Based on a three-dimensional simulation platform, this study investigated the effects of N2, CO2, and O2 as dilution gases on the combustion and emission performance of methanol engines under constant total fuel quantity, dilution ratio of 10%/20%, and blending ratio of 0%/10%/20% conditions. In terms of combustion performance, diluting the syngas can shorten the ignition delay period and combustion duration, advance the combustion center, increase the peak cylinder pressure and temperature, and the effect is most significant when CO2 is diluted (for example, in the 20% dilution ratio CO2 condition, the 20% blending ratio is 3.5 °CA shorter than 0%); the ignition delay period and combustion duration become longer, the cylinder pressure decreases under CO2 conditions, and slightly increases under N2 and O2 conditions. In terms of emission performance, NOX emissions are CO2 < N2 < O2 (at a 10% dilution ratio and 10% blending ratio, CO2 relative to O2 reduces NOX by 98.5%); under O2 conditions, syngas blending reduces CO emissions, while under N2 and CO2 conditions, it increases, and under CO2 with a 20% dilution ratio, CO sharply increases; syngas blending reduces the emissions of HC, CH3OH, and CH2O, and under O2 dilution, the CO emissions are the lowest, and they increase as the dilution ratio increases. In summary, N2 dilution is beneficial for the coordinated optimization of combustion and NOX emissions.
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