Engine knock is one of the most limiting factors for modern Spark-Ignition (SI) engines to achieve high efficiency targets. The stochastic nature of knock in SI units hinders the predictive capability of RANS knock models, which are based on ensemble averaged quantities. To this aim, a knock model grounded in statistics was recently developed in the RANS formalism. The model is able to infer a presumed log-normal distribution of knocking cycles from a single RANS simulation by means of transport equations for variances and turbulence-derived probability density functions (PDFs) for physical quantities. As a main advantage, the model is able to estimate the earliest knock severity experienced when moving the operating condition into the knocking regime. In this paper, improvements are introduced in the model, which is then applied to simulate the knock signature of a single-cylinder 400cm3 direct-injection SI unit with optical access; the engine is operated with two spark timings, under knock-safe and knocking conditions respectively. The statistical prediction of knock resulting from the presented knock model is compared to the experimental evidence for both investigated conditions. The agreement between the predicted and the measured knock distributions validates the proposed knock model. Finally, limitations and some unprecedented possibilities given by the model are critically discussed, with particular emphasis on the meaning of RANS knock prediction
The increasing limitations in engine emissions and fuel consumption have led researchers to the need to accurately predict combustion and related events in gasoline engines. In particular, knock is one of the most limiting factors for modern SI units, severely hindering thermal efficiency improvements. Modern CFD simulations are becoming an affordable instrument to support experimental practice from the early design to the detailed calibration stage. To this aim, combustion and knock models in RANS formalism provide good time-to-solution trade-off allowing to simulate mean flame front propagation and flame brush geometry, as well as “ensemble average” knock tendency in end-gases. Still, the level of confidence in the use of CFD tools strongly relies on the possibility to validate models and methodologies against experimental measurements.In the paper, two sets of cycle-resolved flame visualizations are available from a single-cylinder 400 cm3 direct-injection spark-ignition (DISI) unit with optical access. The engine is operated at two spark timings, ranging from knock-safe to light-knock conditions.On this basis, a numerical analysis is carried out to reproduce flame kernel growth and propagation using the well-known ECFM-3Z combustion model for all the operating conditions. CFD results are compared in terms of enflamed volume and flame morphology against cycle averaged experimental data. In addition, average knock is simulated by means of the in-house built UniMORE Knock Model [1] in terms of knock onset location and phasing.The agreement between predicted and measured position of the flame front and knock inception location for the two different operating conditions confirms the validity of the adopted models and proves their predictive capability for engine design and optimizatio
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