CHAPTER 5
ENGINE PERFORMANCE AND EFFICIENCY
Your prime concern as an Engineman is to
keep the machinery for which you are responsi-
ble operating in the most efficient manner possi-
ble. From your past experience and training, you
know that engine efficiency and performance
depend upon much more than just operating the
throttle and changing oil at prescribed intervals.
The preceding chapters have covered many of the
casualties which may occur to reduce the power
output of an engine. You have learned how to pre-
vent the occurrence of many of these casualties.
As you gain experience and understanding, you
will probably have to train other people. The peo-
ple you will train will frequently come up with
many questions about why an engine does or does
not perform efficiently. Will you be able to answer
their questions?
To understand the various factors that
influence engine performance and efficiency, a
thorough knowledge of the internal combustion
process is necessary. Once the combustion
process is understood, it will be much easier for
you to appreciate the part played by such factors
as engine design, engine operating conditions, fuel
characteristics, fuel injection, ignition, pressures
and temperatures, and compression ratios. This
chapter provides some of the information
necessary for a better understanding of the many
factors that affect engine performance and effi-
ciency. As an Engineman, you will be able to gain
complete understanding of such factors only
through continued study and practical experience.
You should know how the power which an
engine can develop is limited by such factors as
the mean effective pressure, the length of piston
stroke, the cylinder bore, and the engine speed.
You must also know how these factors are used
in determining the power developed by an engine.
You must learn how heat losses, efficiency of
combustion,
volumetric efficiency, and the
proper mixing of fuel and air limit the power
which a given engine cylinder can develop. You
must become familiar with the factors which cause
overloading of an engine and unbalance between
engine cylinders. You should know the symptoms,
causes, and effects of cylinder load unbalance and
the steps that are necessary to maintain an equal
load on each cylinder.
You must know what is meant by engine
efficiency and know how the various types of
efficiencies and losses are used in analyzing the
internal combustion process. You must also be
familiar with those factors which may cause the
efficiencies to increase or decrease, and with the
ways these variations affect engine performance.
Parts of this chapter may serve as a brief
review, but most of the information provided
deals with those factors that influence engine per-
formance and efficiency.
ENGINE PERFORMANCE
In addition to mechanical difficulties, any
engine performance may be affected by other
causes, such as engine design and operator’s per-
formance. A comparison of the principal condi-
tions which influence the performance of
internal combustion engines is given in table 5-1.
Note that the performance conditions for the two
types of engines (diesel and gasoline) are
somewhat similar, except for some differences due
to factors dealing with fuel and ignition.
POWER LIMITATIONS
The design of an engine limits the amount of
power that an engine can develop. Other limiting
5-1
