Apparatus for measuring the particulate matter in the flue gas or exhaust gas from a combustion process

An apparatus for measuring particulate matter in a flue gas or exhaust gas of a combustion process comprising means for transmitting a beam of light between an optical transmitter and an optical receiver. The light passes through two diametrically opposed light slits closed by transparent bodies in the wall of the tail pipe and crosses through a flow of exhaust gas carried in the tail pipe. The attenuation of intensity of the light beam is a measure of the particulate matter. When the measuring apparatus is used to measure a soot concentration in an exhaust gas of internal combustion engines of a vehicle, it is mounted directly on the tail pipe, and the transparent bodies are heated in the vicinity of the faces acted upon by the exhaust gas flow to a temperature above the soot burn-off temperature. As a result, adulteration of the measured value by soot deposits in the measurement path to the transparent bodies is prevented, and continuous operation of the measuring apparatus is assured.

BACKGROUND OF THE INVENTION 
The invention relates to an apparatus for measuring the particulate matter 
in the flue gas or exhaust gas from a combustion process. 
In a measuring instrument of this kind, the light emitted by an optical 
transmitter passes through the exhaust pipe and is converted by an optical 
receiver into an electrical signal. The particles located in the exhaust 
gas lead to a decrease in the amount of light, which is expressed as 
extinction or turbidity and is a standard for the soot emission of a 
combustion device. 
In a known apparatus of this kind (German Utility Model 81 28 634), bodies 
transparent to light are made in the form of windows, each closing off a 
passage, inserted into the light opening or slit, spaced apart from the 
tubular wall of the exhaust pipe. The result is a hydraulically idle 
volume in front of each window, which causes considerable deposits of 
particles on the windows. To eliminate the measurement errors originating 
in the dirt on the windows, one window has a retroreflector associated 
with it, which is protected in a chute against being soiled and is located 
outside the beam of light. At certain time intervals, the reflector is 
briefly moved into the beam path. It then reflects the beam of light from 
the optical transmitter that passes through the window back to the optical 
receiver. There a signal is obtained that is equivalent to the reduction 
in transmission caused by soiling of the window and provides a correction 
value for further measurement once the retroreflector is removed. This 
kind of recalibration of the measuring instrument is complicated and is 
suitable only for stationary combustion systems, but not for mobile ones 
such as internal combustion engines in motor vehicles. 
In internal combustion engines equipped with fuel injection pumps, such 
measuring equipment is used so that from the soot concentration in the 
exhaust gas, a control variable for fuel injection is derived, with which 
the full-load injection quantity can be metered in such a way that the 
maximum soot emission prescribed by law is not exceeded (U.S. Pat. No. 
3,744,461). 
In a known measuring instrument for internal combustion engines (British 
Patent 1,334,472), an electrical detector is installed on the 
circumference of the exhaust pipe carrying the flow of exhaust gas, and 
the detector receives the signal from an electrical transmitter that is 
likewise disposed on the circumference of the exhaust pipe, directly 
opposite the detector. The intensity of the signal received by the 
detector increases or decreases--depending on the type of 
detector--whenever the soot concentration in the exhaust gas flow 
increases. The electrical output signal of the detector is amplified and 
forms a direct control signal for adjustment of a valve. For maximum 
prevention of adulteration of the measurement from soot concentration on 
the detector and transmitter, a curtain of flushing air is installed 
between the exhaust gas, on the one hand, and the active elements of the 
detector and transmitter, on the other. Nevertheless, soot deposits on the 
active elements cannot be prevented, so that over the long term a drift in 
the measurement values occurs. 
OBJECT AND SUMMARY OF THE INVENTION 
The apparatus according to the invention has an advantage as with the 
measuring apparatus for stationary combustion systems, the transparent 
bodies prevent direct deposits of soot on the active elements of the 
transmitter and receiver; but in contrast to these combustion systems, the 
transparency of the bodies is assured by avoiding soot deposition, by 
means of the heater. This prevents other factors besides the soot 
concentration in the exhaust gas from causing turbidity, and hence 
adulterating the measurement signal, in the optical signal received. 
