A method of operating a compoundable engine that includes a turbine having a turbine shaft and an intermittent internal combustion engine having an engine shaft. The engine shaft is rotated at a first rotational speed. The turbine is driven by exhaust gases of the intermittent internal combustion engine to rotate the turbine shaft while the engine shaft rotates independently from the turbine shaft. A rotatable load is driven with the turbine shaft. A rotational speed of the engine shaft is increased from the first rotational speed until the turbine shaft reaches a predetermined rotational speed. After the turbine shaft has reached the predetermined rotational speed, the rotational speed of the engine shaft is adjusted until the turbine shaft and the engine shaft are drivingly engageable with each other, and the turbine shaft with the engine shaft are engaged such that both are in driving engagement with the rotatable load.