Tài liệu Diezel 1410 P1

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In order that the function and operation of submarine diesel engines may be thoroughly understood, it is necessary to describe briefly the history and development leading to modern design.

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Nội dung Text: Tài liệu Diezel 1410 P1

1 DIESEL ENGINE PRINCIPLES A. DEVELOPMENT 1A1. General. In order that the The next notable achievement in function and operation of submarine improving the efficiency of the diesel engines may be thoroughly internal combustion engine was the understood, it is necessary to Hornsby-Ackroyd engine produced in describe briefly the history and England a short while later. It was development leading to modern among the first early designed engines design. that used a liquid fuel derived from crude oil. This engine employed the It is significant that the diesel engine Brayton principle of controlled fuel is an outgrowth of the early struggle injection and compressed the air in to improve the efficiency of existing the cylinder prior to ignition. The types of other internal combustion compression heat thus generated, plus engines. Today's fleet type the use of a hot surface, induced submarine diesel engines are ignition. Since this engine employed indirectly the result of widespread hydraulic force to inject the fuel, it is experimentation in both the Otto now considered the first example of (gasoline) engine field and the more an engine using mechanical or solid recently developed diesel engine injection. field. Basically, however, the In 1893, Dr. Rudolf Diesel, a principles of operation have not changed materially since the first Bavarian scientist, patented a design practical models of the early for an internal combustion engine designs. which was termed a Diesel engine. He considered previous failures and Among the contributors to progress applied himself to designing an in the development of diesel engines engine to operate on an entirely has been the Submarine Service of different thermodynamic principle. the United States Navy. Keen interest and untiring effort, not to Using the mechanics of the 4-stroke mention risk in experimentation, cycle, Dr. Diesel proposed that only testing, and correcting design, have air be drawn into the cylinder during given unparalleled impetus toward the suction or intake stroke. The improved design. compression stroke was to compress the air in the cylinder to a sufficiently 1A2. History of diesel engine high temperature to induce ignition development. The reciprocating and combustion without the use of internal combustion engine was added heat. Like Brayton's engine, introduced in theory as far back as this engine was to inject fuel at a 1862 by Beau de Roches in France. controlled rate. It was Dr. Diesel's A few years later, Otto, of Germany, theory that if the rate of injection were made the first practical application properly controlled during the of Beau de Roches's theory in an combustion phase, combustion could actual working model. Otto's engine be made to occur at a constant was practicable and fairly reliable temperature. Since fuel would have to compared to other earlier attempts. be injected against high compression It employed a 4-stroke cycle of pressures in the cylinder, Dr. Diesel's
operation using gas as a fuel. Thus, design called for fuel injection to be the 4-stroke cycle of a gas engine accomplished by a blast of highly became popularly known as an Otto compressed air. Essentially, this was cycle. air injection. Dr. Diesel further theorized that the temperature drop George Brayton, an American, during the expansion phase of the introduced a new principle of fuel cycle would be efficient to make injection in 1872. Brayton used an external cooling of the combustion internal combustion gas engine in chamber unnecessary. his experiments. He demonstrated that prolonging the combustion A single-cylinder working model was phase of the cycle, by injecting fuel constructed and first experiments at a controlled rate, produced more were conducted using coal dust as a power per unit of fuel consumed. fuel. All efforts to operate However, much of the efficiency gained by this method was lost due to the lack of an adequate method of compressing the fuel mixture prior to ignition. 1 a working model on the cycle and, in addition, many of these proposed by Dr. Diesel resulted in engines gave off considerable carbon explosions and failure. Further monoxide fumes, creating a menace attempts to experiment along this to personnel. same line were abandoned. In the meantime, MAN built and Consequently, an engine operating experimented with 2-stroke cycle entirely on the theoretical cycle diesel engines for submarine proposed by Dr. Diesel was never propulsion. However, insufficient produced. This cycle subsequently progress had been made in metallurgy became known as the diesel cycle. to provide metals capable of Many designers realized the value of withstanding the greater heat and the practical elements in the cycle of stress inherent in engines of this type. operation outlined by Dr. Diesel. MAN then turned its efforts toward Subsequently, experimenters began production of a 4-stroke cycle diesel to achieve