Effect of Composition and Octane Sensitivity of Gasoline Surrogates on PAH Emissions

Primary reference fuels consisting of a mixture of n-heptane and iso-octane have traditionally been used as surrogates to mimic the combustion and emission behavior of gasoline and diesel fuels. However, several recent studies have demonstrated the need to develop more complex surrogates with three or more components in order to better represent the behavior of both conventional and newly emerging fuels, and for a wide range of combustion modes. This is due to the fact that PRF surrogates cannot be used to rate gasoline in terms of RON (research octane number) and octane sensitivity (S) since different PRF surrogates are needed to match the gasoline behavior at different operating conditions. This chapter discusses research dealing with the effects of fuel composition and S on PAHs emissions in laboratory-scale configurations, which include counterflow diffusion and partially premixed flames, and jet-stirred reactor focusing on fuel pyrolysis. Several surrogates with a RON = 70 and S = 0–5.6, and with two, three, and four components containing different amounts of n-heptane, iso-octane, toluene, and ethanol, are considered. For counterflow diffusion flames with binary mixtures (n-heptane/toluene and iso-octane/toluene), results indicate a non-monotonic variation of PAH emission with respect to toluene fraction in the mixture, implying a synergistic effect on PAH emissions at higher toluene content. For the ternary blends investigated, results indicate that RON and S are not true indicators of PAHs emissions. Rather the PAHs emissions are related to the blend composition. On the other hand, for quaternary blends, results indicate a correlation between S and PAH emissions. Thus, an important observation is that the composition of multi-component fuel mixture has a major effect on PAH and soot emissions. In contrast, the effect of S on PAH and soot emissions are not very clear and requires further studies that can explore fuel blends for a wider range of RON and S values.