Exhaust gas recirculation regulation on combustion cyclic variation and near-zero emission characteristics of a turbocharged direct-injection hydrogen engine under high-speed/high-load conditions

To explore the coupled trade-off between combustion stability and NO _x emissions of turbocharged direct-injection hydrogen engines under high-speed and high-load conditions and fill the research gap in existing studies, this paper investigates the in-cylinder pressure, indicated mean effective pressure (IMEP), combustion phase (CA10/CA50), combustion duration, and NO _x emissions at different EGR ratios (6.0 %, 9.0 %, 12.0 %, and 15.0 %) under high-speed (1700 r·min⁻¹) and high-load (1700 N·m) conditions. Results display that: combustion stability deteriorates with increasing EGR ratio, with the optimal range being 6.0 % ∼ 9.0 %; The cyclic variation of IMEP (COV_IMEP) increases from 3.88 % at 6.0 % EGR to 5.55 % at 9.0 % EGR and rises sharply to 7.42 % at 15.0 % EGR; The maximum in-cylinder pressure ( p _max) increases from 147.2 bar (6.0 % EGR) to 169.1 bar (15.0 % EGR), while the COV of p _max (COV _{p max}) fluctuates, reaching its lowest value of 7.02 % at 9.0 % EGR; NO _x emissions exhibit a significant downward trend, decreasing from 0.18 g·(kW·h)⁻¹ (6.0 % EGR) to 0.07 g·(kW·h)⁻¹ (15.0 % EGR), meeting the near-zero emission standard (<20 ppm). It is verified that an EGR ratio of 9.0 % optimizes the balance between combustion stability (COV_IMEP = 5.55 %, COV _{p max} = 7.02 %) and NO _x reduction (0.13 g·(kW·h)⁻¹), providing critical quantitative guidance for the optimization of combustion control strategies and the breakthrough of zero-emission technologies in heavy-duty hydrogen engines.