favorable results by engine capable of developing 1000 eliminating the impractical elements hp. While fairly successful, these and by altering the cycle of engines eventually developed operation. Successful experiments structural weaknesses at the were conducted by the Machinen- crankcase. fabrik-Augsburg-Nurnberg (commonly called MAN) concern in By 1914 the MAN 4-stroke cycle diesel had been partially redesigned Germany. and strengthened, producing the By this time the more volatile SV45/42, 1200-hp engine used in the petroleum fuels were in common use majority of German submarines and diesel engines utilizing liquid during World War I. Following World fuel were designed. These engines War I, the United States Navy operated on a cycle in which the acquired a number of these engines
combustion phase occurred at for use in the earlier S-class boats. A constant pressure rather than at copy of this engine was produced by constant temperature. Experience the New York Navy Yard and used in also disclosed that it was essential to other early S-class submarines. cool the combustion chamber The Electric Boat Company, which externally. Early diesel engines was formerly the Holland Torpedo operating on the constant pressure Boat Company, became licensee in cycle, were efficient enough to make the United States for the MAN commercial production feasible. Company of Germany. Later, the Progress in diesel engine design has Electric Boat Company consolidated been rapid since the early models with the New London Ship and were introduced. The impetus of war Engine Company. Shortly before demands, progress in metallurgy, World War I, the Electric Boat fabrication, and engineering, and Company developed the well-known refinements in fuels and lubricants NELSECO engine. During, and have all served to produce modern, subsequent to World War I, a number high-speed diesel engines of of United States submarines of the O, exceptional efficiency. R, and S classes were equipped with these NELSECO engines. In fact, the 1A3. History of submarine engine principal installations in United States development. The first United submarines were 6- and 8-cylinder States submarines utilizing internal NELSECO's until about 1934. combustion engines for propulsion were powered by 45-horsepower, 2- Prior to 1930 the engines used in most cylinder, 4-stroke cycle gasoline submarines of all the larger naval engines produced by the Otto powers, with the exception of Great Company of Philadelphia. Britain, were 4-stroke cycle diesel Meanwhile, the English Submarine engines. The United States Navy, Service made use of 12- and 16- however, experimented with a 2- cylinder gasoline engines in their stroke cycle Busch-Sulzer engine and earlier submarines. equipped a number of boats with this type of engine. Since then, the The inherent hazards accompanying majority of engines designed for the use of such a highly volatile fuel United States submarine use have as gasoline were quickly realized. been of the 2-stroke cycle type. Stowage was a constant problem and handling of the fuel was extremely dangerous. Internal explosions were frequent 2 Prior to 1929, all engines in the 1A4. How submarine requirements United States Submarine Service affect engine design. The fact that were of the air injection type. submarines are both subsurface and Shortly after 1929, mechanical or surface vessels places definite solid type injection was employed restrictions upon size, hull design, and on MAN engines. The advantages to shape. Total weight, too, is a factor be obtained with this type of having considerable bearing on injection were immediately underwater operations. Hull
apparent. By using solid type characteristics restrict engine size and injection, the weight of the engines location of the engine compartments. could be considerably reduced. The Engine weight must bear a elimination of the air compressor proportionate relationship to the alone accounted for a saving in weight and displacement of the vessel weight of approximately 14 percent. as well as to power requirements. The advantages derived from the use In the first engine-powered of mechanical injection were submarines, the engines were numerous and included: mechanically connected directly to the propeller shafting. This design, known as direct drive, developed 1. simplification of design immediate operational problems. The 2. reduction in length of the hull characteristics definitely fixed the engine angle of the propeller shafting. This 3. greatly reduced weight per restriction also determined engine horsepower position and location. Also, the most 4. reduced fuel consumption efficient propeller speeds did not 5. improved load balance in the correspond with the most efficient engine engine speeds. In direct drive 6. far greater reliability installations, critical speeds (or 7. less maintenance synchronous torsional vibrations) The need for more powerful engines which were inherent in the early model engines, were transferred became apparent with the through the direct drive into shafting development of the fleet type and propellers. At times, the exact submarine. The three engines that cruising speed desired could not be seemed to fulfill submarine obtained, as it was necessary to pass requirements were the Winton V- the engines through critical speeds in type, now known as the General Motors engine; the Fairbanks-Morse the desired operating range as rapidly as possible. Two major problems opposed piston type; and the were brought to the foreground by Hooven-Owen-Rentschler double- these early models: acting type engine. Of these, the HOR was later removed from 1. How to power the propellers and submarines in favor of the General yet separate engines and propeller Motors and Fairbanks-Morse shafting so that no mechanical unity engines which are now the two existed. standard submarine engines. 