CFD Simulation and Flame Analysis of Combustion and Knock in a DISI Optically Accessible Research Engine

Paper #:
  • 2017-01-0555

Published:
  • 2017-03-28
Abstract:
The increasing limitations in engine emissions and fuel consumption have led researchers to the need to predict combustion and knock processes in gasoline engines. In particular, knock is one of the most limiting factors for modern SI engines, preventing further thermal efficiency gain. Modern CFD simulations are becoming an affordable instrument to help experiments from design to calibration processes in engine engineering. 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 knock tendency in end-gases. Three sets of cycle-resolved flame visualizations are available from a single-cylinder 400cm3 direct-injection spark-ignition (DISI) unit with optical access operated at three spark timings, from knock-safe to hard-knock conditions respectively. On this basis, a numerical analysis is carried out to reproduce flame kernel growth and propagation using the ECFM-3Z combustion model for all the operating conditions. The results are compared in terms of enflamed volume and flame morphology against averaged cycle experimental data. In addition, average knock is simulated by means of the in-house built UniMORE Knock Model in terms of knock onset location and phasing. The agreement between predicted and measured position of the flame front and knocking zone position in three different operating conditions validates the adopted models and proves the predictive accuracy for engine design and optimization.
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