Exploring the EGR Dilution Limits of a Pre-Chamber Ignited Heavy-Duty Natural Gas Engine Operated at Stoichiometric Conditions - An Optical Study

Pre-chamber spark ignition (PCSI) systems have been proven to improve combustion stability in highly-diluted and ultra-lean natural gas (NG) engine operation by providing spatially distributed ignition initiated by multiple turbulent flame-jets that lead to faster combustion compared to conventional spark ignition. This work investigates the physico-chemical processes that drive the ignition and subsequent combustion in the presence of combustion residuals (internal EGR) within the pre-chamber at varying EGR levels. The over-arching goal is to improve the dilution tolerance of PCSI systems for stoichiometric-operation of on-road heavy-duty natural gas engine. To this end, experiments were performed in a heavy-duty, optical, single-cylinder engine to explore the EGR dilution limits of a pre-chamber, spark-ignited, NG engine operated under stoichiometric conditions. A special skip-fire sequence is utilized to distinguish the effects of in-cylinder combustion residuals from external EGR. Optical diagnostics involving high-speed OH* chemiluminescence imaging and infrared (IR) imaging are acquired simultaneously to probe the ignition, development of pre-chamber jets and the subsequent combustion behavior of the main-chamber charge under various EGR dilution rates. The imaging results and the thermodynamic analysis show that the presence of combustion residuals within the pre-chamber lead to slower spark kernel development resulting in lower ΔP, retarded main-chamber ignition and subsequently slower main-chamber combustion. Cycle-to-cycle variations and tendency to misfire increased with increasing EGR dilution, with combustion residuals making this effect more pronounced. Advancing the spark timing, despite lowering ΔP (due to reduced fuel mass in the pre-chamber and increasing back pressure) can sufficiently compensate for reduced [O₂] at low to moderate EGR levels. Failed re-ignition of pre-chamber jets is the main cause of increased cycle-to-cycle fluctuations and misfire under the tested conditions.