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1、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 shift in an automati
2、c 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 out that the way a
3、n automatic transmission does it is absolutely amazing!In this article, well work our way through an automatic transmission. Well start with the key to the whole system: planetary gear sets. Then well see how the transmission is put together, learn how the controls work and discuss some of the intri
4、cacies involved in controlling a transmission.Some BasicsJust 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. Without a transmission, cars would be limited to on
5、e 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 never tried driving a manual transmission car using onl
6、y 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 able. So the transmission uses gears to make more
7、effective use of the engines torque, and to keep the engine operating at an appropriate speed. 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
8、an automatic transmission, the same set of gears produces all of the different gear ratios. The planetary gear set is the device that makes this possible in an automatic transmission. GearsThis automatic transmission uses a set of gears, called a compound planetary gear set, that looks like a single
9、 planetary gear set but actually behaves like two planetary gear sets combined. It has one ring gear that is always the output of the transmission, but it has two sun gears and two sets of planets. Lets look at some of the parts: First GearIn first gear, the smaller sun gear is driven clockwise by t
10、he turbine in the torque converter. The planet carrier tries to spin counterclockwise, but is held still by the one-way clutch (which only allows rotation in the clockwise direction) and the ring gear turns the output. The small gear has 30 teeth and the ring gear has 72, so the gear ratio is: Ratio
11、 = -R/S = - 72/30 = -2.4:1So the rotation is negative 2.4:1, which means that the output direction would be opposite the input direction. But the output direction is really the same as the input direction - this is where the trick with the two sets of planets comes in. The first set of planets engag
12、es the second set, and the second set turns the ring gear; this combination reverses the direction. You can see that this would also cause the bigger sun gear to spin; but because that clutch is released, the bigger sun gear is free to spin in the opposite direction of the turbineSecond GearThis tra
13、nsmission does something really neat in order to get the ratio needed for second gear. It acts like two planetary gear sets connected to each other with a common planet carrier. The first stage of the planet carrier actually uses the larger sun gear as the ring gear. So the first stage consists of t
14、he sun (the smaller sun gear), the planet carrier, and the ring (the larger sun gear). The input is the small sun gear; the ring gear (large sun gear) is held stationary by the band, and the output is the planet carrier. so the formula is: 1 + R/S = 1 + 36/30 = 2.2:1The planet carrier turns 2.2 time
15、s for each rotation of the small sun gear. At the second stage, the planet carrier acts as the input for the second planetary gear set, the larger sun gear (which is held stationary) acts as the sun, and the ring gear acts as the output, so the gear ratio is:1 / (1 + S/R) = 1 / (1 + 36/72) = 0.67:1
16、To get the overall reduction for second gear, we multiply the first stage by the second, 2.2 x 0.67, to get a 1.47:1 reduction. This may sound wacky, but it works. Third GearMost automatic transmissions have a 1:1 ratio in third gear. Youll remember from the previous section that all we have to do t
