Chapter 22. -DIESEL AND GASOLINE ENGINES 



OTTO CYCLE 



DIESEL CYCLE 



75.5 



Figure 22-5.— Pressure-volume diagrams 

 for theoretical combustion cycles. 



combustion, induced by spark ignition, occurs 

 at constant volume. The Otto cycle and its prin- 

 ciples serve as the basis for modern gasoline 

 engine designs. 



Compression (see line A-B, figure 22-5) of 

 the charge in the cylinder is adiabatic. Spark 

 ignition occurs at B, and, due to the volatility 

 of the mixture, combustion practically amounts 

 to an explosion. Combustion, represented by 

 line BC, occurs (theoretically) just as the piston 

 reaches TDC. During combustion, there is no 

 piston travel; thus there is no change in the vol- 

 ume of the gas in the cylinder. This accounts for 

 the descriptive term, constant volume. During 

 combustion, there is a rapid rise of temperature 

 followed by a pressure increase which performs 

 the work during the expansion phase, represented 

 by line CD. The removal of gases, represented by 

 line DA, is at constant volume. 



TRUE DIESEL (CONSTANT-PRESSURE) 

 CYCLE.— This cycle may be defined as one in 

 which combustion, induced by compression ig- 

 nition, theoretically occurs at a constant pres- 

 sure. Adiabatic compression (represented by 

 line AB, fig. 22-5) of the air increases its tem- 

 perature to a point where ignition occurs auto- 

 matically when the fuel is injected. Fuel injec- 

 tion and combustion are so controlled as to give 

 constant-pressure combustion (represented by 

 line BC). This is followed by adiabatic expansion 

 (represented by line CD) and constant volume 

 (represented by the line DA). 



In the true diesel cycle, the burning of the 

 mixture of fuel and compressed air is a relatively 

 slow process when compared with the quick, ex- 

 plosive-type combustion process of the Otto 

 cycle. The injected fuel penetrates the 



compressed air, some of the fuel ignites, then 

 the rest of the charge burns. The expansion of 

 the gases keeps pace with the change in volume 

 caused by piston travel; thus combustion is said 

 to occur at constant pressure (represented by 

 line BC). 



MODIFIED COMBUSTION CYCLES.- The 

 preceding discussion covers the theoretical 

 combustion cycles which serve as the basis for 

 modern engines. In actual operation, modern 

 engines operate on modifications of the theo- 

 retical cycles. However, characteristics of the 

 true cycles are incorporated in the cycles of 

 modern engines. This is pointed out in the follow- 

 ing discussion of examples representing the 

 actual cycles of operation in gasoline and diesel 

 engines. 



The following examples are based on the 4- 

 stroke mechanical cycle since the majority of 

 gasoline engines use this type of cycle; thus, a 

 means of comparing the cycles found in both 

 gasoline and diesel engines is provided. Differ- 

 ences existing in diesel engines operating on the 

 2- stroke cycle are pointed out. 



The illustrations in figures 22-6 and 22-7 

 represent the changing conditions in a cylinder 

 during engine operation. Some of the events are 

 exaggerated in order to show more clearly the 

 change which takes place and, at the same time, 

 to show how the theoretical and actual cycles 

 differ. 



The compression ratio situation and a pres- 

 sure-volume diagram for a 4- stroke Otto cycle 

 is shown in figure 22-6. Illustration A shows 

 the piston on BDC at the start of an upstroke, 

 (In a 4- stroke cycle engine, this stroke could 

 be either that identified as the compression 

 stroke or the exhaust stroke.) Notice that in 

 moving from BDC to TDC (illustration B), the 

 piston travels 5/6 of the total distance ab. In 

 other words, the volume has been decreased to 

 1/6 of the volume when the piston was at 

 BDC. Thus, the compression ratio is 6 to 1. 



Illustration C shows the changes in volume 

 and pressure during one complete 4- stroke 

 cycle. Note that the lines representing the com- 

 bustion and exhaust phases are not straight as 

 they were in the theoretical diagram. As in the 

 diagram of the theoretical cycle, the vertical 

 line at the left represents cylinder pressure in 

 psi. Atmospheric pressure is represented by a 

 horizontal line called the atmospheric pressure 

 line. Pressures below this line are less than 

 atmospheric pressures, while pressures above 



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