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7.
AUTOMATIC
DECOMPRESSION
SYSTEM
EH12-2, 17-2, 25-2 engines are employing the automatic decompression system
as
a standard feature.
This enables easy and light start of the engine.
The automatic decompression system releases the compression
of
the engine by lifting up the exhaust
valve at the cranking. Following are the explanation using
type
“D”
engine
as
a sample how the system
works. The components
of
the systems are different for the type
“D”
and type
“B”
engines, however, the
principle
of
the function
is
same.
At the end
of
the compression process, the release lever lifts
up
the tappet which in turns opens up the
exhaust valve slightly
to
release the compression. The release lever has a flyweight
on
its end and another
end
of
the lever
is
a
crescent
cam.
When the engine is cranked, the crescent cam projects the camshaft cam profile and lifts up the tappet
because the gravity force on the weight is larger than the centrifugal force
on
the weight.
i
TAPPET,
I
LEVER
CRESCENT
EXHAUST
Fig.
7-1
When the crank speed reaches up to
a
certain revolution, the crescent cam
is
retracted into the camshaft
cam profile because the centrifugal force applied onto the flyweight becomes larger than the gravity force
and the weight.and
is
shifted
to
the position shown in the illustration.
FLYWEIGHT
CRESCENT
CAM
EXHAUST
CAM
EXHAUST
-Y-T3
CAM
CAMSHAFT
LEVER
,”-
Fig.
7-2
-
46
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