Combined experimental and numerical analysis of knock in spark Ignition engines

none3Knocking combustions limit Spark Ignition engines efficiency, preventing the achievement of the optimal combustion phase: the knock phenomenon is intrinsically cause of inefficiencies, and in addition, it forces to keep the combustion with a safe – but often inefficient – retarded phase. A precise cycle-by-cycle knock monitoring strategy is required to improve the performance without increasing the risk of causing damages to the engine. The paper presents a novel approach to knock detection, with the objective of defining a damage-related and operating conditions-independent index. The methodology is based on the analysis of the Cumulated Heat Release (CHR), estimated by means of the in-cylinder pressure sensor signal. Knocking combustions cause a high heat flux through the combustion chamber walls, increasing the heat losses and lowering the IMEP: this effect can be observed by evaluating IMEP or CHR, that are based on the low-frequency content of the in-cylinder signal spectrum. CHR, however, is better related to the increase in heat losses during knocking combustions: a knock detection index can then be based on the statistical analysis of the cumulated heat release values for a given sample of engine cycles. CFD simulations have been used in order to simulate knock effect on the in-cylinder pressure trace. In fact, the in-cylinder pressure signal holds information about waves propagation and heat losses: it is crucial to relate local pressure oscillations to knock severity. To do so, a CFD model has been implemented, able to predict the combustion evolution with respect to Spark Advance, from non-knocking up to heavy knocking conditions. The knock model validation has been carried out by comparing simulated and measured Knock Onset and pressure oscillation amplitude. A deep insight on knock effects can be achieved by forcing different knock intensity levels, thus allowing a parametrical analysis of the phenomenon. The CFD analysis has been used to assess the CHR-based index sensitivity to the intensity of the phenomenon, optimizing the index definition, in terms of signal filtering, windowing, etc. The index has then been applied to different types of engine, running in different engine running conditions: the results show that the index does not depend on engine running conditions, and on the type of engine, coherently with the damage-based definition.ASME PAPER No. ICES2009-14102 ASME – ICEF 2009noneG.M. Bianchi; C. Forte; E. CortiG.M. Bianchi; C. Forte; E. Cort