Impacts of the horizontal swirl and axial tumble on the turbulent kinetic energy and combustion process of a natural gas engine

Abstract

ABSTRACT Natural gas (NG) engines are restricted by slow combustion rates. An appropriate flow condition is considered to be the most promising way to improve the combustion rate of NG. In this study, decoupling analysis of the impacts of swirl and tumble on turbulent kinetic energy (TKE) and combustion process of a spark ignition (SI) NG engine was conducted by simulation. A 3D model was established and boundary conditions of the most economical operating point (1200 r/min, 100% load) were obtained through tests, and the rationality of the case was verified. Then, the flow field of the original engine was decoupled at the intake valve closing time to obtain the decoupled swirl field and the decoupled tumble field. Finally, the swirl field and the tumble field were amplified and applied to the SI NG engine. The results showed that high swirl or tumble can obtain high TKE, but the contribution mechanism of them to TKE is significantly different. According to the analysis of the spatial motion structure and energy conversion, the TKE in tumble field is much higher than that in swirl field with the same intensity. In addition, the impacts of swirl and tumble on the combustion process of the SI NG engine are both divided into two phases, and high swirl or tumble can obtain higher thermal efficiency. The combustion rate of NG first increased with the increase in initial flow field strength. When the swirl ratio at intake valve closing time (SR0) and the tumble ratio at intake valve closing time (TR0) exceeded 2.0 and 1.25, respectively, the combustion rate is kept in a fluctuating equilibrium state. At this time, the improvement of swirl and tumble on NG combustion process was brought into full play, the maximum indicated thermal efficiency reached 43.18% and 44.39%, respectively. Finally, the conclusion was drawn by comparing swirl and tumble in different aspects where high tumble shows greater potential in achieving super-high thermal efficiency.