Experimental and numerical analysis of the knock phenomenon inside ammonia–hydrogen internal combustion engine

Ammonia is one of the most promising carbon-free fuels for decarbonising sectors reliant on thermal energy conversion, such as power generation and transportation. However, its distinct combustion properties – low laminar burning velocity, narrow flammability limits, and high autoignition temperature – present challenges for stable and efficient ignition in internal combustion engines. To address these limitations, high-compression-ratio engines are often paired with ignition promoters such as hydrogen, which can be produced on-board through ammonia cracking. Nevertheless, the addition of hydrogen increases the risk of knock occurrence under such conditions. This study aims to improve the understanding of knock formation by investigating the chemical kinetics in the unburned gases during the engine cycle. Before engine testing, ignition delay times (IDTs) were measured in a Rapid Compression Machine for ammonia and partially cracked ammonia (PCA) mixtures (90% NH₃, 10% $PCA$: 7.5% H${}_2+2.5$% N${}_2$) under engine-representative conditions (40–70 bar, 950–1000 K, $\Phi =$ 0.5–1.5). Combined with previous IDT data for pure NH₃ and NH₃/10% H₂, the Stagni 2023 mechanism was selected for detailed chemical analysis. Experiments were conducted on a spark-assisted compression ignition engine (CR = 16.4) with fuel blends of pure NH₃, NH₃/10% H₂, and 10% $PCA$. Only the hydrogen-containing blends exhibited knock. The Chemkin Pro SI Engine Zonal Model was employed to simulate in-cylinder chemical evolution. Results indicated that the key reactions driving knock are the consumption of H₂ via H${}_2$ $+$ NH${}_2$ $\leftrightarrow$ H $+$ NH${}_3$ and H${}_2$ $+$ OH $\leftrightarrow$ H $+$ H${}_2$O in the end gases. These pathways contribute significantly to pre-ignition heat release, triggering knock, and explain the absence of knock in pure NH₃ cycles unless sufficient hydrogen is present through residuals or in-situ cracking.