RECIPROCATING INTERNAL-COMBUSTION ENGINE

The engines with which you will be working will convert heat energy into work by burning fuel in a confined chamber within the engine; thus the term INTERNAL COMBUSTION. Because they have pistons that employ a back-and-forth motion, diesel and gasoline engines are also classified as RECIPROCATING engines. The occupational standards for advancement in the Engineman rating require you to know a great deal about reciprocating internal-combustion engines. Some of the required knowledge has been introduced and explained in the training manual, Fireman, NAVEDTRA 10520-H. This chapter provides additional information to help you to understand the differences between the various types of engines and the principles by which an internal-combustion engine operates.

CYCLES OF OPERATION

The operation of an internal-combustion engine involves the admission of fuel and air into a combustion space and the compression and ignition of the charge. The combustion process releases gases and increases the temperature within the space. As temperature increases, pressure increases, and the expansion of gases forces the piston to move. The movement is transmitted through specially designed parts to a shaft. The resulting rotary motion of the shaft is used for work. Thus, the expansion of the gases within the cylinder is transformed into rotary mechanical energy. In order for the process to be continuous, the expanded gases must be removed from the combustion space, a new charge must be admitted, and combustion must be repeated. In the process of engine operation, beginning with the admission of air and fuel and following through to the removal of the expanded gases, a series of events or phases takes place. The term cycle identifies the sequence of events that takes place in the cylinder of an engine for each power impulse transmitted to the crankshaft. These events always occur in the same order each time the cycle is repeated. The number of events occurring in a cycle of operation depends upon whether the engine is diesel or gasoline. Table 2-1 shows the events and their sequence in one cycle of operation of each of these types of engines.

The principal difference, as shown in the table, in the cycles of operation for diesel and gasoline engines involves the admission of fuel and air to the cylinder. While this takes place as one event in a gasoline engine, it involves two events in a diesel engine. Consequently, there are six main events that take place in the cycle of operation of the diesel engine and five main events that take place in the cycle of the gasoline engine. The number of events that take place is NOT identical to the number of piston strokes that occur during a cycle of operation. Even though the events of a cycle are closely related to piston position and movement, ALL of the events will take place during a cycle regardless of the number of piston strokes involved. We will discuss the

Table 2-1.Sequence of Events in a Cycle of Operation in a Diesel and Gasoline Engine

relationship of events and piston strokes later in this chapter.

A cycle of operation in either a diesel or gasoline engine involves two basic factorsheat and mechanics. The means by which heat energy is transformed into mechanical energy involves many terms such as matter, molecules, energy, heat, temperature, the mechanical equivalent of heat, force, pressure, volume, work, and power. (If you need to review these terms, see the appropriate sections in Fireman, NAVEDTRA 10520-H.)

The method by which an engine operates is referred to as the MECHANICAL, or operating, CYCLE of an engine. The heat process that produces the forces that move engine parts is referred to as the COMBUSTION CYCLE. Both mechanical and combustion cycles are included in a cycle of operation of an engine.