The present invention is directed to apparatus for monitoring engine exhaust, comprising a chamber for receiving such exhaust, an inlet of the chamber adapted to be connected to the engine exhaust and an outlet from the chamber to enable exhaust to escape therefrom, so that exhaust from the engine can flow through the chamber continuously, a source of electromagnetic radiation which is of a wavelength within a range comprising the infrared, visible and ultraviolet wavelengths, detector means which detects the amount of such radiation it receives, the source and the detector being so arranged that radiation emitted by the source is received by the detector after the radiation has passed through the chamber, and electrical means connected to the detector to provide an electrical signal which is indicative of the attenuation of the radiation as a result of the presence of particles within the exhaust in the chamber, in which the detector means is such as to be able to provide respective different measures of radiation which it receives for at least two different wavelengths of such radiation, and the electrical means are such as to provide respective electrical signals which are indicative of the attenuation of the radiation, as a result of the presence of particles within the exhaust in the enclosure, for the said at least two different wavelengths.
One shortcoming of such apparatus is that it takes no account of the effects of nitrogen dioxide present in the exhaust. This is a brown gas which absorbs radiation in much of the visible and UV range. Its concentration may fluctuate, and its opacity as a function of wavelength is not simple. Furthermore, oxidation catalysts now fitted to many vehicles increases the amount of nitrogen dioxide present in the engine exhaust as much as tenfold. At the same time, engines are becoming more efficient, so that soot concentration levels have become low. These things very much increase the adverse effect of nitrogen dioxide on the apparatus.