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AEROSPACE STANDARDS


A New Standard In Aviation Emissions


SAE E-31 Aircraft Exhaust Emissions Measurement committee addresses aircraft engine emissions using alternative measurement methods for the development of new aviation standard.


C urrently,


there are over 1,800 SAE standards that are in use


for the development of a typical aircraft, helping to improve product performance and safety as well as to promote global market acceptance of new technologies. The SAE E-31 Aircraft


committee to deliver a presentation on the measurement of engine emissions. Specifically, Clements was asked to discuss the advantages of hot vacuum


❱ ❱ James Clements of Signal Group believes the vacuum


Exhaust Emissions Measurement committee exists to consider all facets of aircraft exhaust emissions measurement, including tools, methods, processes and equipment. It is responsible for standardising the measurement methods of emissions from aircraft. So far, there hasn’t been an aerospace engine emissions standard so the committee want to be sure they recommend or specify the best technique for the job when the standard is released.


CONSULTATION As part of its policy of using industry expertise to assist in its decision making, the committee recently enrolled Signal Group to provide support on defining the most appropriate testing method. Signal Group has been developing and manufacturing gas analysers since 1977 and the company’s staff possess a high level of knowledge in the sampling and analysis of gaseous emissions. Signal Managing Director James


Clements travelled to Germany in response to an invitation from the


12 /// Aerospace Test & Validation Vol 2 No. 1


method of chemiluminescence is ideally suited to emissions testing


chemiluminescence in comparison with cold non- vacuum chemiluminescence and other techniques.


CHEMILUMINESCENCE Chemiluminescence is the recognised standard reference method for the


measurement of total oxides of Nitrogen (NOx) in emissions. This relies on the measurement of light produced by the gas-phase titration of nitric oxide and ozone. NO is a relatively unstable molecule which oxidises to NO2 in the presence of ozone (O3). This reaction produces a quantity of light for each NO molecule which reacts. This light can be measured using a photomultiplier tube or solid state device. If the volumes of sample gas and


excess ozone are carefully controlled, the light level in the reaction chamber is proportional the concentration of NO in the gas sample. The chemiluminescence technique


may be used to measure NOx by passing the sample over a heated catalyst to reduce all oxides of nitrogen to NO. This is undertaken within the instrument, immediately prior to the reaction chamber. In addition, Signal has developed a dual detector version for continuous measurement of both


❱ ❱ Quasar Gas Analyser provides accurate measurements of aircraft engine emissions


NO and NOx, which also enables the fast, continuous, indirect measurement of NO2.


VACUUM VS NON-VACUUM METHOD The chemiluminescence technique can be split into a vacuum method or a non- vacuum method. The vacuum method measures NO in wet sample gas, which is passed through a NOx converter and measured without further treatment However, the non-vacuum method measures NO as dry gas and the sample is passed through a NOx converter and then a drier before measurement. When asked to compare the two


methods to the SAE committee, Clements explained that the vacuum method requires a high quality and somewhat expensive pump. However, heated vacuum chemiluminescence offers higher sensitivity with minimal quenching effects from CO2 and moisture. Also, a heated reaction chamber facilitates the processing of hot, wet sample gases without condensation, an attribute that lowers the chances of reduced instrument accuracy and potentially causing harm to internal components. The detection method is continuous with a fast response time making it ideal for real-time reporting applications. The SAE committee wil now consider the recommendation for using heated vacuum chemiluminescence for achieving high sensitivity with minimal quenching effects. If the committee agrees with the recommendations, then this technique will be included in the next standard for aircraft engine emissions measurement.


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