There is provided an engine water-cooling device that can increase warming-up efficiency of an engine. The engine water-cooling device includes a thermostat housing that houses a thermostat. The thermostat housing is mounted to a front wall of a cylinder head in one side portion in a width direction of the cylinder head. A cooling water pump is mounted to a front wall of a cylinder block in a central portion in a width direction of the cylinder block. A bypass passage includes an intra-head bypass passage in the cylinder head, and the intra-head bypass passage includes a width-direction passage portion extending from a position behind the thermostat housing to a position behind and above the cooling water pump.

A coolant passage device 3 includes coolant intake pipes 11 and 12 that take in coolant from an engine, a delivery pipe 17 to a radiator communicating with the coolant intake pipes, and a delivery pipe 18 to the heater core branched from a central passage 16 connecting the coolant intake pipes with the delivery pipe to the radiator. A branch port 18a leading to the delivery pipe 18 to the heater core is opened in an upper portion in the central passage 16 in a state where the coolant passage device 3 is mounted to the engine, and the branch port 18a has a wall surface 21 surrounding the branch port and hanging down into the central passage 16. The wall surface 21 prevents bubbles contained in the coolant from entering the branch port 18a. And the coolant passage device consequently prevents coolant flow noise from occurring.

A coolant passage device 3 includes coolant intake pipes 11 and 12 that take in coolant from an engine, a delivery pipe 17 to a radiator communicating with the coolant intake pipes, and a delivery pipe 18 to the heater core branched from a central passage 16 connecting the coolant intake pipes with the delivery pipe to the radiator. A branch port 18a leading to the delivery pipe 18 to the heater core is opened in an upper portion in the central passage 16 in a state where the coolant passage device 3 is mounted to the engine, and the branch port 18a has a wall surface 21 surrounding the branch port and hanging down into the central passage 16. The wall surface 21 prevents bubbles contained in the coolant from entering the branch port 18a. And the coolant passage device consequently prevents coolant flow noise from occurring.

An air/oil-cooled internal combustion engine capable of increasing accuracy in detection of the temperature of the air/oil-cooled internal combustion engine by a temperature sensor and carrying out the warm-up operation after the start-up of the air/oil-cooled internal combustion engine favorably. The air/oil-cooled internal combustion engine includes cooling fins that are provided on the circumferences of a cylinder block and a cylinder head; and a cooling passage that is provided in a combustion chamber upper wall covering a combustion chamber of the cylinder head and is used for cooling the combustion chamber upper wall with lubrication oil. The air/oil-cooled internal combustion engine has a temperature sensor configured to detect the temperature of the air/oil-cooled internal combustion engine by detecting the temperature of oil. The temperature sensor faces a cooling oil outlet portion of the cooling passage.

An air/oil-cooled internal combustion engine capable of increasing accuracy in detection of the temperature of the air/oil-cooled internal combustion engine by a temperature sensor and carrying out the warm-up operation after the start-up of the air/oil-cooled internal combustion engine favorably. The air/oil-cooled internal combustion engine includes cooling fins that are provided on the circumferences of a cylinder block and a cylinder head; and a cooling passage that is provided in a combustion chamber upper wall covering a combustion chamber of the cylinder head and is used for cooling the combustion chamber upper wall with lubrication oil. The air/oil-cooled internal combustion engine has a temperature sensor configured to detect the temperature of the air/oil-cooled internal combustion engine by detecting the temperature of oil. The temperature sensor faces a cooling oil outlet portion of the cooling passage.