An arrangement for converting thermal energy to mechanical energy in a vehicle (1). A working medium is vaporized by a heat source (3) in the vehicle (1) and is thereafter expanded through a turbine (13) generating mechanical energy. A control unit (31) receives information indicating the vehicle (1) is to be braked and connects the cooling system (21, 39) of the vehicle to the vehicle's power train (2, 5-9) to cool a refrigerant to a low temperature. The control unit (31) receives information that the vehicle (1) requires extra propulsive force and, uses the cooled refrigerant to subject the working medium in the line circuit (10) to a second step of cooling before it is led to the evaporator (12). The condensation temperature of the working medium may thus be lowered and more mechanical energy may be generated in the turbine (13).