1、一 Introduction to How Automatic Transmissions WorkIf you have ever driven a car with an automatic transmission, then you know that there are two big differences between an automatic transmission and a manual transmission:There is no clutch pedal in an automatic transmission car. There is no gear shi
2、ft in an automatic transmission car. Once you put the transmission into drive, everything else is automatic. Both the automatic transmission (plus its torque converter) and a manual transmission (with its clutch) accomplish exactly the same thing, but they do it in totally different ways. It turns o
3、ut that the way an automatic transmission does it is absolutely amazing!Purpose of an Automatic TransmissionJust like that of a manual transmission, the automatic transmissions primary job is to allow the engine to operate in its narrow range of speeds while providing a wide range of output speeds.
4、Without a transmission, cars would be limited to one gear ratio, and that ratio would have to be selected to allow the car to travel at the desired top speed. If you wanted a top speed of 80 mph, then the gear ratio would be similar to third gear in most manual transmission cars. Youve probably neve
5、r tried driving a manual transmission car using only third gear. If you did, youd quickly find out that you had almost no acceleration when starting out, and at high speeds, the engine would be screaming along near the red-line. A car like this would wear out very quickly and would be nearly undrive
6、able. So the transmission uses gears to make more effective use of the engines torque, and to keep the engine operating at an appropriate speed. When towing or hauling heavy objects, your vehicles transmission can get hot enough to burn up the transmission fluid. In order to protect the transmission
7、 from serious damage, drivers who tow should buy vehicles equipped with transmission coolers. The key difference between a manual and an automatic transmission is that the manual transmission locks and unlocks different sets of gears to the output shaft to achieve the various gear ratios, while in a
8、n automatic transmission, the same set of gears produces all of the different gear ratios. The planetary gearset is the device that makes this possible in an automatic transmission The Planetary GearsetFrom left to right: the ring gear, planet carrier, and two sun gears When you take apart and look
9、inside an automatic transmission, you find a huge assortment of parts in a fairly small space. Among other things, you see: An ingenious planetary gearset A set of bands to lock parts of a gearset A set of three wet-plate clutches to lock other parts of the gearset An incredibly odd hydraulic system
10、 that controls the clutches and bands A large gear pump to move transmission fluid around The center of attention is the planetary gearset. About the size of a cantaloupe, this one part creates all of the different gear ratios that the transmission can produce. Everything else in the transmission is
11、 there to help the planetary gearset do its thing.An automatic transmission contains two complete planetary gearsets folded together into one component. See How Gear Ratios Work for an introduction to planetary gearsets. Any planetary gearset has three main components: The sun gear The planet gears
12、and the planet gears carrier The ring gear Each of these three components can be the input, the output or can be held stationary. Choosing which piece plays which role determines the gear ratio for the gearset. Clutches and Bands in an Automatic TransmissionIn this transmission, when overdrive is en
13、gaged, a shaft that is attached to the housing of the torque converter (which is bolted to the flywheel of the engine) is connected by clutch to the planet carrier. The small sun gear freewheels, and the larger sun gear is held by the overdrive band. Nothing is connected to the turbine; the only inp
14、ut comes from the converter housing. To get the transmission into overdrive, lots of things have to be connected and disconnected by clutches and bands. The planet carrier gets connected to the torque converter housing by a clutch. The small sun gets disconnected from the turbine by a clutch so that
15、 it can freewheel. The big sun gear is held to the housing by a band so that it could not rotate. Each gear shift triggers a series of events like these, with different clutches and bands engaging and disengaging. Bands In this transmission there are two bands. The bands in a transmission are, liter
16、ally, steel bands that wrap around sections of the gear train and connect to the housing. They are actuated by hydraulic cylinders inside the case of the transmission.One of the bands In the figure above, you can see one of the bands in the housing of the transmissionThe gear train is removed. The m
17、etal rod is connected to the piston, which actuates the band.Above you can see the two pistons that actuate the bands. Hydraulic pressure, routed into the cylinder by a set of valves, causes the pistons to push on the bands, locking that part of the gear train to the housing. The clutches in the tra
18、nsmission are a little more complex. In this transmission there are four clutches. Each clutch is actuated by pressurized hydraulic fluid that enters a piston inside the clutch. Springs make sure that the clutch releases when the pressure is reduced. Below you can see the piston and the clutch drum.
