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1、How Automatic Transmissions Workby Karim NiceIntroduction 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 auto
2、matic 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) accomplish exactly the same thing, but they
3、 do it in totally different ways. It turns out that the way an automatic transmission does it is absolutely amazing!Automatic Transmission Image GalleryIn this article, well work our way through an automatic transmission. Well start with the key to the whole system: planetary gearsets. Then well see
4、 how the transmission is put together, learn how the controls work and discuss some of the intricacies involved in controlling a transmission. 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 provid
5、ing a wide range of output speeds.Photo courtesy DaimlerChryslerMercedes-Benz CLK, automatic transmission, cut-away modelWithout 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
6、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 only third gear. If you did, youd quickly find out that you had almost no acceleration when starting out, and at high speeds, the e
7、ngine would be screaming along near the red-line. A car like this would wear out very quickly and would be nearly undriveable.So the transmission uses gears to make more effective use of the engines torque, and to keep the engine operating at an appropriate speed. The key difference between a manual
8、 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 planetary gearset is the device th
9、at makes this possible in an automatic transmission. Lets take a look at how the planetary gearset works. Planetary Gearsets & Gear RatiosWhen you take apart and look inside an automatic transmission, you find a huge An ingenious planetary gearset A set of bands to lock parts of a gearset A set of t
10、hree 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 cantaloupe, this one part creates all of the di
11、fferent 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 screwdriver article. An automatic transmission co
12、ntains two complete planetary gearsets folded together into one component. See How Gear Ratios Work for an introduction to planetary gearsets. From left to right: the ring gear, planet carrier, and two sun gearsAny planetary gearset has three main components: The sun gear The planet gears and the pl
13、anet 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. Lets take a look at a single planetary gearset. One of the planetary gearsets from our transmission
14、 has a ring gear with 72 teeth and a sun gear with 30 teeth. We can get lots of different gear ratios out of this gearset.InputOutputStationaryCalculationGear RatioASun (S)Planet Carrier (C)Ring (R)1 + R/S3.4:1BPlanet Carrier (C)Ring (R)Sun (S)1 / (1 + S/R)0.71:1CSun (S)Ring (R)Planet Carrier (C)-R/
15、S-2.4:1Also, 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 the input speed. The second is an overdrive - the output speed is faster than the input
16、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 automatic transmission. You can try these out in the animation below:So this one set of gears can produce all of these different gear ratios w
