Hydrogen production in ammonia-fueled spark ignition engines

Anhydrous ammonia (NH₃) is a promising carbon-free fuel for internal combustion engines (ICEs) that can be generated from renewable energy sources and stored at high energy density. Ammonia is typically mixed with a small fraction of hydrogen (H₂), which serves as combustion promoter to achieve stable combustion over a large range of engine operation. To overcome the difficulties of storing hydrogen, ammonia can be converted to hydrogen onboard by means of a catalytic reactor fed by waste thermal energy from the engine exhaust or by partial oxidation. This work explores an alternative method of producing hydrogen by partial oxidation of fuel-rich ammonia–air mixtures to hydrogen in an engine cylinder while generating useful work. Such an approach eliminates the need for costly catalyst materials and poses the possibility of a dedicated reforming cylinder to provide hydrogen for other cylinders in a multi-cylinder engine. Experiments were conducted in a single-cylinder spark-ignition (SI) engine with parametric sweeps of intake temperature, intake pressure, equivalence ratio, and spark timing. Experimental results show that 6% exhaust gas hydrogen concentration is attainable and 40% of the unburned fuel energy can take the form of hydrogen. Rich equivalence ratios produced higher concentrations of hydrogen and elevated intake temperatures extended the rich stable-operation limit of the engine. Low order chemical modeling yields insights into hydrogen production throughout the engine cycle, revealing that initial consumption of ammonia is advanced of hydrogen production and that competing elementary reactions for the formation and destruction of hydrogen drive its net quantity in the exhaust.