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Claims  |
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We claim:
1. A method of on-line gas analysis of a multicomponent gas emission flow,
comprising:
(a) continuously sequestering a sample flow from said gas emission flow,
diluted to lower its dew point to below room temperature, and changed in
either temperature and/or pressure to be substantially the same in
temperature and pressure as that of gases used to create reference
transmission frequency spectral data deployed in step (d);
(b) continuously irradiating said sample flow with an electromagnetic
radiation beam while mounting the amplitude of infrared frequencies in the
audio frequency range of said beam, either prior to or immediately
subsequent to irradiation of said sample flow, the produce electromagnetic
signals having discernible amplitude variations resulting from spectral
interference patterns;
(c) detecting and collecting said signals at a sufficiently high rate to
substantially completely distinguish between adjacent spectral pattern
amplitude peaks without mutual spectral interference and to permit
analysis of said signals and
(d) analyzing said signals by (i) mathematically manipulating said signals
in accordance with Beer's Law to create reformed background-corrected
data, and (ii) applying reference transmission frequency spectral data to
said reformed data for each suspected gaseous component to give a linear
quantitative measure of the presence of each and every suspected gas
component in said gas emission flow.
2. The method as in claim 1, in which in step (b) said amplitude modulation
is carried out by use of an interferometer to generate spectral
interference patterns.
3. The method as in claim 1, in which said step (d)(i) is carried out by
taking the negative logorithm of the ratio of corrected known background
spectral data to the corrected sample flow transmission spectral data to
create reformed data.
4. The method as in claim 1, in which in step (d)(ii) the reference
spectral data is in the form of a data mask control which is applied to
multiply the data values to retain strong absorption signals while
eliminating weak nonabsorbing signals for each suspected gaseous
component.
5. The method as in claim 1, in which said quantitative measure of step (d)
is rendered within at least four seconds from the irradiation of said
sample flow.
6. A method of on-line gas analysis of a multicomponent gas emission flow,
comprising:
(a) continuously sequestering a sample flow from said gas emission flow and
affecting the sample flow by (i) filtering to substantially eliminate
solid or liquid particles, (ii) diluting to lower its dew point to below
room temperature, and (iii) changing said sample flow temperature and
pressure to be substantially the same as that used to collect reference
transmission frequency spectral data deployed in step (c);
(b) continuously irradiating the diluted cooled and filtered sample flow
with an electromagnetic radation beam, which beam, after having emerged
from the sample flow, is split into two parts with one part deviated from
the path of the other part to experience a different path length and
thence such one part is recombined with said other part in a manner to
generate electromagnetic signals resulting from spectral interference
patterns;
(c) detecting and collecting said signals at a minimum of 8000 measurements
per each centimeters of path length difference to substantially completely
distinguish adjacent spectral pattern peaks without mutual spectral
interference; and
(d) mathematically manipulating the signals to make interference-free by
(i) converting an interval of detected and collected signals to base
transmission frequency spectrum data, and (ii) applying reference
transmission frequency spectral data to said base transmission frequency
spectrum data for thereby rendering a concentration per unit time for each
and substantially all of the components of the gas emission flow.
7. The method as in claim 6, in which in step (c) said signals are detected
and collected during a time interval in which said beam parts experience a
minimum of 4 cm of path length difference.
8. The method as in claim 7, in which steep (d) is carried out
substantially simultaneously during step (c) in real time.
9. The method as in claim 8, in which the component concentrations per unit
time are rendered in an elapsed time of no greater than four seconds after
sequestering said sample flow.
10. The method as in claim 6, in which said radiation beam employed in step
(b) is infrared.
11. The method as in claim 6, in which in step (a) said sequestered sample
flow is taken in a manner to be proportional to the gas emission flow's
known mass flow rate.
12. The method as in claim 11, in which the component concentration is
divided by the mass flow rate obtained from measuring the exhaust mass
flow to thereby provide a mass emission rate per unit of time.
13. The method as in claim 6, in which proportionality of step (a) is
achieved by placing a laminar flow element across the main emission flow
and placing a similar laminar flow element across the sample flow.
14. The method as in claim 6, in which said dilution is achieved by
admitting nitrogen into the sample flow over a ratio range of 5:1 to 70:1.
