In this study, a detailed 1D simulation model of the hydrogen-enriched natural gas SI engine was built according to realistic engine layout in the lab with practical boundary conditions, and validated against the experimental data. Four different types of EGR systems were comprehensively investigated, including a low-pressure (LP) EGR, a high-pressure (HP) EGR, their combinations, and an internal EGR, using the validated 1D simulation model of the hydrogen-enriched natural gas SI engine for their combustion, performance, and emissions character...