PRINCIPLES OF AN INTERNAL
LEARNING OBJECTIVE: Explain the principles of operation, the different
classifications, and the measurements and performance standards of an internal
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.