MoTeC Knock OKM - Manual (Page 31)

MoTeC Appendices 31
Appendix 5 Detonation Explained
Detonation (also called "spark knock") is an erratic form of combustion that
can cause catastrophic engine failure. Detonation occurs when excessive
heat and pressure in the combustion chamber causes the air/fuel mixture to
self ignite. This produces a sudden rise in cylinder pressure accompanied by
a sharp metallic pinging or knocking noise. The hammer-like shock waves
created by detonation subject the head gasket, piston, rings, spark plug and
rod bearings to severe overloading. Mild or occasional detonation can occur in
almost any engine and normally causes no harm. But prolonged or heavy
detonation can be very damaging.
Detonation is the result of an amplification of pressure waves, such as sound
waves, occurring during the combustion process when the piston is near top
dead centre (TDC). The actual "knocking" or "ringing" sound of detonation is
due to pressure waves pounding against the insides of the combustion
chamber and the piston top. It is not due to 'colliding flame fronts' or 'flame
fronts hitting the piston or combustion chamber walls.'
Normal Combustion
This is the burning of a fuel and air mixture charge in the combustion
chamber. It should burn in a steady, even fashion across the chamber,
originating at the spark plug and progressing across the chamber in a three
dimensional fashion. Similar to the ripples spreading out when a pebble is
thrown into a pond with a glass smooth surface, the flame front should
progress in an orderly fashion. The burn moves all the way across the
chamber and quenches (i.e.: cools) against the walls and the piston crown.
The burn should be complete with no, or very little, remaining unburnt fuel-air
mixture. Note that the mixture does not "explode" but burns in an orderly
fashion.
During combustion, the location of peak pressure (LPP) can be measured
with an in-cylinder pressure transducer. When the spark is fired at optimum
timing the burn is initiated at the spark plug and will progress evenly through
the chamber to reach peak pressure shortly after top dead centre depending
on the chamber design and the burn rate. Ideally, the LPP should occur
between 12 and 15 degrees after top dead centre.