Tài liệu Diezel 1410 P6

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All of our modern submarine diesel engines are of the 2-stroke cycle type.

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6 INTAKE AND EXHAUST SYSTEMS A. GENERAL 6A1. Intake systems. All of our These methods are illustrated in Figures modern submarine diesel engines are of 6-1 to 6-4. In port direct scavenging, the the 2-stroke cycle type. The purpose of exhaust ports are on one side of the the intake systems in these engines is to cylinder and the scavenging ports on the force out the exhaust gases of other. In port loop scavenging, the combustion as effectively as possible exhaust and scavenging ports are on the and to recharge the cylinder with fresh same side of the cylinder. In uniflow port air in order to support combustion for scavenging, the air enters at ports at the the next succeeding cycle. The supply lower end of the cylinder and passes out of air must be in excess of that required through ports in the upper end of the to just support combustion since the cylinder. fuel is thoroughly mixed with only part of the air compressed within the In valve uniflow scavenging, air enters cylinder. The ratio of air to fuel in most the cylinder through ports in the bottom diesel engines is approximately 20 to 1 and passes out through exhaust valves in at full load. the cylinder head, carrying the burned exhaust gases with it. 6A2. Scavenging. The term scavenging is used to describe the process of The ports used for the inlet of scavenging ridding the cylinder of burned exhaust air are usually constructed so as to give gases during the latter part of the the air a whirling motion or turbulence to expansion stroke and the early part of clear out all possible exhaust gases and the compression stroke of the 2-stroke fill the entire cylinder with a charge of cycle engine. Scavenging is fresh air. accomplished by admitting fresh air under a pressure of about 1 to 5 psi into In scavenging air systems, it is possible the cylinder while the exhaust valves or to supercharge the cylinder during the air ports are open. This pressure usually is intake. This is done by closing the developed by means of a scavenging air exhaust ports or valves slightly ahead of blower. These blowers are driven from the inlet port closure. This allows the air the engines themselves and generally pressure in the cylinder to build up to are of the lobed rotor type, the rotors scavenging air pressure, increasing the revolving together in closely fitting amount of air, the air-fuel ratio, and the housings. The process of scavenging combustion efficiency. If the amount of must be carried out in an extremely fuel injected is increased to give the same short period of time, depending upon air-fuel ratio as before supercharging, the the speed of the engine. The burned effect of supercharging is to give more gases must be blown out of the cylinder power output to the cylinder. In the and a fresh charge of air admitted present submarine type engines, the F-M during the time that the ports or valves engine is supercharged, but the GM are open. For example, in an engine engine is not. making 750 rpm with the exhaust ports open for 140 degrees of crank angle, 6A3. Intake system components. The the elapsed time the ports are open each intake systems consist of the following revolution is only (140/360) x (60/750) parts: or approximately 1/32 of a second. a. Air intake silencers and strainers. The scavenging air must be so directed Intake air for submarine engines is drawn as to remove the burned gases from the from the engine room compartments by remote parts of the cylinder. The the scavenging air blower through air
methods used may be classified as silencers and strainers. If some type of air follows: port scavenging (direct, loop, silencer were not used, the noise of the and uniflow), and valve scavenging intake air would be almost unbearable (uniflow ). because of its high-pitched whistling sound. Strainers are installed to remove any dirt or other foreign matter that would otherwise enter the scavenging blower or engine and cause damage. 119 Figure 6-1. Port direct scavenging. Figure 6-3. Valve uniflow scavenging. Figure 6-2. Port loop scavenging. Figure 6-4. Cross section of F-M cylinder with uniflow port scavenging. 120 b. Scavenging air blower. The the reciprocating motion of the pistons. scavenging air blower furnishes air under pressure to the intake headers The exhaust headers or belts conduct the and receivers and eventually to the exhaust gases from the exhaust valves or cylinder inlet ports. ports to the atmosphere through an inboard and an outboard exhaust valve and muffler. The exhaust manifold and c. Air intake headers, receivers, and necessary piping. The air headers and exhaust elbows (if used) are usually
