Two ignition transition modes at small and large distances between electrodes of a lean primary reference automobile fuel/air mixture at 373 K with Lewis number >> 1

Laminar and turbulent minimum ignition energies (MIE_L and MIE_T) of a lean primary reference automobile fuel (PRF95; 95% isooctane/5% n-heptane) and air mixture at the equivalence ratio of 0.8 at 373 K with large Lewis number Le ≈ 2.95 >> 1 using small and large pin-to-pin electrode spark gaps (d_gap) are measured in a dual-chamber, constant temperature/pressure, fan-stirred 3D cruciform burner capable of generating near-isotropic turbulence. Each MIE datum is statistically determined at 50% ignitability by 18–30 repeated runs over a range of ignition energy (E_ig). We find two ignition transition (IT) modes. (1) A non-monotonic IT at small d_gap = 0.8 mm with the lowest MIE_T = 13.9 mJ (< MIE_L = 30.1 mJ) occurring at a critical u′_c ≈ 1.82 m/s due to the coupling effects between differential diffusion and turbulence, where u′ is the r.m.s. turbulent fluctuating velocity. When u′ > u′_c, MIE_T increases drastically (>> MIE_L), because turbulence re-asserts its dominant role. (2) A regular IT at large d_gap = 2 mm is found where MIE_L is only 2.05 mJ, in which the increasing slopes of MIE_T with u′ change from gradually to exponentially when u′ > u′_c ≈ 2.3 m/s. In the post-transition when u′ ≥ u′_c, the averaged ratio of MIE at d_gap = 0.8-mm and 2-mm (MIE_0.8/MIE_2.0) is 1.64 > 1, suggesting that using large d_gap = 2-mm has a higher ignition probability than using small d_gap = 0.8-mm for the lean-burn gasoline surrogate in intense turbulence. Finally, we estimate the uncertainties of burning velocities at u′ = 0 and 2.76 m/s, which are respectively less than 2% and 6% when using small/large E_ig ≈ MIE/70 mJ and/or d_gap = 0.8-mm/2-mm. These results are relevant to spark ignition gasoline engines operated in lean-burn turbulent condition.