Tài liệu Diezel 1410 P9

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Tài liệu Diezel 1410 P9

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Combustion. Engine efficiency is a comparison of the amount of power developed by an engine to the energy input as measured by the heating value of the fuel consumed

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  1. 9 ENGINE PERFORMANCE AND OPERATION A. COMBUSTION, AND EFFICIENCY 9A1. Combustion. Engine efficiency is 1. The fuel must enter the cylinder at the, a comparison of the amount of power proper time. That is, the fuel injection developed by an engine to the energy valve must open and close in correct input as measured by the heating value relation to the position of the piston. of the fuel consumed. In order to understand the various factors 2. The fuel must enter the cylinder in a responsible for differences in engine fine mist or fog. efficiency, it is necessary to have some knowledge of the combustion process 3. The fuel must mix thoroughly with the which takes place in the engine. air that supports its combustion. In the diesel engine, ignition of the fuel 4. Sufficient air must be present to assure is accomplished by the heat of complete combustion. compression alone. To support combustion, air is required. 5. The temperature of compression must Approximately 14 pounds of air are be sufficient to ignite the fuel. required for the combustion of 1 pound of fuel oil. However, to insure complete Figure 9-1 is a reproduction of a combustion of the fuel, an excess pressure-time diagram of a mechanical amount of air is always supplied to the injection engine. The lower curvy part of cylinders. The ratio of the amount of air which is a dotted line, is the curve of supplied to the quantity of fuel injected compression and expansion when no fuel during each power stroke is called the is injected. At A the injection valve air-fuel ratio and is an important factor opens, fuel enters the combustion in the operation of any internal- chamber and ignition occurs at B. The combustion engine. When the engine is pressure from A to B should fall slightly operating at light loads there is a, large below the compression curve without excess of air present, and even when fuel due to absorption of heat by the fuel the engine is overloaded, there is an from the air. The period from A to B is excess of air over the minimum the ignition delay. From B the pressure required for complete combustion. rises rapidly until it reaches a maximum at C. This maximum, in some instances, The injected fuel must be divided into may occur at top dead center. At D the small particles, usually by mechanical injection valve closes, the fuel is cut off, atomization, as it is sprayed or injected but burning of the fuel continues to some into the combustion chamber. It is undetermined point along the expansion imperative that each of the small stroke. particles be completely surrounded by sufficient air to effect complete The height of the diagram from B to C is combustion of the fuel. To accomplish called the firing pressure rise and the this, the air in the cylinder must be in slope of the curve between these two motion with good fuel atomization, points is the rate at which the fuel is combined with penetration and burned. distribution. In mechanical injection engines this is accomplished by forcing Poor combustion of the fuel is usually scavenging air into the cylinder with a indicated by a smoky exhaust, but some whirling motion to create the necessary smoke may be the result of burning turbulence. This is usually done, in the lubricating oil that has passed the rings 2-cycle engine, by shaping the intake into the combustion chamber. Incomplete air ports, or by casting them so that combustion is indicated by black smoke,
  2. their centers are slightly tangential toor if the fuel is not igniting, it may appear as blue smoke. Immediately after starting the axis of the cylinder bore. an engine, when running at light loads or Before proceeding with the study of the at overloads, or when changing from one load to another, smoke is likely to combustion process, the conditions appear. considered essential to good combustion should be reviewed: A smoky exhaust from the engine does not indicate whether one or all the cylinders are 174 Figure 9-1. Pressure-time diagram of combustion process. causing it A black-smoking cylinder radiation and convection to the usually shows a higher exhaust surrounding air. temperature which can be observed from pyrometers installed in the 2. Heat rejected and lost to the individual exhaust lines from the atmosphere in the exhaust. cylinders. Opening the indicator cock on each cylinder to observe the color of 3. Inefficient combustion or lack of the exhaust is another check. Still perfect combustion. another method is cutting off the fuel supply to one cylinder at a time to see A loss due to imperfect or incomplete what effect it has on the engine combustion is an important item, because exhaust. This latter should never be such losses have a serious effect on the done when the engine is operating at power that can be developed in the full load as overloading of the other cylinder as shown by the pressure- cylinders will result if the engine is volume diagram or indicator card. governor controlled. Complete combustion is not possible in the short time permitted in modern 9A2. Engine losses. It is obvious that engine design. However, these losses not all of the heat content of a fuel can may be kept to a minimum if the engine be transferred into useful work during is kept adjusted to the proper operating the combustion process. The many condition. Incomplete combustion can different losses that take place in the frequently be detected by watching transformation of heat energy into work exhaust temperatures, noting the exhaust may be divided into two classes, color, and being alert for unusual noises thermodynamic and mechanical. The
