A single-cylinder natural gas fueled engine equipped with a Exhaust Gas Recirculation (EGR) system was ignited using a laser. The broadband emission from the spark kernel was spectrally resolved and the peaks corresponding to Hα, N and O atoms were measured for a range of conditions with global equivalence ratios ranging between 0.6 and 1.0, and for Exhaust Gas Recirculation fractions up to 29%. The (Hα/O) and (Hα/N) peak intensity ratios from the spectral scans correlated extremely well (R2 > 0.97) with local oxygen based equivalence ratios. Appropriate relations were developed to relate such values to global equivalence ratios and the EGR rate. For homogenous intake charge, the present LIBS diagnostic enables estimation of one of the two values, global equivalence ratio or EGR rate, with the knowledge of the other.

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