Klaus Hrastnik

Description of the discharge process in spark plugs and its correlation with the electrode erosion patterns

The phases of the discharges in spark plugs were studied with a high-speed camera and an oscilloscope. The discharges were done using samples of nickel alloys and platinum as cathode in air at pressures ranging from 100 to 900 kPa. For low pressures (100 kPa), a glow discharge occurs after the breakdown. For higher pressures, an arc discharge follows the breakdown and changes into a glow discharge when the current decreases. The damage produced on the cathode surface was analyzed with scanning electron microscopy and white light interferometry and was correlated with the corresponding discharge. The craters on the surface are mainly produced by the breakdown and arc discharge. The glow discharge delivers energy to the cathode in a large area and produces a negligible material damage. The movement of the arc hot spot produces further craters that are commonly overlapped. Transitions from glow to arc modes produce new small craters, which in some cases can be arranged along polishing traces. This work is relevant for the development of new electrode materials for spark plugs and electrical contacts.