Numerical analysis of the CR-EGR trade-offs for cleaner combustion in a diesel engine with an MD/n-decane biodiesel surrogate

This study uses high-fidelity 3D computational fluid dynamics in ANSYS Forte to investigate the combustion and emission characteristics of biodiesel blends (B20, B50, B75) relative to a baseline diesel (D100). Using a well-characterized n-decane/methyl decanoate binary surrogate, the research systematically explores the thermodynamic impacts of a wide compression ratio (CR) sweep (11:1 to 18:1) from the chemical dilution effects of varying exhaust gas recirculation (EGR) levels. Results indicate that higher biodiesel fractions extend the physical ignition delay and reduce brake power, with the B75 blend exhibiting a 17.48% reduction compared to diesel. Increasing the CR to 18:1 significantly elevates peak cylinder pressure and temperature, with B20 reaching 13.59 MPa and 2913 K. Conversely, applying 20% EGR to the B50 blend at the baseline CR effectively suppresses these thermodynamic peaks by 14.01% and 20.62%, respectively. This EGR-induced thermal suppression achieves a minimum NOx emission of 0.61 g/kg-fuel for B50, representing a 99.58% reduction relative to the baseline. Furthermore, the inherent oxygenated nature of the B50 blend demonstrates significant reductions in carbon monoxide (46.3%) and unburned hydrocarbon (50.18%) emissions compared to D100. While elevated CR and EGR levels generally dampen the peak heat release rate (HRR), the B50 blend demonstrates a notable 52.21% HRR intensification at CR 15 due to complex shifts in spatial combustion phasing. Ultimately, these findings highlight the fundamental 3D in-cylinder trade-offs between thermal efficiency and the formation of kinetic emissions, establishing the need to co-optimize CR and EGR strategies for advanced biodiesel-fueled engines.