| description abstract | Particle image velocimetry (PIV) system was used to measure the tumble structure of the incylinder airflow in a fourvalve optical gasoline direct injection (GDI) engine. The tumble ratio was controlled by a flap in the manifold and a baffle in the intake port. With proper orthogonal decomposition (POD) method, the velocity field was decomposed into four parts, i.e., the mean, coherent, transitional, and turbulent. The effect of tumble motion on the cycletocycle variation (CCV) of airflow and spray was investigated by calculating the shear strain vorticity. The results indicate that the flow structure can be effectively changed through the combination of flap and baffle by forming a single largescale tumble flow with the tumble ratio three times higher than the original one. According to POD analysis, it is revealed that the largescale strong tumble motion leads to the energy occupation ratio of the mean part greatly increase by up to 30%, while the energy transferred to the coherent part is reduced. The above process also decreases the CCV of the coherent part by 50%; thus, the CCV of the whole airflow in the cylinder can be suppressed. A single largescale tumble increases the maximum shear strain rate up to 2400 s−1. Meanwhile, the maximum vorticity increases to about 6000 s−1 by rolling up of the airflow. The contact area between spray droplets and air becomes larger, and the momentum exchanges between them contribute to wider sprays cone angle and shorter penetration distance when the flap is closed. The statistics of the measurements illustrate that a single largescale tumble can promote the formation of homogeneous mixture and reduce the fluctuation between multicycles. | |
