Assessing the retrofitability of direct-injection hydrogen technology for off-road engines: Numerical and experimental investigations

Hydrogen-powered internal combustion engines offer a viable pathway toward low-carbon propulsion while leveraging existing engine architectures. This study presents the conversion of a state-of-the-art 2.4 L diesel engine for off-road applications to direct-injection hydrogen operation through an integrated numerical-experimental approach. Three-dimensional CFD simulations were employed to optimize hydrogen injection and mixture formation, with the scope of reducing mixture stratification, while requiring only minor design modifications ( Φ σ / Φ Avg from values greater than 45% to approximately 15–25%). Based on these results, a prototype engine was developed and experimentally tested. The hydrogen engine achieved performance comparable to the baseline diesel unit, reaching 305 Nm and 55 kW with peak indicated thermal efficiency above 40%. Over the LSI-NRTC homologation cycle, engine-out NOₓ emissions reached 0.68 g/kWh, well below EU Stage V and EPA Tier 5 limits. With the integration of an SCR system, tailpipe NOₓ emissions were further reduced to 0.03 g/kWh, approaching near-zero levels, with only a limited torque derating (< 5%). • Diesel-to-H₂ conversion with minimal engine modifications. • CFD-driven optimization reduces mixture stratification. • Diesel-equivalent performance: 305 Nm, 55 kW, ITE > 40%. • Ultra-low NOx emissions: 0.68 g/kWh (engine-out), 0.03 g/kWh with SCR.