Методические указания к контрольным заданиям для студентов агробиологических и агроинженерных направлений заочной формы обучения


НазваниеМетодические указания к контрольным заданиям для студентов агробиологических и агроинженерных направлений заочной формы обучения
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ТипМетодические указания
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POWER TRANSFORMERS

The power transformer is an electric machine, without moving parts, for transforming alternating voltage from a higher to a lower voltage or from a lower to a higher voltage. It consists of two electric circuits called the primary and secondary and a steel con (which forms a magnetic circuit). Either coil transformer may be used as primary or secondary. Most transformers are designed for widely different primary and secondary voltages. The coil with the high voltage is called the high-tension winding and the one with low voltage the low-tension winding.

When a high voltage coil is the primary and the low voltage is secondary the transformer is called a step-down transformer. When the reverse connection is used, the transformer is called a step-up transformer. The ratio of the high-side voltage to the low-side voltage the ratio of transformation.

Generally speaking, transformers may he designed for any desired ratio of transformation. Common ratios are 5: I, 10: 1, 20: 1.

Transformer Name Plate. The name plate on the transformer gives the voltage and kva ratings of the transformer and shows how the coils are connected. It is common practice to build the lowvoltage winding in two coils and bring four lead; to the outside of the transformer case. The coils may be connected in series or parallel. If the coils are for 115 or 230 volts, they will supply rated kva at either voltage.

The transformer should always be connected as shown on the nameplate. Otherwise a short circuit which would damage the windings may result.

Transformer Connections. Single-phase transformers may be used for either single-phase, two-phase or three-phase connections. One single-phase transformer may he used to supply three-wire 115-230 volts or 230 460 volts

Since the secondary winding is electrically insulated from the primary, any point of the secondary may be connected to ground. In single-phase, 115-230-volt circuits it is common practice to ground the centre of the winding. If the system is a three wire, 230-volt circuit with the centre of the coil grounded, there is a potential of 115 volts from the ground to either wire. The transformer case should be grounded.

Notes:

1resistance - сопротивление

2capacitance - емкость

3alternator – генератор переменного тока

4equipment - оборудование

5horsepower rating – мощность в лошадиных силах

Инженерно-технологический факультет

Вариант №1

AUTOMOTIVE ENGINES AND TRACTORS HISTORY AND DEVELOPMENT OF ENGINES
During the early part of the twentieth century, some steam-operated1 automobiles were in use. Since the steam engine can be stopped and started or reversed at will, no clutch or transmission was required. However, the troubles encountered in the operation of the steam engine, along with the development of internal combustion engines2 and storage batteries3, resulted in the gradual elimination4 of the steam-propelled automobile. The development of the automobile in the 1890's did much to further the development of the internal combustion engine. Gradual acceptance of the automobile into our form of living brought about a demand for dependability, economy, smoothness and quietness of operation, and increased power and speeds. Early internal combustion engines used in automobiles had few cylinders, had low-compression pressures, and developed little power. The engines were large and of considerable weight, and were quite noisy and rough in operation.

In order to improve the performance of the automobile, it was necessary to increase the power developed by the engine. This was generally accomplished by increasing the size or number of cylinders or the length of the stroke5. However, such improvements usually increased the weight and cumbersomeness6 of the engine. Regardless of the number or size of the cylinders, these were all low-compression engines approximately, 60 psi (pounds pressure per square inch). Top engine speeds seldom exceeded 2,000 rpm (revolutions per minute).

The acceptance by the public of the automobile as a means of transportation produced a demand for the improvement and development of roads. As more people began to use and operate automobiles, the demand increased for greater reliability, higher speeds, and economy of operation.

Improvements in fuels permitted higher compression pressures, resulting in increases in power and engine speeds. Higher engine speeds, along with the improved fuels, required changes in the combustion chamber7 design, resulting in a more even burning of the fuel mixture within the cylinder. As engine speeds increased, the timing of the valves8 became more critical. High operating speeds required that the valves be held open for longer periods of time in order to allow sufficient time for the fuel charge to enter the cylinder and the burned gases to be expelled.

The demand for better materials resulted in the rapid development of the various sciences. While science, has contributed much to the modern automobile engine, the engine has likewise contributed much to science by creating the need for better materials. The requirements for interchangeable parts to permit mass production have revolutionized manufacturing processes. While it can be said that modern manufacturing methods make possible the present-day automobile engine, it is equally true that the automobile engine has made possible the present methods and standards in manufacturing.

While the internal combustion engine has undergone numerous changes in design and construction, its basic principle of operation has not changed. Many engines operate on the four-stroke-cycle9 principle patented by Dr. Otto in 1876.

Others operate on the two-stroke-cycle principle. Aside from improvements in materials and methods, the chief changes that have been made in these engines are:

(1) Increased compression ratios.

(2) Improved valve timing.

(3) Better balance of moving parts.

(4) Better mixing and distribution of fuel.

