Effects of EGR rate on combustion and emission characteristics of a diesel-piloted hydrogen/diesel dual-fuel free-piston engine

Aiming at the coordinated optimization of combustion stability and exhaust emission control in free-piston hydrogen engines, a dual-piston free-piston hydrogen engine was investigated in this study. A coupled thermodynamic and chemical-kinetic simulation model was established and validated against experimental measurements obtained under one representative medium-load baseline condition, and the effects of the EGR rate on engine combustion, in-cylinder flow motion, heat transfer, and emission characteristics were analyzed using a three-dimensional CFD method. The results show that, under the unconstrained operating strategy with fixed diesel injection timing and intake pressure, as the mass-based EGR rate increases from 0% to 25%, the peak in-cylinder pressure, peak temperature, peak heat-release rate, and cumulative heat release gradually decrease, indicating that EGR weakens the in-cylinder combustion reactivity. Meanwhile, the in-cylinder flow velocity, turbulent kinetic energy, and wall heat-transfer intensity decrease with increasing EGR rate, and the total cylinder-wall heat loss per cycle decreases from approximately 88 J to 75 J. In terms of emissions, the specific NO emission decreases significantly from approximately 0.295 to 0.0271 g/kWh as the EGR rate increases from 0% to 25%. By contrast, the specific SOOT emission initially decreases and subsequently increases, reaching its minimum value of approximately 2.58 × 10 −10 g/kWh at an EGR rate of 15%. Although a high EGR rate further reduces the specific NO emission, it weakens combustion intensity, inhibits SOOT oxidation, and reduces combustion completeness. A four-dimensional dimensionless multi-objective evaluation model incorporating specific NO emission, specific SOOT emission, cyclic cumulative heat release, and indicated thermal efficiency was developed. The results show that an EGR rate of 15% provides the best overall performance under the unconstrained operating strategy and the adopted multi-objective evaluation criteria, achieving a favorable balance among stable heat release, high thermal efficiency, and simultaneously low NO and SOOT emissions. Under fixed-CA50 and constant-indicated-power constraints, the diesel injection timing and intake pressure were coordinately adjusted, maintaining CA50 within 379.70–379.77 °ECA and limiting the total indicated-power error to less than 1%. Under these constraints, peak in-cylinder pressure and temperature exhibited non-monotonic variations with increasing EGR rate, the specific NO emission no longer decreased monotonically, and a pronounced NO–SOOT trade-off remained. These results indicate that the EGR rate should be optimized jointly with diesel injection timing and intake boosting.