receivers carry the air from the water jacketed to permit cooling of the scavenging air blowers to the inlet ports piping and manifolds. The cooling water of the cylinders. In most installations, normally comes from the engine fresh scavenging air headers and receivers water system. Cooling of these parts are built into the cylinder block. Drains keeps down the temperature of the metal, are placed in the scavenging air headers thus prolonging its life and reducing its to drain off any liquids that may have expansion to a minimum. In most accumulated. Spring-loaded covers are exhaust systems, drains are provided to also furnished in the scavenging air allow drainage of any accumulated header to allow the venting of excess liquids from the exhaust belts. pressure in case of emergency. In submarine installations, the gases of d. Intake air ports. The intake air ports combustion are piped from the exhaust are in the cylinder liner and permit the headers to the outside of the submarine scavenging air to pass from the through an in board and outboard main scavenging air receivers into the engine exhaust valve and muffler. The cylinder when the ports are open. The inboard exhaust valve is inside the ports are usually tangentially pressure hull of the submarine and is constructed so as to give the air a hand operated. The outboard exhaust whirling motion as it enters the valve is located outside the pressure hull cylinders. They are usually opened and and is operated either by hand or by closed by the reciprocating motion of hydraulic power, the controls for the valve being at the throttleman's station at the piston. the engine. Both inboard and outboard exhaust valves are water cooled, the 6A4. Exhaust systems. The purpose of former usually by water from the engine the exhaust system is to convey the fresh water system, the latter by water burned exhaust gases of combustion from the cylinders to the atmosphere as from the engine salt water system. silently as possible. The system includes exhaust valves and ports, Mufflers are placed in the exhaust headers and pipes, main inboard and system. This is necessary, because in a 2- outboard exhaust valves, and engine stroke cycle engine the uncovering of the exhaust ports releases a pressure of 20 to mufflers. 40 psi in the exhaust system and this The exhaust valves or ports, as the case produces a noise that can be heard for miles if not muffled by some form of may be, are properly timed so as to silencer. These mufflers are usually of permit the gases of combustion to escape from the cylinder at the correct cast or sheet iron construction with a system of baffles that break up the noise point of the cycle. In the GM engine, without producing back pressure. There this is accomplished by means of are two general types of mufflers in use, exhaust valves; in the F-M engine, by means of exhaust ports. Due to the heat the wet type and the dry type. In both types, circulating water is used to reduce that must pass through these exhaust the temperature of the exhaust gases as valves or ports, they must be made of much as possible. The difference special material or be thoroughly cooled to prevent distortion and pitting. between the two is that in the dry type the exhaust gases do not come in contact Valves are usually made of a high with the cooling water, whereas in the silicon heat-resistant alloy steel. In wet type the gases are expanded in the some large engine installations, the exhaust valves may be water or sodium muffler in the presence of a water spray. The exhaust gases in passing through the cooled. A thermocouple is usually placed at the exhaust elbow to measure water spray are cooled, condensed, decreased in volume, and the exhaust temperature of each cylinder. When exhaust valves are used, they are opened and closed by
means of rocker arm and camshaft assemblies. The exhaust ports, if used, are opened and closed by 121 Figure 6-5. Typical exhaust system piping. 122 effectively silenced. Under normal muffler, the exhaust gases are passed out operation, the smoke is also eliminated. into the atmosphere through a section of Submarine installations use the wet piping known as the tail pipe. type of muffler. From the B. GENERAL MOTORS INTAKE AND EXHAUST SYSTEM 6B1. General description. The The exhaust gases are released from the General Motors engine employs the cylinder when the exhaust valves are uniflow valve method of scavenging. opened by action of the camshaft and The blower, mounted at the forward rocker arm assembly. The exhaust valves end of the engine crankcase and driven are opened ahead of the inlet ports to by the engine, takes air from the allow the pressure of the exhaust gases to atmosphere through an attached be partially released before the low- silencer and forces it under pressure pressure scavenging air is admitted to the into the air box. The air box consists of cylinder. The exhaust gases pass through the frame space in the engine included the exhaust valves into the water-cooled between the two legs of the V- cylinder head and thence into the exhaust construction and the open space elbow connecting each cylinder head between the upper and lower deckplates with the main exhaust manifold. This of each bank. The air from the air box manifold extends longitudinally along the goes through the cylinder inlet ports top centerline of the engine with elbow
