1、AUTOMATIC TRANSMISSIONThe modern automatic transmission is by far，the most complicated mechanical component in todays 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
2、.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 pos
3、ition.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 unt
4、il 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 transaxl
5、e 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 th
6、e 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 fron
7、t 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
8、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。 The modern automa
9、tic 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 m
10、any 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 th
11、e 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 se
12、ts.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
13、 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 cant move.In this scenario，when we turn the ring gear，the planets will “walk” along the sun gear （ which is
14、 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 t
15、he 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 tu
16、rn 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” clut
17、ch 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 instanc
18、e，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
19、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 combinatio
20、ns 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
21、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
22、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 l
23、ike 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 firs
24、t 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
25、 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 w
26、hen 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 sp
27、eed 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 o
28、f 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 engines crankshaft and turns at engine spee
29、d.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 p
30、recisely 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
31、 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
32、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
33、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 wont engage unless the transmission
