A Simple CFD Model for Knocking Cylinder Pressure Data Interpretation: Part 1

Knock is one of the main limitations on Spark-Ignited (SI) Internal Combustion Engine (ICE) performance and efficiency and so has been the object of study for over one hundred years. Great strides have been made in terms of understanding in that time, but certain rather elementary practical problems remain. One of these is how to interpret if a running engine is knocking and how likely this is to result in damage. Knocking in a development environment is typically quantified based on numerical descriptions of the high frequency content of a cylinder pressure signal. Certain key frequencies are observed, which Draper [1] explained with fundamental acoustic theory back in 1935. Since then, a number of approaches of varying complexity have been employed to correlate what is happening within the chamber with what is measured by a pressure transducer. Whilst such phenomena can be well described by 3D Computational Fluid Dynamics (CFD) with moving meshes, small time-steps and chemical kinetics, such an approach is computationally intensive. Analytical calculations or Finite Element Methods (FEM) on the other hand, can estimate modal frequencies but not their likelihood of occurrence. In the present work, a simple stationary 3D CFD model, taking inspiration from an experiment by Draper [1] in 1934, is implemented in STAR CCM+ software. One or more autoignition events are simulated, and the corresponding frequency spectra and modal pressure distributions are described. It is shown that the model can reproduce the expected knocking frequencies from numerical analysis and experimental data. Sensitivity to autoignition and pressure transducer location is commented upon. Time Frequency Analysis (TFA) is applied to moving mesh data and demonstrates that little accuracy is lost in considering the stationary case. The current model is considered to be an appropriate means for analysis of knocking cycles with trace and moderate intensity, and can be used to bridge the gap between what is measured by a pressure transducer and what is occurring in the combustion chamber.