Effects of Spark Plug Operating Conditions on Electrode Erosion and Surface Deformation

An experimental study of the spark ignition process for SI engines was conducted to study spark plug erosion and the effect of breakdown voltage/energy on electrode surface deformation. The experiments were conducted outside of an engine, in both a pressurized constant volume optical chamber and in a high-pressure vessel heated within a furnace with gas temperatures as high as 730°C. J-gap spark plugs designed for natural gas engines were studied at elevated temperature and under a range of pressures to investigate electrode wear characteristics. Both iridium-alloy and platinum-alloy cathode (center electrode) and anode (ground strap) spark plugs were investigated. In addition, single spark events were performed on polished platinum cathode surfaces to allow the visualization of craters from individual spark events in order to quantify how their size and shape were affected by energy deposition and breakdown characteristics. The spark plug electrodes were investigated using optical profilometry, scanning electron microscopy (SEM), and energy dispersive X-ray (EDX) analysis. The furnace erosion tests were run with up to 30 million spark events for a given spark plug. The breakdown voltage was found to generally increase, as expected, with increasing electrode wear as the number of spark events increased. The rate of increase of the breakdown voltage was greatest during the early portion of a trial, presumably as the relatively sharp edges of the center electrode experienced wear. Consistent with prior studies, platinum-based spark plugs were found to erode significantly more than iridium-based plugs under identical scenarios.