Environment protection issues regarding CO2 emissions as well as customers requirements for fun-to-drive and fuel economy explain the strong increase of Diesel engine on European market share in all passenger car segments. To comply future purposes of emission regulations, particularly dramatic decrease in NOx emissions, technology need to keep upgrading; the reduction of the volumetric compression ratio (VCR) is one of the most promising research ways to allow a simultaneous increase in power at full load and NOx / PM trade-off improvement at part load.This study describes the combustion effects of the reduction of compression ratio and quantifies improvements obtained at full load and part load running conditions on a HSDI Common Rail engine out performance (power, fuel consumption, emissions and noise). Potential and limitations of a reduced compression ratio from 18:1 to 14:1 are underlined. Moreover, this paper aims to define initially the head guidelines for future combustion system designs in focusing particularly on the specific evolution required by the reduction of the compression ratio, including width to depth ratio of piston bowl, number of nozzle holes and swirl.Combustion tests show that the reduction of RVC from 18 to 14 allows an increase of 12% of power at 4000 rpm as well as near zero NOx and PM emissions at 1640 rpm - BMEP 3,7 bar with nevertheless a significant increase of CO and HC emissions. Moreover, whatever the compression ratio, full load performance has been achieved with larger bowl with lower hole number for the nozzle injector and finally a lower swirl level. As opposed to these results, at low load the reduction of emissions promoted by a thin pulverization of fuel and the best homogenization of fuel/air mixing has required an antagonist design.
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