CFD Analysis and Knock Prediction into Crevices of Piston to Liner Fireland of an High Performance ICE

The paper aims at defining a methodology for the prediction and understanding of knock tendency in internal combustion engine piston crevices by means of CFD simulations. The motivation for the analysis comes from a real design requirement which appeared during the development of a new high performance SI unit: it is in fact widely known that, in high performance engines (especially the turbocharged ones), the high values of pressure and temperature inside the combustion chamber during the engine cycle may cause knocking phenomena. “Standard” knock can be easily recognized by direct observation of the in-cylinder measured pressure trace; it is then possible to undertake proper actions and implement design and control improvements to prevent it with relatively standard 3D-CFD analyses. Some unusual types of detonation may occur somewhere else in the combustion chamber: knocking inside piston/liner crevices belongs to the latter category and damages on the piston top land (very similar to pitting) are one of the evidence of knock onset in this region. The very localized regions of damage onset, the cycle to cycle variability and the very short duration of the phenomena do not allow to obtain fully reliable experimental data concerning the investigated problem. A new methodology is therefore implemented in CFD to drive the root causes identification and understanding the impact of crevice design. A preliminary CFD 3D in-cylinder analysis is performed, in order to understand the criticalities in the piston to liner fireland due to local pressure and temperature temporal evolution. Then a “model reduction” is proposed, which is necessary in order to study the problem with reasonable computational costs and times. A 2D simplified model is developed which is able to maintain the possibility to correctly represent the local thermo fluid dynamic effects, especially the auto-ignition conditions. Finally, new geometries are studied in order to prevent local knocking and retard auto-ignition such to improve the KLSA.