The influence of split injection ratio on efficiency and NOx emissions in a pilot diesel-ignited hydrogen direct injection engine

This study investigates the impact of 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 burning. Experiments were conducted on a 1-litre single-cylinder diesel engine equipped with an additional hydrogen injector operating at 35 MPa. Hydrogen of 95% total energy was injected at 150 and 60 °CA bTDC for the first and second pulses, which were selected as high efficiency injection timings from the previous equal split injection tests. The 5% diesel energy was injected near TDC to fix CA50 at 10 °CA aTDC for all tested split injection ratios. While varying the split ratio between the two hydrogen injections, in-cylinder pressure/aHRR profiles, engine efficiency/power output and engine-out emissions of NOx and CO2 were evaluated. Results showed that the hydrogen split ratio does not make a significant impact on IMEP/efficiency, which consistently achieved 17.2% increase over the diesel baseline. Detailed analysis of burn duration and peak combustion pressure indicated it was due to compounding effects of increased mixture homogeneity and thus slower burning for higher first hydrogen fraction but increased peak combustion pressure related to higher hydrogen gas compression during the compression stroke. While CO2 showed a very low level simply due to 95% hydrogen energy operation with no measurable impact of the hydrogen injection split ratio, NOx emissions exhibited very high sensitivity to the hydrogen injection split ratio. The increased first hydrogen injection fraction up to 30% led to reduced NOx, which was related to decreased locally rich mixtures formed from late second hydrogen injection and increased lean mixture homogeneity from the early first hydrogen injection – i.e. slower burning effect. For the hydrogen first injection fraction higher than 30%, however, NOx was increased as the combustion temperature became higher due to increased hydrogen gas compression and thus TDC pressure – i.e. higher combustion pressure effect.