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Mega-Knock

thecardoc3thecardoc3 Posts: 5,258
edited October 2015 in Ford
Turbocharged, small displacement GDI engines have the potential to experience a detonation/pinging that the system cannot control. In common episodes of pinging, the GDI system can resort to using multiple injection events. The first injection event partially charges the cylinder causing an air/fuel ratio that is too lean to support combustion, and then a second event that fully charges the cylinder to the correct air/fuel ratio. The second injection event is immediately followed by the spark ignition and the classic "knocking" is then controlled. However there is a condition called super knock, or mega knock that is the result of auto ignition of very fine oil droplets whether from the PCV system or from oil that gets past the rings. Large oil droplets can handle the heat of compression and do not auto-ignite. But very tiny ones cannot and it is up to additives in the oil to try to prevent the condition. Here is just one white paper on the topic.
http://papers.sae.org/2010-01-1456/ This pdf will also add some insight. http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=4&ved=0CDoQFjADahUKEwjKv-W_zrrIAhVBGz4KHdAJB9Q&url=http://www.cmclinnovations.com/wp-content/uploads/2012/03/cmcl_user_story_07_knock.pdf&usg=AFQjCNHwqfN-lpHLi5k19xHKDUwpC-38oA&sig2=qj8RzY68-EYrTYM8PP7mpw

This by the way is just one more issue that helps explain why manufacturers have oil specifications that exceed the current API and ILSAC ratings. (SN GF5)
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