  3. net useful work delivered by an engine in the engine. is the result obtained by deducting the total losses from the heat energy input. Heat energy losses from both the cooling water systems and lubricating oil system are always present. Some heat is Thermodynamic losses are caused by: conducted through the engine parts and 1. Loss to the cooling system and losses radiated to the atmosphere or picked up by the surrounding air by convection. by The effect of these losses varies according to the part of the cycle in which they occur. The 175 heat appearing in the jacket cooling water is not a true measure of cooling loss because this heat includes: 1. Heat losses to jackets during compression, combustion, and expansion phases of the working cycle. 2. Heat losses during the exhaust stroke. 3. Heat losses absorbed by the walls of the exhaust passages. 4. Heat generated by piston friction on cylinder walls. Figure 9-2. Heat balance for a diesel engine. Heat losses to the atmosphere through the exhaust are inevitable because the pumping losses caused by operation of engine cylinder must be cleared of the water pumps, lubricating oil pumps, and still hot exhaust gases before another scavenging air blowers, power required fresh air charge can be introduced and to operate valves, and so forth. Friction another power stroke begun. The heat losses cannot be eliminated, but they can lost to the exhaust is determined by the be kept at a minimum by maintaining the temperature within the cylinder when engine in its best mechanical condition. exhaust begins. It depends upon the Bearings, pistons, and piston rings should amount of fuel injected and the weight be properly installed and fitted, shafts of air compressed within the cylinder. must be in alignment, and lubricating and Improper timing of the exhaust valves, cooling systems should be at their highest whether early or late, will result in operating efficiency. increased heat losses. If early, the valve releases the pressure in the cylinder 9A3. Compression ratio and before all the available work is efficiencies. a. Compression ratio. The obtained; if late, the necessary amount term compression ratio is used quite of air for complete combustion of the extensively in connection with engine next charge cannot be realized, performance and various types of although a small amount of additional efficiencies. It may be defined as the work may be obtained. The timing of ratio of the total volume of a cylinder to the exhaust valve is a compromise, the the clearance volume of the cylinder. It best possible position of opening and may be best explained by reference to the closing being determined by the engine
  4. designer. It is essential that the valve be pressure-volume indicator card of a tight and properly timed in order to diesel cylinder. In Figure 9-3, the volume maintain the loss to the exhaust at a is reduced from square root(C) + square minimum. This is also true for air inlet root(D) to square root(C) during compression. The compression ratio is valve setting on 4-cycle type engines. then equal to (square root(C) + square root(D))/square If an indicator card is taken of a diesel root(C) engine cylinder, it is possible to calculate the horsepower developed within the cylinder. This calculation does not take into account the power loss resulting from mechanical or friction losses, as will be discussed later, but it reflects the actual work produced within the cylinder. Mechanical losses are of several kinds, not all of them present in every engine. The sum total of these mechanical losses deducted from the indicated horsepower developed in the cylinders will give the brake horsepower finally delivered as useful work by the engine. These mechanical or friction losses include bearing friction, piston and piston ring friction, and 176 the fuel would fire or detonate before the piston could reach the correct firing position. The temperature-entropy (T-S) diagram of any particular cycle indicates the amount of heat input and the amount of heat rejected. For example, in Figure 9-4, the T-S diagram of a modified diesel cycle, the heat input is represented by the area FBDG and the heat rejected to the exhaust by the area FAEG. The heat represented in doing useful work is represented by the difference between Figure 9-3. Compression ratio. these two, or area ABDE. The efficiency of the cycle can then be expressed as Compression ratio influences the (H1-H2)/H1 where H1 is the heat input thermal efficiency of an engine. along lines BC and CD (the lines Theoretically the thermal efficiency representing the constant volume and increases as the compression ratio is constant pressure combustion), and H2 is increased. The minimum value of a the heat rejected along line EA (the line diesel engine compression ratio is representing the constant volume determined by the compression exhaust). Since heat and temperature are required for starting, which, to large proportional to each other, the cycle extent is dependent on the type of fuel efficiency is actually computed from used. The maximum value of the measurements made of the temperature. compression ratio is not limited by the
  5. fuel used but is limited by the strength The specific heat of the mixture in the of the parts of the engine and the cylinder is either known or assumed, and when combined with the temperature, the allowable engine wgt/bhp output. heat content can be calculated at any instant. Thus, it is seen that temperature b. Cycle efficiency. The efficiency of is a measure of heat, and that the heat is any cycle is equal to the output divided proportional to the temperature of the by the input. The diesel cycle shows gas. one of the highest efficiencies of any engine yet built because of the higher compression ratio carried and because c. Volumetric efficiency. The volumetric of the fact that combustion starts at a efficiency of an engine is the ratio of the higher temperature. In other words, the volume that would be occupied by the air heat input is at a higher average charge at atmospheric temperature and temperature. Theoretically, the gasoline pressure to the cylinder displacement (the engine using the Otto or constant product of the volume cycle would be more efficient than the diesel if it could use compression ratios as high as the latter. The gasoline engine operating on the Otto cycle cannot use a compression ratio comparable to the diesel engine due to the fact that the fuel and air are drawn in together and compressed. If high compression ratios were used, Figure 9-4. Temperature-entropy diagram of modified diesel cycle. 177 area of the bore times the stroke of the calculated as previously explained, the piston). The volumetric efficiency indicated thermal efficiency can be determines the amount of air available computed. for combustion of the fuel, and hence influences the maximum power output Indicated thermal efficiency = of the engine. (Indicated hp X 42.42 Btu per minute per hp) / (Rate of heat input of fuel in Btu per Volumetric efficiency is actually the minute) X 100 percent completeness of filling of the cylinder with fresh air at atmospheric pressure. In like manner the over-all thermal The volumetric efficiency of an engine efficiency can be found from the brake may be increased by enlarging the areas horsepower or the actual power available of intake and exhaust valves or ports, at the engine shaft.* and by having all valves properly timed so that as much air as possible will Over-all thermal efficiency = enter the cylinders. Since any burned Brake horsepower / Heat input of fuel X gases will reduce the charge of fresh 100 percent air, the supercharging effect gained by early closing of the exhaust valves or e. Mechanical efficiency. The mechanical ports will reduce the volumetric losses in an engine decrease the efficiency. In some engines, the efficiency of the engine and represent the volumetric efficiency is also increased skill with which the engine parts were by using special apparatus to utilize air designed as well as the skill with which at 2 to 3 psi over the atmospheric the operator maintains the engine. As pressure. This procedure is commonly previously stated, the brake horsepower
