycle, which can be seen on the shock wave file http://static.howstuffworks.com/flash/two-stroke.swf [10]. The internal combustion design can instantly convert the power generated by the explosion of burning fuel into high pressure air to push the piston. Unlike the steam engine, for an internal combustion engine to move the piston faster and faster, more and more fuel is needed to generate higher pressure. In other words, for an internal combustion engine, high pressure is needed to keep the piston running at a high speed, while at low speed, only low pressure is necessary. This is just opposite to the function of the steam engine. Even though it solves the dimension and slow start issues of the steam engine, the internal combustion engine generates another serious problem. When the piston is running at high speed, the pressure needed is also high, which violates the physics rule of motion. Running an engine at
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high speed with high pressure is not efficient, and also decreases the engine life. To solve this problem, the transmission system was invented. To transfer engine power efficiently, the gear ratio between the engine and wheels plays a very important role. When we use a screwdriver, the portion we hold has a larger diameter, while the portion contacting with the screw has smaller diameter. This design makes users use less force to unscrew a screw while applying force on a larger diameter portion of the screw driver. Therefore, attaching a smaller gear to the engine side and connecting it to a larger gear to deliver power to wheels helps overcome friction when moving a static vehicle. Figure 2 shows that the large gear of the wheels needs less force to drive it. However, it also shows that when the engine gear turns one circle, the wheel gear only turns about one half. The car won‟t run as fast as possible.
Figure 2
Consider the following situation from Figure 3: the wheel gear has a smaller size, which needs more force to move it while the car is static. It won‟t even be possible to move the car if the engine power is not large enough. However, when the engine gear turns 1 cycle, the wheel gear may turn 2, which makes the car run faster.
Figure 3
Based on the physics rule of motion, after the object starts moving, the driving force needed becomes 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.
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Different Kinds of Transmission Systems Used for the Automobile The most common transmission systems that have been used for the automotive industry are manual transmission, automatic transmission, semi-automatic transmission, and continuouslyvariable transmission (CVT). The first transmission invented was the manual transmission system. The driver needs to disengage the clutch (see Figure 4) to disconnect the power from the engine first, select the target gear, and engage the clutch again to perform the gear change. This will challenge a new driver. It always takes time for a new driver to get used to this skill.
Figure 4 The transmission system delivers the engine power to wheels. Image is from “How Manual Transmissions Work,” HowStuffWorks, Inc. http://auto.howstuffworks.com/transmission.htm
An automatic transmission uses a fluid-coupling torque converter to replace the clutch to avoid engaging/disengaging clutch during gear change. A completed gear set, called planetary gears, is used to perform gear ratio change instead of selecting gear manually. With the invention of the automatic transmission, a driver no longer needs to worry about gear selection during driving. It makes driving a car much easier, especially for a disabled or new driver. However, the indirect gear contact of the torque converter causes power loss during power transmission, and the complicated planetary gear structure makes the transmission heavy and easily broken. A semi-automatic transmission tries to combine the advantages of the manual and automatic transmission systems, but avoid their disadvantages. However, the complicated design of the semi-automatic transmission is still under development, and the price is not cheap. It is only used for some luxury or sports cars currently. CVT has been used for low-powered machinery like scooters for a long time due to its highly efficient gear change. However, it is a challenge to install it on high power machinery because of the strength of the driving belt. With the progress of materials technology, engineers have been successfully installed it on automobiles, making the power transmission efficient.
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Manual Transmission Manual transmission is also referred to as stick shift transmission because you need to use the transmission stick every time you change the gears. To perform the gear shift, the transmission system must first be disengaged from the engine. After the target gear is selected, the transmission and engine are engaged with each other again to perform the power transmission. Figure 5 shows the components of the manual transmission gearbox, and Figure 6 shows how the clutch works. From Figure 5, the gear selector fork can be moved forward and backward to move the collar to engage it to the tar5 Two-speed manual transmission gearbox. Image is get gears (blue). The gears (blue) are driven by Figure from “How Manual Transmissions Work,” HowStuffWorks, the engine, while the collar (purple) connects Inc. http://auto.howstuffworks.com/transmission.htm to the wheels. To engage the collar to the gear, the power from the engine must be temporarily removed 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 synchronize the spinning speed between collar 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 friction Figure 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 that the collar is meshed into the gear very smoothly without grinding (see Figure 7). For some cars without synchronizers, a driving skill called “double clutching” must be performed during gear shifting to avoid gear grinding.
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For a standard 5-speed manual transmission system, three different 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 engine, is shown in Figure 6. When the thrust pad is pushed, it lifts the pressure pad so the driven pad (clutch plate) is disengaged 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 spinning. The whole procedure is shown on the following Shockwave file: http://static.howstuffworks.com/flash/clutchfig5.swf. The driven pad is made of frictional materials like brake pads, which use friction to drive the flywheel spinning. Like the brake system, the pad will become thinner and thinner, and eventually it will need to be replaced. The life of the driven pad really depends on the driver‟s skill and how carefully the clutch is maintained. Like brake pads, if signs of wear occur, a proper adjustment is needed. Regular adjustment is the best way to keep the clutch and brakes in good shape. With the progress of technology, a new mechanism was invented called a self-adjusting clutch, which uses hydraulic pressure, instead of the traditional school linkage, to push the thrust pad [4]. This idea came from the hydraulic brake, which was first invented in 1927 by Magura located in Bad Urach, Germany and used for BMW motorcycles. The company also invented the 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 Manual changed 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/sequentialbuilt later than 10 years ago were equipped with gearbox.htm/printable
Figure 7 The synchronizer drives the gear to spin at the same speed as the collar to avoid grinding when meshing. Image is from “How Manual Transmissions Work,” HowStuffWorks, Inc. http://auto.howstuffworks.com/transmissi on.htm
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a self-adjusting clutch, which is said to last longer than 120,000 miles without breaking down.
