Effects of hydrogen peroxide in NH3/H2 homogeneous charge compression ignition engine

To address the strong resistance of NH3/H2 mixtures to autoignition, this experimental study explores the use of aqueous hydrogen peroxide (H2O2aq) as an ignition promoter in a homogeneous charge compression ignition engine with a compression ratio of 16.4:1. Experimental results show that the injection of 0.67% by vol. H2O2aq (relative to the NH3/H2/air mixture): (i) significantly reduces the burn delay by approximately 12.9% through faster fuel oxidation, (ii) raises the in-cylinder pressure peak by 15.4%, and (iii) accelerates the heat release. When the H2 fraction is reduced from 12% to 8% by vol., CoVimep rises from 4.7% to 8.0%. A 42.5% increase in the injected H2O2aq mass then reduces CoVimep by 30% and improves the indicated mean effective pressure by 4%. An increase of 202.4% in the injected H2O2aq mass reduces CoVimep to 4.1% and restores the indicated mean effective pressure to 8.0 bar , while lowering the exhaust gas temperature by 26.1 K . Finally, promising results are obtained by extending the lean limit of the NH3/H2 mixture, reducing the equivalence ratio from 0.6 to 0.5. A 0D kinetic analysis of the ignition delay time under relevant engine conditions reveals that the addition of H2O2aq increases the availability of OH and HO2 radicals. Early OH production is promoted via H2O2(+M)↔2OH(+M). The greater OH availability shifts NH3 consumption toward the OH-driven pathway, i.e., NH3+OH↔NH2+H2O, while the contribution of NH3+H↔NH2+H2 decreases. The resulting NH2 pool is mainly consumed through the NH2 →H2NO→HNO→NO→N2 route, with NH2+NO2 ↔H2NO+NO becoming increasingly relevant, whereas the N2O production route remains secondary. Sensitivity analysis further confirms the role of NH2+NO2 ↔H2NO+NO in promoting ignition delay time reduction, while NH3+HO2 ↔NH2+H2O2 shifts toward an inhibiting effect by partially limiting the direct conversion of H2O2 into OH radicals. Novelty and significance statement: The high resistance to autoignition of ammonia (NH3)/hydrogen (H2) blends represents a major challenge for compression ignition (CI) engines, requiring very high compression ratios and elevated intake thermodynamic conditions. For the first time, this study experimentally investigates the use of aqueous hydrogen peroxide (H2O2aq) as an ignition promoter to enhance the combustion of NH3/H2 mixtures in a Homogeneous Charge Compression Ignition (HCCI) engine. The engine experiments reported in this manuscript provide the first comprehensive analysis of the influence of H2O2aq on the performance of an HCCI engine fueled with NH3/H2 under various operating conditions. The results demonstrate that H2O2aq significantly enhances NH3/H2 mixture reactivity, reducing burn delay and stabilizing engine operation under conditions that would otherwise lead to unstable combustion, such as reduced H2 fraction or lean mixture operation. These findings contribute to advancing low-carbon combustion strategies and optimizing engine performance, supporting the transition toward a net-zero carbon society.