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This report describes a new, maintenance-free exhaust gas analysis system for automobile quality control. It incorporates non-dispersive infrared. (NDIR) gas analyzers employing a cross-flow modulation method which provides virtually drift-free performance and eliminates the need for optical adjustment, Analyzer modulation is by means of alternating the flow of sample gas and reference gas into two cells with a rotary valve of simple construction. Microcomputers are used for system control and to process the data. This system measures oxides of nitrogen (NOx), total hydrocarbon (THC) and carbon monoxide/dioxide (CO/CO2) with three analyzers. Full scale ranges of 50 ppm for NOx and 20 ppm for THC are feasible with cells merely 35 mm long. In each case the signal-to-noise ratio is 100. In actual operation, the system drift was so low that it required no span calibration over a period of three months.

This paper reports on real-time mass concentration measurements using the technique of photoacoustic spectroscopy. The measurements were made using a differential photoacoustic cell in which infrared absorption by diesel particulates at the 10.59 μm wavelength was measured with a 2.2 watt CO2 laser. Infrared absorption of the diesel particulates at 10.59 μm wavelength was treated as Rayieigh absorption since the mean mass diameter is around 0.1 μm and small enough to neglect the size effects. A dual cell method allows the separation of the particulate absorption signal from that of interfering gases present in diesel engine exhaust. In practice the sample gases from the dilution tunnel were introduced into two cells, one with and one without a particulate filter. The signals were calibrated by simultaneous measurement using the diesel particulate filter method of the U.S. EPA.