Multijet prechamber ignition using hydrogen and methane: effects of fuel composition and orifice diameter on ammonia combustion in a large-bore chamber

• A large-bore optical chamber was used to visualize multijet prechamber ignition. • Hydrogen and methane were independently supplied to the prechamber to assess ignition effects. • Ammonia or methane was used in the main chamber to evaluate combustion characteristics. • The influence of orifice diameter on flame propagation and combustion stability was quantified. • The results provide design guidance for ammonia-fueled large-bore marine engines. The combustion characteristics of ammonia in a large-bore combustion chamber simulating a marine engine with a multiple turbulent jet ignition system were investigated to clarify the effects of the prechamber fuel composition and orifice diameter on the ignition and flame propagation under interactions between multiple prechamber jets. The combustion behavior was analyzed using high-speed imaging and pressure measurements in a constant-volume combustion chamber. In addition, emission characteristics were evaluated using Fourier-transform infrared (FTIR) spectroscopy. The results indicate that hydrogen-fueled prechamber jets enhance ammonia ignition, reducing ignition delay and combustion duration. Conversely, methane-fueled prechamber jets result in ineffective ignition and delayed combustion. The study also revealed that an optimal orifice diameter stabilizes combustion, improves flame penetration, and minimizes instability caused by flame quenching, re-ignition, and shockwave interactions. Specifically, the results showed that a 3.5 mm orifice provides the optimal balance between penetration and stability among tested orifice diameters. In contrast, the 2.5 mm orifice suffered from excessive cooling losses at the orifice wall and unstable flame propagation due to strong, narrow jets, while the 5.0 mm orifice exhibited short penetration and unburned regions near the chamber wall caused by reduced jet velocity and strong jet–shockwave interference. These findings support the optimization of ammonia combustion in large-bore marine engines and serve as a valuable reference for future computational fluid dynamics and engine optimization studies, contributing to the advancement of ammonia-fueled marine engine technology.