Erosion processes of electrodes – Experiments and modeling

The scope of this work was to identify and examine lifetime limiting processes of spark plugs as used in stationary gas engine applications. Experimental investigations and analytic methods were applied in order to analyze and understand the wear process, especially in terms of electrode material. Physical parameters to describe rate of oxidation and diffusion processes were obtained and employed to set up a simulation model capable of lifetime prediction. Parameters not directly measurable, such as temperature load where the arc strikes the electrode, are extracted from a separate local, in-depth thermal analysis. Furthermore, ignition parameters such as level of ignition current and spark duration were systematically varied to determine their impact on the ignition system and to refine the wear model. Field tests on a Jenbacher Type 6 Gas engine have been conducted to observe the erosion of electrodes in gas engines due to the main contributing factors: combustion, spark energy input and oxidation. Using a specific spark plug test rig where combustion does not take place, the processes can be reduced to either spark energy impact or oxidation. Long-term testing as well as single spark experiments provide greater insight into the wear rate. Following this test rig step, the erosion of the electrode material caused by oxidation alone was investigated in a furnace using thermogravimetry. Before and after the tests, optical inspections (DOF), computer tomography (CT), energy dispersive x-ray spectroscopy (EDX), x-ray diffraction (XRD) and differential thermal analysis (DTA) were applied to analyze the electrode material. The understanding of wear phenomena obtained from these experiments in combination with the new simulation model developed provide the opportunity to predict lifetime and to evolve more durable spark plug designs.