Figure 9 The Top-View of The Standard 5-Speed Manual Transmission System. Image, from “How Manual Transmissions Work,” HowStuffWorks, Inc. http://auto.howstuffworks.com/sequentialgearbox.htm/printable
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 synchronizers 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 directly, it is necessary to shift to neutral first, release the clutch, step on the gas when down-shifting or release 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, and finally release the clutch and step on the gas [5]. This link of a video clip from youtube, 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 double clutching is no longer necessary. However, you still can use this technique when driving to enhance the synchronizers and clutch pad‟s lives.
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Automatic Transmission
Figure 10 Planetary Gear Set. Image is from “The Transmission Bible,” Carbible.com, http://www.carbibles.com/transmission_bible.html
Figure 11 Compound Planetary Gear Set. Image is from “The Transmission Bible,” Carbible.com, http://www.carbibles.com/transmission_bible.html
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 engine and clutch. The gear set is the same as those in a manual transmission box; however, a vacuum cylinder or a hydraulic cylinder is used to perform 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 changed automatically (between 1 and 2 or between 3 and 4) during driving [6].
The automatic system for current automobiles uses a planetary gear set instead of the traditional manual transmission gear set. The planetary gear set contains four parts: sun gear, planet gears, planet carrier, and ring gear (see Figure 10.) Based on this planetary set design, sun gear, planet carrier, and ring gear spin centriProQuest Discovery Guides
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fugally. By locking one of them, the planetary set can generate three different gear ratios, including one reverse gear, without engaging and disengaging the gear set. The shockwave file http://static.howstuffworks.com/flash/automatic-transmission-planetary.swf shows how this mechanism works when you click on the left buttons. Normally an automatic transmission system has two planetary gear sets with different sizes of sun gears with their planet gears intermeshed (see Figure 11.) Only one planet carrier is used to connect both sets of planet gears. This is called a compound planetary gear set. This design can generate four different gear ratios and one 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
Planet Carrier
72-tooth Ring
36-tooth Sun
0.67:1
Total 2nd
1.47:1
2nd
3rd
30- and 3672-tooth Ring 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
1.0:1
Table 1 In order to lock the gears to perform gear ratio change, a band and clutches are used. The band is connected to an actuator piston by way of a lever link (see Figure 12). The piston pushes the lever link to force the band to lock the gear.
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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 (pressure plates), and clutch discs (friction plates).
Figure 12 The structure of the actuator piston, lever link, and band system. Images are from “The Transmission Bible,” Carbible.com, http://www.carbibles.com/transmission_bible.html and Martin L. Culpepper, “2.000 How and Why Machines Work [3-3-3], http://pcsl.mit.edu/2.000/start.html
From different auto makers, several different types of automatic transmission systems were designed and developed. Figure 14 shows one of the simple automatic transmission 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 connected to the input shaft (blue), which transfers power from the engine. The planet carrier (green) is connected to the output shaft (green), which transfers power into the wheels. Based on this design, when in neutral, 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 planet gears drive the sun gear to spin freely. In this situation, the input shaft is not able to transfer power to the output shaft. When
shifting to 1st gear, the band locks the sun gear by locking the drum. The ring gear drives the planet carrier to spin. In this situation, the ring gear (input shaft) spins faster than the planet carrier (output shaft). To shift to higher gear, the band is released and the clutch is engaged to force the sun gear and planet carrier (output shaft) to spin at the same speed. The input shaft will also spin at the same speed as the output shaft, which makes the car run faster than in 1st gear. Using a compound planetary gear set generates more gear ratios with a special gear ratio, over-drive gear,
Figure 13 Clutch Packs. Image is from Charles Ofria, “A Short Course on Automatic Transmission,” The Family Car Web Magazine, http://www.familycar.com/
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whose 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 automatically? Both the band and clutch piston are pressurized by the hydraulic system (see Figure 15). The part connecting the band or clutches to the hydraulic system is called the shift valve, while the Figure 14 Planetary Gear Sets with Band and Clutches. Image is from Charles Ofria, “A Short one connecting the hyCourse on Automatic Transmission,” The Family Car Web Magazine, draulic system to the http://www.familycar.com/ output shaft is called the 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 the fluid goes through and the higher the pressure applied on the shift valve. Therefore, each band and clutch can be pushed to lock the gear based on a specific spin speed detected by the governor from the output shaft. To make the hydraulic system work efficiently, a complex maze of passages was designed to replace a large number of tubes (see Figure 17). For modern cars, an electronic controlled (computer controlled) solenoid pack is used to detect throttle position, vehicle speed, engine speed, engine load, brake pedal position, etc., and to automatically choose the best gear for a Figure 15 Hydraulic system. Image is from HowStuffWorks, Inc., http://auto.howstuffworks.com moving vehicle.
