Experimental Investigation of Polycyclic Aromatic Hydrocarbons Growth Characteristics of Gasoline Mixed with Methanol, Ethanol, or n‑Butanol in Laminar Diffusion Flames

Alcohols have been regarded as being able to reduce particulate emissions from GDI engines, which in turn requires a deeper understanding of the effect of fuel chemistry on the formation and growth of polycyclic aromatic hydrocarbons (PAHs). This work focused on combustion and soot-forming progress and thus studied the distribution characteristics of PAHs with different ring sizes by PLIF and the combustion characteristics by OH and CH luminescence in laminar diffusion flames. The effect of methanol, ethanol, and n-butanol addition to gasoline on PAHs was evaluated at the same mixing ratio and the same oxygen content, and the effect of alcohol ratio from 0% to 80% was investigated. The results showed that at the same mixing ratio the ability of alcohols to reduce all size PAHs is methanol > ethanol > n-butanol. As the alcohol ratio increases, all size PAHs show a monotonous decreasing trend. The ability of methanol or ethanol to reduce large-ring aromatics is always far greater than that to reduce one-ring aromatics (A1); even the percentage reduction of large-ring aromatics is always greater than the alcohol ratio, while the percentage reduction of all size PAHs by n-butanol is always lower than its mixing ratio. At the same oxygen and carbon content, the ability to decrease A1 is n-butanol > ethanol > methanol, indicating that dilution plays a key role in A1 reduction. The ability to decrease A4–A5 (450 nm) is consistent with the results at the same mixing ratio, inferring that the molecular structure rather than oxygen and carbon contents dominates the ability to reduce large PAHs. As the aromatic ring size increases, its high-concentration region gradually evolves from the flame center at lower position to the two wings of the flame at higher position. The flame lift-off length based on the OH chemiluminescence and the intensity of the high-temperature reaction near the flame lift-off height show a monotonous decreasing trend with the alcohol chain length and alcohol mixing ratio. CH mainly locates at the interface between periphery OH and inner flame in the middle of the flame, and its luminescence intensities show a monotonous decreasing trend with the alcohol ratio.