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AUTOMATIC TRANSMISSION
The modern automatic transmission is by far , the most complicated mechanical component in today’s automobile . It is a type of transmission that sifts itself . A fluid coupling or torque converter is used instead of a manually operated clutch to connect the transmission to the engine .
There are two basic types of automatic transmission based on whether the vehicle is rear wheel drive or front wheel drive . On a rear wheel drive car , the transmission is usually mounted to the back of the engine and is located under the hump in the center of the floorboard alongside the gas pedal position . A drive shaft connects the transmission to the final drive which is located in the rear axle and is used to send power to the rear wheels . Power flow on this system is simple and straight forward going from the engine , through the torque converter , then trough the transmission and drive shaft until it reaches the final drive where it is split and sent to the two rear transmission .
On a front wheel drive car , the transmission is usually combined with the final drive to form what is called a transaxle . The engine on a front wheel drive car is usually mounted sideways in the car with the transaxle tucked under it on the side of the engine facing the rear of the car . Front axles are connected directly to the transaxle and provide power to front wheels . In this example , power floes from the engine , through the torque converter to a larger chain that sends the power through a 180 degree turn to the transmission that is along side the engine . From there , the power is routed through the transmission to the final drive where it is split and sent to the two front wheels through the drive axles .
There are a number of other arrangements including front drive vehicles where the engine is mounted front to back instead of sideways and there are other systems that drive all four wheels but the two systems described here are by far the most popular . A much less popular rear and is connected by a drive shaft to the torque converter which is still mounted on the engine . This system is found on the new Corvette and is used in order to balance the weight evenly between the front and rear wheels for improved performance and handling . Another rear drive system mounts everything , the engine , transmission and final drive in the rear . This rear engine arrangement is popular on the Porsche.
Automobile available more than a century, especially from the mass production of motor vehicles and the automotive industry since the development of large, Car has been the economic development of the world for mankind to enter the modern life and have had a tremendous impact on the immeasurable, The progress of human society has made indelible contributions to the great, epoch-making set off arevolution. From From the vehicle as a power plant using internal combustion engine to start, auto transmission has become an important component. Is Generation is widely used in automotive reciprocating piston internal combustion engine with a small size, light weight, reliable operation and the use of The advantages of convenience, but its torque and speed range of smaller changes, and complex condition requires the use of motor vehicles Traction and the speed can be considerable changes in the scope. Therefore, its performance and vehicle dynamics and economy of There are large inter-contradictions, which contradictions of modern automotive internal combustion engine by itself is insoluble. Because Here, in the automotive power train set up the transmission and main reducer in order to achieve the purpose of deceleration by moment. Speed The main function of performance: ⑴ change gear ratio of motor vehicles, and expand the wheel drive torque and rotational speed of the Fan Wai, in order to adapt to constantly changing driving cycle, while the engine in the most favorable conditions within the scope of work; ⑵ no change in the direction of engine rotation, under the premise of the realization of cars driving back; ⑶ the realization of the free, temporary Interruption of power transmission, in order to be able to start the engine, idling, etc.. Variable-speed drive transmission by the manipulation of institutions and agencies. Change the transmission ratio by way of transmission is divided into There are class-type, non-stage and multi-purpose three. Have class most widely used transmission. It uses gear drive, with a number of transmission ratio setting. Stepless transmission Continuously Variable Transmission (CVT) transmission ratio of a certain The framework of multi-level changes may be unlimited, there is a common type of power and torque (dynamic fluid-type) and so on. Continuously Variable Transmission Transmission development is the ultimate goal, because only it can make the most economical engine in working condition Can provide the best vehicle fuel economy and optimal power in order to provide the most comfortable By the feeling. Today's CVT is a typical representative of the CVT and IVT, however as a result of the reliability of Poor, non-durable materials and high cost issues, development is not very good. Comprehensive refers to transmission torque converter and the mechanical components have the level of transmission variable hydraulic mechanical Speed, the transmission ratio can be between the maximum and minimum range of a few discontinuous change for no class, but its Significantly lower transmission efficiency than the efficiency of gear drives. 2 By manipulation, transmission control type can be divided into mandatory, automatic and semi-automatic control to manipulate three - Species . Mandatory on the driver to manipulate the direct transmission gear shift control for the majority of motor vehicles used Also known as Manual Transmission Manual Transmission (MT). Automatic transmission control selection of the transmission ratio (transmission) is carried out automatically. Just add the driver to manipulate Speed pedal, you can control the speed, also known as Automatic Transmission Automatic Transmission (AT). It is According to the speed and load (throttle pedal travel) for two-parameter control, stall in accordance with the above two Parameters to automatically take-off and landing. AT and MT in common is that they are level transmission, but AT According to the speed of the speed shift automatically, you can eliminate the manual transmission "setback" of the shift feel. However, AT also have many drawbacks, such as body complex, mechanical efficiency is not high, high cost, reliability and control Sensitivity remains to be increasing . AMT (Automated Mechanical Transmission) is in the traditional dry clutch and manual transmission gear based on the transformation of form, mainly to change the part of the manual gearshift control. That is, the overall structure of the MT cases the same switch to electronically controlled automatic transmission to achieve. Semi-automatic control, there are two forms of transmission. A number of stalls is a common automatic control, and the remaining stalls manipulated by the driver; the other is pre-style, that is, pre-selected pilot stalls, the clutch pedal in the down or release the accelerator pedal, the for retirement or an electromagnetic device to shift the hydraulic device. In recent years, with advances in vehicle technology and road traffic density increased, the performance requirements of the transmission is also getting higher and higher. A large number of automotive engineers in improving the performance of automobile transmission study a great deal of effort devoted to the rapid transmission of technology development, such as AT, AMT, DCT, CVT and the emergence of IVT.