15. The method as in claim 6, in which said reference absorbance frequency
data is masked to contain only the minimum and maximum spectral data
frequencies.
16. The method as in claim 6, in which step (d)(i) is carried out by the
use of fourier-transform spectroscopic techniques.
17. A method of measuring the multicomponent constituency of a gas emission
flow, comprising:
(a) continuously sequestering a sample flow of said gas emission flow which
(i) has been filtered to substantially eliminate solid or liquid
particles, (ii) is proportional to the gas emission flow's known mass flow
rate, (iii) has been diluted sufficient to lower its dew point to below
room temperature, and (iv) has been changed in temperature and pressure to
be substantially the same as that used to collect reference transmission
frequency spectral data;
(b) continuously irradiating said diluted sample flow with an infrared
electromagnetic radiation beam to generate test spectral data;
(c) while said steps (a) and (b) are simultaneously proceeding, detecting
said test spectral information with a resolution of at least 0.25
cm.sup.-1 ; and
(d) mathematically manipulating the signals to make interference-free by
(i) converting an interval of detected signals to base transmission
frequency spectral data, and (ii) applying reference transmission
frequency spectral data to render a component concentration per unit time.
18. The method as in claim 17, in which said reference transmission
frequency spectral data is masked to contain only minimum and maximum data
frequencies.
19. The method as in claim 17, in which step (d)(i) is carried out by the
use of fourier-transform spectroscopic techniques.
20. The method as in claim 17, in which a tracer gas is introduced at known
rate and mixed with said gas emission flow to enable measurement of the
mass flow rate of the sample flow.
21. The method as in claim 17, in which in step (d)(ii) the application is
carried out by (1) ratioing known background transmission frequency
spectral data to base transmission frequency spectral data, (2) taking the
negative log of the ratio to create reformed absorbance spectral data, and
(3) substrating reference transmission frequency spectral absorbance data,
for each suspected component, from the reformed absorbance spectral data
to render a component concentration per unit time interval.
22. A gas sampler device, comprising:
(a) a channel having a length of 3 feet or less and an average aspect ratio
(length to diameter) of 24 to 9 for conducting a gas emission flow
therethrough between an entrance and exit of said channel;
(b) means for introducing and mixing a tracer gas at a known rate into said
gas emission flow adjacent the entrance thereof;
(c) means extracting a sample flow from the said channel adjacent the exit
thereof which is proportional to the known mass flow rate of said gas
emission flow; and
(d) means for diluting the sample flow with an inert gas to lower its dew
point to below room temperature.
23. The device of claim 22, in which said means of (c) comprises a laminar
flow element extending across said channel and a similar laminar flow
element extending across the sample flow.
24. The device as in claim 22, in which said tracer gas is carbon
tetrachloride.
25. An on-line gas measurement apparatus, comprising:
(a) dilution tube means for sequestering a sample flow from a gas emission
flow, the sample flow being filtered to substantially eliminate solid or
liquid particles, diluted to lower its dew point to below room
temperature, and changed in either temperature and/or pressure to be
substantially the same in temperature and pressure so that of gases used
to create reference transmission frequency spectral data;
(b) FTIR apparatus effective to produce electromagnetic signals with
discernibl amplitude variations resulting from irradiating said sample
flow;
(c) computer means for (i) detecting and collecting said signals at a
sufficiently high rate to substantially completely distinguish between
adjacent spectral amplitudes without mutual spectral interference, and
(ii) analyzing said signals in accordance with Beer's Law to create
reformed background-corrected data and applying reference transmission
frequency spectral data to said reformed data for each suspected gas
component to give an linear quantitative measure of the presence of each
and every suspected gas component in said gas emission flow. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
1. Technical Field
This invention relates to the art of gas analysis and, more particularly,
to instantaneous on-line analysis of gas flows having multicomponents.
2. Description of the Prior Art
Gas analysis has wide ranging utility, from the measurement of respiration
of humans or animals to the measuremetn of the effluence of combustion
chambers, including automotive emissions. Gas analysis has conventionally
been accomplished by the use of dilution tubes and by the use of liquids
or solids off-line from the flow of gases under analysis. These techniques
are inadequate fro modern purposes because of the inability to provide
instantaneous dynamic information and measure only a single component per
technique. These techniques are unable to process increasingly larger
volumes of data.