2. How to design a drive in which At the present time, the General different and varied rotative speeds Motors Corporation manufactures could be selected for both engines and 16-cylinder, single-acting engines propellers. rated at 1600 brake horsepower (bhp) for main engine installations, Various types and combinations of and 8-cylinder engines for auxiliary drives were designed and tested. Over installations. Fairbanks Morse and a period of time it became apparent Company manufactures 9- and 10- that the electric drive installations cylinder, opposed piston engines (commonly referred to as diesel- rated at 1600 bhp for main engine electric drive) were the practical installations, and 7-cylinder, solution. This type of design solved opposed piston engines for auxiliary
installations. These engines have both of the major problems. The proved most efficient. They weigh engines were coupled only to the as little as 15 to 20 pounds per bhp generators that supplied power to the including auxiliary equipment. electric motors. The propeller shafting Standardizing on only two designs was driven by the motors through has also made it possible to mass reduction gears or directly produce engines with a minimum amount of delay and difficulty. 3 by slow-speed electric motors. The 1. The engine should furnish only connections between engine maximum amount of power with power and propeller shafting were minimum weight and space electrical. Hence, vibrations requirement. developed by the engines could not 2. The engine should possess the be conducted to the propeller ability to develop occasionally more shafting and propellers, and the than full load rating. various stresses encountered by the propellers could not be transmitted 3. The engine should have the ability directly to the engines as was the to run continuously at slightly less case with mechanical couplings. than full load rating. In addition, the rotative speed of the 4. The engine should operate with engine was no longer limited by the small fuel consumption per unit of rpm of the propellers. Consequently, horsepower. the engines could be designed for any desired speed within a selected 5. The engine should have a small range. Likewise, the propellers lubricating oil consumption. could be operated independently of engine speed within the speed limits 6. All wearing parts should be readily of their design. The diesel-electric accessible for quick replacement. drive gave greater latitude to designers with respect to operating 7. There should be perfect balance speed, size, and location of engines. with respect to primary and secondary It also gave the boat designers forces and couples. greater freedom in placement of engine compartments. 8. Major critical speeds within the operating ranges of the engine should There are eight major requirements be eliminated. that a submarine diesel engine should fulfill: B. PRINCIPLES Of DESIGN AND OPERATION charge of fuel and air is admitted, and 1B1. Reciprocating internal combustion engines. An engine that the process is repeated. The above converts heat energy into work by sequence of events is called a cycle of burning fuel in a confined chamber operation. is called an internal combustion 1B2. Cycles of operation. The word engine. Such an engine employing
back-and-forth motion of the pistons cycle enters into the description of the is called a reciprocating type operation of any internal combustion internal combustion engine. The engine. As applied to internal diesel engine and the gasoline combustion engines, it may be engine are the most familiar defined as the complete sequence of examples of reciprocating internal events that occur in the cylinder of an combustion engines. engine for each power stroke or impulse delivered to the crankshaft. The basic principle of operation of Those events always occur in the an internal combustion engine is same order each time the cycle is relatively simple. The space in the repeated. cylinder in which the fuel is burned is called the combustion chamber. Each cycle of operation is closely Fuel and air are admitted to the related to piston position and combustion chamber and ignited. movement in the cylinder. Regardless The resulting combustion increases of the number of piston strokes the temperature within the involved in a cycle, there are four combustion chamber. Gases, definite events or phases that must released by combustion, plus the occur in the cylinders. increase in temperature, raise the 1. Either air or a mixture of air and pressure which acts on the piston fuel must be taken into the cylinder crown, forcing the piston to move. and compressed. Movement of