1. A method for classifying vehicles that emanate low frequency electro-magnetic (EM) signals, comprising: a) passively sensing low frequency EM signals emanated from a vehicle; b) computing the temporal frequency response of the EM signals; c) extracting features from the temporal frequency response; d) using the extracted features to determine a set of vehicle characteristics; and e) using the set of vehicle characteristics to classify the vehicle; wherein the vehicles comprise a 4-cycle combustion engine having a rotating crankshaft and a plurality of cylinders that fire at ignition frequencies F.sub.I, equal to one-half their crankshaft frequency F.sub.C times the number of cylinders N in that vehicle, said engine emanating relatively weak EM signals at the harmonic frequencies of the crankshaft frequency F.sub.C and relatively strong intensity levels at the harmonic frequencies of the ignition frequency F.sub.I, said harmonic features being extracted by identifying a first harmonic interrelationship between the relatively weak signal emanations and a second harmonic interrelationship between relatively strong signal emanations, said first and second harmonic interrelationships being used to compute the crankshaft and ignition frequencies, respectively, from which the number of cylinders N in the vehicle's engine is computed and used to classify the vehicle.