Numerical Study on Knock for an SI Engine by Thermally Coupling Combustion Chamber and Cooling Circuit Simulations

The present research focuses on the understanding and improved prediction of knock at full load in a four-cylinder passenger car spark-ignition (SI) engine using computational fluid dynamics (CFD) methodology. The emphasis is on the possibility of controlling the knock limit via optimised engine cooling mechanisms. To date, CFD simulations of the combustion chamber and cooling circuit are performed separately, while chamber wall temperatures are derived from either experiments or experience. This, however, entails the risk of employing inadequate boundary and hence in-cylinder conditions for a combustion and knock simulation.

CFD simulations are performed for all four combustion chambers and metal components, including the cooling circuit. Both types of simulations are thermally coupled via the conditions on the chamber walls. Several engine cycles are simulated with the knock model switched off to converge in terms of wall temperatures and in-cylinder conditions, therefore allowing for more appropriate conditions in the combustion chambers. Thereafter one engine cycle is calculated including the knock model.

A sensitivity study for wall temperatures on knock was performed. The CFD results were compared against local wall temperature measurements on the cylinder head and engine block. The predictions reveal a highly non-uniform temperature distribution on the chamber walls. It is also demonstrated that knock is influenced primarily by the wall temperatures via the resulting thermodynamic state of the in-cylinder mixture due to wall heat transfer rather than via local wall temperature effects.