Experimental Evaluation of Various Gasoline Surrogates Based on Soot Formation Characteristics

The evaluation of soot tendency of gasoline surrogates in simple atmosphere environments is essential for understanding soot formation processes and developing accurate soot models to represent real fuel chemistry in computational fluid dynamics simulations of gasoline engines. In this work, several surrogates were evaluated on the basis of soot precursor and soot characteristics in laminar diffusion flames. The relative concentrations of polycyclic aromatic hydrocarbons (PAHs) with different ring sizes and soot were measured using laser-induced fluorescence (LIF) and laser-induced incandescence (LII) techniques. The OH and CH luminescence intensities were also recorded using an intensified charge-coupled device. The results showed that the gasoline surrogates failed to represent the formation characteristics of small-ring aromatics (320/360 nm) for gasoline flames but have certain characterization ability for large-ring aromatics (400/450 nm). The relationship between the PAH LIF signal and the toluene content is not monotonically increasing. On the basis of the relative concentrations of larger PAHs and soot, the surrogate (¹/₃ n-heptane, ¹/₃ isooctane, and ¹/₃ toluene) can characterize the formation characteristics of soot precursors and soot in gasoline laminar diffusion flames best. On the basis of the OH and CH luminescence intensities, the surrogate (¹/₃ n-heptane, ¹/₃ isooctane, and ¹/₃ toluene) can best characterize the flame structure and development of gasoline. The CH trend is consistent with the PAH trend, inferring that there exists a strong correlation between CH intensity and PAHs in the diffusion flames. As the aromatic ring number increases, its PAH LIF signal peaks at a higher position and the high concentration region gradually evolves from the flame center to the two wings of the flame, which eventually leads to the characteristic of the two-wing distribution of soot. For the binary mixture of n-heptane-toluene, the 320 nm PAH LIF signal increases monotonically with the toluene ratio. The maximum PAH LIF signals of 360-450 nm exhibit a non-monotonic tendency, which reach the peak at a 50% toluene ratio. Furthermore, there exists a tolerance in terms of the toluene mixing ratio (10%), below which the effect of toluene on larger ring aromatic (A4-A5) formation can be neglected.