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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.

Latest emissions standards impose very low NOx and particle emissions that have led to new Diesel combustion operating conditions, such as low temperature combustion (LTC). The principle of LTC is based on enhancing air fuel mixing and reducing combustion temperature, reducing raw nitrogen oxides (NOx) and particle emissions. However, new difficulties have arisen. LTC is typically achieved through high dilution rates and low CR, resulting in increased auto-ignition delay that produces significant noise and deteriorates the combustion phasing. At the same time, lower combustion temperature and reduced oxygen concentration increases hydrocarbon (HC) and carbon oxide (CO) emissions, which can be problematic at low load. Therefore, if LTC is a promising solution to meet future emission regulations, it imposes a new emissions, fuel consumption and noise trade-off. For this, the injection strategy is the most direct mean of controlling the heat release profile and fuel air mixture.