Page 14 Technical Information 08.2005
TI_08_2005_11_01_02.fm
5.4.2 Top dead centre
The top dead centre (TDC) is defined as the point at
which the piston ends its upward stroke and starts
its downward stroke. In a 4-MIX engine, the power
stroke is executed once in every two revolutions of
the crankshaft. The piston consequently passes
through the top dead centre twice in order to
execute one power stroke. Refer to the STIHL
Service Training System "Carburetor", under the
heading four-stroke principle.
Top dead centre, valve overlap
Cam (1) points upwards.
The rocker arms (2) and valve stems (3) move up
and down (see arrows above) when the crankshaft
is turned back and forth a few degrees.
This represents the change of cycle between intake
and delivery.
Top dead centre, ignition
Cam (1) points downward.
The rocker arms (2) and valve stems (3) do not
move up and down when the crankshaft is turned
back and forth a few degrees.
This represents the change of cycle between
compression and power stroke.
The valve clearance must be set at the top dead
centre for ignition. The inlet and exhaust valves
are only closed simultaneously in this position.
254TI010 KN
2
1
3
254TI009 KN
2
1
3
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