Investigations on combustion system optimization of a heavy-duty natural gas engine

•Two intake ports and four combustion chambers were studied on spark ignition natural gas engines.•Optimized chemical model was used for computation acceleration while keeping accuracy.•Effective mixed-flow intake ports were proposed to improve the in-cylinder turbulence.•Eccentric hemispherical combustion chamber scheme with mixed intake flow showed the most optimal combustion performance.•Eccentric hemispherical combustion chamber scheme exhibited increased NOx emissions while negligible CO and CH4 emissions. Nowadays, the carbon-free policies and the crisis of crude oil make natural gas get increasing attention. However, spark-ignited natural gas engines continually suffer from problems of thermal efficiency and NOx emissions. Optimizing intake and combustion systems are effective ways to improve combustion and emission performance, but comprehensive work involving both intake and combustion systems is still lacking, especially for heavy-duty commercial engines. In this work, both intake and combustion systems of spark-ignited heavy-duty natural gas engines were investigated using muti-dimensional numerical simulations. Two intake ports and four combustion chambers were considered. An optimized chemical model was employed to accelerate the computation efficiency of combustion processes. The optimal combination of intake and combustion systems was obtained, with impressive thermal efficiency and acceptable emission performance. The results show that the mixed-flow intake port performs better than the swirl intake port for the natural gas engine with premixed combustion mode, manifesting increased in-cylinder tumble ratio and turbulent kinetic energy. Meanwhile, the eccentric hemispherical combustion chamber (EHCC) combined with mixed-flow intake ports presents the best optimal combustion performance, exhibiting an effective thermal efficiency beyond 41.53%. However, the EHCC scheme shows an increased NOx emission due to fast combustion speed and high combustion temperature while negligible CO and CH4 emissions. Despite this, natural gas engines equipped with three-way catalysis after-treatment can still meet emission regulations under stoichiometric operating conditions.