| Abstract:In order to improve the indicated thermal efficiency of a large-bore natural gas engine with stoichiometric operation, a concept of organizing an in-cylinder air motion with weaker swirl, stronger tumble and higher turbulence intensity was proposed. According to this concept, different shapes of intake ports and combustion chambers were designed. Then, the effects of the shape of the intake port and combustion chamber on the in-cylinder flow and combustion were studied using three-dimensional numerical simulation. Results showed that in comparison with the original engine, when changing the original helical intake port to a straight intake port and adopting the original reentrant combustion chamber and a new vertical-cave, open-cave, and hemispherical-cave combustion chamber in turn, the in cylinder swirl flow was significantly weakened, and the tumble flow was obviously enhanced. Especially, the maximum tumble flow ratio during the compression stroke could reach 0.76, 0.83, 1.03 and 1.71, respectively, and the in-cylinder average turbulent kinetic energy at the ignition timing was increased by 35.27%, 36.26%, 50.33%, and 135.99%, respectively, further accelerating the combustion rate and increasing the indicated thermal efficiency. Therefore, the match of the hemispherical-cave combustion chamber with the straight intake port was more beneficial to the formation of a stronger in-cylinder tumble flow and the improvement of turbulence intensity at the ignition timing. Finally, the indicated thermal efficiency was increased from 39.51% of the original engine to 42.14%. |