Characterization of Spark- and Laser-Ignition of Bio- and Natural Gas

This paper presents a comparative investigation of the early phase of combustion initiated by a focused pulsed laser beam and a conventional spark-discharge. The comparative approach is applied to the characterization of combustible mixtures of natural gas and biogas with varying CO₂ content. Interferometry and Schlieren imaging are used to probe flame kernel formation and its subsequent transition to a self-sustained flame. The pressure rise in the chamber is also recorded by means of a fast-response piezo-electric pressure transducer. From these diagnostics, differences between the early phases of laser-induced and spark-ignition are revealed. Unlike in laser-induced ignition, spark-ignition features a delay in the transition from the early flame kernel to a self-sustained flame (attributed to heat loss), which leads to failed ignition for mixtures with high CO₂ content. The role of CO₂ as an ignition retardant is observed by comparing fuel/air mixtures with and without CO₂.