Cooling water passage structure in cylinder head of internal combustion engine

A cooling water passage structure in a cylinder head of an internal combustion engine, comprising:

• a cooling water passage extending from a cooling water inlet port to a cooling water outlet port, the cooling water inlet port being formed at one end side of the cylinder head in a lengthwise direction of the cylinder head, the cooling water outlet port being formed at another end side of the cylinder head in the lengthwise direction, the cooling water passage comprising:
• a main cooling water passage extending in the lengthwise direction above a plurality of combustion chambers of the internal combustion engine, the internal combustion engine including an exhaust collecting portion formed in the cylinder head to collect exhaust gases exhausted from the plurality of combustion chambers which are arrayed in a line;
• a first exhaust-side cooling water passage and a second exhaust-side cooling water passage arranged to sandwich the exhaust collecting portion and extending in the lengthwise direction;
• and
• an exhaust-side communication passage communicating the main cooling water passage with the first exhaust-side cooling water passage and with the second exhaust-side cooling water passage;
• a ridge extending in the lengthwise direction to adjust a flow speed of cooling water to flow through the main cooling water passage, the ridge being provided on a first wall surface of a main passage defining portion to define the main cooling water passage on a side away from the plurality of combustion chambers;
• and
• an exhaust-side throttle portion extending in a direction substantially perpendicular to the lengthwise direction to reduce a passage cross-sectional area of the exhaust-side communication passage, the exhaust-side throttle portion being provided on a second wall surface of an exhaust-side communication passage defining portion to define the exhaust-side communication passage on a side away from the plurality of combustion chambers.