Experimental and Modeling Study of Spark Plug Electrode Heat Transfer and Thermal Energy Deposition 2021-01-0480

Spark plug electrode heat transfer and its relationship with the thermal energy deposition from the spark plasma to the gas in the spark gap was studied under quiescent non-combusting conditions. The thermal energy deposition to the gas (N2) was measured with a spark plug calorimeter as a function of pressure, up to 30 bar. The measurements were carried out for two gap distances of 0.3 mm and 0.9 mm, for three nominally identical spark plugs having different electrode surface area and/or surface thermal conductivity. The unmodified baseline spark plug had a nickel center electrode (cathode) 2.0 mm in diameter, the first modified spark plug had both the ground and center electrodes shaved to a diameter of approximately 0.5 mm, and the second modified spark plug had copper inserts bonded to both electrodes. The experimental results were compared with multi-dimensional simulations of the conjugate heat transfer to the gas and to the metal electrodes, conducted using CONVERGE CFD. Consistent with the literature, the measurements showed the thermal energy deposition to the gas increased with both increasing pressure and spark gap distance. The thermal energy deposition to the gas was found similar for both the unmodified and the shaved fine-wire electrode plugs, however the delivered electrical energy to the gap was approximately one third less for the fine-wire electrode plug, resulting a higher energy conversion efficiency for the fine-wire plug. The simulations indicated that the temperature rise of the metal electrode surfaces was mostly confined to the immediate area of contact with the plasma arc and that heat loss to other parts of the spark plug and to the calorimeter walls was negligible over the time-scale of the arc duration. For a steel electrode with an assumed arc diameter of 0.1 mm, the maximum predicted rise in surface temperature was approximately 175 ^oC. The simulations indicated that the high thermal conductivity of a copper surface resulted in locally lower temperature peaks and, as expected, more rapid diffusion of the heat affected zone such that the 1/e time for the temperature dissipation was approximately 0.6 ms after the end of the spark. Experimentally, the high thermal conductivity copper surfaces had no measureable effect on the thermal energy deposition. The results showed that spark plug electrode surface area had only a small effect on the thermal energy deposition to the gas and little effect on the amount of heat transfer from the arc to the electrodes under the investigated conditions where the gap fluid motion was small. An implication may be that if spark plug electrode size and geometry affects flame initiation, it is due to heat loss from the nascent flame kernel rather than from the spark plasma.