A study of laser induced ignition of methane-air mixtures inside a Rapid Compression Machine

Presented herein is a fundamental study of laser ignition of methane/air mixtures at temperatures and pressures representative of an internal combustion engine. An Nd:YAG laser operating at λ = 1064 nm was used to ignite methane/air mixtures at equivalence ratios of 0.4 ≤ Φ ≤ 1 in a Rapid Compression Machine (RCM). Experiments were conducted to study the lean limit, minimum spark energy (MSE), and minimum ignition energy (MIE). The results show that laser ignition exhibits a stochastic behavior which must be interpreted statistically. A 90% probability of occurrence was used to evaluate the MSE and MIE, which resulted in MSE90 =2.3 mJ and MIE90 =7.2 mJ at an equivalence ratio Φ = 0.4 at compressed pressure and temperature of Pcomp = 29 bar and Tcomp = 750 K, respectively. The lean limit was characterized based on the fraction of chemical energy converted into thermal energy, which was determined by calculating the apparent rate of heat release as derived from RCM high speed pressure data. A lean limit for 90% chemical energy conversion was found to correspond to an equivalence ratio of 0.47 (Tcomp = 782 K). Schlieren photography was employed as a diagnostics tool to visualize the flame initiation and propagation inside the RCM.