Abstract:
An apparatus for monitoring the exhaust of an engine includes a flow-through chamber for receiving exhaust, a source of electromagnetic radiation and a detector. The source provides electromagnetic radiation in a range comprising the infrared, visible and ultraviolet wavelengths. The source and a detector are arranged so that radiation passing through the chamber is incident on the detector. An electronic circuit is connected to the detector to provide a signal indicative of the attenuation of the radiation by particles in the exhaust in the chamber. The detector provides respective measures of radiation which it receives for at least two different wavelengths of the radiation, and the electronic circuit provides corresponding electrical signals indicating the attenuation of the two different wavelengths by particles within the exhaust in the chamber. The wavelengths are selected to be those for which the attenuation caused by nitrogen dioxide in the exhaust is substantially the same.

Description:
TECHNICAL FIELD 
   This invention relates to the an of apparatus for monitoring the exhaust of internal combustion engines. 
   BACKGROUND OF THE INVENTION 
   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. 
   SUMMARY OF THE INVENTION 
   The present invention seeks to overcome this problem. 
   Accordingly, the present invention is directed to apparatus having the construction set out in the opening paragraph of the present specification in which the said at least two different wavelengths are wavelengths at which the attenuation of electromagnetic radiation caused by the presence of nitrogen dioxide in the exhaust is substantially the same. 
   This provides the advantage that the apparatus will indicate if the particle size is sufficiently small to make a difference in the attenuation of the electromagnetic radiation at the respective different wavelengths, independently of the nitrogen dioxide content of the exhaust. This in turn is significant from the point of view of harmful pollution because particles of a small size are more readily absorbed by the body, especially lung tissue, than particles of a larger size. 
   Preferably, one of the said two different wavelengths is substantially 300 nm, and the other is substantially 500 nm. This benefits from a relatively large difference between the wavelengths, to increase the sensitivity of the apparatus to the presence of small soot particles within the exhaust. 
   Preferably, the detector means are such as to be capable of providing different measures of radiation which it receives for more than two different wavelengths, preferably three. The third wavelength is preferably that for red light, more preferably substantially 680 nm, at which the attenuation caused by the presence of nitrogen dioxide is substantially negligible. This enables the apparatus to give a clearer assessment of one dominant particle size in the exhaust, without being unduly adversely affected by nitrogen dioxide. 
   Preferably, there are at least two detectors in the detector means for each different wavelength that is detected. This reduces the likelihood of spurious measurements from inadequately mixed exhaust. 
   Preferably, blowers are provided to blow a curtain of air or other gas across the operative faces of the source and/or the detector means to reduce the extent to which they might become permanently marked by materials within the exhaust. 
   This reduces the likelihood of any error owing to such a mark. Preferably, the operative faces of the source and/or the detector means are flat and/or smooth to facilitate the passage of such a curtain of air or other gas. 
   Preferably, the light source is a single broadband source which emits electromagnetic energy across the range of wavelengths which comprises the said at least two different wavelengths. 
   It is desirable for the electrical means to comprise an Analog to Digital Converter to convert signals obtained from the detector means to a stream of pulses providing respective measures of the attenuation of the electromagnetic energy at the said at least two different wavelengths at regular intervals in real time. 
   Preferably, the electrical means calculate the k value for each sensor of the detector means given by the equation:
 
 k= 1/ L* log e (1−0.01 *N )
 
where L is the smoke column length, being the effective optical path length for the electromagnetic radiation through exhaust filled regions in passing from the source to the detector means, and N is the opacity for that sensor given by the equation:
 
 N =100*( V−V   o )/ V   100   −V   o )
 
V being the value of the signal from the sensor at any given time, V 100  being the reading from the sensor when the chamber is totally opaque, or when there is no electromagnetic radiation passing through it, and V 0  being the reading from the sensor for full transmission when there is no exhaust in the chamber.
 
