ТОП 10 на сайтеПриготовление дезинфицирующих растворов различной концентрации
Техника нижней прямой подачи мяча.
Франко-прусская война (причины и последствия)
Организация работы процедурного кабинета
Смысловое и механическое запоминание, их место и роль в усвоении знаний
Коммуникативные барьеры и пути их преодоления
Обработка изделий медицинского назначения многократного применения
Образцы текста публицистического стиля
Четыре типа изменения баланса
Задачи с ответами для Всероссийской олимпиады по праву
ЗНАЕТЕ ЛИ ВЫ?
Влияние общества на человека
Приготовление дезинфицирующих растворов различной концентрации
Практические работы по географии для 6 класса
Организация работы процедурного кабинета
Изменения в неживой природе осенью
Уборка процедурного кабинета
Сольфеджио. Все правила по сольфеджио
Балочные системы. Определение реакций опор и моментов защемления
THE FOUR-STROKE-CYCLE ENGINE.
Prime Movers. We have what are called prime movers doing a lot of useful work for us. For instance, there is the windmill which takes advantage of the energy of moving air to pump water. There is the hydraulic turbine which uses the velocity and consequent energy of moving water to create power. The water wheel uses the energy of falling water; the steam engine, the energy of expanding steam. These are examples of physical changes adding to man's source of power.
External Combustion Engine. Another important example of a prime mover is the heat engine. Certain organic compounds which contain carbon are caused to burn and give off heat. This heat furnishes power in one of two ways. In the external type of heat engine, the fuel is burned in a chamber or firebox; the heat is applied to some liquid such as mercury or water which, in turn, is piped to the engine as it changes from a liquid to a gas. The heat has caused the molecules of the liquid to separate so far from each other that they form a vapor which may be considered as a gas. The enormous energy of the moving molecules of the gas creates a tremendous pressure against a reciprocating piston or against the blades of a turbine. However, it must be noted that the fuel is burned on the outside of the engine itself; it is, therefore, of the external combustion type. The steam engine and steam turbine are good examples. The steam engine has been inefficient in the conversion of heat into the energy of movement. The average steam engine of the reciprocating type has only been able to convert about 5 per cent of the energy of the fuel into, useful work. That is one of the reasons why the diesel engine with a comparable efficiency of approximately 30 per cent has been displacing the steam locomotive engine.
The Internal Combustion Engine. The most important type of prime mover is the internal combustion engine. Most of the engines of this type use liquids as their fuel, although some burn gas from manufactured or natural sources. The two most common fuels used are gasoline and a less volatile fuel called diesel or fuel oil.
Engine Principles. In the internal combustion piston engine, the fuel is forced into the space between the cylinder head and the piston which is usually called the combustion chamber or space. The size of this space is variable, that is it depends upon the position of the piston. If the piston is at the head end (HEDC), the space is small. If the fuel is in this space and is ignited, it will tend, to expand. As the head is securely fastened to the block and as the cylinder walls are either in the block or are a part of the block, the only object that can move is the piston. This it does, going as far as it can toward the crankcase end (CEDC).Thus the internally burning fuel causes expansion, and movement in the internal combustion engine.
Engine Types. There are several types of such engines, depending upon cycle, location of parts, and other items. A partial designation of engine types could be as follows:
1. Type of fuel burned - gasoline, kerosene, fuel oils, and gases.
2. Method of cooling - liquid or gas (air).
3. Arrangement of cylinders.
4. Valve location.
5. Cycle - four-stroke and two-stroke.
6. Type of valves.
7. Ignition - spark or compression.
8. Moving parts - piston or blade (turbine).
9. Compression - low, medium, high.
One of the most important engine designation is by the cycle used. The usual outboard boat engine is of the two-stroke cycle. The usual tractor engine and all known automobile engines are of the four-stroke type.
The four-stroke-cycle engine is sometimes improperly called the four'-cycle engine. This is not a correct technical designation. It is simply an abbreviation of the longer descriptive term; it does not indicate that there are four piston strokes per cycle.
