Impact of hydrogen injection strategies on ammonia internal combustion engines ignited with active pre-chambers

The ongoing global energy transition towards renewable zero-carbon energy carriers demands a disruptive evolution of the combustion process inside internal combustion engines (ICEs). In many ways, ammonia (NH3) is an ideal candidate as future energy carrier due to the absence of carbon content, a well-established renewable production process, and high liquid energy density. However, ammonia has significantly higher minimum ignition energy and lower combustion speed than fossil fuels, representing a significant research challenge for traditional premixed combustion systems. In this work, an active pre-chamber ignition concept is explored on a premixed ammonia-air engine configuration, with hydrogen as the directly-injected fuel into the pre-chamber. This solution combines the advantages of volumetric ignition (turbulent jet ignition) with high-reactivity of hydrogen, overcoming the high ignition energy and low flame speed of ammonia. Specifically, this investigation is focused on the impact of H2 injection strategies on main-chamber NH3 combustion development. First, an experimental activity is conducted on a flexible research engine configuration, modified for active pre-chamber operation. Then, a 3D computational fluid-dynamics (CFD) analysis examines complex phenomena affecting the dual-fuel, inhomogeneous premixed combustion process in terms of flame development and highlight challenges related to H2 injection strategies. Results show that H2 injection timing strongly influences the pre-chamber combustion process. Delayed injection timing promotes retention of H2 inside the pre-chamber, producing overly rich local equivalence ratios around the spark plug, leading to misfire. Injecting H2 into the pre-chamber earlier allows H2 to emerge from the pre-chamber nozzles and distribute throughout the main-chamber prior to ignition, which accelerates combustion in the cylinder. Additionally, the duration of the H2 injection mainly impacts the quantity of H2 entering into the main-chamber, modifying the auto-ignition limit of the engine. Therefore, in any practical implementation of the active H2 pre-chamber concept, the H2 injection strategy is a critical parameter to be optimized. Novelty and Significance Statement The novelty of this research is the understanding of the impact of active pre-chamber hydrogen injection on the turbulent jet ignition of a premixed ammonia-air mixture, and the subsequent turbulent combustion propagation inside the main chamber of an internal combustion engine. An elongated injection duration favors auto-ignition phenomena, promoting greater thermal and combustion efficiencies, while a delayed start of injection leads to unstable main-chamber ignition and possible misfire. This insight is achieved through a combined experimental-numerical research study, where 3D CFD is employed as diagnostic tool for a deeper understanding of the experimental findings. This research is significant because it demonstrates how hydrogen direct-injection enables actively-fueled pre-chamber ammonia ICEs. This technology is extremely promising in the energy transition context because it minimizes the need for high levels of hydrogen blending, enabling on-board hydrogen generation from catalytic dissociation of ammonia, a more efficient and economic solution than hydrogen storage.