The purpose of this research was to analyze numerically the effect of adding nitrogen, hydrogen, reformer gas and hydrogen and nitrogen mixtures on the combustion and exhaust emissions properties of a natural gas-diesel dual-fuel reactivity controlled compression ignition (RCCI) engine at different engine loads. By using natural gas, emissions and engine performance suffer at low loads due to its lower reactivity and higher ignition delay compared to gasoline. However, the use of hydrogen and syngas (reformer gas) as an additive can improve the combustion process of the engine at low loads. On the other hand, RCCI engine operation is limited to low-medium loads due to high peak pressure rise rates and high ringing intensity at high engine loads. The use of nitrogen as a dilution gas can mitigate this behavior at engine high load operation. The results obtained indicate that combustion was improved at low engine load by using hydrogen and syngas and CO and HC emissions are reduced. In addition, it was found that stable combustion and a low pressure rise rate could be achieved at 17 bar IMEP at the expense of higher carbon emissions with nitrogen-rich intake air. The effect of adding hydrogen, reformer gas, nitrogen, and a mixture of hydrogen and nitrogen on ignition delay, combustion duration, combustion efficiency, gross indicated efficiency, pressure rise rate, and ringing intensity was also investigated. It was indicated that how the gases produced by catalytic fuel reformer will allow RCCI engine to operate at various loads with low emissions. (C) 2017 Elsevier Ltd. All rights reserved.