The paper reports a combined experimental and numerical investigation of a small unit displacement two-stroke SI engine operated with either Gasoline and Natural Gas (CNG). It is widely recognized that for two-stroke, crankcase scavenged, carbureted engines the scavenging patterns (fuel short-circuiting, residual gas distribution, point wise lambda field, etc.) plays a fundamental role on both engine performance and tailpipe emissions. To properly characterize the engine behavior in terms of scavenging patterns and combustion, a detailed multi-cycle 3D-CFD analysis of the scavenging process is at first performed starting from preliminary 1D computed boundary conditions provided by a in-house developed 1D model of the whole engine. In order to assess the accuracy of the adopted numerical approach, comparisons between numerical forecasts and experimental measurements of the instantaneous in-cylinder pressure history for steady-state operations of the engine are at first performed and shown in the paper. Subsequently, the activity is focused on the investigation of knock occurrence. In order to limit the computational cost of the simulations, calculations are at first carried out within the 1D modeling framework, where customized quasi-dimensional combustion and knock models are used. In particular, the 1D model is used to compute a numerical knock index which can be useful to address the tuning of the spark advance, given a prescribed and controlled percentage of knock released heat. At the end of the simulation process, the 1D knock index is qualitatively compared to results obtained from full 3D knocking analyses for different in-cylinder compositions and spark timings. The intrinsic knock-resistance of the CNG fuel is finally numerically exploited, through variations of both compression ratio and spark advance.
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