Exploring the effects of varying pre-chamber geometry in a heavy-duty natural gas optically accessible engine under dilution conditions

Pre-chamber combustion (PCC) is an advanced ignition strategy that has been shown to enhance spark ignition (SI) engine combustion stability by providing distributed ignition sites from turbulent jets. PCC has been proven especially advantageous compared to SI in dilute and ultra-lean operating conditions. This work involves experimental and computational investigation of the effects of varying pre-chamber geometry on pre-chamber and main chamber combustion under simulated exhaust gas recirculation dilution conditions in a heavy-duty, single-cylinder, optically accessible natural gas engine at stoichiometric fuel-air ratio. Pre-chambers with varying nozzle number (5 – 12), diameter (1.3 – 2.5 mm), and nozzle angle (0 – 20°) are examined, keeping the nozzle area to pre-chamber internal volume (A/V) ratio constant. Pressure-based diagnostics are used to probe pre-chamber and subsequent main chamber combustion, and optical diagnostics including high-speed OH* chemiluminescence and infrared imaging probe the development of pre-chamber jets and resulting ignition of the main chamber charge. Heat release analysis of the in-cylinder pressure data indicates that the pre-chamber with the smallest nozzle diameter has delayed main chamber ignition despite comparable pre-chamber pressure rise, under diluted conditions. This effect is more pronounced for cycles with in-cylinder combustion residuals in addition to external dilution. Conversely, the smallest nozzle diameter pre-chamber has the fastest main chamber ignition over the entire range of spark timing for no dilution. Additionally, it was observed that pre-chambers with swirling nozzles have a faster pressure rise in the pre-chamber and main chamber. Misfires and cycle-to-cycle variations increase with retarding spark times mainly due to failed re-ignition of pre-chamber jets. Computational simulations were carried out to investigate the differences in flame development inside the pre-chamber that lead to observed main chamber combustion characteristics for the various cases considered in the present work.