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DIESEL ENGINE PRINCIPLES
 
A. DEVELOPMENT
 
1A1. General. 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.

It is significant that the diesel engine is an outgrowth of the early struggle to improve the efficiency of existing types of other internal combustion engines. Today's fleet type submarine diesel engines are indirectly the result of widespread experimentation in both the Otto (gasoline) engine field and the more recently developed diesel engine field. Basically, however, the principles of operation have not changed materially since the first practical models of the early designs.

Among the contributors to progress in the development of diesel engines has been the Submarine Service of the United States Navy. Keen interest and untiring effort, not to mention risk in experimentation, testing, and correcting design, have given unparalleled impetus toward improved design.

1A2. History of diesel engine development. The reciprocating internal combustion engine was introduced in theory as far back as 1862 by Beau de Roches in France. A few years later, Otto, of Germany, made the first practical application of Beau de Roches's theory in an actual working model. Otto's engine was practicable and fairly reliable compared to other earlier attempts. It employed a 4-stroke cycle of operation using gas as a fuel. Thus, the 4-stroke cycle of a gas engine became popularly known as an Otto cycle.

George Brayton, an American, introduced a new principle of fuel injection in 1872. Brayton used an internal combustion gas engine in his experiments. He demonstrated that prolonging the combustion phase of the cycle, by injecting fuel at a controlled rate, produced more power per unit of fuel consumed. 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.

  The next notable achievement in improving the efficiency of the internal combustion engine was the Hornsby-Ackroyd engine produced in England a short while later. It was among the first early designed engines that used a liquid fuel derived from crude oil. This engine employed the Brayton principle of controlled fuel injection and compressed the air in the cylinder prior to ignition. The compression heat thus generated, plus the use of a hot surface, induced ignition. Since this engine employed hydraulic force to inject the fuel, it is now considered the first example of an engine using mechanical or solid injection.

In 1893, Dr. Rudolf Diesel, a Bavarian scientist, patented a design for an internal combustion engine which was termed a Diesel engine. He considered previous failures and applied himself to designing an engine to operate on an entirely different thermodynamic principle.

Using the mechanics of the 4-stroke cycle, Dr. Diesel proposed that only air be drawn into the cylinder during the suction or intake stroke. The compression stroke was to compress the air in the cylinder to a sufficiently high temperature to induce ignition and combustion without the use of added heat. Like Brayton's engine, this engine was to inject fuel at a controlled rate. It was Dr. Diesel's theory that if the rate of injection were properly controlled during the combustion phase, combustion could be made to occur at a constant temperature. Since fuel would have to be injected against high compression pressures in the cylinder, Dr. Diesel's design called for fuel injection to be accomplished by a blast of highly compressed air. Essentially, this was air injection. Dr. Diesel further theorized that the temperature drop during the expansion phase of the cycle would be efficient to make external cooling of the combustion chamber unnecessary.

A single-cylinder working model was constructed and first experiments were conducted using coal dust as a fuel. All efforts to operate

 
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a working model on the cycle proposed by Dr. Diesel resulted in explosions and failure. Further attempts to experiment along this same line were abandoned. Consequently, an engine operating entirely on the theoretical cycle proposed by Dr. Diesel was never produced. This cycle subsequently became known as the diesel cycle.

Many designers realized the value of the practical elements in the cycle of operation outlined by Dr. Diesel. Subsequently, experimenters began to achieve favorable results by eliminating the impractical elements and by altering the cycle of operation. Successful experiments were conducted by the Machinen-fabrik-Augsburg-Nurnberg (commonly called MAN) concern in Germany.

By this time the more volatile petroleum fuels were in common use and diesel engines utilizing liquid fuel were designed. These engines operated on a cycle in which the combustion phase occurred at constant pressure rather than at constant temperature. Experience also disclosed that it was essential to cool the combustion chamber externally. Early diesel engines operating on the constant pressure cycle, were efficient enough to make commercial production feasible.

Progress in diesel engine design has been rapid since the early models were introduced. The impetus of war demands, progress in metallurgy, fabrication, and engineering, and refinements in fuels and lubricants have all served to produce modern, high-speed diesel engines of exceptional efficiency.

1A3. History of submarine engine development. The first United States submarines utilizing internal combustion engines for propulsion were powered by 45-horsepower, 2-cylinder, 4-stroke cycle gasoline engines produced by the Otto Company of Philadelphia. Meanwhile, the English Submarine Service made use of 12- and 16-cylinder gasoline engines in their earlier submarines.

