WO1993010438A1

WO1993010438A1 - Spark-excited fluorescence sensor

Info

Publication number: WO1993010438A1
Application number: PCT/US1992/009672
Authority: WO
Inventors: Hiroshi Kimura; Tetsuo Hadeishi; Harold M. Olsen; Chan S. Bak
Original assignee: Hughes Aircraft Company
Priority date: 1991-11-15
Filing date: 1992-11-09
Publication date: 1993-05-27
Also published as: US5333487A; JPH06503429A; EP0567624A1; IL103716A0

Abstract

A spark-excited fluorescence sensor (10) is provided, which enables monitoring of various gas species (14), such as H2, COx, NOx, O2, N2, NHx and hydrocarbons added to a system as source fuels and/or additive agents, or discharged from a system as exhaust including pollutants, for more efficient use of fuels for optimizing performance of the system, and, also, for reducing pollutants in the atmosphere. The spark-excited fluorescence sensor of the invention comprises a spark plug (12) to excite molecules of the gaseous species, an optical fiber window (18) and an optical fiber bundle (20) to collect and transmit, respectively, the fluorescence, bandpass filters (24) to select predetermined wavelengths corresponding to the gases to be detected, detectors (26), and signal processor (28). The output from the signal processor is then used to improve overall performance of the system.

Description

SPARK-EXCITED FLUORESCENCE SENSOR

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the monitoring of gas species as atmospheric pollutants, and also as source fuels, additive agents and emissions of various systems and to improve the operating efficiency of the various systems.

2. Description of Related Art

Monitoring of intake and output gases of systems, such as, but not limited to, fuel cells, smoke stacks, burn-boxes, fume hoods, and in particular, internal combustion systems and sources of pollution would allow the efficient use of fuels and the control and reduction of pollution discharged into the atmosphere.

Unfortunately, although there are solid state sensors to monitor individual gases, such as NO, CO, H₂O, their response times are too long to enable monitoring instantaneous variations in the concentrations of the gas species to provide feedback for control. Also, these prior art sensors tend to suffer from poisoning by various gas molecules in the exhaust. Thus, there is a need for a reliable sensor which can monitor and control systems which use and discharge various gaseous species with a fast response time.

SUMMARY OF THE INVENTION

In accordance with the invention, a spark-excited fluorescence sensor is provided, which enables monitoring of various intake and output gas species of the above- mentioned systems. The sensor according to the invention is particularly sensitive to various gas species, such as ^H2 ^C0 _X' ^N0 _X' °₂ > ^N ₂' ^NHx hydrocarbons and additive agents. The monitoring by the sensor provides more efficient use of gases as fuel sources, optimization of performance and, also, can be used, in conjunction with associated controls, to reduce pollutants discharged into 5 the environment.

The spark-excited fluorescence sensor of the invention comprises:

(a) excitation means to excite molecules of the various gas species from a ground state to excited states,

^■•C whereby the molecules in the excited state emit fluorescence upon decay to the ground state;

(b) an optical collection means to collect the fluorescence emitted;

(c) an optical transmission means to transmit the 5 collected fluorescence as an optical signal;

(d) filter means to select pre-determined bands of wavelengths corresponding to the gaseous species to be detected;

(e) detection means for converting the optical 0 signals to corresponding electrical signals; and

(f) signal processing means to provide output signals corresponding to the concentration of each gaseous species detected.

