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 TM 9-8000
Section VII. ENGINE EFFICIENCY
within the pulley system.
2-37. Frictional Losses (Fig. 2-66). Efficiency
The engine also is subject to many power losses.
is the relationship between results obtained and
The two categories of losses are thermal and
the effort required to obtain those results. Effi-
mechanical. The overall efficiency of an engine
ciency Is expressed as follows:
will be calculated in the next three paragraphs.
output
Efficiency =
2-38. Thermal Efficiency. Thermal efficiency Is
Input
the relationship between actual heat energy
System efficiency can be calculated using this
stored within the fuel and the power produced In
formula. For example, if a 90-lb. box was lifted with
the engine (indicated horsepower). The thermal
a rope and pulley, it would require a force of 100
efficiency figure Indicates how much of the
lb. Therefore:
potential energy contained in the fuel actually Is
used by the engine to produce power and how
output
much energy is lost through heat. There Is an
Efficiency =
x100 =
Input
extremely large amount of energy from the fuel
that is lost through heat In an internal combustion
engine. This unused heat that is produced while
90 lb.
the engine Is producing power Is of no value to the
= 90 percent.
100 lb.
engine and must be removed from It,
The above results simply mean that only 90
a. The heat Is dissipated in the following ways.
percent of the total effort used for lifting the box
actually went to that task. The remainder, or 10
(1) The cooling system removes heat from
the engine to control engine operating
temperature.
(2) A major portion of the heat produced by
the engine exits through the exhaust system.
(3) The engine radiates a portion of the heat
to the atmosphere.
4) A portion of this waste heat may be
channeled to the passenger compartment to heat
it.
(5) The lubricating oil in the engine re-
moves a portion of the waste heat.
b. In addition to energy lost through wasted
heat, there are the following inherent losses in the
piston engine.
(1) Much energy is consumed when the
piston must compress the mixture on the com-
pression stroke.
(2) Energy from the gasoline is consumed
to pull the Intake mixture Into the
Figure 2-66. Pulley System with a 90 Percent
cylinder.
Efficiency.
TA233350
percent of the effort, was lost to frictional forces
2-42
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