Simulation Study of Cathode Spot Formation on Spark Plug Electrodes Leading to Electrode Erosion

A multi-dimensional cathode spot generation model is proposed to study the interaction between the plasma arc and cathode surface of a spark plug during the ignition process. The model is focused on the instationary (high current) arc phase immediately following breakdown, and includes detailed physics for the phenomena during spot formation such as ion collision, thermal-field emission, and metal vaporization, to simulate the surface heat source, current density and surface pressure. The spot formation for a platinum cathode is simulated using the VOF (volume of fluid) model within FLUENT, where the local metal is melted and deformed by pressure differences on the surface. A random walk model has been integrated to consider the movement of the arc center, resulting in the formation of different types of spots. The simulation results show: it takes approximately 100 ns for the arc to discharge the electric charge of the spark plug side capacitance and form the spot in the instationary arc phase; the ion collisions are the dominant heating source for the spot generation, and thermal-field emission of electrons is the dominant process for current density and surface cooling rather than conduction to the metal cathode. The moving radius in the random walk model determines the different spot types and the surface profiles. The effects of different working conditions (pressure and temperature) are presented from simulation results while keeping breakdown voltage the same, and pressure has a very strong influence on spot formation. A preliminary estimation for the erosion rate due to oxidation is provided in the paper and its importance in different phases during ignition is discussed.