SAE International Relating Knocking Combustions Effects to Measurable Data 2015-24-2429

Knocking combustions heavily influence the efficiency of Spark Ignition engines, limiting the compression ratio and sometimes preventing the use of Maximum Brake Torque (MBT) Spark Advance (SA). A detailed analysis of knocking events can help in improving the engine performance and diagnostic strategies. An effective way is to use advanced 3D Computational Fluid Dynamics (CFD) simulation for the analysis and prediction of the combustion process. The standard 3D CFD approach based on RANS (Reynolds Averaged Navier Stokes) equations allows the analysis of the average engine cycle. However, the knocking phenomenon is heavily affected by the Cycle to Cycle Variation (CCV): the effects of CCV on knocking combustions are then taken into account, maintaining a RANS CFD approach, while representing a complex running condition, where knock intensity changes from cycle to cycle. The focus of the numerical methodology is the statistical evaluation of the local air-to-fuel and turbulence distribution at the spark plugs and their correlation with the variability of the initial stages of combustion. CFD simulations have been used to reproduce knock effect on the in-cylinder pressure trace. For this purpose, the CFD model has been validated, proving its ability to predict the combustion evolution with respect to SA variations, from non-knocking up to heavy knocking conditions. The CFD model allowed relating measurable data (i.e., the simulated cylinder pressure signal) to other factors, representative of the phenomena actually taking place during knocking combustions: for each cell used in the CFD simulation, information such as pressure, heat release, etc. are available and can be traced over the angular domain. Furthermore, the analysis refers to hundredths of engine cycles, leading to a comprehensive correlation between standard cylinder pressure-based knock indexes and other indexes (only available in a simulation environment), more representative of the actual knock intensity.