1、外文资料 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
2、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 that t
3、he way an 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
4、the intricacies involved in controlling a transmission. Some Basics Just 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 li
5、mited 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 never tried driving a manual transmission ca
6、r 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 able. So the transmission uses gears to
7、 make more 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
8、, while in 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. Gears This automatic transmission uses a set of gears, called a compound planetary gear set, that looks l
9、ike a single planetary gear set but actually behaves like two 外文资料 6 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 Gear In first gear, the smaller sun gear is d
10、riven clockwise by the 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
11、gear ratio is: Ratio = -R/S = - 72/30 = -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
12、 set of planets engages 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 turbi
13、ne Second Gear This transmission 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 fi
14、rst stage consists of 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. so the formula is: 1 + R/S = 1 + 36/30 = 2.2:1 The plane
15、t 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 ring gear acts as the output, so the gear ratio is: 1 / (1 + S/R) =
16、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 sectio
17、n that all we have to do to 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 direct
18、ion, the planet gears lockup because 外文资料 7 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. Overdrive By definition, an overdrive has a faster output speed than input speed. Its a speed increase. In th
19、is transmission, engaging the overdrive 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
20、 the transmission. In this transmission, 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 over
21、drive band. Nothing is connected to 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) = 0.67:1 So the output spins once
22、for every two-thirds of a rotation 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. Reverse Reverse is very similar to first gear, except that instead of th
23、e small sun gear being driven by the 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:
24、1 So the ratio in reverse is a little 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: Gear Input Output Fixed Gear Ratio 1st 30-tooth sun 72-tooth ring Planet carrier 2.4:1 2nd 30-tooth sun Planet carrier 36-tooth ring 2.2:1
