Influences of isomeric butanol addition on anti-knock tendency of primary reference fuel and toluene primary reference fuel gasoline surrogates

The anti-knock tendency of blends of butanol isomers and two gasoline surrogates (primary reference fuels and toluene primary reference fuels) was studied on a single-cylinder cooperative fuel research engine. The effects of butanol molecular structure (n-butanol, i-butanol, s-butanol and t-butanol) and butanol addition percentage on fuel research octane numbers were investigated. The experimental results revealed that butanol addition to either PRF80 or TPRF80 increased research octane numbers, and the research octane numbers of fuel blends showed higher linearity with the molar percentage than with the volumetric percentage of butanol addition. Furthermore, the research octane number boosting effects of butanol isomers were observed to change with the fuel compositions, that is, i-butanol >s-butanol >n-butanol >t-butanol for primary reference fuels and i-butanol >s-butanol >t-butanol >n-butanol for toluene primary reference fuels. In addition, butanol/primary reference fuel blends exhibited higher research octane numbers than butanol/toluene primary reference fuel blends. We thereafter tried to elucidate the underlying fuel combustion kinetics for the observed anti-knock quality of different butanol/gasoline surrogate blends. It was found that the measured research octane numbers of fuel blends showed the best correlation with the calculated ignition delay times at the thermodynamic condition of 770 K and 2 MPa, and the reaction sensitivity analysis in auto-ignition at this condition revealed that the H-abstraction reactions of butanol isomers by OH radical suppressed fuel reactivity, thus elevating the fuel research octane numbers when butanol was added to the gasoline surrogates. Compared with the butanol/primary reference fuel blends, the positive sensitive reactions related to n-heptane were of higher importance, while the inhibitive effects of sensitive reactions related to butanol and iso-octane decreased for the toluene primary reference fuel/butanol blends, thus leading to lower research octane numbers of the toluene primary reference fuel/butanol blends than those of the butanol/primary reference fuel blends.