Spark discharge ignition can be effectively enhanced by increasing the plasma volume and its duration, but evaluation and realization of an optimal ignition performance under engine working conditions are big challenges due to the complex in-cylinder flow field and thermodynamic environment. In this investigation, ignition strategies with various spark discharge parameters were tested, and their effects on the initial flame kernel formation and propagation in the early stage of combustion were compared and analyzed in a single-cylinder optical engine. The experimental results indicate that the combustion under stoichiometric conditions is not sensitive to the spark discharge settings; it mainly depends on the fast chemical reaction rate of the combustible mixture and the flow field inside of the cylinder. However, as the mixture becomes leaner, the increased ignition delay makes the ignition difficult gradually. Once the ignition capacity is insufficient, a flame displacement would occur during the flame propagating at the early stage of the combustion; the ignition performance is not fully released. A new spark discharge around the unburned zone can further improve ignition performance. Larger flame displacement means a later combustion phase and worse combustion performance. By boosting the discharge current and duration, the flame displacement and its duration are reduced, and the initial flame kernel formation and propagation can be effectively improved. The fitting results for the flame displacement indicate that each discharge current corresponds to a critical discharge duration; longer discharge duration than the critical discharge can achieve an optimal ignition performance of internal combustion (IC) engines. |