Analyses without liquids or solids have included chemiluminescence, flame
ionization, and total hydrocarbon analysis, all without the use of
infrared spectroscopy. These modes have proved inadequate because (a) the
analysis is of a single component, (b) takes too long, sometimes weeks,
(c) the data for separate components has no commonality in response time
and thus cannot be readily combined, (d) the sensed data suffers from
cross-interferences of the added chemicals, and (e) some gaseous compounds
cannot be analyzed.
One of the most recent adaptation for gas analysis has been the use of
infrared spectroscopy. Although infrared specftroscopy has been used as a
quality control technique to obtain information on the composition of
chemical products for many years, it has been used essentially off-line
and primarily for measurement of nongases. Samples are typically prepared
as thin films or solutions and measured in a quality control room with a
laboratory instrument. Unfortunately, inherent time delays between actual
material production and analytical results can typically range from a few
hours to several days, which can result in costly waste and production of
unacceptable material. Fourier-transform, infrared spectrometric
techniques have been applied to particles suspended in gas flows (see U.S.
Pat. No. 4,652,755).
In those prior art applications where infrared spectroscopy was applied to
gas analysis, there was no dilution of the gas sampler and therefore the
gas itself had to be heated to a temperature in excess of 100.degree. C.
to accommodate samples with high water vapor. If other reference
information was applied to such detected information, the reference
information had to be taken at identical elevated temperatures, which made
the entire methodology extremely complex, delicate and difficult to
calibrate. In U.S. Pat. No. 4,549,080, filters were used to look at
isolated wavelengths, again without dilution.
The task of measuring emissions from vehicles has become increasingly more
difficult. Demands for lower detection limits have arisen from the
development of more efficient catalytic converters. Greater versatility is
required for work with alternate fuels as new and as yet uncharacterized
gas species are encountered. In addition to these requirements, a need for
more efficient engines with lower emission rates necessitates the
development of fast, on-line instrumentation, capable of analysis during
transient engine operation. Such new analysis will permit in-depth
examination of the combustion process in lieu of the current cumulative
information obtained from conventional emissions instrumentations having
expensive exhaust handling equipment including constant volume sampling.
The inventors herein have applied infrared spectroscopy to the on-line
analysis of gases, particularly auto emissions. Our earlier work, as
described in scientific publication "On-Line Characterization of Vehicle
Emissions by FTIR and Mass Spectrometry", Butler et al, SAE Paper #810429
(1981), describes a system for dynamic analysis of vehicle emissions; the
analysis system was comprised of a fourier transform, infrared
spectrometer, a quadropolemass spectrometer, and a total hydrocarbon
analyzer. Although it allowed on-line measurement of regulated and
nonregulated emissions from a steady-state gas stream, the system needed
to be calibrated with some difficulty. The three major apparatus
components were significantly expensive; but, most importantly, an
unusually large size, constant volume sampling apparatus was required for
dilution of the sample gas. The speed at which such an integrated system
operated was at the rate of three second. However, the data was analyzed
off-line, rendering an analysis not in real time (while the test if
on-going). This introduces an analysis time which is not considered
sufficiently fast for the demands of new applications. It the total
hydrocarbon analyzer, quadropole mass spectrometer, and constant volume
sampling unit could be eliminated, the cost of the system would be
significantly reduced. If the remaining components could be improved in
response time, the speed of data collection could be increased
significantly. Furthermore, if the data could be processed in real time
(during the test), the utility of information would be greatly enhanced
because adjustments can be made immediately and effects of the adjustments
can be seen.
SUMMARY OF THE INVENTION
A principal object of this inventino is to provide an improved gas analysis
method and a simplified, unique combination of elements that would
eliminate the need for extended apparatus such as mass spectrometers,
air/fuel ratio sensors, and hydrocarbon analyzers while, at the same time,
generating extremely high resolution, high volume spectral data, and
extremely fast speeds for measuring and exhibiting data on a myriad of
gaseous components in real time.
Another object of this invention is to provide an improved gaseous sampling
device that eliminates water condensation contamination and ensures
proportionality of mass flow rate of the sampled gas.
