1、Automobile Transmissions and Power Steering Automobiles, trucks, buses, and tractors all depend on transmissions to deliver power from the engine to the wheels, The gasoline and diesel engines that power these vehicles cannot be connected directly to the wheels, because the engines must keep turning
2、 at a certain seed to keep from stalling. Also, different amount of torque (turning force) must be delivered to the wheels at different times. A large amount of force is needed to get a car moving from a standstill. Less force is needed to keep the car moving once it is rolling. Going up a hill of d
3、riving at high speed requires still other amount of force. It is the job of the transmission to deliver the particular amount of force that is needed. The transmission also allows a motor vehicle to back up -gasoline and diesel engines can run in only one direction, but the transmission can reverse
4、the direction of the force. And when a car stops, the transmission lets the engine keep running without moving the wheels. The heart of any transmission even an automatic one is a set of gears. Gears do not change the power of an engine, but they can increase the torque by decreasing the speed. They
5、 can also increase the speed by decreasing the torque. The automobile transmission uses a series of gears which enables the engine to continue to operate at maximum efficiency when the vehicles speed is altered. Setting an automobile in motion requires a large amount of power to overcome the inertia
6、 of the vehicles weight. This process requires high engine speed, needed for high power, and a gradual increase in a vehicles speed to avoid a jerky start. To do this, a low gear ratio is allows the crankshaft to revolve several times in order to turn the real axle once. The low gear ratio is used f
7、or starting, climbing steep slops, and other situations in which maximum power is required .As power needs are reduced, a second, higher gear ratio is used which rotates the rear axle with fewer revolutions of the crankshaft. As the cars speed increases, successively higher gear ratios are used unti
8、l the drive from the engine to the rear wheels passes through the transmission without reduction. Two principle types of transmission are used, manual and automatic. The manual transmission system permits the driver to select the desired gear ratio by manipulating a shift lever. Besides the forward
9、speed gears, additional gearing is incorporated to permit the vehicle to operate in reverse. Manually operated passenger car transmission used in the United States and Canada usually have there speeds forward and one in reverse. Trucks, tractors, buses, and other heavy-duty vehicles have as many as
10、10 forward speeds and 2 in reverse. These units are basically five-speed transmissions with a two-speed auxiliary gearbox. Transmission that are to be shifted with the vehicle in motion incorporate synchromesh units to prevent gears from clashing as they are meshed. The synchromesh unit synchronizes
11、 the speed of the gears so that they revolve at the same speed as they slide into engagement. The automatic transmission system changes gear ratios automatically in response to changes in engine speed or throttle setting. The use of automatic transmissions increased tremendously after World War , an
12、d they are installed in more than half of the automobile produced in the United States. Automobiles equipped with an automatic transmission have a control lever which allows the driver to select neutral, low, drive, and reverse. The engine is started in neutral, and the lever is moved to “drive” for
13、 normal operation when moving forward. In “drive” position the vehicles can accelerate from rest to maximum speed by simply depressing the accelerator. The “low” position prevent the transmission from shifting out of the lower gear ratios. It is used for climbing steep grades, in mud, or at other to
14、mes when maximum power is needed. Some units have a “park” position, which locks the transmission to prevent a parked car from rolling. The automatic transmission makes it easier to drive a car, but it is less efficient than a manually shifted unit and increase gasoline consumption. For this reason,
15、 the automatic transmission is not as common in Europe, where economy of operation is a prime sales factor. Four basic types of automatic transmissions have been developed to the point where they have been installed in production vehicles. The first consists of a standard mechanical transmission and
16、 clutch which is automatically shifted by pneumatic, hydraulic or electric power units. The second type uses a hydraulic torque converter plus a planetary gear system to increase engine torque. The third system combines a hydraulic coupling with an automatically shifted mechanical gearbox to provide
17、 torque amplification. The fourth type uses one or more stages of hydraulic torque conversion to provide torque multiplication. Automatic transmission shift in response to signals from speed sensing and throttle position sensors. The units incorporating hydraulic torque converters use the hydraulic
18、fluid, under pressure, to engage and disengage planetary gear trains. Power steering system 也是汽车重要的一部分， The car of today is larger and heavier than earlier cars； the tyres are wider, further apart and inflated to lower pressures .In addition, the trend of development has been to place more than half
19、 the weight on the front wheels, especially the weight of the engine , which itself is larger and heavier than in the early days. To make cars easier to steer, the gear ratio in the steering box at the end of the steering column was changed to that turning the wheel required less torque, but this in
20、creased the number of turns of the steering wheel required on modern cars without power steering compared to 2.5 or 3 turns for cars built before 1940. Modern cars with power steering only require about three turns. Power assisted steering was first developed in the 1920s； one of the first devices w
21、as developed by an engineer at Pierce Arrow, an American make of luxury cars. The Cadillac division of General Motors was going to offer power steering as optional equipment on some models in the early 1930s, but the depression interfered with development. During World War power steering was fitted
22、to military vehicles； in 1952 Chrysler began offering it , and it is now standard equipment on many of the biggest American cars . Electric devices were tried , but power steering today is always hydraulic , with oil pressure of perhaps 1000 psi (70kg/cm2) maintained by a pump driven by the engineer
23、 of the car . The system is a servomechanism, or servo loop, which makes a correction to compensate for the torque applied to the steering wheel by the driver. It consists of an actuator and a control valve. The actuator is a hydraulic cylinder with a piston, or ram, which is free to travel in eithe
24、r direction from the center. The function of the control valve is to respond to the torque from the steering wheel by actuating smaller valves at each end of the cylinder. The system is designed to assist the steering linkage, rather than to replace it, and it does not do all of the work of steering
25、 , but leaves some of it for the driver. Thus if the hydraulics fail the car can still be steered , though with greater effort, and at all times the feel of the road is mechanically transmitted from the front wheels to the hands of the hands of the driver on the steering wheel, an essential element
26、of safe driving. The power steering makes a positive contribution to safe driving in that if the driver hits a small obstacle in the road or has a flat tyre at speed, the power unit makes it easier to keep the car under control. Many large cars fitted with wide, stiff radial ply tyres would be nearl
27、y impossible to steer at parking speeds without power steering. Hydrostatic systems, designed for off-the-road vehicles, are exception to some of this, because they dispense with the steering column and the steering box , and the steering wheel and the steered wheels are connected only by hydraulic
28、tubes or hoses. The power steering system includes a reservoir to hold the oil. Oil pressure is always provided when the engine is running, but when the system is at rest, that is when the steering wheel is not being turned, equal pressure is available to each side of the piston in the actuator, so that it does not move. There are basically two types of power steering system: those which have the control valve located within the steering box, in which case it is usually a rotary valve, and those in which the valve is integral with actuator, when it is an axial spool valve.
