Air intake passage structure for multi-cylinder engine

An air intake passage structure for a multi-cylinder engine comprising:

• a plurality of cylinders that are arranged in an array;
• a plurality of intake ports that respectively communicate with the plurality of cylinders;
• and
• an air intake passage that is connected to the intake ports, wherein
• the air intake passage has:
• a plurality of independent passages that are respectively connected to the plurality of intake ports;
• a surge tank, to which upstream end portions of the plurality of independent passages arranged in an array according to an alignment order of the corresponding intake ports are connected;
• and
• an upstream passage that is connected to the surge tank so as to introduce gas into the surge tank,
• a bottom surface of the surge tank has such a shape that, in a vehicle-mounted state, the bottom surface is positioned below a lower surface at an upstream end of each of the plurality of independent passages and comes closer to the lower surface at the upstream end of each of the plurality of independent passages in a vertical direction farther away from a connected portion between the surge tank and the upstream passage in a cylinder array direction,
• when seen in a perpendicular cross section to the cylinder array direction, the surge tank is formed with an inclined surface that is interposed between the bottom surface of the surge tank and the respective lower surface at the upstream end of each of the independent passages and that is inclined upward from a surge tank side to an independent passage side, and
• the inclined surface is formed to be more gradually inclined farther away from the connected portion between the surge tank and the upstream passage in the cylinder array direction.