17、o get a 1:1 output is lock together any two of the three parts of the planetary gear. With the arrangement in this gear set it is even easier - all we have to do is engage the clutches that lock each of the sun gears to the turbine. If both sun gears turn in the same direction, the planet gears lock
18、up because they can only spin in opposite directions. This locks the ring gear to the planets and causes everything to spin as a unit, producing a 1:1 ratio. OverdriveBy definition, an overdrive has a faster output speed than input speed. Its a speed increase. In this transmission, engaging the over
19、drive accomplishes two things at once. If you read How Torque Converters Work, you learned about lockup torque converters. In order to improve efficiency, some cars have a mechanism that locks up the torque converter so that the output of the engine goes straight to the transmission. In this transmi
20、ssion, when overdrive is engaged, 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 t
21、o the turbine; the only input comes from the converter housing. Lets go back to our chart again, this time with the planet carrier for input, the sun gear fixed and the ring gear for output. Ratio = 1 / (1 + S/R) = 1 / ( 1 + 36/72) = 0.67:1So the output spins once for every two-thirds of a rotation
22、of the engine. If the engine is turning at 2000 rotations per minute (RPM), the output speed is 3000 RPM. This allows cars to drive at freeway speed while the engine speed stays nice and slow. ReverseReverse is very similar to first gear, except that instead of the small sun gear being driven by the
23、 torque converter turbine, the bigger sun gear is driven, and the small one freewheels in the opposite direction. The planet carrier is held by the reverse band to the housing. So, according to our equations from the last page, we have: Ratio = -R/S = 72/36 = 2.0:1So the ratio in reverse is a little
24、 less than first gear in this transmission. Gear Ratios This transmission has four forward gears and one reverse gear. Lets summarize the gear ratios, inputs and outputs: GearInputOutputFixedGear Ratio1st30-tooth sun72-tooth ringPlanet carrier2.4:12nd30-tooth sunPlanet carrier36-tooth ring2.2:1Plane
25、t carrier72-tooth ring36-tooth sun0.67:1Total 2nd1.47:13rd30- and 36-tooth suns72-tooth ring1.0:1ODPlanet carrier72-tooth ring36-tooth sun0.67:1Reverse36-tooth sun72-tooth ringPlanet carrier-2.0:1Hydraulic SystemThe automatic transmission in your car has to do numerous tasks. You may not realize how
26、 many different ways it operates. For instance, here are some of the features of an automatic transmission: If 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, shi
27、fts will occur at lower speeds than if you accelerate at full throttle. If you floor the gas pedal, the transmission 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 go
28、ing too fast, it will wait until the car slows down and then downshift. If you put the transmission in second gear, it will never downshift or up shift 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 t
29、he brain of the automatic transmission, managing all of these functions and more. The passageways you can see 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 th
30、e various parts of the transmission. First, well discuss the key components of the hydraulic system; then well see how they work together.The PumpAutomatic 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
31、the bottom of the transmission and feeds it to the hydraulic system. It also feeds the transmission cooler 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
32、 as the gears rotate, fluid is drawn up from the sump on one side of the crescent and forced out into the hydraulic system on the other side. The GovernorThe 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,