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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/
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Unlike a manual transmission system, automatic transmission doesn‟t use a clutch to disconnect power from the engine temporarily when shifting gears. Instead, a device called a torque converter was invented to prevent power from being temporarily disconnected from the engine and also to prevent 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 Figure 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 (impeller), turbine, stator, and transmission fluid. The pump is connected to the engine and transfers engine power to the transmission fluid. The fluid causes the turbine, which is connected to the input shaft, to spin. The stator is used to redirect fluid returning from the turbine before it hits the pump, again to increase the efficiency. In this design, even when the vehicle is in gear but not moving (the turbine is forced to stop), the pump can 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 modern 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 of Figure 19 The Structure of a Torque Converter. Image is from automatic transmission with a computer “What is a Torque Converter?” Pacific Driveline, controlled automatic shift. The driver can http://www.pacificdriveline.com/torque.htm switch the transmission to manual mode, which lets her shift the gear at her wish sequentially up (+) or down (-) (see Figure 20) without disengaging the clutch. This works just like a manual transmission; however, it still uses a torque converter to transfer power from the engine. Unfortunately, this is less efficient than a manual transmission. ProQuest Discovery Guides
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Figure 20 Tiptronic transmission. Image is from “Test Drive: 2002 Audi A4 1.8T,” Canadian Driver, http://www.canadiandriver.com/articles/pw/02a4cvt.htm
Continuously-Variable Transmission (CVT) The continuously-variable transmission is also an automatic transmission system, which changes the diameters of input shaft and output shaft directly, instead of going through several gears to perform gear ratio change. This design can generate an infinite number of possible gear ratios. Unlike the complicated planetary automatic transmission 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 infinite number of gear ratios design, it is said that Figure 21 Pulley Based CVT. Image is from “How 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 and two rollers (wheels) instead of a belt, compared with a pulley based CVT. Figure 22 shows that the 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 larger ProQuest Discovery Guides
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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 engines like lawn mowers or snow blowers. It was Figure 22 Nissan Extroid toroidal CVT. Image is from also used for some small engine vehicles under “How CVT Works?” HowStuffWorks Inc., 1,500 cc. With the progress of material technolhttp://auto.howstuffworks.com/cvt2.htm ogy, Nissan has been able to put a pulley based CVT into a 3,500 V6 Altima, which dramatically improves the fuel efficiency for a large engine vehicle. Because CVT is basically designed to perform automatic transmission without manually engaging/disengaging the clutch, it generates another issue: How to prevent the engine from stalling when the car is not moving? CVT has several clutch designs to achieve this goal. Figure 23 is a CVT clutch assembly normally used for a scooter. The CVT clutch for an automobile is much more complicated but the idea is similar. The CVT clutch has Figure 23 CVT Clutch Assembly for 152QMI & 157QMJ two parts, a clutch disc with frictional materials at- Engine. Image is from “CVT Transmission Parts for Taishan & Geely Scooters,” ScootUSA.com, tached on the side, and a clutch cover (drum). The Qingqi, http://www.scootusa.com/CVT-transmission.htm cover encircles the disc, with the frictional materials barely touching the inside wall. The cover is connected to the drive pulley, while the disc is connected to the engine. When the engine is in idle, the clutch disc spins slowly, and the frictional pad doesn‟t contact with the inside wall of the cover. At high speed, the centrifugal force causes the frictional pad to move away from its spinning axis to touch the inside wall of the cover, and drives 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 transmission with a torque converter. Semi-automatic Transmission A semi-automatic transmission is a very advanced system, which still uses a clutch to perform the gear shift instead of a torque converter. Unlike the manual transmission, the computer does ProQuest Discovery Guides
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all of the clutch disengaging, gear shifting, and clutch engaging. This not only makes the gear shifting faster than manual transmission, but also prevents the vehicle from stalling when the car is stationary. Like a tiptronic transmission, a semi-automatic transmission can also be switched to manual mode to perform gear shifting at the drivers‟ wish. The two most common semi-automatic transmissions are direct shift transmission (aka dual-clutch transmission) and electrohydraulic manual transmission (aka sequential transmission). The direct shift transmission was designed 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, synchronizer, 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 different 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 conventional manual transmission gear boxes in Figure 24 6-Speed Direct-Shift Gear Box. Image is from “How Dualone. To automatically shift from 1st gear clutch Transmissions Work?” HowStuffWorks Inc., http://auto.howstuffworks.com/dual-clutch-transmission.htm to 2nd gear, first the computer detects that the spinning speed of the input shaft is too high, and engages the 2nd gear‟s collar to the 2nd gear. The clutch then disengages from 1st gear‟s input shaft, and engages the 2nd gear‟s input shaft. Controlled by computer, the gear shift becomes extremely fast compared with a conventional manual transmission. Using direct contact of the clutch instead of fluid coupling also improves power transmission efficiency. Another advanced technology used for direct shift transmission allows it to perform “double clutching” by shifting the gear to neutral first, adjusting the spinning speed of the input shaft, and then shifting to the next gear. This makes gear shifting very smooth.
