A comparative study of the effects of EGR on combustion and emission characteristics of port fuel injection and late direct injection in hydrogen internal combustion engine

Hydrogen internal combustion engine (H2ICE), heralded as the most promising application of hydrogen energy, represents an excellent pathway for achieving energy transformation and carbon neutrality objectives. However, constrained by the primary obstacles of H2ICE, including abnormal combustions and NOX emissions, the port fuel injection hydrogen internal combustion engine (PFI-H2ICE) exhibits some shortcomings in power output despite its relatively lower cost. In comparison, the direct injection strategy is unaffected by air displacement and can achieve stratified combustion by delaying the injection timing to organize the incylinder mixture, known as the Late Direct Injection (LDI), which further enhances power and economy. However, due to the stratified combustion, the NOX emissions of LDI increase significantly compared to the PFI. On the other hand, Exhaust Gas Recirculation (EGR), as the simplest strategy to implement, demonstrates robust effectiveness in balancing power and emissions, and reducing the risk of abnormal combustions. To alleviate concerns about emissions, the integration of EGR is one of the most promising directions for H2ICE. Therefore, this study compares and analyzes the effects and differences of PFI and LDI combined with EGR on combustion and emissions characteristics. The experimental results indicate that the PFI strategy characterized by a homogeneously mixed air-fuel mixture exhibits advantages in maintaining relatively high brake thermal efficiency (BTE) and reducing NOX emissions under low loads due to the implementation of lean burn. The addition of EGR slows down the flame speed and reduces the in-cylinder temperature, leading to a significant reduction in NOX emissions as the EGR rate increases, especially under high loads. On the other hand, due to the air displacement effect, the PFI strategy consumes part of the air entering the cylinder, resulting in a comprehensively lower cylinder pressure and heat release rate (HRR) than LDI, along with equally diminished overall levels in break mean effective pressure (BMEP) and NOX emissions. Nonetheless, when combining EGR and LDI's stratified combustion under high loads, not only an 84.4% reduction in NOX emissions is achieved, but also a comparable or even lower NOX emission level is reached compared to PFI under the same lambda conditions, while maintaining relatively high BTE and BMEP. Therefore, the combination of EGR and LDI is more promising in balancing the power performance and NOX emissions of H2ICE. Moreover, Coefficient of Variation in Indicated Mean Effective Pressure (COVIMEP) are more significantly influenced by spark timing, with relatively minor effects attributed to EGR rates. Given the adoption of Minimum Spark Advance for Best Torque (MBT) and near-MBT in spark timing, the COVIMEP remains below 1.3% across most operational conditions. For H-2 emissions, generally increase with the rise in EGR rate and lambda due to the decrease in flame speed and temperature.