PRINCIPLES OF NAVAL ENGINEERING 



GAS 

 PRESSURE 



SIDE SIDE 



THRUST THRUST 



75.57 



Figure 22-18.— Side thrust of a trunk-type 

 piston, single-acting engine. 



speed of tiie shaft tends to increase, the flywheel 

 absorbs energy, and when the speed tends to 

 decrease, the flywheel gives up energy to the 

 shaft in an effort to keep shaft rotation uniform. 

 In doing this, a flywheel (1) keeps variations in 

 speed within desired limits at all loads; (2) limits 

 the increase or decrease in speed during sudden 

 changes of load; (3) aids in forcing the piston 

 through the compression event when an engine is 

 running at low or idling speed; and (4) helps 

 bring the engine up to speed when it is being 

 cranked. 



An important group of engine parts consists 

 of the bearings. Some bearings remain stationary 

 in performing their function while others move. 

 One principal group of stationary bearings in an 

 engine is that which supports the crankshaft. 

 These bearings are generally called main engine 

 bearings. (See fig. 22-13). Main bearings in most 

 engines are of the sliding contact, or plain type, 

 consisting of two halves or shells. 



Main bearings are subjected to a fluctuating 

 load. This is also true of the crankpin bearings 



and the piston-pin bearings. However, the man- 

 ner in which main journal bearings are loaded 

 depends upon the type of engine in which they are 

 used. 



In a 2- stroke cycle engine, a load is always 

 placed on the lower half of the main bearings and 

 the lower half of the piston pin bearings in the 

 connecting rod; meanwhile the load is placed upon 

 the upper half of the connecting rod bearings at 

 the crankshaft end of the rod. This is true 

 because the forces of combustion are greater 

 than the inertia forces created by the moving 

 parts. 



In a 4- stroke cycle engine, the load is applied 

 first on one bearing shell and then on the other. 

 The reversal of pressure is the result of the 

 large forces of inertia imposed during the intake 

 and exhaust strokes. In other words, inertia 

 tends to lift the crankshaft in its bearings during 

 the intake and exhaust strokes. 



There is a definite reversal of load appli- 

 cation on the main bearings of a double-acting 

 engine. In this case, the reversal is caused by 

 combustion taking place first on one end of the 

 piston and then on the other. 



The bearings used in connection with piston 

 pins are of three types: the integral bearing, the 

 sleeve bearing or bushing, and the needle type 

 roller bearing. The bearings in the bosses (hubs) 

 of most pistons are of the sleeve bushing type. 

 However, in a few cases, the boss bearings are 

 an integral part of the piston. In such cases, the 

 bearing surface is precision bored directly in 

 the bosses. Pistons fitted with stationary piston 

 pins require no bearing surfaces in the bosses. 



Even though the piston pins in most engines 

 are equipped with bushing type bearings, some 

 have been fitted with bearings of the needle 

 roller type. 



The types of bearings used for main bearings 

 and in connection with piston-pin assemblies are 

 representative of those used at other points in an 

 engine where bearing surfaces are required. 



All of the parts which make a complete 

 engine have by no means been covered in the 

 preceding section of this chapter. Since many 

 engine parts and accessories are commonly 

 associated with the systems of an engine , 

 functions of some of the principal components 

 not covered to this point are considered with 

 the applicable system which they affect. 



ENGINE AIR SYSTEMS.- Parts and acces- 

 sories which supply the cylinders of an engine 

 with air for combustion, and remove the waste 

 gases after combustion and the power events 



564 