   The present invention extends to a method of monitoring engine exhaust comprising passing the exhaust through a chamber, directing electromagnetic radiation through the exhaust in the chamber, and providing a measure of the attenuation of such radiation for at least two different wavelengths thereof at which the attenuation of such radiation caused by the presence of nitrogen dioxide in the exhaust is substantially the same. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An example of apparatus embodying the present invention will now be described by way of example with reference to the accompanying drawings in which: 
       FIG. 1  shows a diagram of the apparatus and its interior; 
       FIG. 1   b  shows in greater detail a part of the apparatus shown in  FIG. 1  viewed in the direction indicated by the arrow ‘A’ shown in  FIG. 1 ; 
       FIG. 2  shows a diagram of circuitry used with the apparatus shown in  FIG. 1 ; 
       FIG. 3  shows a diagram of the steps taken in the programme executed by a processor in the circuitry shown in  FIG. 2 ; and 
       FIGS. 4 to 8  show respective different explanatory graphs. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The apparatus  10  shown in  FIG. 1  comprises an enclosure  12  defined by a generally cylindrical closed housing  14 , having an inlet  16  positioned halfway along the housing  14 , and which is adapted to be connected to an engine exhaust, and an outlet  18 , which is also positioned halfway along the length of the housing  14 , but which is diametrically opposite the inlet  16 . The outlet  18  is for the extraction of waste exhaust/airmix from the enclosure  12 . The inlet  16  is connected to feed exhaust through a wall of a cylinder  20 , which defines a measuring chamber, at a position equidistant from the ends thereof. The cylinder  20  is open at both ends. As a result a path is provided for exhaust which starts at the inlet  16 , progresses in both directions therethrough outwardly towards the respective ends of the cylinder  20  close to the closed ends of the housing  14 , back along the enclosure  12  towards a position halfway along the housing  14  around the outside of the cylinder  20  and thence out through the outlet  18 . 
   The outlet  18  comprises a simple aperture  22  within the side wall of the housing  14 , and a hollow box construction  24  mounted around that aperture  22 , the box  24  being provided with a further aperture  26  in its side. The box  24  houses an extractor fan (not shown) to expel exhaust/airmix through the aperture  26 . 
   A source of white light in the form of a tungsten lamp  28  is seated in the enclosure  12  on the interior of one of the ends of the housing  14  and is oriented as to direct radiation along the interior of the cylinder  20 . 
   Seated on the interior of the other end of the housing  14  is a detector block  30  having a multiplicity of sensors  32  in an operative face  34  of the block  30 . This operative face  34  is directed to receive radiation which has been emitted from the lamp  28  and which has passed through the interior of the cylinder  20 . 
   Each sensor  32  is selectively sensitive to a particular wavelength of electromagnetic radiation. Thus, two may be selectively sensitive to radiation of the wavelength 300 nm, two may be selectively sensitive to radiation of wavelength 500 nm, and two may be selectively sensitive to red light of wavelength 680 nm. The selective sensitivity may be effected by means of respective optical filters in the operative face  34 . 
   Blocks  36  and  38  are mounted within the enclosure  12  to direct curtains of air across the operative faces of the source  28  and detector block  30 , respectively. The spacing between the air curtains defines the effective optical path length for the measurement of opacity, and it will be appreciated that this somewhat exceeds the length of the cylinder  20 . It will also be appreciated that the introduction of air by the blocks  36  and  38  significantly dilutes the exhaust with clean air before it exits through the aperture  26 . 
   Circuitry is provided for the sensors  32  as shown in  FIG. 2 . This comprises a plurality of amplifiers  40 , connected respectively to the sensors  32 . The outputs from the amplifiers  40  are connected to an Analog to Digital Converter  42 , an output of the Analog to Digital Converter is connected to an input of a processor  44  which in turn provides output signals at its output  46 . These signals indicate in digital form the respective outputs of the sensors  32  every 50 ms. 
   The processor  44  is programmed to perform the steps set out in  FIG. 3 . Thus, at step  50 , every 50 ms it cycles through the digital values of the sensor outputs and, during each cycle calculates the opacity N for each sensor n given by the equation:
 
 N   n =100*( V   n   −V   no )/ V   n100   −V   no ).
 
To enable it to complete this calculation, it receives stored values held in the processor  44  for each sensor  32  of the end of scale values V no , being the digital output from the sensor  32  when there is no exhaust in the chamber and the source  28  is switched on, and at the other end of the scale, the value of V n100  for the sensor n when there is no light reaching the sensor n, for example when the lamp  28  is switched off. (These values are created and stored in the processor  44  during a calibration procedure).
 
   At step  52 , the processor  44  calculates the k value for each sensor n as given by the equation:
 
 k   n =1/ L* log e (1−0.01* N   n ).
 
   This provides a stream of signals representing the k value for green light at output  54  every 50 ms, and a corresponding stream at output  56  for a combination of all the k factors weighted towards those for shorter wavelengths. This output  54  may be considered the output for k standard, and the output  56  for k corrected, since it takes account of particles which might be invisible to green light. 
   The processor  44  is further programmed to execute step  58  which examines the relative k values of the different sensors to provide an estimate of the dominant particle size on the basis of the different k values. These further results are provided as a stream of output signals at an output  60  which gives the dominant particle size. Outputs  54  and  56  are further processed at step  62  to provide a stream of weighted k values given by the equation:
 
 k   w   =w ( k   c   −k   s )+ k   s  
 
in which k w  is the weighted k value, w is the weighting factor (for example the value 5), k c  is the corrected k value and k s  is the standard k value. The stream of k w  values is provided at output step  64 .
 
