A syngas/diesel dual-fuel reactivity-controlled compression ignition (RCCI) engine was numerically investigated by an improved multidimensional model coupled with a reduced chemical mechanism. In the test RCCI engine, the syngas was premixed with air in the intake manifold, while the diesel was directly injected into the cylinder well before top dead center (TDC). The effect of the syngas composition, the premixed ratio of the syngas, the initial in-cylinder temperature at intake valve closing (IVC), and the hydrogen (H2) proportion in the syngas on the RCCI combustion and emission characteristics were investigated. The results indicate that the utilization of the syngas/diesel dual-fuel strategy in the RCCI engine with lean and premixed combustion is capable of simultaneously reducing the emissions of nitrogen oxides (NOx) and soot. Compared with the gasoline/diesel RCCI combustion, the combustion characteristics of the syngas/diesel RCCI is much more complicated due to the complex composition of syngas,...