To see the measurement of emission with our own eyes we organised an excursion into the TÜ V Bayern Motor Vehicle Research Institute in Prague. We were present when car emissions were measured on the test rollers by a precisely designed methodology, which is given in the Appendix. We inspected the measuring apparatus and were instructed as to their principles.
The exhaust gases are taken via an elastic tube into a lead container in which they are diluted with air at a precisely defined ratio, and aliquots are taken for the assay of individual components. The analysis covers CO, CO2, HC, NO, NO2 and O2. The monitoring instruments are calibrated every day by pure gases or mixtures with exactly defined composition.
Principles of assay of individual components
CO, CO2 are measured by an infrared analyser based on the
absorption of infrared radiation. The intensity of radiation passing through
the cuvette with the tested gas is compared with that of radiation passing
through a cuvette with nitrogen as reference gas. The wavelength set for
CO assay is 4.5 m m, for CO2 assay
4.2 m m.
HC are determined by using a flame ionisation detector employed in gas chromatography.
HC can be measured also by infrared analysers at wavelengths of 3 - 3.5 m m.
O2 is determined by an electrochemical probe.
NO, NO2 are assayed by a very interesting method based on the principle of photo-multiplier measurement of chemiluminescence. The first measurement concerns only NO.
A part of the instrument is an ozone developer. The released ozone reacts with NO according to the equation
The second measurement gives the sum of both oxides. NO2 is first reduced by metallic tungsten to NO, which reacts with ozone as shown above. The difference of the two measurements gives the content of NO2.
