A quantum model of a heat engine is analyzed. This engine is constructed from two coupled oscillators in interaction with a warm and cold reservoir. Power is extracted by an external periodic driving force. As a function of control parameters a maximum in power is obtained, and a decline of thermodynamic efficiency below the ideal Carnot value. This irreversibility is a consequence of the mechanism devised to extract power in its perturbing the energy level structure of the engine. In the limit of weak coupling to the driving force the efficiency at maximum power obtains the value of η=1-((Ta/Tb))1/2.