Knocking Cylinder Pressure Data Interpretation for Modern High-Performance Engines—A Computational Fluid Dynamics Informed Approach

Knock has been studied by internal combustion engine researchers for well over a century. It remains perhaps the main limit on spark-ignition engine efficiency today. In an engine development environment, knock is typically described through quantification of the high-frequency signal content of cylinder pressure measurements. A cylinder pressure transducer gives a point measurement in the combustion chamber volume. In non-knocking combustion cycles, there is little pressure variation across the chamber; hence, this point measurement adequately represents the average gas pressure acting on the piston. This is not the case for knock where autoignition leads to strong pressure gradients and standing wave behavior or even supersonic shock wave propagation. The resulting pressure signal is complex to interpret. Knocking phenomena can be simulated in Computational Fluid Dynamics (CFD), ideally using a combination of Large Eddy Simulations (LES), chemical kinetics, moving meshes, and small timesteps. Such approaches are computationally intensive, however, and may not be feasible to apply over the wide range of conditions for which an engine must be calibrated. A simpler model that could aid in cylinder pressure data interpretation would be invaluable as a support to calibration engineers.

To this end, a new methodology is proposed. This again uses CFD but concentrates on the pressure distributions in a combustion chamber following a localized exothermic event. The model is applied to modern engine geometry of high-performance Normally Aspirated (NA) and turbocharged Direct Injection (DI) gasoline engines. Output from the CFD model is compared to experimental data at high-load and beyond borderline knock conditions. It is shown that this approach can give new insight into experimental results interpretation and allow firmer conclusions to be drawn on the relationship between knocking cylinder pressure measurements and the phenomena that are driving them.