Методические указания к контрольным заданиям для студентов агробиологических и агроинженерных направлений заочной формы обучения
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Engine bearings91. The rotating parts of an engine generally are supported in plain bearings, the journals turning within a bearing of antifriction metal. The antifriction metal em ployed in engine bearings is an alloy such as babbitt92, copper-lead, cadmium-silver, and others. Bearing metals are selected for their low coefficient of friction and their ability to withstand heavy bearing loads, high surface speeds, and high temperatures without seizure and excessive wear of the crankshaft. Engine bearings are either replaceable or cast directly in the crankcase or connecting rods. Replaceable bearings usually are composed of thin steel shells lined with a thin layer of bearing metal. To provide ease in assembly and replacement, the main and connecting rod bearings are usually the “split” or two-piece type. On some types of engines, ball or roller bearings are employed for main and connecting rod bearings. Valves93. In most engines, intake and exhaust valves of the poppet type are employed to open and close openings or ports through which the gases enter and leave the cylinders. Each cylin der in the four-stroke-cycle engine has at least one intake and one exhaust valve. The valves are located either in the cylinder block or in the cylinder head and are supported in valve guides. A camshaft opens each valve at the proper time and a valve spring closes the valve. In two-stroke-cycle gasoline engines, the fuel mixture is admit ted and the exhaust gases expelled through ports in each side of -the cylinder, the ports being opened and closed by the action of the piston. Two-stroke-cycle diesel engines generally have one port opening in the cylinder and one cam-actuated poppet valve through which the air is admitted into the cylinder and the exhaust gases expelled. Camshaft94. A camshaft opens the valves against the ten sion of the valve springs at the proper time and holds them open for the required interval. A separate cam is provided on the camshaft for the operation of each valve. Some opposed engines have each intake cam operate two intake valves. The camshaft is driven from the crankshaft through timing gears, or a timing chain and sprockets. In four stroke-cycle engines, the camshaft revolves at one-half crankshaft speed, and each valve opens and closes once every two, revolutions of the crankshaft. In a two-stroke-cycle diesel engine, the camshaft revolves at crankshaft speed, and each valve opens and closes with each revolution of the crankshaft, Valve Lifters95. Valve lifters or tappets are employed between the camshaft and the valve stem to open the valves. Valve stems expand when they become heated; and in most, engines a definite clearance must be provided between the valve stem and the valve lifter. In some engines, valve lifters are provided with adjusting screws to regulate the clearance. Some engines are equipped with self-adjusting hydraulic, valve lifters which operate with no clearance between the valve stem and valve lifter. Manifolds96. Manifolds are employed to conduct the gases into and out of the cylinders. An intake manifold is connected be tween the carburetor and the intake valve ports leading into the cylinders. The exhaust manifold connects the exhaust ports to the exhaust system. The intake and exhaust manifolds may be separate castings bolted together, or both may be cast together. Exhaust gas usually is utilized to heat the intake manifold, thus assisting in vaporizing the incoming fuel charge. In diesel engines, the intake manifold conducts air to the cylin ders, the fuel oil being sprayed directly into the cylinder at the pro per time by a fuel injector. Two-stroke-cycle gasoline engines have no intake manifold. The crankcase is utilized as a receiver for the fuel mixture. Вариант №3 Transport for Tomorrow One thing is certain about the public transport of the future: it must be more efficient than it is today. The time is coming when it will be quicker to fly across the Atlantic to New York than to travel from home to office. The two main problems are: what vehicle shall we use and how can we plan our use of it? There are already some modern vehicles which are not yet in common use, but which may become a usual means of transport in the future. One of these is the small electric car: we go out into the street, find an empty car, get into it, drive to our destination, get out and leave the car for the next person who comes along. In fact, there may be no need to drive these cars. With an automatic guid ance system for cars being developed, it will be possible for us to se lect our destination just as today we select a telephone number, and our car will move automatically to the address we want. For long journeys in private cars one can also use an automatic guidance system. Arriving at the motorway, a driver will select the lane97 he wishes to use, switch over to automatic driving, and then relax — dream, read a newspaper, have a meal, flirt with his pas senger — while the car does the work for him. Unbelievable? It is already possible. Just as in many ships and aircraft today