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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.
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Figure 4-1
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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.
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Figure 4-4
Automatic Transmission
The modern automatic transmission is by far the most complicated mechanical component in today’s automobile. Automatic transmissions contain mechanical systems, hydraulic systems, electrical systems and computer controls, all working together in perfect harmony.
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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.
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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.
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Figure 4-2 Front wheel drive layout
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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.
UNIT 4 Introduction to Drive Train
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Part ITechnical and Practical Reading
Passage A Drive Train (I)
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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.
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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.