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TM 9-8000
c.  Duration. Duration Is the amount of crank-
begins at approximately 15 to 20 degrees before
bottom dead center and ends at approximately
shaft rotation (in degrees) that a given valve will
15 to 20 degrees after bottom dead center. These
remain open. In figure 2-54, a typical intake valve
two positions are illustrated in figure 2-56. These
cycle is illustrated. The duration that it remains
two periods of crankshaft rotation are utilized
open is a total of 279 degrees.
when establishing a valve timing sequence as
d. Valve Overlap. Valve overlap is a period in
the four-stroke cycle when the intake valve opens
(1)During the period that occurs at top
before the exhaust valve closes. Typical valve
dead center, valve overlap is introduced to in-
overlap is Illustrated In figure 2-55. The intake
crease volumetric efficiency. By opening the
valve Is opening 28 degrees before top dead
intake valve before the exhaust valve Is closed, the
center for the beginning of the intake stroke. The
intake mixture is pulled in by the momentum of the
exhaust valve Is remaining open for 45 degrees
exiting exhaust gas. The intake mixture coming In
after top dead center and into the intake stroke.
also helps to sweep or scavenge the cylinder of
This gives a valve overlap of 73 degrees.
exhaust gases. Because the overlap occurs
a. Valve Timing Considerations. Throughout
during one of the periods of little piston move-
ment, there Is no problem with exhaust being
the crankshaft revolution, the speed of the piston
pushed Into the Intake port or exhaust gas being
changes. From a stop at the bottom of the stroke,
pulled into the cylinder through the exhaust port by
the piston will reach its maximum speed halfway
the piston.
through the stroke and gradually slow to a stop as It
reaches the end of the stroke.
(2)  During the period that occurs at bottom
The piston will behave exactly the same on the
dead center, the pressure remaining In the
downstroke. There are two periods of crankshaft
cylinder at the end of the power stroke Is utilized by
rotation In which there Is almost no perceptible
opening the exhaust valve early. When the
movement of the piston. One of these periods
exhaust valve opens, the pressure In the cylinder
begins at approximately 15 to 20 degrees before
starts pushing the exhaust gas out of the cylinder.
top dead center and ends at approximately 15 to 20
degrees after top dead center. The other pedod
Figure 2-54. Valve Opening Duration


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