Project report on power transmission in automobiles, Bài đọc hiểu tiếng anh chuyên ngành ô tô, bao gồm tất cả các phần và hệ thống trên xe ô tô như: gầm (phanh, treo lái, hộp số), động cơ, điện. Project report on power transmission in automobiles, Bài đọc hiểu tiếng anh chuyên ngành ô tô, bao gồm tất cả các phần và hệ thống trên xe ô tô như: gầm (phanh, treo lái, hộp số), động cơ, điện.
Power Transmission in Automobiles CHAPTER 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 The Fig in right hand side (Fig 1) shows Newcomen steam engine 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 Department Of Mechanical Engineering I.A.C.R Engineering College Power Transmission in Automobiles 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, 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 Because the combustion room is located outside the engine, the steam engine is also called the external combustion engine 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 Department Of Mechanical Engineering I.A.C.R Engineering College Power Transmission in Automobiles power based on the physics rule of motion; however, it takes a while 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 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 First, the piston moves downward and at the same time gasoline is injected into the cylinder through inlet valve 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 exhaust valve and moves the piston upward again While fired once every two cycles for a four-stroke cycle internal combustion engine, a twostroke combustion cycle internal engine is fired once per cycle The internal combustion design can instantly convert the power Department Of Mechanical Engineering I.A.C.R Engineering College Power Transmission in Automobiles 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 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 Department Of Mechanical Engineering I.A.C.R Engineering College Power Transmission in Automobiles connecting it to a larger gear to deliver power to wheels helps overcome friction when moving a static vehicle The figure 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 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 cycle, the wheel gear may turn 2, which makes the car run faster Based on the physics rule of motion, after the object starts moving, the driving force needed becomes smaller Therefore, if Department Of Mechanical Engineering I.A.C.R Engineering College Power Transmission in Automobiles 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 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, Continuously-variable transmission (C.V.T.) Manual Transmission: The first transmission invented was the manual transmission system The driver needs to disengage the clutch to disconnect the Department Of Mechanical Engineering I.A.C.R Engineering College Power Transmission in Automobiles 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 Automatic Transmission: 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 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 Semi-Automatic Transmission: 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, Department Of Mechanical Engineering I.A.C.R Engineering College Power Transmission in Automobiles and the price is not cheap It is only used for some luxury or sports cars currently Continuously (C.V.T.):- Variable Transmission The Continuously Variable Transmission (C.V.T.) is a transmission in which the ratio of the rotational speeds of two shafts, as the input shaft and output shaft of a vehicle or other machine, can be varied continuously within a given range, providing an infinite number of possible ratios The other mechanical transmissions described above only allow a few different gear ratios to be selected, but this type of transmission essentially has an infinite number of ratios available within a finite range It provides even better fuel economy if the engine is constantly made run at a single speed This transmission is capable of a better user experience, without the rise and fall in speed of an engine, and the jerk felt when changing gears Department Of Mechanical Engineering I.A.C.R Engineering College Power Transmission in Automobiles CHAPTER MANUAL TRANSMISSION SYSTEM Manual transmissions also referred as stick shift transmission or just ‘stick', 'straight drive', or standard 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 Manual transmissions are characterized by gear ratios that are selectable by locking selected gear pairs to the output shaft inside the transmission Fig: The transmission system delivers the engine power to wheels The main components of manual transmission are: Clutch Gear box Department Of Mechanical Engineering I.A.C.R Engineering College Power Transmission in Automobiles U- joint Shafts Differential gear box 10 Clutch: Clutch is a device which is used in the transmission system of automobile to engage and disengage the engine to the transmission or gear box It is located between the transmission and the engine When the clutch is engaged, the power flows from the engine to the rear wheels in a rear-wheel-drive transmission and the vehicle moves When the clutch is disengaged, the power is not transmitted from the engine to the rear wheels and vehicle stops even if engine is running Department Of Mechanical Engineering I.A.C.R Engineering College Power Transmission in Automobiles 81 large number of engine revolutions producing a small number of output revolutions) for better low-speed acceleration As the car accelerates, the pulleys vary their diameter to lower the engine speed as car speed rises This is the same thing a conventional automatic or manual transmission does, but while a conventional transmission changes the ratio in stages by shifting gears, the CVT continuously varies the ratio hence its name Types of C.V.T • Variable-Diameter Pulley (V.D.P.) Or Reeves Drive In this most common CVT