The inherent hazards accompanying the use of such a highly volatile fuel as gasoline were quickly realized. Stowage was a constant problem and handling of the fuel was extremely dangerous. Internal explosions were frequent

  and, in addition, many of these engines gave off considerable carbon monoxide fumes, creating a menace to personnel.

In the meantime, MAN built and experimented with 2-stroke cycle diesel engines for submarine propulsion. However, insufficient progress had been made in metallurgy to provide metals capable of withstanding the greater heat and stress inherent in engines of this type. MAN then turned its efforts toward production of a 4-stroke cycle diesel engine capable of developing 1000 hp. While fairly successful, these engines eventually developed structural weaknesses at the crankcase.

By 1914 the MAN 4-stroke cycle diesel had been partially redesigned and strengthened, producing the SV45/42, 1200-hp engine used in the majority of German submarines during World War I. Following World War I, the United States Navy acquired a number of these engines for use in the earlier S-class boats. A copy of this engine was produced by the New York Navy Yard and used in other early S-class submarines.

The Electric Boat Company, which was formerly the Holland Torpedo Boat Company, became licensee in the United States for the MAN Company of Germany. Later, the Electric Boat Company consolidated with the New London Ship and Engine Company. Shortly before World War I, the Electric Boat Company developed the well-known NELSECO engine. During, and subsequent to World War I, a number of United States submarines of the O, R, and S classes were equipped with these NELSECO engines. In fact, the principal installations in United States submarines were 6- and 8-cylinder NELSECO's until about 1934.

Prior to 1930 the engines used in most submarines of all the larger naval powers, with the exception of Great Britain, were 4-stroke cycle diesel engines. The United States Navy, however, experimented with a 2-stroke cycle Busch-Sulzer engine and equipped a number of boats with this type of engine. Since then, the majority of engines designed for United States submarine use have been of the 2-stroke cycle type.

 
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Prior to 1929, all engines in the United States Submarine Service were of the air injection type. Shortly after 1929, mechanical or solid type injection was employed on MAN engines. The advantages to be obtained with this type of injection were immediately apparent. By using solid type injection, the weight of the engines could be considerably reduced. The elimination of the air compressor alone accounted for a saving in weight of approximately 14 percent.

The advantages derived from the use of mechanical injection were numerous and included:

1. simplification of design
2. reduction in length of the engine
3. greatly reduced weight per horsepower
4. reduced fuel consumption
5. improved load balance in the engine
6. far greater reliability
7. less maintenance

The need for more powerful engines became apparent with the development of the fleet type submarine. The three engines that seemed to fulfill submarine requirements were the Winton V-type, now known as the General Motors engine; the Fairbanks-Morse opposed piston type; and the Hooven-Owen-Rentschler double-acting type engine. Of these, the HOR was later removed from submarines in favor of the General Motors and Fairbanks-Morse engines which are now the two standard submarine engines.

At the present time, the General Motors Corporation manufactures 16-cylinder, single-acting engines rated at 1600 brake horsepower (bhp) for main engine installations, and 8-cylinder engines for auxiliary installations. Fairbanks Morse and Company manufactures 9- and 10-cylinder, opposed piston engines rated at 1600 bhp for main engine installations, and 7-cylinder, opposed piston engines for auxiliary installations. These engines have proved most efficient. They weigh as little as 15 to 20 pounds per bhp including auxiliary equipment. Standardizing on only two designs has also made it possible to mass produce engines with a minimum amount of delay and difficulty.

  1A4. How submarine requirements affect engine design. The fact that submarines are both subsurface and surface vessels places definite restrictions upon size, hull design, and shape. Total weight, too, is a factor having considerable bearing on underwater operations. Hull characteristics restrict engine size and location of the engine compartments. Engine weight must bear a proportionate relationship to the weight and displacement of the vessel as well as to power requirements.

In the first engine-powered submarines, the engines were mechanically connected directly to the propeller shafting. This design, known as direct drive, developed immediate operational problems. The hull characteristics definitely fixed the angle of the propeller shafting. This restriction also determined engine position and location. Also, the most efficient propeller speeds did not correspond with the most efficient engine speeds. In direct drive installations, critical speeds (or synchronous torsional vibrations) which were inherent in the early model engines, were transferred through the direct drive into shafting and propellers. At times, the exact cruising speed desired could not be obtained, as it was necessary to pass the engines through critical speeds in the desired operating range as rapidly as possible. Two major problems were brought to the foreground by these early models:

1. How to power the propellers and yet separate engines and propeller shafting so that no mechanical unity existed.

2. How to design a drive in which different and varied rotative speeds could be selected for both engines and propellers.

