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 itlf . A fluid coupling or torque converter is ud instead of a manually operated clutch to connect the transmission to the engine .
There are two basic types of automatic transmission bad 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 ud to nd 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 nt 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 nds 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 nt 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 ud 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 automatic transmission consists of many components and systems that designed to work together in a symphony of planetary gear ts , the hydraulic system, als 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 ud simple , generic explanation where possible to describe the systems .
Planetary gear ts
Automatic transmission contain many gears in various combinations . 北京最好的大学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 u of planetary gear ts .
The basic planetary gear t 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 ud 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 cau 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 u a planetary gear t is by locking the planet carrier from moving , then applying power to the ring gear which will cau the sun gear to turn in opposite direction giving us rever 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 ud , 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 relead , 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 relead 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 ts connected in various way to provide the different forward speeds and rever that are found in modern automatic transmission .
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 .
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 ud 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 relea 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 ca , 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 ud whereas in Low , a clutch pack or a band is ud .
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 clo to speed of the powered fan . The difference with a torque converter is that instead of using air it ud oil or transmission fluid , to be more preci .
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 precily direct the flow of oil through the converter .
With the engine running , transmission fluid is pulled into the pump ction and is pushed outward by centrifugal force until it reaches the turbine ction 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 increa . 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 increa , 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 .
Hydraulic System
The hydraulic system is a complex maze of passage and tubes that nds transmission fluid under pressure to all parts of the transmission and torque converter . Transmission fluid rves a number of purpo including : shift control ,general lubrication and transmission cooling . Unlike the engine , which us oil primary for lubrication , every aspect of transmission’s functions is dependant on a constant supply of fluid under pressure , a portion of the fluid under pressure . In order to keep the transmission at normal operating temperature a portion of the fluid is nt through one of two steel tubes to 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 average of ten quarts of fluid between the transmission are constantly , torque submerged in fluid including the clutch packs and bands . The friction surfaces on the designed to operate properly only when they are submerged in oil .
Oil Pump
The transmission oil pump ( not to be confud with the pump element inside the torque conquer converter ) is responsible for producing all the oil pressure that is required in the transmission . The oil pump is mounted to the front of the transmission ca and is directly connected to the engine crankshaft , the pump will produce pressure whenever the engine is running as long as there is a sufficient amount of transmission fluid available . The oil enters the pump through a filter that is located at the bottom of the transmission oil pan and travels up a pickup tube directly to the oil pump . The oil is then nt , under pressure to the pressure regulator , the valve body and the rest of the components , as required .
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 then activate the appropriate clutch pack or band rvo to smoothly shift to the appropriate gear for each driving situation . Each of the many valves in the valve body has a specific purpo and is named for that function . For example the 2-3 shift valve activates the 2nd gear to 3 rd gear up-shift or 3-2 shift timing valve which determines when a downshift should occur . 治疗失眠的食物
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 placed in . When you place the gear shift in drive , for instate , the manual valve directs fluid to the clutch pack(s) that activates 1st gear . It also ts force for the 1-2 shift . On computer controlled transmissions , you will also have electrical under computer control to more precily control shift points .
Computer Controls
The computer us nsors on the engine and transmission to the detect such things as throttle position , vehicle speed , engine speed , engine load , stop light switch position , to control exact shift points as well as soft of firm the shift should be . Some computerizes transmission even learn your driving style and constantly adapt to it so that every shift is timed precily when you would need it .
Becau of computer controls , sports models are coming out with the ability to take manual control of the transmission as though it were a stick shift , allowing the driver to lect gears manually . This is accomplished on some cars by passing the shift lever through a special gate , then tapping it in one direction or the other in order to up-shift or down-shift at will . The computer monitors this activity to make sure that the driver do not lect a gear that could over speed the engine and damage it .
Another advantage to the “smart” transmission is that they have a lf diagnostic mode which can detect a problem early on and warn you with an indicator light on the dash . A technician can then plug test equipment in and retrieve a list of trouble codes that pinpoint where the problem . | |
