Investigation of Ignition Specification for Lean Limit Expansion of Pre-chamber Combustion

To reduce CO₂ and pollutant emissions from internal combustion engines worldwide, both electrification and efficiency improvement of internal combustion engines are necessary. Pre-chamber (PC) combustion is one of the promising technologies for realizing the above objectives. This article investigates the ignition specification for the PC system with the swirled orifice layout to extend the stability limit of the lean burn. The computational fluid dynamics (CFD) simulation results showed that the discharge channel would be exposed to the direction-fluctuating velocity, where the discharge channel moves around on the electrode surface of the spark plug (SP) and stays at the electrode gap area without channel stretching. To keep a stable discharge channel against the flow in the PC and promote the initial flame propagation in a short time, the high current coil with a short discharge duration was proposed. For the concept validation, a single-cylinder engine test was conducted with four ignition coils with different specifications. The ignition coil with a high current and short discharge duration showed the equivalent effect of the lean limit expansion to the high secondary energy coil. Meanwhile, the ignition coil with a lower current showed the worst lean limit, and signal oscillations in the secondary current were observed, which indicated the disturbance of the spark channel. Furthermore, to estimate the effective energy ratio of the secondary energy which has contributed to the flame propagation, the secondary energy generated before the mass fraction burn of 2% (MFB02) was calculated as the effective secondary energy, and the rest of the energy was regarded as marginal secondary energy. It is expected that the marginal secondary energy has a correlation with the combustion stability, provided that enough amount of the secondary current was given. Controlling the marginal energy amount depending on the combustion state will be the key to efficient energy use while maximizing the combustion performance. Based on the investigations, the secondary current was the essential parameter contributing to the discharge channel stability. The ignition coil with the high current was identified as the preferred ignition specification for the PC.