Since the intake valve close timing (IVC) directly determines the amount of displacement backflow and the amount of fresh charge trapped in the cylinder, optimizing the IVC is important to improve the performance of the diesel engine. In this paper, the relationship between the IVC and the displacement backflow of the cylinder at the high-speed condition was studied by establishing a one-dimensional (1D) gas dynamic model of a single-cylinder diesel engine. The results show that the forward airflow mass of intake and the backflow increase as the IVC retards, and the airflow mass trapped in cylinder increases at first and then decreases. It is interesting to find that the backflow does not equal zero when the air mass trapped in cylinder is the largest, which is different from the traditional optimizing strategy on the IVC. That is to say, there exists a misalignment between the maximum-volume-efficiency IVC and the none-backflow IVC. To further verify this interesting misalignment, the airflow characteristics at the optimized IVC condition are studied by establishing a three-dimensional (3D) simulation. It is found that the appearance of backflow is a gradual process, and there exists an overall backflow when the engine volume efficiency reaches its maximum value. In addition, the misalignment is reduced as the mean valve-closing velocity increases. The misalignment equals to 0 only if the mean valve-closing velocity approaches infinity.

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