Gas Phase Measurements

W. Addy Majewski, Hannu Jääskeläinen

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• Techniques of Gas Analysis
  • Overview
  • Non-Dispersive Infra-Red
  • Flame Ionization Detector
  • Chemiluminescence Analyzer
  • Fourier Transform Infra-Red
  • Laser Diode Spectrometer
  • Magnetic Methods
• Analysis of Engine Exhaust Gases
  • Carbon Monoxide and Carbon Dioxide
  • Hydrocarbons
  • Nitrogen Oxides
  • Ammonia

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.

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. These methods are typically used for emission 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.

In addition to methods listed in Table 1 and Table 2, electrochemical cells have been developed for the measurement of many gas species. 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:

• Cross-sensitivities. Electrochemical cells may be sensitive to other compounds that are present in the measured gas. For example, NO cross-sensitivity of a CO cell may be as high as 30%, depending on the quality of the cell.
• Deterioration of the cell. The life span of electrochemical cells is usually 1-2 years and a gradual decline in performance is frequently observed.
• Simple sampling systems in portable instruments may cause several errors related to temperature or humidity of the measured gas. If condensation occurs in the sampling line acid gases will dissolve in the condensate and disappear from the gas phase.

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