Substitution of diesel fuel with natural gas in heavy-duty diesel engines offers significant advantages in terms of operating cost, as well as NOx, PM emissions and greenhouse gas emissions. However, the challenges of high THC and CO emissions, combustion stability, exhaust temperatures and pressure rise rates limit the substitution levels across the engine operating map and necessitate an optimized combustion strategy.Reactivity controlled compression ignition (RCCI) combustion has shown promise in regard to improving combustion efficiency at low and medium loads and simultaneously reducing NOx emissions at higher loads. RCCI combustion exploits the difference in reactivity between two fuels by introducing a less reactive fuel, such as natural gas, along with air during the intake stroke and igniting the air-CNG mixture by injecting a higher reactivity fuel, such as diesel, later in the compression stroke. Recent studies to optimize dual fuel diesel-CNG RCCI combustion have primarily focused on the simultaneous reduction of NOx and soot emissions. However, further investigation is needed to outline the in-cylinder conditions that are required in order for RCCI combustion to proceed. In addition, the THC emissions produced under dual fuel diesel-CNG RCCI operation need to be analyzed to better understand how to address this limitation of the technology.The current study builds on the dual fuel diesel-CNG study previously presented by the same set of authors by analyzing the experimental RCCI combustion results achieved on a heavy-duty diesel engine at 6 bar BMEP and multiple engine speeds. The study evaluates the impact of various control variables, such as CNG substitution, EGR rate and injection strategy on achieving RCCI combustion at 6 bar BMEP, thereby establishing a general framework for in-cylinder mixture properties required in realizing RCCI combustion. The conclusions at 6 bar BMEP are supported by 3D simulations of the complete combustion chamber using Converge CFD software. CFD results are also used to highlight the causes of high CH4 and CO emissions with dual fuel diesel-CNG RCCI operation. Further, the paper analyzes the experimental RCCI combustion results at 14 bar BMEP and multiple engine speeds to lay out the challenges in achieving RCCI combustion at increased engine load.