A water-cooled internal combustion engine

A water-cooled internal combustion engine (1), for equipping scooters and motorcycles, having an engine block (2), obtained from one only single-piece melting and with a driving axle (A) substantially perpendicular to a longitudinal development of the internal combustion engine, and a cooling system comprising a radiator (8), a relative fan (9) and a water circulating pump (11) comprising an intake nozzle (12) and a discharge nozzle (13), wherein the fan (9) of the radiator (8) is mounted directly on a crankshaft (3) from which a driving force for said cooling water circulation pump (11) is derived, the radiator (8) being arranged laterally to the engine block (2), with the fan (9) lying on a plane substantially parallel to said longitudinal development,

• said circulating pump (11) is arranged on an opposite side of the engine block (2) with respect to the radiator (8), wherein said cooling system comprises:
• a first cooling circuit having:
• a discharge line (16) and a extraction line (17) connecting the radiator (8) to said engine block (2), said discharge and extraction lines (16, 17) being constituted by tubes outside the engine block (2) with respective discharge openings (18), formed in said engine block (18), and intake opening (23) connected to said intake nozzle (12) of the pump (11);
• an inner circuit portion (24, 25) for extracting heat from the engine;
• a second cooling circuit wholly included in said engine block (2) for recirculating cooling water, comprising a by-pass line (26), connecting said discharge opening (18) to the intake nozzle (12) of the pump (11), and said inner circuit portion (24, 25) for extracting the heat from the engine (1);
• and
• a by-pass valve (27), arranged on the engine block (2) at said discharge opening (18), actuated by a thermostat (29) sensible to the water temperature, allowing the water circulation in the first cooling circuit above a reference temperature.