A method for monitoring various gaseous species is al- 5 so provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a spark-excited flu¬ orescence sensor of the invention;

FIG. 2 is a schematic diagram depicting use of the sensor of the invention in an automobile;

FIG. 3, on coordinates of intensity ratio and concen¬ tration in parts per million, is a calibration plot of the ratio of the fluorescence intensity of CO to N₇ as a func¬ tion of concentration of CO in N₂; FIG. 4 is a calibration plot similar to that of FIG 3, but at lower CO concentrations;

FIGS. 5a-d, on coordinates of intensity (in arbitrar units) and wavelength (in nm) are spectra showing the posi tions of fluorescence bands in CO (FIG. 5a), N₂ (FIG. 5b), CH (FIG. 5c), and 0₂ (FIG. 5d) ; and

FIG. 6, on coordinates of intensity and wavelength i nm, is a plot of the fluorescence intensity of CO in N₂ a a function of wavelength.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1, there is shown a spark-ex cited fluorescence sensor 10 of the invention, which com- prises means 12 to excite molecules, shown generally at 14. An excitation source 16 is used to activate the excitation means 12.

Molecules excited from a ground state to upper excite states emit fluorescent light upon decay to the groun state. This fluorescent light is at a wavelength that is unique and characteristic for each molecule.

A light collection means 18 and a light transmissive means 20 permit the fluorescence generated by the excita¬ tion means 12 to be collected and transmitted, respective- ly, to a plurality of bandpass filters 22 for selecting out undesired wavelengths and for permitting only pre-selected wavelengths to pass through to a plurality of detectors 24. In this manner, only those fluorescent energies associated with particular molecules are detected, each by its own de- tector. For example, the bandpass filters 22 could each be set to pass through fluorescent energies associated with one of the species of CO, NO, and CH₄. The energy associat¬ ed with that species is then detected by one of the detec¬ tors, 24a, 24b, 24c. While three detectors are shown, there is no limit to the number of detectors that may be employed, each set to detect the fluorescent energy associ¬ ated with a particular gaseous species. The signal from each detector 24 is then amplified by an amplifier 26, and the amplified signal is subjected to further signal processing by processing means 28 to provide an output 30, such as a real-time indication of the guanti- 5 ties of various intake/output gases and/or pollutants for making adjustments in the system.

In the present invention, gas species added to or discharged from the system, such as fuel cells, smoke stacks, burn-boxes, fume hoods, and in particular, 0 internal combustion systems and sources of pollution, are excited by the spark discharge from the ground state to the excited states. Intensities of the fluorescence emitted at certain frequencies, when molecules, atoms, or ions relax to their ground state, are proportional to the concentrations of the species. By selecting a high oscillator-strength band whose intensity is proportional to the concentrations, and is free from interferences and quenching, then concentrations can be determined with good sensitivity. In practice, as shown in a preferred embodiment in FIG. 2, the spark plug 12 and light collection and transmissive means 18, 20 are installed in the exhaust pipe 32 of the automobile 34 at a location where the combustion of the exhaust is representative of the combustion products before the occurrence of decomposition, association, or disproportionation of the gas species. In particular, the sensor 10 is inserted in the exhaust pipe 32 between the engine 36 and the catalytic converter 38. The excitation source 16 for 30 activating the spark plug 12 is conventional.

The light collection means, preferably an optical fi¬ ber window, 18 is installed near the spark plug 12 to col¬ lect the fluorescence. The fiber optic window is encased in a guard housing 40 to protect against mechanical damage

-> R and is shielded from deposition of foreign particles.

The light transmissive means, preferably a fiber optic bundle, 20 is connected to the window 18 to transmit the fluorescences to bandpass filters 22. The bandpass filters 22 are chosen to transmit onl the selected fluorescence bands. For each of the bandpas filters 22, an optical detector 24 is interfaced to conver the optical signals to electrical signals. The electrical signals are amplified, if necessary, b amplifier 26 and transmitted to the signal processor 28, where they are converted to concentrations. Because of th dependency of fluorescence intensity on excitation energy, an internal standard must be chosen to accurately determin the concentrations of the molecules. The use of a standar also monitors, and compensates for, the attenuation o light due to changes in the surface condition of the windo 18. The internal standard is selected from molecules i the exhaust gas whose concentrations are relatively con stant, independent of the combustion process; N₂ is a goo candidate.

