Abstract:

Dual-fuel combustion is a promising method for achieving high-efficiency clean combustion in internal combustion engines. Most current research focuses on the effects of dual-fuel injection on engine performance and emissions. Our understanding of dual-fuel in-cylinder combustion processes needs further investigation. In this study, an optical diagnostic system was established to determine the intermediate products during in-cylinder combustion; the system enabled simultaneously qualitative two-dimensional measurements of formaldehyde and OH radicals. To confirm the feasibility of using this laser diagnostic system, laser-induced fluorescence (LIF) spectra and images of formaldehyde and OH radicals in a laminar premixed methane flame were acquired; the excitation laser wavelengths for formaldehyde and OH radicals were verified to be 355 and 292.85 nm, respectively. Non-simultaneous determination of formaldehyde and OH radicals in the combustion chamber was performed to analyze the two-stage heat release process and distribution regions of formaldehyde and OH radicals during dual-fuel combustion. In this investigation, the engine speed was kept at 1200 r·min^-1 and the total equivalent fuel quality was 30 mg of n-heptane. Isooctane was injected in intake manifold at the beginning of the intake stroke and n-heptane (9 mg) was directly injected into the cylinder at 10° crank angle before compression top dead center. The results indicate that formaldehyde is formed in the low-temperature heat-release stage and is mainly located in the region near the spray jet; formaldehyde then fills most of the combustion chamber. When the high-temperature heat-release stage is initiated, formaldehyde located at the edge of the combustion chamber is consumed first, followed by consumption of formaldehyde in the center region. Accompanied with the disappearance of formaldehyde during the high-temperature heat-release stage, OH radicals first emerge at the edge of the combustion chamber and later fill the whole combustion chamber. Finally, simultaneous measurements of formaldehyde and OH radicals were conducted. Formaldehyde consumption is spatially accompanied by the formation of OH radicals. In general, the distributions of formaldehyde and OH radicals are separate spatially, but there are some regions where formaldehyde and OH radicals exist simultaneously.

Key words: Laser-induced fluorescence, Engine, Dual-fuel combustion, Formaldehyde, OH radical