Chapter 22. -DIESEL AND GASOLINE ENGINES 



The amount of fuel mixed with the air must be 

 carefully regulated, and must change with the 

 engine's different speeds and loads. The amount 

 of fuel required by an engine which is warming- 

 up is different from the amount required by an 

 engine which has reached operating tempera- 

 ture. Special fuel adjustment is needed for rapid 

 acceleration. All of these varying requirements 

 are met automatically by the modern carburetor. 



Engine Ignition Systems.— The methods by 

 which the fuel mixture is ignited in the cylinders 

 of diesel and gasoline engines differ as much as 

 the methods of obtaining a combustible mixture 

 in the cylinders of the two engines. An ignition 

 system, as such, is not commonly associated with 

 diesel engines. There is no one group of parts in 

 a diesel engine which functions only to cause ig- 

 nition, as there is in a gasoline engine. However, 

 a diesel engine does have an "ignition system"; 

 otherwise, combustion would not take place in 

 the cylinders. 



In a diesel engine, the parts which may be 

 considered as forming the ignition system are 

 the piston, the cylinder liner, and the cylinder 

 head. These parts are not commonly thought of 

 as forming an ignition system since they are 

 generally associated with other functions such as 

 forming the combustion space and transmitting 

 power. Nevertheless, ignition in a diesel engine 

 depends upon the piston, the cylinder, and the 

 head. These parts not only form the space where 

 combustion takes place but also provide the 

 means by which the air is compressed to gen- 

 erate the heat necessary for self- ignition of the 

 combustible mixture. In other words, both the 

 source (air) of ignition heat and its generation 

 (compression) are wholly within a diesel engine. 



This is not true of a gasoline engine because 

 the combustion cycles of the two types of engines 

 are different. In a gasoline engine, even though 

 the piston, the cylinder, and the head form the 

 combustion space, as in a diesel engine, the heat 

 necessary for ignition is caused by energy from 

 a source external to the combustion space. The 

 completion of the ignition process, involving the 

 transformation of mechanical energy into elec- 

 trical energy and then into heat energy, requires 

 several parts, each performing a specific func- 

 tion. The parts which make the transformation 

 of energy and the system which they form are 

 commonly thought of when reference is made to 

 an ignition system. 



The spark which causes the ignition of the 

 explosive mixture in the cylinders of a gasoline 



engine is produced when electricity is forced 

 across a gap formed by two electrodes in the 

 combustion chamber. The electrical ignition 

 system furnishes the spark periodically to each 

 cylinder, at a predetermined position of piston 

 travel. In order to accomplish this function, an 

 electrical ignition system must have, first of 

 all, either a source of electrical energy or a 

 means of developing electrical energy. In some 

 cases, a storage battery is used as the source of 

 energy; in other cases, a magneto generates 

 electricity for the ignition system. The voltage 

 from either a battery or a magneto is not suf- 

 ficiently high enough to overcome the resistance 

 created by pressure in the combustion chamber 

 and to cause the proper spark in the gap formed 

 by the two electrodes in the combustive cham- 

 ber. Therefore, it is essential that an ignition 

 system include a device which increases the 

 voltage of the electricity supplied to the system 

 sufficiently to cause a "hot" spark in the gap 

 of the spark plug. The device which performs 

 this function is generally called an ignition coil 

 or induction coil . 



Since a spark must occur momentarily in 

 each cylinder at a specific time, an ignition 

 system must include a device which controls the 

 timing of the flow of electricity to each cylinder. 

 This control is accomplished by interrupting the 

 flow of electricityfromthe source to the voltage- 

 increasing device (ignition coil). The interrup- 

 tion of the flow of electricity also plays an im- 

 portant part in the process of increasing voltage. 

 The interrupting device is generally called the 

 breaker assembly. A device which will dis- 

 tribute electricity to the different cylinders in 

 the proper firing order is also necessary. The 

 part which performs this function is called the 

 distributing mechanism. Spark plugs to provide 

 the gaps and wiring and switches to connect the 

 parts of the system are essential to complete an 

 ignition system. 



All ignition systems are basically the same, 

 except for the source of electrical energy. The 

 source of energy is frequently used as a basis 

 for classifying ignition systems; thus the bat- 

 tery-ignition system and the magneto-ignition 

 system. 



Engine Cooling Systems.— A great amount 

 of heat is generated within an engine during 

 operation. Combustion produces the greater 

 portion of this heat; however, compression of 

 gases within the cylinders and friction between 

 moving parts add to the total amount of heat 



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