PRINCIPLES OF NAVAL ENGINEERING 



by the action of the piston. The timing of the 

 valves in a supercharged 4-stroke cycle engine 

 is also different from that in a similar engine 

 which is not supercharged. In the supercharged 

 engine the intake-valve opening is advanced and 

 the exhaust-valve closing is retarded so that 

 there is considerable overlap of the intake and 

 exhaust events. This overlap increases power, 

 the amount of the increase depending upon the 

 supercharging pressure. The increased over- 

 lap of the valve openings in a supercharged 4- 

 stroke cycle engine also permits the air pres- 

 sure created by the blower to be used in removing 

 gases from the cylinder during the exhaust 

 event. How the opening and the closing of the 

 intake and exhaust valves or ports affect both 

 scavenging and supercharging, and the differ- 

 ences in these processes as they occur in 

 supercharged 2- and 4-stroke cycle engines, 

 can be seen by studying the diagrams in figure 

 22-22. 



As in the case of the diagrams used in con- 

 nection with the discussion of engine operating 

 principles, the circular pattern in figure 22-22 

 represents crankshaft rotation. Some of the 

 events occurring in the cycles are shown in 

 terms of degrees of shaft rotation. However, 

 the numbers (of degrees) shown on the diagrams 

 are for purposes of illustration and comparison 

 only. When these diagrams are being studied, 

 it must be kept in mind that the crankshaft of a 

 4-stroke cycle engine makes two complete revo- 

 lutions in one cycle of operation while the shaft 

 in a 2-stroke cycle engine makes only one revo- 

 lution per cycle. It should also be remembered 

 that the exhaust and intake events in a 2-stroke 

 cycle engine do not involve complete piston 

 strokes as they do in a 4-stroke cycle engine. 



Even though the primary purpose of a diesel 

 engine intake system is to supply the air re- 

 quired for combustion, the system generally 

 has to perform one or more additional functions. 

 In most cases, the system cleans the air and 

 reduces the noise created by the air as it enters 

 the engine. In order to accomplish the functions 

 of intake, an intake system may include an air 

 silencer, an air cleaner and screen, an air box 

 or header, intake valves or ports, a blower, an 

 air heater, and an air cooler. All of these parts 

 are not common to every intake system. An 

 intake system in which only a silencer, a screen, 

 a blower, an air box, and intake ports provide 

 a clean supply of air, with minimum noise, to 

 the combustion spaces is shown in figure 22- 

 20. 



T.D.C. 



FOUR 



STROKE 



CYCLE 



TWO 



STROKE 



CYCLE 



B.D.C. 



54.19:.20B 

 Figure 22-22.— Scavenging and supercharging in 

 diesel engines. 



The system which functions primarily to con- 

 vey gases away from the cylinders of an engine 

 is called the exhaust system . In addition to this 

 principal function, an exhaust system may be 

 designed to perform one or more of the follow- 

 ing functions: muffle exhaust noise, quench 

 sparks, remove solid material from exhaust 

 gases, and furnish energy to a turbine-driven 

 supercharger. The principal parts which may 

 be used in combination to accomplish the func- 

 tions of an engine exhaust system are shown in 

 figures 22-20 and 22-23. 



568 



