Effect of Ignition Energy and Hydrogen Addition on Laminar Flame Speed, Ignition Delay Time, and Flame Rising Time of Lean Methane/Air Mixtures

A series of experiments were performed to investigate the effect of ignition energy (Eig) and hydrogen addition on the laminar burning velocity (Su 0 ), ignition delay time (tdelay), and flame rising time (trising) of lean methane−air mixtures. The mixtures at three different equivalence ratios (φ) of 0.6, 0.7, and 0.8 with varying hydrogen volume fractions from 0 to 50% were centrally ignited in a constant volume combustion chamber by a pair of pin-to-pin electrodes at a spark gap of 2.0 mm. In situ ignition energy (Eig ∼2.4 mJ ÷ 58 mJ) was calculated by integration of the product of current and voltage between positive and negative electrodes. The result revealed that the Su 0 value increases non-linearly with increasing hydrogen fraction at three equivalence ratios of 0.6, 0.7, and 0.8, by which the increasing slope of Su 0 changes from gradual to drastic when the hydrogen fraction is greater than 20%. tdelay and trising decrease quickly with increasing hydrogen fraction; however, trising drops faster than tdelay at φ = 0.6 and 0.7, and the reverse is true at φ = 0.8. Furthermore, tdelay transition is observed when Eig > Eig,critical, by which tdelay drastically drops in the pre-transition and gradually decreases in the post-transition. These results may be relevant to spark ignition engines operated under lean-burn conditions.