A review of comparative studies on injection strategies of spark ignition internal combustion engines with ammonia-hydrogen blends

Ammonia-hydrogen fuels are promising for low-carbon spark-ignition (SI) engines, but their performance strongly depends on fuel-delivery and combustion-control strategy. This review compares port fuel injection (PFI), direct injection (DI), PFI + DI, ammonia-hydrogen dual direct injection (AHDI), cooperative multi-injection (CMI), and active pre-chamber ignition within a unified framework. PFI promotes homogeneous mixtures and stable combustion but offers limited stratification control and higher flashback risk at high H 2 fractions. DI-based strategies enable controllable mixture stratification and stronger regulation of peak pressure and heat release. CMI and active pre-chamber ignition can shorten combustion duration, extend lean operation, and improve indicated thermal efficiency. Emission responses are strongly condition-dependent: optimized stratification or multiple injection can reduce NO x and NH 3 slip, whereas hydrogen-rich high-temperature combustion may increase NO x . No single strategy is optimal for all operating conditions.