(5) More accurate timing of fuel charge ignition

These changes have resulted in compact, powerful, highly efficient engines, of light weight that can operate at extremely high speeds. The weight per horsepower developed has been reduced to but a fraction10 of the weight of early engines. Nevertheless, in principle, the modern automobile engine is the same as originally conceived.
Who invented the car?

Two Germans, Carl Benz and Gottlieb Daimler, both took credit for making the first car. Eventually the car companies that they founded combined to become the Daimler-Benz Company, which still produces Mercedes-Benz automobiles more than 100 years later.

shareCarl Benz was born in 1844, 10 years after Gottlieb Daimler. Both Germans were mechanical engineers who worked to develop internal combustion engines that ran on liquid gasoline. Then they installed the engines on bicycles, carriages and other vehicles that were normally powered by people or animals. When these early experiments worked well, the inventors continued to improve their designs until the first cars were ready to race.

Carl Benz used a gasoline-powered engine with one cylinder to power a three-wheeled car in 1885, but he did not get it patented until 1886. His car had a tiller to steer the front wheel. Carl Benz began selling his cars in 1887. Gottlieb Daimler patented his internal combustion engine in 1885. He also invented a carburetor. Gottlieb Daimler sold one of his first cars to the Sultan of Morocco in 1889. In 1926, the two companies became the Daimler-Benz Company.

Other European and American inventors added innovations to make other versions of early automobiles. Emile Levassor, a French man, was the first person to think of putting the engine in the front of the car. This allowed a more powerful engine than those used by either Benz or Daimler. Levassor's engine had two cylinders. He showed off his automobile in a 700-mile race in France, which he won in 49 hours.

Automobile races became popular ways for American car makers to test market their designs. They also helped get the onlookers interested in perhaps becoming car owners someday. One famous car race happened in Chicago on Thanksgiving Day in 1895. Two car makers from Springfield, Massachusetts, Charles and Frank Duryea, entered one of the two cars that finished the 49-mile race. The other winner was one of Carl Benz's cars from Germany.

It did not take many years before cars became affordable to regular people. In 1900, 48,000 people attended the first National Automobile Show in New York's Madison Square Garden. In 1908, Henry Ford started the Ford Motor Company, which became famous for building the Model T, which sold for $850 at first. It got cheaper, though - 8 years later, a Model T Ford only cost $360. New 2009 Fords range in price from $17,000 to more than $34,000.
Вариант №2

A COMPARISON OF SPARK-IGNITION11 AND

COMPRESSION-IGNITION12 ENGINES

CI and SI Engine Uses. The gasoline engine has been most useful in automobiles, light trucks, a type tractor. The diesel or oil-burning engine has been most popular in boats, in locomotives, and in powerhouses. As the speed of the diesel engine has been increased, it has been used in more farm tractors, particularly in those of the crawler type. Because of other, characteristics the diesel engine is gaining favor for the row or wheel type of farm tractor.

What, essentially, are the differences between the CI and SI engines?

The SI Fuel System. One of the main differences is how the fuel is mixed. In the SI engine we must provide a carburetor or mixer. This is a device for mixing air with fuel. There are several hundred different carburetors in use. Some are quite simple and others are extremely complicated. One characteristic of most carburetors is that they must be in operation almost on a level or the liquid fuel will leak out. This sometimes causes faulty operation when the tractor SI engine is tipped at a considerable angle or when the farm truck goes around a corner.

The CI Fuel System. The CI engine does not have a carburetor. The fuel is not mixed with the air outside of the cylinder space as in the SI engine. As a substitute for the carburetor, a pump13 and an injector14 are used. There are not as many injectors in use as there are carburetors, but injectors and pumps are very precise instruments. There is an injector or nozzle15 for every cylinder. Their purpose is to prevent the air within the combustion chamber16 from leaking out but at the same time inject a stream of fuel into the hot compressed gases at the correct time and interval. It is, therefore, apparent that the air and fuel must mix within the combustion space. The correct mixing of the fuel and air in the space above the piston is a difficult design problem.

Injectors and Carburetors. In the SI engine, air and fuel are compressed together. In the CI engine, the air alone is compressed in the cylinder space the fuel is compressed as a liquid by a pump at the side of the engine. This pump must develop a pressure greater than that of the air within the cylinder. The usual pressure is about 2700 psia (pounds per square inch, absolute). As the pressures in the cylinder before combustion will seldom be higher than 800 psia the liquid fuel can be forced into the cylinder at the proper time. Of course, this pump requires a force to operate it, and it is negative work; that is, it does not add to our output of power. The ordinary carburetor does not require any power to operate; there is some friction loss, however.

Terminology. Usually the term SI means or implies that a carburetor is being used. The CI engine never has a carburetor, - as the fuel is forced into the cylinder space by an injector. The injection of the fuel is sometimes misnamed17 solid injection. As the fuel is not in solid form, this is a misnomer18. A better name is liquid injection.

