Influence of injection mode and hydrogen enrichment on in-cylinder phenomena in CNG/HCNG engines: a simulation study

This study compares CNG and HCNG combustion under direct and indirect injection using high-fidelity CFD simulations in a standard spark-ignition engine model. The objective is to understand how injection mode and hydrogen enrichment affect in-cylinder thermodynamics, mixture formation, heat release, and emissions in order to identify configurations that improve efficiency while managing nitrogen oxides (NOx) and other pollutants. Four fuel/injection configurations are examined: CNG-DI, CNG-IDI, HCNG-DI, and HCNG-IDI in a 1.6 MPI engine, a compression ratio of 14:1, and a swept volume of 1598 cc. In this engine, we applied CNG mass of 4.96496 mg and hydrogen mass corresponding to 20% in HCNG, i.e., 0.23504 mg. The governing mass, momentum, energy, and species equations are solved with detailed chemistry to capture ignition delay, flame propagation, and combustion stability. Key performance indicators include cylinder pressure and temperature histories, heat release rate (HRR), mean heat transfer coefficient (HTC) at the piston head, combustion fluctuation, and emissions indices (CO₂, NO) across the crank-angle range. The results show that direct injection (DI) yields faster energy release, higher peak pressures, and higher peak temperatures due to improved local mixing and earlier flame development. Hydrogen enrichment (HCNG) amplifies these effects. HCNG-DI exhibits the most significant pressure increase, the earliest and highest peak temperatures (approximately 1900–2000 K), and the highest HRR. Conversely, IDI produces a smoother energy release with lower peak pressures and broader ignition timing sensitivity. This offers enhanced operability and reduced carbon buildup, though it typically results in lower peak efficiency. HCNG-IDI increases NO formation due to extended high-temperature residence and stronger heat release; however, CO₂ production also increases, indicating improved combustion completeness under HCNG. These findings inform engine design regarding injection strategies and hydrogen enrichment, guiding the timing, duration, and spray characteristics for various operating conditions. The study underscores the importance of simulation-driven evaluations in selecting DI or IDI configurations and HCNG blending levels to balance power output, emissions, and reliability in future clean vehicle applications.