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Another famous semi-automatic transmission is an electrohydraulic manual transmission (aka sequential transmission). The gear set is almost the same as the conventional transmission system, except that the shifting of the selector 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, fork can move forward and backward http://www.carbibles.com/transmission_bible.html in turn, which makes gear selection sequential. Therefore, it is impossible for an electrohydraulic manual transmission to perform a gear shift from 1st to 3rd or 4th to 2nd. The shifting must be sequential, like 1st2nd3rd4th, or 4th3rd2nd1st. Comparison of Transmission Systems The traditional automatic transmission system is much more complicated than the manual transmission system. It occupies more space, so normally a small car can easily be designed up to 5-speed manually, but only up to 3- or 4speed automatically. The automatic transmission system is also heavier than the manual transmission system, which wastes more gasoline. The fluid torque converter also loses some power transferred from the engine due to 26 Collar and Fork Move When Drum is Turning. indirect gear contact. The worst part of the Figure Image is from “How Sequential Gearboxes Work?” automatic transmission is that it is easily bro- HowStuffWorks Inc., http://auto.howstuffworks.com/sequential-gearbox.htm ken due to its complicated design, and when there 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, because engaging/disengaging the clutch by foot is not necessary, and the car will never stall during gear shifting. The CVT uses solid coupling instead of fluid coupling, which not only solves the power loss problem but also prevents the clutch from engaging/disengaging to perform the gear ratio change. However, the CVT is also heavy, and the strength of the driving belt is the ProQuest Discovery Guides
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critical issue which determines the CVT‟s performance. The infinite number of gear ratios makes gear change smooth, and the result is better fuel efficiency. However, some drivers who enjoy quick acceleration may not like the CVT‟s slow and smooth acceleration. Two different semi-automatic transmission systems are the most advanced transmission systems used in race cars and luxury cars. They use computers to perform gear shift faster than conventional manual transmission, and also with better fuel economy. However, the direct shift transmission has too many machine parts, and also a complex clutch set, which makes the gear box heavy, and easily broken. The high repair cost is also an issue. The electrohydraulic manual transmission has a simpler gear structure with a rotating drum to perform gear ratio change. The drum 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. That is why it‟s also used for manual transmission on motorcycles. However, if you need to make a sudden stop at high speed, it‟s impossible to shift from 6th gear directly to 1st gear. You will need to go through all gears from 5th to 2nd. Without a clutch design, the gear shift is not smooth. 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 Systems It is likely that, with the progress of new material technology, the CVT will gradually replace the conventional automatic transmission due to its high fuel efficiency and smooth gear shift. The technology of semi-automatic transmission systems will also be improved to perform smooth gear shift and extend the cars‟ lifetime, without losing fast acceleration and fuel efficiency. The torque converter with fluid coupling may be improved, or may no longer be used for cars in the future due to its low-efficiency power transfer. The gear shift of the manual transmission will be controlled by computer instead of engaging/disengaging the clutch and moving the gear selector by hand. For some modern cars, using push-buttons behind the steering wheel, instead of a conventional shift lever or stick, also saves gear shift time when shifting manually. An adaptive transmission control (ATC) has also been invented by using a computer to recognize and memorize different drivers‟ styles, and determining the best shifting timing for different drivers. As mentioned in the Introduction, a transmission system is needed for a vehicle due to the internal combustion engine‟s property of running at high pressure at high speed but low pressure at low speed. If someday an engine with different properties is invented, the transmission system may no longer be necessary, but can still get the vehicle to reach its maximum speed in a couple of seconds.
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References 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.htm
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Automotive Transmissions: Efficiently Transferring Power from Engine to Wheels By Chao-Hsu Yao Introduction Before the steam engine was invented, all of the physically demanding jobs like construction, agriculture, shipping, and even traveling, were done by strong animals or human beings themselves. The invention of the steam engine prompted the Industrial Revolution, at which time human beings started using automated machines to reduce human work load and increase job efficiency. In 1705 Thomas Newcomen invented the first version of the steam engine, which is also called atmospheric engine [2]. Figure 1 shows the animation of how the Newcomen 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 move up, which causes the pressure inside the cylinder to decrease. Gravity will push the water from the upper tank to open the left valve, and splash the water into the cylinder to cool steam. The steam inside the cylinder therefore is condensed, which turns the cylinder vacuum and sucks back the piston. The descending piston shuts two valves and finishes one cycle.
Figure 1 Newcomen Steam Engine. Image is from “Newcomen Steam Engine,” Answers Corporation. http://www.answers.com/topic/newcomen-steamengine
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, is boiled and vaporized, and turns into high pressure steam. This steam pushes the piston, generating a forward and backward motion (see http://static.howstuffworks.com/flash/steam-engine.swf for an animated picture) [3]. Because the combustion room is located outside the engine, the steam engine is also called the external combustion engine.
