An experimental investigation of Reactivity Controlled Compression Ignition (RCCI) combustion was conducted in a small single-cylinder HSDI diesel generator engine and compared to standard Direct Injection (DI) diesel combustion to assess the validity of this combustion strategy for high efficiency operation and simultaneous NOx and soot emission reduction in cylinder for this type of engine. A Yanmar L70AE engine was modified from its unit injector mechanical fuel system to operate with a more flexible, electrically controlled common rail DI fuel system in order to achieve the high level of injection event control required for RCCI combustion.RCCI combustion was realized using split, early DI diesel fuel and Port Fuel Injected (PFI) gasoline for 25%, 50% and 75% engine loads (~3, 4.3 and 5.5 bar IMEPn). The effects of intake air temperature, DI injection timing and combustion phasing on engine efficiency, emissions and combustion stability were explored.The results indicated that RCCI combustion was effective at reducing NOx and soot emissions, but suffered from increased HC and CO emissions, and poor combustion efficiency at light loads, and higher levels of combustion variation compared to conventional diesel combustion. Thermal efficiency of RCCI combustion suffered at low loads due to poor combustion efficiency. At 75% load, RCCI attained nearly a 3% efficiency improvement over standard diesel combustion, indicating that there is significant opportunity to reduce light load fuel consumption with further optimization effort focused on combustion efficiency.