PRINCIPLES OF NAVAL ENGINEERING 



As one of the principal parts in the power 

 transmitting assembly, the piston must be so 

 designed and must be made of such materials 

 that it can withstand the extreme heat and 

 pressure of combustion. Pistons must also be 

 light enough to keep inertia loads on related 

 parts to a minimum. The piston aids in the 

 sealing of the cylinder to prevent the escape 

 of gas and transmits some of the heat through 

 the piston rings to the cylinder wall. In ad- 

 dition to serving as the unit which transmits 

 the force of combustion to the connecting rod 

 and conducts the heat of combustion to the 

 cylinder wall, a piston serves as a valve in 

 opening and closing the ports of a two-stroke 

 cycle engine. The nomenclature for the parts of 

 a typical trunk type piston is given in figure 

 22-16. 



Piston rings are particularly vital to engine 

 operation in that they must effectively perform 

 three functions: seal the cylinder, distribute and 

 control lubricating oil on the cylinder wall, and 



COMPRESSION 



RING GROOVES 



AND LANDS 



PISTON HUB 

 BUSHING 



75.47 



Figure 22-16.— Piston nomemclature. 



transfer heat fromthepistontothecylinderwall. 

 All rings on a piston perform the latter function, 

 but two general types of rings— compression and 

 oil— are required to perform the first two func- 

 tions. There are numerous types of rings in each 

 of these groups, contructed in different ways 

 for particular purposes. Some of the variations 

 in ring design are illustrated in figure 22-17. 



In trunk-type piston assemblies, the connec- 

 tion between the piston and the connecting rod is 

 usually the piston pin (sometimes referred to as 

 the wrist pin ) and its bearings. These parts 

 must be of especially strong construction be- 

 cause the power developed in the cylinder is 

 transmitted from the piston through the pin to 

 the connecting rod. The pin is the pivot point 

 where the straight-line or reciprocating motion 

 of the piston changes to the reciprocating and 

 rotating motion of the connecting rod. Thus, the 

 principal forces to which a pin is subjected are 

 the forces created by combustion and the side 

 thrust created by the change in direction of 

 motion. (See fig. 22-18.) 



The connecting link between the piston and 

 crankshaft or the crankshaft and the crosshead 

 of an engine is the connecting rod . In order that 

 the forces created by combustion can be trans- 

 mitted to the crankshaft, the rod changes the 

 reciprocating motion of the piston to the rotating 

 motion of the crankshaft. 



Most marine engines in Navy service use the 

 trunk-type piston connected directly to the con- 

 necting rod. 



The camshaft is a shaft with eccentric pro- 

 jection, called cams, designed to control the 

 operation of valves, usually through various 

 intermediate parts as described later in this 

 chapter. Originally cams were made as separate 

 pieces and fastened to the camshaft. However, 

 in most modern engines the cams are forged or 

 cast as an integral part of the camshaft. 



To reduce wear and to help them withstand 

 the shock action to which they are subjected, 

 camshafts are made of low-carbon alloy steel 

 with the cam and journal surfaces carburized 

 before the final grinding is done. 



The cams are arranged on the shaft to 

 provide the proper firing order of the cylinders 

 served. The shape of the cam determines the 

 point of opening and closing, the speed of open- 

 ing and closing, and the amount of the valve 

 lift. If one cylinder is properly time, the 

 remaining cylinders are automatically in time. 

 All cylinders will be affected if there is a change 

 in timing. 



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