A Comparison of Fourier Transform Infrared Spectroscopy with Traditional Analyzers for Enhanced Inspection and Maintenance Testing 950219

The IM240[1] inspection and maintenance test is being implemented in a number of cities throughout the country. This test, which requires the vehicle to be run on a chassis dynamometer, is designed to detect excessive exhaust emissions during different types of driving conditions. The present IM240 test procedure requires the use of four separate analyzers to provide modal information for CO, NOx, CO2 and total hydrocarbon from “in-use” vehicles. There are significant concerns about the calibration and complexity of this instrumentation for the level of use that is expected.
A single FT-IR analyzer is capable of rapidly measuring the concentrations of many of the species found in tailpipe emissions. In this paper we will describe the results of a series of evaluation tests that we performed to compare the precision and accuracy of an FT-IR analyzer with the four analyzers presently used for the IM240 test.
Repetitive tests were performed on a number of vehicles where both the traditional analyzers and the FT-IR were sampling from the same dilution system (CVS). Some of tests were run on each vehicle without the specified warm up time so that the catalytic converter was not fully operational. This “cool start” test provided a data set where the concentrations greatly exceeded the defined failure levels.
A statistical analysis of the data showed excellent correlation for all of components including hydrocarbon. Because the FT-IR calculates the actual concentrations of the low molecular weight hydrocarbons a non-methane hydrocarbon value can also be measured each second during the test.


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