A RANS CFD 3D Methodology for the Evaluation of the Effects of Cycle By Cycle Variation on Knock Tendency of a High Performance Spark Ignition Engine 2014-01-1223

Knocking combustions heavily limits the efficiency of Spark Ignition engines. The compression ratio is limited in the design stage of the engine development, letting to Spark Advance control the task of reducing the odds of abnormal combustions.

A detailed analysis of knocking events can help improving engine performance and diagnosis strategies. An effective way is to use advanced 3D CFD (Computational Fluid Dynamics) simulation for the analysis and prediction of combustion performance. Standard 3D CFD approach is based on RANS (Reynolds Averaged Navier Stokes) equations and allows the analysis of the mean engine cycle. However knocking phenomenon is not deterministic and it is heavily affected by the cycle to cycle variation of engine combustions. A methodology for the evaluation of the effects of CCV (Cycle by Cycle Variability) on knocking combustions is here presented, based on both the use of Computation Fluid Dynamics (CFD) tools and experimental information. 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. The pressure signal holds information about waves propagation and heat losses: it is crucial to relate local pressure oscillations to knock severity. For this purpose, a CFD model has been implemented, able to predict the combustion evolution with respect to Spark Advance, from non-knocking up to heavy knocking conditions. The CFD model validation phase is essential for a correct representation of both regular and knocking combustions: the operation has been carried out by means of an accurate statistical analysis of experimental in-cylinder pressure data. The methodology is applied to a high performance engine, equipped with both mono-spark and twin-spark configurations, and proved to be an useful tool for the evaluation of knock tendency of the two different settings in Maximun Brake Torque condition.