Engine emissions standards committee invites Signal participation

Heated vacuum chemiluminescence analyser for engine emissionsAs a developer and manufacturer of gas analysers Signal possesses a high level of knowledge in the sampling and analysis of gases in industrial processes and emissions, and was recently invited to make submissions to the SAE E-31 Aircraft Exhaust Emissions Measurement committee.

Signal Managing Director James Clements travelled to Germany in response to an invitation from the committee to deliver a presentation on the measurement of engine emissions. Specifically, James was asked to discuss the advantages of hot vacuum chemiluminescence in comparison with cold non-vacuum chemiluminescence and other techniques.
 
James believes that the committee is reviewing the test methods because so far there hasn’t been an aerospace engine emissions standard, so they want to be sure they recommend or specify the best technique for the job when the standard is released.
 
Chemiluminescence is the recognised standard reference method for the measurement of NOx in emissions.
 
The chemiluminescence method for gas analysis of oxides of nitrogen 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 reaction of NO to NO2:
NO + O3 -> NO2 + O2 + hv
 
The chemiluminescence technique may be used to measure total oxides of nitrogen (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 NO and NOx, which also enables the fast, continuous, indirect measurement of NO2.
 
James was asked to compare the two chemiluminescence methods:
 
1.  Non-vacuum method – measures NO as dry gas. The sample is passed through a NOx converter and then a drier before measurement.
2.  Vacuum method – measures NO in wet sample gas. The wet sample is passed through a NOx converter and measured without further treatment.
 
Comparing the two methods, James 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, which can lower instrument accuracy and potentially harm internal components. This method is continuous with a fast response time making it ideal for real-time reporting applications.
 
Despite recommending the vacuum method, James explained that some customers prefer the non-vacuum method, and for this reason, Signal Group offers both types of instrument.
 
Over 1,800 SAE standards are used in the development of a typical aircraft, helping to improve product performance and safety; promote global market acceptance of new technologies; reduce costs; and decrease time to market. The SAE E-31 Aircraft Exhaust Emissions Measurement committee addresses all facets of aircraft exhaust emissions measurement – tools, methods, processes, and equipment. It is responsible for standardizing measurement methods of emissions from aircraft, including isolated combustor systems. The group is comprised of four subcommittees dedicated to creating, preparing, and maintaining all relevant specifications, standards, and requirements for aircraft exhaust emissions measurement.
 
Following the presentation, the committee will decide which method would be ideal for NOx measurement (hot-wet, or cold-dry chemiluminescence) and include it in the next standard for aircraft engine emissions measurement. James was also invited to be part of the committee and to take part in future discussions.