Various types and combinations of drives were designed and tested. Over a period of time it became apparent that the electric drive installations (commonly referred to as diesel-electric drive) were the practical solution. This type of design solved both of the major problems. The engines were coupled only to the generators that supplied power to the electric motors. The propeller shafting was driven by the motors through reduction gears or directly

 
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by slow-speed electric motors. The only connections between engine power and propeller shafting were electrical. Hence, vibrations developed by the engines could not be conducted to the propeller shafting and propellers, and the various stresses encountered by the propellers could not be transmitted directly to the engines as was the case with mechanical couplings.

In addition, the rotative speed of the engine was no longer limited by the rpm of the propellers. Consequently, the engines could be designed for any desired speed within a selected range. Likewise, the propellers could be operated independently of engine speed within the speed limits of their design. The diesel-electric drive gave greater latitude to designers with respect to operating speed, size, and location of engines. It also gave the boat designers greater freedom in placement of engine compartments.

There are eight major requirements that a submarine diesel engine should fulfill:

  1. The engine should furnish maximum amount of power with minimum weight and space requirement.

2. The engine should possess the ability to develop occasionally more than full load rating.

3. The engine should have the ability to run continuously at slightly less than full load rating.

4. The engine should operate with small fuel consumption per unit of horsepower.

5. The engine should have a small lubricating oil consumption.

6. All wearing parts should be readily accessible for quick replacement.

7. There should be perfect balance with respect to primary and secondary forces and couples.

8. Major critical speeds within the operating ranges of the engine should be eliminated.

 
B. PRINCIPLES Of DESIGN AND OPERATION
 
1B1. Reciprocating internal combustion engines. An engine that converts heat energy into work by burning fuel in a confined chamber is called an internal combustion engine. Such an engine employing back-and-forth motion of the pistons is called a reciprocating type internal combustion engine. The diesel engine and the gasoline engine are the most familiar examples of reciprocating internal combustion engines.

The basic principle of operation of an internal combustion engine is relatively simple. The space in the cylinder in which the fuel is burned is called the combustion chamber. Fuel and air are admitted to the combustion chamber and ignited. The resulting combustion increases the temperature within the combustion chamber. Gases, released by combustion, plus the increase in temperature, raise the pressure which acts on the piston crown, forcing the piston to move. Movement of the piston is transmitted through other parts to the crankshaft whose rotary motion is utilized for work. The expended gases are ejected from the cylinder, a new

  charge of fuel and air is admitted, and the process is repeated. The above sequence of events is called a cycle of operation.

1B2. Cycles of operation. The word cycle enters into the description of the operation of any internal combustion engine. As applied to internal combustion engines, it may be defined as the complete sequence of events that occur in the cylinder of an engine for each power stroke or impulse delivered to the crankshaft. Those events always occur in the same order each time the cycle is repeated.

Each cycle of operation is closely related to piston position and movement in the cylinder. Regardless of the number of piston strokes involved in a cycle, there are four definite events or phases that must occur in the cylinders.

1. Either air or a mixture of air and fuel must be taken into the cylinder and compressed.

2. The fuel and air mixture must be ignited, or fuel must be injected into the hot compressed air to cause ignition.

 
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3. The heat and expansion of gases resulting from combustion must perform work on the piston to produce motion.

4. The residual or exhaust gases must be discharged from the cylinder when expansion work is completed.

The cycles of operation in each type of internal combustion engine are characterized both by the mechanics of operation and the thermodynamic processes. The three most commonly known cycles are the Otto cycle, the diesel cycle, and the modified diesel cycle.

1B3 Thermodynamics. To explain thermodynamics as used in an engineering sense, it is first necessary to define the term and the related terms used with it.

Thermodynamics is the science that deals with the transformation of energy from one form to another. A basic law of thermodynamics is that energy can neither be created nor destroyed but may be changed from one form to another. In diesel engineering, we are concerned primarily with the means by which heat energy is transformed into mechanical energy or work.

Force is that push or pull which tends to give motion to a body at rest. A unit of force is the pound.

Pressure is force per unit area acting against a body. It is generally expressed in pounds per square inch (psi).

Work is the movement of force through a certain distance. It is measured by multiplying force by distance. The product is usually expressed in foot-pounds.

Power is the rate of doing work, or the amount of work done in unit time. The unit of power used by engineers is the horse power (hp). One horsepower is equivalent to 33,000 foot-pounds of work per minute or 33,000/60 = 550 foot-pounds per second.

Energy is the ability to perform work. Energy is of two types: kinetic, which is energy in motion, and potential, which is energy stored up.

  Matter is anything having weight and occupying space. Solids, liquids, and gases are matter.

