Engine Knock: Characteristics and Mechanisms
Knock in SI engines limits both power density and fuel conversion efficiency. The main mechanisms with the potential to generate the cylinder pressure fluctuations generally observed during knocking combustion are discussed. It is reasoned that some mechanisms of flame acceleration can further amplify a shock wave generated by autoignition. The mechanisms causing knock damage, knock suppression, and the application of knock classification methods are discussed.
Five co-authored SAE papers form a part of this dissertation. Their major topics are summarized as follows. To test the autoignition theory of knock, end-gas temperature measurements close to a wall during knocking cycles were made using dual-broadband rotational coherent anti-Stokes Raman spectroscopy (DB-RCARS) in a single-cylinder research engine. The engine was modified to allow line-of-sight optical access to the end-gas region (SAE 971669). The wavelet transform was tested for filtering the cylinder pressure and was found to give good results. It is a very sensitive tool for the analysis of nonstationary signals with changing frequency content, as SI engine knock (SAE 971670). Eight different knock intensity classification methods were compared. There is no indication of any knock intensity measure that has an advantage in performance over the maximum amplitude of the band-pass filtered cylinder pressure method (SAE 972932). Based on measurements with both a single-cylinder research engine and two production engines, it is reasoned that the number of consecutive knocking cycles for accurate knock analysis be on the order of at least 100 - 200 (SAE 982475). A new knock form generating a gradual increase in cylinder pressure oscillation during light knock is reported and it is concluded that the amplification of the oscillations is generated by a crevice mechanism (SAE 982589).