Four-stroke-cycle Engine. In order to simplify the explanation of the operation of the four-stroke engine, one of the gasoline-burning type will be used. This is the usual type found on garden tractors, lawn mowers, and most wheeled farm tractors.
Cycle Processes. By necessity we must start with one of the four events. The most logical one for beginning is the intake of the fuel. As the piston moves down (in the vertical engine), there is a tendency for a vacuum to form in the cylinder space. This is caused by the expansion of the gas (air only, for our beginning stroke). For instance, if one volume of air occupies six times as much space, say in an engine with a compression ratio of 6:1, there should be about one-sixth the absolute atmospheric pressure, according to Boyle's gas law. This means that the pressure on the outside is much greater than on the inside. If now the intake valve is opened, there will be a rush of air into the cylinder space. If by carburetion some fuel is mixed with air, we have a combustible mixture in the engine. If the intake valve now closes, the mixture is trapped. If this mixture is ignited at this point, no work wi11 be done as the piston is down as far as it can go. Also the fuel would not have much force as the molecules of fuel and oxygen (in the air) are quite widely separated. Therefore the piston is moved to the top of the cylinder by moving the flywheel or hand crank. This will compress the fuel-air mixture into approximately one-sixth of its former volume. The mixture will now be under six times the pressure it was formerly, more if the temperature of the mixture rose during compression. If both valves are left closed and the mixture is ignited, the trapped and compressed gases will exert a tremendous force while burning. This force will cause the piston to move to the crank end of the cylinder, thus causing motion and doing work with heat energy.
To complete the cycle, the exhaust valve is opened when the power of the rapid combustion is mostly spent, and, as the piston moves toward the head, the exhaust gases are pushed from the combustion chamber. Energy stored in the flywheel as momentum provides the power to exhaust the products of combustion, to provide the engine with another charge of fuel and air, and to compress the mixture ready for ignition and another power stroke.
Briefly the events are as follows: intake, one valve open; compression, both valves closed; burning or combustion, both valves closed; exhaust, one valve open. Thus one cycle of four events with four strokes of the piston has been completed (ignition could be a fifth event).
If you were asked, "What is the most important job in caring for a tractor?" you would probably say lubrication. If any one job can be considered more important than another, you would be right. The different lubricants used in the tractor perform these essential functions:
Conduct away excessive heat.
Reduce friction and wear between moving parts.
Seal compression between pistons and cylinder walls.
Cushion the loads on bearings of the power-transmitting system.
Because tractor work is so severe - long hours, heavy loads, dusty fields, rough ground, steep hills, and all kinds of weather – we have to be very careful about lubrication. Fortunately, however, the actual job of properly lubricating a tractor has been made quite easy. The petroleum industry has developed excellent oils and greases. Manuals tell what kinds and grades to use, and where and how often to apply them. The lubrication system holds and circulates an ample supply.
Your job in properly lubricating a tractor can be summarized as follows:
1. Selecting the kinds of lubricants recommended by the manufacturer.
2. Applying these in correct amounts at recommended intervals.
3. Maintaining proper quantities within the tractor.
4. Changing lubricants as required.
5. Keeping these lubricants clean and free from contamination and dilution.
Engine Lubrication System. You should understand the construction of the engine lubrication system and know how it circulates oil to all parts of the engine. In most tractors, oil is circulated and distributed, within the engine by a gear-type oil pump. This maintains sufficient pressure to supply oil in proper quantities to all the working parts. This is a full-pressure-type lubrication system.
The oil intake of the pump and is of the floating type, composed of afloat with a metal screen, which prevents the entrance of large foreign particles that would damage the pump gears. The intake is hinged in a way that permits the float to remain near the surface of the oil in the oil pan in order to draw in the cleanest oil. The pump forces the oil, under pressure, to the principal distribution passages drilled into the cylinder block. This pump also has a pressure regulating valve which maintains the specified pressure to protect the lubricating system and the pump mechanism when the lubricants are thickened by cold temperature.
