Effects of dimethyl ether on soot formation in premixed laminar flame by laser induced incandescence method

The control of PM emission is a rather complicated problem to be solved both for diesel engine and GDI engines. There are vast factors affecting PM emission, the fuel, A/F, combustion temperature and so on. But undoubtedly, most efforts are paid to exhaust aftertreatment, rather than the control of generation. This paper is to investigate the effect of dimethyl ether (DME, CH₃OCH₃) on soot generation in flame. Therefore, a laser induced incandescence (LII) measurement system for soot volume concentration measurements in flame is used.Two DME gaseous mixtures were prepared with equivalence ratios 1 and 2. To highlight its effect on soot formation, acetylene known as the precursor of soot, is added in different ratios. Similarly, CO₂ is mixed to simulate exhaust gas recycling (EGR) effect in engines. The experimental results indicate that the fuel property and A/F ratio are the dominating factors for the generation of soot during combustion. The combustion of DME doesn't emit soot obviously even under rich mixture condition. When burning the mixture of DME and acetylene (C₂H₂) in different proportion, there is barely soot emission at equivalence ratio of 1. The soot emission increases as the proportion of C₂H₂ increases at equivalence ratio 2, and the DME addition reduces the soot emission of C₂H₂ flame. CO₂ dilution doesn't lead to the increase of soot when burning DME at equivalence 1 and 2. Soot emission is lower when the mixture of DME and acetylene is diluted by CO₂, though soot volume concentration increases slightly as the proportion of C₂H₂ increases.The kinetic analysis indicates that the combustion of DME does not produce the precursor of soot. Under fuel-rich combustion conditions, C₂H₂ reacts with H radical to form C₂H₃, which leads to soot. When DME is added, it competes for H radical, which reduces the formation of C₂H₃ and thus reduces soot formation.The research carried out in this paper indicates that DME fuel has obviously power to reduce soot formation during combustion, no matter whether it is diluted by CO₂ or mixed with soot precursor substance C₂H₂.