the piston is transmitted through other parts to 2. The fuel and air mixture must be the crankshaft whose rotary motion ignited, or fuel must be injected into is utilized for work. The expended the hot compressed air to cause gases are ejected from the cylinder, ignition. a new 4 3. The heat and expansion of gases Matter is anything having weight and resulting from combustion must occupying space. Solids, liquids, and perform work on the piston to gases are matter. produce motion. A molecule is the smallest division of 4. The residual or exhaust gases a given matter, which, when taken must be discharged from the alone, still retains all the properties cylinder when expansion work is and characteristics of the matter. completed. Heat is a form of energy caused by The cycles of operation in each type the molecular activity of a substance. of internal combustion engine are Increasing the velocity of molecular characterized both by the mechanics activity in a substance increases the of operation and the thermodynamic amount of heat the substance processes. The three most contains. Decreasing the velocity of commonly known cycles are the molecular activity in a substance Otto cycle, the diesel cycle, and the decreases the amount of heat the modified diesel cycle. substance contains. 1B3 Thermodynamics. To explain Temperature is a measure of the
thermodynamics as used in an intensity of heat and is recorded in engineering sense, it is first degrees by a thermometer. The two necessary to define the term and the temperature scales most commonly related terms used with it. used are the Fahrenheit and centigrade scales. Thermodynamics is the science that deals with the transformation of Volume may be described as the energy from one form to another. A amount of space displaced by a basic law of thermodynamics is that quantity of matter. energy can neither be created nor 1B4. The mechanical equivalent of destroyed but may be changed from heat energy. The function of an one form to another. In diesel internal combustion engine is to engineering, we are concerned transform heat energy into mechanical primarily with the means by which energy. Recalling the basic law of heat energy is transformed into thermodynamics we know that energy mechanical energy or work. cannot be destroyed. It is possible to Force is that push or pull which convert mechanical energy to heat tends to give motion to a body at completely, and by delicate physical rest. A unit of force is the pound. experiments it has been found that for every 778 foot-pounds of mechanical Pressure is force per unit area acting energy so converted, one Btu of heat against a body. It is generally will be obtained. Because of expressed in pounds per square inch fundamental limitations, it is usually (psi). not possible to convert heat completely to work, but for every Btu Work is the movement of force that is converted, 778 foot-pounds through a certain distance. It is will be realized. This important measured by multiplying force by constant is known as the mechanical distance. The product is usually equivalent of heat. expressed in foot-pounds. 1B5. Relationship of pressure, Power is the rate of doing work, or temperature, and volume. Figure 1- the amount of work done in unit 1A illustrates a simple cylinder with a time. The unit of power used by reciprocating piston. A dial pressure engineers is the horse power (hp). gage at the top of the cylinder One horsepower is equivalent to registers pressure inside the cylinder. 33,000 foot-pounds of work per Temperature inside the cylinder is minute or 33,000/60 = 550 foot- recorded by a thermometer. The pounds per second. thermometer at the side registers room temperature. The piston is at outer Energy is the ability to perform dead center in its stroke. At this stage, work. Energy is of two types: the pressure inside kinetic, which is energy in motion, and potential, which is energy stored up. 5
Figure 1-1. Pressure, temperature, and volume relationship in a cylinder. the cylinder is the same as conditions found in the compression atmospheric pressure outside, and stroke of a modern submarine diesel the dial of the pressure gage engine. The temperature of the registers 0. Also, the temperature compressed air within the cylinder has inside the cylinder is the same as been raised to a sufficient degree to room temperature, or approximately cause automatic ignition on the 70 degrees F. injection of fuel oil into the cylinder. In Figure 1-1B, force has been Thus, in summation, we see that applied to the piston, moving it during a cycle of operation, volume is about a third of the distance of its constantly changing due to piston compression stroke. Air trapped in travel. As the piston travels toward the cylinder is compressed. As the the inner dead center during the volume of this air is decreased, the compression stroke, the air in the pressure is increased to about 155 cylinder is reduced in volume. psi. The temperature rises from 70 Physically, this amounts to reducing degrees F to about 300 degrees, the space occupied by the molecules indicating that heat has been added of air. Thus, the pressure of the air to the air in the cylinder. This shows working against the piston crown and that mechanical energy, in