A molecule is the smallest division of a given matter, which, when taken alone, still retains all the properties and characteristics of the matter.

Heat is a form of energy caused by the molecular activity of a substance. Increasing the velocity of molecular activity in a substance increases the amount of heat the substance contains. Decreasing the velocity of molecular activity in a substance decreases the amount of heat the substance contains.

Temperature is a measure of the intensity of heat and is recorded in degrees by a thermometer. The two temperature scales most commonly used are the Fahrenheit and centigrade scales.

Volume may be described as the amount of space displaced by a quantity of matter.

1B4. The mechanical equivalent of heat energy. The function of an internal combustion engine is to transform heat energy into mechanical energy. Recalling the basic law of thermodynamics we know that energy cannot be destroyed. It is possible to convert mechanical energy to heat completely, and by delicate physical experiments it has been found that for every 778 foot-pounds of mechanical energy so converted, one Btu of heat will be obtained. Because of fundamental limitations, it is usually not possible to convert heat completely to work, but for every Btu that is converted, 778 foot-pounds will be realized. This important constant is known as the mechanical equivalent of heat.

1B5. Relationship of pressure, temperature, and volume. Figure 1-1A illustrates a simple cylinder with a reciprocating piston. A dial pressure gage at the top of the cylinder registers pressure inside the cylinder. Temperature inside the cylinder is recorded by a thermometer. The thermometer at the side registers room temperature. The piston is at outer dead center in its stroke. At this stage, the pressure inside

 
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Figure 1-1 A and B. Pressure, temperature, and volume relationship in a cylinder.
Figure 1-1. Pressure, temperature, and volume relationship in a cylinder.
the cylinder is the same as atmospheric pressure outside, and the dial of the pressure gage registers 0. Also, the temperature inside the cylinder is the same as room temperature, or approximately 70 degrees F.

In Figure 1-1B, force has been applied to the piston, moving it about a third of the distance of its compression stroke. Air trapped in the cylinder is compressed. As the volume of this air is decreased, the pressure is increased to about 155 psi. The temperature rises from 70 degrees F to about 300 degrees, indicating that heat has been added to the air in the cylinder. This shows that mechanical energy, in the form of force supplied to the piston, has been transformed into heat energy in the compressed air.

In Figure 1-1C, more force has been applied to the piston, raising the pressure in the cylinder to about 300 psi, and the temperature to nearly 700 degrees F.

Figure 1-1D shows the final stage of the compression stroke as the piston arrives at inner dead center. Pressure is in the neighborhood of 470 psi and the temperature is about 1000 degrees F. This illustration closely approximates the

  conditions found in the compression stroke of a modern submarine diesel engine. The temperature of the compressed air within the cylinder has been raised to a sufficient degree to cause automatic ignition on the injection of fuel oil into the cylinder.

Thus, in summation, we see that during a cycle of operation, volume is constantly changing due to piston travel. As the piston travels toward the inner dead center during the compression stroke, the air in the cylinder is reduced in volume. Physically, this amounts to reducing the space occupied by the molecules of air. Thus, the pressure of the air working against the piston crown and walls of the cylinder is increased and the temperature rises as a result of the increased molecular activity. As the piston nears inner dead center, the volume is reduced rapidly and the temperature increases to a point sufficient to support the automatic ignition of any fuel injected.

Combustion changes the injected fuel to gases. After combustion, the liberation of the gases with a very slight increase in volume causes a sharp increase in pressure and

 
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Figure 1-1 C and D. Pressure, temperature, and volume relationship in a cylinder.
Figure 1-1. Pressure, temperature, and volume relationship in a cylinder.
temperature. During the power stroke, volume increases rapidly, and toward the end of the stroke, pressure and temperature decrease rapidly.

1B6. Pressure-volume diagrams. Various methods and devices are used for measuring and recording the pressures at various piston positions during a cycle of operation in an engine cylinder. The result may be graphically illustrated by a diagram such as that shown in Figure 1-2. Such diagrams are known as pressure-volume diagrams. In practice, they are referred to as indicator cards.

Pressure-volume diagrams give the relationship between pressures and piston positions, and may be used to measure the work done in the cylinder. Also, if the speed of the engine and the time involved in completing one cycle are known, the indicated horsepower may be computed by taking pressure-volume diagrams on each cylinder and converting the foot-pounds per unit of time into horsepower. This method of determining horsepower, however, is not practicable on modern fleet type submarine engines.

