Optical Diagnostics of Methanol Active-Thermal Atmosphere Combustion in Compression Ignition Engine

Abstract

Methanol has become one of research hotspots of internal combustion engine in recent years. A method for methanol compression ignition is that create higher in-cylinder active-thermal atmosphere to ignite direct injection (DI) methanol due to higher resistance to auto-ignition. However, the combustion essences and related mechanisms in methanol active-thermal atmosphere combustion (ATAC) have not been well comprehended. In this study, the methanol ATAC characteristics were investigated on an optical engine using different optical diagnostic methods. The port injection (PI) fuel is polyoxymethylene dimethyl ethers (PODE 2 ) or n-heptane, and DI fuel is methanol. The experiment results indicate that methanol ATAC with PI of PODE 2 are divided into two combustion process, i.e., one is PODE 2 premixed combustion and methanol partially premixed combustion (PPC), which manifests that the premixed blue flames PODE 2 and the partially premixed yellow flames of methanol. The other is PODE 2 premixed combustion and methanol diffusion combustion, which manifests that the premixed blue flames PODE 2 and the yellow spray diffusion flames of methanol. The combustion mode of methanol ATAC and the flame structure can be adjusted by changing DI timing. The increase of PI energy proportion results in more pronounced PODE 2 premixed blue flame being observed. The DI of methanol is driven by spray diffusion flames, unless the methanol injection timing is earlier than high temperature heat release of PODE 2 . The capability of PODE 2 to ignite methanol are obviously superior compared with n-heptane. Kinetic analysis reveals that the ignition delay times of PODE 2 are lower than that of n-heptane even at lower equivalence ratio. Furthermore, the required initial temperature of PODE 2 is lower than that of n-heptane under the same equivalence ratio and ignition delay time. The KL factor of methanol diffusion flame ranges from 0.02 to 0.04, which is remarkable lower than diesel spray flames. The methanol ATAC can achieve higher substitution rate of methanol.

Keywords: Methanol, Active-thermal atmosphere combustion, Polyoxymethylene dimethyl ethers (PODE2), n-Hpetane, Optical diagnostics