Optical diagnostics on the combustion characteristic of ammonia pre-chamber ignition under different thermodynamic boundary conditions

The carbon-neutral property of ammonia positions it as a highly promising fuel for engines. However, ammonia exhibits poor flammability and low flame speed. Pre-chamber ignition (PCI) technology can generate multiple distributed ignition sites and significantly improve engine combustion. A passive pre-chamber with a narrow throat was designed. Natural flame luminosity imaging was applied to investigate the effect of thermodynamic boundary conditions (i.e., spark timings, intake temperatures (Tin) and compression ratios (CR)) on the combustion characteristics and jet flame development mechanism in ammonia pre-chamber ignition mode. The results present better combustion performance when the spark timings are between −20 crank angle degree (°CA) and −30 °CA after top dead center for ammonia PCI mode. Too early or late ignition may cause jet flame quenching during the jet discharge period. The combustion performance of the ammonia PCI mode can be significantly improved by increasing Tin (from 25 °C to 100 °C), which accelerates the jet flame penetration and shortens both the jet discharge period and combustion duration. A lower CR of 11 results in severe misfires, and increasing CR to 14.5 effectively improves the combustion performance of ammonia PCI mode, accompanied by more uniform jet flame propagation.