whenever the individual pistons connections into each cylinder head. uncover the ports at the end of the Thermocouples for measuring the expansion or power stroke. This temperature of the exhaust gases for each scavenging air forces out the exhaust cylinder are located in each exhaust gases and charges the cylinder with elbow. Both exhaust elbows and exhaust fresh air. Figure 6-6. GM cylinder intake and exhaust. 123 manifold are water jacketed for cooling purposes. From the main exhaust manifold, the gases pass into a vertical pipe which leads to the inboard exhaust valve. From this valve, the gases pass outside the pressure hull, through exhaust piping which leads to the hydraulically operated main engine outboard exhaust valve, and thence to the atmosphere by way of the muffler and tail pipe. The inboard exhaust valve is cooled by water from the engine fresh water system, while the outboard valve is cooled by the engine salt water system. Figure 6-7. Cutaway of blower assembly, GM. Drains are provided in the piping between the inboard and outboard exhaust valves so that any salt water that may have leaked past the outboard exhaust valve can be drained into the engine room bilges. On a submarine it is extremely important that this space be drained before starting an engine after surfacing from submerged operations, otherwise the engine may be flooded. 6B2. Scavenging air blower. The
scavenging air blower is of the positive displacement type consisting of a pair of rotors revolving together in a closely fitted housing. Each rotor has three helical lobes which produce a continuous and relatively uniform displacement of air. The rotors do not touch each other or the surrounding housing. Air enters the housing at the top and fills the spaces between the rotor lobes as they roll apart. The air is carried around the cylindrical sides of the housing, in the closed spaces between the lobes and the housing. It is forced under pressure to the bottom of the housing as the lobes roll together. Figure 6-8. Front view of blower, GM. Then the air passes through the space between the inner and outer wall of the blower housing and into the air box around the cylinder liners. Each rotor is carried on a tubular serrated shaft. Endwise movement is prevented by two taper pins. No gaskets are used between the end plates and the housing due to the importance of maintaining the correct rotor end clearance. A fine silk thread around the housing and inside the stud line, together with a thin coat of nonhardening gasket compound, provides an air tight seal. Babbitted bearings in the end plates locate the rotors in the two half-bores of the housing. These bearings permit clearances to be held to 124 a minimum between the rotor tips and the housing bores. Both ends of the rotor bearings have thrust surfaces at the gear end of the blower. The thrust surfaces locate the rotors endwise and prevent contact between the rotors and the end plates. The blower is driven from the crankshaft through a quill shaft and through a train of helical spur gears. The quill shaft is driven through a serrated quill shaft coupling on the crankshaft, and drives the main driving
gear in the train through a serrated Figure 6-9. Air silencer. connection in the gear hub. The main drive gear transmits power directly to the blower rotor driver gear. The quill shaft coupling is fastened to the end of the crankshaft and is driven through large dowel pins. The rotor driver and driven gear are closely fitted and rigidly attached to both rotor shafts to prevent the rotors from touching as they revolve. Each gear hub is pressed on the serrated rotor shaft. A hexagon head lockscrew in the rotor shaft holds a thrust collar as a spacer between the gear hub and the end of the rotor. The Figure 6-10. Cutaway of typical air collar maintains clearance between silencer. rotors and blower end plate. The blower rotor gears are bolted to the gear hub flanges and are located angularly by dowel pins. Due to the importance of having the rotors roll together without touching, yet with the least possible clearance, it is necessary to locate the dowel pins during assembly for a given set of gears and hubs. Oil passages in the end plates conduct lubricating oil under pressure to the blower bearings. Oil seals are provided at each bearing to prevent oil from entering the rotor housing. 6B3. Intake silencer. The air is drawn into the blower through an intake silencer mounted on the blower intake adapter. The silencer is a double sheet metal case with screened openings at the top. Felt padding is cemented between the double layers of metal at the top and sides of the case. To minimize the noise caused by the entering air, a perforated metal tube is welded through the center of the case, and the upper space between the outer shell and intake tube is filled with sound-deadening material. 6B4. Air maze. A breather system is used to prevent contamination of the engine room atmosphere by heated or fume-laden air which 125
would otherwise escape from the removes the oil from the vapor being crankcase. This ventilation of the drawn into the blower. crankcase also reduces the formation of sludge in the oil and prevents any The air maze element consists of a accumulation of combustible gases in number of fine steel and copper wire the crankcase and oil pan. screens that remove the oil from the oil- laden air as it is drawn through the air Atmospheric air for the breather system maze screens. The oil deposited on the enters the engine through the cylinder wire drips to the bottom of the air maze head cover breathers. The blower housing, and then drains back to the suction draws air from the crankcase sump tank through a tube. through the air maze which C. FAIRBANKS-MORSE INTAKE AND EXHAUST SYSTEM 6C1. General description. The inlet or piping which leads the exhaust gases up scavenging air system supplies the to the inboard exhaust valve. The exhaust fresh air that blows the exhaust gases belts and exhaust nozzles are cooled by out of each cylinder at the end of the fresh water from the engine fresh water