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TM 9-8000
(3) Energy from the gasoline Is consumed
to push the exhaust gases out of the cylinder.
Indicated horsepower(Btu's)
Fuel input(Btu's)
c.  The combination of all of the factors in a
piston engine that use and waste energy leaves
= 0.23x100 = 23percent.
the average engine approximately 20 to 25
percent thermally efficient. To calculate the ther-
mal efficiency, the potential energy In the gaso-
2-39.  Mechanical Efficiency. Mechanical effi-
line and the indicated horsepower must be
ciency within the engine is the relationship be-
expressed In a common unit of measure. Both of
tween the actual power produced in the engine
these factors may be converted to British thermal
(Indicated horsepower) and the actual power
units (Btu). The conversion factors are:
delivered at the crankshaft (brake or shaft horse-
power). The actual power at the crankshaft Is
always less than the power produced within the
One Btu = 778ft Ib of work.
engine. This is due to the following:
Energy value of 1 Ib of gas = 18,800Btu.
a. Frictional losses between the many moving
For example: A given engine delivers 90 Indicated
b. In four-stroke cycle engines, a great deal
horsepower for a period of 1 hour, and in that time
of horsepower Is used to drive the valve train.
consumes 50 lb. (7 1/2 gal.) of gasoline. Given
Mechanical efficiency Is calculated by dividing the
this data, the thermal efficiency of the engine can
brake or shaft horsepower by the Indicated
be calculated by doing the following:
horsepower. For example: When checked on a
dynamometer, the engine with the Indicated
(1)  Calculate how many Btu's of work are
horsepower of 90, referred to In paragraph 2-38,
performed by 1 hp In 1 hour by doing the following:
produces a brake horsepower of 85. To calculate
the mechanical efficiency, do the following:
33,000 ft lb. per minute x
60 minutes
brake horsepower
Mechanical efficiency = x indicated horsepower
1 hphr =
= 2445 Btu.
778 ft lb. per Btu
(2)  Calculate the Btu's of work output for 90
100 =
90 =94percent.
hp hr using the Information from (1) by doing the
2-40.  Overall Efficiency. In paragraphs 2-38
and 2-39, the thermal and mechanical losses of
2445 Btu's x 90 = 22,050 Btu's.
the engine were calculated. Use the following
procedure to calculate the overall efficiency of the
(3)  Calculate how many Btu's were inputted
engine. The overall efficiency Is the relationship
bydolng the following:
between the power Input and the true output of the
engine (brake or shaft horsepower). The input
18,800 Btu's x 50 lb. = 940,000 Btu's input
will be expressed, as in paragraph 2-38, in Btu's.
per hour.
The power output also must be converted to Btu's.
Begin by using the information calculated in
(4)  Calculate the thermal efficiency by
doing the following:
Total power lnput = 940,000 Btu's per
1 hp hr = 2,445 Btu's.


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