Reactivity Controlled Compression Ignition (RCCI) is a promising dual-fuel Low Temperature Combustion (LTC) mode with significant potential for reducing NOx and particulate emissions while improving or maintaining thermal efficiency compared to Conventional Diesel Combustion (CDC) engines. The large reactivity difference between diesel and Natural Gas (NG) fuels provides a strong control variable for phasing and shaping combustion heat release. In this work, the Brake Thermal Efficiencies (BTE), emissions and combustion characteristics of a light duty 1.9L, four-cylinder diesel engine operating in single fuel diesel mode and in Diesel-NG RCCI mode are investigated and compared. The engine was operated at speeds of 1300 to 2500 RPM and loads of 1 to 7 bar BMEP. Operation was limited to 10 bar/deg Maximum Pressure Rise Rate (MPRR) and 6% Coefficient of Variation (COV) of IMEP. The engine performance was investigated using a combination of RCCI control variables including NG/diesel Blend Ratio (BR), diesel injection fuel split, and Start of Injection (SOI) timing for diesel injections. The RCCI map was generated using different injection strategies (single and double injections) and up to 20% EGR Exhaust Gas Recirculation (EGR) at higher loads to obtain the best brake thermal efficiency. In addition, the majority of the required energy (more than 80%) in RCCI operating points was provided from NG. The results showed a maximum of 5% increase in brake thermal efficiency and 92% reduction in NOx in RCCI combustion mode compared to the CDC mode.