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Experiments were conducted on two direct injection diesel engines: a standard production 2.5 litre engine, and a single cylinder research engine fitted with a high-pressure electronically controlled unit injector. The influence of some operating parameters (such as load, speed, injection timing and swirl-ratio) on the total mass and composition of particulate emissions sampled in a mini-dilution tunnel was investigated. A data acquisition system was used to record and analyse the cylinder pressure, fuel line pressure and fuel injection needle-lift diagrams. Analysis of the HC and NOx emissions as well as the Bosch smoke number was helpful in the understanding of how the fuel and air mixing process, the flame type and the combustion chamber temperature and pressure affect the amount and composition of diesel particulate emissions. A good correlation between the Bosch smoke number and the exhaust carbon concentration was also found.