Chapter 9 Intake And Exhaust Systems Navybmr-PDF Download

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Scavenging must take place in a relatively short, portion of the operating cycle The duration of the. process differs in two and four stroke cycle,engines In a two stroke cycle engine the. process takes place during the latter part of the,downstroke expansion and the early part of the. upstroke compression In a four stroke cycle,engine scavenging takes place when the piston. is nearing and passing top dead center TDC,during the latter part of an upstroke exhaust and.
the early part of a downstroke intake The intake,and exhaust openings are both open during this. interval of time The overlap of intake and,exhaust permits the air from the blower to pass. through the cylinder into the exhaust manifold,cleaning out the exhaust gases from the cylinder. and at the same time cooling the hot engine,When scavenging air enters the cylinder of an. engine it must be so directed that the waste,gases are removed from the remote parts of the.
cylinder The two principal methods by which,removal is accomplished are referred to as port. uniflow scavenging and valve uniflow scavenging,In the uniflow method of scavenging both the air. and the burned gases flow in the same direction,This action causes a minimum of turbulence and. improves the effectiveness of the scavenging,action An example of a port uniflow system is. shown in Figure 9 1,Valve uniflow Figure 9 2 scavenging and.
supercharging are not common to all diesel,engines For instance in some four stroke cycle. engines the air enters the cylinder as a result of. a pressure difference created by the piston as it,moves away from the combustion space during. the intake event This type of intake is sometimes,referred to as the suction type or naturally. aspirated intake however the air is actually,forced into the cylinder because of the greater. pressure outside the cylinder An increase in,airflow into the cylinders of an engine can serve.
to increase power output in addition to being Figure 9 1 Port uniflow system. used for scavenging Because the power of an, engine comes from the burning of fuel an increase in power requires more fuel The increased fuel. in turn requires more air because each pound of fuel requires a certain amount of air for combustion. The supplying of more air to the combustion spaces than can be supplied through the action of. atmospheric pressure and piston action in four stroke cycle engines or scavenging air in two stroke. cycle engines is called supercharging,In some two stroke cycle. diesel engines the,cylinders are supercharged,during the air intake simply. by an increase in the,pressure of scavenging air,The same blower is used. for supercharging and,scavenging Scavenging is,done when air is admitted.
under low pressure into the,cylinder while the exhaust. valves or ports are open,Supercharging is done with. the exhaust ports or valves,closed a condition that. enables the blower to force,air under pressure into the. cylinder and thereby,increase the amount of air,available for combustion A.
supercharged engine,occurs when the manifold,pressure exceeds the. atmospheric pressure The,increase in pressure,resulting from the. compression action of the, blower will depend on the Figure 9 2 Valve uniflow system in a two stroke cycle diesel. type of installation With the engine, increase in pressure and amount of air available for combustion there is a corresponding increase in. combustion efficiency within the cylinder An engine of a given size that is supercharged can develop. more power than an engine of the same size that is not supercharged. For a four stroke diesel engine to be supercharged a blower must be added to the intake system. because exhaust and intake in an unsupercharged engine are performed by the action of the piston. The timing of the valves in a supercharged four stroke cycle engine is also different from that in a. similar engine that is not supercharged In a supercharged engine the closing of the intake valve is. slowed down so that the intake valves or ports are open for a longer time after the exhaust valves. close The increased time that the intake valves are open after the exhaust valves close allows. more air to be forced into the cylinder before the start of the compression event. The amount of additional air that is forced into the cylinder and the resulting increase in horsepower. depends on the pressure in the air box or intake manifold The increased overlap of the valve. openings also permits the air pressure created by the blower to remove gases from the cylinder. during the exhaust event Study Figure 9 3 frames 1 and 2 so that you will understand how the. opening and closing of the intake and exhaust valves or ports affect both scavenging and. supercharging Also note the differences in these processes as they occur in supercharged two and. four stroke cycle engines, Figure 9 3 Scavenging and supercharging in diesel engines.
In Figure 9 3 frames 1 through 3 the circular pattern represents crankshaft rotation Some of the. events occurring in the cycles are shown in degrees of shaft rotation for purposes of illustration and. easier comparison only When working with the timing of a specific engine check the appropriate. instructions, When studying Figure 9 3 frames 1 through 3 keep in mind that the crankshaft of a four stroke cycle. engine makes two complete revolutions in one cycle of operation while the shaft in a two stroke cycle. engine makes only one revolution per cycle Also keep in mind that the exhaust and intake events in. a two stroke engine do not involve complete piston strokes as they do in a four stroke engine. Four Stroke Cycle Scavenging and Supercharging, Figure 9 3 frame 1and 2 is based on the operation of a four stroke cycle engine that uses a. centrifugal type blower turbocharger to supply the cylinders with air under pressure. In a supercharged four stroke cycle engine the duration of each event differs somewhat from the. length of the same events in a non supercharged four stroke engine The intake and exhaust valves. are open much longer in a supercharged engine and the compression and power events are shorter. permitting a longer period for scavenging When the exhaust event is complete the turbocharger fills. the cylinder with fresh air under pressure before the compression event begins The turbocharger. supercharges the cylinders, To understand the relationship of scavenging and supercharging to the events of the cycle look again. at Figure 9 3 frame 1and 2 and follow through the complete cycle Start your study of the cycle at. TDC the beginning of the power event At this point peak compression has been reached fuel. injection is nearly completed and combustion is in progress Power is delivered during the. downstroke of the piston for 125 of crankshaft rotation At this point in the downstroke at 55 before. bottom dead center BDC the power event ends and the exhaust valves open. The exhaust valves remain open throughout the rest of the downstroke 55 throughout all of the. next upstroke 180 and throughout 85 of the next downstroke a total of 320 of shaft rotation At a. point 75 before the piston reaches TDC the intake valves open and the turbocharger begins forcing. fresh air into the cylinder For 160 of shaft rotation the air passes through the cylinder and out of the. exhaust valves clearing the waste gases from the cylinder The rapid flow of gases escaping through. the exhaust manifold drives the turbocharger The process of scavenging continues until the exhaust. valves close at 85 past TDC, The intake valves remain open after the exhaust valves close for an additional 140 of shaft rotation. 45 past BDC From the time the exhaust valves close until the piston reaches approximately BDC. the cylinder is being filled with air from the turbocharger During this interval the increase in pressure. is too small to be considered because of the increasing volume of the cylinder space The piston is in. the downstroke When the piston reaches BDC and starts the upstroke the volume of the space. begins to decrease as the turbocharger continues to force air into the cylinder The result is a. supercharging effect During the remainder of the upstroke after the intake valves close the. supercharged air is compressed Fuel injection begins several degrees before TDC and ends shortly. after TDC The actual length of the injection period in a specific engine depends on the speed and. load of the engine When the piston reaches TDC a cycle two complete crankshaft revolutions and. four strokes of the piston has taken place and the engine is ready to repeat the cycle. Two Stroke Cycle Scavenging and Supercharging, When comparing Figure 9 3 frames 1 through 3 note that the length of the supercharging and.
