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The arc breakdown phase in automotive spark-plugs is a sub-microsecond event that precedes the main spark event. This phase is typically characterized by strong non-equilibrium plasma phenomena with high voltage and currents. The nature of the initial breakdown phase has strong implications for the successful spark formation and the electrode erosion/lifetime. There are evidently very few studies that seek to characterize this phase in detail. The goal of this work is to investigate this non-equilibrium plasma arc breakdown phase, using high-fidelity computational modeling. We perform studies using the VizGlow non-equilibrium plasma modeling tool. During the early breakdown phase, the plasma forms thin filamentary streamers that provide the initial conductive channel across the gap. Once the streamers bridge the gap, the plasma begins to transition to a thermal arc.

A high-fidelity arc plasma simulation tool has been developed that can model arc physics coupled with electrode erosion. The arc physics is modelled using an equilibrium, resistive magneto-hydrodynamic (MHD) model. Solid electrodes are modelled using an immersed boundary method that allows for the electrodes to deform dynamically without modifying the original mesh or the underlying governing equations. First, a partial validation of the arc physics is performed by comparing arc stretch in cross-flow with experimental results across a 1.1 mm spark gap. Subsequently, the ability to model arc physics in a coupled manner with ablating electrodes is demonstrated in 2D using a 0.7 mm electrode gap with iron electrodes. The objective of this work is to illustrate modeling of coupled arc physics with eroding electrodes; a capability that will be extended for the realistic prediction of spark plug lifetimes in future work.

Spark plug erosion is critical in determining the overall efficiency of a spark ignition engine. Over its lifetime, a spark plug is subject to millions of firings. Each spark event results in material erosion due to several mechanisms such as melting, vaporization, sputtering and oxidation. With electrode wear, the inter-electrode spacing increases and a larger voltage difference is required to initiate the spark. The probability of engine misfires also increases with electrode erosion. Once a critical gap is reached, the energy in the ignition coil is not enough to cause a spark breakdown, and the spark plug must be replaced. Due to the long relevant time scales over which erosion occurs, and the difficulty of analyzing the spark plug environment during operation, determining spark plug lifetime typically requires extensive field testing. A high fidelity commercial thermal plasma solver, VizSpark is used simulate electrode erosion due to spark events.