# Experimental Activities on a Hydrogen-Fueled Spark-Ignition Engine for Light-Duty Applications
## 1 Introduction  
Hydrogen stands out as one of the alternatives in the pursuit of decarbonizing the automotive sector due to its carbon-free composition, making it a compelling choice for curbing greenhouse gas (GHG) emissions [1]. The EURO VII regulation, designed for the future, not only targets the reduction in nitrogen oxide (NO\({}_{x}\)) emissions but also proposes a significant reduction in carbon dioxide (CO\({}_{2}\)) emissions from vehicle fleets. Specifically, it aims to achieve a 15% reduction from 2021 levels by 2025 and an even more ambitious goal of up to 37.5% by 2030. For light-duty vehicles, the targets are set at 15% and 31%, respectively [2]. Various methods exist for hydrogen production, with green hydrogen being a noteworthy example. Green hydrogen is generated through electrolysis powered by renewable energies and is deemed a crucial component of the broader effort to decarbonize the economy [3, 4]. Beyond its role as a potential fuel for the automotive sector, hydrogen is also being explored as an energy vector to manage the variable energy outputs from renewable sources [5]. However, it is important to note that the majority of hydrogen currently originates from fossil resources. The predominant method of production is steam methane reforming (SMR) [6], a process that contributes to GHG emissions by releasing carbon in the form of CO\({}_{2}\) into the atmosphere. To mitigate this environmental impact, efforts areunderway to integrate carbon capture mechanisms into SMR production [7]. Nevertheless, carbon capture technology remains an ongoing subject of research and development. Hydrogen can find various applications within the automotive industry, primarily in two ways: it can be employed in fuel cells (FCs) or in internal combustion engines (ICEs). FC vehicles boast impressive tank-to-wheel efficiencies, hovering around 50% [8], and produce zero NOx emissions, emitting only water vapor (H2O) as exhaust. Despite these advantages, the dynamic behavior of FC systems in automotive applications poses challenges, significantly reducing the durability of FC stacks [9] and leading to catalyst corrosion. Some well-established engine manufacturers, including Toyota, Honda, and Hyundai, have already introduced commercial FC vehicles. However, the production cost remains high, partly due to the lack of economies of scale [8]. Nevertheless, the reliance on batteries to distribute energy generated by the FC stack comes with its drawbacks, such as the involvement of rare-earth elements and battery degradation, akin to those of battery electric vehicles (BEVs). On the other hand, when hydrogen is applied to ICEs [10], it exhibits a higher tolerance for purity, reduces the use of rare-earth materials, and allows for a relatively straightforward adaptation of existing engine models to replace conventional carbon-based fuels [11]. This adaptability makes this technology a compelling option for the future of the carbon-free automotive sector [12]. Also, the absence of carbon virtually eliminates unburned hydrocarbons (HC), carbon monoxide (CO), and CO2 emissions, with these pollutants being generated only in very small proportions through the combustion of lubricant in the combustion chamber [13]. Recent research efforts have concentrated on adapting engine designs to leverage the unique properties of hydrogen, entailing modifications in fuel supply, combustion chambers, and ignition systems to ensure safety and efficiency. Safety concerns stemming from hydrogen's low ignition energy and wide flammability range have prompted research into secure handling and storage, especially in vehicle applications [14]. Beyond engine adjustments, efforts have been directed towards broader infrastructure considerations, including hydrogen production, transportation, and distribution for widespread use [15]. The ongoing research is also focused on advancements in materials science, catalyst development, and general engine technology to enhance hydrogen compatibility as a fuel in spark-ignition engines.