Optical investigations on lean combustion improvement of natural gas engines via turbulence enhancement

In the global background of “Carbon Peak” and “Carbon Neutral”, natural gas engines show great advantages in energy-saving and pollution reduction. However, natural gas engines suffer from the issues of combustion instabilities when operating under lean burning conditions. In this paper, the role of turbulence enhancement in improving the lean combustion of natural gas was investigated in an optical SI engine with high compression ratios. Variable swirl control valves (SCV) were designed and intake tumble and swirl were combined to regulate turbulent motion and turbulent intensity. Particle image velocimetry was employed to measure in-cylinder turbulence, and transient pressure acquisition and high-speed photography were synchronously performed to quantify combustion evolutions. The results show that in-cylinder turbulent intensity is enhanced significantly through reducing SCV closing angles. Such that flame propagation speed and thermal efficiency are significantly improved with an increment of turbulent intensity, which indicated that mean effective pressures are not sensitive to spark timing. The analysis of flame images shows that the combined turbulence increases in the radial orientation from the spark plug to the cylinder wall, leading to an earlier flame kernel formation and a faster burning rate. Therefore, the combined turbulence has the potential in reducing the cyclic variations of lean combustion in natural gas engines.