Thanks to a high specific torque, associated to a low fuel consumption, the market share of the Diesel engine has not stopped increasing during the last decade. Nevertheless, due to the strong emissions regulations to come, the challenge of the Diesel will be to save its advantage in CO2 and to drastically reduce NOx and particulate emissions, whilst offering the save driving pleasure.
Hence new concepts of combustion are in development in order to keep NOx emissions as low as possible at an acceptable cost. The homogeneous combustion, by generating a lower pollutant level directly in the combustion chamber, seems to be a very promising way in this direction. Especially, the NADI™ concept, developed by IFP has already shown a high potential in NOx and particulate simultaneous reduction, but up to now its relative low specific power at full load compared to the next generation of conventional Diesel engines could be a major drawback for a global application.
This work presents how a combustion chamber dedicated to HCCI Diesel combustion at part loads has been optimized in order to reach a high specific power with a conventional turbocharging system, a low smoke number and an acceptable cylinder peak pressure by complementary CFD studies combined to single cylinder engine tests. Main results show how injector hydraulic flow and piston bowl shape, especially the combustion bowl width to depth ratio, have to be combined in order to obtain the best power output. Main results notably demonstrate that with a narrow bowl, the level of performance grows with the injector hydraulic flow increase (like for conventional applications), but they also put into evidence the interest of associating a low hydraulic flow to a wide bowl.