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 TM 9-8000
however, this Is not the case. Even though the
for the pressure to build in the combustion
compressed mixture burns very quickly, it does
chamber. There are timing advance mechanisms
take a certain amount of time for the pressure to
built into most Ignition systems. They will be
rise in the combustion chamber. Because of this
discussed in detail in paragraph 15-8.
time lag, it is desirable to have the spark occur
before the piston reaches top dead center. This
2-26.
Valve Timing.
way there is time for the pressure In the combus-
tion chamber to rise to its maximum as the piston
a. General. Valve timing is a system devel-
reaches top dead center. This, In turn, allows the
oped for measuring In relation to the crankshaft
piston to get the maximum push from the combus-
position (in degrees), the points when the valves
tion. If the spark occurs too late into the power
open, how long they stay open, and when they
stroke, very little of the push from the combustion
close. Valve timing is probably the single most
is utilized. This Is because the piston already will
important factor in tailoring an engine for specific
be moving away by the time the gases start ex-
needs. By altering valve timing, an engine can be
panding from the combustion. This will reduce
made to produce its maximum power in a variety of,
greatly the amount of pressure that Is exerted on
speed ranges. The following factors together
the piston and much power would be lost. The
make up a valve operating sequence.
limiting factor to timing advance is that It can
cause detonation. Detonation is covered in detail
b. Opening and Closing Point. The opening
In paragraph 440.
and closing points are the positions of the
crankshaft (in degrees) when the valve Just
d. Ignition Timing and Engine Speed. It is
begins opening and just finishes closing. Typical
desirable to have Ignition progressively advance
opening and closing points are illustrated in figure
as engine speed increases. This Is because there
2-53. Note the Intake valve opening 28 degrees
Is less time for the pressure to build In the
before top dead center and closing at 71 degrees
combustion chamber as the speed increases. By
after bottom dead center.
making the spark happen earlier (as much as 30
degrees btdc) at high speeds, there Is more time
Figure 2-53. Opening and Closing Points of the Valve.
2-34 Change 1
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