Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGAs one of the principal parts in the powertransmitting assembly, the piston must be sodesigned and must be made of such materialsthat it can withstand the extreme heat andpressure of combustion. Pistons must also belight enough to keep inertia loads on relatedparts to a minimum. The piston aids in thesealing of the cylinder to prevent the escapeof gas and transmits some of the heat throughthe piston rings to the cylinder wall. In additionto serving as the unit which transmitsthe force of combustion to the connecting rodand conducts the heat of combustion to thecylinder wall, a piston serves as a valve inopening and closing the ports of a two-strokecycle engine. The nomenclature for the parts ofa typical trunk type piston is given in figure22-16.Piston rings are particularly vital to engineoperation in that they must effectively performthree functions: seal the cylinder, distribute andcontrol lubricating oil on the cylinder wall, andCOMPRESSIONRING GROOVESAND LANDSPISTON HUBBUSHING75.47Figure 22-16.— Piston nomemclature.transfer heat fromthepistontothecylinderwall.All rings on a piston perform the latter function,but two general types of rings— compression andoil— are required to perform the first two functions.There are numerous types of rings in eachof these groups, contructed in different waysfor particular purposes. Some of the variationsin ring design are illustrated in figure 22-17.In trunk-type piston assemblies, the connectionbetween the piston and the connecting rod isusually the piston pin (sometimes referred to asthe wrist pin ) and its bearings. These partsmust be of especially strong construction becausethe power developed in the cylinder istransmitted from the piston through the pin tothe connecting rod. The pin is the pivot pointwhere the straight-line or reciprocating motionof the piston changes to the reciprocating androtating motion of the connecting rod. Thus, theprincipal forces to which a pin is subjected arethe forces created by combustion and the sidethrust created by the change in direction ofmotion. (See fig. 22-18.)The connecting link between the piston andcrankshaft or the crankshaft and the crossheadof an engine is the connecting rod . In order thatthe forces created by combustion can be transmittedto the crankshaft, the rod changes thereciprocating motion of the piston to the rotatingmotion of the crankshaft.Most marine engines in Navy service use thetrunk-type piston connected directly to the connectingrod.The camshaft is a shaft with eccentric projection,called cams, designed to control theoperation of valves, usually through variousintermediate parts as described later in thischapter. Originally cams were made as separatepieces and fastened to the camshaft. However,in most modern engines the cams are forged orcast as an integral part of the camshaft.To reduce wear and to help them withstandthe shock action to which they are subjected,camshafts are made of low-carbon alloy steelwith the cam and journal surfaces carburizedbefore the final grinding is done.The cams are arranged on the shaft toprovide the proper firing order of the cylindersserved. The shape of the cam determines thepoint of opening and closing, the speed of openingand closing, and the amount of the valvelift. If one cylinder is properly time, theremaining cylinders are automatically in time.All cylinders will be affected if there is a changein timing.562