we are piloted automatically for the greater part of the journey, so in the fu ture we can also have this luxury in our own cars. A decade ago, the only thing electronic on most automobiles was the radio. But at present sophisticated electronics is playing a big part in current automotive research. For example, in every gasoline-powered98 car that General Motors Corporation makes there is a small computer continuously monitoring the exhaust. The device, about the size of a pack of cigarettes, adjusts the vehi cle carburetor fuel intake99 to get the best fuel economy. Ford cars are equipped with an electronic instrument panel that, among other things100, will calculate how far one can drive on the fuel left in the tank. It will also estimate the time of arrival at destination and tell the driver what speed he has averaged101 since turning on the ignition. According to specialists these features made possible by micro electronics are only the beginning. Radar may control the brakes to avoid collisions, and a display screen may show the car's position on the road. Recently a radar to be mounted on lorries and cars has been designed in the USA. The radar aerial looks like a third head light placed directly above the bumper. Having summed up the in formation about the speed and distance of various objects ahead, the computer detects all possible dangers and their nature. A third com ponent in the system is a monitor on the instrument panel. The radar only observes objects ahead of the vehicle. It is automatically turned on when the speed exceeds ten miles an hour. The green light on the panel indicates that the system is on. The yellow light warns of sta tionary objects ahead, or something moving slower than the car. The red light and buzzer warn that the speed should go down. Another red light and sound signal make the driver apply the brakes. A Japanese company is designing a car of a new generation. When completed, the new model will have a lot of unusual charac teristics. The car's four-wheel control system will ensure move ment diagonally and even sideways, like a crab, at right angles to the longitudinal axis. This is especially important when leaving the car in parking places. To help the driver get information while con centrating on the road, the most important data will be projected on the wind screen. A tourist travelling in such a car will not lose his way even in Sahara with its impassable roads: a navigation Earth satellite will indicate the route. A new ceramic engine has been developed in Japan. Many im portant parts as pistons, pressure rings102, valves and some others have been made of various ceramic materials, piston rings103 made of silicon materials being in many respects better than those of steel. They withstand temperatures up to 1,000 °C. Therefore, the engine does not need a cooling system. The Running Gear The running gear104 of the car includes the wheel-sus pension system, the stabilizers, and the wheels and tyres. The frame of the car may be considered the integrating member of the running gear. It is attached to the rear axle and to the front wheels by springs. These springs, along with the axles, the control and support arms, and the shock absorbers, constitute the wheel-suspension system. In modern cars the front wheels are independ ently suspended from the frame in a manner that per mits either wheel to change its plane without appreci ably affecting the other. This type of front-wheel sus pension is known popularly as independent suspension105. The stabilizers consist of spring-steel bars, connected between the shock-absorber arms by levers, to decrease body roll and improve steerability. The Control System Steering106 is controlled by a hand wheel, mounted on an inclined column and attached to a steering tube inside the column. The other end of the tube is connected to the steer ing gear, which is designed to provide maximum ease of operation. Power steering, adapted for passenger cars in the early 1950s, is generally a hydraulic mechanism used as a booster to reduce the effort of steering. A car has two sets of brakes: the hand or emergency brake and the foot brake. The emergency brake gener ally operates on the rear wheels only. The foot brake in modern cars is always of the four-wheel type, operating on all wheels. Hydraulic brakes on cars and hydraulic vacuum, air, or power brakes on lorries apply the brak ing force to the wheels with much less force on the brake pedal than is required with ordinary mechanical brakes. The wheel brakes are generally of the internally expand ing type, in which a convex strip of material is forced against a concave steel brake drum. Вариант №4 CONSTRUCTION OF AN AUTOMOBILE The primary components of a car are the power plant, the power transmission, the running gear, and the con trol system. These constitute the chassis, on which the body is mounted. The power plant includes the engine and its fuel, the carburettor, ignition, lubrication, and cooling systems, and the starter motor. The Engine107. The greatest number of cars use piston engines. The four-cycle piston engine requires four strokes of the pis ton per cycle. The first downstroke draws in the petrol mixture. The first upstroke compresses it. The second