   The signals from all the outputs  54 ,  56 ,  60  and  64  may either be stored for subsequent use or fed directly to a screen via appropriate screen drivers to provide a graph of the values of the outputs plotted against time. 
     FIG. 4  shows the Mie theory extinction graph for absorbing particles. This shows a light extinction factor Q (a value which is normalised as regards concentration of particles and the radiation wavelength) plotted against the particle, the parameter of which is given by the value of the natural constant π multiplied by the diameter d of the particle divided by the wavelength λ of the radiation. 
     FIG. 5  shows particular values on the extinction curve. Thus, the small squares show the positions on the curve for particles of size 50 nm for the wavelengths 680 nm (corresponding to red light), 560 nm (corresponding to green light), 440 nm (corresponding to blue light), and 310 nm (corresponding to ultra-violet light), progressing from the left to the right of the diagram. The diamond shapes show these values for particles of 100 nm, the circle for particles of 150 nm, the triangles for particles of size 200 nm and the large squares for particles of size 250 nm. It will be seen from this diagram that there is very little difference in the extinction factor for particles the size of which, relative to the wavelength which is considered, is large. Once the particle size is small compared to the wavelength considered, the extinction factor is very low. The greater the spread in the readings for the different sensors for the apparatus shown in  FIG. 1 , the closer is the particle size parameter to the range 0.5 to 1.0, where the divergence is greatest. 
     FIG. 6  shows the k values with respect to time as given by the apparatus shown in  FIG. 1  with the latter being connected to the exhaust of a diesel engine having a common rail. The peaks represent free accelerations and the spacing for the different colours indicates a dominant particle size of between 100 and 150 nm. 
     FIG. 7  shows a plot of the k value as a function of wavelength of light being attenuated by the presence of 150 parts per million of nitrogen dioxide in nitrogen gas. It will be noted that at the red end of the spectrum, where the wavelength is long, there is substantially no attenuation. This means that red light, with a wavelength of substantially 680 nm, is substantially unaffected by nitrogen dioxide. At the same time, because of the shape of the curve, being very roughly an inverted V for the range 300 nm to 500 nm, there are two different wavelengths for which the attenuating effect of nitrogen dioxide is substantially the same. For maximum resolution, therefore, selection of 300 nm and 500 nm as the two different wavelengths for the illustrated apparatus to work in accordance with the invention is desirable. 
     FIG. 8  shows the ratio (k 300 −k 500 )/k 680  as a function of particle size, where k n  is the k value for wavelength n nm, the ratio being substantially independent of the concentration of nitrogen dioxide in the exhaust, and substantially independent of particle concentration. This function may be used to determine the dominant particle size in step  58 . 
   It will thereby be appreciated that the apparatus shown in  FIG. 1  provides an indication of the density of particles in the exhaust and an indication of the dominant particle size in that exhaust, substantially independently of the nitrogen dioxide content of the exhaust. A given threshold value for these readings, for example a given threshold value for the output signals of the output  64  may be agreed to determine whether or not the engine passes or fails an emissions test. 
   The apparatus shown in  FIG. 1  may be modified in various different ways without taking the resulting construction outside the scope of the present invention. For example, there may be fewer sensors  32 , perhaps as low as only two sensors providing measures of attenuation at different wavelengths. Conversely, there may be more than the six sensors shown in  FIG. 1   b , with a cluster of sensors on each wavelength to enable a better mean value to be obtained for that wavelength. The path of the exhaust may be in a meandering fashion instead of the longitudinal contra-flow arrangement shown in  FIG. 1 . 
   The illustrated apparatus can be readily mounted on the rear of a vehicle, with the inlet  16  attached via a flexible hose (not shown) to the vehicle engine exhaust. It is suitable for in-service particulate testing of diesel vehicles, and can be used to assess oil burning in gasoline vehicles. The electrical means may be provided with the apparatus  10  or at least partially by a portable computer. The apparatus has a rapid response enabling it to measure transient effects. 
   All the output values at outputs  54 ,  56 ,  60  and  64  can be smoothed by processing them in accordance with the equation:
 
k av 0.9k old +0.1k new  
 
in which k new  is the latest value, k old  is the last calculated averaged or smoothed value, and k av  is the averaged or smoothed k value. Peak values may be captured for transient tests.