Still another object of this invention is to provide an improved on-line
measuring apparatus for multicomponent gas emission flows, the apparatus
being characterized by improved data collection speed, greater freedom
from false or interference data, and is much less costly to fabricate.
The method steps of this invention comprise: (a) continuously sequestering
a sample flow, from a gas emission flow, the sample flow bing (i) filtered
to substantially eliminate solid or liquid particles, (ii) diluted
sufficiently to lower the dew point of such sample flow to below room
temperature, and (iii) changed in either temperature and/or pressure to be
substantially the same in temperature and pressure as that of gases used
to create reference transmission frequency spectral data; (b) continuously
irradiating the sample flow with an electromagnetic radiation beam, while
modulatin the amplitude of infrared frequencies in the audio frequency
range of the beam either prior to or immediately subsequent to irradiation
of the sample flow to generate electromagnetic signals having discernible
amplitude variations resulting from spectral interference patterns; (c)
detecting and collecting said signals at a sufficietnly high rate to
substantailly completely distinguish adjacent spectral pattern peaks
without mutual spectral interference and to permit analysis of the siganls
in real time; and (d) analyzing the signals in real time by (i)
mathematically manipulating said signals in accordance with Beer's Law to
create reformed background corrected data, and (ii) applying reference
transmission frequency spectral data to the reformed data for each
suspected component to give a linerar quantitative measure of the presence
of each and every suspected gas component in the gas emission flow. Beer's
Law is often recited as:
##EQU1##
where I=intensity of light coming out of absorbance cell
I.sub.o =intensity of light going into absorbance cell
l=gase of natural log, i.e., 2.71828
.alpha.=particle density x pathlength
s=absorbance spectrum
f=frequency or wavenumber
More particularly, step (b) advantageously comprises continuously
irradiating the sample flow with a light beam, splitting the partially
absorbed beam emerging from the sample flow into portions, sending one
portion over a fixed length and the other over a variable length
determined by movement of a mirror over a predetermined stroke, and
recombining the beam portions to generate light signals resulting from
spectral interference patterns. More particularly, step (c) preferably
comprises detecting and collecting the signals at a minimum of 8000
measurements per each centimeter of path length difference of the beam and
for a minimum of 4cm of path length difference. More particularly, step
(d) advantageously comprises mathematically manipulating the signals by
(i) converting an interval of detected signals to base transmission
frequency spectrum data, (ii) rationing known background transmission
frequency spectral data to the base data and taking the negative data,
(iii) subtracting reference transmission frequency spectral absorbance
data, for each suspected component, from the reformed absorbance spectral
data to render a component concentration per unit time interval. If
desired, the concentration values can be converted to mass per mile units
by dividing the mass flow rate of the sequestered sample flow into the
concentration value to render a mass per unit of time or engine usage.
Preferably, the sample flow is made proportional to the main emission
flow's mass flow rate. Such proportionality of the sample flow can be
achieved by placing a laminar flow element across the main flow as well as
placing a laminar flow element across the sample flow. Dilution may
preferably be achieved by admitting nitrogen gas into the sample over a
flow ratio range of 5:1 to 70:1.
Preferably, the mathematical conversion step is carried out by the use of
fourier-transform to provide transmission frequency spectral data. To
achieve interference-free signals, reference data consisting principally
of maximum/minimal signal frequency regions for a specific component is
used to extract concentration information from the expanded spectral data.
SUMMARY OF THE DRAWINGS
FIG. 1 is a schematic illustration of the apparatus components effective to
carry out the steps of the method of this invention and depicts a novel
gas sampling device and displays a unique appartus combination for the
total gas analysis measuring system;
FIG. 2 is a graphical representation of voltage signals plotted against
time, such signals eminating from the interferogram;
FIG. 3 is a graphical representation of absorbance data plotted against
wavenumber eminating from fourier-transform;
FIGS. 4, 4a and 4b are graphical representations of absorbance data against
wavenumber after applying Beer's Law; and
FIG. 5 is a graphical representation of absorbance data plotted against
wavenumber showing a spectral mask for NO.sub.x developed from the data
points in a spectrum of points from 1920-1925.
