Four-stroke cycle internal combustion engine

A four-stroke cycle internal combustion engine, comprising:

• a crankshaft provided within a crank chamber;
• an oil reservoir arranged below said crank chamber for storing lubricating oil;
• a valve chamber in communication with said oil reservoir through an oil mist passage, oil mist being formed in said oil reservoir and supplied to an inside of said crank chamber and to said valve chamber through said oil mist passage;
• wherein said oil mist passage comprises a pre-separation oil mist passage section formed in said crankshaft, said pre-separation oil mist passage section being in fluid communication with said oil reservoir at one end and said crankshaft at another end;
• a cylindrical centrifugal-separation chamber surrounding an outer-periphery of said crankshaft where said other end is formed;
• a post-separation oil mist passage section extending from said centrifugal-separation chamber in an opposite direction of a rotational direction of said crankshaft, said post-separation oil mist passage section being in fluid communication with said valve chamber at one end and said centrifugal-separation chamber at another end;
• and an oil return passage in fluid communication with said centrifugal-separation chamber and said oil reservoir, said oil return passage extending from said centrifugal-separation chamber in the same direction as the rotation direction of said crankshaft.