Engine Knock in an SI Engine with Hydrogen Supplementation under Stoichiometric and Lean Conditions 2014-01-1220
In this work, the effects of engine operational parameters, λ, spark timing, and compression ratio, on knock tendency and intensity as well as H2 supplementation are studied. We postulated, verified and eventually used the duration from ignition to 70% mass fraction burnt (MFB0-70%) as an explanatory variable to describe the knock tendency and intensity. In this manner, the physical factors and fuel factors that are introduced by the differences in test conditions can be differentiated.
Practically, in terms of percentage of knocking cycles or the spark timing at audible knock, knock tendency decreases as λ increases and increases with H2 supplementation. However, when MFB duration is taken into account, then for the same MFB duration, knock tendency increases as λ increases and decreases with H2 supplementation.
Knock intensity can be represented by both peak-to-peak pressure variations and integrated power spectral density of the oscillating pressure, and these are strongly correlated. By using peak-to-peak oscillating pressure as the knock index, it was found that knock intensity increases as MFB0-70% duration decreases. Additionally, knock intensity only depends on the MFB0-70% duration relative to the MFB0-70% duration at threshold knock. The smaller is the relative MFB0-70% duration, the greater is the knock intensity, and vice versa.