From the standpoint of production engineering, sheathed-element glow plugs which are used in an internal combustion engine have a predefined tolerance of the temperature to be set at the sheathed-element glow plug for a nominal voltage. Thus, for example, for a ceramic sheathed-element glow plug, given a nominal voltage of 7 volts with which the sheathed-element glow plug is activated, a temperature of 1,200° C. is indicated as glow temperature, which allows a tolerance of +/−50 K. This means that the maximum temperature of the sheathed-element glow plug is not allowed to exceed 1,250° C. If control units which activate the sheathed-element glow plugs in the internal combustion engine are applied, then it there can be deviations with respect to the desired temperature of 1,200° C., for example. Since the control unit is unable to react to changing tolerances during operation of the sheathed-element glow plugs in the internal combustion engine, the maximum sheathed-element glow plug temperature of 1,200° C. is predefined definitively in the control unit.
German Published Patent Application No. 10 2008 040 971 describes a method and a device for regulating the temperature of sheathed-element glow plugs in an internal combustion engine, in which during a reference operation of the internal combustion engine, a mathematical correlation is formed between measured temperatures and measured resistances of at least one sheathed-element glow plug, the resistances being obtained from an actually applied voltage and an actual current flow at this sheathed-element glow plug, and this mathematical correlation being adapted dynamically over the entire service life of the plug and being utilized in the overall operation of the internal combustion engine. In this method, for improved application quality, the changing operating conditions of the internal combustion engine are taken into account. This hampers the application of the control unit for the installed sheathed-element glow plug, since great tolerances are considered during use of the sheathed-element glow plug in the internal combustion engine.