  6. is equal to the indicated horsepower called supercharging. minus the mechanical losses. The ratio of brake horsepower to indicated d. Thermal efficiency. Thermal horsepower, then, is the mechanical efficiency may be regarded as a efficiency of the engine which increases measure of the efficiency and as the mechanical losses decrease. completeness of combustion of the injected fuel. Thermal efficiencies are generally considered as being of two Mechanical efficiency = kinds, indicated thermal efficiency and Brake horsepower / Indicated horsepower over-all thermal efficiency. X 100 percent If all the potential heat in the fuel were delivered as work, the thermal efficiency would be 100 percent. This * This power referred to as shaft is not possible in practice, of course. To horsepower, is the amount available for determine the values of the above useful work. It is the power available at efficiencies the amount of fuel injected the propeller. There is a further loss of is known, and from its heating value, or power between the main propulsion Btu per pound, the total heat content of engine (measured as brake horsepower) the injected fuel can be found. From the and shaft horsepower due to the friction mechanical equivalent of heat (778 in the reduction gears, hydraulic or foot-pounds are equal to 1 Btu), the electric type couplings, line shaft number of foot-pounds of work bearings, stuffing boxes, stern tube contained in the fuel can be computed. bearings, and strut bearings. These losses If the amount of fuel injected is in some cases are considerable and the measured over a period of time, the rate total loss may be as high as 7 or 8 at which the heat is put into the engine percent. Therefore, they should not be can be converted into potential power. neglected in making computations. Then, if the indicated horsepower developed by the engine is 178 B. ENGINE PERFORMANCE 9B1. Engine performance. a. General. which the engine will operate with a Many factors affect the engine smoky exhaust. performance of an engine. Some of these factors are inherent in the engine f. Injection rate. The rate of injection is design; others can be controlled by the important because it determines the rate operator. The following list of variable of combustion and influences engine conditions affecting the performance of efficiency. Injection should start slowly a diesel engine is not complete, but so that a limited amount of fuel will contains all the important factors that accumulate in the cylinder during the should be familiar to operating initial ignition lag before combustion personnel. begins. It should proceed at such a rate that the maximum rise in cylinder pressure is moderate, but it must b. Fuel characteristics. The cetane number of the fuel has an important introduce the fuel as rapidly as effect on engine performance. Fuels permissible in order to obtain complete with low cetane rating have high combustion and maximum expansion of ignition lag. A considerable amount of the combustion products. fuel collects in the combustion space before ignition occurs, with the result g. Atomization of fuel. The average size
  7. that high maximum pressures are of the fuel particles affects the ignition reached, and there is a tendency toward lag and influences the completeness of knocking. This tends to increase wear combustion. Small-sized particles are of the engine and reduce its efficiency. desirable because-they burn more Fuels with high cetane ratings have low rapidly. Opposed to this requirement is auto-ignition temperatures and hence the fact that small particles have a low are easier starting than fuels with low penetration, and there is therefore a cetane ratings. Therefore, diesel engine tendency toward incomplete mixing of performance is improved by the use of the fuel and the combustion air, which high cetane number fuel oils. leads to incomplete combustion. c. Air temperature. The temperature of h. Combustion chamber design. The the air in the cylinder directly affects amount of turbulence present in the the final compression temperature. A combustion chamber of an engine affects high intake temperature results in the mixing of the fuel and the decreased ignition lag and facilitates combustion air. High turbulence is an aid easy starting, but is generally to complete combustion. undesirable because it decreases the volumetric efficiency of the engine. 9B2. Power. Engine performance of an internal-combustion engine may be d. Quantity of fuel injected per stroke. measured in terms of torque, or power The quantity of fuel injected determines developed by the engine. The power that the amount of energy available to the any internal-combustion engine is engine, and also (for a given volumetric capable of developing is limited by mean effective pressure, length of stroke, efficiency) the air-fuel ratio. cylinder bore, and the speed of the engine e. Injection timing. The injection timing in revolutions per minute (rpm). has a pronounced effect on engine performance. For many engines, the a. Mean indicated pressure. The average optimum is between 5 degrees to 10 or mean pressure exerted on the piston degrees before top dead center, but it during each expansion or power stroke is varies with engine design. Early known as the mean indicated pressure. injection tends toward the development Mean indicated pressure is of great of high cylinder pressures, because the importance in engine design. It can be fuel is injected during a part of the obtained from indicator cards cycle when the piston is moving slowly mathematically or directly from the and combustion is therefore at nearly planimeter. Excessive mean pressures constant volume. Extreme injection result in overloading the engine and advance will cause knocking. Late consequent high temperatures. injection tends "to decrease the mean Temperatures greater than those indicated pressure (mip) of the engine contemplated in the engine design may and to lower the power output. cause cracked cylinder heads, liners, and Extremely late injection tends toward warped valves. There are two kinds of incomplete combustion, as a result of mean effective pressures. One, mip, or mean 179 indicated pressure is that developed in single-acting, 2-stroke cycle engine, there the cylinder and can be measured. The is a power stroke for each revolution. other is bmep or brake mean effective pressure and is computed from the bhp Having defined the factors influencing delivered by the engine. the power capable of being developed, the general formula for calculating NOTE. Maximum pressure developed