The concentration of a species is determined relative to the concentration of the internal standard. The signals are used to control the efficient operation of the system, or for example, the combustion conditions for a more efficient combustion or to reduce pollutants.

FIG. 3 is a calibration plot of the fluorescence in tensity ratio, I(CO)/I(N₂) as a function of the concentra tion of CO in N₂ (in ppm) . This plot, which was obtaine from apparatus substantially identical to that shown i FIG. 1, provides calibration of CO as a function of concen tration, and shows that the system behaves in a monotoni fashion. In FIG. 4, the calibration of CO in N₂ represente by FIG. 3 is extended to lower concentrations. Advantageously, the fluorescence spectra for carbo monoxide, nitrogen, methane, and oxygen have high intensit bands at sufficiently different wavelengths to provide choice of selection. FIGS. 5a-d show the positions i wavelength of fluorescence bands and their intensities in arbitrary units for CO, N₂, CH , and 0 . Any one of the bands free of interferences from other gases in the automo¬ bile exhaust emissions may be selected for monitoring. For example, in the case of a gas mixture where CO, CH₄, and N₂ are present, the band at approximately 800 nm must be cho sen for CO analysis. For CH₄, a narrow bandpass filter mus be employed to avoid interferences from CO and N₂-

Also, the relative concentrations of gases in the mix ture must be considered for the selection of the band. Th fluorescence intensities as shown in FIGS. 5a-d are fo pure gases. In the dilute state, the relative intensities may vary widely between gases, and, therefore, the sensi¬ tivity of each band must be determined. It is evident from FIGS. 5a-d that there is a strong band for CO at about 400 nm that is suitably employed in the practice of the invention. FIG. 6 shows that the CO peak, at about 400 nm, varies proportionally with concen¬ tration, as shown in the Table below.

Band selection for the determination of concentrations of molecules in the atmosphere or in the exhaust from in- ternal combustion engines is as follows: For the determi¬ nation of CO concentration in the absence of CH₄ or when the concentration of CH₄ is low relative to CO, the fluorescent band at approximately 402 nm is selected. This band is highly sensitive and does not exhibit saturation in a wide concentration range as shown in FIGS. 3 and 4; concentra¬ tions as low as 20 ppm and as high as 10,000 ppm (1%) can be measured by this technique. For N₂, the internal stan¬ dard, the band at approximately 337 nm is selected. For the determination of concentrations of multiple chemical species, their fluorescence intensities are normalized^•to that of the internal standard, N₂. For the determination of nitric oxide, NO, the band at 872 n , which is free of interferences from other molecules, is selected. This is, also, calibrated and shows that concentrations wide range, from 20 to 10,000 ppm, are measurable.

From the information in FIG. 6, it is seen that important to normalize the data to N₂, as is done in

3 and 4, since the spark intensity varies. For CO ana in the absence of CH , two optical bandpass filters, on the CO band at about 400 nm and the other for the N₂ at about 337 nm, and two detectors 24a, 24b are requir quantitatively determine the concentration of CO. In termining the concentration, the signal from the det for CO is divided by that from the detector for N₂ by signal processor 28, and the ratio is converted to co tration by a calibration curve, such as the one show FIG. 3, installed in the processor.

More specifically, the conversion of optical sig to concentrations of gaseous species is achieved by following processes:

1. The optical signals (the fluorescence gases are transmitted by optical fibers 20 to the band filters 22, where only the selected band is transmitte the photodetectors 24.

2. The photodetectors 24 convert the optical nals to electrical signals, which are amplified by the plifier 26 and then transmitted to the microprocessor

3. The microprocessor 28 takes the electr signal of each species and normalizes it with the signa the N2 gas, for example.