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According to the physics rule of motion, when an object is in static status it needs a larger force to overcome friction. When the object starts moving, the needed driving force becomes smaller and smaller, and the speed becomes faster and faster. Therefore, to move the piston in a steam engine from static position, very high pressure must be generated to push the piston. When the piston starts moving, the pressure decreases, because it is released from the exhaust by the movement of the piston, before it can be compressed into high pressure air. At low speed, the engine creates high pressure steam to push the piston, while at high speed, the steam pressure becomes low. That‟s why the old steam powered locomotives start very slowly, but still can reach a very high speed. The steam engine is very efficient at generating power based on the physics rule of motion; however, it takes awhile before the machine can reach its highest efficiency. Another drawback is that the steam engine occupies too much space. Therefore, scientists tried to develop an engine with smaller size, but that can instantly generate the power needed. The internal combustion engine, which has been used for most machinery including vehicles, was invented. Several kinds of internal combustion engines have been widely used for vehicles, for example, in the two-stroke combustion cycle, four-stroke combustion cycle, and rotary engines. The first engine to use a four-stroke combustion cycle successfully was built in 1867 by N. A. Otto [9]. The design of the internal combustion engine is much more complicated than the steam engine, however. All internal combustion engines need to go through the following procedures to finish the combustion cycle: intake, compression, combustion, and exhaust. The shock wave file http://static.howstuffworks.com/flash/engine.swf [11] shows how the four-stroke internal combustion 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 moves the piston downward again. Finally, the fired air is exhausted through „L‟ and moves the piston upward again. While fired once every two cycles for a four-stroke cycle internal combustion engine, a two-stroke combustion cycle internal engine is fired once per cycle, which can be seen on the shock wave file http://static.howstuffworks.com/flash/two-stroke.swf [10]. The internal combustion design can instantly convert the power generated by the explosion of burning fuel into high pressure air to push the piston. Unlike the steam engine, for an internal combustion engine to move the piston faster and faster, more and more fuel is needed to generate higher pressure. In other words, for an internal combustion engine, high pressure is needed to keep the piston running at a high speed, while at low speed, only low pressure is necessary. This is just opposite to the function of the steam engine. Even though it solves the dimension and slow start issues of the steam engine, the internal combustion engine generates another serious problem. When the piston is running at high speed, the pressure needed is also high, which violates the physics rule of motion. Running an engine at
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high speed with high pressure is not efficient, and also decreases the engine life. To solve this problem, the transmission system was invented. To transfer engine power efficiently, the gear ratio between the engine and wheels plays a very important role. When we use a screwdriver, the portion we hold has a larger diameter, while the portion contacting with the screw has smaller diameter. This design makes users use less force to unscrew a screw while applying force on a larger diameter portion of the screw driver. Therefore, attaching a smaller gear to the engine side and connecting it to a larger gear to deliver power to wheels helps overcome friction when moving a static vehicle. Figure 2 shows that the large gear of the wheels needs less force to drive it. However, it also shows that when the engine gear turns one circle, the wheel gear only turns about one half. The car won‟t run as fast as possible.
Figure 2
Consider the following situation from Figure 3: the wheel gear has a smaller size, which needs more force to move it while the car is static. It won‟t even be possible to move the car if the engine power is not large enough. However, when the engine gear turns 1 cycle, the wheel gear may turn 2, which makes the car run faster.
Figure 3
Based on the physics rule of motion, after the object starts moving, the driving force needed becomes 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.
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Different Kinds of Transmission Systems Used for the Automobile The most common transmission systems that have been used for the automotive industry are manual transmission, automatic transmission, semi-automatic transmission, and continuouslyvariable transmission (CVT). The first transmission invented was the manual transmission system. The driver needs to disengage the clutch (see Figure 4) to disconnect the power from the engine first, select the target gear, and engage the clutch again to perform the gear change. This will challenge a new driver. It always takes time for a new driver to get used to this skill.
Figure 4 The transmission system delivers the engine power to wheels. Image is from “How Manual Transmissions Work,” HowStuffWorks, Inc. http://auto.howstuffworks.com/transmission.htm
An automatic transmission uses a fluid-coupling torque converter to replace the clutch to avoid engaging/disengaging clutch during gear change. A completed gear set, called planetary gears, is used to perform gear ratio change instead of selecting gear manually. With the invention of the automatic transmission, a driver no longer needs to worry about gear selection during driving. It makes driving a car much easier, especially for a disabled or new driver. However, the indirect gear contact of the torque converter causes power loss during power transmission, and the complicated planetary gear structure makes the transmission heavy and easily broken. A semi-automatic transmission tries to combine the advantages of the manual and automatic transmission systems, but avoid their disadvantages. However, the complicated design of the semi-automatic transmission is still under development, and the price is not cheap. It is only used for some luxury or sports cars currently. CVT has been used for low-powered machinery like scooters for a long time due to its highly efficient gear change. However, it is a challenge to install it on high power machinery because of the strength of the driving belt. With the progress of materials technology, engineers have been successfully installed it on automobiles, making the power transmission efficient.