  8. has no bearing on mep. horsepower is as follows: b. Length of stroke. The distance the IHP = (P X L X A X N) / 33,000 piston travels from one dead center to its opposite dead center is known as the P = Mean indicated pressure, in psi length of stroke. This distance is one of L = Length of stroke, in feet the factors that determines the piston A = Effective area of the piston in square speed which is limited by the frictional inches heat generated and the inertia of the N = Number of power strokes per minute moving parts. In modern engines, piston speed reaches approximately The horsepower developed within the 1600 feet per minute. If the length of cylinder as a result of combustion of the stroke is too short, excessive side thrust fuel can be calculated by measuring the will be exerted on a trunk type piston. mean indicated pressure and engine The length of stroke, however, cannot speed. Then with the bore and stroke be too great because of the lack of known, the horsepower can be computed overhead space available on submarine for the type of engine being used. This type engines. power is called indicated horsepower because it is obtained from the pressure c. Cylinder bore. The cylinder bore is measured from an engine indicator card. its diameter, and from this the cross- It does not take into account the power sectional area of the piston is loss due to friction, as will be discussed determined. It is upon this area that the later. Example: gas pressure acts to create the driving force. This pressure is the mean Given a 12-cylinder, 2-cycle, single- indicated pressure referred to above, acting engine having a bore of 8 inches expressed and calculated for an area of and a stroke of 10 inches. Its rated speed 1 square inch. The ratio of length of is 720 rpm. When running at full load stroke to cylinder bore is somewhat and speed, the mean indicated pressure is fixed in engine design. There are a few measured and is found to be 105 psi. instances in which the stroke has been What is the indicated horsepower less than the bore, but in almost every developed by the engine? case the stroke is longer than the bore. This ratio in a modern trunk-piston type Solution: engine is about 1.25, while in a crosshead type engine in use today it is From the formula about 1.50. IHP = (P X L X A X N) / 33,000 d. Revolutions per minute. This is the speed at which the crankshaft rotates, P = 105 and since the piston is connected to the L = 10 / 12 A = 3.1416 (8/2)2 shaft, it determines, with the length of stroke, the piston speed. Since the N = 720 piston moves up and down each IHP = (105 X (10 /12) X 3.1416 (8/2)2) revolution, the piston speed is equal to twice the stroke times the revolutions X 720 per minute (rpm), and is usually expressed in feet per minute. If the IHP = 96.96 stroke is 10 inches, and the speed of rotation is 750 rpm, the piston speed is Since this is just the horsepower developed in one cylinder, if the load is 750 X 2 X (12/10) = 1,250 feet per perfectly balanced among all cylinders, minute. the total indicated horsepower of the engine is The power developed by the engine
  9. depends upon the engine's speed and IHP = 12 X 96.96 = 1163.5 the type of engine. If it is a single- acting, 4-stroke cycle engine there will be one power stroke for every two revolutions of the crankshaft. If it is a 180 e. Brake horsepower. As stated above, be determined from the indicated brake horsepower is the power horsepower under varying conditions of delivered by the engine in doing useful operation. It should be noted that as a work. Numerically, it is equal to the rule, indicator cards taken on engines indicated horsepower minus the having a speed over 450 rpm are not mechanical losses. reliable and therefore no indicator motions are provided. BHP = IHP minus the mechanical losses. 9B3. Engine performance limitations. The power that can be developed by a From the example above, the IHP was given size cylinder whose piston stroke is fixed is limited only by the piston speed found to be 1163.5. If the brake and the mean effective pressure. The horsepower of this engine was 900 as determined in a test laboratory, then the piston speed is limited by the inertia forces set up by the moving parts and the mechanical losses would be problem of lubrication due to frictional heat. 1163.5 - 900 = 263.5 horsepower The mean indicated pressure is limited or by: (263.5 / 1163.5) X 100 = 22.6 percent 1. Heat losses and efficiency of of the indicated horsepower developed combustion. in the cylinders 2. Volumetric efficiency or the amount of or 90 / 1163.5 = 77.4 percent air charged into the cylinder and the mechanical efficiency. degree of scavenging. Engine power is frequently limited by 3. Complete mixing of the fuel and air the maximum mean pressure allowed. which requires fine atomization, To find the bmep of the above engine, sufficient penetration, and a properly first obtain the power developed in one designed combustion chamber. cylinder. Thus, The limiting mean effective pressures, 900 / 12 = 75.0 bhp both brake and indicated, are prescribed by the manufacturer or the Bureau of From the general formula for Ships and should never be exceeded. In a horsepower, direct-drive ship, the mean effective pressures developed are determined by HP = (P X L X A X N) / 33,000 the rpm of the shaft. In electric-drive 2 75 = P X (10/12) X 3.1416 X (8/2) 720 ships, the horsepower and mep can be determined readily from the electrical /33,000 readings, taking into account generator P = (75 X 33,000) / (10/12 X 3.1416 X efficiency. (8/2)2 X 720) The diesel operator should remember that the term overloading means exceeding
