Evaluation of CNG engine conversion to hydrogen fuel for stationary and transient operations

This study investigates the use of hydrogen (H 2 ) as a substitute for compressed natural gas (CNG) in a heavy-duty (HD) six-cylinder engine, focusing on both port fuel injection (PFI) and direct injection (DI) systems. Numerical modeling in a 0D/1D environment was employed, simulating engine operation under stationary conditions and during the worldwide harmonized transient cycle (WHTC) and worldwide harmonized vehicle cycle (WHVC) homologation cycles. Results indicated a reduction in torque (7% for direct injection and 21.5% for port fuel injection) and power (32% for direct injection and 35.5% for port fuel injection) when switching from CNG to H 2 . Efficiency slightly decreased, primarily due to knocking at high engine loads and speeds during H 2 operation. The reduced torque and power were mainly attributed to the turbocharger being undersized for H 2 , given its low density and the lean mixture combustion strategy used. Upgrading the turbocharger or implementing a two-stage compressor could restore or even improve torque and power levels compared to CNG. Heat transfer losses in the H 2 engine were lower than with CNG, due to the lower in-cylinder temperature resulting from the lean mixture strategy, which also contributed to a significant reduction in nitrogen oxides (NO x ) emissions, approximately 2.5 times lower than those with CNG. Despite a notable exhaust energy loss during H 2 operation, caused by delayed combustion due to knocking, the lower NO x emissions and absence of carbon emissions are crucial for reducing pollution. During vehicle cycles, selecting an optimal gear-shift strategy is critical to mitigating the performance gap resulting from reduced torque and power with H 2 fueling. • Direct and port fuel injection are compared in a HD SI engine fueled by H 2 and CNG. • High injection flow rates are essential due to hydrogen’s low density. • Hydrogen fueling eliminates carbon emissions and significantly reduces NO x emissions. • Cylinder filling must be improved with better compressors or a two-stage system. • The autoignition of H 2 affects combustion phasing with an increase of exhaust losses.