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Manual Transmission Manual transmission is also referred to as stick shift transmission because you need to use the transmission stick every time you change the gears. To perform the gear shift, the transmission system must first be disengaged from the engine. After the target gear is selected, the transmission and engine are engaged with each other again to perform the power transmission. Figure 5 shows the components of the manual transmission gearbox, and Figure 6 shows how the clutch works. From Figure 5, the gear selector fork can be moved forward and backward to move the collar to engage it to the tar5 Two-speed manual transmission gearbox. Image is get gears (blue). The gears (blue) are driven by Figure from “How Manual Transmissions Work,” HowStuffWorks, the engine, while the collar (purple) connects Inc. http://auto.howstuffworks.com/transmission.htm to the wheels. To engage the collar to the gear, the power from the engine must be temporarily removed 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 synchronize the spinning speed between collar 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 friction Figure 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 that the collar is meshed into the gear very smoothly without grinding (see Figure 7). For some cars without synchronizers, a driving skill called “double clutching” must be performed during gear shifting to avoid gear grinding.
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For a standard 5-speed manual transmission system, three different 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 engine, is shown in Figure 6. When the thrust pad is pushed, it lifts the pressure pad so the driven pad (clutch plate) is disengaged 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 spinning. The whole procedure is shown on the following Shockwave file: http://static.howstuffworks.com/flash/clutchfig5.swf. The driven pad is made of frictional materials like brake pads, which use friction to drive the flywheel spinning. Like the brake system, the pad will become thinner and thinner, and eventually it will need to be replaced. The life of the driven pad really depends on the driver‟s skill and how carefully the clutch is maintained. Like brake pads, if signs of wear occur, a proper adjustment is needed. Regular adjustment is the best way to keep the clutch and brakes in good shape. With the progress of technology, a new mechanism was invented called a self-adjusting clutch, which uses hydraulic pressure, instead of the traditional school linkage, to push the thrust pad [4]. This idea came from the hydraulic brake, which was first invented in 1927 by Magura located in Bad Urach, Germany and used for BMW motorcycles. The company also invented the 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 Manual changed 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/sequentialbuilt later than 10 years ago were equipped with gearbox.htm/printable
Figure 7 The synchronizer drives the gear to spin at the same speed as the collar to avoid grinding when meshing. Image is from “How Manual Transmissions Work,” HowStuffWorks, Inc. http://auto.howstuffworks.com/transmissi on.htm
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a self-adjusting clutch, which is said to last longer than 120,000 miles without breaking down.
Figure 9 The Top-View of The Standard 5-Speed Manual Transmission System. Image, from “How Manual Transmissions Work,” HowStuffWorks, Inc. http://auto.howstuffworks.com/sequentialgearbox.htm/printable
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 synchronizers 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 directly, it is necessary to shift to neutral first, release the clutch, step on the gas when down-shifting or release 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, and finally release the clutch and step on the gas [5]. This link of a video clip from youtube, 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 double clutching is no longer necessary. However, you still can use this technique when driving to enhance the synchronizers and clutch pad‟s lives.
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Automatic Transmission
Figure 10 Planetary Gear Set. Image is from “The Transmission Bible,” Carbible.com, http://www.carbibles.com/transmission_bible.html
Figure 11 Compound Planetary Gear Set. Image is from “The Transmission Bible,” Carbible.com, http://www.carbibles.com/transmission_bible.html
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 engine and clutch. The gear set is the same as those in a manual transmission box; however, a vacuum cylinder or a hydraulic cylinder is used to perform 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 changed automatically (between 1 and 2 or between 3 and 4) during driving [6].
The automatic system for current automobiles uses a planetary gear set instead of the traditional manual transmission gear set. The planetary gear set contains four parts: sun gear, planet gears, planet carrier, and ring gear (see Figure 10.) Based on this planetary set design, sun gear, planet carrier, and ring gear spin centriProQuest Discovery Guides
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fugally. By locking one of them, the planetary set can generate three different gear ratios, including one reverse gear, without engaging and disengaging the gear set. The shockwave file http://static.howstuffworks.com/flash/automatic-transmission-planetary.swf shows how this mechanism works when you click on the left buttons. Normally an automatic transmission system has two planetary gear sets with different sizes of sun gears with their planet gears intermeshed (see Figure 11.) Only one planet carrier is used to connect both sets of planet gears. This is called a compound planetary gear set. This design can generate four different gear ratios and one 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
Planet Carrier
72-tooth Ring
36-tooth Sun
0.67:1
Total 2nd
1.47:1
2nd
3rd
30- and 3672-tooth Ring 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
1.0:1
Table 1 In order to lock the gears to perform gear ratio change, a band and clutches are used. The band is connected to an actuator piston by way of a lever link (see Figure 12). The piston pushes the lever link to force the band to lock the gear.
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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 (pressure plates), and clutch discs (friction plates).