the form walls of the cylinder is increased and of force supplied to the piston, has the temperature rises as a result of the been transformed into heat energy in increased molecular activity. As the the compressed air. piston nears inner dead center, the volume is reduced rapidly and the In Figure 1-1C, more force has been temperature increases to a point applied to the piston, raising the sufficient to support the automatic pressure in the cylinder to about 300 ignition of any fuel injected. psi, and the temperature to nearly 700 degrees F. Combustion changes the injected fuel to gases. After combustion, the Figure 1-1D shows the final stage of liberation of the gases with a very the compression stroke as the piston slight increase in volume causes a arrives at inner dead center. Pressure sharp increase in pressure and is in the neighborhood of 470 psi and the temperature is about 1000
degrees F. This illustration closely approximates the 6 Figure 1-1. Pressure, temperature, and volume relationship in a cylinder. temperature. During the power 1B7. Pressure-volume diagrams for stroke, volume increases rapidly, the Otto cycle, diesel cycle, and and toward the end of the stroke, modified diesel cycle. Figure 1-2 pressure and temperature decrease shows typical pressure-volume rapidly. diagrams for the three types of engine cycles. Each pressure-volume 1B6. Pressure-volume diagrams. diagram is a graphic representation of Various methods and devices are cylinder pressure as related to used for measuring and recording cylinder volume. In the diagrams the the pressures at various piston ordinate represents pressure and the positions during a cycle of operation abscissa represents volume. In actual in an engine cylinder. The result practice, when an indicator card is may be graphically illustrated by a taken on an engine, the vertical plane diagram such as that shown in is calibrated in pressure units and the Figure 1-2. Such diagrams are volume plane is calibrated in inches. known as pressure-volume The volume ordinate of the diagram diagrams. In practice, they are then shows the length of stroke of the referred to as indicator cards. piston which is proportional to the volume. Pressure-volume diagrams give the relationship between pressures and Letters are located on each of the piston positions, and may be used to figures in the diagrams. The distance measure the work done in the between two adjacent letters on the cylinder. Also, if the speed of the figures is representative of a phase of engine and the time involved in the cycle. Comparing the diagrams completing one cycle are known, the provides a visible means of indicated horsepower may be comparing the variation in the phases computed by taking pressure- between the three cycles. volume diagrams on each cylinder and converting the foot-pounds per 1B8. The Otto cycle. The Otto cycle
unit of time into horsepower. This (Figure 1-2) is more commonly method of determining horsepower, known as the constant volume cycle however, is not practicable on and its principles form modern fleet type submarine engines. 7 Figure 1-2. Pressure-volume diagrams. the basis for all modern automobile oils will ignite automatically with gasoline engine designs. In this sufficient air at a temperature of about cycle, combustion is timed to occur 480 degrees F, ignition occurs as soon theoretically just as the piston as the fuel oil spray reaches the hot arrives at top dead center. Ignition is air. This is called compression accomplished by a spark, and, due to ignition. the volatility of the fuel-air mixture, This combustion process (or burning combustion practically amounts to of the fuel and compressed air) is a an explosion. Combustion is relatively slow process compared with completed with virtually no piston the quick, explosion type combustion travel and hence, little, if any, process of the Otto cycle. The fuel change in volume of the gas in the combustion chamber. This gives rise spray penetrates the compressed air, some of the fuel ignites, then the rest to the description constant-volume cycle. During combustion there is a of the fuel charge burns. In the true quick rise of the temperature in the diesel cycle, the expansion of gases cylinder, immediately followed by a keeps pace with the change in volume occasioned by piston travel during the pressure rise which performs the combustion phase. Thus combustion work during the power stroke. is said to occur at constant pressure. The Otto cycle may be defined as a cycle in which combustion induced The diesel cycle may be defined as a cycle in which combustion induced by by spark ignition theoretically compression ignition theoretically occurs at constant volume. occurs at a constant pressure. 1B9. The diesel cycle. In the true diesel cycle, only air is compressed 1B10. Modified diesel cycle. We in the cylinder prior to ignition. This have previously described the Otto cycle as one in which combustion normally produces a final compression pressure of about 500 occurs theoretically at constant
