| When
an internal combustion engine is torn down for rebuilding and
badly burned and distorted pistons and valves are found, it
is very likely caused by extremely high combustion chamber temperatures
and pressures from detonation or pre-ignition. Generally speaking,
most machine shop men know that such damage is caused from abnormal
engine operation. Too often, the customer does not know what
causes the costly damage, yet the machine shop man may be unjustly
blamed for an engine "that didn't stand up".
Detonation and pre-ignition are forms
of abnormal combustion in the combustion chamber. During normal
operation of the engine, the burning of the fuel-air charge
produces a steady, smooth push on the pistons of each cylinder.
At the instant of ignition by the spark plug, the flame of
combustion moves rapidly outward from the plug very much like
the waves when a stone is dropped into a pool of water (see
fig. 1).
Abnormal operation may allow
combustion pressures to develop so fast that the heat and
pressure will "explode" the remaining unburned fuel.
This produces the knock, often called "ping", carbon
knock, etc. Actually this is detonation. The knock
results from the violent explosion when the normal flame front
runs into the secondary flame front. Detonation will cause
piston and ring damage, top ring groove wear, scoring, sticking
rings, loose head gaskets and possible complete engine failure.
(Photo of detonation
damaged piston). Fig. 2 |
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Detonation
can be caused by:
- Lean fuel mixture
- Fuel octane too low
- Improper ignition timing
- Lugging
- Carbon deposits
- Excessive milling of heads or block,
which will increase compression ratio.
Pre-ignition, as the term suggests,
is the ignition of the fuel-air mixture before the regular
ignition spark from the spark plug. If the regular spark occurs
shortly after the pre-ignition, the colliding of the two flame
fronts will cause a pinging noise. Preignition causes loss
of engine power and can cause severe damage to pistons, rings
and valves.
(Photo of pre-ignition
damaged piston). Fig 3
Detonation and pre-ignition are so closely related that it is en difficult to distinguish
one from the other by sound. Each can lead to the other and
either condition can cause extensive engine damage. Inspection
of damaged pistons and rings quite often indicate which caused
the damage.
Damaged pistons and rings usually mean
replacement. The same damage can occur again unless the cause
of the detonation or pre-ignition is corrected.
Pre-ignition can be caused by:
- Carbon deposits that remain incandescent
- Spark plugs too hot a heat range
- Spark plugs not firmly seated against
gasket
- Detonation or the condition leading
to it
- Sharp edges in combustion chamber
- Valves operating at higher than
normal temperature because of excessive guide clearance
or improper seal with valve seats.
- Overheating
- Ignition crossfiring. Induced voltage
in spark plug wires that run parallel to each other for
long distances
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