llibgrebnek
Ready to race!
So, on the topic of LSPI (I am directly involved in combustion research on the topic)...
Two normal events have to come together to create an LSPI event - preignition and knock. Knock can ONLY occur after the spark and preignition can occur anytime.
Preignition is a slower heat release than standard combustion that can be promoted by many things, migrated oil droplets in the combustion chamber are but one possible source.
When preignition begins, the heat release creates a pressure rise in the cylinder which can influence an otherwise benign engine speed/load point into knocking. Even if the engine does not experience knock, it will experience cylinder pressures which exceed design limits.
Now, if knocking does occur, due to the onset of a PI event, EVEN with high octane fuel, that is where stuff breaks in one or two combustion cycles. That is sometimes referred to as megaknock, when the statistical worst of PI and knock come together. That is why an otherwise sound engine suddenly goes boom one day. Nothing changed but statistics caught up with it...strong PI met up with moderate knock.
PI (manifest in LSPI) can result from oil droplets which make it past the rings into combustion chamber, partially or completely unvolatilized fuel components which act like diesel combustion and spontaneously combust, combustion chamber deposits flying across the chamber acting as glow plugs, fuel and oil additives such as calcium, higher carbon aromatics...and others.
It is a topic of great research and growing understanding. Theoretically, high enough octane can minimize megaknock events, by removing the knock portion of the equation, but high octane can come from sources which promote LSPI such as aromatics, and oxygenates like ethanol.
Ethanol has a higher laminar flame speed than gasoline so is prone to PI (flex fuel cars can experience this). But combine it with other factors, and the tendency for LSPI can increase. An example - ethanol is higher octane than most gasoline components, but also has higher latent heat, so it cools the combustion chamber more effectively. Good thing, no? Yes and no. The “no” part is that the cooled chamber then inhibits higher boiling point fuel components from volatilizing, thus INCREASING LSPI chances. But if enough ethanol is added, (like >E50) then sufficient energy is removed from the chamber to hinder larger heat release from PI AND the octane is much higher as well.
Interesting topic, and in a nutshell, a safety factor is a great idea. Best safety factor is to run water injection (not water meth) as water has very high phase change heat and also transforms the particulate matter nature and provides a greater number of crankshaft degrees over which heat release is spread. If leaner A/F and optimal spark timing are employed as part of the water injection, more power, efficiency AND safety may be realized.
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