Hydrocarbon flame ionization detector

Details Hydrocarbon flame ionization detector Hydrocarbon flame ionization detector

1990-02-01

1991-12-17

Wieder, Kenneth A.

Electricity: measuring and testing

Using ionization effects

For analysis of gas, vapor, or particles of matter

324464, 422 54, 436154, G01N 2762

Patent

active

050737538

DESCRIPTION:

BRIEF SUMMARY
The present invention relates to the detection of fuel concentration and unburnt hydrocarbons associated with combustion processes, in particular, a hydrocarbon flame ionization detector for the measurement of such parameters in internal combustion engines.
Meeting the stringent emissions legislation that exists in many countries requires very careful engine design. With the additional desirability of good fuel economy, new methods of examining the management of fuel within the combustion process are of great importance.
During the combustion of hydrocarbon (HC) fuels ions are produced to the extent of-about 1 ion pair for every 10.sup.6 carbon atoms in the fuel The level of ionization is sufficient for detection and forms the basis of the well-known flame ionization detector (FID) which is used almost universally for the routine measurement and legislative assessment of HC emissions from vehicles.
In an FID a sample of the gas under study is burnt in a hydrogen flame and the resulting ions are collected by means of an electrically biased electrode. The current in the electrode is found to be nearly exactly proportional to the rate of supply of HC molecules to the flame. The hydrogen flame is supported by an oxidant, normally air, the hydrogen flame itself producing negligible ionization. Mixtures of hydrogen and buffer gases (typically helium or nitrogen) are often used as the fuel gas for the FID, and in this specification, the term hydrogen will be used for simplicity when referring to the fuel gas as a mixture or just hydrogen.
Although conventional FID's work well, they have a drawback if rapid response is required, since they conventionally have a time constant of in the order of 1 second. This is due to the fact that the sample is led through a long tube from the sample origin (typically an engine exhaust) to the detector unit which may be several metres away. Diffusion processes in the sampling system then result in the poor time constant. In 1980 J. E. Fackrell published a paper (see J.Phys. E. Sci. Instru., Vol 13, 1980) showing how the basic frequency response could be improved dramatically by using a very short sample line, and by introducing the sample right at the tip of the burner within the FID chamber. In this way frequency responses of up to 300 Hz were achieved.
In Fackrell's design the flow of hydrocarbons into the FID chamber is proportional to the pressure difference between the pressure in the chamber and that in the surrounding atmosphere. Thus the measurement of hydrocarbon concentration is dependent on a steady ambient pressure at the sample point.
Fackrell's modification to the conventional FID is thus only suitable for use in constant pressure environments, and is unsuitable for use effectively for internal combustion engine studies, since, in all cases, the source of the hydrocarbons under study experiences considerable pressure fluctuations In addition, the design of the FID used by Fackrell does not embody certain improvements made possible by modern electronics and is of a more complex mechanical design than necessary as a result.
The present invention relates to a system for obtaining high frequency response measurements from sources with pressure fluctuations, and to improvements over existing designs of FID chambers.
According to the present invention there is provided a flame ionization detector comprising a housing forming a flame chamber for burning hydrogen, an oxidant, and a sample gas; the flame chamber including a nozzle at the tip of which the mixture of gases is burned; means for igniting the gas emitted from the nozzle; inlets for hydrogen and oxidant; an exhaust outlet through which the products of combustion are removed; a collector electrode electrically insulated from the housing of the flame chamber; the nozzle and flame chamber housing being electrically grounded and the collector electrode, in use, being maintained at a predetermined potential relative to ground; and pre-chamber means adjacent to the flame chamber for maintaining the pressure of the sampl

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