and the number of parts of the engine system is reduced the number of parts of the engine cooling structure is small since the radiation passage system 71 can simply be constructed the cooling passage system 101 is connected to the radiation passage system 71 through the crankshaft 13 therefore the cooling medium can be transferred between the cooling passage system 101 and the radiation passage system 71 without increasing the number of components used the engine cooling structure 10 employs the engine cooling oil as the medium for cooling the engine 1 therefore the various portions of the engine 1 are lubricated and cooled at the same time and the engine lubricating oil is effectively utilized the flywheel 37 is made up of the two members 43 45 which can be divided in the axial direction of the crankshaft 13 the radiation passage system 71 is defined jointly by the confronting surfaces of the flywheel members 43 45 therefore the passage system 71 is constructed with ease the flywheel member 45 is made of a material having a high thermal conductivity whereas the other flywheel member 43 is made of a magnetic material consequently the heat radiation efficiency of the radiation passage system 71 is increased and the flywheel 37 itself doubles as the rotor of the generator the number of parts of the engine 1 is thus further reduced fig3 shows a cooling structure.