  10. P = 82.1 psi the limiting mean effective pressure. Hence, for the above engine under the 9B4. Operation. All submarine type conditions stated the bmep is 82.1 diesel engines are rated at a given horsepower and a given speed by the while the mip is 105 psi. manufacturer. These factors should ordinarily never be exceeded in the The brake horsepower is the power operation of the engine. Using the rated available at the engine shaft for useful speed and bhp, it is possible to determine work. Brake horsepower cannot usually a rated bmep which each individual be measured after an engine is installed cylinder should never exceed, otherwise in service, unless the engine drives an that cylinder will become overloaded. electric generator. The brake The rated bmep holds only for rated horsepower is determined by actual speed. If the speed of the engine drops tests in the shops of the manufacturer down below rated speed, then the before delivery of the engine. Frictional cylinder bmep which should not be losses are quite independent of the load exceeded generally drops down to a on the engine. Hence, unless the brake lower value due to propeller horsepower has been measured at characteristics. The bmep should never various loads and speeds, the exceed the normal mep at lower engine mechanical losses cannot speed. Usually it 181 should be somewhat lower if the engine Diesel engines do not operate well at speed is decreased. exceedingly low bmep such as that occurring at idling speed. This type of engine running tends to gum up pistons, Navy type engines are generally rated rings, valves, and exhaust ports. If an higher for emergency use than would normally be the case with commercial engine is run at idling speed for long periods of time, it will require cleaning engines. The economical speed for most Navy type diesel engines is found and overhaul much sooner than if it had been run at 50 percent to 100 percent of to be about 90 percent of rated speed. load. For this speed the optimum load conditions have been found to be from 70 percent to 80 percent of the rated Some engine manufacturers design their load or output. Thus, we speak of engine fuel systems so that it is running the engines at an 80-90 impassible to exceed the rated bmep to combination which will give the engine any great extent. This is done by limiting parts a longer life and will keep the the maximum throttle or fuel control engine itself much cleaner and in better setting by means of a positive stop. This operating condition. The 80-90 means regulates the maximum amount of fuel that we are running the engine with 80 that can enter the cylinder and therefore percent of rated load at 90 percent of the maximum load of the cylinder. rated speed. C. LOAD BALANCE 9C1. Indications. Load balance means from individual cylinders indicate an the adjustment of the engine so that the overloaded condition of these cylinders. load will be evenly distributed among A high common exhaust temperature in all the cylinders of the engine. Each the exhaust header indicates a probable cylinder must produce its share of the overloading of the whole engine. These total work done by the engine in order conditions are indicated by pyrometers to have a balanced load. If the engine is installed in all modern engines. A
  11. developing its rated full load, or nearly constant check on the pyrometer readings so, and one cylinder or more is will indicate accurately when any producing less than its share of the cylinder is firing properly and carrying load, the remainder of the cylinders its correct share of the load. Any sudden obviously must be doing more than change in the reading of the exhaust their share of the total work and hence temperature of any cylinder should be are overloaded. investigated immediately. The difference in exhaust temperatures between any two cylinders should not exceed 25 degrees F An overloaded condition of an engine, for a well-balanced engine. However a or of one or more of its cylinders, may certain tolerance is allowed; usually 50 be indicated by: degrees to 75 degrees is permissible. 1. Black smoke in the exhaust. Thermometers are provided in the lubricating oil and cooling water systems. 2. High exhaust temperature. Modern diesel engines have thermometers installed in the cooling 3. High lubricating oil and cooling systems of individual cylinders. An water temperature. abnormal rise in any of these temperatures may indicate an overloaded 4. Hot bearings and high temperatures condition and should be investigated as of other engine parts (in general, a hot quickly as possible. running engine). In general, excessive heat in any part of 5. Excessive vibration of the engine. the engine may indicate overloading. An overheated bearing may be the result of 6. Unusual sound of the engine. overloading a cylinder. An abnormally hot crankcase could result from When black smoke is observed in the overloading the engine as a whole. exhaust from the mufflers, it is not Excessive temperatures of some engine possible to determine immediately parts can be checked by touch. whether the entire engine or just one of the cylinders is overloaded. However, If all cylinders are not doing an equal by opening the indicator cocks on the individual cylinders, the color of their exhausts can be determined. High temperatures of the exhaust gases 182 amount of work, the force exerted by out should be within 10 to 20 psi of each individual pistons will be unequal. In other in all cylinders of a properly this event, the unequal forces may adjusted engine. cause an uneven turning moment to be exerted on the crankshaft and vibrations In order to have the load equally will be set up. The skilled operator can distributed, each cylinder must receive tell by the feel and the sound of an the same amount of fuel. It is here that engine when a poor distribution of load the effect of an improperly adjusted fuel exists. This, of course, comes from long pump is evident. A cylinder receiving experience, but it is important that the more fuel than necessary for a given load beginner avail himself of every will develop more power than required. opportunity to observe engines running under all conditions of loading and Any adjustment of the fuel pump must be performance. undertaken only by a person thoroughly familiar with the type of pump being
  12. used. He should first determine beyond all doubt that the engine is in proper 9C2. Causes of unbalance. In the mechanical condition. A great many preceding section some of the general factors may cause the cylinder to fire causes of unequal load distribution unevenly. Some of these causes are a were discussed. To prevent unbalance clogged or improperly timed fuel in an engine, the foremost injection valve, improperly timed air consideration is that the engine must be intake or exhaust valve, air or water in in excellent mechanical condition. A the fuel system, improper rocker arm leaky valve or fuel injector, leaky valve clearance, dirt or other foreign compression rings, or any other such matter in the fuel oil which may be mechanical difficulties will make it plugging up the strainers and filters, and impossible for the operator to balance any other factor that contributes to poor the load unless he secures the engine combustion. If a cylinder is firing and dismantles at least a part of it. incorrectly, always check the above Therefore, the engine must be placed in conditions before making any proper mechanical condition before the adjustments to the fuel pump. load can be balanced. Changing the amount of fuel being Since the heat of compression is relied delivered by adjusting the pump should upon to ignite the fuel injected in the be done only when it is certain that the diesel engine, the amount of this cause of the trouble is in the