Optimization of turbocharged direct-injection hydrogen internal combustion engine for high-altitude operation: a two-stage turbocharging approach

This study investigates a two-stage turbocharging system designed to enable efficient and clean high-altitude operation for a 2.0 L direct-injection H₂ICE. A combined experimental and simulation approach was employed. Ground-level engine tests compared the single-stage and the proposed two-stage turbocharging configurations across a wide operating range. A validated one-dimensional simulation model, calibrated with experimental data, was then used to evaluate performance across altitudes from 0 to 8000 m. Experimental results at sea level demonstrated that the two-stage system significantly enhanced performance. At the maximum power point, it increased BTE from 32.2% to 33.7% and power output from 140.4 kW to 145.6 kW. Crucially, it enabled a much leaner combustion, which drastically reduced NOx emissions by approximately 50% by lowering combustion temperatures. Simulation results revealed the system's exceptional altitude-holding capability. The two-stage H₂ICE maintained over 97% of its sea-level power up to 8 km altitude, whereas the single-stage system suffered over 10% power loss by 5 km. Furthermore, at the 2500 rpm high-efficiency cruise point, the two-stage system sustained a BTE above 41% up to 8 km, outperforming the single-stage system's peak efficiency. The proposed two-stage turbocharging system effectively overcomes the critical altitude limitations of H₂ICEs. It ensures minimal power degradation, sustains high efficiency, and enables ultra-lean combustion for drastic NOx reduction across the flight envelope. This work provides a validated technical pathway for the development of high-altitude and zero‑carbon aviation propulsion systems in environmentally sensitive airspace.