Application of hydrogen single and split injection strategies in a direct-injection pilot-diesel ignition engine

• 95% hydrogen energy achieved in a 1 L/cylinder diesel engine. • Hydrogen injection timing switches combustion mode related to mixture formation. • Advancing second hydrogen injection timing improves power output and efficiency. • Late hydrogen second injection timing effectively reduces NO x emissions. • Up to 95.2 % CO 2 and 84.8 % NO x reduction achieved compared to the diesel baseline. This study applies hydrogen direct injection to a pilot diesel ignition engine with a 1-litre/cylinder displacement volume for its versatile and wide variety of use in transport, power generation and construction/agricultural machines and thus significant impact on decarbonisation. Previously proven effective in a smaller 0.5-litre/cylinder engine, the technology has been scaled up with increased hydrogen injection pressure from 20 to 35 MPa while keeping the two-injector approach with a conventional centrally mounted diesel common-rail injector and a separate hydrogen direct injector installed on one side of the cylinder head. This new study was required considering increased injection momentum and hydrogen jet developing in a larger piston-bowl space, which is expected to affect hydrogen combustion initiated by diesel pilot flame. The hydrogen injection timing was varied in the entire compression stroke, and the diesel injection timing was adjusted near top dead centre to fix the combustion phasing, which showed a successful engine operation with 95 % hydrogen by energy as a main fuel and 5 % diesel as a pilot flame. The control of hydrogen injection timing showed a combustion mode switch between premixed burn and diffusion burning, resulting in the efficiency-NO x trade-off with the hydrogen diffusion flame effective in NO x reduction at the expense of power output/efficiency. Splitting the hydrogen injection amount by half and fixing the first hydrogen injection timing early at 150 °CA bTDC with varied second injection timing exhibited that a split injection strategy further improves power output and thermal efficiency compared to the single injection strategy through enhanced partially premixed burn. In comparison, the fixed second injection timing late at TDC with varied first injection timing showed a substantial NO x reduction while still maintaining comparable thermal efficiency thanks to a diffusion burning dominated combustion mode. The results suggest simultaneous efficiency increase and NO x reduction is possible in a hydrogen fuelled diesel combustion engine while keeping the significant benefit of CO 2 reduction.