Chapter 22. -DIESEL AND GASOLINE ENGINES 



during a cycle of operation. This oversight 

 sometimes leads to confusion when the op- 

 erating principles of an engine are being con- 

 sidered. 



This discussion points out the relationship 

 of events to strokes by covering the number of 

 events occurring during a specific stroke, the 

 duration of an event with respect to a piston 

 stroke, and the cases where one event overlaps 

 another. The relationship of events to strokes 

 can be shown best by making use of graphic rep- 

 resentation of the changing situation occurring in 

 a cylinder during a cycle of operation. Figure 

 22-1 illustrates these changes for a 4-stroke 

 cycle diesel engine. 



The relationship of events to strokes is more 

 readily understood, if the movements of a piston 

 and its crankshaft are considered first. In part A 

 of figure 22-1, the reciprocating motion and 

 stroke of a piston are indicated and the rotary 

 motion of the crank during two piston strokes is 

 shown. The positions of the piston and crank at 

 the start and end of a stroke are marked "top" 

 and "bottom, " respectively. If these positions 

 and movements are marked on a circle (part B, 

 fig 22-1) the piston position, when at the top of 

 a stroke, is located at the top of a circle. When 

 the piston is at the bottom of a stroke, the piston 

 position is located at the bottom center of the 

 circle. Note in parts A and B of figure 22-1 that 

 the top center and bottom center identify points 

 where changes in direction of motion take place. 

 In other words, when the piston is at top center, 

 upward motion has stopped and downward motion 

 is ready to start or, with respect to motion, 

 the piston is "dead." 



The points which designate changes in di- 

 rection of motion for a piston and crank are 

 commonly called top dead center (TDC) and 

 bottom dead center (BDC). 



If the circle illustrated in B is broken at 

 various points and "spread out" (part C, fig. 

 22-1), the events of a cycle and their relation- 

 ship to the strokes and how some of the events 

 of the cycle overlap can be shown. TDC and 

 BDC should be kept in mind since they identify 

 the start and end of a stroke and they are the 

 points from which the start and end of events 

 are established. 



By following the strokes and events as illus- 

 trated, it can be noted that the intake event starts 

 before TDC, or before the actual down stroke 

 (intake) starts, and continues on past BDC, or 

 beyond the end of the stroke. The compression 

 event starts when the intake event ends, but the 



upstroke (compression) has been in process 

 since BDC. The injection and ignition events 

 overlap with the latter part of the compression 

 event, which ends at TDC. The burning of the 

 fuel continues a few degrees past TDC. The 

 power event or expansion of gases ends several 

 degrees before the down (power) stroke ends 

 at BDC. The exhaust event starts when the power 

 event ends and continues through the complete 

 upstroke (exhaust) and past TDC. Note the over- 

 lap of the exhaust event with the intake event of 

 the next cycle. The details on why certain 

 events overlap and why some events are shorter 

 or longer with respect to strokes will be covered 

 later in this chapter. 



From the preceding discussion, it can be 

 seen why the term "stroke" is sometimes used 

 to identify an event which occurs in a cycle of 

 operation. However, it is best to keep in mind 

 that a stoke involves 180° of crankshaft rotation 

 (or piston movement between dead centers) while 

 the corresponding event may take place during a 

 greater or lesser number of degrees of shaft 

 rotation. 



The relationship of events to strokes in a 

 2-stroke cycle diesel engine is shown in figure 

 22-2. Comparison of figures 22-1 and 22-2 re- 

 veals a number of differences between the two 

 types of mechanical or operating cycles. These 

 differences are not too difficult to understand if 

 one keeps in mind that four piston strokes and 

 720° of crankshaft rotation are involved in the 

 4-stroke cycle while only half as many strokes 

 and degrees are involved in a 2-stroke cycle. 

 Reference to the cross-sectional illustrations 

 (fig. 22-2) will aid in associating the event with 

 the relative position of the piston. Even though 

 the two piston strokes are frequently referred 

 to as power and compression, they are identified 

 as the "down stroke" (TDC to BDC) and "up 

 stroke" (BDC to TDC) in this discussion in order 

 to avoid confusion when reference is made to an 

 event. 



Starting with the admission of air, (1) figure 

 22-2, we find that the piston is in the lower half 

 of the down stroke and that the exhaust event (6) 

 is in process. The exhaust event started (6') a 

 number of degrees before intake, both starting 

 several degrees before the piston reached BDC. 

 The overlap of these events is necessary in 

 order that the incoming air (1') can aid in 

 clearing the cylinder of exhaust gases. Note that 

 the exhaust event stops a few degrees before the 

 intake event stops, but several degrees after the 

 upstroke of the piston has started. (The exhaust 



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