Figure 4-5
Automatic transmissions contain many gears in various combinations. In a manual transmission, gears slide along shafts as you move the shift lever from one position to another, engaging various sized gears as required in order to provide the correct gear ratio.
Figure 4-1
Some types of drive train layouts use a “transaxle”, which is simply a combination of the transmission and the differential. These are usually found on front wheel drive cars, but are also used on rear-engine cars. See figure 4-2.
This is accomplished through a system that works by oil pressure. Each shift of the gears is controlled by a shift valve; and the gear changes depend on speed, the road, and load conditions. Both manual transmission (with its clutch) and automatic transmission (plus its torque converter) accomplish exactly the same thing, but they do it in totally different ways. See figure 4-3.
UNIT 4 Introduction to Drive Train
Part ITechnical and Practical Reading
Passage
A
Drive
Train (I)
The drive train serves two functions: it transmits power from the engine to the drive wheels, and it varies the amount of torque. “Power” is the speed at which work is performed. “Torque” is twisting force.
All automatic transmissions have a shifter that has at least 4 positions, park, reverse neutral and drive. Many units also have a low range to assist in engine braking on long downhill runs. Others have special positions where the driver can manually shift if desired. See figure 4-5.
In an automatic transmission, however, the gears are nays engaged to the same gears. This is accomplished through the use of planetary gear sets. See figure 4-6.
The drive train includes: transmission that is connected to the back of the engine and sends the power from the engine to the drive wheels; driveshaft that transmits the power from transmission to differential; and differential that sends the power to the two wheel axles. See figure 4-1.
3. Each shift of the gears is controlled by a shift valve; and the gear changes depend on speed, the road, and load conditions. 每次 换挡由换挡阀控制；挡位变换取决于车速、 路况和载量。
5. Automatic transmissions contain mechanical systems, hydraulic systems, electrical systems and computer controls, all working together in perfect harmony. 自动变速器包含机械系统、液压系统、 电器系统和电脑控制系统、所有这些彼此协调共 同运作。
6. In an automatic transmission, however, the gears are never physically moved and are always engaged to the same gears. 但是， 在自动变速器上，齿轮本身从不移动，始 终与相同齿轮啮合。
Figure 4-3
Manual Transmission
Manual transmissions usually have four or five speeds, and often have “overdrive”, which means that the output shaft can turn faster than the input shaft for fuel economy on the highway. In general, transmissions with more speeds provide better fuel economy and smoother shifting, but they cost more to manufacture, so small sub-compact cars will often have optional 4 speed transmissions, while more upscale cars will have 5 or 6 speed transmissions. See figure 4-4.
Notes：
1. The drive train includes: transmission that is connected to the back of the engine and sends the power from the engine to the drive wheels; driveshaft that transmits the power from transmission to differential; and differential that sends the power to the two wheel axles. 传动系包 括变速器（与发动机尾部连接将发动机的动力传 递到驱动轴）、传动轴（将动力从变速器传递到 差速器）、差速器（将动力传递到两个半周）。
7. The planet gears are connected to each other through a common carrier that allows the gears to spin on shafts called “pinions” attached to the carrier. 行星齿轮 通过共用的行星齿轮架相互连接，行星齿 轮架使行星齿轮在被称为“小齿轮”的轴 上旋转，“小齿轮”与行星齿轮架相连接。
Figure 4-2 Front wheel drive layout
Transmission Type
There are two types of transmissions: manual and automatic. If you have a manual transmission, you have to shift the gears yourself, usually with a stick located on your console and the clutch pedal. If you have an automatic transmission, the mechanism changes without any help from you.
4. In general, transmissions with more speeds provide better fuel economy and smoother shifting, but they cost more to manufacture, so small sub-compact cars will often have optional 4 speed transmissions, while more upscale cars will have 5 or 6 speed transmissions. 一般来说，变速 越多的变速器燃油经济性就越好，换挡就越平顺， 但是，制造成本也就越高。所以，小型紧凑型汽 车经常选用4速变速器，而更高端汽车用5速或6 速变速器。
2. If you have a manual transmission, you have to shift the gears yourself, usually with a stick located on your console and the clutch pedal. 如 果是手动变速器，必须自己换挡，通常用中控台 上的操纵杆和离合器踏板。