<VV> Detonation vs. Pre-ignition

[Sethracer at aol.com]

In a message dated 5/12/2007 2:57:17 PM Pacific Daylight Time,  
richard at widman.biz writes:

Pinging  is from hot deposits igniting the mixture too far before TDC.
Deposits  maintain heat and reduce the cylinder space, increasing compression
to  where you either need to get rid of them or move up to higher octane gas
to  retard the rate of ignition.  Pings cause damage to engine by  pushing
backwards on the crankshaft.




Richard I disagree with some of what you said. Pre-ignition is what you  
first described and that, indeed, is usually due to deposits in the engine,  on a 
otherwise regularly operating car. On carbureted cars it can also  cause 
cylinder-based engine run-on. Even if ignition power is removed, the  deposits can 
keep some cylinders firing a bit, causing the engine to keep  turning over 
(slowly) and suck more fuel/air in, burn it and keep the  hot spot hot. (Fuel 
Injection cuts off the fuel, of course, run-on it  theoretically impossible.) 
Detonation is not pre-ignition, and can happen  in a motor with no deposits at 
all. As you describe, it is due to the rate  of burning changing from slow 
(burning) to fast (exploding) - which is the sound  you hear - the "Ping" of 
detonation. It can happed before or after TDC. And  it can hurt power for the reason 
you describe if exploding on the up  stroke, but the effect of detonation is 
to expend all the energy of the fuel  rapidly, like a hammer on the piston 
top, instead of pushing down on the piston  all the way to bottom dead center. So 
it hurts power even on the down  stroke. And has some bad effects on the 
piston top and rings as well.  Pre-ignition was often a by-product of using low 
quality (not low octane) fuel  or excess oil being ingested and burned, leaving 
bad deposits in the motor.  Detonation is a by product of low octane fuel use, 
or contamination by a  low-octane addition, such as engine oil or 
Transmission fluid. If either is  leaking past into the air inlet system and is ingested, 
it will lower the  effective octane and cause pinging (usually accompanied by 
smoke!) In addition,  when the Corvair engine gets extremely hot, like after 
losing a fan belt and  still being driven, the heat of the cylinder head and 
the cylinder with parts  getting very hot, it will overcome any possible octane 
capability of the  fuel, and burn (or detonate) it upon ingesting it into the 
motor. It is quite  possible to get both detonation and pre-ignition at the 
same time.  -Seth  Emerson



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