Chapter 3 Engine Classifi cation, Parts Identifi cation 55
Valve-to-Piston Clearance
Some engines have high compression, which
squeezes the air-fuel charge into a smaller than
normal area. In order to have this small area
and still allow room for the valves to open, it
is sometimes necessary for the pistons to have
small indentations cut into their tops to allow the
valves to clear the piston at top dead center (TDC),
Figure 3-16. These indentations, or grooves, keep
the piston head from striking the valves at the
top of the exhaust stroke when both valves may
be partially open. If the piston and valve(s) make
contact, extensive engine damage would result.
Detonation and Preignition
When a highly compressed air-fuel charge
is fi red, the fl ame travels from the plug in an
outward direction. As the charge continues to
burn, chamber pressure greatly increases. If the
combustion chamber pressure increases faster
in another part of the cylinder before the fl ame
reaches it, this area will become hot enough to
fi re itself. This will produce two fl ame fronts,
which increases the fuel charge burn rate.
Instead of fi ring smoothly, there is a violent
explosion that literally slams the piston down.
This is called detonation, which is very damag-
ing to an engine.
Detonation can be caused by an excessively
high compression ratio or by a low grade of gas-
oline. An overheated valve or piece of carbon
can ignite the remaining air-fuel mixture and
cause the formation of the second fl ame front,
Figure 3-17. Preignition is caused when a glow-
ing piece of carbon, an overheated spark plug,
or a piece of metal extending into the combus-
tion chamber ignites the fuel charge before the
spark plug fi res. When preignition occurs, the
premature explosion attempts to drive the pis-
ton down against the direction of rotation. Deto-
nation and preignition can destroy an engine if
not corrected.
Lock nut
Adjusting screw
Rocker arm
Valve
clearance
Valve
Valve lifter
Figure 3-14.
Rocker arm with adjustment screw used to set proper
valve clearance or “lash.”
British-Leyland
Oil supply
Rocker shaft
Valve push rod
Valve stem
Seal
Figure 3-15.
This non-adjustable rocker arm depends upon precise
parts so that the hydraulic lifter plunger will be at midpoint
of travel upon assembly.
Dodge
Indentations for valve clearance
Figure 3-16.
Piston indentations used for valve clearance. The piston
has small indentations to allow the valves to clear. The
pistons in such engines are clearly marked to ensure that
the indentations are correctly positioned during engine
reassembly.
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