Calibration measurements can be dispensed with. Continuous, 
interruption-free operation is assured. The transparent bodies may be 
embodied as disks or bars of sapphire or quartz, which are sintered, 
cemented or pressed in the ceramic bodies. In individual cases, especially 
with disks or bars that are flush with the exhaust pipe, the prevention of 
soiling of the transparent bodies can be further reinforced by providing, 
in a feature of the invention, that the bodies are acted upon, on their 
surface facing into the flow of exhaust gas, with pulses of flushing air. 
Substantial advantages of the apparatus are attained in that in a preferred 
embodiment of the invention, the optical transmitter and receiver are 
connected to the one light slit via fiber optical wave guides; a 
reflection surface is disposed behind the other light slit, and the 
transparent bodies are located between the wave guide and the reflection 
face, and between the wave guide and the flow of exhaust gas, 
respectively. With these provisions, the length of the beam of light 
penetrating the flow of exhaust gas is doubled while the diameter of the 
exhaust pipe is unchanged; thus greater measurement accuracy is attained 
without enlarging the exhaust pipe. The reflection face may advantageously 
be formed by a retroreflector, which in turn is embodied by the suitably 
shaped, heated sapphire or quartz body. A separate fiber optical wave 
guide leads from the optical transmitter and the optical receiver to the 
one light slit. Both wave guides terminate in a common terminal piece. The 
undesirable reflection from the transmitted light on the side oriented 
toward the transmitter wave guide can be reduced by embodying the 
transparent body as a double cone. 
In a further feature of the invention, if the emitted light is divided into 
a measurement beam and a reference beam, then the temperature drift, aging 
and other harmful effects of a light emitting diode or LED forming the 
light source of the optical transmitter can be detected and compensated 
for in an evaluation circuit. If the reference diode is disposed in the 
receiver, then a third fiber optical wave guide must couple the reference 
diode to the transmitter. If the reference diode is disposed in a side 
channel behind the transmitter diode, the third fiber optical wave guide 
can be omitted. 
The aforementioned pulses of flushing air are advantageously made 
available, in a further feature of the invention by a diaphragm pump that 
is driven by the pressure pulses of the exhaust. Thus, the flushing air 
supply requires no additional energy. 
The measurement apparatus can be embodied structurally simply and 
economically if, in a preferred feature of the invention, a heating coil 
is applied directly to the transparent bodies, which are embodied as glass 
rods. The ceramic bodies can then be omitted as supports for the heating 
coils, which translates into reduced production costs. The glass rods are 
then preferably manufactured from quartz glass. 
The embodiment of the transparent bodies as elongated glass rods also makes 
it possible, in a further advantageous feature of the invention, to 
dispose the transmitter and receiver directly on the exhaust pipe, and 
thus to dispense with the fiber optical wave guides. This further reduces 
the production costs. Accommodating the optical transmitter and receiver 
on the same side of the exhaust pipe and disposing a retroreflector on the 
other side of the exhaust pipe is extremely advantageous from the 
standpoint of control technology and also makes it possible to provide a 
compact structure of the measuring instrument. 
A structural simplification is attained if in a further feature of the 
invention the LED, a photoelectric diode and a reference photoelectric 
diode are disposed on a common substrate. Then the finished diodes can be 
cast on the substrate, or the semiconductors can be bonded to the 
substrate. A mirror or a metal wafer provides shielding of light between 
the adjacent transmitter and receiver diodes. 
The invention will be better understood and further objects and advantages 
thereof will become more apparent from the ensuing detailed description of 
preferred embodiments taken in conjunction with the drawings.