Figure 12 The structure of the actuator piston, lever link, and band system. Images are from “The Transmission Bible,” Carbible.com, http://www.carbibles.com/transmission_bible.html and Martin L. Culpepper, “2.000 How and Why Machines Work [3-3-3], http://pcsl.mit.edu/2.000/start.html
From different auto makers, several different types of automatic transmission systems were designed and developed. Figure 14 shows one of the simple automatic transmission 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 connected to the input shaft (blue), which transfers power from the engine. The planet carrier (green) is connected to the output shaft (green), which transfers power into the wheels. Based on this design, when in neutral, 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 planet gears drive the sun gear to spin freely. In this situation, the input shaft is not able to transfer power to the output shaft. When
shifting to 1st gear, the band locks the sun gear by locking the drum. The ring gear drives the planet carrier to spin. In this situation, the ring gear (input shaft) spins faster than the planet carrier (output shaft). To shift to higher gear, the band is released and the clutch is engaged to force the sun gear and planet carrier (output shaft) to spin at the same speed. The input shaft will also spin at the same speed as the output shaft, which makes the car run faster than in 1st gear. Using a compound planetary gear set generates more gear ratios with a special gear ratio, over-drive gear,
Figure 13 Clutch Packs. Image is from Charles Ofria, “A Short Course on Automatic Transmission,” The Family Car Web Magazine, http://www.familycar.com/
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whose 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 automatically? Both the band and clutch piston are pressurized by the hydraulic system (see Figure 15). The part connecting the band or clutches to the hydraulic system is called the shift valve, while the Figure 14 Planetary Gear Sets with Band and Clutches. Image is from Charles Ofria, “A Short one connecting the hyCourse on Automatic Transmission,” The Family Car Web Magazine, draulic system to the http://www.familycar.com/ output shaft is called the 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 the fluid goes through and the higher the pressure applied on the shift valve. Therefore, each band and clutch can be pushed to lock the gear based on a specific spin speed detected by the governor from the output shaft. To make the hydraulic system work efficiently, a complex maze of passages was designed to replace a large number of tubes (see Figure 17). For modern cars, an electronic controlled (computer controlled) solenoid pack is used to detect throttle position, vehicle speed, engine speed, engine load, brake pedal position, etc., and to automatically choose the best gear for a Figure 15 Hydraulic system. Image is from HowStuffWorks, Inc., http://auto.howstuffworks.com moving vehicle.
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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/
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Unlike a manual transmission system, automatic transmission doesn‟t use a clutch to disconnect power from the engine temporarily when shifting gears. Instead, a device called a torque converter was invented to prevent power from being temporarily disconnected from the engine and also to prevent 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 Figure 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 (impeller), turbine, stator, and transmission fluid. The pump is connected to the engine and transfers engine power to the transmission fluid. The fluid causes the turbine, which is connected to the input shaft, to spin. The stator is used to redirect fluid returning from the turbine before it hits the pump, again to increase the efficiency. In this design, even when the vehicle is in gear but not moving (the turbine is forced to stop), the pump can 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 modern 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 of Figure 19 The Structure of a Torque Converter. Image is from automatic transmission with a computer “What is a Torque Converter?” Pacific Driveline, controlled automatic shift. The driver can http://www.pacificdriveline.com/torque.htm switch the transmission to manual mode, which lets her shift the gear at her wish sequentially up (+) or down (-) (see Figure 20) without disengaging the clutch. This works just like a manual transmission; however, it still uses a torque converter to transfer power from the engine. Unfortunately, this is less efficient than a manual transmission. ProQuest Discovery Guides
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Figure 20 Tiptronic transmission. Image is from “Test Drive: 2002 Audi A4 1.8T,” Canadian Driver, http://www.canadiandriver.com/articles/pw/02a4cvt.htm
Continuously-Variable Transmission (CVT) The continuously-variable transmission is also an automatic transmission system, which changes the diameters of input shaft and output shaft directly, instead of going through several gears to perform gear ratio change. This design can generate an infinite number of possible gear ratios. Unlike the complicated planetary automatic transmission 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 infinite number of gear ratios design, it is said that Figure 21 Pulley Based CVT. Image is from “How 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 and two rollers (wheels) instead of a belt, compared with a pulley based CVT. Figure 22 shows that the 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 larger ProQuest Discovery Guides
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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 engines like lawn mowers or snow blowers. It was Figure 22 Nissan Extroid toroidal CVT. Image is from also used for some small engine vehicles under “How CVT Works?” HowStuffWorks Inc., 1,500 cc. With the progress of material technolhttp://auto.howstuffworks.com/cvt2.htm ogy, Nissan has been able to put a pulley based CVT into a 3,500 V6 Altima, which dramatically improves the fuel efficiency for a large engine vehicle. Because CVT is basically designed to perform automatic transmission without manually engaging/disengaging the clutch, it generates another issue: How to prevent the engine from stalling when the car is not moving? CVT has several clutch designs to achieve this goal. Figure 23 is a CVT clutch assembly normally used for a scooter. The CVT clutch for an automobile is much more complicated but the idea is similar. The CVT clutch has Figure 23 CVT Clutch Assembly for 152QMI & 157QMJ two parts, a clutch disc with frictional materials at- Engine. Image is from “CVT Transmission Parts for Taishan & Geely Scooters,” ScootUSA.com, tached on the side, and a clutch cover (drum). The Qingqi, http://www.scootusa.com/CVT-transmission.htm cover encircles the disc, with the frictional materials barely touching the inside wall. The cover is connected to the drive pulley, while the disc is connected to the engine. When the engine is in idle, the clutch disc spins slowly, and the frictional pad doesn‟t contact with the inside wall of the cover. At high speed, the centrifugal force causes the frictional pad to move away from its spinning axis to touch the inside wall of the cover, and drives 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 transmission with a torque converter. Semi-automatic Transmission A semi-automatic transmission is a very advanced system, which still uses a clutch to perform the gear shift instead of a torque converter. Unlike the manual transmission, the computer does ProQuest Discovery Guides