psi. At such a pressure the volume, and the diesel cycle as one in temperature of the compressed air which may range from 900 degrees to 1050 degrees F. Since most fuel 8 combustion occurs theoretically at After the fuel is injected into the constant pressure. In actual cylinder, combustion converts it into operation, a gasoline engine does gases. This conversion is a not follow the true Otto cycle, nor thermodynamic change. A does the diesel engine follow the thermodynamic change during which true diesel cycle. In fact, the the temperature remains constant is operation of a medium- or high- called an isothermal process. A speed diesel engine follows the thermodynamic change during which modified diesel cycle (Figure 1-2). the temperature may vary but during This cycle involves phases of both which heat is neither received nor the Otto cycle and the diesel cycle in rejected is called an adiabatic that the combustion phase takes process. place at both constant volume and In a strict sense the thermodynamic constant pressure. cycles outlined below are not true The modified diesel cycle, as thermodynamic cycles. In a true cycle applied to diesel engines, may be the process is reversible. The working defined as a cycle of operation in substance is heated, does work, is which the combustion phase, cooled, and is heated again. In the induced by compression ignition, cycle of an actual engine, the residue begins on a constant-volume basis of the combustion process is and ends on a constant pressure exhausted at the end of the expansion basis. stroke and a new charge is taken into the cylinder for the next cycle of All submarine main and auxiliary events. However, the true engines used today employ the thermodynamic cycle is useful for modified diesel cycle. The studying the thermodynamic fundamental differences between the processes in actual engine operation. Otto and the modified diesel cycles are: a. The Otto cycle. This is the thermodynamic cycle used as a basis 1. The methods of mixing fuel and for the operation of all modern air. This is accomplished before and gasoline engines. The cycle (Figure 1- during compression in the Otto cycle 2) consists of the adiabatic and usually near the end of the compression of the charge in the compression phase in the modified cylinder along the line AB, the diesel cycle. constant-volume combustion and heating of the charge from B to C, the 2. The methods of ignition. Spark adiabatic expansion of the gases from ignition is used in the Otto cycle and C to D, and the constant-volume compression ignition is used in the rejection of gases from the cylinder modified diesel cycle. along DA. The term diesel cycle has become b. The diesel cycle. In the original
popularly associated with all diesel cycle proposed by Dr. Diesel, compression-ignition or diesel the combustion phase of the engines. In actual practice, this is a thermodynamic cycle was to be a misnomer when applied to modern, constant-temperature or isothermal medium-speed or high-speed diesel process. However, no engine was ever engines, because practically all operated on this cycle. As a result of diesel or compression-ignition his experimentation, however, a engines in this category operate on constant-pressure thermodynamic the modified diesel cycle. cycle was developed. All early type, slow-speed diesel engines 1B11. Thermodynamics of the approximated this cycle, although it is Otto cycle, every diesel cycle, and in little use today. modified diesel cycle. In every thermodynamic cycle there must be In this cycle (Figure 1-2), adiabatic a working substance. With internal compression occurred along AB, to combustion engines, some form of provide the temperature necessary for substance must undergo a change in the ignition of the fuel. Fuel injection the cylinder in order to convert heat and combustion were so controlled as energy into mechanical energy. The to give constant-pressure combustion working substance in the cylinder of a compression-ignition engine is fuel oil. 9 along BC. This was followed by 2. With the intake valve closed, the adiabatic expansion from C to D. piston makes an upward stroke, Rejection of the gases from the compressing the air. Pressure is cylinder was constant volume from generally around 500 psi with D t o A. resultant temperatures as high as 900 degrees to 1050 degrees F, depending c. The modified diesel cycle. This is on the design of the engine. At about the cycle (Figure 1-2) used in all the end of this stroke, the fuel is fleet type submarine diesel engines injected into the hot compressed air, and in practically all modern diesel and ignition and combustion occur engines. In this thermodynamic over a relatively short period of piston cycle, compression is adiabatic from travel. A to B. Combustion is partly constant volume from B to C and 3. The expansion of combustion gases partly constant pressure from C to forces the piston downward through D. Expansion is adiabatic from D to one stroke. This is called the power E. Rejection of gases from the stroke. As the piston nears the end of cylinder is constant volume along this stroke, the exhaust valve opens, EA. permitting some of the burned gases to escape. 1B12. Thermal efficiency. The thermal efficiency of an internal 4. The piston makes another upward combustion engine may be stroke in which the remaining exhaust considered the percentage of gases are forced out of the cylinder. efficiency, in converting the total This completes the cycle. potential heat energy available in the