  1B7. Pressure-volume diagrams for the Otto cycle, diesel cycle, and modified diesel cycle. Figure 1-2 shows typical pressure-volume diagrams for the three types of engine cycles. Each pressure-volume diagram is a graphic representation of cylinder pressure as related to cylinder volume. In the diagrams the ordinate represents pressure and the abscissa represents volume. In actual practice, when an indicator card is taken on an engine, the vertical plane is calibrated in pressure units and the volume plane is calibrated in inches. The volume ordinate of the diagram then shows the length of stroke of the piston which is proportional to the volume.

Letters are located on each of the figures in the diagrams. The distance between two adjacent letters on the figures is representative of a phase of the cycle. Comparing the diagrams provides a visible means of comparing the variation in the phases between the three cycles.

1B8. The Otto cycle. The Otto cycle (Figure 1-2) is more commonly known as the constant volume cycle and its principles form

 
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Figure 1-2. Pressure-volume diagrams.
Figure 1-2. Pressure-volume diagrams.
the basis for all modern automobile gasoline engine designs. In this cycle, combustion is timed to occur theoretically just as the piston arrives at top dead center. Ignition is accomplished by a spark, and, due to the volatility of the fuel-air mixture, combustion practically amounts to an explosion. Combustion is completed with virtually no piston travel and hence, little, if any, change in volume of the gas in the combustion chamber. This gives rise to the description constant-volume cycle. During combustion there is a quick rise of the temperature in the cylinder, immediately followed by a pressure rise which performs the work during the power stroke.

The Otto cycle may be defined as a cycle in which combustion induced by spark ignition theoretically occurs at constant volume.

1B9. The diesel cycle. In the true diesel cycle, only air is compressed in the cylinder prior to ignition. This normally produces a final compression pressure of about 500 psi. At such a pressure the temperature of the compressed air may range from 900 degrees to 1050 degrees F. Since most fuel

  oils will ignite automatically with sufficient air at a temperature of about 480 degrees F, ignition occurs as soon as the fuel oil spray reaches the hot air. This is called compression ignition.

This combustion process (or burning of the fuel and compressed air) is a relatively slow process compared with the quick, explosion type combustion process of the Otto cycle. The fuel spray penetrates the compressed air, some of the fuel ignites, then the rest of the fuel charge burns. In the true diesel cycle, the expansion of gases keeps pace with the change in volume occasioned by piston travel during the combustion phase. Thus combustion is said to occur at constant pressure.

The diesel cycle may be defined as a cycle in which combustion induced by compression ignition theoretically occurs at a constant pressure.

1B10. Modified diesel cycle. We have previously described the Otto cycle as one in which combustion occurs theoretically at constant volume, and the diesel cycle as one in which

 
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combustion occurs theoretically at constant pressure. In actual operation, a gasoline engine does not follow the true Otto cycle, nor does the diesel engine follow the true diesel cycle. In fact, the operation of a medium- or high-speed diesel engine follows the modified diesel cycle (Figure 1-2). This cycle involves phases of both the Otto cycle and the diesel cycle in that the combustion phase takes place at both constant volume and constant pressure.

The modified diesel cycle, as applied to diesel engines, may be defined as a cycle of operation in which the combustion phase, induced by compression ignition, begins on a constant-volume basis and ends on a constant pressure basis.

All submarine main and auxiliary engines used today employ the modified diesel cycle. The fundamental differences between the Otto and the modified diesel cycles are:

1. The methods of mixing fuel and air. This is accomplished before and during compression in the Otto cycle and usually near the end of the compression phase in the modified diesel cycle.

2. The methods of ignition. Spark ignition is used in the Otto cycle and compression ignition is used in the modified diesel cycle.

The term diesel cycle has become popularly associated with all compression-ignition or diesel engines. In actual practice, this is a misnomer when applied to modern, medium-speed or high-speed diesel engines, because practically all diesel or compression-ignition engines in this category operate on the modified diesel cycle.

1B11. Thermodynamics of the Otto cycle, every diesel cycle, and modified diesel cycle. In every thermodynamic cycle there must be a working substance. With internal combustion engines, some form of substance must undergo a change in the cylinder in order to convert heat energy into mechanical energy. The working substance in the cylinder of a compression-ignition engine is fuel oil.

  After the fuel is injected into the cylinder, combustion converts it into gases. This conversion is a thermodynamic change. A thermodynamic change during which the temperature remains constant is called an isothermal process. A thermodynamic change during which the temperature may vary but during which heat is neither received nor rejected is called an adiabatic process.

