Study on in-cylinder combustion energy distribution and emissions formation characteristics of a low-carbon methanol/diesel RCCI engine

Methanol/diesel RCCI mode is a low-carbon combustion technology for engines, but it faces issues such as poor combustion stability and high unconventional emissions. To ascertain in-cylinder combustion mechanism and emission formation processes of methanol/diesel RCCI mode, this study used experimental and simulation methods to analyze the effects of methanol substitution rate (MSR) on in-cylinder components, combustion stability, energy distribution and emission characteristics from a methanol/diesel dual fuel engine. The results showed that compared to diesel-only (D100) mode, the in-cylinder combustion temperature in methanol/diesel dual fuel (M40D60) mode declined overall. The main combustion phase dominated NO generation, while HO2 radical generated by methanol oxidation promoted the conversion of NO to NO2. CH3OH and HCHO were distributed in the non-contact area of diesel diffusion flame and the gap volume. As MSR increased, in-cylinder combustion pressure and combustion fluctuation increased, the peak pressure phase advanced, and the heat release rate curve shifted from bimodal to unimodal. Due to faster flame propagation of methanol and improved combustion isochoric efficiency, the effective work proportion significantly increased, and the exhaust and cooling losses were significantly reduced. At full load, the effective work proportion increased from 38.72% to 40.14% as MSR increased from 0 to 40%, while the exhaust and cooling losses decreased from 31.01% to 30.14% and 19.34% to 17.41%. The RCCI mode significantly reduced CO2, NOx and Soot emissions, but led to a rapid increase in HC, NO2 and NO2/NOx ratio. Compared to D100 mode, NO2/NOx ratio in M40D60 increased from 4.97% to 60.74%, respectively.