PRINCIPLES OF NAVAL ENGINEERING 



takes place. The term cycle identifies the 

 sequence of events that takes place in the 

 cylinder of an engine for each power impulse 

 transmitted to the crankshaft. These events 

 always occur in the same order each time the 

 cycle is repeated. The number of events oc- 

 curring in a cycle of operation will depend 

 upon the engine type— diesel or gasoline. The 

 difference in the events occurring in the cycle 

 of operation for these engines is shown in the 

 following table. 



The events and their sequence in a cycle 

 operation for a: 



DIESEL ENGINE 



INTAKE of air 

 COMPRESSION of air 



INJECTION of fuel 

 IGNITION AND COM- 

 BUSTION of charge 

 EXPANSION of gases 

 REMOVAL of waste 



GASOUNE ENGINE 



INTAKE of fuel and air 

 COMPRESSION of fuel- 

 air mixture. 



IGNITION and COM- 

 BUSTION of charge 

 EXPANSION of gases 

 REMOVAL of waste 



The principal difference, as shown in the 

 table, in the cycles of operation for diesel and 

 gasoline engines involves the admission of 

 fuel and air to the cylinder. While this takes 

 place as one event in the operating cycle of a 

 gasoline engine, it involves two events in 

 diesel engines. Thus, insofar as events are 

 concerned, there are six main events taking 

 place in the diesel cycle of operation and five 

 in the cycle of a gasoline engine. This is pointed 

 out in order to emphasize the fact that the 

 events which take place and the piston strokes 

 which occur during a cycle of operation are not 

 identical. Even though the events of a cycle 

 are closely related to piston position and move- 

 ment, all of the events will take place during 

 the cycle regardless of the number of piston 

 strokes involved. The relationship of events 

 and piston strokes is discussed later under a 

 separate heading. 



The mechanics of engine operation is some- 

 times referred to as the mechanical or operating 

 cycle of an engine; while the heat process which 

 produces the forces that move engine parts may 

 be referred to as the combustion cycle . A cycle 

 of each type is included in a cycle of engine op- 

 eration. 



Mechanical Cycles 



In the preceding section, the events taking 

 place in a cycle of engine operation were 

 emphasized. Little was said about piston strokes 

 except that a complete sequence of events would 

 occur during a cycle regardless of the number of 

 strokes made by the piston. The number of 

 piston strokes occurring during any one series 

 of events is limited to either two or four, de- 

 pending upon the design of the engine; thus, the 

 4-stroke cycle and the 2-stroke cycle. These 

 cycles are known as the mechanical cycles of 

 operation. 



Four- and Two-Stroke Cycles.— Both types 

 of mechanical cycles are used in diesel and 

 gasoline engines. However, most large gaso- 

 line engines in Navy service operate on the 

 4-stoke cycle; a greater number of diesel 

 engines operate on the 2-stroke than on the 

 4-stroke cycle. The relationship of the events 

 and piston strokes occurring in a cycle of op- 

 eration involves some of the differences be- 

 tween the 2-stroke cycle and the 4-stroke cycle. 



RELATIONSHIP OF EVENTS AND STROKES 

 IN A CYCLE .—A piston stroke is the distance a 

 piston moves between limits of travel. The 

 cycle of operation is an engine operating on the 

 4-stroke cycle involves four piston strokes— in- 

 take, compression , power , and exhaust . In the 

 case of the 2-stroke cycle, only two strokes 

 apply— power and compression . 



A check of the previous table listing the 

 series of events which take place during the 

 cycles of operation of diesel and gasoline en- 

 gines will show that the strokes are named to 

 correspond to some of the events. However, 

 since six events are listed for diesel engines 

 and five events for gasoline engines, it is evi- 

 dent that more than one event takes place during 

 some of the strokes, especially in the case of 

 the 2-stroke cycle. Even though this is the case, 

 it is common practice to identify some of the 

 events as strokes of the piston. This is because 

 such events as intake, compression, power and 

 exhaust in a 4-stroke cycle involve at least a 

 major portion of a stroke and, in some cases, 

 more than one stroke. The same is true of 

 power and compression events and strokes in a 

 2-stroke cycle. Such association of events and 

 strokes overlooks other events taking place 



544 