DESCRIPTION OF THE EXEMPLARY EMBODIMENTS 
In FIG. 1, an internal combustion engine 10 is shown, in particular a 
Diesel engine, of a motor vehicle, serving as an example of an internal 
combustion engine; the exhaust system 11 is attached to the engine 10. The 
exhaust system comprises the pipe 12 carrying exhaust gas, which includes 
one or two mufflers 13, 14, and a tail pipe 15. The apparatus for 
measuring the soot concentration in the flow of exhaust gas through the 
tail pipe 15 is disposed on the tail pipe 15. To this end, the wall of the 
tail pipe 15 is provided with two diametrically opposed light slits 16, 
17, and in the vicinity of these light slits 16, 17 the tail pipe is 
surrounded by an attachment ring 18, which has radial bores 19, 20 in 
alignment with the light slits 16, 17. The radial bores 19, 20 are 
embodied as stepped bores with the bore diameter of the various bore 
sections 191-193 and 201-203 increasing from the tail pipe 15 outward up 
to the outer circumference of the attachment ring 18. A ceramic body 21 or 
22 is inserted into the two outer bore sections 192, 193 or 202 and 203, 
respectively and adapted to the contours of the respective bore sections 
192, 193 or 202, 203. The annular gap remaining between the ceramic bodies 
21, 22 and the bore sections 192, 193 or 202, 203 is sealed with ring 
seals 23, 24 and 25, 26, respectively, so that no exhaust gas can flow out 
of the attachment ring 18. Each ceramic body 21, 22 has an axial bore, 
which is embodied as a through bore 27 for the ceramic body 21 and as a 
blind bore 28 for the ceramic body 22. Both the through bore 27 and the 
blind bore 28 discharge on the face end of the ceramic body 21 and 22, 
respectively, oriented toward the tail pipe 15, and are there closed off 
by a transparent disk in the form of a sapphire disk 29, 30 sintered into 
the ceramic bodies 21 and 22. A platinum heating coil schematically 
represented at 31 and 32 is disposed on the ceramic bodies 21 and 22 and 
connected to two respective connection wires 33, 34 and 35, 36, which lead 
to a heating circuit 37. With these platinum heating coils 31, 32, the 
sapphire disks 29, 30 are heated to a temperature that is above the soot 
burn-off temperature, so that soot can no longer settle on the sapphire 
disks 29, 30, and so the transparency of the sapphire disks 29, 30 remains 
assured. 
A fiber optical wave guide terminal piece 38, which joins two fiber optical 
wave guides 39, 40, is inserted into the through bore 27 of the ceramic 
body 21. One wave guide 39 leads to an optical transmitter 41, and the 
other wave guide 40 leads to an optical receiver 42. In the optical 
transmitter 41, a light source embodied as a light-emitting diode or LED 
43, is coupled to the wave guide 39. In the optical receiver 42, the wave 
guide 40 is optically coupled with a photodetector, here a simple 
photoelectric diode 44. The receiver 42 is connected to an evaluation 
circuit 45, which from the light intensities retrieved, with turbid and 
unturbid light passed through the exhaust pipe, calculates an absorption 
coefficient K or the turbidity T and compares it with rpm-dependent 
threshold values stored in a performance graph. Deviations from setpoint 
values are used as control variables to correct the quantity adjusting 
device of the fuel injection pump. The light emitted by the LED 43 is 
pulsed, which compensates for the effect of scattered light from the 
environment and the thermal radiation of the hot sapphire disks 29, 30. 
The wavelength of the light is preferably in the infrared range, on the 
one hand because infrared detectors with an elevated light yield are 
available, and on the other hand because with infrared light, the 
influence on the measurement result on the particle size in the exhaust 
gas is less. 
A reflection face 46 in the form of a retroreflector foil is disposed on 
the bottom of the blind bore 28 in the ceramic body 22, so that the light 
originating in the optical transmitter 41, which is carried via the wave 
guide 39 into the attachment ring 18, crosses the tail pipe 15, is 
reflected at the reflection face 46, passes once again through the tail 
pipe 15, and is then carried via the wave guide 40 to the optical receiver 
42. As a result, the beam of light crosses the flow of exhaust gas in the 
tail pipe 15 twice, so that the measurement path that detects the soot 
concentration is twice as long as the diameter of the tail pipe 15. A 
third fiber optical wave guide 47 leads directly from the transmitter 41 
to the receiver 42, where it acts upon a reference diode 56. By means of 
this arrangement, the light emitted by the LED 43 is divided into a 
measurement beam, transmitted via the wave guide 39, and a reference beam, 
transmitted via the wave guide 47. As a result, fluctuations in intensity 
of the LED 43 can be compensated for in a simple manner in the evaluation 
circuit 45. 
