Design of a test rig for fundamental investigations of spark characteristics

A common means to increase efficiency in stationary spark ignited engines is to operate the engine with a higher air/fuel ratio of the mixture in conjunction with a higher turbulence level; however, this generally leads to severe conditions that significantly impact the inflammability of the gas–air mixture and combustion stability. Because the electric arc that forms at the spark plug is a main influencing factor in combustion, detailed research work in the field of electric arc behavior generated at spark plugs is required. This article thus presents a specially tailored test rig that is designed to facilitate an investigation of electric arc behavior under cross-flows at a spark plug typically used in gas engines. The test rig consists of a closed flow circuit for inert gases; its centerpiece is a test cell that provides optical access for high-speed imaging of the electric arc behavior at the spark plug. The required flow velocity at the spark plug is set with a blower. Flow velocities up to 30 m/s, pressures up to 60 bar and temperatures up to 80 °C can be achieved inside the flow system at the location of the spark plug. Postprocessing algorithms have been developed to automatically extract information from the high-speed images. The results reveal that the arc stretches more at a higher flow velocity as indicated by its greater arc length. In addition, it is evident that the cycle-to-cycle variation in arc length increases at higher flow velocities. The secondary voltage history and its cycle-to-cycle variation are strongly influenced by the arc length. This is reflected in the cycle-to-cycle variation of the spark energy input to the flowing gas. These results support the conclusion that spark behavior itself can be a substantial source of cycle-to-cycle variation in the combustion process observed in spark ignited gas engines.