contributions of the individual cylinders of the four cylinder internal combustion engine 1 and to irregularities in the sector wheel 4 these fluctuations are easily recognizable in fig2 in which the rotational speed n is plotted over the time t as a rotational speed curve 7 the average rotational speed 10 is additionally drawn in the graph in fig2 the signal of the sensor 5 is also marked in fig2 as a time series 13 the rectangular signal of this time series shows the passes of the individual teeth of the sector wheel during the pass of a segment of the sector wheel said segment containing 30 teeth in the present example and having a width which is marked by 8 in fig2 the rotational speed fluctuates clearly as a result at a desired instant t 0 or t 1 in the rotational speed of the sector wheel said speed being reflected in the sequence of pulses of the time series 13 a momentary rotational speed was calculated which does not correspond to the average rotational speed therefore at the instant t 0 for example the current average rotational speed is in the order of magnitude of 1 7 u00d7 1000 r min whereas a momentary rotational speed in the order of magnitude of 2 5 u00d7 1000 r min would be obtained from the scanning of a part segment pass taking the form of the last three teeth.