Vehicle exhaust gas analysis system

The contaminants in the exhaust gas emissions from a motor vehicle such as carbon monoxide (CO) and hydrocarbons (HC) are analyzed and the concentration of the contaminants together with other pertinent data is displayed in a digital manner. The exhaust gas emissions are fed into a sample cell contained within a nondispersive infrared analyzer and the absorption of an infrared radiation beam at selected wavelengths by the gas within the cell is measured. A reference cell containing a reference gas is also positioned in the infrared radiation beam path. By means of a rotating chopper disk positioned in the light path, the infrared radiation beam passes alternately through the reference cell and the sample cell and is focused at a plurality of detectors whch are each sensitized to a narrow wave band by a filter and which receive the alternate sample cell and reference cell radiation pulses. Synchronization of the system is provided by a notch in the chopper disk which passes alternately between three light sources and associated photoresponsive devices. Automatic span correction is obtained by an automatic gain control circuit which maintains the reference cell output at a predetermined amplitude. The sample cell is initially filled with ambient air by a gas transport system. By measuring the sample cell and reference cell outputs with ambient air in the sample cell, and then measuring the sample cell and reference cell outputs with exhaust gas in the sample cell, a ratio can be computed which is proportional to the amount of the contaminant. Computation of the ratio in this manner results in automatic zeroing and spanning of the system. The computed ratio is then calibrated for nonlinearities in the system, and linear corrections are made for variations in ambient pressure and for variations in the temperature of the gas passing through the sample tube. Provision is also made for assuring that the vehicle being tested has achieved a predetermined engine speed and that no blockage has occurred in the gas transport system when the measurements are made. After the exhaust gas has been analyzed it is replaced in the sample cell with ambient air to prevent contamination of the system optics from the exhaust gas. A sapphire window is positioned in front of the infrared source to reduce changes in source temperature by air currents produced by the rotating chopper disk.

We claim:

1. A gas analysis system for measuring the amount of a selected contaminant contained within a sample gas comprising

a gas sample cell adapted to contain a gas,

a reference cell containing a reference gas,

means for filling said gas sample cell with ambient air,

means for producing a moving beam of infrared energy and passing said beam alternately through said reference cell and said sample cell,

detector means positioned to receive said energy beam after it has passed through said cells, said detector means including filter means which transmit therethrough only a selected narrow wave band within the spectrum of said infrared energy beam, said detector means producing first and second electrical signals indicative respectively of the absorption of said infrared beam within said reference and sample cells when said sample cell contains ambient air,

means for filling said gas sample cell with a sample gas,

means for passing said moving beam of radiant energy alternately through said sample cell and said reference cell while said sample cell is filled with said sample gas, said detector means producing in response thereto third and fourth electrical signals indicative respectively of the absorption of said infrared beam within said reference and sample cells,

means including a signal processing unit for receiving said first, second, third and fourth signals and producing therefrom an output signal proportional to the concentration of said selected contaminant in said sample gas.

2. A gas analysis system as in claim 1 in which said signal processing unit includes means providing said output signal as a function of the product of said first and fourth signals divided by the product of said second and third signals.

3. A gas analysis system as in claim 1 in which said signal processing unit includes

means for producing a first ratio signal indicative of the ratio of said first signal to said third signal, for producing a second ratio signal indicative of the ratio of said fourth signal to said second signal, and for multiplying said first and second ratio signals to produce said output signal.

4. A gas analysis system as in claim 1 and including means for compensating said output signal according to a predetermined nonlinear function to produce a compensated output signal equal to the magnitude of said selected gas contaminant.

5. A gas analysis system as in claim 1 in which said detector means includes first and second infrared detectors, each detector producing said electrical signals in response to said infrared beam.

6. A gas analysis system as in claim 5 in which said filter means includes first and second infrared filters positioned respectively between said first and second detectors and said infrared energy beam.

7. A gas analysis system as in claim 6 in which said first filter passed therethrough, infrared radiation only in a wave band in which carbon monoxide absorbs said radiation.

8. A gas analysis system as in claim 6 in which said second filter passes therethrough infrared radiation only in a wave band in which hydrocarbons absorb said radiation.

9. A gas analysis system as in claim 1 in which said means for producing a moving beam of infrared energy comprises

a source of infrared energy,

mirror means for focusing said source of infrared energy on said detector means,

a disk member having a slot therein and adapted for rotation interposed between said source and said mirror means,

and means for producing rotation of said disk member.

10. A gas analysis system as in claim 9 and including means for synchronizing the rotation of said disk member with the generation of said electrical signals.

11. A method for analyzing a sample gas and determining the concentration of a selected contaminant therein comprising the steps of

generating a rotating beam of infrared radiation,

passing said radiation beam alternately through a sample gas cell filled with a sample gas and a reference gas cell filled with a reference gas positioned in the path of said rotating beam,

filling said sample cell with ambient air,

detecting the absorption of said infrared beam within a selected wave band by the ambient air within said sample cell and by the reference gas within reference cell and producing respectively first and second electrical signals indicative thereof,

removing the ambient air from said sample cell and filling said sample cell with a sample gas,

detecting the absorption of said infrared beam within a selected wave band by the sample gas within said sample cell and by the reference gas within said reference cell and producing respectively third and fourth electrical signals indicative thereof,

and computing a ratio signal by multiplying the ratio of said third and first electrical signals by the ratio of said second and fourth electrical signals, said ratio signal being a function of the concentration of said selected contaminant in said sample gas.

12. A method as in claim 11 and including the step of compensating said ratio signal according to the curve of FIG. 12.

13. A method as in claim 11 and including the step of compensating said ratio signal according to the curve of FIG. 13.

14. A method as in claim 11 and including the step of detecting the absorption of said infrared beam by the gases within said sample and reference cells within two distinct wave bands and producing said plurality of electrical signals representative of the absorption of said infrared beam by the gases within said cells for each of said two wave bands.

15. A method as in claim 14 in which the step of detecting the absorption within two distinct wave bands includes the steps of

positioning a first detector in the path of said infrared beam and interposing a first filter between said infrared beam and said first detector, said first filter passing therethrough infrared radiation only in a wave band in which carbon monoxide absorbs said radiation,

and positioning a second detector in the path of said infrared beam and interposing a second filter between said infrared beam and said second detector, said second filter passing therethrough infrared radiation only in a wave band in which a selected hydrocarbon absorbs said radiation.

16. A method as in claim 11 and including the steps of

compensating said ratio signal in accordance with a predetermined nonlinear function to produce a compensated ratio signal.