With the emergence of stringent emissions standards and needs for fuel diversification, many countries are considering a massive use of natural gas for transportation. In this context, dual fuel diesel-CNG combustion is considered as a promising solution for highly efficient internal combustion engines. This concept offers the possibility to combine a diesel pilot injection as a high energy combustion initiation event, with an indirect injection of methane as main energy source. Low CO2 emissions can be reached thanks to the use of a conventional compression ignition engine with high compression ratio, and thanks to methane's high knocking resistance and low carbon content. Another benefit of dual fuel operation with high diesel substitution rates is the drastic reduction of PM emissions since methane is a very stable molecule containing no soot precursor. Moreover, the additional cost of the gas injection system is counterbalanced by the fuel cost reduction: indeed, CNG is cheaper than diesel fuel in most countries.Until now, dual fuel approach has been applied mainly on heavy duty engines using a basic approach of diesel fuel substitution. This paper presents an example of application based on a production Euro 6 diesel passenger car engine equipped with a modern diesel common rail injection system. A port fuel gas injection system was easily implemented to allow dual fuel functionality. The advantage of this configuration is that full diesel operation can be maintained and no major modification of engine hardware is needed, which is valuable for progressive introduction of dual fuel into the automotive market.The objective of the study was to optimize the dual fuel combustion process on the largest engine operating range and to maximize the methane to diesel fuel ratio. Optimal combustion mode between stoichiometry and lean burn was selected for several operating points taking into account the Euro 6 target for engine-out NOx emissions and the maximization of CO2 savings. A first step, not detailed in this paper, consisted in optimizing the swirl level, the combustion chamber and the compression ratio. On the optimal configuration, the boundaries of dual fuel operating range were determined at part and full load. The main limitations of the concept were identified such as pre-ignition at high load and unburned HC emissions at low load caused by fuel trapped on engine crevices and flame quenching. The optimal dual fuel settings were compared to diesel operation thanks to a split of losses approach allowed by means of an in-house dedicated tool. This methodology allows identifying precisely the gains and the losses for each contributor involved in the combustion process (e.g. heat losses, pumping losses, combustion phasing and duration, etc.). This comparison is well adapted for prospective concepts in order to unlink complex phenomena.