DETAILED DESCRIPTION AND BEST MODE
Obtaining Gas Sample
The first step of the process comprises continuously sequestering a sample
of the gas emission flow and affecting the sample flow to make it (i)
filtered so as to be substantially devoid of solid or liquid particles,
(ii) proportional to the mass flow rate of the gas emission flow, (iii)
diluted sufficiently to lower the dew point of the sample flow to below
room temperature, and (iv) changed in temperature and pressure to
substantially the same temperature and pressure at which reference data
was collected. Preferably, the reference data is collected at room
temperature and at a pressure of 700 Torr; this will usually necessitate
cooling of the sample gas flow to achieve.
As shown in FIG. 1, a sampling device 10 is used to carry out these
functions. The device comprises a stainless steel tube 11, preferably
having an internal diameter of about 21/4 inches and a length of about 3
feet, preferably no greater than 4 feet. At the entrance or upstream end
11a of such tube (which is connected to the tailpipe or exhaust of an
engine), an inlet 12 is located for introduction of a tracer gas,
preferably carbon tetrafluoride; the tracer gas is injected at a known
rate such as, for example, 10 cubic centimeters per minute. The inlet 12
may have a throat diameter of about 1/8 inch and is directed
countercurrent to the main exhaust flow and is coplanar with the baffle 13
to achieve thorough mixing. The necessity for the injection of a tracer
gas is to make possible the measurement of exhaust mass flow independent
of fluctuations in the main emissions flow.
The tractor gas and emissions flow together, pass through the main body of
the tube, are mixed, such as by baffle 13, and encounter a main laminar
flow element 14 extending across the main flow 18 and across the internal
extent of tube 11. The element 14 permits back-pressure to increase in the
main flow in response to an increase in mass flow. A sampling outlet 15 is
provided adjacent the frontal face 14a of the main laminar flow element.
The sampling passage 16 also contains a laminar flow element 17 extending
across the sampling flow 19. A passage 21 carries the diluted flow to a
cell 22 of an infrared optical apparatus 23. A passage 24 is connected
across passage 21 and the outlet end 11b of the tube 11 to set up the same
pressure differences to allow the sample to be withdrawn. The downstream
pressure of both laminar flow elements 14 and 17 are substantially
identical, thus the fraction sampled is proportional to the rate of the
conductance of the two laminar flow elements.
The sampled flow 19 is then diluted by admission of a dilution gas, such as
nitrogen, having a purity of 99.9%, from an inlet at 20 and admitted,
preferably, at a rate of about 30 liters per minute so as to dilute the
sample flow in a ratio ranging from about 5:1 to a ratio of 70:1. Dilution
is significant because it eliminates condensation and allows the test to
take place at substantially atmospheric conditions (preferably 700 Torr),
thus minimizing pressure effects on the infrared spectrum and allowing
calibration using a preexisting reference data base.
IRRADIATION
The diluted sample flow in the cell 22 is then continuously irradiated by
being subjected to an infrared light source to provide test spectral data.
This is accomplished by use of an infrared optical apparatus 23, such as
shown schematically in FIG. 1, wherein through an optical analysis module
arrangement, the infrared light source 26 is directed by mirror 27 through
an iris aperture 25 and then again by mirror 28 through the cell 22 of a
sampling module containing the diluted sample gas flow. The partially
absorbed beam, emerging from cell 22, is directed by mirror 29 into a beam
splitter 30. The beam is split at unit 35 into two portions, one portion
is reflected by the splitter to traverse a fixed distance or length, into
corner-cube mirrors 31 and return to splitter 30 and pass through the
splitting unit 35. The other beam part is allowed to pass through unit 35
and traverse a variable length determined by movement of a sliding or
stroked corner-cube mirror 32. The corner-cube mirror 23 is moved by a
linear magnetic motor 47 operating on a shaft 48 attached to the mirror 38
through a bearing 49. The extent of the mirror cube stroke determines the
deviation of the other beam part from the first beam part. The deviated
and nondeviated beam parts are recombined by unit 35 to form light signals
resulting from spectral interference patterns.
To sense the movement of the movable corner-cube mirror 32 and determine
when to measure detected signals, a helium-neon laser fringe system 38 is
used. The beam emitted from the source at 38 passes through an alignment
device 39 associated with fixed corner-cube mirror 31 and thence is
directed by mirror 45 to pass into alignment device 46 associated with the
movable corner-cube mirror 32. A detector 50 senses the difference or
variance from zero alignment between the mirrors and thus senses the
location and movement of movable corner-cube mirror 32.
