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The correct simulation of combustion process allows to better face several SI engines design problems, not only for innovative mixture formation concepts (stratified or ultra-lean charge), but for traditional homogeneous mixture as well. Even though many commercial codes are able to describe the complex 3-D non reacting fluid dynamics in ICE, the simulation of high turbulent flame propagation does not seem to be a completely solved problem yet. In this work a comparison between two different turbulent combustion models (a characteristic time based one by Abraham and Reitz [2, 15, 16] and a flamelet based one by Cant and AbuOrf [4, 20]) has been performed using KIVA-3V code to assess simulation reliability. Models predictive capabilities have been tested with reference to specific data acquired at the engine test bench of Tor Vergata Mechanical Engineering Department on a Fiat Punto 1242 cc 8 valves SI engine over a wide range of operating conditions.

Ultra lean-burn natural gas engines offer the potential for lower emissions and higher efficiency than conventional SI engines. Combustion instabilities near the lean limit can be addressed by partially stratifying the in-cylinder charge. The Partially Stratified Charge (PSC) approach involves micro-direct-injection of pure fuel, or a fuel-air mixture, to create a rich zone in the region of the spark-plug. This has been demonstrated to improve combustion in an ultra-lean bulk mixture. An experimental premixing apparatus was devised to investigate the effect of changing the stoichiometry of the micro-direct-injected charge. In conjunction, a numerical methodology was used as an aid to understanding the complex in-cylinder processes. Although rich premixed micro-injection improved engine performance over the homogeneous case, the fastest heat release rate was found to occur with a pure fuel PSC charge.

The aim of this work is to assess the accuracy of results obtained from Large Eddy Simulations (LES) of a partially-premixed natural gas spark-ignition combustion process in a Constant Volume Combustion Chamber (CVCC). To this aim, the results are compared with the experimental data gathered at the University of British Columbia. The computed results show good agreement with both flame front visualization and pressure rise curves, allowing for drawing important statements about the peculiarities of the Partially Stratified Combustion ignition concept and its benefits in ultra-lean combustion processes.