In this paper, an experimental and numerical analysis of combustion process and knock occurrence in a small displacement spark-ignition engine is presented. A wide experimental campaign is preliminarily carried out in order to fully characterize the engine behavior in different operating conditions. In particular, the acquisition of a large number of consecutive pressure cycle is realized to analyze the Cyclic Variability (CV) effects in terms of Indicated Mean Effective Pressure (IMEP) Coefficient of Variation (CoV). The spark advance is also changed up to incipient knocking conditions, basing on a proper definition of a knock index. The latter is estimated through the decomposition and the FFT analysis of the instantaneous pressure cycles.
Contemporary, a quasi-dimensional combustion and knock model, included within a whole engine one-dimensional (1D) modeling framework, are developed. Combustion and knock models are extended to include the CV effects, too. Cycle-by-cycle variations are characterized through the introduction of a random variation on a number of parameters controlling the rate of heat release (air/fuel ratio, spark advance, initial flame kernel radius, residual burned gas fraction, turbulence intensity). The intensity of the random variation is specified in order to realize an IMEP CoV similar to the measured one. A kinetic scheme is then solved within the unburned gas zone, characterized by different thermodynamic conditions occurring cycle-by-cycle. The combustion and knock model are applied to compute a numerical knock index. In this way, the spark advance values for borderline knock, at different operation, are evaluated and compared to the experimental findings.
The paper highlights the importance of considering the CV effects for a better identification of the “knock-limited” spark advance. The developed procedure allows to perform, on a theoretical basis and with a reduced set of experimental data, a better choice of the spark advance realizing only a prescribed and controlled percentage of individual knocking cycles.