Chapter 22. -DIESEL AND GASOLINE ENGINES 



Reference to combustion cycles suggests an- 

 other important difference between gasoline and 

 diesel engines— compression pressure. This 

 factor is directly related to the combustion proc- 

 ess utilized in an engine. Diesel engines have a 

 much higher compression pressure than gasoline 

 engines. The higher compression pressure in 

 diesels explains the difference in the methods of 

 ignition used in gasoline and diesel engines. 

 Compressing the gases within a cylinder raises 

 the temperature of the confined gases. The 

 greater the compression, the higher the temper- 

 ature. In a gasoline engine, the compression 

 temperature is always lower than the point where 

 the fuel would ignite spontaneously. Thus, the 

 heat required to ignite the fuel must come from 

 an external source— spark ignition. On the other 

 hand, the compression temperature in a diesel 

 engine is far above the ignition point of the fuel 

 oil; therefore, ignition takes place as a result 

 heat generated by compression of the air within 

 the cylinder— compression ignition. 



The difference in the methods of ignition indi- 

 cates that there is a basic difference in the com- 

 bustion cycles upon which diesel and gasoline 

 engines operate. This difference involves the 

 behavior of the combustion gases under varying 

 conditions of pressure, temperature, and vol- 

 ume. Since this is the case, the relationship of 

 these factors is considered before the combus- 

 tion cycles, 



RELATIONSHIP OF TEMPERATURE, PRES- 

 SURE, ANDVOLUME.-The relationship of these 

 three conditions as found in an engine can be 

 illustrated by considering what takes place in a 

 cylinder fitted with a reciprocating piston. (See 

 fig. 22-4.) 



Instruments are provided which indicate the 

 pressure within the cylinder and the tempera- 

 ture inside and outside the cylinder. Consider 

 that the air in the cylinder is at atmospheric 

 pressure and that the temperatures, inside and 

 outside the cylinder, are about 70°F. (See fig. 

 22-4A.) 



If the cylinder is an airtight container and a 

 force pushes the piston toward the top of the 

 cylinder, the entrapped charge will be com- 

 pressed. As the compression progresses, the 

 volume of the air decreases , the pressure in- 

 creases , and the temperature rises (see B and 

 C). These changing conditions continue as the 

 piston moves and when the piston nears TDC 

 (see D) we find that there has been a marked 

 decrease in volume and that both pressure and 



temperature are much greater than at the be- 

 ginning of compression. Note that pressure has 

 gone from to 470 psi and temperature has in- 

 creased from 70° to about 1000° F, These chang- 

 ing conditions indicate that mechanical energy, 

 in the form of work done on the piston, has been 

 transformed into heat energy in the compressed 

 air. The temperature of the air has been raised 

 sufficiently to cause ignition of fuel injected into 

 the cylinder. 



Further changes take place after ignition. 

 Since ignition occurs shortly before TDC, there 

 is little change in volume until the piston passes 

 TDC. However, there is a sharp increase in 

 pressure and temperature shortly after ignition 

 takes place. The increased pressure forces the 

 piston downward. As the piston moves down- 

 ward, the gases expand, or increase in volume, 

 and pressure and temperature decrease rapidly. 

 The changes in volume, pressure, and tempera- 

 ture, described and illustrated here, are rep- 

 resentative of the changing conditions within the 

 cylinder of a modern diesel engine. 



The changes in volume and pressure in an 

 engine cylinder can be illustrated by diagrams 

 similar to those shown in figure 22-5, Such 

 diagrams are made by devices which measure 

 and record the pressures at various piston posi- 

 tions during a cycle of engine operation. Dia- 

 grams which show the relationship between pres- 

 sures and corresponding piston positions are 

 called pressure-volume diagrams or indicator 

 cards . Examples of theoretical and actual pres- 

 sure-volume diagrams are used in this chapter 

 with the description of combustion cycles. 



On diagrams which provide a graphic repre- 

 sentation of cylinder pressure as related to vol- 

 ume, the vertical line P on the diagram (fig. 

 22-5) represents pressure and the horizontal 

 line V represents volume. When a diagram is 

 used as an indicator card, the pressure line is 

 marked off in units of pressure and the volume 

 line is marked off in inches. Thus, the volume 

 line could be used to show the length of the 

 piston stroke which is proportional to volume. 

 The distance between adjacent letters on each 

 of the diagrams represents an event of a com- 

 bustion cycle— that is, compressionof air, burn- 

 ing of the charge, expansion of gas, and removal 

 of gases. 



The diagrams shown in figure 22-5 provide 

 a means by which the Otto and true diesel com- 

 bustion cycles can be compared. Reference to 

 the diagrams during the following discussion of 

 these combustion cycles will aid in identifying 



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