Spark Discharge Characteristics for Varying Spark Plug Geometries and Gas Compositions

Spark discharge properties were studied and characterized for varying gas compositions and spark plug geometries using a spark calorimeter and constant volume optical vessel. Two different 18 mm natural gas engine spark plugs were used in the experiments. All measurements were recorded under quiescent conditions and with a spark gap of 0.30 mm. The spark plug calorimeter was used for measuring thermal energy deposition to the gas for gas compositions of nitrogen, a stoichiometric mixture of nitrogen and methane, a stoichiometric mixture of nitrogen and methane diluted with 30% carbon dioxide by volume, and for air. Other measurements of interest included breakdown voltage, electrical energy delivered to the spark gap, electrical-to-thermal energy conversion efficiency, and spark duration, for pressures up to 28 bar at 300 K. The optical vessel was used for the combusting mixture of stoichiometric air and methane at pressures up to 28 bar. For these combusting experiments, thermal energy deposition was unable to be measured. All measurements were performed on both an 18 mm natural gas engine spark plug with a J-shaped ground strap and an 18 mm natural gas engine spark plug with four individual ground straps, uniformly distributed around the plug’s center electrode. Experiments showed consistently higher thermal energy deposition and electrical-to-thermal energy conversion efficiency for the J-shaped plug compared to the four-ground strap plug. Experiments also showed significant increases in breakdown voltage for air compared to nitrogen or nitrogen-based mixtures.