The modern automatic transmission consists of many components and systems that designed to work together in a symphony of planetary gear sets , the hydraulic system, seals and gaskets , the torque converter , the governor and the modulator or throttle cable and computer controls that has evolved over the years into what many mechanical inclined individuals consider to be an art from . Here try to used simple , generic explanation where possible to describe these systems .
1）Planetary gear sets
Automatic transmission contain many gears in various combinations . In a manual transmission , gears slide along shafts as you move the shift lever from one position to another , engaging various sizes gears as required in order to provide the correct gear ratio . In an automatic transmission , how ever , the gears are never physically moved and are always engaged to the same gears . This is accomplished through the use of planetary gear sets .
The basic planetary gear set consists of a sun gear , a ring and two or more planet gears , all remaining in constant mesh . The planet gears are connected to each other through a common carrier which allows the gears to spin on shafts called “pinions” which are attached to the carrier .
One example of a way that this system can be used is by connecting the ring gear to the input shaft coming from the engine , connecting the planet carrier to the output shaft , and locking the sun gear so that it can’t move . In this scenario , when we turn the ring gear , the planets will “walk” along the sun gear ( which is held stationary ) causing the planet carrier to turn the output shaft in the same direction as the input shaft but at a slower speed causing gear reduction ( similar to a car in first gear ) .
If we unlock the sun gear and lock any two elements together , this will cause all three elements to turn at the same speed so that to output shaft will turn at the same rate of speed as the input shaft . This is like a car that is third or high gear . Another way we can use a planetary gear set is by locking the planet carrier from moving , then applying power to the ring gear which will cause the sun gear to turn in opposite direction giving us reverse gear .
The illustration in Figure shows how the simple system described above would look in an actual transmission . The input shaft is connected to the ring gear , the output shaft is connected to the planet carrier which is also connected to a “Multi-disk” clutch pack . The sun gear is connected to drum which is also connected to the other half of the clutch pack . Surrounding the outside of the drum is a band that can be tightened around the drum when required to prevent the drum with the attached sun gear from turning .
The clutch pack is used , in this instance , to lock the planet carrier with the sun gear forcing both to turn at the same speed . If both the clutch pack and the band were released , the system would be in neutral . Turning the input shaft would turn the planet gears against the sun gear , but since noting is holding the sun gear , it will just spin free and have no effect on the output shaft . To place the unit in first gear , the band is applied to hold the sun gear from moving . To shift from first to high gear , the band is released and the clutch is applied causing the output shaft to turn at the same speed as the input shaft .
Many more combinations are possible using two or more planetary sets connected in various way to provide the different forward speeds and reverse that are found in modern automatic transmission .
2）Clutch pack
A clutch pack consists of alternating disks that fit inside a clutch drum . Half of the disks are steel and have splines that fit into groves on the inside of the drum . The other half have a friction material bonded to their surface and have splines on the inside edge that fit groves on the outer surface of the adjoining hub . There is a piston inside the drum that is activated by oil pressure at the appropriate time to squeeze the clutch pack together so that the two components become locked and turn as one .
3）One-way Clutch
A one-way clutch ( also known as a “sprag” clutch ) is a device that will allow a component such as ring gear to turn freely in one direction but not in the other . This effect is just like that bicycle , where the pedals will turn the wheel when pedaling forward , but will spin free when pedaling backward .
A common place where a one-way clutch is used is in first gear when the shifter is in the drive position . When you begin to accelerate from a stop , the transmission starts out in first gear . But have you ever noticed what happens if you release the gas while it is still in first gear ? The vehicle continues to coast as if you were in neutral . Now , shift into Low gear instead of Drive . When you let go of the gas in this case , you will feel the engine slow you down just like a standard shift car . The reason for this is that in Drive , one-way clutch is used whereas in Low , a clutch pack or a band is used .
4）Torque Converter
On automatic transmission , the torque converter takes the place of the clutch found on standard shift vehicles . It is there to allow the engine to continue running when the vehicle comes to a stop . The principle behind a torque converter is like taking a fan that is plugged into the wall and blowing air into another fan which is unplugged . If you grab the blade on the unplugged fan , you are able to hold it from turning but as soon as you let go , it will begin to speed up until it comes close to speed of the powered fan . The difference with a torque converter is that instead of using air it used oil or transmission fluid , to be more precise .
