PRINCIPLES OF NAVAL ENGINEERING 



permits the burned gases to enter the exhaust 

 manifold. As the piston moves downward, the in- 

 take ports are uncovered (!') and the incoming 

 air clears the cylinder of the remaining ex- 

 haust gases and fills the cylinder with a fresh 

 air charge (1); thus, the cycle of operation has 

 started again. 



Now what is the difference between the 2- 

 and 4-stroke cycles? From the standpoint of the 

 mechanics of operation, the principal difference 

 is in the number of piston strokes taking place 

 during the cycle of events. A more significant 

 difference is the fact that a 2-stroke cycle 

 engine delivers twice as many power impulses 

 to the crankshaftfor every 720° of shaft rotation. 

 (See fig. 22-3.) 



Diagrams showing the mechanical cycles of 

 operation in gasoline engines would be somewhat 

 similar to those described for diesel engines 

 except that there would be one less event taking 

 place during the gasoline engine cycle. Since air 

 and fuel are admitted to the cylinder of a gasoline 

 engine as a mixture during the intake event, the 

 injection event does not apply. 



The figures shown here representing the 

 cycles of operation are for illustrative purposes 

 only. The exact number of degrees before or 

 after TDC or BDC that an event starts and ends 

 will vary between engines. Information on such 



details should be obtained from appropriate 

 technical manuals dealing with the specific en- 

 gine in question. 



Combustion Cycles 



To this point, the strokes of a piston and the 

 related events taking place during a cycle of 

 operation have been given greater consideration 

 than the heat process involved in the cycle. How- 

 ever, the mechanics of engine operation cannot 

 be discussed without dealing with heat. Such 

 terms as ignition, combustion, and expansion of 

 gases, all indicate that heat is essential to a 

 cycle of engine operation. So far, particular 

 differences between diesel and gasoline engines 

 have not been pointed out, except the number of 

 events occurring during the cycle of operation. 

 Whether a diesel engine or a gasoline engine, 

 the 2- or the 4-stroke cycle may apply. Then, 

 one of the principal differences between these 

 types of engines must involve the heat process 

 utilized to produce the forces which make the 

 engine operate. The heat processes are some- 

 times called combustion or heat cycles. 



The three most common combustion cycles 

 associated with reciprocating internal combus- 

 tion engines are the Otto cycle , the true diesel 

 cycle, and the modified diesel cycle . 



FOUR STROKE CYCLE 



DOWNSTROKE (POWER) | UPSTROKE (EXHAUST) | DOWNSTROKE (INTAKE) | UPSTROKE (COMPRESSION) 



-ONE REVOLUTION OF CRANKSHAFT - 



-ONE REVOLUTION OF CRANKSHAFT - 



- 360°- 



INJECT lON-IGN ITION-COMBUSTION - 



54.19:.20X 



Figure 22-3.— Comparison of the 2- and 4-stroke cycles. 



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