scavenging periods in a two stroke cycle engine is not the same as those in a four stroke cycle. engine Also there is considerable difference in piston location between the times when these. processes take place in the two types of engines In a four stroke cycle scavenging takes place while. the piston is traveling through the latter part of the upstroke and the early part of the downstroke and. supercharging takes place when the piston is in the vicinity of BDC In a two stroke cycle the. processes of scavenging and supercharging both take place while the piston is in the lower part of the. cylinder In a four stroke cycle engine a piston does much of the work of intake and exhaust In a. two stroke cycle engine the piston does very little work in these two processes Therefore many two. stroke cycle engines use a blower to force air into the cylinder and to clear out the exhaust gases. Figure 9 3 frame 3 is based on the two stroke cycle of operation If you compare Figure 9 3 frames. 1 through 3 the differences in the scavenging and supercharging processes in two and four stroke. cycle engines are more apparent Start your study of the cycle with the piston at TDC in Figure 9 3. frame 3 Fuel has been injected ignition has occurred and combustion is taking place The power. developed forces the piston through the power event until the piston is 92 1 2 as compared to 125. for the four stroke cycle in frames 1 and 2 past TDC just a little more than halfway through the. downstroke At this point the exhaust valves open gases escape through the manifold and cylinder. pressure drops rapidly, When the piston reaches a point 48 before BDC the intake ports are uncovered as the piston moves. downward and scavenging begins Compare this motion with the opening of the intake valves in a. four stroke cycle in Figure 9 3 frame 1and 2 The scavenging air under blower pressure swirls. upward through the cylinder and clears the cylinder of exhaust gases This situation in the cylinder. when scavenging starts is approximately the same as that illustrated in Figure 9 2 Note the position. of the piston the open scavenging ports the open exhaust valves and the flow of air through the. cylinder The flow of scavenge air through the cylinder also helps to cool the parts which are heated. by combustion, Study again Figure 9 3 frame 3 Scavenging continues until the piston is 44 1 2 past BDC a total of. 92 1 2 as compared with 160 in the four stroke cycle in Figure 9 3 frame 1and 2 at which point the. exhaust valves close In a two stroke cycle engine the exhaust valves remain open during only 132. as compared with the 320 in the four stroke cycle The scavenge ports remain open for another 3. 1 2 of shaft rotation 45 in the four stroke cycle and the blower continues to force air into the. cylinder The ports are open for only a short interval after the exhaust valves close enough time is. available for the blower to create a supercharging effect before the compression event starts. The piston closes the intake ports at 48 past BDC The compression event takes place during the. remainder of the upstroke with injection and ignition occurring at TDC At this point one cycle is. ended and another is ready to start,INTAKE SYSTEM COMPONENTS. There are many variations in the designs of the engine parts which function as a group to properly. direct clean air to intake valves or ports The function of each kind of part remains basically the same. We will discuss the common types and principal parts of engine air intake systems. Silencers Screens and Cleaners,Diesel engines use a great amount of. air that enters from the intake system,This air must enter as quietly and.
clean as possible Unless a silencer is,installed the air that rushes through. the air cleaning devices will sound like,an extremely high pitched whistle. Consequently silencers are generally,constructed as part of the air cleaning. components,One type of air intake silencer,assembly is shown in Figure 9 4 The. silencer assembly is bolted to the,intake side of the blower Figure 9 2.
A perforated steel partition divides the,silencer lengthwise into two sections. Air enters the end of the silencer and, passes through the inner section into Figure 9 4 Air intake silencer assembly. the blower The noise of the air passes, through the silencer where it is reduced by a sound absorbent flameproof felted cotton waste which. fills the outer section of the silencer Upon leaving the silencer the air enters the blower through an. air intake screen Figure 9 4 The air intake screen prevents particles of foreign material from. CHAPTER 9 INTAKE AND EXHAUST SYSTEMS Combustion requires air fuel and heat Certain ratios of all three are necessary if an engine is to operate This chapter discusses air as it is required to support combustion in the cylinder of an engine the processes of scavenging and supercharging and the group of parts involved in supplying the cylinders of an engine with air and in removing the

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