Fuel slip in hydrogen-powered port-fuel-injected large-bore stationary engines under stable operation and in the presence of combustion anomalies

Hydrogen slip in large-bore, port fuel-injected (PFI) engines limits their efficiency and stable operating range. This study combines experimental data from a single-cylinder engine with 3D CFD simulations to investigate slip sources under stable and anomalous combustion. An optimized operating map is presented, achieving a minimum hydrogen slip of 430 ppm and high indicated efficiencies of up to 46.6%, with stable operation limited to 22.4 bar indicated mean effective pressure. 3D simulations identify valve overlap scavenging and crevices near the piston top land as the primary slip sources. While singular pre-ignition and knocking events show no correlation with increased slip, ongoing backfire leads to exceptionally high emissions of up to 34,600 ppm. The findings suggest that hydrogen slip is a consequence, rather than a trigger, of backfire, providing key insights for the development of robust, hydrogen-emission-optimized PFI engines. • Optimized PFI engine achieves 430 ppm H2 slip and 46.6% indicated efficiency. • Anomalies limit stable PFI operation to a load of 22.4 bar IMEP. • Simulations show top land and spark plug crevices are sources of unburned hydrogen. • Ongoing backfire events increase hydrogen emissions to over 30,000 ppm. • Observations suggest hydrogen slip is a consequence, not a trigger, of backfire.