Numerical Investigation of Spark Ignition Events in Lean and Dilute Methane/Air Mixtures Using a Detailed Energy Deposition Model

It is beneficial but challenging to operate spark-ignition engines under highly lean and dilute conditions. The unstable ignition behavior can result in downgraded combustion performance in engine cylinders. Numerical approach is serving as a promising tool to identify the ignition requirements by providing insight into the complex physical/chemical phenomena. An effort to simulate the early stage of flame kernel initiation in lean and dilute fuel/air mixture has been made and discussed in this paper. The simulations are set to validate against laboratory results of spark ignition behavior in a constant volume combustion vessel. In order to present a practical as well as comprehensive ignition model, the simulations are performed by taking into consideration the discharge circuit analysis, the detailed reaction mechanism, and local heat transfer between the flame kernel and spark plug. The energy profile and the energy source geometry are investigated in detail to represent the physics of electrical discharge. It was observed in the experiments that a sufficiently high ambient pressure is necessary for a successful ignition event in the lean and dilute mixture when the spark plug gap size and primary energy input are held constant. By adopting realistic energy levels, this detailed energy deposition model showed the capability to reasonably present such ignition behavior transition. The unique combination of energy deposition profile and geometry reveals the complexity of electrical discharge during the spark ignition event. The response of the combustible gas to the energy deposition showed dependency on the volumetric energy density, energy source’s surface area, temperature gradient at the energy source boundary, as well as the heat transfer condition local to the flame kernel.