【正文】 外文資料 5 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 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) acplish exactly the same thing, but they do it in totally different ways. It turns out that the way an automatic transmission does it is absolutely amazing! In this article, we39。ll work our way through an automatic transmission. We39。ll start with the key to the whole system: plaary gear sets. Then we39。ll see how the transmission is put together, learn how the controls work and discuss some of the intricacies involved in controlling a transmission. Some Basics Just like that of a manual transmission, the automatic transmission39。s 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 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. You39。ve probably never tried driving a manual transmission car using only third gear. If you did, you39。d quickly find out that you had almost no acceleration when starting out, and at high speeds, the engine would be screaming along near the redline. A car like this would wear out very quickly and would be nearly undrive able. So the transmission uses gears to make more effective use of the engine39。s 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 an automatic transmission, the same set of gears produces all of the different gear ratios. The plaary gear set is the device that makes this possible in an automatic transmission. Gears This automatic transmission uses a set of gears, called a pound plaary gear set, that looks like a single plaary gear set but actually behaves like two 外文資料 6 plaary gear sets bined. It has one ring gear that is always the output of the transmission, but it has two sun gears and two sets of plas. Let39。s look at some of the parts: First Gear In first gear, the smaller sun gear is driven clockwise by the turbine in the torque converter. The pla carrier tries to spin counterclockwise, but is held still by the oneway 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 = 72/30 = :1 So the rotation is negative :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 plas es in. The first set of plas engages the second set, and the second set turns the ring gear。 this bination reverses the direction. You can see that this would also cause the bigger sun gear to spin。 but because that clutc