To provide a turbo compound engine reducing a motive power loss by driving a power turbine from the engine body side in a low load.

This turbo compound engine has a turbocharger for recovering motive power from exhaust gas, and the power turbine for further recovering the motive power from the exhaust gas passing through a turbine of the turbocharger; and transmits the motive power recovered by the power turbine to a crankshaft via a hydraulic coupling 9. A one-way transmission means for reducing efficiency of the inverse directional motive power transmission to the input side from the output side of the hydraulic coupling 9 more than efficiency of the normal motive power transmission, is constituted by slantingly arranging an impeller blade 17 on the input side of the hydraulic coupling 9 so that its outer peripheral side end retreats in the rotational direction, and slantingly arranging an output side turbine blade 15 so that its outer peripheral side end precedes in the rotational direction.

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