PURPOSE:To change operation cycles according to an operation condition of an engine by arranging two kinds of cam shafts for two-cycle operation and four cycle operation, and operating only the cam shaft corresponding to the operated cycle. CONSTITUTION:An engine 1 functions both as a two-cycle engine and as a four-cycle engine by changing opening/closing timings of an intake valve 11 and an exhaust valve 12. A solenoid clutch 6 is arranged between a timing pulley 35 of a cam shaft 3 for four-cycle operation and a driven gear 52. A solenoid clutch 60 is arranged between a timing pulley 45 of a cam shaft 4 for two-cycle operation and a crank gear 51. A controller 7 operates, based on detection signals from a rotary sensor 71 and a load sensor 72, the solenoid clutch 60 to rotate the cam shaft 4 when the two-cycle operation is selected, while operating the solenoid clutch 6 to rotate the cam shaft 3 when the four-cycle operation is selected.
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