The apparatus 23 differs from conventional grating or prism instruments in
that wavelength determination is accomplished by modulating the amplitude
of each wavelength of the emitted radiation at its own unique audio range
frequency via a scanning Michelson interferometer. The interferometer used
as a Mattson SIRIUS 100, equipped with a KBr/Ge beam splitter. The light
source was a conventional ceramic glower emitting a broad band infrared
radiation, which is close to white light (the latter having all
frequencies generally intense). The cell 22 was a Wilks 20 meter variable
path cell used in the 14th order resulting in an effective pathlength of
21.75 meters.
Deviation is used herein to mean the amount of mirror travel or stroke used
in splitting off a part of the light beam for developing spectral
interference patterns. Spectral interference pattern is used herein to
mean the intensity fluctuations imposed on the original beam radiations by
the movement of the stroked mirror. An interference pattern results from
use of an interferometer giving amplitude modulation of each radiation
wavelength at its own unique audio range frequency.
DETECTION
The light signals resulting from spectral interference patterns are
directed by mirror 33 to a detector 34 (a liquid nitrogen cooled HgCdTe
photoconductor). The spectral emissions are received by the detector on a
continuous basis and are converted to an analog voltage. The amplified
voltage signals, being a linear measure of the changes in the detector
conductivity, are digitized using a computer processor 37. To obtain
higher resolution with high volume spectral data eminating from a
multicomponent gas emission flow, the detector must detect and collect the
light signals (as changes in detector conductivity) at a minimum of 8000
measurements per each centimeter of path length difference and for a
minimum of 4 cm of path length difference to create a spectrum of data.
This large spectrum of data in a very short interval permits this process
to substantially completely distinguish adjacent spectral pattern peaks
without mutual spectral interference. A measurement of the detector output
voltage is made at uniform spacings during the 4 cm of stroke (at
substantially the wavelength of the helium-neon laser). Distinguishing
adjacent spectral peaks means employing a long enough corner cube mirror
stroke to produce derived spectral patterns with 0.25 cm.sup.-1 (wave per
centimeter) spectral line widths.
The signals were collected during a three second interval by forming an
interferogram and writing the interferogram onto computer memory in
computer processor 37. The interval (which was three seconds) was
determined by the period needed to fully process the spectral data. The
detector receives the spectral emission signals aas analogue signals per
unit of time or interval, and are converted to continuously varying
digital signals per unit of time and stored in the computer memory. The
signals are received by the detector at a rate of at least 32,000 data
points in a three second interval during sweep of the sliding corner cube
mirror over a 4 cm path length.
MATHEMATICAL CONVERSION
The ability to detect and record such a high accumulation of data points in
such a short period of time is made possible by the use of a
multiprocessor computer 40-41-44 having multimemory feed paths. More
specifically, the multipath accomplished the following. While incoming
signals are being recorded and stored (i) the previously stored signals
can be simultaneously fourier-transformed to yield spectral signals in
computer 40, (ii) the transformed signals can be reduced to gaseous
component concentration values in computers 41-44, and (iii) the computed
values can be displayed on a veiwing apparatus. All of this is
accomplished in real time.
In the first stage of mathematical conversion, a spectrum or an interval of
the detected signals from spectral interference patterns (interferogram as
shown in FIG. 2) is converted to infrared intensity (absorbance) data
varying with wavenumber (frequency) as shown in FIG. 3. This is carried
out by the use of fourier-transform techniques programmed in computer 40.
A detailed description of such techniques used in computer 40 is given in
"Introductory Fourier-Transform Spectroscopy", by R. J. Bell, Academic
Press, New York (1972). Frequency analysis or fourier analysis of the
digitally recorded interferogram leads to the wavelength dependence of the
infrared intensity, the infrared spectrum. The use of the
fourier-transform spectroscopic method offers great speed advantages when
dealing with the very high resolution spectra needed for the quantitative
analysis of gas mixtures. FTIR is sometimes used herein to mean the
operations carried out by apparatus 23 and computer 40.
