The dramatic change in terms of pollutant constraints for diesel engines, with future Euro-6 regulations for example, will probably require the improvement of alternative combustion modes such as homogeneous combustion (Homogenous Charge Compression Ignition - HCCI). These new concepts allow the reduction of NOx and particulate emissions to very low levels for low loads thanks to a high level of external Exhaust Gas Recirculation (EGR) while maintaining CO2 emission advantage of diesel engines. Nevertheless, due to a resultant low combustion temperature, HC and CO emissions rise significantly, especially at low load when the catalyst bed temperature is not sufficient for their aftertreatment. This paper describes three considered ways to potentially overcome this barrier thanks to HCCI combustion improvement. After a preliminary optimization of the combustion system performed on a single cylinder, the work was operated on a mid-size multicylinder engine to reduce engine out HC–CO levels.