SAE International A Study of Methods to Lower HC and CO Emissions in Diesel HCCI 2008-01-0034

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 CO 2 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. Firstly, it focused on the optimization of multi-injection strategies in HCCI combustion: the Heat Release Rate could be split up with a direct impact on the noise level and a reduction of the EGR rate to modify the trade-off with NOx emissions in favor of HC and CO emissions. Secondly, to facilitate their oxidation in the combustion chamber, two concepts were also examined with the aim of a higher in-cylinder temperature. Whereas the study of a short high-pressure-loop EGR without cooler shows that it could be a simple attractive low-cost solution, the evaluation of a Variable Valve Lift system highlights the potential of an exhaust secondary lift by using Internal Gas Residual (IGR) in replacement of an EGR part. Finally, combination of adapted injection strategies and an increased global burnt gas temperature allows extending their respective benefits with an HC-CO emissions reduction of 60% for identical low NOx emissions level compared to standard HCCI results. Further development would be necessary to deal with NVH and transient control.