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In this work, a detailed study on the number of exhaust particles, the particle size distribution and the geometric mean diameter from a 1.5 l gasoline direct-injection engine running with different engine operating parameters was conducted. The air-to-fuel ratio, the spark timing, the injection timing and the exhaust gas recirculation rate were varied at the same engine speed and the same engine load in order to understand how the mixture condition before ignition and the oxygen concentration affect the particulate matter emissions in terms of the total number and the size distribution. The experimental results showed that a lean mixture can significantly reduce the total number of particles by up to 90% compared with the rich mixture condition. When a lean mixture was used, most of the particulate matter was in the nucleation mode. The higher air-to-fuel ratio may have enhanced the oxidation of particles in the combustion process. Delaying the spark timing can also reduce the particulate matter emissions, especially for nucleation-mode particles which decreased by up to 65.41% compared with those for the early-spark-timing condition. The lower in-cylinder temperature may have resulted in the formation of a reduced number of nucleation-mode particles. The injection timing affected both the mixing process and the heat release process and could be optimized to reduce the particulate matter emissions. For a brake mean effective pressure of 3 bar, advanced injection timing reduced the numbers of particles in both the nucleation mode and the accumulation mode. The longer mixing duration improved the mixture homogeneity and led to lower formation rates of particles. Increasing the exhaust gas recirculation rate was effective in reducing the accumulation-mode particle emissions, while increasing the nucleation-mode particle emissions.

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