The Effect of Oil Intrusion on Super Knock in Gasoline Engine

Super knock which occurs in highly boosted spark ignition engines in low speed pre-ignition regime can lead to severe engine damage. However, super knock occurs occasionally, it is difficult to clearly identify the causes. The widely accepted assumption for the cause of this phenomenon is oil intrusion. Most of oils have been proved to have higher cetane number than n-heptane dose, indicating that the intruded oil is very liable to auto-ignition in a boosted engine. Although there have been reported the type of base oil and additive has significant effect on pre-ignition frequency, the oil induced super knock is still so far not supported by any direct evidence.

This paper presents the effect of direct oil intrusion into cylinder on super knock. The experiment was carried out in a single cylinder engine. The diluted oil by gasoline with different ratio was directly injected into cylinder using a modified single-hole injector with 4MPa injection pressure. The results showed that oil intrusion before TDC could indeed induce pre-ignition then cause knock. The effect of oil injection timing on knock intensity varies with oil injection quantity and dilution ratio. For the same dilution ratio, more oil intrusion leads to more severe knock. The slightly diluted oil (75% oil + 25% gasoline, for example) causes the heaviest knock.