Two axial bores 48, 49 extending at right angles to the radial bores 19, 20 
are provided in the attachment ring 18, each extending into one of the 
radial bores 19, 20, in the vicinity of the second bore section 192 or 
202. The axial bores 48, 49 are provided with a flushing air connection 50 
or 51, which discharges into the radial bores and these connections 
communicate via flushing air lines 52, 53 with a diaphragm pump 54. The 
diaphragm pump 54, the structure of which is known per se, communicates 
with the exhaust pipe 12 and is driven by the pressure pulses of the 
exhaust gases. The air aspirated via an air filter 55 is forced into the 
bore sections 192 and 202 in the form of pressure pulses via the flushing 
air lines 52, 53, and in these bore sections acts upon the sapphire disks 
29, 30. This provides additional cleaning action on the sapphire disks 29, 
30. The flushing air is delivered to the tail pipe 15 via the bore 
sections 191 and 201 and leaves the tail pipe along with the flow of 
exhaust gas. 
The above-described apparatus for measuring the soot concentration in the 
engine exhaust gas functions as follows: 
From the LED 43, light is emitted at a defined light intensity .phi.*. The 
beam of light exits the terminal piece 38 of the wave guide 39, passes 
through the light slit 16, crosses the exhaust gas flow, and passes 
through the light slit 17 onto the reflection face 46. The light beam is 
reflected there and takes the same course in the opposite direction back 
to the terminal piece 38, and from there, via the wave guide 40, reaches 
the photoelectric diode 44 of the optical receiver. After passing twice 
through the exhaust gas flow, the beam of light is received in the 
photoelectric diode 44 with an intensity .phi..sub.0, and if there is a 
given soot concentration it is received as more or less turbid at an 
intensity .phi.. The intensity .phi., according to Beer-Lambert's law, 
depends on the length of the measurement path L, in this case twice the 
diameter of the exhaust pipe; on the absorption properties K of the 
exhaust flow; and on the received intensity .phi..sub.0 of the light if 
the exhaust gas is pure, in accordance with the following equation: 
EQU .phi.=.phi..sub.0 .multidot.e.sup.-K.multidot.L 
In the evaluation circuit, from the known received intensity .phi..sub.0 of 
the beam of light with pure exhaust gas and from the light intensity 
.phi..sub.0 measured at the photoelectric diode 44, the absorption 
coefficient K or the turbidity T is calculated in accordance with the 
following equation 
##EQU1## 
The turbidity T is compared with rpm-dependent threshold values stored in a 
performance graph. Deviations in the actual value are compensated for by 
displacement of the governor rod of the fuel injection pump, which varies 
the fuel injection quantity. Heating the sapphire disks 29, 30 and acting 
upon them with flushing air protects the optical systems against soiling, 
so that no adulteration of the measured value arises. 
The measuring apparatus shown in FIG. 2 is in principle identical to what 
is described above, except that some advantageous modifications have been 
made. The attachment ring 18 is secured to the tail pipe 15 by means of a 
ring carrier or welded-on stopper 57. The ceramic bodies 21, 22 are 
inserted into the radial bores 19, 20 of the attachment ring 18 in such a 
way that the transparent bodies close off the light slits 16, 17 flush in 
the interior of the tail pipe 15. The operation of flushing with air is 
dispensed with entirely. The likewise heated transparent bodies are 
embodied in this case as sapphire disks 58, 59; the sapphire disks 58 
disposed in the ceramic body 21 may be embodied as a double cone, to 
reduce undesirable reflections on the side facing the transmitter wave 
guide 39. The other sapphire disk 59 in the ceramic body 22 is embodied as 
a retroreflector 60, which assumes the function of the reflection face 46 
in FIG. 1. 
