Evaluation of CNG Engine Conversion to Hydrogen Fuel for Stationary and Transient Operations

This study investigates the use of hydrogen (H2 ) as a substitute for compressed natural gas (CNG) in a heavyduty (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 H2 . Efficiency slightly decreased, primarily due to knocking at high engine loads and speeds during H2 operation. The reduced torque and power were mainly attributed to the turbocharger being undersized for H2 , 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 H2 engine were lower than with CNG, due to the lower incylinder temperature resulting from the lean mixture strategy, which also contributed to a significant reduction in nitrogen oxides (NOx ) emissions, approximately 2.5 times lower than those with CNG. Despite a notable exhaust energy loss during H2 operation, caused by delayed combustion due to knocking, the lower NOx 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 H2 fueling.