Influence of equivalence ratio and colorless distributed combustion on combustion characteristics and emissions in a heavy-duty natural gas-fueled converted engine

The conversion of heavy-duty diesel engines to natural gas spark-ignition engines offers a promising solution for improving engine performance and sustainability. However, it also introduces the challenge of high NO x emissions because of high compression ratios. This study aims to achieve ultra-low emissions by investigating the combined effects of equivalence ratio and Colorless Distributed Combustion (CDC) strategy with a three-dimensional numerical simulation. Specifically, the research focuses on the performance and emissions characteristics of a heavy-duty diesel engine converted to spark-ignition operation using methane fuel. To address this objective, three equivalence ratios (0.6, 0.7, and 0.8) were examined under varying oxygen concentrations (23%, 21%, 19%, 17%, and 15% by mass), where dilution was achieved by introducing nitrogen (N 2 ) as an inert diluent to promote distributed combustion conditions. A three-dimensional computational fluid dynamics (CFD) model incorporating a reduced methane chemical mechanism was employed to capture in-cylinder flow, heat release, and pollutant formation. The G-equation combustion model was coupled with the RNG k-ϵ turbulence model to simulate the propagation of lean premixed turbulent flames. The CDC regime effectively minimized NO x emissions across all equivalence ratios, with the most favorable balance between efficiency and emissions observed at φ = 0.7 and an oxygen concentration of 19%. These findings provide insights into the optimization of lean CDC strategies in compression ignition engines converted to gas-fueled spark-ignition systems for cleaner combustion.