Exploring the Potential of Hydrogen Opposed Piston Engines for Single-Cylinder Electric Generators: A Computational Study 2023-24-0128

One of the main challenges related to the use of Hydrogen in Internal Combustion Engines is the trade-off between NOx emissions and brake power output: on the one hand, a lean premixed charge (Lambda ≈2.5) is generally able to provide a regular and efficient combustion, yielding near-zero NOx emissions; on the other hand, the power density tends to be very poor, due to the huge amount of air required by the thermodynamic process. As a further penalization, the injection of a gaseous fuel during the intake process has a negative impact on volumetric efficiency. Supercharging can be a solution for addressing the problem, but at the cost of an increase of complexity, cost and overall dimensions. An alternative path is represented by the 2-stroke cycle, and, in particular, by the opposed piston (OP) design. Most of the existing OP engines are compression ignited, but Spark ignition and direct fuel injection can be implemented without relevant modifications to the layout of cylinders. The goal of this paper is to assess the potential of Hydrogen OP engines, by means of a simplified but physically consistent numerical approach, focused on a relatively simple application, i.e. a single cylinder electric generator delivering 45 kW at 3000 rpm. The full modularity of the concept permits to get different power ratings with no change to the cylinder design. Moreover, a higher power density can be achieved by increasing the engine speed. The study is carried out by means of CFD 1D simulations (GT-Power by Gamma Technologies): the model is based on a previous study on OP diesel engines, and it is supported by some specific CFD 3D analyses. The predictive combustion model is calibrated on the basis of experimental data from literature, obtained on a 4-stroke turbocharged H2 engine. The numerical results obtained on the optimized model suggest that the proposed engine can deliver 30 kW at 2000 rpm, with a brake thermal efficiency of about 50%, along with near zero emissions. Further work is obviously required to confirm these encouraging preliminary results: in particular, a more sophisticated approach is necessary to investigate the injection and combustion processes in the specific operating conditions.