Regulatory standards on diesel engines emissions will decidedly become more restrictive in coming years. This has led to the development and implementation of alternative fuels. The objective of this paper is to evaluate the potential of Sasol Fischer-Tropsch (FT) diesel fuel to improve the reduction of emissions in a direct injection diesel engine with a high pressure common-rail injection system (HDI engine from PSA Peugeot-Citroën). In principle, FT diesel fuel shows significant advantages in reducing emissions over a standard diesel fuel due to its low aromaticity, high cetane rating and high H/C rating.Initial tests with two 406 HDI Euro 2 vehicles with standard calibration showed very favourable trends on exhaust emissions in comparaison with reference fuel (CEC RF73-A-93 type). Sasol FT diesel fuel gave significant improvement on specific fuel consumption, and decreased the HC, CO, CO2 and particulate emissions without degrading NOx emissions.A subsequent comprehension study, performed on a single cylinder HDI diesel engine, has confirmed the trends observed in the vehicle tests, and allowed us to investigate and explain the physical phenomena involved in both injection and combustion processes that produce the trends observed.Those two parts of the study (HDI vehicle tests and spray test rig plus HDI single cylinder understanding studies) show the great potential of FT diesel fuel used in a HDI engine with standard calibrations in terms of reducing global exhaust emissions. Furthermore, trends have been identified to improve even more this favourable balance by adapting calibrations to the FT fuel.