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

The laser ignition of CH₄/air mixtures at temperatures and pressures representative of an internal combustion engine was examined using an Nd:YAG laser in a Rapid Compression Machine. Specifically the lean limit minimum spark energy (MSE) and minimum ignition energy (MIE) were investigated. The results show that laser ignition displayed a statistically stochastic behavior. Using a 90% probability of occurrence the obtained figures were MSE₉₀ =2.3 mJ and MIE₉₀ = 7.2 mJ at a 0.4 equivalence ratio 29 bar and 750 K. A lean limit for 90% chemical energy conversion was observed to correspond to a 0.47 equivalence ratio. Schlieren imaging indicated that the adiabatic core assumption after compression is valid and the propagating flame was laminar.