The valve which regulates the pump pressure opens to permit oil to escape into the oil pan through a bypass. An oil pressure gage, which can be seen by the operator, is connected to the main oil passage. From the main oil passage the oil is forced through smaller passages drilled in the block to main crankshaft bearings and other parts of the engine. The excess oil is by-passed from the main oil passage through a smaller passage, is forced through the filter, and is then returned through a metering hole located in the central bolt of the filter, to the oil pan.
In the center bearing of the camshaft there is a small oil slot. As the camshaft revolves, contacting the main oil passage, the oil flows through passage or intermittently lubricating the rocker arms and valves, and the exhaust valve stems and guides; the overflow returns to the oil pan. The intake valve stem guides are lubricated by oil vapors. The oil is forced under pressure from the three main crankshaft bearings through passages drilled in the crankshaft to the four connecting rod bearings. On the larger engines the connecting rods are drilled and the oil from the bearing is forced through this drilled oil passage to lubricate the piston pin and cylinder walls. On the smaller engines the oil lubricates the connecting rod bearings, escapes at the ends, and splash lubricates the cylinder walls, piston pins, etc. The front camshaft bearing contains oil holes and passages which index with other passages at each revolution. As this occurs, the oil is forced intermittently through the drilled oil passages to the timing gears and through the oil passage to the governor assembly (on engines using a carburetor).
The purpose of the oil filter is to remove foreign matter from the oil and other substances which might damage the engine.
Motor oil, or engine oil, is an oil used for lubrication of various internal combustion engines. While the main function is to lubricate moving parts, motor oil also cleans, inhibits corrosion, improves sealing and cools the engine by carrying heat away from moving parts.
Motor oils are derived from petroleum-based and non-petroleum synthesized chemical compounds. Motor oils are today mainly blended by using base oils composed of hydrocarbons, thus organic compounds consisting entirely of carbon and hydrogen.
Motor oil is a lubricant used in internal combustion engines. These include motor or road vehicles such as cars and motorcycles, heavier vehicles such as buses and commercial vehicles, non-road vehicles such as go-karts, snowmobiles, boats (fixed engine installations and outboards), lawn mowers, large agricultural and construction equipment, locomotives and aircraft, and static engines such as electrical generators. In engines, there are parts which move against each other causing friction which wastes otherwise useful power by converting the energy to heat. Contact between moving surfaces also wears away those parts, which could lead to lower efficiency and degradation of the motor. This increases fuel consumption and decreases power output and can, in extreme cases, lead to engine failure.
Lubricating oil creates a separating film between surfaces of adjacent moving parts to minimize direct contact between them, decreasing heat caused by friction and reducing wear, thus protecting the engine. In use, motor oil transfers heat through convection as it flows through the engine by means of air flow over the surface of the oil pan, an oil cooler and through the build up of oil gases evacuated by the Crankcase Ventilation system.
In petrol (gasoline) engines, the top piston ring can expose the motor oil to temperatures of 320°F (160 °C). In diesel engines the top ring can expose the oil to temperatures over 600°F (315 °C).
Coating metal parts with oil also keeps them from being exposed to oxygen, inhibiting oxidation at elevated operating temperatures preventing rust or corrosion. Corrosion inhibitors may also be added to the motor oil. Many motor oils also have detergents and dispersants added to help keep the engine clean and minimize oil sludge build-up.
Второй год обучения.
1. Модальные глаголы can, may, must и их эквиваленты.
2. Временные формы группы Perfect.
3. Неопределенные местоименияsome, any, no, everyи их производные.
4. Употребление местоимений many, much, little, few.
5. Местоименияone (ones), that (those),как заменители существительного.
6. Использование существительного в функции определения. Перевод его на русский язык.
7. Страдательный залог и особенности перевода пассивных конструкций
8. Глаголы to be (to), to have (to), выражающие долженствование.
9. Простые неличные формы глагола.
10. Герундий. Значение, употребление и перевод на русский язык.
Последнее изменение этой страницы: 2016-12-12; Нарушение авторского права страницы
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