Throttling19. The carburetor of the SI engine must deliver the fuel to the manifold20 and to the cylinder in rather definite ratios of fuel to air. If this is not done, the engine will not operate correctly. The usual fuel-to-air ratio would be about 1 lb21 of fuel to 15 lb of air. Not only must the ratio be correct, but also the amount of mixture fed to the cylinder must be controlled. The usual method of controlling the amount of mixture, and therefore the power arid speed of the SI engine, is to install a butterfly valve in the passageway leading from the carburetor to the manifold. By turning this butterfly valve it is possible to regulate the amount of fuel and air from a zero to a maximum. Moving the butterfly valve is called throttling. That is, throttling an engine would imply regulating its speed. A closed throttle would allow the engine to idle a full throttle would allow the engine to develop full power or speed. The manifold throttle is a characteristic of the SI engine. It is not found on the CI engine. It is an important difference between the two types of engines. The throttle has considerable influence on the efficiency of the SI type of engine; for when the throttle is almost fully open, the efficiency is the highest; when the throttle is at idling position; the efficiency is the lowest.

Thus, the SI engine power is varied gaby manifold throttling. This means that the amount of the fuel mixture is varied. In the diesel, only the fuel amount is varied; the air volume remains relatively constant. Thus a diesel can operate on such a small quantity of fuel that the air-to-fuel ratio may became 100:1, which is a very economical ratio. This explains why the diesel will operate economically at light loads. The large amount of surplus air also causes the diesel to operate at lower temperatures. Thus, because of combustion characteristics, the diesel runs more economically, and cooler at the slower speeds. And, because of the possible higher compression ratio, the diesel is able to convert more of the heat energy of the fuel into work energy at any speed.

Heavier Parts. The diesel engine has to be made heavier than the gasoline engine in order to withstand22 the higher pressures. These pressures are against the cylinder walls or liners, against the head and valves, and against the piston head. These additional pressures are transferred through the piston head to the piston pin through the connecting rod and to the crankshaft pin. All these parts must be made heavier. The crankshaft23 is heavier, the rods are stronger, the bearings are thicker and wider, and more bearings are used. Almost every diesel has a main bearing between each crank throw for greater support. All this extra strength requirement means a heavier engine and a more expensive engine.

Вариант №3

COOLING SYSTEMS

The function of the cooling system is to maintain the temperature of the engine at that temperature which the engineers have found advisable and for which they designed the engine. Any variation to any extent above or below this optimum temperature invites trouble and early engine deterioration24. There are two types of liquid cooling systems based on the forces that circulate the coolant25. These two types are the gravity26, natural, or thermosiphon system and the pump or forced system.

The forced type of circulation is the predominating method of circulating the liquid coolant: A centrifugal27 type of pump is used, its pressure depending on its size and the speed of operation. This type of pump is used because, if the regular channels of circulation are cut off by thermostats, the pump merely circulates the water within the pump housing with no damage to the part and no build up of high pressures within the system. A centrifugal pump draws liquid in at the center and throws it to an outlet28 by the spinning effect of the rapidly rotating blades29.

The pump is usually driven by the same shaft that supports the fan as the fan rotates, so does the water pump. Ball bearings are being used in these pumps so that they can be lubricated for life and, therefore, offer little maintenance problems. Others have bushed bearings and utilize grease30 or oil. Spring-loaded31 fiber, rubber32, or leather seals33 keep the lubricant and coolant separated. Important parts of the engine cooling system are the pump, the fan34, radiator, shutters35, thermostat, hose belts, and gages or indicators.

Pumps of the centrifugal types are found in the forced circulation system. They may pump anywhere from 15 to 50 gpm (gallons per minute). Systems using high-capacity pumps will usually use less coolant, and they will, therefore, have a quick warmup36 period as there is less coolant to heat.

The fans are necessary in order to move a large volume of air. This is true where the engine is air-cooled or if it is liquid-and-air-cooled.

Gages37 or indicators indicate the approximate temperature of the coolant.

The most common radiator type has the coolant flowing from top to bottom through round or flattened38 tubing. The flattened tubing has more flexibility in case the coolant freezes and hence will not burst as easily.

The reader may come across the term heat exchanger. The tractor radiator is one form of heat exchanger39, merely serving as a device for the transfer or exchange of heat from the liquid to the gaseous air.

The purpose of the cooling system is to control the temperature of the engine within reasonable limits while operating, and to assist in the rapid warmup of the engine so as to decrease wear and to arrive quickly at an efficient operating temperature. To accomplish this purpose, tractor and automobile engines have been equipped with shutters or thermostats, sometimes both. The cooling system using air entirely as the outside coolant requires very little attention, perhaps just enough to see if all the passages are free.

The engine using the liquid-air combination does require considerable attention. It is just as important to keep the cooling system clean and free from obstructions as it is the lubrication system. The three greatest "saboteurs" of the cooling system are rust40, scale41, and electrolysis42.
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