33、the faster the governor spins. Inside the governor is a spring-loaded valve that opens in proportion to 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 faster the car goes, the more
34、the governor valve opens and the higher the pressure of the fluid it lets through. Valves and ModulatorsThrottle Valve or ModulatorTo shift properly, the automatic transmission has to know how hard the engine is working. There are two different ways that this is done. Some cars have a simple cable l
35、inkage 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 throttle valve. The modulator senses the manifold pressure, which drops when the engine is under
36、a greater load. Manual ValveThe 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 lever is in third gear, it feeds a circuit that prevents overdrive from engaging. Sh
37、ift ValvesShift 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. For instance, the 1 to 2 shift valve determines when to shift from fir
38、st 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 which gear the car runs in. The shift valve will delay a shift if the
39、 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 forces the shift valve over until the first gear circuit is closed, and t
40、he 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 against the shift valve. This means that the pressure from the governor h
41、as 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 2-to-3 shift valve will take over, because the pressure from the gover
42、nor is high enough to trigger that valve. Electronic ControlsElectronically controlled transmissions, which appear on some newer cars, still use hydraulics to actuate the clutches and bands, but each hydraulic circuit is controlled by an electric solenoid. This simplifies the plumbing on the transmi
43、ssion and allows for more advanced control schemes.待添加的隐藏文字内容3自动变速器如何工作 如果你曾经驾驶过一辆带着自动变速器的车, 那么你就知道自动变速器和手动变速器之间有很大的不同: 在一个带着自动变速器的汽车中没有离合器踏板。 在一个带着自动变速器的汽车中没有变动的传输器。 一旦你把变速器放入驾驶车之内，其他每个部件都是自动的。 虽然自动变速器 (加上它的扭矩变换器) 和一个手动变速器(借由它的离合器) 完成同样的工作, 但是他们操作方法完全不同。 自动变速器的操作方法另人觉得非常了不起！在这里，我们将通过自动变速器来研究我们的方法。
44、我们将会从整个系统的关键开始着手:行星齿轮。然后我们将会看到变速器如何被集合,学习控制器如何工作而且讨论一些关于控制变速器方面的文章。基础部分： 正如手动变速器一样, 自动变速器最初的工作是允许机器在很小的速度范围内操作，同时提供各种不同范围的速度。 没有传输器, 汽车对一个齿轮比率会被限制,并且那个比率必须被选择允许汽车渴望旅行的最高速度。如果您想要80里/时的最高速度,那么齿轮比率会与大多数带有手动变速器的汽车的第三个齿轮相似。 你或许从未尝试过驾驶只使用第三个齿轮的带有手动传输器的汽车。如果你尝试过，你很快就会发现当汽车发动时，你几乎没法加速。当你高速行驶时，引擎就会在红线附近发出尖叫声
45、。如果这样的话，汽车很快就会被磨损而且很快就会无法操作。因此变速器使用齿轮可以使引擎的扭矩更加有效的使用，并且使引擎在一个适当的速度操作。手动变速器和自动变速器最关键的区别在于：手动变速器的锁和开不同的套齿轮对输出轴达到各种各样的齿轮比率，同时自动变速器中相同的套齿轮会导致所有不同的齿轮比率。行星齿轮很有可能在自动变速器中成为一个设备。齿轮这一个自动变速器使用一组齿轮, 称作复式的行星齿轮, 像行星齿轮一样的外貌但是实际上像被联合的二行星齿轮的作用。这里有一个作为变速器输出的环齿轮，但是它有二个太阳轮和二组行星。 让我们看一些零件: 一档齿轮在一档齿轮中，较小的太阳轮被扭力变换器的轮机驾驶顺时
46、针运转。 行星台车试着向反时针方向快速旋转,但是被单向离合器 (只允许顺时针方向旋转的装置) 保持静止，而且环齿轮输出。 小的齿轮有30颗牙齿而环齿轮有72颗牙齿,因此齿轮比是: 比 =-半径/S=- 72/30=-2.4:1因此旋转装置是负 2.4:1, 意味着输出方向与输入方向相反。 但是输出方向真的相同于输入方向-这和二组行星的进入有关。 第一组行星答应第二个组,第二组昨星转动环齿轮; 这个结合颠倒了方向。 你能意识到这也会导致较大的太阳轮快速旋转; 但是就因为那一个离合器被释放, 较大的太阳轮自由的往轮机的反方向快速旋转。二档齿轮这个变速器做真正的整洁是为了使比率与二档齿轮相符合。 它
47、运转起来就像两个行星齿轮与一个普通的行星载体互相连接。行星载体的第一阶段实际上使用更大的太阳齿轮作为环齿轮。 因此第一阶段包括太阳(更小的太阳齿轮), 行星载体, 并且环齿轮(更大的太阳齿轮) 。 输入的是小的太阳齿轮; 环齿轮 (大的太阳齿轮) 被传输带所固定，而输出的是行星载体，所以公式是: 1 + R/S=1 + 36/30 =2.2:1小太阳齿轮每转动一次，行星载体转动2.2次。在第二阶段, 行星载体作为第二个星球齿轮集合的输入,较大的太阳齿轮 (被稳固的) 作为太阳，而环齿轮作为输出, 因此齿轮比率是: 1/(1+ S/半径)=1/(1+36/72)=0.67:1为了使整体减少为二档
48、齿轮，我们通过第一阶段和第二阶段相乘（2.2 x 0.67），使它以1.47：1的速率减少，我们在第二, , 之前乘第一个级得到一个 1.47:1 还原。 这可能听起来有点奇怪, 但它可以运转。三档齿轮大多数的自动变速器在三档齿轮中有 1:1的 比率。从早先部分可以看出我们必须使行星齿轮的三个部件中的任何两个结合在一起，都以1：1的比率输出。在齿轮组的安排下，它变得更加容易，我们必须做的就是使所有的都参与离合器，把每个太阳齿轮结合成涡轮。如果两个太阳齿轮在相同的方向运转,那么行星齿轮就要被锁定起来，因为他们仅仅只能在相反的方向快速旋转，把环齿轮与行星齿轮锁定在一起，就会使每个部位快速旋转，就如一个单位,产生1:1 的比率。加速挡由定义，一个加速挡输出速度比输入速度更快，它是速度的增加。在这传输中, 参与加速档可立即完成二件事。 如果你读扭力变换器如何工作, 你就学习扭力变换器的锁定。 为了提高效率, 有些汽车就有一个机制可以锁定扭力变换器，以便引擎的输出直接达到变速器。 在这一个变速器中，当加速挡参与其中，附有扭矩交换器(被闩上引擎飞轮) 的轴由离合器连接到行星载体。小的太阳齿轮轻松快活地运转，而较大的太阳齿轮被加速挡传输带所固定