system, there are two V-belt pulleys that are split perpendicular to their axes of rotation, with a V-belt running between them The gear ratio is changed by moving the two sections of one pulley closer together and the two sections of the other pulley farther apart Due to the V-shaped cross section of the belt, this causes the belt to ride higher on one pulley and lower on the other Doing these changes the effective diameters of the pulleys, this changes the overall gear ratio The distance between the pulleys does not change, and neither does the length of the belt, so changing the gear ratio means both pulleys must be adjusted (one bigger, the other smaller) Department Of Mechanical Engineering I.A.C.R Engineering College Power Transmission in Automobiles 82 simultaneously to maintain the proper amount of tension on the belt • Toroidal Or Roller-Based CVT Toroidal CVTs are made up of discs and rollers that transmit power between the discs The discs can be pictured as two almost conical parts, point to point, with the sides dished such that the two parts could fill the central hole of a torus One disc is the input, and the other is the output (they not quite touch) Power is transferred from one side to the other by rollers When the roller's axis is perpendicular to the axis of the near-conical parts, it contacts the near-conical parts at same-diameter locations and thus gives a 1:1 gear ratio The roller can be moved along the axis of the near-conical parts, changing angle as needed to maintain contact This will cause the roller to contact the near-conical parts at varying and distinct diameters, giving a gear ratio of something other than 1:1 Systems may be partial or full toroidal Full toroidal systems are the most efficient design while partial toroidals may still require a torque converter, and hence lose efficiency • One disc connects to the engine This is equivalent to the driving pulley Department Of Mechanical Engineering I.A.C.R Engineering College Power Transmission in Automobiles • 83 Another disc connects to the drive shaft This is equivalent to the driven pulley • Rollers, or wheels, located between the discs act like the belt, transmitting power from one disc to the other • Infinitely Variable Transmission (I.V.T.):A specific type of C.V.T is the infinitely variable transmission (I.V.T.), in which the range of ratios of output shaft speed to input shaft speed includes a zero ratio that can be continuously approached from a defined "higher" ratio A zero output speed with a finite input speed implies an infinite input-to-output speed ratio, which can be continuously approached from a given finite input value with an IVT Low gears are a reference to low ratios of output speed to input speed This ratio is taken to the extreme with I.V.T.’s, resulting in a "neutral", or non-driving "low" gear limit, in which the output speed is zero, although, unlike neutral in a normal automotive transmission, the output torque may be Department Of Mechanical Engineering I.A.C.R Engineering College Power Transmission in Automobiles 84 non-zero: the output shaft is rigidly fixed at zero speed rather than being freely rotating Most I.V.T.’s result from the combination of a C.V.T with an epicyclic gear system (which is also known as a planetary gear system) which enforces an output shaft rotation speed which is equal to the difference between two other speeds If these two other speeds are the input and output of a C.V.T., there can be a setting of the C.V.T that results in an output speed of zero The maximum output/input ratio can be chosen from infinite practical possibilities through selection of additional input or output gear, pulley or sprocket sizes without affecting the zero output or the continuity of the whole system The I.V.T is always engaged, even during its zero output adjustment I.V.T.’s can in some implementations offer better efficiency when compared to other C.V.T.’s as in the preferred range of operation because most of the power flows through the planetary gear system and not the controlling C.V.T Torque transmission capability can also be increased There's also possibility to stage power splits for further increase in efficiency, torque transmission capability and better maintenance of efficiency of a wide gear ratio range An example of a true I.V.T is the Hydristor because the front unit connected to the engine can displace from zero to 27 cubic inches Department Of Mechanical Engineering I.A.C.R Engineering College Power Transmission in Automobiles 85 per revolution forward and zero to -10 cubic inches per revolution reverse The rear unit is capable of zero to 75 cubic inches per revolution • Ratcheting C.V.T.:The ratcheting C.V.T is a transmission that relies on static friction and is based on a set of elements that successively become engaged and then disengaged between the driving system and the driven system, often using oscillating or indexing motion in conjunction with one-way clutches or ratchets that rectify and sum only "forward" motion The transmission ratio is adjusted by changing linkage geometry within the oscillating elements, so that the summed maximum linkage speed is adjusted, even when the average linkage speed remains constant Power is transferred from input to output only when the clutch or ratchet is engaged, Department Of Mechanical Engineering I.A.C.R Engineering College Power Transmission in Automobiles 86 and therefore when it is locked into a static friction mode where the driving & driven rotating surfaces momentarily rotate together without slippage These C.V.T.’s can transfer substantial torque because their static friction actually increases relative to torque throughput, so slippage is impossible in properly designed systems Efficiency is generally high because most of the dynamic friction is caused by very slight transitional clutch speed changes The drawback to ratcheting C.V.T.’s is vibration caused by the successive