Effects of Injection Timing and Pressure on Combustion Performance and Emissions in Hydrogen Direct Injection Engine

<div class="section abstract"><div class="htmlview paragraph">As the demand for cleaner and more efficient propulsion systems increases, hydrogen internal combustion engines have emerged as a promising solution due to their high thermal efficiency and zero-carbon emissions potential. Achieving ultra-lean combustion conditions (lambda > 2.8) in hydrogen engines significantly improves thermal efficiency while maintaining combustion stability and reducing knock intensity. However, hydrogen injection timing and pressure are crucial factors influencing the combustion and emission characteristics of hydrogen engines.</div><div class="htmlview paragraph">This study investigates the effects of hydrogen injection timing and pressure on the combustion performance and emission characteristics of a direct injection hydrogen engine under different load conditions. Experimental tests were conducted on a multi-cylinder engine equipped with a hydrogen direct injection system, focusing on part-load operation to explore the interplay between injection parameters and engine performance. Results show that retarding the start of injection (SOI) monotonically increased NO<sub>x</sub> emissions, while effective thermal efficiency peaked before declining. At a BMEP of 0.6 MPa, retarding the SOI from 240 deg CA BTDC to 180 deg CA BTDC improved thermal efficiency by approximately 1 percentage point. Additionally, increasing hydrogen injection pressure improved thermal efficiency. However, excessively high pressures reduced mixture homogeneity due to shorter injection durations, longer combustion periods, and increased knock propensity, leading to higher NO<sub>x</sub> emissions. The engine achieved high thermal efficiency at a hydrogen injection pressure of 5 MPa across the range of load conditions tested.</div></div>