CHAPTER 2 PRINCIPLES OF AN INTERNAL COMBUSTION ENGINE

LEARNING OBJECTIVE: Explain the principles of operation, the different classifications, and the measurements and performance standards of an internal combustion engine.

As a Construction Mechanic, you are concerned with repairing and replacing worn or broken parts, making various adjustments to vehicles and equipment, and ensuring that they are serviced properly and inspected regularly. To perform these duties intelligently, you must fully understand the operation and function of the various components of an internal combustion engine. This makes your job of diagnosing and correcting troubles much easier. This, in turn, saves time, effort, and money.

This topic discusses the theory and operation of an internal combustion engine. You also need to become familiar with the terms being used.

INTERNAL COMBUSTION ENGINE

LEARNING OBJECTIVE: Identify the series of events, as they occur, in both a gasoline engine and a diesel engine. Describe the differences between a four-stroke cycle engine and a two-stroke cycle engine. Combustion is the act or process of burning. An "external" or "internal" combustion engine is defined simply as a machine that converts heat energy into mechanical energy. Figure 2-1 shows, in simplified form, an external and an internal combustion engine.

In the internal combustion engine, combustion takes place inside the cylinder and is directly responsible for forcing the piston to move down. With an external combustion engine, such as a steam engine, combustion takes place outside the engine. The external combustion engine requires a boiler to which heat is applied. This combustion causes water to boil to produce steam. The steam passes into the cylinder under pressure and forces the piston to move downward.

The transformation of HEAT ENERGY to MECHANICAL ENERGY by the engine is based on the fundamental law of physics which states that gas expands when heated. The law also states that when gas is compressed, the temperature of the gas increases. if the gas is confined with no outlet for expansion, then the pressure of the gas increases when heat is applied. In the internal combustion engine, the burning of fuel within an enclosed cylinder results in an expansion of gases. This expansion creates pressure on top of the piston, causing it to move downward. In an internal combustion engine, the piston moves up and down

Figure 2-1. - Simple external and internal combustion engines. 2-1