fuel into mechanical energy. We 1B14. The 2-stroke diesel cycle. In have already stated that the this cycle (Figure 1-4) the piston mechanical equivalent of heat makes two strokes to complete the energy is 778 foot-pounds for one cycle. There is one power stroke for Btu of heat. By this equation, it is a every two piston strokes or for each simple matter to figure how much revolution of the crankshaft. An work should be delivered on an ideal engine employing this cycle requires a basis from a given quantity of fuel. scavenging air blower to assist in An engine operating on this- basis clearing the exhaust gases from the would be 100 percent efficient. No cylinder, to replenish the cylinder internal combustion engine, with the necessary volume of fresh however, is 100 percent efficient, air, and to make possible a slight because heat losses, conducted supercharging effect. through the cooling and exhaust Figure, 1-4 shows the two strokes and systems, and friction losses make the sequence of events that occur in the thermal efficiency of any the 2-stroke diesel cycle as follows: internal combustion engine relatively low. 1. Start of compression. The piston has just passed bottom dead center, 1B13. The 4-stroke diesel cycle. In the cylinder is charged with fresh air, the 4-stroke diesel cycle, the piston and both the intake ports and the makes four strokes to complete the exhaust valve are closed. The fresh air cycle. There is one power stroke or is trapped and compressed in the power impulse for every four piston cylinder. strokes, or two complete revolutions of the crankshaft. 2. Injection. At about the end of the compression stroke, the fuel is Figure 1-3 shows the four strokes injected and combustion occurs. and the sequence of events that occur in the 4-stroke diesel cycle. 3. Expansion. Expansion of gases from combustion forces the piston 1. The intake valve opens and a downward through one stroke. As the supply of fresh air is drawn into the piston nears the end of this stroke, the cylinder while the piston makes a exhaust valve is opened slightly in downward stroke. 10 advance of the uncovering of the uncovered, the scavenging air which intake ports. This permits some of is under pressure, rushes into the the burned gases to escape. cylinder. This drives out the remaining exhaust gases and 4. Exhaust. As the intake ports are completes the cycle. C. DIESEL ENGINE TYPES end of the piston. The piston rod 1C1. Single-acting diesel engine. Both the 4-stroke and the 2-stroke extends through the cylinder head of cycle diesel engines illustrated and the lower combustion chamber and described in the previous section passes through a stuffing box to were of the single-acting type prevent leakage of pressure. The
(Figure 1-5). In all single-acting piston rod is attached to a crosshead, engines the pistons used are usually and the connecting rod is attached to of the trunk type, that is, pistons the crosshead so that it may turn whose length is greater than their freely on the crosshead pin. The diameter. One end of the trunk type crosshead has a flat bearing surface piston is closed; this end is called that moves up and down on a the crown. The opposite or skirt end crosshead guide to steady the piston of the piston is open. The connecting rod and piston and prevent uneven rod extends through the open end of wear. the piston and is attached to the Combustion occurs in the upper piston by means of the piston pin. combustion chamber, and the pressure The term single-acting is used to of the gases of combustion is applied describe these engines because the to the top end of the piston during the pressure of the gases of combustion downward stroke. At the completion acts only on one side (the crown) of of this stroke, combustion occurs in the pistons. In the 4-stroke cycle, the bottom combustion chamber and single-acting engines, the power expansion pressure is applied to the stroke occurs only once in every two bottom end of the piston during the revolutions of the crankshaft. In the upward stroke. The downward power 2-stroke cycle, single-acting stroke serves as the compression engines, the power stroke occurs stroke for the lower combustion once in every revolution of the chamber and the upward power stroke crankshaft. All of the main and serves as the compression stroke for auxiliary diesel engines currently the top combustion chamber. Thus the installed in fleet type submarines are power strokes are double that of a of the single-acting type. single acting engine and the engine is referred to as a double-acting type. 1C2. Double-acting diesel engine. A considerable number of double- The 2-stroke cycle, double-acting acting diesel engines (Figure 1-6), engine has a distinct advantage in namely the HOR and MAN engines, power output compared with the were used in installations for fleet single-acting type. With twice as type submarines until recent years. many power strokes as a comparable Lately, however, most of these single acting engine and, with other double-acting engines have been conditions being equal, it develops removed and replaced with 2-stroke practically twice as much power per cycle, single-acting engines. While cylinder. In addition, the