MoTeC Knock OKM - Manual (Page 34)

34 Appendices
microscope the small holes are not unlike those found in Swiss
cheese. The detonation actually mechanically erodes material out of
the piston. Typically the sandblasted look can be expected in the part
of the chamber most distant from the spark plug.
3. Overheating (scuffed piston skirts due to excess heat input or high
coolant temperatures).
Because the pressure spike is very severe and of very short duration,
it can actually shock away the boundary layer of gas that surrounds
the piston. Normal combustion temperatures exceed 900 deg Celsius.
An aluminium piston, subjected directly to that temperature, would
melt.
Under normal combustion it does not melt because of its thermal
inertia and because of a boundary layer of air fuel mixture a few
molecules thick next to the piston top. This thin layer isolates the
piston from the flame and causes the flame to be quenched as it
approaches this relatively cold material. This protects the piston and
chamber from absorbing the heat of the combustion.
However, under extreme conditions the shock wave from the
detonation spike can cause the boundary layer to breakdown. Since
pressure waves created during detonation can sweep away these
unburned boundary layers of air-fuel mixture they leave parts of the
piston top and combustion chamber exposed to the flame front. This,
in turn, causes an immediate rise in the temperature of these parts,
often leading to direct failure or at least to engine overheating.
Detonation Causes
The potential for detonation is influenced by chamber design elements
including: shape, size, geometry, plug location, compression ratio, engine
timing, mixture temperature, cylinder pressure and fuel octane rating. Too
much spark advance ignites the mixture too soon, increasing the pressure
resulting in spontaneously combustion.
Preventing Detonation
With the engine configuration set, detonation can be reduced by
y reducing ignition timing
y reducing air/fuel intake temperature (i.e. making the mixture richer
or using a larger intercooler)
y Reducing Coolant Temperature
y Using a fuel with a higher octane rating