In a strict sense the thermodynamic cycles outlined below are not true thermodynamic cycles. In a true cycle the process is reversible. The working substance is heated, does work, is cooled, and is heated again. In the cycle of an actual engine, the residue of the combustion process is exhausted at the end of the expansion stroke and a new charge is taken into the cylinder for the next cycle of events. However, the true thermodynamic cycle is useful for studying the thermodynamic processes in actual engine operation.

a. The Otto cycle. This is the thermodynamic cycle used as a basis for the operation of all modern gasoline engines. The cycle (Figure 1-2) consists of the adiabatic compression of the charge in the cylinder along the line AB, the constant-volume combustion and heating of the charge from B to C, the adiabatic expansion of the gases from C to D, and the constant-volume rejection of gases from the cylinder along DA.

b. The diesel cycle. In the original diesel cycle proposed by Dr. Diesel, the combustion phase of the thermodynamic cycle was to be a constant-temperature or isothermal process. However, no engine was ever operated on this cycle. As a result of his experimentation, however, a constant-pressure thermodynamic cycle was developed. All early type, slow-speed diesel engines approximated this cycle, although it is in little use today.

In this cycle (Figure 1-2), adiabatic compression occurred along AB, to provide the temperature necessary for the ignition of the fuel. Fuel injection and combustion were so controlled as to give constant-pressure combustion

 
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along BC. This was followed by adiabatic expansion from C to D. Rejection of the gases from the cylinder was constant volume from D to A.

c. The modified diesel cycle. This is the cycle (Figure 1-2) used in all fleet type submarine diesel engines and in practically all modern diesel engines. In this thermodynamic cycle, compression is adiabatic from A to B. Combustion is partly constant volume from B to C and partly constant pressure from C to D. Expansion is adiabatic from D to E. Rejection of gases from the cylinder is constant volume along EA.

1B12. Thermal efficiency. The thermal efficiency of an internal combustion engine may be considered the percentage of efficiency, in converting the total potential heat energy available in the fuel into mechanical energy. We have already stated that the mechanical equivalent of heat energy is 778 foot-pounds for one Btu of heat. By this equation, it is a simple matter to figure how much work should be delivered on an ideal basis from a given quantity of fuel. An engine operating on this- basis would be 100 percent efficient. No internal combustion engine, however, is 100 percent efficient, because heat losses, conducted through the cooling and exhaust systems, and friction losses make the thermal efficiency of any internal combustion engine relatively low.

1B13. The 4-stroke diesel cycle. In the 4-stroke diesel cycle, the piston makes four strokes to complete the cycle. There is one power stroke or power impulse for every four piston strokes, or two complete revolutions of the crankshaft.

Figure 1-3 shows the four strokes and the sequence of events that occur in the 4-stroke diesel cycle.

1. The intake valve opens and a supply of fresh air is drawn into the cylinder while the piston makes a downward stroke.

  2. With the intake valve closed, the piston makes an upward stroke, compressing the air. Pressure is generally around 500 psi with resultant temperatures as high as 900 degrees to 1050 degrees F, depending on the design of the engine. At about the end of this stroke, the fuel is injected into the hot compressed air, and ignition and combustion occur over a relatively short period of piston travel.

3. The expansion of combustion gases forces the piston downward through one stroke. This is called the power stroke. As the piston nears the end of this stroke, the exhaust valve opens, permitting some of the burned gases to escape.

4. The piston makes another upward stroke in which the remaining exhaust gases are forced out of the cylinder. This completes the cycle.

1B14. The 2-stroke diesel cycle. In this cycle (Figure 1-4) the piston makes two strokes to complete the cycle. There is one power stroke for every two piston strokes or for each revolution of the crankshaft. An engine employing this cycle requires a scavenging air blower to assist in clearing the exhaust gases from the cylinder, to replenish the cylinder with the necessary volume of fresh air, and to make possible a slight supercharging effect.

Figure, 1-4 shows the two strokes and the sequence of events that occur in the 2-stroke diesel cycle as follows:

1. Start of compression. The piston has just passed bottom dead center, the cylinder is charged with fresh air, and both the intake ports and the exhaust valve are closed. The fresh air is trapped and compressed in the cylinder.

2. Injection. At about the end of the compression stroke, the fuel is injected and combustion occurs.

3. Expansion. Expansion of gases from combustion forces the piston downward through one stroke. As the piston nears the end of this stroke, the exhaust valve is opened slightly in

 
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advance of the uncovering of the intake ports. This permits some of the burned gases to escape.

4. Exhaust. As the intake ports are

  uncovered, the scavenging air which is under pressure, rushes into the cylinder. This drives out the remaining exhaust gases and completes the cycle.
 
