Increasing global efficiency of direct injection spark ignition (DISI) engine is nowadays one of the main concerns in automotive research. A conventional way to reduce DISI engine fuel consumption is through downsizing. This approach is well suited to the current homologation cycle as NEDC, but has the drawback to induce over-consumptions in customer real driving usage. Moreover, the driving cycles dedicated to EURO 6d and future regulations will evolve towards higher load operating conditions with higher particulate emissions. Therefore, efficiency of current DISI has to be strongly increased, for homologation cycle and real driving conditions. This implies to deeply understand and improve injection, mixing and flame propagation processes. This work proposes an alternative way to improve the thermodynamic efficiency of the combustion system, by coupling an increase of Compression Ratio (CR) with high levels of Exhaust Gas Recirculation (EGR) and the setup of Miller/Atkinson cycle at intake. The study is focused on the understanding of the physical phenomena involved by high CR and Miller/Atkinson type cycle. Particularly, the impact on turbulence level, air-fuel mixture, combustion efficiency and final global efficiency is assessed. Several tools presented here are used to optimize an existing downsized DISI engine: optical diagnostics, 3D simulation, and single cylinder engine optimization. Then, the impacts of the different technological components (CR, Intake valve opening duration, Valve timing, EGR…) is detailed. Finally the obtained results are discussed to draw some perspectives on the best suited engine architecture.