A torque converter is a lager doughnut shaped device that is mounted between the engine and the transmission . It consists of three internal elements that work together to transmit power to the transmission . The three elements of the torque converter are the pump , the Turbine , and the Stator . The pump is mounted directly to the torque housing which in turn is bolted directly to the engine’s crankshaft and turns at engine speed . The turbine is inside the housing and is connected directly to the input shaft of the transmission providing power to move the vehicle . The stator is mounted to a one-way clutch so that it can spin freely in one direction but not in the other . Each of the three elements has fins mounted in them to precisely direct the flow of oil through the converter .
With the engine running , transmission fluid is pulled into the pump section and is pushed outward by centrifugal force until it reaches the turbine section which stars it running . The fluid continues in a circular motion back towards the center of the turbine where it enters the stator . If the turbine is moving considerably slower than the pump , the fluid will make contact with the front of the stator fins which push the stator into the one way clutch and prevent it from turning . With the stator stopped , the fluid is directed by the stator fins to re-enter the pump at a “help” angle providing a torque increase . As the speed of the turbine catches up with the pump , the fluid starts hitting the stator blades on the back-side causing the stator to turn in the same direction as the pump and turbine . As the speed increase , all three elements begin to turn at approximately the same speed . Sine the ‘80s , in order to improve fuel economy , torque converters have been equipped with a lockup clutch which locks the turbine to the pump as the vehicle reaches approximately 40-50 mph . This lockup is controlled by computer and usually won’t engage unless the transmission is in 3rd or 4th gear .
5）Hydraulic System
The hydraulic system is a complex maze of passage and tubes that sends that sends transmission fluid and under pressure to all parts of the transmission and torque converter and . Transmission fluid serves a number of purpose including : shift control ,general lubrication and transmission cooling . Unlike the engine ,which uses oil primary for lubrication ,every aspect of a transmission ‘s function is dependant on a constant supply of fluid is send pressure . In order to keep the transmission at normal operating temperature , a portion of the fluid is send to through one of two steel tubes to a special chamber that is submerged in anti-freeze in the radiator . Fluid passing through this chamber is cooled and then returned to the transmission through the other steel tube . A typical transmission has an avenge of ten quarts of fluid between the transmission , torque converter , and cooler tank , In fact , most of the components of a transmission are constantly submerged in fluid including the clutch packs and bands . The friction surfaces on these parts are designed to operate properly only when they are submerged in oil .
6）Oil Pump
The transmission oil pump ( not to confused with the pump element inside the torque converter ) is responsible for producing all the oil pressure that is required in the transmission . The oil pump is mounted to front of the transmission case and is directly connected to a flange on the engine crankshaft , the pump will produce pressure whenever the engine is running as there is a sufficient amount of transmission fluid available . The oil enters the pump through a filter that is located at bottom of the transmission oil pan and travels up a pickup tube directly to the oil pump . The oil is then sent , under pressure to the pressure regulator , the valve body and the rest of the components , as required .
7）Valve Body
The valve body is the control center of the automatic transmission . It contains a maze of channels and passages that direct hydraulic fluid to the numerous valves which when activate the appropriate clutch pack of band servo to smoothly shift to the appropriate gear for each driving situation . Each of the many valves in the valve body has a specific purpose and is named for that function . For example the 2-3 shift valve activates the 2nd gear up-shift or the 3-2 shift timing valve which determines when a downshift should occur .
The most important valve and the one that you have direct control over is the manual valve. The manual valve is directly connected to the gear shift handle and covers and uncovers various passages depending on what position the gear shift is paced in . When you place the gear shift in Drive , for instance , the manual valve directs fluid to the clutch pack ( s ) that activates 1st gear . It also sets up to monitor vehicle speed and throttle position so that it can determine the optimal time and the force for the 1-2 shift . On computer controlled transmission , you will also have electrical solenoids that are mounted in the valve body to direct fluid to the appropriate clutch packs or bands under computer control to more precisely control shift points .
8）Seals and Gaskets
An automatic transmission has many seals and gaskets to control the flow of hydraulic fluid and to keep it from leaking out . There are two main external seals : the front seal and the rear seal . The front seal seals the point where the torque converter mounts to the transmission case . This seal allows fluid to freely move from the converter to the transmission but keeps the fluid from leaking out . The rear seal keeps fluid from leaking past the output shaft .
A seal is usually made of rubber ( similar to the rubber in a windshield wiper blade ) and is used to keep oil from leaking past a moving part such as a spinning shaft . In some cases , the rubber is assisted by a spring that holds he rubber in close contact with the spinning shaft .
A gasket is a type of seal used to seal two stationary parts that are fasted together . Some common gasket materials are : paper , cork , rubber , silicone and soft metal .
Aside
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