Both the automatic transmission and the manual transmission accomplish exactly the same thing, and the are identical in every respect, except that they do it in totally different ways.

Just like that of a manual transmission, the automatic transmission’s primary job is to allow the engine to operate in its narrow range of speeds while providing a wide range of output speeds to drive the car, whether it is forwards or backwards.

The main components that make up an automatic transmission includes:Planetary Gear Sets,which are the mechanical systems that provide the various,forward gear ratios as well as Hydraulic System,which controls the Clutches and the Bands in order to control the plantary gear,, which are used to keep the oil where it is supposed to be and prevent it from leaking Torque Converter,which acts like a clutch to allow the vehicle to come to,a stop in gear while the engine is still running.

Following is the important part of automatic transmission:Planetary gear sets.

Automatic transmission contain many gears in various a manual transmission,gears slide along shafts as you move the shift lever from one position to another,engaging vatious sized gears as required in order to provide the correct gear an automatic transmission,however,the gears are never physically moved and are always engaged to the same gears. This is accomplished through the use of planetary gear sets.

The basic panetary gear set consists of a sun gear, a ring gear and two or more planet gears, all remaining in constant mesh. The planet gears are connected to each other through a common carrier,which allows the gears to spin on shafts called “pinions” which are attached to the carrier. The planetary gear system gets its name from the fact that that the pinions revolve around the sun gear and rotate at the same time, just as the planets in our solar system rotate and revolve around the sun.

One example of a way that this system can be used is by connecting the ring gear to the input shaft coming from the engine, connecting the planet carrier to the output shaft, and locking the sun gear so that it can’t move. In this scenario, when we turn the ring gear, the planets will “walk” along the sun gear (which is held stationary) causing the planet carrier to turn the output shaft in the same direction as the input shaft but at a slower speed causing gear reduction (similar to a car in first gear).

If we unlock the sun gear and lock any two elements together, this will cause all three elements to turn at the same speed so that the output shaft will turn at the same rate of speed as the input shaft. This is like a car that is in third or high gear. Another way that we can use a Planetary gear set is by locking the planet carrier from moving, then applying power to the ring gear which will cause the sun gear to turn in the opposite direction giving us reverse gear.

How the simple system described above would look in an actual transmission. The input shaft is connected to the ring gear (Blue); The Output shaft is connected to the planet carrier (Green) which is also connected to a “Multi-disk” clutch pack. The sun gear is connected to a drum (yellow) which is also connected to the other half of the clutch pack. Surrounding the outside of the drum is a band (red) that can be tightened around the drum when required to prevent the drum with the attached sun gear from turning.

The clutch pack is used, in this instance, to lock the planet carrier with the sun gear forcing both to turn at the same speed. If both the clutch pack and the band were released, the system would be in neutral. Turning the input shaft would turn the planet gears against the sun gear,but since nothing is holding the sun gear, it will just spin free and have no effect on the output shaft. To place the unit in first gear, the band is applied to hold the sun gear from moving. To shift from first to high gear, the band is released and the clutch is applied causing the output shaft to turn at the same speed as the input shaft.

Many more combinations are possible using two or more planetary sets connected in various ways to provide the different forward speeds and reverse that are found in modern automatic transmissions.

Some of the clever gear arrangements found in four and now, five-speed automatics are complex enough to make a technically astute lay person’s head spin trying to understand the flow of power through the transmission as it shifts from first gear through top gear as the vehicle accelerates to highway speed. On newer vehicles, the vehicle’s computer monitors and controls these shifts so that they are almost imperceptible.

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手动变速器和自动变速器完成同一任务，而且，除了工作原理完全不同以外，其他各方面完全一样。

和手动变速器相比，自动变速器首要解决的问题是：发动机的运行速度范围有限，而汽车行驶和倒车的速度变化范围很大。

自动变速器的主要组成部分是：行星齿轮机构，提高多个前进挡和倒档的机械系统。液压系统，通过控制各个离合器和制动箍带，来控制行星齿轮机构。密封：用来保持油位，放置漏油。液力变矩器，类似于离合器，允许汽车在发动机不熄火的情况下让车停下。

下面就是自动变速器的最主要的机构：行星齿轮机构。

自动变速器内有许多齿轮的组合。在一个手动变速器里，当你操纵换挡杆从一个档位到另一个档位时，齿轮沿轴向移动，使大小不同的齿轮进行啮合从而满足不同的传动比的需要。但是，在自动变速器里，齿轮不需要滑动，所有齿轮之间保持常啮合状态，换挡时通过行星齿轮的控制来实现的。

通常，行星齿轮包括太阳轮，齿圈和两个或两个以上的行星轮，所有齿轮都始终处于常啮合状态。行星齿轮用同一个行星架相互连在一起，每个行星轮在各自的轴上转动，行星轮也成为行星架上的小齿轮。之所以叫行星轮，是由于小齿轮在围绕中心转动的同时也自转，正如太阳系的行星自转并围绕太阳运行一样。

这种系统的一种运行方式是，发动机动力经由输入轴传入齿圈，行星架与输出轴相连，太阳轮固定。在这种动力流中，当我们转动齿圈，行星轮将绕着太阳轮转动，则输入轴与输出轴同向运行，但是输出轴减速，低速运行。

如果我们松开太阳轮，把任意两个构件连为一体，则三个构件就全部以同样转速运转，那么输入轴与输出轴都会以同样的转速运行。这就相当于三档或高速档。行星齿轮的另一种传动方式是行星架固定，动力传入齿圈，齿圈带动太阳轮反向运行，就相当于倒档。

输入轴与齿圈连接，输出轴与行星架相连，行星架上有多片离合器。太阳轮与轮毂相连，轮毂同时也连接离合器的另一半。在轮毂外是箍紧的制动箍带，随时可固定连着太阳轮的轮毂，使其停止转动。

在这种情况下，由离合器将行星架和太阳轮连成一体，使两者以相同转速运转。如果离合器和制动带都松开，那么该系统就处于空挡。输入轴转动可以带动行星齿轮使太阳轮反向转动，但是，由于太阳轮处于非约束状态，所以，太阳轮只是空转，对输出轴没有任何作用。如果，将行星机构置于一档，制动箍带将锁住太阳轮。如果从一档换至高速档，制动带松开，离合器结合，则输出轴和输入轴同速同向旋转。

两排或三排以上的简单行星齿轮机构进行不同的组合，就形成了现代自动变速器各个前进挡和倒档。

现在，还出现了四排和五排行星齿轮机构的智能化自动变速器，结构十分复杂，难以靠大脑对一档到高速档的动力传动进行分析。在较新型的汽车上，车上的计算机监控这些档位，使换挡操作几乎感觉不到。