Effects of gas injection timing on combustion instability for a spark ignition natural gas engine under low load conditions

Experiments are carried out with different GITs under 10% and 25% load conditions to reveal the influence of gas injection timing (GIT) on combustion instability for a spark ignition natural gas engine under low load conditions. The phase space reconstruction and coefficient of cyclic variation (CCV) of the entire cycle in-cylinder pressure time series, the return maps of indicated mean effective pressure (IMEP) and CA50, the distributions of CA5 and CA90 are applied to analyze the combustion instability of the natural gas engine qualitatively and quantitatively. Results show that the combustion process of the natural gas engine presents chaotic characteristics. With increasing GIT from 0 & DEG;CA to 90 & DEG;CA ATDC, the CCV and the separation degree of phase space attractor initially increase and then decrease. The distributions of return map points for IMEP and CA50 initially disperse and then concentrate, and the combustion instability and chaotic characteristics are initially enhanced and then weaken. When the GIT is near 60 & DEG;CA ATDC, the chaotic characteristics of combustion instability for the natural gas engine is the strongest. The reasons for the influence of GIT on combustion instability of the natural gas engine are identified based on 3D computational fluid dynamic (CFD) simulation, showing that the pressure difference between in-cylinder and the intake manifold under different GITs leads to the difference in the mixing degree of the natural gas and air as well as the CH4 concentration distribution of in-cylinder. This finding provides a theoretical basis for improving the lean-burn stability of natural gas engines.