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

Abstract

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 (NH 3 ) 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 H 2 injection strategies on main-chamber NH 3 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 H 2 injection strategies. Results show that H 2 injection timing strongly influences the pre-chamber combustion process. Delayed injection timing promotes retention of H 2 inside the pre-chamber, producing overly rich local equivalence ratios around the spark plug, leading to misfire. Injecting H 2 into the pre-chamber earlier allows H 2 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 H 2 injection mainly impacts the quantity of H 2 entering into the main-chamber, modifying the auto-ignition limit of the engine. Therefore, in any practical implementation of the active H 2 pre-chamber concept, the H 2 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. • Turbulent jet ignition, with actively fueled H 2 pre-chamber, applied to NH 3 engines. • Combustion of stratified H 2 -NH 3 mixtures studied with experiments and simulations. • Analyzed the impact of different H 2 injections on the main-chamber NH 3 combustion. • Long injection duration delivers more H 2 to the cylinder, promoting auto-ignition. • Delayed start of H 2 injection leads to unstable combustion, promoting misfire.