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Thermal Efficiency | Up
Construction Mechanic Basic Volume 01 - Construction methods and practices | Next
Engine Performance |
A portion of this waste heat may be channeled to
the passenger compartment to heat it.
The lubricating oil in the engine removes a
portion of the waste heat.
In addition to energy lost through waste heat, there
are the following inherent losses in the piston engine.
Much energy is consumed when the piston must
compress the mixture on the compression stroke.
Energy from the fuel is consumed to pull the
intake mixture into the cylinder.
Energy from the fuel is consumed to push the
exhaust gases out of the cylinder.
The combination of all these factors in a piston
engine that uses and wastes energy leaves the average
engine approximately 20 to 25 percent thermally
efficient.
LINEAR MEASUREMENTS
The size of an engine cylinder is indicated in terms
of bore and stroke (fig. 2-24). BORE is the inside
diameter of the cylinder. STROKE is the distance
between top dead center (TDC) and bottom dead center
(BDC). The bore is always mentioned first. For
example, a 3 1/2 by 4 cylinder means that the cylinder
bore, or diameter, is 3 1/2 inches and the length of the
stroke is 4 inches. These measurements are used to
figure displacement.
PISTON DISPLACEMENT is the volume of
space that the pistondisplaces, as it moves from one end
of the stroke to the other. Thus the piston displacement
in a 3 1/2-inch by 4-inch cylinder would be the area of a
3 1/2-inch circle multiplied by 4 (the length of the
stroke.) The area of a circle is R2, where R is the radius
(one half of the diameter) of the circle. With S being the
length of the stroke, the formula for volume (V) is the
following:
V = R2 x S
= 3.14
V =
(1 .75)2 x 4
V = 3.14 x 3.06 x 4
V = 38.43 cu in.
If the formula is applied to figure 2-22, the piston
displacement is computed as follows:
R = 1/2 the diameter = 1/2 x 3.5 = 1.75 in.
Figure 2-24.—Bore and stroke of an engine cylinder.
The total displacement of an engine is found by
multiplying the volume of one cylinder by the total
number of cylinders.
38.43 cu in. x 8 cylinders = 307.44 cu in.
The displacement of the engine is expressed as 307
cubic inches in the English system. To express the
displacement of the engine in the metric system, convert
cubic inches to cubic centimeters. This is done by
multiplying cubic inches by 16.39. It must be noted that
16.39 is constant.
307.44 cu in. x 16.39 = 5,038.9416 cc
To convert cubic centimeters into liters, divide the
cubic centimeters by 1,000. This is because 1 liter =
1,000 cc.
5,038.9416 = 5.0389416
1,000
The displacement of the engine is expressed as 5.0 liters
in the metric system.
2-18
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