Advanced Turbulence Model for SI Combustion in a Heavy-Duty NG Engine

In the recent years, the interest in heavy-duty engines fueled with Compressed Natural Gas (CNG) is increasing due to the necessity to comply with the stringent CO2 limitation imposed by national and international regulations. Indeed, the reduced number of carbon atoms of the NG molecule allows to reduce the CO2 emissions compared to a conventional fuel. The possibility to produce synthetic methane from renewable energy sources, or bio-methane from agricultural biomass and/or animal waste, contributes to support the switch from conventional liquid fuels to CNG. To drive the engine development and reduce the time-to-market, the employment of numerical analysis is mandatory. This requires a continuous improvement of the simulation models toward real predictive analyses able to reduce the experimental R&D efforts. In this framework, 1D numerical codes are fundamental tools for system design, energy management optimization, and so on. The present work is focused on the improvement of the turbulence sub-model, originally conceived to describe turbulence evolution in tumble-promoting engines. The turbulence model is here developed with reference to a SI heavy-duty CNG engine derived from a diesel engine. In this architecture, due to the flat cylinder head, turbulence is generated primarily by swirl and squish flow motions unlike conventional tumble-assisted SI engines. To extend the turbulence model, a 3D simulation campaign was carried out aiming at extracting the information for model conceptualization and validation. The turbulence sub-model demonstrated to properly predict turbulence and swirl/tumble evolution under various operating conditions, without the need for any case-dependent tuning. It hence presented the potential for appropriately support the predictive capabilities of any combustion model for SI heavy-duty tumble- and swirl-promoting engines.