Effects of Methanol Injection Pressure and Diesel Injection Timing on Combustion and Emissions Characteristics in a Diesel–Methanol Dual-Direct-Injection Engine

To advance the performance of diesel–methanol dual-direct-injection (DMDDI) engines, a critical low-carbon powertrain solution, this study conducts a systematic experimental investigation into how methanol injection pressure (pM) and main diesel injection timing (θD2) regulate combustion dynamics, thermal efficiency, and pollutant emissions. Key findings reveal that advancing θD2 from −8.0°CA to −12.0°CA aTDC significantly enhances combustion intensity by extending the premixed combustion phase, with the maximum cylinder pressure (pmax) improved, combustion duration (dc) shortened, and indicated thermal efficiency (ηi) increased. Notably, increasing pM from 5.0 to 15.0 MPa optimizes fuel atomization and air–fuel homogeneity, with ηi to a peak of 41.5% and brake-specific indicated soot (BSsoot) to a minimum of 0.062 g·(kW·h)−1; however, further increasing pM triggers severe wall-wetting effects, which degrades ηi and raises BSsoot. For emissions, advancing θD2 or increasing pM (up to 15 MPa) elevates brake-specific indicated NOx (BSNOx), driven by intensified high-temperature premixed combustion, while concurrently reducing brake-specific indicated HC and CO via enhanced fuel–air homogeneity and pollutant oxidation. The optimal operating condition, identified as pM=15.0 MPa and θD2=−12.0°CA aTDC, achieves a superior trade-off between thermal efficiency and pollutant control. This work provides pivotal guidance for the design and calibration of DMDDI engine injection systems, laying a foundation for the industrial application of this low-carbon powertrain technology.