Unlocking ammonia engines: pre-chamber ignition with partial ammonia cracking

Ammonia combustion suffers from poor flammability and sluggish combustion. Pre-chamber ignition (PCI) technology enhances combustion stability and flame speed. Hydrogen addition further augments fuel mixture reactivity. Combining these techniques may synergistically improve the ammonia combustion performance. A passive pre-chamber (PC) featuring narrow throat geometry was developed, and partial ammonia cracking was applied to enhance the ammonia combustion in this study. The impact of partial ammonia cracking on combustion behavior under the PCI and spark ignition (SI) modes was investigated. The results reveal that increasing the ammonia cracking ratio (γ) from 0% to 10 % effectively improves the combustion performance in both ignition modes. For the PCI mode, the combustion process consists of three distinct phases: pre-chamber flame propagation, pre-chamber ignition, and turbulent flame propagation. Increasing γ elevates the pressure difference (ΔP) between the pre- and main chambers, reduces the probability of flame quenching near the PC orifices, and shortens the overall combustion process. Comparative analysis reveals that the combustion performance in the PCI mode is inferior to that of the SI mode under pure ammonia conditions. However, the PCI mode exhibits superior combustion performance with ammonia cracking compared to the SI mode, as evidenced by a significantly higher combustion rate. In the PCI mode, higher ΔP promotes flame development, minimizes ignition delay, and contributes to more stable combustion. Additionally, the impact of spark timing on combustion performance was investigated under high ammonia cracking ratios. Increasing γ postpones the optimal spark timing due to higher combustion rates. Pre-chamber ignition with partial ammonia cracking of only about 10% enables stable and fast ammonia combustion, making it a highly promising pathway for high-efficiency ammonia engines.