Numerical investigation of ammonia flame quenching in pre-chamber engines

Ammonia is a promising carbon-free fuel for internal combustion engines; however, its low flame speed and poor ignition stability limit the engine combustion performance. Passive pre-chamber ignition (PCI) can enhance ignition through multiple reactive jet flames, but the effects of pre-chamber throat and orifice geometries on jet flame stability and quenching under ammonia-fueled conditions remain unclear. In this study, a numerical model of passive pre-chamber engine fueled by pure ammonia was established and experimentally validated against cylinder pressure, heat release rate (HRR), and natural flame luminosity (NFL) of ammonia flames. Three throat/orifice geometries were studied. The impacts of pre-chamber throat and orifice diameters on jet flame temperature, OH radical distribution, flow characteristics, and wall heat transfer within the throat-orifice region were analyzed. Results show that a high pre-chamber to main-chamber pressure difference (ΔP) and jet velocity do not necessarily accelerate ammonia combustion. Compared to the baseline engine with a throat diameter of 5 mm and orifice diameter of 2 mm, reducing the orifice diameter to 1.4 mm or reducing the throat diameter of 3 mm decreases the indicated thermal efficiency (ITE) from about 36% to 34% and 33%, and increases unburned NH3 from 33 to 36 and 38 g/kW·h, respectively. The ammonia combustion rate and stability are significantly affected by flame quenching. Reducing throat or orifice diameter enhances jet momentum but increases wall heat transfer significantly, leading to lower temperature and OH levels at the orifice exit and thereby reducing the jet flame stability. A quenching tendency index (QTI) is proposed to quantify the local competition between chemical heat release and wall heat loss, classifying orifice flames into robust, weakened, and near-extinction regimes. Pre-chambers with lower heat-transfer losses sustain unquenched orifice flames with higher temperatures and OH levels, enabling faster main-chamber combustion. Overall, high pre-chamber jet velocity must be accompanied by unquenched flames at the orifice to achieve high ammonia engine combustion efficiency.