Chapter 22. -DIESEL AND GASOLINE ENGINES 



TDC 



BDC 



ATMOSPHERIC 



PRESSURE 



LINE 



54.19C 



Figure 22-7.— Pressure-volume diagram, diesel 4-stroke cycle. 



In the case of Otto cycle, this event includes 

 the admission of fuel and air. As indicated 

 earlier, the intake event starts before TDC, 

 or at point a, illustration C. Note that pressure 

 is decreasing and after the piston reaches TDC 

 and starts down, a vacuum is created which 

 facilites the flow of the fuel-air mixture into 

 the cylinder. The intake event continues a few 

 degrees past BDC, ending at point b. Since the 

 piston is now on an upstroke, compression takes 

 place and continuesuntil the piston reaches TDC. 

 Note the increase in pressure (xtox') and the de- 

 crease in volume (f to x). Spark ignition at c 

 starts combustion which takes place very rapid- 

 ly. There is some change in volume since the 

 phase starts before and ends after TDC. 



There is a sharp increase in pressure dur- 

 ing the combustion phase. The relative amount 

 is shown by the curve cd. The increase in 

 pressure provides the force necessary to drive 

 the piston down again. The gases continue to 

 expand as the piston moves toward BDC, and 

 the pressure decreasesas the volume increases, 

 from d to e. The exhaust event starts a few 

 degrees before BDC, ate, and the pressure drops 

 rapidly until the piston reaches BDC. As the 



piston moves toward TDC, there is a slight drop 

 in pressure as the waste gases are discharged. 

 The exhaust event continues a few degrees past 

 TDC to point g so that the incoming charge 

 aids in removing the remaining waste gases. 



The modified diesel combustion cycle is one 

 in which the combustion phase, induced by com- 

 pression ignition, begins on a constant-volume 

 basis and ends on a constant -pressure basis. 

 In other words, the modified cycle is a combi- 

 nation of the Otto and true diesel cycles. The 

 modified cycle is used as the basis for the 

 design of practically all modern diesel engines. 



An example of a pressure-volume diagram 

 for a modified 4-stroke cycle diesel engine is 

 shown in figure 22-7. Note that the volume line 

 is divided into 16 units, indicating a 16 to 1 

 compression ratio. The higher compression 

 ratio accounts for the increased temperature 

 necessary to ignite the charge. By comparing 

 this illustration with illustration C of figure 

 22-6, it will be found that the phases of the 

 diesel cycle are relatively the same as those of 

 the Otto cycle, except for the combustion phase. 

 Fuel is injected at point c and combustion is 

 represented by line cd. While combustion in the 



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