downstroke - the power stroke - following the combus tion of the fuel, supplies the power, and the second upstroke evacuates the burned gases. Intake and exhaust valves in the cylinder control the intake of fuel and the release of burned gases. At the end of the power stroke the pressure of the burned gases in the cylinder is 2.8 to 3.5 kg/sq cm. These gases escape with the sudden open ing of the exhaust valve. They rush to a silencer (muf fler), an enlarged section of piping containing expand ing ducts and perforated plates through which the gases expand and are released into the atmosphere. Greater smoothness of operation of the four-cycle en gine were provided by the development of the four-cyl inder engine, which supplies power from one or another of the cylinders on each stroke of the cycle. A further increase in power and smoothness is obtained in engines of 6,8,12, and 16 cylinders, which are arranged in either a straight line or two banks assembled in the form of a V. Carburation108. Air is mixed with the vapour of the petrol in the car burettor. To prevent the air and the carburettor from becoming too cold for successful evaporation of the fuel, the air for the carburettor is usually taken from a point close to a heated part of the engine. Modern carburet tors are fitted with a so-called float-feed chamber and a mixing or spraying chamber. The first is a small cham ber in which a small supply of petrol is maintained at a constant level. The petrol is pumped from the main tank to this chamber, the float rising as the petrol flows in until the desired level is reached, when the inlet closes. The carburettor is equipped with such devices as accel erating pumps and economizer valves, which automati cally control the mixture ratio for efficient operation under varying conditions. Level-road driving at constant speed requires a lower ratio of petrol to air than that needed for climbing hills, for acceleration, or for start ing the engine in cold weather. When a mixture ex tremely rich in petrol is necessary, a valve known as the choke cuts down the air intake, permitting large quanti ties of unvaporized fuel to enter the cylinder. Ignition109. The mixture of air and petrol vapour delivered to the cylinder from the carburettor is compressed by the first upstroke of the piston. This heats the gas, and the higher temperature and pressure facilitate ignition and quick combustion. The next operation is that of igniting the charge by a spark plug. One electrode is insulated by por celain or mica; the other is grounded through the metal of the plug, and both form part of the secondary circuit of an induction system. The principal type of ignition now commonly used is the battery-and-coil system. The current from the bat tery flows through the coil and magnetizes the iron core. When this circuit is interrupted at the distributor points by the interrupter cam, a current is produced in the pri mary coil with the assistance of the condenser. This in duces a high-voltage current in the secondary winding. This secondary high voltage is needed to cause the spark to jump the gap in the spark plug. The spark is directed to the proper cylinder by the distributor, which connects the secondary coil to the spark plugs in the several cylin ders in their proper firing sequence. The interrupter cam and distributor are driven from the same shaft, the number of breaking points on the interrupter cam being the same as the number of cylinders. The electrical equipment controls the starting of the engine, its ignition system, and the lighting of the car. It consists of the battery, a generator for charging it when the engine is running, a starter and the necessary wiring. Electricity also operates various automatic de vices and accessories, including windscreen wipers, di rectional signals, heating and air conditioning, cigarette lighters, powered windows and audio equipment. Lubrication110. In the force-feed system, a pump forces the oil to the main crankshaft bearings and then through drilled holes in the crankpins. In the full-force system, oil is also forced to the connecting rod and then out to the walls of the cylinder at the piston pin. Cooling111. At the moment of explosion, the temperature within the cylinder is much higher than the melting point of cast iron. Since the explosions take place as often as 2,000 times per minute in each cylinder, the cylinder would soon become so hot that the piston, through ex pansion, would «freeze» in the cylinder. The cylinders are therefore provided with jackets, through which water is rapidly circulated by a small pump driven by a gear on the crankshaft or camshaft. During cold weather, the water is generally mixed with a suitable antifreeze, such as alcohol, wood alcohol, or ethylene glycol. To keep the water from boiling away, a radiator forms part of the engine-cooling system. Radiators vary in shape and style. They all have the same function, how ever, of allowing the water to pass through tubing with a large area, the outer surface of which can be cooled by the atmosphere. In air cooling of engine cylinders, vari ous means are used to give the heat an outlet and carry it off by a forced draught of air. |
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