19、 Notice the rubber seal on the piston - this is one of the components that is replaced when your transmission gets rebuilt. The next figure shows the alternating layers of clutch friction material and steel plates. The friction material is splined on the inside, where it locks to one of the gears. T
20、he steel plate is splined on the outside, where it locks to the clutch housing. These clutch plates are also replaced when the transmission is rebuilt.The clutch plates The pressure for the clutches is fed through passageways in the shafts. The hydraulic system controls which clutches and bands are
21、energized at any given moment. Automatic Transmissions: Hydraulics, Pumps and the Governor Hydraulics The automatic transmission in your car has to do numerous tasks. You may not realizehow many different ways it operates. For instance, here are some of the features of an automatic transmission: . I
22、f the car is in overdrive (on a four-speed transmission), the transmission will automatically select the gear based on vehicle speed and throttle pedal position. .If you accelerate gently, shifts will occur at lower speeds than if you accelerate at full throttle. .If you floor the gas pedal, the tra
23、nsmission will downshift to the next lower gear. .If you move the shift selector to a lower gear, the transmission will downshift unless the car is going too fast for that gear. If the car is going too fast, it will wait until the car slows down and then downshift. .If you put the transmission in se
24、cond gear, it will never downshift or upshift out of second, even from a complete stop, unless you move the shift lever.Youve probably seen something that looks like this before. It is really the brain of the automatic transmission, managing all of these functions and more. The passageways you can s
25、ee route fluid to all the different components in the transmission. Passageways molded into the metal are an efficient way to route fluid; without them, many hoses would be needed to connect the various parts of the transmission. First, well discuss the key components of the hydraulic system; then w
26、ell see how they work together. The Pump Automatic transmissions have a neat pump, called a gear pump. The pump is usually located in the cover of the transmission. It draws fluid from a sump in the bottom of the transmission and feeds it to the hydraulic system. It also feeds the transmission coole
27、r and the torque converter. The inner gear of the pump hooks up to the housing of the torque converter, so it spins at the same speed as the engine. The outer gear is turned by the inner gear, and as the gears rotate, fluid is drawn up from the sump on one side of the crescent and forced out into th
28、e hydraulic system on the other side. The Governor The governor is a clever valve that tells the transmission how fast the car is going. It is connected to the output, so the faster the car moves, the faster the governor spins. Inside the governor is a spring-loaded valve that opens in proportion to
29、 how fast the governor is spinning - the faster the governor spins, the more the valve opens. Fluid from the pump is fed to the governor through the output shaft. The governorAutomatic Transmissions: Valves and Modulators To shift properly, the automatic transmission has to know how hard the engine
30、is working. There are two different ways that this is done. Some cars have a simple cable linkage connected to a throttle valve in the transmission. The further the gas pedal is pressed, the more pressure is put on the throttle valve. Other cars use a vacuum modulator to apply pressure to the thrott
31、le valve. The modulator senses the manifold pressure, which increases when the engine is under a greater load. The manual valve is what the shift lever hooks up to. Depending on which gear is selected, the manual valve feeds hydraulic circuits that inhibit certain gears. For instance, if the shift l
32、ever is in third gear, it feeds a circuit that prevents overdrive from engaging. hift valves supply hydraulic pressure to the clutches and bands to engage each gear. The valve body of the transmission contains several shift valves. The shift valve determines when to shift from one gear to the next.
33、For instance, the 1 to 2 shift valve determines when to shift from first to second gear. The shift valve is pressurized with fluid from the governor on one side, and the throttle valve on the other. They are supplied with fluid by the pump, and they route that fluid to one of two circuits to control
34、 which gear the car runs in. The shift valve will delay a shift if the car is accelerating quickly. If the car accelerates gently, the shift will occur at a lower speed. Lets discuss what happens when the car accelerates gently. As car speed increases, the pressure from the governor builds. This for
35、ces the shift valve over until the first gear circuit is closed, and the second gear circuit opens. Since the car is accelerating at light throttle, the throttle valve does not apply much pressure against the shift valve. When the car accelerates quickly, the throttle valve applies more pressure aga
36、inst the shift valve. This means that the pressure from the governor has to be higher (and therefore the vehicle speed has to be faster) before the shift valve moves over far enough to engage second gear. Each shift valve responds to a particular pressure range; so when the car is going faster, the
37、2-to-3 shift valve will take over, because the pressure from the governor is high enough to trigger that valve. Electronically Controlled Transmissions Electronically controlled transmissions, which appear on some newer cars, still use hydraulics to actuate the clutches and bands, but each hydraulic
38、 circuit is controlled by an electric solenoid. This simplifies the plumbing on the transmission and allows for more advanced control schemes. Electronically controlled transmissions have even more elaborate control schemes. In addition to monitoring vehicle speed and throttle position, the transmis
39、sion controller can monitor the engine speed, if the brake pedal is being pressed, and even the anti-lock braking system. Using this information and an advanced control strategy based on fuzzy logic - a method of programming control systems using human-type reasoning - electronically controlled tran
40、smissions can do things like: Downshift automatically when going downhill to control speed and reduce wear on the brakes Upshift when braking on a slippery surface to reduce the braking torque applied by the engine Inhibit the upshift when going into a turn on a winding road Lets talk about that las
41、t feature - inhibiting the upshift when going into a turn on a winding road. Lets say youre driving on an uphill, winding mountain road. When you are driving on the straight sections of the road, the transmission shifts into second gear to give you enough acceleration and hill-climbing power. When y
42、ou come to a curve you slow down, taking your foot off the gas pedal and possibly applying the brake. Most transmissions will upshift to third gear, or even overdrive, when you take your foot off the gas. Then when you accelerate out of the curve, they will downshift again. But if you were driving a manual transmission car, you would probably leave the car in the same gear the whole time. Some automatic transmissions with advanced control systems can detect this situation after you have gone around a couple of the curves, and “learn“ not to upshift again.