4. The normalized intensity of each specie then converted to concentration by the calibration c installed in the microprocessor. For examples of cali tion curves, see FIGS. 3 and 4. The installation of c bration curves in microprocessors is well-known and not form a part of this invention. 5. The concentration signals are transmitte the main computer, which is programmed to provide inst tions via output 30 to optimize the monitored system. The instructions include, but are not limited to, sending an alarm signal or shutting down the system when concentrations reach a certain level; providing a warning indicator signal to replace a failed component, such as 5 the catalytic converter; adjusting the intake gas levels which are monitored by the invention in a feedback loop with the analysis of the output gas levels; and adjusting fuel/air mixture ratios. All of these actions provide optimum operation of the systems for efficient use of 10 fuels and reduction of pollutants.

The data supporting the present application were ac¬ cumulatedon a test apparatus, analogous to that depicted in FIG. 1, comprising a conventional spark plug, such as AC, Bosch, or NGK, using a conventional distributor module and ⁱ⁵ coil from a 1990 model Chevrolet S-10 pickup in a 6 cylin¬ der, 4.3 liter engine. The pick up coil in the distribu¬ tor, which was driven mechanically, was replaced with a pulse generator through a pulse transformer. Using a maxi¬ mum red-line speed of 6,000 rpm engine speed, the coil ⁰ fired at 600 times/sec. The test set could be varied to fire from 5 times/sec to 4,000 times/sec. The data herein was collected at 500 cps (times/sec). The battery connec¬ tion to the automobile coil was replaced with a 12 volt DC power supply. 5

A test chamber replaced the actual environment, such as the exhaust pipe in the automobile. A gas inlet and gas outlet capability were included, as well as a heater element to simulate exhaust gas temperatures. As the test _Q gases passed the excited spark plug, optical emission spectra were seen by a monochromator via a fiber optic window (Suprasil) and either an optical lens or an optical fiber. The window did not absorb the optical wavelengths of the spectra under study. The spectral beam could then be concentrated by using the optical lens or the fiber optic cable to transmit the optical signal to the monochromator. Data from the monochromator was then fed to a computer for analysis and calibration. Thus, there has been disclosed a spark-excite fluorescence detector for detecting gaseous species i various systems. Although the embodiment described was a automobile exhaust system, the invention should not b limited thereto and the invention is intended to apply t many systems which use and/or discharge gaseous specie during operation, such as fuel cells, smoke stacks remediation systems, and internal combustion system generally. It will be appreciated that various change and modifications of an obvious nature may be made withou departing from the spirit of the invention. For example, the invention may include a chamber to sample the gases i a smoke stack and provide the sample to the excitatio means. The invention may include monitoring gaseou levels of additive agents, such as ammonia, added to remediation system to react with NO_x, to reduce NO_x in th atmosphere. A feedback loop between intake additive level and the output additive level and NO_x level provides constant monitoring at both ends. The molecules of the additive agent that are excited have characteristic fluorescence at wavelengths which are determined in the same way as described above for the other gaseous species. The invention senses a characteristic wavelength or band with an appropriate bandpass filter, as described above. These and other such changes and modifications are considered to fall within the scope of the invention as defined by the appended cϊaims.

Claims

CLAIMS What Is Claimed Is:

1. A spark-excited fluorescence sensor for monitoring gaseous species comprising:

(a) excitation means to excite molecules of said gaseous species from a ground state to excited states, whereby said molecules in said excited state emit fluorescence upon decay to said ground state; (b) an optical collection means to collect said fluorescence emitted;

(c) an optical transmission means to transmit said collected fluorescence as an optical signal;

(d) filter means to select pre-determined bands of wavelengths from said optical signal, corresponding to said gaseous species to be detected;

(e) detection means for converting said filtered optical signals to corresponding electrical signals; and

(f) signal processing means to process said elec- trical signals and to provide output signals corresponding to the concentration of each gaseous species detected.

2. The sensor of Claim 1 further including amplifica¬ tion means to amplify said electrical signals from said de- tection means.

