Dynamic Measurement of Carbon Monoxide Concentrations in Automotive Exhaust Using Infrared Diode Laser Spectroscopy

Infrared diode laser spectroscopy was used to make the first measurements of carbon monoxide (CO) concentrations in automotive exhaust with a time response fast enough to be useful in the analysis of engine and emission control system dynamics. Currently used measurement techniques are too slow to follow rapidly changing CO levels. Infrared diode laser spectroscopy can selectively measure CO in the presence of large amounts of water vapor, permitting the direct analysis of the full exhaust flow. Carbon monoxide concentrations were measured before and after a three-way catalytic converter. The response time of the laser system (for a change in amplitude of 10%–90% of fullscale) was 25 ms, more than adequate to resolve all CO transients of interest. The instrument was capable of resolving concentration changes on the order of 0.1 vol % before the catalyst and 0.02 vol % after it. A minicomputer was used to simultaneously collect data on CO concentrations before and after the catalyst, and also the output of an oxygen sensor located in the exhaust line. These signals were recorded under various engine operating conditions: 1) constant A/F, 2) step changes in A/F (rich to lean and lean to rich), and 3) oscillation of the A/F at frequencies on the order of 1 Hz.