C. DIESEL ENGINE TYPES
 
1C1. Single-acting diesel engine. Both the 4-stroke and the 2-stroke cycle diesel engines illustrated and described in the previous section were of the single-acting type (Figure 1-5). In all single-acting engines the pistons used are usually of the trunk type, that is, pistons whose length is greater than their diameter. One end of the trunk type piston is closed; this end is called the crown. The opposite or skirt end of the piston is open. The connecting rod extends through the open end of the piston and is attached to the piston by means of the piston pin.

The term single-acting is used to describe these engines because the pressure of the gases of combustion acts only on one side (the crown) of the pistons. In the 4-stroke cycle, single-acting engines, the power stroke occurs only once in every two revolutions of the crankshaft. In the 2-stroke cycle, single-acting engines, the power stroke occurs once in every revolution of the crankshaft. All of the main and auxiliary diesel engines currently installed in fleet type submarines are of the single-acting type.

1C2. Double-acting diesel engine. A considerable number of double-acting diesel engines (Figure 1-6), namely the HOR and MAN engines, were used in installations for fleet type submarines until recent years. Lately, however, most of these double-acting engines have been removed and replaced with 2-stroke cycle, single-acting engines. While double-acting engines have no place in current installations, it is well for the student to be familiar with their general design and operation.

In double-acting diesel engines, the piston proper is usually shorter and is described as the crosshead type. The piston is closed at both ends and has a rigid piston rod extending from the lower end. Both ends of the cylinder are closed to form a combustion chamber at each

  end of the piston. The piston rod extends through the cylinder head of the lower combustion chamber and passes through a stuffing box to prevent leakage of pressure. The piston rod is attached to a crosshead, and the connecting rod is attached to the crosshead so that it may turn freely on the crosshead pin. The crosshead has a flat bearing surface that moves up and down on a crosshead guide to steady the piston rod and piston and prevent uneven wear.

Combustion occurs in the upper combustion chamber, and the pressure of the gases of combustion is applied to the top end of the piston during the downward stroke. At the completion of this stroke, combustion occurs in the bottom combustion chamber and expansion pressure is applied to the bottom end of the piston during the upward stroke. The downward power stroke serves as the compression stroke for the lower combustion chamber and the upward power stroke serves as the compression stroke for the top combustion chamber. Thus the power strokes are double that of a single acting engine and the engine is referred to as a double-acting type.

The 2-stroke cycle, double-acting engine has a distinct advantage in power output compared with the single-acting type. With twice as many power strokes as a comparable single acting engine and, with other conditions being equal, it develops practically twice as much power per cylinder. In addition, the operation is smoother due to the fact that the expansion stroke in one combustion chamber of the cylinder is balanced or cushioned by the compression stroke in the opposite combustion chamber.

There are two principal difficulties encountered in adapting double-acting engines to submarine use. First, the crosshead type of piston construction requires considerably more length than that of single-acting engine types. As a

 
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Figure 1-3. The 4-stroke diesel cycle.
Figure 1-3. The 4-stroke diesel cycle.
 
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Figure 1-4. The 2-stroke diesel cycle.
Figure 1-4. The 2-stroke diesel cycle.
 
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Figure 1-5. Single-acting diesel principle.
Figure 1-5. Single-acting diesel principle.
consequence, the engines must be built too high and bulky for practical use in the confined spaces available aboard submarines. Secondly, many difficulties are encountered in effecting a tight seal where the piston rod passes through the stuffing box.

1C3. Opposed piston engine. The opposed piston engine (Figure 1-7) is designed with two pistons in each cylinder. The pistons are arranged in opposed positions in the cylinder. Piston action is so timed that at one point of travel the two pistons come into close proximity to each other near the, center of the cylinder.

  As the pistons travel together they compress air between them. The space between the two pistons thus becomes the combustion chamber. The point at which the two pistons come into closest proximity is called combustion dead center. Just prior to combustion dead center, fuel is injected and the resultant expansion caused by combustion drives the pistons apart.

The scavenging air ports are located in the cylinder walls at the top of the cylinder and are opened and closed by the movement of the upper piston. The exhaust ports are located near the bottom of the cylinder and are opened and closed by the movement of the lower piston.

 
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Figure 1-6. Double-acting diesel principle.
Figure 1-6. Double-acting diesel principle.
 
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Figure 1-7. Opposed piston principle.
Figure 1-7. Opposed piston principle.

All the upper pistons are connected by connecting rods to the upper crankshaft. All the lower pistons are connected by connecting rods to the lower crankshaft. In Fairbanks 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.

  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 exhaust ports and most of the expanded gases escape. Just before reaching outer dead center, the upper piston uncovers the scavenging air ports and scavenging air rushes into the cylinder, cleaning out the remaining exhaust gases.

