Experimental and numerical investigation of ammonia-water direct injection nozzle configurations for knock suppression in gasoline spark-ignition engines

This study experimentally and numerically evaluates the impact of ammonia-water direct injection (DI) nozzle configurations on knock suppression in gasoline spark-ignition (SI) engines under high-load conditions. The original six-hole injector was modified into two single-hole variants: DI-1 (nozzle holes oriented toward the piston) and DI-2 (holes directed toward the cylinder head). Experiments analyzed 12 end-of-injection (EOI) timings (−330°CA to 0°CA ATDC) and varying direct injection ratios (DIr) to assess knock suppression, combustion stability, and temperature distribution. Numerical simulations using CONVERGE software validated experimental results and provided insights into in-cylinder spray dynamics and flame propagation. Findings revealed that DI-1 significantly outperformed DI-2, achieving a broader effective knock suppression range (KI reduction up to 42 %) at minimal pulse widths (DIr = 3.5 %) due to enhanced cooling of the end-gas mixture via piston-directed spray alignment with tumble flow. Early-phase injection near bottom dead center (BDC, −180°CA ATDC) optimized cooling efficiency by maximizing spray dispersion during compression, whereas delayed injection post-BDC diminished suppression due to altered spray-wall interactions. The original six-hole injector, despite requiring higher DIr (9.5 %), demonstrated robust cooling performance but faced challenges in precise low-flow control. Combustion analysis highlighted that ammonia-water injection reduced peak cylinder pressure (by 8–12 %) and delayed combustion phasing (CA50 by 4–6°CA), necessitating a balance between DIr and ignition timing to avoid excessive retardation. Temperature field simulations confirmed DI-1's superior peripheral cooling, reducing high-temperature regions (>730 K) by 18 % compared to DI-2. This work underscores the viability of ammonia-water as a knock inhibitor and proposes nozzle orientation optimization as a critical pathway for minimizing inhibitor consumption while maintaining thermal efficiency. The results provide actionable insights for designing high-performance DI systems in downsized, high-efficiency SI engines.