An arrangement and a method for converting thermal energy to mechanical energy includes a circulation unit (4) a refrigerant in the a circuit (3), an evaporator (6) for the refrigerant, a turbine (9) driven by vaporised refrigerant, a condenser (12) cooling the refrigerant to condense, and an accumulator tank (14) for storage of the refrigerant is not being circulated in the line circuit (3). A control device estimates the degree of filling of the line circuit (3) with refrigerant at which the turbine (9) achieves a substantially optimum effect, and controls the flow of refrigerant between the line circuit (3) and the accumulator tank (14) to achieve the estimated degree of filling the line circuit (3) with refrigerant.