Gas Phase Measurements

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Abstract: The principles of the techniques most often used in exhaust gas analysis include infra-red (NDIR and FTIR), chemiluminescence, flame ionization detector (FID and fast FID), and paramagnetic methods.

Techniques of Gas Analysis

Overview

Methods which are commonly used for the analysis of exhaust gas constituents are listed in Table 1 [119]. All of these techniques are based on a selected physical property of the measured gas. In some cases, the measured species undergoes a chemical reaction in the analyzer but even then only an accompanying physical property is directly measured. Concentrations of gases are calculated by comparing the instrument signal with that generated using a calibration gas of precisely known concentration.

Table 1
Techniques of Emission Analysis
Technique Gas Typical Range 90% Response Time
Non-dispersive infra-red (NDIR) CO 0-3000 ppm 2-5 s
CO2 0-20% 2-5 s
Chemiluminescence detector (CLD) NOx 0-10,000 ppm 1.5-2 s
Flame ionization detector (FID) Total HC 0-10,000 ppm 1-2 s
Fourier transform infra-red (FTIR) NOx, some HC, etc. various 5-15 s
Paramagnetic analyzer O2 0-25% 1-5 s

The analytical techniques listed above have been developed or adopted for measurements of exhaust gases from internal combustion engines. They offer reasonably high accuracy and insensitivity to mutual interference between gas species. The use of these methods (with the exception of FTIR) has been typically required by exhaust emission regulations for certification and compliance testing.

Because of their slow response, analyzers used for certification and compliance testing are not very suitable for true transient engine testing. Analyzers designed for steady-state work must be accurate, sensitive and stable and thus tend to have slow response times and to be well dampened. Analyzers for transient work must respond very quickly, preferably within a few milliseconds. A further problem in transient testing arises from the time and distance lags associated with the positioning of the exhaust gas sampling points, located downstream of the engine. Mathematical techniques are used for reconstructing the true signal from the instrument signal, taking into account sampling delays and instrument response characteristics [120].

Fast response gas analyzers for total HC, NO/NO2, CO and CO2 have been developed, as outlined in Table 2. The fast response analyzers are miniaturized developments of the respective conventional instruments, capable of response times measured in milliseconds and may thus be used for true transient testing as well as for in-cylinder and exhaust manifold measurements. These instruments are used primarily for research and development efforts; they have not yet been adopted for emission certification and compliance testing.

Table 2
Fast Response Gas Analyzers
TechniqueGasMax Concentration10-90% Response
Fast FIDTotal HC1,000,000 ppm1 ms
Fast CLDNO/NO220,000 ppm2-10 ms
Fast NDIRCO & CO2250,000 ppm8 ms

In addition to methods listed in Table 1 and Table 2, electrochemical cells for measuring many gases have been developed. Many portable analyzers utilize such electrochemical sensors. Instruments of this type are commonly used in occupational health and safety practice for ambient air quality evaluation. Portable electrochemical cell exhaust gas analyzers are also available for emission measurements in field conditions. Electrochemical sensor instruments are usually less accurate and, as a rule, are not allowed by regulated emission test procedures. The basic sources of error when using electrochemical instruments are:

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