Test results demonstrate the human inability to reliably assess the intensity and opacity of diesel exhaust smoke. This is true not only under unfavorable road conditions, but also in the laboratory where adverse side effects can widely be eliminated.
This inability to discriminate is due to the fact that the subjective assessment of exhaust smoke is determined by two separate, unconsciously combined visual sensations, namely the opacity and the size of the exhaust cloud, i.e. the gas flow rate.
Mathematical relationships were established, in which the dependence of both sensations on the exhaust gas flow rate is expressed by exponents 0.33 and 0.5, respectively.