pump. This compression must be maintained within point cannot be emphasized too strongly. fixed limits. In order to have the same For instance, if the failure of a cylinder to type of combustion in each cylinder, fire correctly was due to a clogged fuel the degree of compression in all injection valve tip and the operator cylinders should be approximately the increased the fuel supply to the cylinder same. For example, low compression with the intention of increasing the power pressure in one cylinder may prevent developed by that particular cylinder, the all the fuel from burning, or may even increase in fuel might wash the valve prevent ignition of the fuel in that clean and cause the cylinder to become cylinder. This would result in a reduced badly overloaded from the excess fuel amount of work or no work being done supplied. The correct procedure would by this cylinder. The common causes of have been to replace the clogged low compression are: injection valve with a spare and to clean the one that was removed. The decrease 1. Sticking compression rings. in power delivered by a cylinder may also be due to some foreign matter under 2. Excessive ring or cylinder wear. a valve or piston ring, and once cleared, the cylinder would become overloaded if 3. Leaky cylinder head gasket or the fuel supply had, in the meantime, cracked cylinder head. been increased. 4. Leaky valve in cylinder head. The operator who always maintains his plant in good mechanical condition will 5. Cracked cylinder liner. be required to make few, if any, adjustments to the 6. Excessive clearance volume. In correcting these, it is generally necessary to replace the defective part. However, in some cases such as a sticking ring or valve, it is necessary only to clean the part and replace it. These cold compression pressures with fuel cut
  13. 183 fuel system while it is running. The fuel developed within a cylinder is directly supply to an individual cylinder should proportional to the power produced by not be adjusted until after an exhaustive that cylinder, any increase in one will search has revealed that every other cause a corresponding increase in the condition is normal in all respects. other. Hence, if the power is not evenly distributed throughout the cylinders, the After an overhaul in which piston rings mean indicated pressures in the individual cylinders will vary. of cylinder liners have been renewed, Temperature varies directly as the considerable adjustment of the engine may be necessary. Lubricating oil will pressure, so that a decrease in pressure will result in a corresponding decrease in leak by the rings into the combustion space until after the rings have properly temperature, The quality of combustion obtained depends upon the heat, and heat seated. The compression will also upon the temperature, so that with a increase as the seal between the rings decrease in pressure, combustion will not and the liner becomes more effective. be so good as before. This poor The lubricating oil will burn in the cylinder, giving an incorrect indication combustion will lower the thermal of fuel oil combustion, and if the pump efficiency, and the output of the engine has been properly set when the engine will be reduced. was started, the engine will be overloaded, or at least unbalanced. As If an engine is developing 600 bhp, and the compression rises to normal its mechanical efficiency is 80 percent, pressure, the power developed will the indicated horsepower being increase as also will the conditions of developed is 750. If the engine has 10 pressure and temperature under which working cylinders, each cylinder should the combustion takes place. Hence, be producing 75 indicated horsepower. when an overhaul has been completed, When this is not the case the engine is the engine must be carefully watched unbalanced. The effect here would be to until the rings are seated, and the increase the mean indicated pressure of compression set to the level specified in those cylinders doing less than their share the instructions for that type of engine. of the work, and to decrease that of those This adjustment will be facilitated by cylinders producing more than 75 the use of frequent compression tests. If indicated horsepower. the engine is not fitted so that the compression can be readily varied, the The turning moment acting on the engine should be run under light load crankshaft is proportional to the force until it is certain that the rings have acting on the piston. This force, in turn, seated. is the result of the mean indicated pressure developed in the cylinder. If 9C3. Effect of unbalance. In general, these forces from different cylinders are the effect of unbalance is an overheated not equal, there is an uneven turning engine. Clearances are established by moment acting along the length of the the engine designer to allow for crankshaft, and vibrations result. These sufficient expansion of moving parts vibrations, if sufficiently severe, may when operating at the designed shake the engine loose in its foundation, temperatures. Consequently, an engine crack the engine housing, framework, operating at temperatures in excess of and bedplate, destroy the bearings, and those for which it was designed may even break the crankshaft. It is obvious suffer many casualties. Excessive that a badly vibrating engine can result in expansion of the moving parts will serious damage and should be stopped cause seizures and a burning up of the immediately. engine. If the temperatures rise above
  14. the flash point of the lubricating oil To avoid all the harmful effects of vapors in the crankcase, an explosion overloading and unbalancing of load, the may result. The high temperatures may load on a diesel engine should be equally destroy the lubricating oil film between distributed among the working cylinders; adjacent surfaces of the moving parts and no cylinder, or the engine itself, and result in further increased should ever be overloaded. In conclusion, temperatures due to the increased the correct procedure to follow in friction. In fact, the effect is the same balancing an engine is: as for overheating from any cause. 1. Maintain the engine in proper Since the mean indicated pressure mechanical condition. 184 2. Adjust the fuel system in accordance 4. Keep the cylinder temperatures and with the manufacturer's instructions. pressures as evenly distributed as possible. 