The Influence of Split Injection Ratio on Efficiency and NO _x Emissions in a Pilot Diesel-Ignited Hydrogen Direct Injection Engine

This study investigates the impact of the hydrogen split injection ratio on the combustion of pilot diesel-ignited hydrogen direct-injection engines, which is expected to affect hydrogen-air mixture conditions and thus flame propagation and diffusion flame developments. Experiments were conducted on a 1-litre single-cylinder diesel engine equipped with an additional hydrogen injector operating at 35 MPa. Hydrogen accounting for 95% of total input energy was injected at 150 and 60 °CA bTDC for the first and second pulses, which were selected as high-efficiency injection timings from previous equal-split injection tests. The 5% diesel energy was injected near TDC to control CA50 at 10 °CA aTDC. While varying the split ratio between the two hydrogen injections, in-cylinder pressure/aHRR profiles, engine efficiency/power output and engine-out emissions of NO_x and CO₂ were evaluated. Results showed that the hydrogen split ratio does not significantly affect IMEP/efficiency, which consistently achieved a 17.2% increase over the diesel baseline. While CO₂ emissions remained at a very low level due to high substitution of hydrogen energy, they showed no dependency on the split ratio. By contrast, NO_x emissions were highly sensitive to the hydrogen injection split ratio. Increasing the first hydrogen injection fraction to 30% reduced NO_x, attributed to decreased locally rich mixtures formed by late second hydrogen injection and increased lean mixture homogeneity from early first hydrogen injection, leading to a slower burning effect. However, further increasing the first injection fraction led to higher NOₓ emissions due to increased hydrogen compression, which raised TDC and combustion pressure.