# Nozzle Design of Plug-and-Play Passive Pre-Chamber Ignition Systems for Natural Gas Engines
## 1 Introduction  
Driven by increasingly stringent fuel consumption and emission regulations [1], the powertrain industry has striven for innovation to meet the ambitious greenhouse gas and CO\({}_{2}\) reduction targets [2; 3]. For long-haul transportation, heavy-duty engines are still expected to dominate the market in the foreseeable future. Further efforts, however, still need to be focused on the improvement of thermal efficiency and pollution emissions. Natural gas is an attractive alternative fuel, due to its low carbon emissions and less expensive after-treatment device [4]. An inherent advantage over other fossil fuels is that natural gas can be renewable, and even has a potentially negative carbon impact [5]. The disadvantage lies in its lower efficiency, which is due to knocking limits and low combustion speed. In recent years, different technologies for improving thermal efficiency have been proposed, such as cooled exhaust gas recirculation (EGR), direct injection, high-energy ignition, lean-burning, miller cycle combined with high compression ratio, pre-chamber (PC), etc. [6; 7; 8]. The lean-burning technology is considered a promising solution, thanks to its higher specific heat ratio and lower auto-ignition tendencies. However, its application is limited due to the requirement of specific three-way catalysts in its after-treatment systems. The PC ignition concept (or turbulent jet ignition) can shift the flammability limits towards a higher dilution tolerance, due to enhanced turbulence and increased flame propagation [9; 10], which leads to considerably improved ignition and combustion performance under lean-burning conditions. Depending on whether there is a supply of auxiliary fuel or not, jet ignition is classified into two forms, i.e., active and passive schemes [11; 12; 13; 14]. To investigate the ignition and-flame dynamics, various experimental [15; 16; 17] and numerical studies [18; 19] have been carried out. Different single-fuel [20; 21; 22; 23] and dual-fuel concepts [24] have been validated, and a new reactivity stratification with a hydrogen turbulent jet to enhance ammonia combustion has also been proposed recently [25; 26]. [PERSON] [27] assessed the effects of the PC on combustion characteristics under lean-burning conditions and found that a larger PC volume allowed faster pressure buildup but led to higher heat transfer losses. [PERSON] [28] evaluated the effect of orifice diameter on the jet characteristics and main chamber (MC) combustion in an optically accessible single-cylinder SI engine. High-speed broadband chemiluminescence imaging was used to track the jet penetration and flame front propagation. Despite the highest flame development angle, the PC with a 1.0 mm orifice diameter presented the lowest combustion duration. [PERSON] [29] investigated three passively fueled spark plugs with a stoichiometric mixture and different geometries, including orifice diameter, volume, area, and area-to-volume (A/V) ratio. They found that asymmetric orifice sizes yielded slightly more significant variability and delayed flame development. [PERSON] [30] numerically investigated the effects of passive PC geometry, and their results demonstrated that the highest engine efficiency was achieved at a medium-to-high PC volume and large diameter, both in stoichiometric and lean-burn conditions. [PERSON] [31] explored the EGR dilution limits of a PC-ignited engine operated at stoichiometric conditions and found that the presence of residuals within the PC led to slower spark kernel development. Despite these studies, the effects of the main geometrical parameters of a passive PC on the scavenge and subsequent combustion processes are still not fully understood, especially for heavy-duty gas engine fueling with stoichiometric mixtures and EGR.