knock?
03-10-2007 | 09:47 AM
#4
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Posts: 70,853
From: Hell was full so they sent me to NJ
"Knock" is usually used to describe detonation. Detonation arises when a second flame front is created after the spark-initiated flame front. When the two flame fronts collide, there is a huge pressure spike, followed by a deteriorating resonance. That's generally where the noise comes from.
"Ping" is usually used to describe pre-ignition. Pre-ignition occurs when the pressure of the rising piston auto-ignites the A/F mixture in advance of the spark ignition. Again, two flame fronts collide and go bang.
From the Gasoline FAQ:
Good reading:
http://www.faqs.org/faqs/autos/gasoline-faq/part1/
"Ping" is usually used to describe pre-ignition. Pre-ignition occurs when the pressure of the rising piston auto-ignites the A/F mixture in advance of the spark ignition. Again, two flame fronts collide and go bang.
From the Gasoline FAQ:
Simply put, the octane rating of the fuel reflects the ability of the
unburnt end gases to resist spontaneous autoignition under the engine test
conditions used. If autoignition occurs, it results in an extremely rapid
pressure rise, as both the desired spark-initiated flame front, and the
undesired autoignited end gas flames are expanding. The combined pressure peak arrives slightly ahead of the normal operating pressure peak, leading to a loss of power and eventual overheating. The end gas pressure waves are superimposed on the main pressure wave, leading to a sawtooth pattern of pressure oscillations that create the "knocking" sound.
unburnt end gases to resist spontaneous autoignition under the engine test
conditions used. If autoignition occurs, it results in an extremely rapid
pressure rise, as both the desired spark-initiated flame front, and the
undesired autoignited end gas flames are expanding. The combined pressure peak arrives slightly ahead of the normal operating pressure peak, leading to a loss of power and eventual overheating. The end gas pressure waves are superimposed on the main pressure wave, leading to a sawtooth pattern of pressure oscillations that create the "knocking" sound.
http://www.faqs.org/faqs/autos/gasoline-faq/part1/
Last edited by Injuneer; 03-10-2007 at 09:53 AM.
03-16-2007 | 08:48 PM
#9
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Joined: May 2006
Posts: 592
From: new orleans, la
Follow up question: sort of off-topic. Why do boosed applications often retard timing? Would retarding timing not add to DCR and increase the already abnormal cylinder pressures present in boosed engines? I know its to prevent knock, but how?
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