What causes spark knock and how do you get rid of it?
Drawing on left shows completion of normal combustion. Cutaway
on right shows a detonating cylinder, where the last portion of the
air/fuel mixture self-ignites and collides with the normal
combustion front.
Spark knock (detonation) is an erratic form of combustion that
occurs when multiple flame fronts occur simultaneously inside a
combustion chamber. Detonation occurs because fuel is subjected to
either too much pressure, too much heat or both. It usually happens
during acceleration when the engine is heavily loaded and cylinder
pressures are at their peak.
Instead of a single flame front growing outward smoothly like an
expanding balloon from the point of ignition, multiple flame fronts
are generated spontaneously throughout the combustion chamber as the
fuel automatically ignites from heat and pressure. The multiple
flame fronts collide, creating shock waves that produce a sharp
metallic pinging or knocking noise.
Mild detonation can occur in almost any engine and will not cause
damage. Prolonged heavy detonation can crack pistons and rings, blow
out head gaskets, damage spark plugs and valves, and flatten rod
bearings.
Any of the following can cause detonation:
1) Too Much Compression: An accumulation of carbon deposits in
the combustion chambers, on piston tops and valves can increase
compression to the point where it exceeds fuel octane rating. If a
top cleaner fuel additive fails to remove deposits, a new
alternative is to blast the deposits loose by blowing crushed walnut
shells through the spark plug hole. Otherwise, the head will have to
be removed so the deposits can be scraped off.
2) Overadvanced Ignition Timing: Too much spark advance causes
cylinder pressure to rise too rapidly. If resetting the timing to
stock specifications does not help, retarding timing a couple of
degrees may be necessary to eliminate knock.
3) Engine Overheating: A hot engine is more likely to suffer spark
knock than one which runs at normal temperature. Overheating can be
caused by low coolant, a defective fan clutch, too hot a thermostat,
a bad water pump, etc. A buildup of lime and rust deposits in the
head and block can also reduce heat transfer
4) Overheated Air: The thermostatically controlled air cleaner
provides the carburetor with hot air to aid fuel vaporization during
engine warm-up. If the air control door sticks shut so that the
carburetor continues to receive heated air after the engine is warm,
detonation may occur, especially during hot weather. Check the
operation of the air flow control door in the air cleaner to see
that it opens as the engine warms up. No movement may mean a loose
vacuum hose or a defective vacuum motor or thermostat.
5) Lean Fuel Mixture: Rich fuel mixtures resist detonation while
lean ones do not. Air leaks in vacuum lines, intake manifold
gaskets, carburetor gaskets or fuel injection intake plumbing
downstream of the throttle can all admit extra air into the engine
and lean out the fuel mixture. Lean mixtures can also be caused by
dirty fuel injectors, carburetor jets clogged with fuel deposits or
dirt, a restricted fuel filter, or a weak fuel pump.
6) The air/fuel ratio can also be affected by changes in altitude. A
carburetor calibrated for high altitude driving will run too lean if
driven at a lower elevation. Altitude changes are generally
compensated for on computer cars by the barometric pressure sensor.
A lean fuel condition can be diagnosed by watching for lean misfire
on an ignition scope, or by using a four-gas infrared analyzer and
watching exhaust oxygen levels. A reading over about 3% to 4% oxygen
would indicate a lean fuel condition.
Spark Plug Too Hot: The wrong heat range plug can cause detonation
as well as pre-ignition. Copper core plugs are less likely to cause
detonation than standard spark plugs.
Loss of Exhaust Gas Recirculation (EGR): EGR keeps combustion
temperatures down, reducing the tendency to detonate. If the EGR
valve is inoperative or someone has disconnected or plugged its
vacuum hose, higher combustion temperatures can cause pinging.
Low Octane Fuel: Burning cheap gas may be one way to save pennies,
but switching to a higher grade of fuel may be necessary to
eliminate a persistent knock problem.
Defective Knock Sensor: The knock sensor responds to frequency
vibrations produced by detonation (typically 6 - 8 kHz), and signals
the computer to momentarily retard ignition timing until detonation
stops. A knock sensor can usually be tested by rapping a wrench on
the manifold near the sensor (never hit the sensor itself). If there
is no timing retard, the sensor may be defective.
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