operation is double-acting engines have no place smoother due to the fact that the in current installations, it is well for expansion stroke in one combustion the student to be familiar with their chamber of the cylinder is balanced or general design and operation. cushioned by the compression stroke in the opposite combustion chamber. In double-acting diesel engines, the piston proper is usually shorter and There are two principal difficulties is described as the crosshead type. encountered in adapting double-acting The piston is closed at both ends and engines to submarine use. First, the has a rigid piston rod extending crosshead type of piston construction from the lower end. Both ends of the requires considerably more length cylinder are closed to form a than that of single-acting engine combustion chamber at each types. As a
11 Figure 1-3. The 4-stroke diesel cycle. 12
Figure 1-4. The 2-stroke diesel cycle. 13
Figure 1-5. Single-acting diesel principle. consequence, the engines must be As the pistons travel together they built too high and bulky for practical compress air between them. The use in the confined spaces available space between the two pistons thus aboard submarines. Secondly, many becomes the combustion chamber. difficulties are encountered in The point at which the two pistons effecting a tight seal where the come into closest proximity is called piston rod passes through the combustion dead center. Just prior to stuffing box. combustion dead center, fuel is injected and the resultant expansion 1C3. Opposed piston engine. The caused by combustion drives the opposed piston engine (Figure 1-7) pistons apart. is designed with two pistons in each cylinder. The pistons are arranged in The scavenging air ports are located opposed positions in the cylinder. in the cylinder walls at the top of the Piston action is so timed that at one cylinder and are opened and closed by point of travel the two pistons come the movement of the upper piston. into close proximity to each other The exhaust ports are located near the near the, center of the cylinder. bottom of the cylinder and are opened and closed by the movement of the lower piston. 14
Figure 1-6. Double-acting diesel principle. 15
1. Both pistons are on the return travel from outer dead center, the upper piston has covered the scavenging air ports, the lower piston has covered the exhaust ports, and compression has begun. 2. Just as both pistons approach combustion dead center, fuel is injected. 3. Injection has been completed, expansion has begun, and both pistons are moving toward outer dead center. 4. Expansion of gases from combustion drives the pistons apart, causing the crankshafts to turn. This is the power stroke of the cycle. 5. As the pistons approach outer dead center, the lower piston uncovers the Figure 1-7. Opposed piston exhaust ports and most of the principle. expanded gases escape. Just before All the upper pistons are connected reaching outer dead center, the upper by connecting rods to the upper piston uncovers the scavenging air crankshaft. All the lower pistons are ports and scavenging air rushes into connected by connecting rods to the the cylinder, cleaning out the lower crankshaft. In Fairbanks remaining exhaust gases. Morse, opposed piston, submarine engines, the upper and lower crankshafts are connected by a vertical gear drive. The power from the upper crankshaft not used to drive auxiliaries is transmitted through this drive to the lower crankshaft and ultimately to the engine final drive. Figure 1-8 shows the various phases in a 2-stroke cycle of operation in an opposed piston engine. Figure 1-8. Opposed piston cycle. 16
6. The lower piston has covered the With the 12-degree lower crankshaft exhaust ports and scavenging air lead, the lower piston has advanced supercharges the cylinder until the the crankshaft through a 12-degree arc upper piston covers the scavenging of travel in the expansion phase of the air ports. cycle by the time the upper piston has reached inner dead center. This causes Figure 1-13 shows how the lower the lower piston to receive, at full crankshaft leads the upper engine load, the greater part of the crankshaft by 12 degrees in the expansion work, with the result that Fairbanks-Morse submarine diesel about 70 percent of the total power is engine. This lower crankshaft lead delivered by the lower crankshaft. has a definite effect both upon scavenging and power output. For submarine use, the opposed piston engine has three distinct advantages. Since the lower crankshaft leads the upper, the exhaust ports at the lower 1. It has higher thermal efficiency end of the cylinder are covered than engines of comparable ratings. slightly before upper piston travel 2. It eliminates the necessity of covers the intake ports. Thus, for a cylinder heads and intricate valve brief interval, the exhaust ports are mechanisms with their cooling and closed while the intake parts are lubricating problems. open. By the time the intake port is covered, the cylinder has been 3. There are fewer moving parts. charged with fresh air well above atmospheric pressure. Thus, through the lower crankshaft lead and scavenging action, a supercharging effect is achieved in this engine. Figure 1-8. Opposed piston cycle. 17