Computational Investigation of Reactive Flow and Combustion Characteristics of Biogas-Diesel Dual Fuel in a Compression Ignition Engine

The dual-fuel combustion of biogas and diesel in compression ignition engines offers a pathway for reducing diesel fuel consumption and carbon emissions in power generation applications relevant to rural Nigeria, where biogas production from organic waste is expanding. This study conducts a computational investigation of reactive flow and combustion characteristics in a compression ignition engine operating on biogas-diesel dual fuel at pilot diesel substitution rates of 20%, 40%, and 60% biogas energy fraction. ANSYS Fluent with the G-equation turbulent combustion model and a validated chemical kinetics mechanism for methane-air combustion was used for three-dimensional in-cylinder CFD simulation at engine speed of 1,500 rpm and full load. Biogas compositions of 60% and 70% methane were simulated to reflect typical anaerobic digestion outputs in Nigeria. In-cylinder pressure, heat release rate, peak pressure, ignition delay, combustion duration, indicated thermal efficiency, NOx formation, and soot index were computed for all operating conditions. Results showed that increasing biogas substitution to 40% improved indicated thermal efficiency by 3.1 percentage points over neat diesel, attributed to higher effective octane rating and improved heat release phasing. NOx emissions decreased by 18.4% at 40% substitution due to reduced peak flame temperatures, while soot formation decreased by 31.2% due to hydrogen-rich methane combustion. At 60% substitution, combustion instability was predicted under cold ambient conditions, suggesting a practical operating ceiling. The study provides computational design guidance for dual-fuel engine calibration in Nigerian biogas-to-power applications. Keywords: biogas-diesel dual fuel, CFD, combustion, compression ignition, NOx emissions