3. Operate the engine within the temperature limits specified in the 5. Train yourself to detect a bad instructions. condition by the senses of touch and hearing. D. ENGINE DYNAMICS AND VIBRATIONS 9D1. Balancing. It is not possible to excessive vibration in service. This is due balance out all the forces producing to the low speeds used with the balancing vibration in an engine. However, the machines. Diesel engines in the service primary or principal forces may be must operate over a wide speed range almost entirely balanced by the addition usually, and for this reason they are not of weights to the crankshaft or accepted until after they have been tried connecting rods at the proper places. at all speeds at which they must operate Balancing by the addition of weights so when installed in service. as to create forces equal and opposite to those of inertia is known as In any event, all rotating parts of the counterbalancing. Usually, after engine should be as accurately balanced counterbalancing, there are still some as possible. small forces remaining that have not been completely balanced out. These 9D2. Flywheels. A flywheel stores up remaining forces are produced by the energy, the amount of which depends reciprocating parts, since it is possible upon the rotating speed, the weight, and to completely counterbalance all the diameter of the wheel. In most marine primary rotating forces. engines heavy flywheels are not necessary, as the other rotating masses on All rotating parts are subjected to two the shaft serve the same purpose. These kinds of unbalance. They are called masses are the clutch and generator, and static unbalance and dynamic with a large number of cylinders firing, unbalance. The unbalanced condition the power stroke is smoother, and there is in both cases can be readily determined less need for a flywheel. and corrected by counterbalancing. The flywheel serves three purposes, A static balancing test is conducted by namely: placing the two ends of the rotating part on perfectly smooth, horizontal, and 1. To prevent the engine from stalling parallel rails. If statically unbalanced, when running at idling speed. the part will roll on the rails until its
  15. center of gravity reaches its lowest 2. To reduce the variations in speed at all position and then it will come to rest. loads. If, however, its center of gravity lies along its axis it will remain at rest when 3. To help carry the engine over centers placed in any position, and it is then in when starting. static balance. When the speed of the shaft tends to It frequently occurs that the center of increase, the flywheel absorbs energy. gravity of a body lies in its axis of When it tends to decrease, the flywheel rotation but that its irregular shape or gives up its energy to the shaft in an composition generates a disturbing effort to keep it rotating at a uniform force when the body is rotated. In this speed. case the body would be in static balance and in dynamic unbalance. In 9D3. Torsional vibrations. The twisting general, before balancing, most rotating and untwisting of the shaft system result parts are in both static and dynamic in torsional vibrations. All shafts have unbalance. some flexibility and with weights attached to them, such as pistons, gears In all cases, complete balancing can be and camshafts in diesel engines, they obtained by attaching weights to the have what is known as a natural fixed rotating body, if the position and degree frequency. When the frequency of the of unbalancing are known. For power stroke impulses coincides with the determining this unbalance all naval natural frequency of the entire shaft shipyards are equipped with balancing system, a torsional vibration is produced, machines. Experience with large and and the shaft is then said to be high-speed machinery has shown that balancing machines show good results but do not insure against 185 rotating at a critical speed. This critical bedplate, crankcase, or similar members, speed is dependent on the dimensions results in flexural vibrations. The cause of the crankshaft, the number of of flexural vibration lies in the faulty cylinders, all rotating masses of the balance of the rotating and reciprocating engine, other shafting and masses masses of the engine and the presence of including the propeller, the number of the so-called free forces or rocking power strokes per minute, the couples. It may be manifest in the arrangement of the cylinders (whether horizontal or vertical planes and may in they are in line or in a V), and the turn be the cause of vibration of cylinder firing order. surrounding structures, such as the ship's hull in marine installations. This type of vibration does not depend on the way the Without going into further detail, it is engine is coupled to its load, and if an sufficient to say that torsional critical engine does not vibrate on test, no speeds depend upon the number of vibrations will develop after it is placed power impulses per revolution and the in service. natural rate of vibration of the combined shaft system. Special instruments are available for 9D5. Torsional vibration dampening. determining the degree of torsional There are certain forces acting in vibration and the natural frequency of resistance to torsional vibrations. These forces are due to the friction of the any particular shaft system. bearings that carry the shafting and the work absorbed in the metal of the shaft in To change the range or point of resisting the twisting called hysteresis. maximum vibration of the critical
  16. speeds for a given installation it is Propellers in the water are the most necessary to make a change in the influential factor. All of these forces may masses on the elastic shaft system. It is be said to be the result of natural causes, evident therefore that in engines and they act to dampen out, or reduce the operating at a constant speed, it is much amplitude of the torsional vibrations. In simpler to change the natural frequency addition to these natural forces, there are in order to avoid dangerous critical other methods employed to reduce or speeds than it is in a marine engine eliminate the severity of the vibrations. requiring a wide range of operating This may be accomplished by changing the firing order of the cylinders in the speeds. engine, or by changing the rotating weights, or the flexibility of the Critical speeds and mode of vibrations shaftings. are determined with the aid of an instrument recording torsional vibrations. The engine builders In addition to the above dampening calculate the critical speeds and furnish factors and methods there are various a guarantee that, with the engine types of commercial torsional vibration coupled to the load for which it is dampeners, such as that used on the F-M designed, no dangerous critical speeds 38D 8 1/8 10-cylinder engine. Each such dampener must be designed for a specific will occur within the operating speeds. shaft system operating with a particular type engine. Vibration dampeners are Torsional vibrations need not usually located at or near a point of necessarily shake the framing of the engine and may not even be noticeable maximum torsional vibration amplitude along the shaft, generally at the forward to the operator. This fact has