To simplify manufacture, the axial bore in the ceramic body 22 is embodied 
not as a blind bore but rather as a through bore 28'. 
The reference diode 56 is disposed not in the receiver 42 but in the 
transmitter 41, in a side channel downstream, in the transmission 
direction, of the LED 43, so that the reference diode is likewise acted 
upon by some of the transmitted light. The reference diode 56 continues to 
be connected to the evaluation circuit 45. Its purpose is the same, but 
the third wave guide 47 present in 41 is not needed here. 
Instead of the sapphire bars or disks, quartz bars or disks may be used. 
The measuring apparatus shown in FIG. 3 is compact in structure and is 
disposed directly on the tail pipe 15 of the exhaust system of the engine. 
Two radially offstanding connection necks 61, 62 are welded to the tail 
pipe 15, surrounding two diametrical openings in the pipe wall. A housing 
65 and 66, provided with respective cooling fins 63 and 64, is threaded 
into each connection neck 61, 62 with an axially protruding, 
hollow-cylindrical connection tang 67 and 68. The interior openings of the 
connection tangs 67, 68 form the light slits 16, 17, which are closed off 
flush by the transparent bodies, in this case embodied as elongated glass 
rods 69, 70. The glass rods 69, 70, extending axially through each 
respective housing 65, 66 and made of quartz glass, have a heating coil 71 
and 72 on the forward portion of the circumference. Each heating coil 71, 
72 is attached directly to the glass rod 69, 70 and insulated with respect 
to the housing 65 and 66. The electrical contact of the heating coils 71, 
72 is effected via spring clamps 94, 95, which press against the heating 
coil ends and are extended continuously to the rear. 
The glass rods 69, 70 are each kept spaced apart from the housing 64, 65, 
along the end portion remote from the light slits 16, 17, in an axial 
recess of an insulating block 77 and 78 made of teflon disks. While the 
end of the glass rod 70 is provided with a retroreflection face 79 that is 
preferably joined with the glass rod 70 to make a one-piece retroreflector 
of circular or rectangular cross section, the glass rod 69 is embodied in 
prismatic fashion, with a rectangular cross-sectional face, as a beam 
splitter. The LED 80 of the optical transmitter 41, the photoelectric 
diode 81 and the reference photoelectric diode 82 of the optical receiver 
42 are disposed at the end of the beam splitter remote from the light slit 
16. These elements are fitted into suitable bores of the insulating block 
77 in such a way that the transmission direction of the LED 80 is in 
alignment with the axis 83 of the glass rod 69, and the receiving 
directions of the LED 80 and of the photoelectric diode 81 are aligned at 
right angles to the axis 83 of the rod. 
As can be seen better in the enlarged view of the optical components in 
FIG. 4, the end face of the glass rod 69 remote from the light slit 16 is 
divided into a light inlet face 84, extending approximately to the rod 
axis 83 and extending at right angles to the rod axis 83, and a reflection 
face 85 extending at an angle of 45.degree. to the rod axis 83. 
The photoelectric diode 81 and the reference photoelectric diode 82 face 
one another on the glass rod 69, so that they are respectively oriented 
toward the front and back of the reflection face 85. The photoelectric 
diode 81 is disposed such that the light reflected by the retroreflector 
70, 79 is at least partly diverted from the reflection face 85 toward this 
diode 81, while the reference photoelectric diode 82 is illuminated by 
some of the transmitted light of the LED 80, reflected by the reflection 
face 85. The course of the beam of light is represented schematically in 
FIG. 4. The end face of the glass rod 69 is illuminated by the LED 80. 
