Principles of naval engineering - Historic Naval Ships Association

Chapter 22. -DIESEL AND GASOLINE ENGINESReference to combustion cycles suggests anotherimportant difference between gasoline anddiesel engines— compression pressure. Thisfactor is directly related to the combustion processutilized in an engine. Diesel engines have amuch higher compression pressure than gasolineengines. The higher compression pressure indiesels explains the difference in the methods ofignition used in gasoline and diesel engines.Compressing the gases within a cylinder raisesthe temperature of the confined gases. Thegreater the compression, the higher the temperature.In a gasoline engine, the compressiontemperature is always lower than the point wherethe fuel would ignite spontaneously. Thus, theheat required to ignite the fuel must come froman external source— spark ignition. On the otherhand, the compression temperature in a dieselengine is far above the ignition point of the fueloil; therefore, ignition takes place as a resultheat generated by compression of the air withinthe cylinder— compression ignition.The difference in the methods of ignition indicatesthat there is a basic difference in the combustioncycles upon which diesel and gasolineengines operate. This difference involves thebehavior of the combustion gases under varyingconditions of pressure, temperature, and volume.Since this is the case, the relationship ofthese factors is considered before the combustioncycles,RELATIONSHIP OF TEMPERATURE, PRES-SURE, ANDVOLUME.-The relationship of thesethree conditions as found in an engine can beillustrated by considering what takes place in acylinder fitted with a reciprocating piston. (Seefig. 22-4.)Instruments are provided which indicate thepressure within the cylinder and the temperatureinside and outside the cylinder. Considerthat the air in the cylinder is at atmosphericpressure and that the temperatures, inside andoutside the cylinder, are about 70°F. (See fig.22-4A.)If the cylinder is an airtight container and aforce pushes the piston toward the top of thecylinder, the entrapped charge will be compressed.As the compression progresses, thevolume of the air decreases , the pressure increases, and the temperature rises (see B andC). These changing conditions continue as thepiston moves and when the piston nears TDC(see D) we find that there has been a markeddecrease in volume and that both pressure andtemperature are much greater than at the beginningof compression. Note that pressure hasgone from to 470 psi and temperature has increasedfrom 70° to about 1000° F, These changingconditions indicate that mechanical energy,in the form of work done on the piston, has beentransformed into heat energy in the compressedair. The temperature of the air has been raisedsufficiently to cause ignition of fuel injected intothe cylinder.Further changes take place after ignition.Since ignition occurs shortly before TDC, thereis little change in volume until the piston passesTDC. However, there is a sharp increase inpressure and temperature shortly after ignitiontakes place. The increased pressure forces thepiston downward. As the piston moves downward,the gases expand, or increase in volume,and pressure and temperature decrease rapidly.The changes in volume, pressure, and temperature,described and illustrated here, are representativeof the changing conditions within thecylinder of a modern diesel engine.The changes in volume and pressure in anengine cylinder can be illustrated by diagramssimilar to those shown in figure 22-5, Suchdiagrams are made by devices which measureand record the pressures at various piston positionsduring a cycle of engine operation. Diagramswhich show the relationship between pressuresand corresponding piston positions arecalled pressure-volume diagrams or indicatorcards . Examples of theoretical and actual pressure-volumediagrams are used in this chapterwith the description of combustion cycles.On diagrams which provide a graphic representationof cylinder pressure as related to volume,the vertical line P on the diagram (fig.22-5) represents pressure and the horizontalline V represents volume. When a diagram isused as an indicator card, the pressure line ismarked off in units of pressure and the volumeline is marked off in inches. Thus, the volumeline could be used to show the length of thepiston stroke which is proportional to volume.The distance between adjacent letters on eachof the diagrams represents an event of a combustioncycle— that is, compressionof air, burningof the charge, expansion of gas, and removalof gases.The diagrams shown in figure 22-5 providea means by which the Otto and true diesel combustioncycles can be compared. Reference tothe diagrams during the following discussion ofthese combustion cycles will aid in identifying549