TM 5-4240-501-14&P
THEORIES OF OPERATION
Compression
Section 14
THEORIES OF OPERATION
COMPRESSION
The general subject of compression is a familiar one to
most mechanics. It has been discussed in detail by
valve
manufacturers,
ring
manufacturers,
piston
manufacturers, and by makers of
valve
grinding
equipment. The home mechanic, or handy-man, thinks
nothing of getting out his grinding compound, lapping in
the valves and putting a new set of rings on the piston -
all without knowledge of proper fit or tolerance. Whether
he does the job right or not, he thinks it is easy. And, it is
easy. There is nothing difficult or mysterious about
compression, and the nice part is that a good job that will
create lasting customer satisfaction is about as easy to
do as a poor job.
We must keep in mind, however, that the Briggs &
Stratton engine is an air-cooled, single cylinder engine.
The rules that hold true on liquid cooled, multi-cylinder
engines do not always apply to Briggs & Stratton
engines. For example:
The operating temperature of a liquid-cooled engine is
quite constant. The operating temperature of an air-
cooled engine, however, may vary greatly with changes
in air temperature, the load, and the speed. This
necessitates differences in tolerances and clearances of
parts like pistons, which must be fitted to Briggs &
Stratton’s established clearances. These can differ from
those used in most automotive engines.
The advantages of a n air-cooled engine are many.
There is no need for a complicated cooling system. The
engine is lighter in weight and occupies less space than
its liquid-cooled counterpart, and is comparatively easy
to repair.
Before we get into the mechanics of the subject, let us
clarify some of the terms in common use.
On
single
cylinder
engines
we
think
of
good
compression, not in terms of pounds of pressure per
square inch, but in terms of horsepower output. If the
engine produces the power for which it was designed, we
believe the compression must be good. It is extremely
difficult to make an accurate compression test on a
small, one cylinder engine without expensive machinery.
The reasons for this are the lack of a starter to crank the
engine at a constant speed and the small displacement
of the cylinder. Therefore, we do not publish any
compression pressure figures. As a simple compression
test, give the flywheel a quick spin. If the flywheel
rebounds on the compression stroke, the compression is
at least good enough to start the engine.
We talk about " compression" stroke and "power stroke".
What are they? The Briggs & Stratton engine is a four
stroke cycle engine, or as it is commonly called, a four
cycle engine. It operates on the same principle as an
automobile engine. The crankshaft makes two complete
revolutions to each power stroke of the piston.
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