The Mitsubishi GDI engine has adopted a pair of upright intake ports, to induce a rotating in-cylinder flow, reverse tumble, and control air fuel mixing with this flow. The port design of the GDI engine was optimized for achieving a high intensity of the reverse tumble while maintaining a high charge coefficient, by means of modeling of in-cylinder flow and experiment with a steady flow rig.
First of all, the ideal design of the upright ports was discussed. It was found that for enhancing the reverse tumble, it is more effective to arrange a pair of the ports parallel, than to arrange them convergent. The parallel arrangement leads to the smoother flows passing through the intake sides of the intake valves, and then descending on the cylinder liner, that is turning toward the rotation direction of the reverse tumble, because of less impingement of the flows through a pair of the valves.
Actually, however, the completely parallel arrangement of the ports is not adoptable, because of interference of the ports with the spark plug located at the center of the cylinder. Therefore, the port design for the GDI engine was modified to evade the plug, as well as to maintain the effectiveness of the parallel port arrangement, so that it was composed mainly of two parts, an upstream parallel part and a downstream convergent part. It was confirmed that the ports can parallel flows in them, in spite of their convergent part.
In addition, the port design was refined to more enhance the flows through the intake sides of the intake valves. A flat slope was installed on the exhaust-side inner wall of each of the ports, to guide a flow in the port toward the intake side of the valve. A small fin was installed downstream from each of the valve stems, as a rectifier of the flow through the intake side of the valve. The fin is effective in improving both an intensity of the reverse tumble and a charge coefficient