Holding down the influence of engine rpm fluctuation, it offers the crank angular detection device of the engine which raises the reliability of detection of crank angular standard position.As ring gear 42 is locked in bracket axle 41 of one cylinder engine 30, in the margin of the said ring gear 42 plural projections (tooth) 43 are provided at regular interval, it forms unequal interval section (tooth being lacking section) 44 in 1 places, it has the crank angular sensor 3 in order to detect the stand rising and fall of the both ends of projection 43 of the said ring gear 42, the said crank angular sensor 3, detecting the face width of each projection, 43 and the interval with the projection which it adjoins with this the aforementioned each projection recognizes 43 and unequal interval section 44 by calculating the ratioIt did, as piston 31 of the aforementioned engine 30 the aforementioned unequal interval section 44 is detected at the time of the bottom dead center vicinity, the aforementioned crank angular sensor it arranged 3 and unequal interval section 44.
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