transition in speed required to accelerate the element which must supplant the previously operating & decelerating, power transmitting element Ratcheting C.V.T.’s are distinguished from V.D.P.’s and rollerbased C.V.T.’s by being static friction-based devices, as opposed to being dynamic friction-based devices that waste significant energy through slippage of twisting surfaces An example of a ratcheting C.V.T is one prototyped as a bicycle transmission protected under U.S Patent #5516132 in which strong pedaling torque causes this mechanism to react against the spring, moving the ring gear/chain wheel assembly toward a concentric, lower gear position When the pedaling torque relaxes to lower levels, the transmission self-adjusts toward higher gears, accompanied by an increase in transmission vibration Department Of Mechanical Engineering I.A.C.R Engineering College Power Transmission in Automobiles • 87 Hydrostatic C.V.T.: - Hydrostatic transmissions use a variable displacement pump and a hydraulic motor All power is transmitted by hydraulic fluid These types can generally transmit more torque, but can be sensitive to contamination Some designs are also very expensive However, they have the advantage that the hydraulic motor can be mounted directly to the wheel hub, allowing a more flexible suspension system and eliminating efficiency losses from friction in the drive shaft and differential components This type of transmission is relatively easy to use because all forward and reverse speeds can be accessed using a single lever An integrated hydrostatic transaxle (I.H.T.) uses a single housing for both hydraulic elements and gear-reducing elements This type of transmission, most commonly manufactured by Hydro-Gear has been effectively applied to a variety of inexpensive and expensive versions of ridden lawn mowers and garden tractors Many versions of riding lawn mowers and garden tractors propelled by a hydrostatic transmission are capable of pulling a reverse tine tiller and even a single bladed plow One class of riding lawn mower that has recently gained in popularity with consumers is zero turning radius mowers These mowers have traditionally been powered with wheel hub mounted hydraulic motors driven by continuously variable pumps, but this Department Of Mechanical Engineering I.A.C.R Engineering College Power Transmission in Automobiles 88 design is relatively expensive Hydro-Gear, created the first costeffective integrated hydrostatic transaxle suitable for propelling consumer zero turning radius mowers Some heavy equipment may also be propelled by a hydrostatic transmission; e.g agricultural machinery including foragers combines and some tractors A variety of heavy earth-moving equipment manufactured by Caterpillar Inc., e.g compact and small wheel loaders, track type loaders and tractors, skid-steered loaders and asphalt compactors use hydrostatic transmission Hydrostatic CVTs are usually not used for extended duration high torque applications due to the heat that is generated by the flowing oil • Variable Toothed Wheel Transmission:A variable toothed wheel transmission is not a true C.V.T that can alter its ratio in infinite increments but rather approaches C.V.T capability by having a large number of ratios, typically 49 This transmission relies on a toothed wheel positively engaged with a chain where the toothed wheel has the ability to add or subtract a tooth at a time in order to alter its ratio with relation to the chain it is driving The "toothed wheel" can take on many configurations including ladder chains, drive bars and sprocket teeth The huge advantage of this type of C.V.T is that it is a positive mechanical Department Of Mechanical Engineering I.A.C.R Engineering College Power Transmission in Automobiles 89 drive and thus does not have the frictional losses and limitations of the Roller based or V.D.P C.V.T.’s The challenge in this type of C.V.T is to add or subtract a tooth from the toothed wheel in a very precise and controlled way in order to maintain synchronized engagement with the chain This type of transmission has the potential to change ratios under load because of the large number of ratios resulting in the order of 3% ratio change differences between ratios, thus a clutch or torque converter is only necessary for pull away No C.V.T.’s of this type are in commercial use probably because of above mentioned development challenge Cone CVT:This category comprises all CVTs made up of one or more conical bodies which function together along their respective generatrices in order to achieve the variation In the single cone type, there is a revolving body (a wheel) that moves on the generatrix of the cone, thereby creating the variation between the inferior and the superior diameter of the cone In a C.V.T with oscillating cones, the torque is transmitted via friction from a variable number of cones (according to the torque to be transmitted) to a central, barrel-shaped hub The side surface of the hub is convex according to a determined radius of curvature, which is smaller than the concavity radius of the cones Department Of Mechanical Engineering I.A.C.R Engineering College Power Transmission in Automobiles 90 In this way, there will be only one (theoretical) contact point between each cone and the hub A revolutionary new CVT using this technology, the Warko, was presented in Berlin during the 6th International CTI Symposium of Innovative Automotive Transmissions, on 3-7 December 2007 A particular characteristic of the Warko is the absence of a clutch: the engine is always connected to the wheels, and the rear drive is obtained by means of an epicyclic system in output This system, named “power split”, allows the condition of geared neutral or "zero Dynamic": when the engine turns (connected to the sun gear of the epicyclic system), the variator (which rotates the ring of the epicyclic system in the opposite sense to the sun gear), in a particular