engine control based on the smooth or rough running of the engine is performed by obtaining a signal indicative of crankshaft rotation and by digital measurement of the period of revolution . the periods of three successive revolutions are compared to obtain a second differential , related to acceleration and hence to combustion chamber pressure . this second differential is compared digitally to a nominal value , taken from an rpm - dependent function of the engine smoothness , derived by frequency synthesis of the frequencies from individual stages in one or more of the counters .