汽车实用英语作业3

Engine Fuel System

The fuel system has the job of supplying a combustible mixture of air and fuel to the engine. The fuel system must vary the proportions of air and fuel to suit different operating conditions. When the engine is cold, for example, then the mixture must be rich ( have a high proportion of fuel ). The reason for this is that the fuel does not vaporize rapidly at low temperatures. Therefore, extra fuel must be added to the mixture so that there will be enough vaporized fuel to form a combustible mixture.

The fuel system consists of the fuel tank, fuel pump, fuel filter, carburetor, intake manifold, and fuel lines, or tubes, connecting the tank, pump, and carburetor. Some gasoline engines use a fuel-injection system; in this system, a fuel-injection pump replaces the carburetor.

The fuel tank, in which gasoline is stored, is normally located at the rear of the vehicle.(1) It is made of sheet metal and is attached to the frame.

A fuel pump delivers fuel from the tank to the carburetor. There are two general types of fuel pump, mechanical and electric.

The fuel system has filters and screens to prevent dire in the fuel from entering the fuel pump or carburetor. Dirt could, of course, prevent normal operation of these units and cause poor engine performance.(2)

The carburetor is essentially. a mixing device which mixes liquid gasoline with air. In this process, it throws a fine spray of gasoline into air passing through the carburetor on its way to the engine. (3) The gasoline vaporizes and mixes with the air to form a highly combustion mixture. This mixture then enters the engine combustion chambers, where it is ignited. It burns, causing the engine to produce power. (4) The mixture must be of varying degrees of richness to suit engine operating conditions. It must be rich (have a higher percentage of fuel ) for starting. Acceleration, and high-speed operation. And it should lean out (become less rich ) for operation at intermediate speed with a warm engine. The carburetor has several different circuits, or passages, through which fuel and air-fuel mixture flow under different operating conditions to produce the varying richness of the air-fuel mixture. (5)

Tire

Tires have two functions. First, they interpose a cushion between the road and the car wheels to absorb shocks resulting from irregularities in the road. The tires flex, or give, as bumps are encountered, thus reducing the shock effect to the passengers in the car. (1) Second, the tires provide frictional contact between the wheels and the road so that good traction is secured. This permits the transmitting of power through the tires to the road for rapid accelerating, combats the tendency of the car to skid on turns, and allows quick stops when the brakes are applied. (2)

Tires are of two basic types, solid and pneumatic (air-filled). Solid tires have very limited usage, being confined largely to specialized industrial applications. (3) Pneumatic tires are of two types, those using an inner tube and the tubeless type. The amount of air pressure used depends on the type of tire and operation. (4) Passenger-car tires are inflated to about 275 to 413 kPa. Air is introduced into the tire ( or inner tube ) through a valve that opens when the chuck on the air hose if applied. On the tire with an inner tube, the valve is mounted on the tube. On the tubeless tire, the valve is mounted on the wheel rim.

Suspension System

There are two basic suspension systems in use today. One is the solid axle, leaf spring type; the other is the independent suspension using long and short swinging arms. There are various adaptations of these systems, but all use the same basic principle.

The solid axle suspension uses a solid steel dead axle (does not turn with wheels ) with a leaf spring at each side. The wheels swivel on each end via a pivot arrangement between the axle and the wheel spindle. (1)

With independent suspension, each wheel is free to move up and down with minimum (least attainable ) effect on the other wheel. There is also far less twisting motion imposed on the frame.

Rear Suspension

The leaf spring mounted transversally does require additional bracing; control arms and strut rods are therefore used. (3) With coil springs, the rear-suspension system requires some method of holding the axle housing in place.

The coil springs are assembled between spring seats in the car frame and pads on the axle housing. Two control arms, or links, are used. They are attached between the rear-axle housing and the car frame and permit upward or downward movement of the axle housing with respect to the car frame. They prevent side movement or forward and backward movement.

Front Suspension

The suspension of the front wheels is more complicated than the suspension of the rear wheels. Not only must the front wheels move up and down with respect to the car frame ( for spring action ), but also the must be able to swing at various angles to the car frame for steering. (4) In order to permit the front wheels to swing to on side or the other for steering, each wheel is supported on a spindle which is part of a steering knuckle. The steering knuckle is then supported through ball joints, by upper and lower control arms which are attached to the car frame. (5) Independent Front Suspension

Practically all passenger cars now use the independent type of front suspension system in which each front wheel is independently supported by a coil, torsion-bar, or leaf spring. (6) The coil-spring arrangement is the most common. There are three types of coil-spring front suspension. In one, the coil spring is located between the upper and lower control arms, and the lower control arm has one point of attachment to the car frame. In the second type, the coil spring is located between the upper and lower control arms, and the lower control arm has two points of attachment to the car frame. In the third type, the coil spring is between the upper control arm and spring tower or housing that is part of the front-end sheet-metal work.