Experimental and numerical study on the performance and exhaust emissions of a Single-Cylinder Natural Gas Large Bore Engine equipped with Passive Pre-chamber

The maritime industry is among the most energy-intensive sectors, and achieving fleet decarbonization is crucial to significantly reduce greenhouse gas emissions. As a transitional fuel, natural gas (NG) presents a viable short-to-midterm solution. Compared to conventional marine fuels, NG has the potential to lower carbon dioxide emissions by approximately 20–30%. However, to fully leverage this potential on carbon footprint reduction, substantial advancements in combustion technologies are required. One promising approach to enhance the efficiency of spark-ignited (SI) NG engines is the implementation of Passive Pre-Chamber (PPC) technology. This strategy enables leaner combustion, improving thermal efficiency, mitigating the occurrence of knocking, and reducing NOx emissions. This study presents both experimental and numerical investigations to analyze the impact of charge dilution and ignition timing on the performance and emissions of a single-cylinder prototype NG PPC SI engine for marine application, retrofitted from baseline diesel architecture. Numerical simulations combining 1D and 3D CFD approaches were used to guide the combustion system and engine component design, optimizing valve timing, compression ratio, and fuel injection parameters to mitigate knock and improve thermal efficiency. Based on numerical simulations, excess air effects on thermodynamic efficiency and flame speed were evaluated. The experimental tests were conducted at 1500 rpm constant engine speed under different load conditions. Variation of the air-to-fuel ratio and spark advance were performed to characterize their effects on engine operation. The results were utilized to validate a 1D model, which demonstrated a high level of accuracy in reproducing the combustion evolution. PPC technology enabled high charge dilution (λ≈1.7). The coefficient of variation of IMEP remained below 1.5% throughout a wide range of λ values and combustion phasing conditions, indicating stable combustion. The engine exhibited indicated efficiencies of over 45%, marginally exceeding the predictions obtained from numerical simulations. This research underscores the potential of PPC technology in enhancing the efficiency and sustainability of NG-fueled marine engines, offering valuable insights to optimize combustion strategies for future low-emission propulsion systems.