including China, Chile, Australia, the United States, and Argentina. This reliance on specific geographic regions also has considerable implications for global geopolitics [1; 5]. From this perspective, internal combustion engines still offer an excellent solution for a rapid transition toward zero emissions. The key to this technology is represented by alternative fuels, such as e-fuels, hydrogen, and ammonia [6]. Although hydrogen enables clean combustion due to its carbon-free nature, it presents a range of non-trivial technological challenges. One of the primary issues derives from its low density. In fact, when hydrogen is injected into the intake manifold (port fuel injection), an approximate 30% reduction in volumetric efficiency can be observed compared to gasoline. To maintain comparable performance levels, direct injection is necessary. This is still the case even if it could jeopardize the integrity of the engine, since due to the wide flammability range of hydrogen, it cannot be injected with the intake valves open to avoid possible backfire [7]. NOx formation during hydrogen combustion still persists despite the absence of carbon in hydrogen molecules, as it depends solely on oxygen and nitrogen in the atmosphere and is not related to the type of fuel. NOx formation is strictly linked to the combustion temperature, which can be reduced by using a lean mixture. Now, the wide flammability range of hydrogen becomes a significant advantage, enabling the use of ultra-lean mixtures (\(\lambda>2.5\)) and leading to nearly-zero NOx emissions. However, operating with such ultra-lean mixtures inevitably leads to a significant drop in specific performance [8]. The challenges mentioned above pave the way for the investigation into new engine architectures or the revisiting of shelved designs [9]. One architecture that was very popular in the aeronautical and marine sectors in the nineteenth and twentieth centuries is the two-stroke opposed-piston (OP) engine [10]. An OP engine consists of two crank mechanisms that share the same cylinder with pistons that move in opposite directions (Figure 1). Specifically, one piston manages the intake phase and the other the exhaust phase by appropriately opening and closing suitable intake and exhaust ports. The movement of these pistons is mainly managed in two ways: with a shaft and a system of connecting rods and levers, or with two crankshafts, one for each crank mechanism, which are mechanically connected using gears, belts, or chains. In this work, attention was focused on the two-shaft configuration. Two-stroke OP engines were first developed in Germany during the late 19 th century and later refined in various countries for numerous applications. Their appeal lay in their straightforward manufacturability, high power-to-weight ratio, and competitive efficiency. These attributes enabled two-stroke OP engines to find widespread use in fields such as aviation, marine propulsion, military vehicles, heavy-duty trucks, and locomotives, securing their relevance through much of the 20 th century. Despite their advantages, two-stroke OP engines have encountered heavy issues related to emissions control, fuel economy, cost-effectiveness, and durability [11]. Over time, four-stroke engines have proven to be more capable of addressing these issues, leading to their dominant position in the global engine market [10; 12]. Never  
Figure 1: 3D model of a generic OP crank mechanism. theless, the application of hydrogen as fuel can open new design strategies for the use of OP engine architectures, as they allow for more air to be introduced into the combustion chamber than conventional reciprocating engines. Furthermore, the 2-stroke operating cycle helps to mitigate performance losses associated with ultra-lean combustion.