To increase the speed of electronic assimilation of such high volume/rate
signals, an array processor was used as part of the multiprocessor
computer 40. The multiprocessor computer also comprised a Mass comp
minicomputer 2M Byte memory, 85M Byte Winchester disks, 1.2M Byte floppy
disks, and a 40M Byte magnetic tape system to handle the data processing
display and archiving. The software package included a special purpose,
fast fourier-transform routines for comparing, combining, displaying,
plotting, analyzing and otherwise manipulating spectral files.
The resulting intensity-frequency data (as shown in FIG. 3) was corrected
(within electronic computer means 41 for data reformation) for the
contribution of room temperature stray radiation by referencing a
previously determined room temperature background. A reformed absorbance
spectrum was thus generated by calculating the negative logorithm of the
ratio of corrected known transmission spectrum of the dilution air
backgroun (i.e., taken at 700 Torr and room temperature for the dilution
tube air in the cell) to the corrected transmission spectrum of the
sample. This eliminates effects of CO.sub.2, H.sub.2 O, and trace
hydrocarbons in the ambient air.
The resulting reformed absorbance spectra (as shown in FIG. 4) were then
analyzed for components of interest by computer 44. The difficulty of
analyzing simply the reformed absorbance data to arrive at a concentration
value is demonstrated in FIGS. 4a and 4b. It should be noted that for
these absorbance spectra, Beer's Law implies that the gas component
concentration is linearly related to the spectral line strength. Only by
considerable expertise can the spectral line strengths be identified as a
specific gas species. To add the line strengths to arrive at a
concentration value is fraught with difficulty.
To analyze for components of interest by this invention, reference masks
data (contained electronically in computer means 42) are applied to
reformed absorbance data (the room temperature corrected spectral
information) in computer 44 for each suspected component to render a
component concentration.
A mask is explained as follows and by reference to FIG. 5. A linear measure
of the concentration of a gas is given by the strength of a line in its
true absorbance spectrum. When noise is present in the spectrum, a more
reliable measure is provided by summing the strengths of many lines. Such
measure is also provided by the height of a narrow line projecting above a
broader absorbance feature. The area under the absorbance curve or trace
is the measure of the gas species. The method of approximating the area
under such curve is speeded up by use of linear algebra in the form of
masks. A simulated spectrum or "mask" is prepared consisting of segments
60 made up of 1.0's at spectral positions corresponding to narrow
absorbance lines (spectral regions of strong, but not saturating unique
absorption for that gas species), segments 61 made up of several small
negative fractions at positions surrounding the 1.0's, and segments 62
made up of zero elsewhere. The negative fractions (segments 61) have value
(number of 1.0's)/(sum of negatives) and correspond to adjacent localized,
nonabsorbing spectral regions. Their purpose is first to establish the
average base level above which the narrow line protrudes, and then used to
subtract that level from each of the 1.0's. The above sum can then be
computed by multiplying together the spectrum from computer 41 (FIG. 4)
and the mask, i.e., taking their "dot" product, regarding them to be
vectors. This task is accomplished very quickly by the computer's "array"
or "vector" processor, particularly if spectal regions where the mask is
zero are ignored entirely. The constant or proportionality relating the
sum and the concentration of the gas can be determined by employing gases
of known concentration, i.e., "standard" gases.
To analyze a mixture of gases, the mask for each species present is applied
to the absorbance spectrum of the mixture. For the ideal case in which
(mask of gas A) x (spectrum of gas B)=0
for A=B, the result, upon applying the known constants of proportionality,
gives directly the quantitative composition of the sample gas. In
practice, the masks are not ideal, but interferences can be accounted for
since the response of each mask to the spectrum of each pure gas is known.
The construction of such masks depends upon the availability of a library
of reference absorbance spectra for all gases comprising the mixture to be
analyzed, in a concentration near that at which they are present in the
mix. Each mask is prepared manually with constant referral to the library,
the need to maximize response to the subject gas being weighed carefully
against introduction of either interferences from other species or
unwanted noise. The partial mask was made up of only the essential or
unique spectral distinguishing feature points of a known gas species. This
step was calibrated to give actual concentration values by applying the
partial mask to the spectrum of a carefully selected and prepared standard
sample of known concentration.
