Quote:
Originally Posted by theunchosen
I would argue with that. Its not really uncontrolled any more than normal. It is suspected that it happens if/when the flame front strikes a pocket of stoich mix while the surrounding area is lean. the stoich burns faster and causes an uneven pressure spike on the bottom of the cylinder(against the piston) which ignites that fuel and creates very small pockets that detonate under the intense pressure and heat.
All said and done the combustion event is no less controlled(as there are no means for controlling the flame front).
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theunchosen,I'm relying on memory going back to the 1970s,with my I.C.Engine Lab class.While "ping" was acceptable under load,"knock" was considered dangerous to the engine and is the reason for anti-knock additives in gasoline,as tetra-ethyl lead,Methyl Tertiary Butyl Ether,and Ethanol.Engine manufacturers will recommend an octane rating for a particular engine to protect it from the knock caused by detonation which is created by a non-uniform flame front propagating from the spark source.The octane helps guarantee "controlled" combustion at maximum spark advance,allowing the engine to achieve peak efficiency for its compression ratio.