3. The sensor of Claim 1 wherein said excitation means comprises a spark plug activated by an excitation source.

4. The sensor of Claim 1 wherein said optical collec¬ tion means comprises an optical fiber window.

5. The sensor of Claim 1 wherein said optical trans¬ mission means comprises an optical fiber bundle.

6. The sensor of Claim 1 wherein said filter means comprises a plurality of bandpass filters, each bandpass filter set to pass a unique band associated with a pre¬ determined gaseous species.

7. The sensor of Claim 6 wherein said detection means comprises a plurality of detectors, each detector opera- tively associated with one of said bandpass filters.

8. The sensor of Claim 1 wherein a gas comprising CO substantially free of CH is detected by selecting as said pre-determined band about 402 nm.

9. The sensor of Claim 1 wherein a gas comprising NO is detected by selecting as said pre-determined band about 872 nm.

10. The sensor of Claim 1 wherein N₂ is used as an in¬ ternal standard and is detected by selecting as said pre¬ determined band about 337 nm.

11. The sensor of Claim 10 wherein said signal pro¬ cessing means normalizes the optical signal from each said gaseous species with respect to the optical signal from N₂ to provide a normalized intensity for each said gaseous species and then converts the normalized intensity to con- centration by means of a calibration curve installed in said signal processing means.

12. A spark-excited fluorescence sensor for monitoring intake and -output gaseous species of environmental systems comprising: (a) a spark plug to excite molecules in said ve¬ hicular exhaust from a ground state to excited states, whereby said molecules in said excited state emit fluores¬ cence upon decay to said ground state;

(b) an optical fiber window to collect said flu- orescence emitted;

(c) an optical fiber bundle to transmit said col¬ lected fluorescence as an optical signal; (d) a plurality of bandpass filters, each band¬ pass filter set to pass a unique band associated with a pre-determined gaseous species;

(e) a plurality of detectors for converting said optical signals to corresponding electrical signals, each detector operatively associated with one of said bandpass filter; and

13. The sensor of Claim 12 further including amplifi¬ cation means to amplify said electrical signals from said detectors.

14. The sensor of Claim 12 wherein a gas comprising CO substantially free of CH₄ is detected by selecting as said pre-determined band about 402 nm.

15. The sensor of Claim 12 wherein a gas comprising NO is detected by selecting as said pre-determined band about 872 nm.

16. The sensor of Claim 12 wherein N₂ is used as an in- ternal standard and is detected by selecting as said pre¬ determined band about 337 nm.

17. The sensor of Claim 16 wherein said signal pro¬ cessing means normalizes the optical signal from each said gaseous species with respect to the optical signal from N₂ to provide a normalized intensity for each said gaseous species and then converts the normalized intensity to con¬ centration by means of a calibration curve installed in said signal processing means.

18. A method for monitoring various gaseous species to generate information for providing more efficient system performance, comprising:

(a) exciting said various gaseous species from a ground state to excited states, whereby said molecules in said excited state emit fluorescence upon decay to said ground state;

(b) collecting said fluorescence emitted;

(c) transmitting said collected fluorescence as an optical signal;

(d) selecting pre-determined bands of wavelengths corresponding to said gaseous species to be detected;

(e) detecting said pre-determined bands of wave¬ lengths and converting said optical signals to correspond- ing electrical signals; and

(f) processing said electrical signals to provide output signals corresponding to the concentration of each gaseous species detected.

19. The method of Claim 18 wherein a gas comprising CO substantially free of CH₄ is detected by selecting as said pre-determined band about 402 nm.

20. The method of Claim 18 wherein a gas comprising NO is detected by selecting as said pre-determined band about

872 nm.

21. The method of Claim 18 wherein N₂ is used as an in¬ ternal standard and is detected by selecting as said pre- determined band about 337 nm.

22. The method of Claim 21 wherein said signal pro¬ cessing means normalizes the optical signal from each said gaseous species with respect to the optical signal from N₂ to provide a normalized intensity for each said gaseous species and then converts the normalized intensity to concentration by means of a calibration curve installed in said signal processing means .