Regulated emissions and fuel consumption are the main constraints affecting internal combustion engine (ICE) design. Over the years, many techniques have been used with the aim of meeting these limitations. In particular, exhaust gas recirculation (EGR) has proved to be an invaluable solution to reduce NOx emissions in Diesel engines, becoming a widely used technique in production engines. However, its application has a direct effect on fuel consumption due to both the changes in the in-cylinder processes, affecting indicated efficiency, and also on the air management. An analysis, based on the engine Global Energy Balance, is presented to thoroughly assess the behavior of a HSDI Diesel engine under variable EGR conditions at different operating points. The tests have been carried out keeping constant the conditions at the IVC and the combustion centering. The analysis includes a combination of theoretical (0-D and 1-D modelling) and experimental tools (heat rejection and wall temperature measurement) that have been used to ensure control of in-cylinder conditions and to provide detailed explanation of the different phenomena affecting engine efficiency when the EGR is modified. Based on these tools, the impact of EGR on the engine performance, the heat transfer in the chamber and the global energy split of the engine are analyzed in detail. Main conclusions obtained show that indicated efficiency is mainly controlled by two factors, the changes in the combustion process and the heat transfer, while pumping losses dominate brake thermal efficiency.