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all of the clutch disengaging, gear shifting, and clutch engaging. This not only makes the gear shifting faster than manual transmission, but also prevents the vehicle from stalling when the car is stationary. Like a tiptronic transmission, a semi-automatic transmission can also be switched to manual mode to perform gear shifting at the drivers‟ wish. The two most common semi-automatic transmissions are direct shift transmission (aka dual-clutch transmission) and electrohydraulic manual transmission (aka sequential transmission). The direct shift transmission was designed 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, synchronizer, 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 different 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 conventional manual transmission gear boxes in Figure 24 6-Speed Direct-Shift Gear Box. Image is from “How Dualone. To automatically shift from 1st gear clutch Transmissions Work?” HowStuffWorks Inc., http://auto.howstuffworks.com/dual-clutch-transmission.htm to 2nd gear, first the computer detects that the spinning speed of the input shaft is too high, and engages the 2nd gear‟s collar to the 2nd gear. The clutch then disengages from 1st gear‟s input shaft, and engages the 2nd gear‟s input shaft. Controlled by computer, the gear shift becomes extremely fast compared with a conventional manual transmission. Using direct contact of the clutch instead of fluid coupling also improves power transmission efficiency. Another advanced technology used for direct shift transmission allows it to perform “double clutching” by shifting the gear to neutral first, adjusting the spinning speed of the input shaft, and then shifting to the next gear. This makes gear shifting very smooth.
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Another famous semi-automatic transmission is an electrohydraulic manual transmission (aka sequential transmission). The gear set is almost the same as the conventional transmission system, except that the shifting of the selector 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, fork can move forward and backward http://www.carbibles.com/transmission_bible.html in turn, which makes gear selection sequential. Therefore, it is impossible for an electrohydraulic manual transmission to perform a gear shift from 1st to 3rd or 4th to 2nd. The shifting must be sequential, like 1st2nd3rd4th, or 4th3rd2nd1st. Comparison of Transmission Systems The traditional automatic transmission system is much more complicated than the manual transmission system. It occupies more space, so normally a small car can easily be designed up to 5-speed manually, but only up to 3- or 4speed automatically. The automatic transmission system is also heavier than the manual transmission system, which wastes more gasoline. The fluid torque converter also loses some power transferred from the engine due to 26 Collar and Fork Move When Drum is Turning. indirect gear contact. The worst part of the Figure Image is from “How Sequential Gearboxes Work?” automatic transmission is that it is easily bro- HowStuffWorks Inc., http://auto.howstuffworks.com/sequential-gearbox.htm ken due to its complicated design, and when there 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, because engaging/disengaging the clutch by foot is not necessary, and the car will never stall during gear shifting. The CVT uses solid coupling instead of fluid coupling, which not only solves the power loss problem but also prevents the clutch from engaging/disengaging to perform the gear ratio change. However, the CVT is also heavy, and the strength of the driving belt is the ProQuest Discovery Guides
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critical issue which determines the CVT‟s performance. The infinite number of gear ratios makes gear change smooth, and the result is better fuel efficiency. However, some drivers who enjoy quick acceleration may not like the CVT‟s slow and smooth acceleration. Two different semi-automatic transmission systems are the most advanced transmission systems used in race cars and luxury cars. They use computers to perform gear shift faster than conventional manual transmission, and also with better fuel economy. However, the direct shift transmission has too many machine parts, and also a complex clutch set, which makes the gear box heavy, and easily broken. The high repair cost is also an issue. The electrohydraulic manual transmission has a simpler gear structure with a rotating drum to perform gear ratio change. The drum 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. That is why it‟s also used for manual transmission on motorcycles. However, if you need to make a sudden stop at high speed, it‟s impossible to shift from 6th gear directly to 1st gear. You will need to go through all gears from 5th to 2nd. Without a clutch design, the gear shift is not smooth. 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 Systems It is likely that, with the progress of new material technology, the CVT will gradually replace the conventional automatic transmission due to its high fuel efficiency and smooth gear shift. The technology of semi-automatic transmission systems will also be improved to perform smooth gear shift and extend the cars‟ lifetime, without losing fast acceleration and fuel efficiency. The torque converter with fluid coupling may be improved, or may no longer be used for cars in the future due to its low-efficiency power transfer. The gear shift of the manual transmission will be controlled by computer instead of engaging/disengaging the clutch and moving the gear selector by hand. For some modern cars, using push-buttons behind the steering wheel, instead of a conventional shift lever or stick, also saves gear shift time when shifting manually. An adaptive transmission control (ATC) has also been invented by using a computer to recognize and memorize different drivers‟ styles, and determining the best shifting timing for different drivers. As mentioned in the Introduction, a transmission system is needed for a vehicle due to the internal combustion engine‟s property of running at high pressure at high speed but low pressure at low speed. If someday an engine with different properties is invented, the transmission system may no longer be necessary, but can still get the vehicle to reach its maximum speed in a couple of seconds.
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References 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.htm
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