Analysis of the nonlinear dynamic characteristics of the combustion instability in a methanol/diesel dual-fuel engine

Abstract

Methanol/diesel dual-fuel engine is prone to unstable combustion phenomena of cylinder pressure fluctuation at high methanol alternative ratio (MAR) under light-speed and low load operations. In this paper, in order to investigate the combustion instability of a methanol/diesel dual fuel engine, experiments were conducted with MAR from 0 % to 22.5 % under an engine speed of 1000 r/min and load rate of 25 % to assess their influences on the cycle-to-cycle variations (CCV) of a premixed methanol/diesel dual-fuel engine. Using nonlinear dynamics analysis, including phase space reconstruction and return mapping technologies, the effects of MAR on the nonlinear dynamic behavior of the test engine have been examined. The results show that a higher MAR leads to pressure fluctuations of the methanol/diesel dual-fuel engine. Specifically, the CCV of cylinder pressure, CA5 and CA50 increase with the increase of MAR, which is 11.51 %, 8.89 % and 9.59 %, respectively. As the MAR increases, the separation of phase space attractor trajectories increases, and the phase point distributions of return maps for cylinder pressure become more decentralized, corresponding to the nonlinear growth process of combustion instability. In addition, the correlation coefficients between the CCV of cylinder pressure and combustion phase (CA5 and CA50) were also analyzed. The correlation coefficients between CCVp and CCV_CA5 is 0.89 for MAR of 22.5 %, while it is 0.91 between CCVp and CCV_CA50, which means that the cylinder pressure fluctuation is strongly related to CA50 combustion phase. The study provides a theoretical basis for enhancing the combustion stability in a methanol/diesel dual-fuel engine.