Automatic Decompression System - Robin America EH12-2, EH17-2, EH25-2 Service Manual

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
C A M
CAMSHAFT
LEVER
, " -
Fig. 7-2
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46
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