Some of the light is reflected by the reflection face 85 onto the 
reference photoelectric diode 82. The rest of the light, via the light 
inlet face 84, enters the glass rod 69, passes through it and the exhaust 
gas flow in the tail pipe 15, and passes through the glass rod 70, is 
reflected back by 180.degree. at the retroreflection face of the glass rod 
70, passes once again through the glass rod 70 and the exhaust gas flow 
and the glass rod 69, and is reflected by the reflection face 85 toward 
the photoelectric diode 81, where the light intensity is measured in the 
manner described above. 
In the exemplary embodiments shown in FIGS. 5 and 6 of the measuring 
apparatus, the optical component on the transmitter and receiver side is 
modified. The LED 80 of the optical transmitter 41 and the photoelectric 
diode 81 and the reference photoelectric diode 82 are disposed on a common 
substrate 86. This can be done by casting the already manufactured diodes 
80-82 together on the substrate 86, or by bonding the semiconductors on 
the substrate 86. Between the photoelectric diode 81 and the LED 80, a 
metal wafer 87 is provided for light shielding. The end face of the glass 
rod 69' remote from the light slit 16 is embodied as flat. The substrate 
86 is mounted directly on the end face, and the end face covers the region 
of the substrate 86 occupied by the photoelectric diode 81 and the LED 80. 
The region of the substrate 86 that includes the reference photoelectric 
diode 82 disposed beside the LED 80 is opposite the glass rod 69'. The 
course of the beam, indicated by arrows in FIG. 5, is once again such that 
some of the light transmitted by the LED 80 is reflected at the interface 
between the two optical media and is directed to the reference 
photoelectric diode 82, while the remainder of the light, after passing 
twice through the glass rods 69' and 70 and exhaust gas flow, reaches the 
photoelectric diode 81. 
The embodiment of the measuring apparatuses in FIG. 6 differs from that of 
FIG. 5 in a modified shaping of the glass rod 69". The LED 80, 
photoelectric diode 81 and reference photoelectric diode 82 are again 
accommodated on a common substrate 86, and the LED 80 is flanked by the 
photoelectric diode 81 and the reference photoelectric diode 82 in the 
same manner. The photoelectric diode 81 is shielded from the LED 80 by the 
metal wafer 87. The substrate 86 is again mounted directly on the end face 
of the glass rod 69" and extends with about two-thirds of its total area 
over the end face of the glass rod 69. The LED 80 is opposed by a portion 
90 of the end face of the glass rod 69" that extends approximately as far 
as the rod axis 83 and is inclined relative to the rod axis 83 by an acute 
angle. This angle of inclination is adjusted such that some of the light 
is reflected by the surface portion 90 to the reference photoelectric 
diode 82, while the majority of the light enters the glass rod 69 at a 
small angle of incidence. The remaining surface portion 91 of the end 
face, as before, extends at a right angle to the rod axis 83 and is 
immediately adjacent the region of the substrate 86 covered by the 
photoelectric diode 81. 
The end face of the glass rod 69" located at the light slit is also divided 
into two surface portions 92 and 93. Each surface portion 92, 93 extends 
as far as the rod axis 83. The surface portion 92 is located opposite the 
surface portion 91 of the other end face and extends parallel to it, or in 
other words perpendicular to the rod axis 83. The surface portion 93 is 
again inclined by an acute angle with respect to the rod axis 83, and the 
angle of inclination is opposite the angle of inclination of the opposed 
surface portion 90, but is equal in size. The light exiting from the glass 
rod 69" at the surface portion 93 again extends parallel to the rod axis 
83, passes through the exhaust gas flow in the tail pipe 15, and enters 
the glass rod 70 approximately parallel to it axis. After reflection at 
the retroreflection face 79, the beam of light, now having been deflected 
twice by 90.degree., passes through the glass rods 69", 70 parallel to 
their axes and finally reaches the photoelectric diode 81. The course of 
the beam is shown in FIG. 6 by arrows. 
The foregoing relates to a preferred exemplary embodiment of the invention, 
it being understood that other variants and embodiments thereof are 
possible within the spirit and scope of the invention, the latter being 
defined by dependent claims.