Experimental minimization of pilot diesel injection for stable hydrogen combustion in compression ignition engines

• A diesel engine was modified to operate with hydrogen as the primary fuel. • Hydrogen ignition was controlled by a pilot injection of diesel fuel. • The novelty lies in experimentally characterising the minimum pilot diesel amount. • Thermal efficiency was improved, and NOx emissions and knocking were reduced. • Water injection is a key strategy for enabling the operation of hydrogen engines. Hydrogen has emerged as a promising alternative fuel due to its zero-carbon combustion. However, its use in compression ignition engines presents challenges, including abnormal combustion phenomena, which limit the maximum attainable load, and increased nitrogen oxides emissions. This study explores the use of hydrogen as the primary fuel in a compression ignition engine, implementing a novel strategy where diesel is employed solely as a minimized pilot ignition source. This approach departs from conventional dual-fuel combustion methods by significantly reducing diesel usage while maintaining stable combustion. The main novelties consist of determining the lowest feasible diesel quantity required for stable combustion and demonstrating the improvement in performance resulting from reduced diesel pilot injection. The experimental investigation was conducted on a modified turbocharged compression ignition engine, incorporating water injection as a strategy to mitigate abnormal combustion and nitrogen oxides formation. The study analyses the effects of key operating parameters, including start of injection timing, compression ratio, and engine speed, on combustion stability and performance. The results highlight the critical role of pilot diesel injection in achieving stable combustion while minimizing carbon dioxide emissions. Furthermore, maximizing hydrogen energy share leads to enhanced thermal efficiency without compromising engine operability. Additionally, water injection effectively reduces nitrogen oxides emissions and delays the onset of knock combustion, despite its slight negative impact on efficiency. Compared to other operational strategies, the approach proposed here improves thermal efficiency (by 1–3 %), reduces knock tendency (peak pressure rise rate < 5 bar/CAD), and lowers nitrogen oxides emissions (by 30–50 %).