power stroke and recharges and system. From the inboard exhaust valve, supercharges the cylinder for the next the gases pass outside the pressure hull compression stroke. The air is drawn through the outboard exhaust valve, from the engine room into the muffler, and tail pipe to the atmosphere. scavenging blower through an air As in the GM installation, a drain is intake silencer. From the scavenging air placed in the exhaust piping between the blower, the air is forced into two outboard and inboard exhaust valves. exhaust belts and receivers, one Both inboard and outboard exhaust extending along each side of the valves are cooled by water from the engine. These receivers conduct the air engine salt water system. up to the cylinder block compartments which surround the cylinder liners at 6C2. Scavenging air blower. their inlet ports. These ports direct the Scavenging air is supplied to the scavenging air tangentially into the cylinders under a pressure of from 2 to 5 cylinder when the upper piston psi by a positive displacement type uncovers the scavenging air ports. This blower. The blower consists of the air clears out the exhaust gases of housing which has inlet and outlet combustion and fills the cylinder with a passages and encloses two three-lobe charge of fresh air. As the lower crank spiral impellers. The impellers are leads the upper crank by 12 degrees, interconnected by timing gears driven by the exhaust ports are uncovered by the a gear drive from the upper crankshaft. lower piston before the inlet ports are uncovered by the upper piston. The Scavenging air from the atmosphere is delay allows most of the pressure of the drawn through the air silencer and enters cylinder to escape through the exhaust the inlet passage of the blower. It is ports before the relatively low pressure moved by the lobes along the walls of the of the scavenging air is admitted. The blower housing and forced through the lower crank lead also causes the lower outlet passages and through piping to the piston to cover the exhaust ports before air receiver compartments on each side of the upper piston has covered the inlet the cylinder block. ports. This allows the inlet air to be built up in the cylinder to the Due to the design of the impeller lobes, scavenging air pressure, resulting in a the scavenging air is discharged from the certain degree of supercharging. blower at a uniform velocity. Efficient operation is possible due to the small
clearances between the impellers, the impellers and the blower housing, and From the exhaust ports, the exhaust the impellers and the bearing plates. Oil gases pass into the exhaust belt which should never be allowed to leak into the encloses the lower part of each cylinder blower housing or the air receivers. To liner to a height slightly above the liner permit removal of any water that may exhaust ports. The gases then pass into enter the blower air passages through the two exhaust manifolds, one on each air silencer, or indirectly through the side of the engine, along the manifolds exhaust manifold, drain tubes and a to the control end of the engine, thence through two exhaust nozzles or elbows to the exhaust 126 drain tube cock are provided. This cock should be opened before starting the engine if any abnormal condition is suspected. Opening of this cock will drain the outlet air passages of the blower and the lowest part of the housing. Each impeller is cast on a splined shaft. Each shaft turns in two roller bearings, the outer bearing taking the shaft thrust. The bearings are held by retainer rings in the end plates which also locate the impellers with radial relation to each other. The thrust bearings prevent contact between the ends of the impellers and the housing. Figure 6-11. Cross section through F- M scavenging air blower. Figure 6-13. Blower assembly, timing Figure 6-12. Blower impellers and gear end, F-M. timing gears, F-M. Power to drive the blower is transmitted from the upper crankshaft through a flexible gear drive that meshes with a drive pinion. The drive pinion drives the blower driving timing gear, on the end of the lower impeller, which in turn transmits power to the driven timing gear
on the end of the upper impeller. The flexible drive gear and drive pinion on the upper crankshaft are lubricated by oil sprayed through nozzles from the engine lubricating system. The blower timing gears and the inner and outer bearings are lubricated by oil through tubes from the engine lubricating system. Oil is 127 collected between the end cover and the 6C4. Oil separator.The upper crankshaft inner housing of the blower and drained and the lower crankcase compartments to the vertical drive housing from are vented by means of a pipe connected which it returns to the engine oil sump. to the suction side of the blower. In the Gaskets between the bearing plates and vertical drive compartment this vent line blower housing form an oiltight seal. passes through an oil separator, in which a copper ribbon screen prevents oil from being carried into the blower with air 6C3. Intake silencer. The intake from the crankcase. Any leakage of silencer, through which air is drawn lubricating oil from the side covers of the before entering the scavenging air lower crankcase is an indication that the blower, is similar to the GM unit in separator needs cleaning. design and construction. It is mounted directly over the inlet opening of the blower. 128