Exhaust Gas Analysis for Automobile Quality Control by a New Non-Dispersive Infrared Gas Analyzer

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. By use of various self-diagnostic features, this system achieved virtually maintenance-free operation; the first of its kind in such a measuring system. By providing accurate automobile exhaust gas measurement on an automobile production line, the system contributed greatly to quality control.