Optimizing hydrogen utilization in RCCI and dual-fuel engines with hybrid electric integration

• Hybridization enables 90 % hydrogen share under varying torque demands in engines. • Engine-only mid-load limited to 40 % HER with high NOx. • HEV mode demonstrated superior efficiency and negligible emissions. • Parallel hybrids ensure optimal performance and low emissions across all loads. • Hydrogen-fueled HEV systems offer a viable pathway for sustainable transportation. This study presents an experimental approach to enhancing hydrogen utilization in hydrogen-diesel dual-fuel and reactivity-controlled compression ignition combustion strategies by integrating a parallel hybrid electric system. At higher engine loads, hydrogen substitution becomes constrained by severe knocking and elevated NOₓ emissions. To overcome this constraint, a diesel engine was modified into a cost-effective parallel hybrid configuration using a belt-driven torque coupling mechanism and electromagnetic clutches, allowing three operational modes: engine-only, motor-only, and hybrid mode. The experimental investigation was conducted at load conditions of 7.2 Nm, 14.5 Nm, 21 Nm and 29 Nm, selected to represent low and mid-load operations. Hydrogen was incrementally introduced under consistent injection parameters derived from previous research. In engine-only mode, results showed that low-load operation supported up to 90 % hydrogen energy share with low emissions but moderate efficiency. At mid-load, 40 % hydrogen energy share yielded higher efficiency but was accompanied by increased NOx and knock, limiting further hydrogen substitution. Incorporating the hybrid system enabled higher load operations with a 90 % hydrogen energy share, where the motor supplemented engine torque. This configuration eliminated knock and facilitated ultra-low emission levels while significantly improving thermal efficiency. The belt-driven hybrid mechanism provided a flexible and economical platform for studying torque supplementation in dual-fuel systems. The findings underscore the effectiveness of hybridization in addressing hydrogen integration challenges in medium-duty engines. This approach supports the development of clean, high-efficiency propulsion systems and advances the viability of hydrogen as a sustainable transportation fuel.