Abstract: A system of an internal combustion engine (VM) with rpm actuator (DRS), a refrigerator (AM) and a direct drive mechanically coupling the engine to the refrigerator, and a control unit including a thermostat for maintaining the internal temperature of the refrigerator at a predetermined set point value (SW), is improved by (a) a refrigerator thermostat setting, (b) a refrigerator external temperature sensor coupled, and (c) an engine speed tachometer (DM), all coupled to the control unit; (d) the control unit determining an optimal engine rpm from the inputs and characteristic function stored in a memory device (ST) of the control unit; and (e) the rpm actuator adjusting the rpm to the optimal rpm value.