been borne out in several casualties in which end of an engine. the crankshaft broke without warning. Excessive wear of gears or of attached There are several different types of auxiliaries and repeated breakage of dampeners. All, however, accomplish the shafting or other parts attached to it can same purpose. They tend to reduce the very well be caused by torsional swinging motion of the shaft. This is vibrations. Most installations in naval accomplished by having a freely rotating vessels have been checked and tested to disk or disks acting against a fixed disk determine the exact location of which creates friction and thereby acts as torsional vibrations, their amplitudes, a brake. This prevents the shaft, from twisting and untwisting while rotating on and frequencies. its axis. 9D4. Flexural vibrations. The bending of the parts of the engine framing such as the 186 E. ENGINE PRESSURE INDICATOR 9E1. General. Efficient uninterrupted horizontal distance will represent piston performance of the engine depends movement. As an example, in a two- upon the maintenance of equal correct stroke cycle engine, one complete compression without fuel, and firing revolution or cycle would produce a pressures with fuel among the various diagram like the one shown in the cylinders. Poor engine compression illustration. This diagram is called an causes loss of power, poor acceleration, indicator card. If the indicator spring is smoky exhaust, and starting difficulties. calibrated so that the number of pounds An abnormally high firing pressure in of pressure required to raise the pencil 1
  17. one or more cylinders may cause inch is known, then to read the pressure engine wear, uneven running, and at any point on the card all that is overheating. These compression necessary is to measure the distance in pressures may be measured by inches from the atmospheric line X-Y on instruments known as pressure the diagram to the point at which the indicators. Compression readings amount of pressure is desired, and without fuel are taken after the engine multiply this by the calibration number of is warmed up and the fuel cut off on the spring. The total length of the that particular cylinder. Firing pressure diagram represents the stroke of the readings are taken with the engine piston. This horizontal scale then can be warmed up and operating under a stated laid off in inches, feet, piston stroke, or load at a stated speed. volume of piston displacement. This type of indicator is little used by 9E2. Types of engine indicators. There are two general classes of engine operating personnel on fleet type pressure recording indicators. In the submarines today, mainly because there first, the instrument measures is no provision made on modern engines graphically the cylinder pressure and at for the attachment of the equipment the same time indicates the position of necessary to take the indicator card, and the piston at any point of its stroke or also because there are no means of cycle. In other words, the indicator compression adjustment other than draws a diagram of the pressure in the complete overhaul of the engine. cylinder with respect to the movement of the piston. Since the movement of The other type of indicator (indicating the piston is a measure of the volume maximum pressures only) is used to displaced, the diagram is drawn to the some extent for taking maximum ordinates of pressure and volume. In cylinder pressures, to check against the second general class, the indicator manufacturer's test data and previous records the maximum pressures only. shipboard pressure tests. The two most commonly used indicators of this type Figure 9-5 shows the fundamental are the Premax indicator and the Kiene principle of the operation of an engine indicator. indicator in which the movement of the piston is recorded. The indicator The method normally used to check the equipment includes a small cylinder equal distribution of power among the that can be attached to the main various cylinders is to compare the working cylinder of the engine, a piston exhaust gas temperatures of the cylinders and rod that work in this small cylinder, by means of thermocouples placed in the with a pencil on the end of the rod. The exhaust elbows of each cylinder. pencil point bears on the paper tacked Pyrometer readings have proved to be a to the drum which is moved by hook good check on the general running and string over a pulley. Any pressure conditions of an engine, and the records in the working cylinder enters the of exhaust gas temperatures are of great indicator cylinder and forces the small value in conjunction with indicator indicator piston and pencil in a vertical readings as aids in getting the best results direction at the same time the main from a diesel engine. However, even piston moves the card in a horizontal though the exhaust temperatures are direction by means of the string and normal, the engine at times may not pulley. develop its rated horsepower. It is readily seen that any vertical 9E3. Premax indicator. The Premax distance on the diagram will represent indicator is an instrument for determining pressure, and the cylinder 187
  18. Figure 9-5. Principle of engine indicator. Figure 9-7. Kiene pressure indicator. Figure 9-6. Premax pressure indicator. 188 compression and firing pressures. The 9E4. Kiene indicator. The Kiene diesel indicator consists essentially of a piston indicator is a pressure indicator gage for subject to cylinder pressure, a spring measuring the compression and firing against which the piston acts and the pressure of an engine while it is running. tension of which is adjustable by means The complete unit consists of a pressure of an index sleeve, a control switch, gage and an air-cooled pressure chamber and a neon light circuit that shows if which is attached to the cylinder the piston is moving. It is attached to indicator cock. the cylinder indicator cock in the same way as any other indicator. The The cylinder discharge passes through pressure acting on one side of the the indicator plug up through the filler, piston in the indicator is gradually screen, and seat piece. This raises the increased by increasing the spring valve, allowing the gas to pass through tension with the index sleeve until this the drilled holes in the guide piece into spring pressure is equal to the the pressure chamber and on to the gage. maximum cylinder pressure which acts The action of the gas in the curved tube on the opposite side of the piston.
  19. When the two pressures are equal, the of the gage tends to straighten the tube, piston stops moving, as shown by thereby moving the gage needle and stopping of the neon light flashes. The recording the pressure on a calibrated pressure reading is then read on the scale. scale sleeve. 189


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