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 transmission efficiency restraining means for reducing motive power transmission efficiency of the hydraulic coupling 9 in the low load, comprises a normally opening control valve 17 arranged in the middle of a supply line 16 for supplying engine oil 15 as a working fluid to the hydraulic coupling 9, and a control device 18 for throttling the oil volume by controlling opening of the control valve 17 by an opening command signal 17a so as to become small in the low load.

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