CHAPTER 3
PRINCIPLES OF INTERNAL COMBUSTION ENGINES
cylinder inside the engine and is directly responsible
INTRODUCTION
for forcing the piston to move down. In external
As an Aviation Support Equipment Technician,
combustion engines, such as steam engines,
you will be concerned with repairing and replacing
combustion takes place outside the engine. Figure 3-1
worn or broken parts, making various adjustments to
shows, in simplified form, external and internal
vehicles and equipment, and seeing that vehicles and
combustion engines.
equipment are properly serviced and regularly
The external combustion engine requires a boiler to
inspected. To perform these duties intelligently, it is
which heat is applied. This combustion causes water to
important that you fully understand the principles of
boil to produce steam. The steam passes into the engine
internal combustion engine operation and the function
cylinder under pressure and forces the piston to move
of the various components that make up the engine.
downward.
This knowledge will make your job of diagnosing
trouble and making corrections much easier.
The transformation of heat energy to mechanical
energy by the engine is based on a fundamental law of
This chapter discusses basic principles of engine
physics that states "gas will expand when heated." The
operation and briefly explains some of the related
law also states that "when a gas is compressed, the
terms. You will hear many of these terms used around
temperature of the gas will increase." If the gas is
the shop, and you will see them in maintenance and
confined with no outlet for expansion, the pressure of
repair manuals.
the gas will be increased when heat is applied. In the
internal combustion engine, the burning of a fuel within
COMBUSTION ENGINES
a closed cylinder results in an expansion of gases. This
LEARNING OBJECTIVE: Define the term
expansion creates pressure on top of a piston, causing it
"combustion." Define the main principles
to move downward. In an internal combustion engine,
governing the operation of gasoline and diesel
the piston moves up and down within a cylinder.
engines. Identify the four basic engine strokes.
This up-and-down motion is known as reciprocat-
Combustion is the act or process of burning. An
ing motion. This reciprocating motion (straight line
internal or external combustion engine is defined
motion) must be changed to rotary motion (turning
simply as a machine that converts this heat energy to
motion) in order to turn the wheels of a vehicle. A
mechanical energy. To fulfill this purpose, the engine
crankshaft and a connecting rod change this reciprocat-
may take one of several forms. In the internal
ing motion to rotary motion.
combustion engine, combustion takes place within a
ASf03001
Figure 3-1.--Simple external and internal combustion engines.
3-1
