The invention is related to a method for controlling the rotational speed of an engine operating as the power source of a work machine, preferably an agricultural harvester, wherein a change in the load required from the engine during operation of the machine results in the controlled change of the engine speed towards a target value defined by a control curve, wherein the controlled change takes place according to one of a plurality of active control characteristics, depending on the degree of change in the engine speed detected as a direct consequence of the load change. A faster or slower speed change may for example be selected, and/or the target value may be continuously updated. According to a further embodiment, the control curve is referenced to the crop load, being the part of the load related to crop processing. The invention is related also to a machine equipped with a control mechanism configured to implement the method of the invention.