This work investigates the effects of the piston bowl geometry on RCCI (Reactivity Controlled Compression Ignition) heat transfer and combustion losses and its repercussion on the engine efficiency. For this purpose, three piston geometries with compression ratio 14.4:1 have been studied and compared by means of computational modeling. In addition, the engine operating conditions proposed at low, medium and high load were also validated experimentally in a heavy-duty single-cylinder engine adapted for dual fuel operation. The engine speed was kept constant at 1200 rev/min during the research. Results suggest that heat flux through the piston surface represent the major portion of the heat transfer energy. Thus, the comparison of the three geometries demonstrates that reduced piston surface area and reduced charge motion, are the key factors to improve the gross indicated efficiency over the different engine loads. Moreover, it is found that a shallow piston geometry with a smooth transition from the center to the squish region, with a 16% reduced surface area, strongly improves the gross work at low load. However, this gain diminishes due to increased combustion losses as engine load increases. Finally, an intermediate geometry was confirmed as the best balanced piston geometry for RCCI operation over the three different loads. (C) 2016 Elsevier Ltd. All rights reserved.