C H A P T E R 5
Principles of Engine
Operation, Two- and
Four-Stroke Engines
Learning Objectives Key Terms
bottom dead center
compression ratio
compression stroke
exhaust stroke
four-stroke engine
intake stroke
internal combustion
power stroke
scavenge loss
top dead center
two-stroke engine
valve overlap
After studying this chapter, you will be able to:
Explain simple engine operation.
Explain why gasoline is atomized in the
small engine.
Describe four-stroke engine operation and
explain the purpose of each stroke.
Explain the concept of valve timing.
Compare the lubrication system in a four-cycle
engine to the system in a two-stroke engine.
Describe two-stroke engine operation and
explain the principles of two-cycle operation.
List the advantages and disadvantages of
two-stroke and four-stroke engines.
Principles of Engine
A gasoline-fueled engine is a mechanism
designed to transform the chemical energy of
burning fuel into mechanical energy. In operation,
it controls and applies this energy to mow lawns,
cut trees, propel tractors, and perform many other
laborsaving jobs.
The small gasoline engine is called an internal
combustion engine because an air/fuel mixture is
ignited (fi red) and burned inside the engine. See
Figure 5-1.
For the engine to operate effi ciently, the gasoline
must be broken into small particles that will ignite
easily and burn quickly. In addition, the energy
produced by the burning gasoline must be con-
trolled in some way so it can perform useful work.
Gasoline Must Be Atomized
The more surface area of gasoline exposed to the
air, the faster a given amount will burn. Gasoline
placed in a container and ignited will produce a hot
fl ame, yet it will not burn fast enough to produce
the rapid release of heat necessary to run an engine.
Even though a considerable quantity of fuel may be
involved, a large fl ame will not necessarily result.
See Figure 5-2.
Under no circumstances should experiments illustrated
in this chapter be performed. Gasoline can be a very
dangerous fuel and must be handled with caution.
Illustrations and examples discussed here are meant
to demonstrate how gasoline is prepared and used in
an engine.
U d i h ld i ill d
Copyright Goodheart-Willcox Co., Inc.
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