Partially premixed combustion (PPC) is a promising way to achieve high thermal efficiency and low emissions, especially by using multiple injection strategies. The mechanisms behind PPC efficiency are still to be explained and explored. In this paper, multiple injections have been used to affect the gross indicated efficiency in an optical PPC engine modified from a Volvo MD13 heavy-duty diesel engine. The aim is both to improve and impair the gross indicated efficiency to understand the differences. The combustion natural luminosity is captured by a high-speed camera, and the distribution of fuel, oxygen, and temperature during the combustion process has been further explored by CFD simulation. The results show that with the right combination of the pilot, main, and post injection the gross indicated efficiency can be improved. Using a post injection in a triple-injection case show to have less effect on the combustion phasing than pilot injection in a double-injection case, while it can significantly affect combustion efficiency. The later of the double-injection cases tested (c30/16), has less heat transfer losses since the high-temperature region transported away from the cylinder head and piston bowl wall, which can be seen in the CFD-simulations. The highest gross indicated efficiency among the tested cases is given by the triple-injection case d38/24/6 as it reaches the best balance between the mixing and the local temperature through the jet-jet interactions and combustion-jet interactions.