Figure 1-8. Opposed piston cycle, 1 and 2.
Figure 1-8. Opposed piston cycle.

 
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6. The lower piston has covered the exhaust ports and scavenging air supercharges the cylinder until the upper piston covers the scavenging air ports.

Figure 1-13 shows how the lower crankshaft leads the upper crankshaft by 12 degrees in the Fairbanks-Morse submarine diesel engine. This lower crankshaft lead has a definite effect both upon scavenging and power output.

Since the lower crankshaft leads the upper, the exhaust ports at the lower end of the cylinder are covered slightly before upper piston travel covers the intake ports. Thus, for a brief interval, the exhaust ports are closed while the intake parts are open. By the time the intake port is covered, the cylinder has been 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.

  With the 12-degree lower crankshaft lead, the lower piston has advanced the crankshaft through a 12-degree arc of travel in the expansion phase of the cycle by the time the upper piston has reached inner dead center. This causes the lower piston to receive, at full engine load, the greater part of the expansion work, with the result that about 70 percent of the total power is delivered by the lower crankshaft.

For submarine use, the opposed piston engine has three distinct advantages.

1. It has higher thermal efficiency than engines of comparable ratings.

2. It eliminates the necessity of cylinder heads and intricate valve mechanisms with their cooling and lubricating problems.

3. There are fewer moving parts.

Figure 1-8. Opposed piston cycle.
Figure 1-8. Opposed piston cycle.
 
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Figure 1-9. GM 16-278A, outboard side, control end, right-hand engine.
Figure 1-9. GM 16-278A, outboard side, control end, right-hand engine.
 
Figure 1-10. GM 16-278A, inboard side, blower end, right-hand engine.
Figure 1-10. GM 16-278A, inboard side, blower end, right-hand engine.
 
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Figure 1-11. F-M 10-cylinder 38D 8 1/8, outboard side, blower end, left-hand engine.
Figure 1-11. F-M 10-cylinder 38D 8 1/8, outboard side, blower end, left-hand engine.
 
Figure 1-12. F-M 10-cylinder 38D 8 1/8, Inboard side, control end, right-hand engine.
Figure 1-12. F-M 10-cylinder 38D 8 1/8, Inboard side, control end, right-hand engine.
 
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Figure 1-13. Lower crank lead.
Figure 1-13. Lower crank lead.

1C4. Modern fleet type submarine diesel engines. Modern diesel engines currently used

  in fleet type submarine installations vary in design but all are of the 2-stroke cycle type. Following is a list of engines normally found on fleet type submarines:

a. Main engines.

1. General Motors V-16 type. There are two engine designs in this category, the 16-278A and 16-248. Each engine has two banks of 8 cylinders, each arranged in a V-design with 40 degrees between banks. Each engine is rated at 1600 bhp at 750 rpm. Both engines are equipped with mechanical or solid type injection and have a uniflow valve and port system of scavenging.

2. Fairbanks-Morse opposed piston type, Model 38D 8 1/8. This model number includes two engines, one a 10-cylinder and the other a 9-cylinder engine. Both engines are rated at 1600 bhp at 720 rpm. Both engines are equipped with mechanical or solid type injection and have a uniflow port system of scavenging.

b. Auxiliary engines.

1. General Motors, Model 8-268. This engine is an 8-cylinder, in-line type. When operated in a generator set at 1200 rpm, it has a power output of 300 kilowatts. This engine is equipped with mechanical or solid type injection and has a uniflow valve and port system of scavenging.

2. Fairbanks-Morse opposed piston type, Model 38E 5 1/4. This is a 7-cylinder, opposed piston type engine. When operated in a generator set at 1200 rpm, it has a power output of 300 kilowatts. This engine is equipped with mechanical or solid type injection and has a uniflow port system of scavenging.

 
D. SUBMARINE DIESEL ENGINE INSTALLATIONS
 
1D1. Submarine diesel engine installations. Figure 1-14 shows a typical main and auxiliary engine installation aboard a modern, diesel-electric drive, fleet type submarine. Each engine is coupled with a generator to form a generator set. Through the main control cubicle, the current supplied by main generator sets may be   directed to charging the batteries or powering the main motors. The auxiliary generator set may be used directly either to charge the batteries or to power the auxiliary equipment. It may also be used indirectly for powering the main motors. Main motors are used for propulsion and may be powered either by the batteries or by the main generator sets.
 
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Figure 1-14. CUTAWAY OF FLEET TYPE SUBMARINE SHOWING ENGINE INSTALLATIONS.

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