engine knock...
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From: Ben Lomond, Ca, USA
Re: engine knock... (d16lxciv)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by d16lxciv »</TD></TR><TR><TD CLASS="quote">comb. knock.</TD></TR></TABLE>
There are two forms of combustion knock. One is called pre-ignition and the other is post-igniton. They are both grouped under the term 'detonation'.
During pre-ignition the air-fuel mixture in the combustion chamber ignites before the spark plug fires. This is usually caused by an overly hot area in the combustion chamber such as an incandescant piece of carbon or sharp metal edges which are glowing red hot. Once the glowing piece of carbon or whatever lights off the air-fuel mixture a flame front starts expanding radially around it. Some time after this, the spark plug is fired as it normally would and starts a second flame front. The two flame fronts race toward each other and when they collide the pressure in the cylinder raises dramatically and causes the engine to resonate and cause a 'ping' noise. If your engine was damaged by pre-ignition you will see pock marks on the top of the pistons or maybe even a hole.
During post-ignition (this is what is usually refered to as combustion knock) the spark plug has already fired and a flame front has begun moving radially away from the plug. The unburnt gases in front of the flame front, called 'end gases', are compressed and because of this their temperature raises dramatically. If the end gases reach the temperature where the gasoline will ignite, the end gases will basically all burn at once and cause a huge pressure rise - usually sounding like a 'knock' sound. To cure this you may use higher 'octane' fuel which will effectively raise the temperature at which the end gases will ignite. You can also try retarding the ignition timing which will lower the pressure in the end gases and therefore lower the temperature. The mechanical design of the engine may cause post-ignition also. Having a short duration cam in a high comression ratio engine will cause it to have post-ignition detonation (BTW, there is no direct relationship between the 'static compression ratio' and detonation despite what 'they' have told you.). If your engine has been destroyed by post-ignition you will notice that the ring lands on the piston have been blown away and the rings are usually damaged or melted.
The terms pre-ignition, post-ignition, and combustion knock are usually just lumped into the term 'detonation'. While this is not the technically correct term it is what people understand so I usually just use the word detonation.
There are two forms of combustion knock. One is called pre-ignition and the other is post-igniton. They are both grouped under the term 'detonation'.
During pre-ignition the air-fuel mixture in the combustion chamber ignites before the spark plug fires. This is usually caused by an overly hot area in the combustion chamber such as an incandescant piece of carbon or sharp metal edges which are glowing red hot. Once the glowing piece of carbon or whatever lights off the air-fuel mixture a flame front starts expanding radially around it. Some time after this, the spark plug is fired as it normally would and starts a second flame front. The two flame fronts race toward each other and when they collide the pressure in the cylinder raises dramatically and causes the engine to resonate and cause a 'ping' noise. If your engine was damaged by pre-ignition you will see pock marks on the top of the pistons or maybe even a hole.
During post-ignition (this is what is usually refered to as combustion knock) the spark plug has already fired and a flame front has begun moving radially away from the plug. The unburnt gases in front of the flame front, called 'end gases', are compressed and because of this their temperature raises dramatically. If the end gases reach the temperature where the gasoline will ignite, the end gases will basically all burn at once and cause a huge pressure rise - usually sounding like a 'knock' sound. To cure this you may use higher 'octane' fuel which will effectively raise the temperature at which the end gases will ignite. You can also try retarding the ignition timing which will lower the pressure in the end gases and therefore lower the temperature. The mechanical design of the engine may cause post-ignition also. Having a short duration cam in a high comression ratio engine will cause it to have post-ignition detonation (BTW, there is no direct relationship between the 'static compression ratio' and detonation despite what 'they' have told you.). If your engine has been destroyed by post-ignition you will notice that the ring lands on the piston have been blown away and the rings are usually damaged or melted.
The terms pre-ignition, post-ignition, and combustion knock are usually just lumped into the term 'detonation'. While this is not the technically correct term it is what people understand so I usually just use the word detonation.
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