TM 5-4240-501-14&P
Figure 3.
What is "piston displacement"? It is the space displaced
by the piston in its up and down movement or the volume
shown above the piston in Figure 3. The bigger the bore
and the longer the stroke, the greater the piston
displacement.
Displacement
is
computed
by
the
following formula:
Displacement = (Bore)2 X p X Stroke
4
Let us compute the displacement of a Model 6 engine
which has a 2" bore and a 2" stroke. Using the above
formula:
2X2
Displacement =
X 3.1416 X 2
4
Displacement = 6.2832 cubic inches
Our specification sheets show 6.28 cubic inches as the
displacement for the Model 6 engine.
The model numbers of the current engines indicate the
approximate piston displacement. Model 60000 has 6.65
cubic inches; Model 14 has 14.21 cubic inches, etc.
Piston displacement indicates the relative size of the
engine, and usually horsepower is in direct proportion to
size.
THEORIES OF OPERATION
Compression
Figure 4.
What do we mean when we say an engine has a 6 to 1
compression ratio? We mean that the space in the
cylinder when the piston is at the top of the stroke is only
one-sixth as great as when the piston is at the bottom of
the stroke.
Compression ratios do not tell us the horse-power of an
engine. They do have a meaning as regards the
efficiency of an engine.
Generally, the higher the compression ratio, the greater
the efficiency. However, as compression ratios are
increased, the loads and stresses upon engine parts
become more severe. Premium fuels may be required
with high compression ratios. Experience has proven
that compression ratios in the range of 5 -1 to 6 -1,
currently used in Briggs & Stratton engines, are the best
for the work and the conditions under which these
engines must operate. Therefore premium fuel is not
needed and "regular" is recommended.
It is generally conceded that the valves are the most
important factor in good compression. They operate
under more severe conditions than any other parts of the
engine. This is particularly true of the exhaust valve.
Figure 5.
The valves open and close in a little less than one
revolution. When the engine is operating at 3000 RPM,
each valve opens and closes in about 1/50 of a second.
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