Misfire detection based on crankshaft fluctuation

An arrangement for suppressing disturbances when detecting misfires in an internal combustion engine, the arrangement comprising:

• first computation means for computing uneven-running values which are a measure for the changes of the crankshaft rpm of the engine which are caused by misfires;
• second computation means for computing the spacing W between bands of uneven-running values;
• a memory which inputs an uneven-running threshold value (SW.sub.-- P or SW.sub.-- 0), a threshold spacing (W.sub.-- SW.sub.-- P or W.sub.-- SW.sub.-- 0) and a negative uneven-running threshold value (SW.sub.-- N) with these values being dependent in general upon the instantaneous engine operating state;
• and,
• comparator means for comparing the computed band spacing W to the pregiven threshold spacing and, in the case of a band spacing greater than the threshold spacing (W.sub.-- SW.sub.-- P or W.sub.-- SW.sub.-- O), outputting an indication as to the presence of misfires;
• whereas, said comparator means otherwise diagnosing a disturbance of the crankshaft rpm for a simultaneous drop below the negative uneven-running threshold value by the uneven-running values and interrupting the detection of misfires.