position of its range, will compensate for the engine rotation, having zero turns in output (planetary = the output of the system) As a consequence, the satellite gears roll within an internal ring gear Modularity, wide ratio range (= 9), high efficiency (95%), high torque capability (up to 500 Nm) and compactness (less than 36 cm length for 31 cm diameter and 60 kg) are the most important characteristics of the Warko The same device, with the same identical cone but in different configurations, covers 90% of the engines produced all over the world, with a power range that goes from 60 to 200 Hp, gasoline Department Of Mechanical Engineering I.A.C.R Engineering College Power Transmission in Automobiles 91 and diesel Because millions will be manufactured, its production costs will be comparable to mechanical transmission costs The motion is transmitted to the output shaft by means of an internal gearing The lateral surface of the hub is convex according to a given radius of curvature, which is inferior to the radius of concavity of the cones In this way, there will be only a (theoretical) contact point between a cone and the hub Since the cone can oscillate on the hub, it realizes all the possible couplings with the diameters of the same hub The contact between the satellite cones and the hub is kept and forced by a pneumatic (or hydraulic) system (not shown) which pushes all the satellite cones against the hub and the outside ring named Reaction Ring The concavity radius of the satellite cones and the convexity radius of the hub are Department Of Mechanical Engineering I.A.C.R Engineering College Power Transmission in Automobiles 92 calculated in such a way so as to keep the external diameter constant = the internal diameter of the Reaction Ring • Radial Roller C.V.T.:- The working principle of this CVT is similar to that of conventional oil compression engines, but, instead of compressing oil, common steel rollers are compressed The motion transmission between rollers and rotors is assisted by an adapted traction fluid, which ensures the proper friction between the surfaces and slows down wearing thereof Unlike other systems, the radial rollers not show a tangential speed variation (delta) along the contact lines on the rotors From this, a greater mechanical efficiency and working life are obtained The main advantages of this CVT are the manufacturing inexpensiveness and the high power efficiency Advantages of the CVT Engines not develop constant power at all speeds; they have specific speeds where torque (pulling power), horsepower (speed power) or fuel efficiency are at their highest levels Because there are no gears to tie a given road speed directly to a given engine speed, the CVT can vary the engine speed as needed to access Department Of Mechanical Engineering I.A.C.R Engineering College Power Transmission in Automobiles 93 maximum power as well as maximum fuel efficiency This allows the CVT to provide quicker acceleration than a conventional automatic or manual transmission while delivering superior fuel economy Disadvantages of the CVT The CVT's biggest problem has been user acceptance Because the CVT allows the engine to rev at any speed, the noises coming from under the hood sound odd to ears accustomed to conventional manual and automatic transmissions The gradual changes in engine note sound like a sliding transmission or a slipping clutch - signs of trouble with a conventional transmission, but perfectly normal for C.V.T Flooring an automatic car brings a lurch and a sudden burst of power, whereas CVTs provide a smooth, rapid increase to maximum power To some drivers this makes the car feel slower, when in fact a CVT will generally out-accelerate an automatic Automakers have gone to great lengths to make the CVT feel more like a conventional transmission Most CVTs are set up to creep forward when the driver takes his or her foot off the brake This provides a similar feel to a conventional automatic, and Department Of Mechanical Engineering I.A.C.R Engineering College Power Transmission in Automobiles 94 serves as an indicator that the car is in gear Other CVTs offer a "manual" mode that simulates manual gear changes Because early automotive CVTs were limited as to how much horsepower they could handle, there has been some concern about the long-term reliability of the CVT Advanced technology has made the CVT much more robust Nissan has more than a million CVTs in service around the world and uses them in powerful cars such as the 290 horsepower Maxima, and says their long-term reliability is comparable to conventional transmissions REFERENCES Prof R.B Gupta, Automobile Engineering, Satya Prakshan, New Delhi,2003 K.M Gupta, Automobile Engineering, Umesh Publications, New Delhi, 2001 R.K.Rajput, Automobile Engineering, Laxmi Publications, New Delhi, 2005 Chao-Hsu Yao , Automotive Transmissions: Efficiently Transferring Power from Engine to Wheels , ProQuest Discovery Guides, Jan 2008 Department Of Mechanical Engineering I.A.C.R Engineering College Power Transmission in Automobiles 95 Sites Visited: http://www.auto.howstuffworks.com http://www.google.com http://en.wikipedia.org http://www.csa.com/discoveryguides/discoveryguides-main.php http://www.answers.com/ 10 http://www.familycar.com/ Department Of Mechanical Engineering I.A.C.R Engineering College ... Engineering I.A.C.R Engineering College Power Transmission in Automobiles Department Of Mechanical Engineering I.A.C.R Engineering College 20 Power Transmission in Automobiles 21 Cone Clutch: Cone... Manual transmission, Automatic transmission, Semi-automatic transmission, Continuously-variable transmission (C.V.T.) Manual Transmission: The first transmission invented was the manual transmission. .. Mechanical Engineering I.A.C.R Engineering College Power Transmission in Automobiles Department Of Mechanical Engineering I.A.C.R Engineering College 17 Power Transmission in Automobiles 18 Multi-plate