Individual masks were manually constructed for the best mode by a computer
operator from a reference spectrum of a molecule of interest using as
guides, in the choice of unique absorption bands, both the spectrum of the
exhaust sample to be measured and reference spectra of all other molecules
suspected of being present. Separate sets of masks were prepared to handle
each of the various combinations of species in concentrations encountered
in different experiments. A response matrix was then generated for each
set of masks by applying each mask successively to the reference spectrum
of each of the molecules respresented in that set. Ideally, this would be
a unit matrix, but inevitably the masks were imperfect. Such imperfections
were eliminated from the final result by multiplication with the inverse
of the appropriate response matrix, all in accordance with the
prescriptions of linear response theory. The summation value generated by
component analysis in computer 44 can be displayed in unit 43.
When a sample is introduced by means of the sampling system shown in FIG.
1, the mass emission rates are preferably obtained by combining the
component signal with the signal strength of the carbon tetrafluoride
signal and dividing by the mass injection rate of the carbon tetrafluoride
tracer.
More specifically, the exhaust mass flow of the engine may be computed by
measuring the equivalence ratio as taught in U.S. Pat. No. 4,389,881 and
combining this measurement with the hydrogen carbon ratio of the fuel, the
oxygen carbon ratio of the fuel, the carbon dioxide fraction in the
diluted sample as measured by FTIR, the carbon tetrafluoride mass
injection flow, and the carbon tetrafluoride fraction in the diluted
exhaust as measured by FTIR. The following illustrates mathematically how
this is carried out.
EXHAUST GAS FLOW (BASED ON MINI-CVS)
Lean Case (.lambda..gtoreq.1):
##EQU2##
Rich Case (.lambda.<1):
##EQU3##
where N=hydrogen/carbon ratio of fuel (no units)
P=oxygen/carbon ratio of fuel (no units)
.lambda.=oxygen equivalence of exhaust gas (no units)
Y=CO.sub.2 fraction in final diluted sample (measured by FTIR (no units)
D=CF.sub.4 tracer flow (STP volume/time)
W=CF.sub.4 fraction by FTIR (no units)
X=engine exhaust flow (STP volume/time)
Alternately, the mass emission rate is determined by sampling from a
dilution tube and by first obtaining the average dilution tube flow (in
scfm) for each test by use of a flowmeter 70. This tunnel flow was then
converted to liters per three seconds to match the time interval of the
FTIR data interval. Based on the tunnel flow and the concentration data,
the emissions in milligrams per three second interval are computed for
each gaseous component.
Table I shows an actual computer listing of compound concentrations
obtained from testing as identified.
While particular embodiments of the invention have been illustrated and
described, it will be noted by those skilled in the art that various
changes and modifications may be made without departing from the
invention, and it is intended to cover in the appended claims all such
modifications and equivalents as fall within the true spirit and scope of
the invention.
TABLE I
______________________________________
Computer Generated Quantitative Analysis
of the Spectrum of a Methanol Fuel FTP Test
Estimated Error
Compound Concentration of Measurement
______________________________________
1. H.sub.2 O 0.27 0.1%
2. CO.sub.2 0.33 0.1%
3. COHI 207.65 10.0 ppm
4. HCIC 14.26 8.0 ppmC
5. NO 27.74 0.3 ppm
6. NO.sub.2 LO
3.72 0.3 ppm
7. N.sub.2 O -0.01 0.1 ppm
8. HONO 0.33 0.1 ppm
9. HCN -0.03 0.2 ppm
10. NH.sub.3 Q
-0.06 0.1 ppm
11. SO.sub.2 -0.14 0.2 ppm
12. CH.sub.4 0.94 0.1 ppmC
13. C.sub.2 H.sub.2
-0.36 0.2 ppmC
14. C.sub.2 H.sub.4 Q
0.52 0.5 ppmC
15. C.sub.2 H.sub.6
0.06 0.2 ppmC
16. C.sub.3 H.sub.6
0.74 1.0 ppmC
17. IC.sub.4 H.sub.8 Q
1.52 1.0 ppmC
18. CH.sub.2 O
6.93 0.1 ppmC
19. HCOOH -0.08 0.1 ppmC
20. CH.sub.3 OHI
150.98 0.1 ppmC
______________________________________
Total HC = 175.52 ppmC.sub.3
Total NOX = 31.79 ppm
* * * * *
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