Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGconsideration lias been given to the source ofpower or to the mechanisms which cause theseparts to operate.In many cases, the mechanism which transmitspower for the operationof the engine valvesand blower may also transmit power to partsand accessories which are components of variousengine systems. For example, such itemsas the governor; fuel, lubricating, and waterpumps; and overspeed trips, are, in some engines,operated by the same mechanism. Sincemechanisms which transmit power to operatespecific parts and accessories may be relatedto more than one engine system, such operatingmechanisms are considered here before theremaining engine systems are discussed.The parts which make up the operating mechanismsof an engine may be divided into twogroups: the group which forms the drive mechanismsand the group which forms the actuatingmechanisms . The source of power for the operatingmechanisms of an engine is the crankshaft.As used in this chapter, the term drivemechanism identifies the group of parts whichtakes power from the crankshaft and transmitsthat power to various engine parts and accessories.In engines, the drive mechanismsdoes not change the type of motion, but it maychange the direction of motion. For example,the impellers or lobes of a blower are drivenor operated as a result of rotary motion whichis taken from the crankshaft and transmitted tothe impellers or lobes by the drive mechanism,an arrangement of gears and shafts. While thetype of motion (rotary) remains the same, thedirection of motion of one impeller or lobe isopposite to that of the other impeller or lobeas a result of the gear arrangements withinthe drive mechanism.A drive mechanism may be of the gear,chain or belt type. Of these, the gear type isthe most common; however, some engines areequipped with chain assemblies, A combinationof gears and chains is used as the drivingmechanism in some engines.Some engines have a single drive mechanismwhich transmits power for the operation of engineparts and accessories. Inother cases, theremay be two or more separate mechanisms.When separate assemblies are used, the onewhich transmits power for the operation of theaccessories is called the accessory drive . Someengines have more than one accessory drive.A separate drive mechanism which is used totransmit power for the operationof engine valvesis generally called the camshaft drive or timingmechanism .The camshaft drive, as the name implies,transmits power to the camshaft of the engine.The shaft, in turn, transmits the power througha combination of parts which causes the enginevalves to operate. Since the valves of an enginemust open and close at the proper moment(with respect to the position of the piston) andremain in the open and closed positions fordefinite periods of time, a fixed relationshipmust be maintained between the rotational speedsof the crankshaft and the camshaft. Camshaftdrives are designed to maintain the properrelationship between the speeds of the two shafts.In maintaining this relationship, the drive causesthe camshaft to rotate at crankshaft speed in a2-stroke cycle engine; and at one-half crankshaftspeed in a 4-stroke cycle engine.The term actuating mechanism, as used inthis chapter, identifies that combination of partswhich receives power from the drive mechanismand transmits the power to the engine valves.In order for the valves (intake, exhaust, fuelinjection, air starter) to operate, there must bea change in the type of motion. In other words,the rotary motion of the crankshaft and drivemechanism must be changed to a reciprocatingmotion. The group of parts which, by changingthe type of motion, causes the valves of an engineto operate is generally referred to as thevalve actuating mechanism . A valve-actuatingmechanism may include the cams, cam followers,push rods, rocker arms, and valvesprings. In some engines, the camshaft is solocated that the need for push rods is eliminated.In such cases, the cam follower is a part of therocker arm. (Some actuating mechanisms aredesigned to transform reciprocating motion intorotary motion, but in internal combustion enginesmost actuating mechanisms change rotarymotion into reciprocating motion.)There is considerable variation in the designand arrangement of the parts of operating mechanismsfound in different engines. The size ofan engine, the cycle of operation, the cylinderarrangement, and other factors govern the designand arrangement of the components aswell as the design and arrangement of the mechanisms.Three types of operating mechanismsare shown in figures 22-24, 22-25, and 22-26.The mechanisms which supply power for theoperation of the valves and accessories of gasolineengines are basically the same as those570