The positive external stretch inhibits the flame propagation when the Lewis number of the deficient reactant is above unity. Enrichment of hydrogen into iso-octane/air mixture can enhance the flame propagation in regimes I and II: hydrogen addition can greatly promote radical generation and heat release from chemical reaction; and it can reduce the Lewis number. This fact is demonstrated by Fig. 10(b) which shows the normalized flame propagating speed as a function of normalized stretch rate for three different values of α. For each case, the ignition Sulfo-Cy5 NHS ester is set to be slightly larger (<1%) than MIE of each α to ensure successful ignition. It is seen that the propagation speed of the small ignition kernel (corresponding to large stretch rate) is greatly enhanced with the increase of hydrogen addition. Therefore, the MIE is greatly reduced by hydrogen addition when α is small (α < 0.6). At the same hydrogen blending level, the MIE decreases with the increase of the equivalence ratio as shown in Fig. 9. This is due to the fact that the Markstein length decreases with the increase of equivalence ratio (see Fig. 8) and the MIE becomes smaller at lower value of Lb  and .