1、 附录二 专业外文及翻译 How Automatic Transmissions Work If 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
2、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 out
3、that the way an automatic transmission does it is absolutely amazing! Planetary Gearsets & Gear Ratios When you take apart and look 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 band
4、s 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 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 cantalou
5、pe, this one part creates all of the different gear ratios that the transmission can produce. Everything else in the transmission is there to help the planetary gearset do its thing. This amazing piece of gearing has appeared on HowStuffWorks before. You may recognize it from the electric screwdrive
6、r article. 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 and the planet gears carrier The ring g
7、ear 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. Lets take a look at a single planetary gearset. One of the planetary gearsets from our transmission has a ring gear with 72 teet
8、h and a sun gear with 30 teeth. We can get lots of different gear ratios out of this gearset. Also, locking any two of the three components together will lock up the whole device at a 1:1 gear reduction. Notice that the first gear ratio listed above is a reduction - the output speed is slower than t
9、he input speed. The second is an overdrive - the output speed is faster than the input speed. The last is a reduction again, but the output direction is reversed. There are several other ratios that can be gotten out of this planetary gear set, but these are the ones that are relevant to our automat
10、ic transmission. This automatic transmission uses a set of gears, called a compound planetary gearset, that looks like a single planetary gearset but actually behaves like two planetary gearsets combined. It has one ring gear that is always the output of the transmission, but it has two sun gears an
11、d two sets of planets. Next you can see the inside of the planet carrier. The shorter gears are engaged only by the smaller sun gear. The longer planets are engaged by the bigger sun gear and by the smaller planets. First Gear In first gear, the smaller sun gear is driven clockwise by the turbine in
12、 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 = -R/S = - 7
13、2/30 = -2.4:1 So 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 engages the secon
14、d 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 turbine (counterclockwise). Second Gea
15、r This transmission does something really neat in order to get the ratio needed for second gear. It acts like two planetary gearsets connected to each other with a common planet. The first stage of the planet carrier actually uses the larger sun gear as the ring gear. So the first stage consists of
16、the 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. For this stage, with the sun as input, planet carrier as output, and the ring
17、gear fixed, the formula is: 1 + R/S = 1 + 36/30 = 2.2:1 The planet carrier turns 2.2 times 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 rin
18、g gear acts as the output, so the gear ratio is: 1 / (1 + S/R) = 1 / (1 + 36/72) = 0.67:1 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 Gear Most automatic transmissions have a 1:1 ratio in third gear. Youll remember from the previous section that all we have to do to get a 1:1 output is lock together any two of
