| Abstract | Despite recent advancements in regulatory standards for aircraft turbine engines, significant uncertainty remains in accurately quantifying non-volatile particulate matter (nvPM) number and mass emissions due to the complexity of sampling, measurement, and calibration procedures. The transition to new engine technologies and sustainable aviation fuels can also reduce regulatory nvPM levels towards the limit of quantification (LOQ), necessitating better quantification and mitigation of these uncertainties.
This study, initiated by the European Union Aviation Safety Agency (EASA) and funded under the EU Horizon 2020 RAPTOR and SAMPLE IV research programmes, investigated uncertainties in instrument drift, system-to-system variability, nvPM mass LOQ, and system operability within current regulatory nvPM measurement, sampling, and calibration standards. To assess these uncertainties, two comparative tests of the European (EUR) and Swiss (CH) regulatory-compliant reference systems were performed over a twelve-month calibration cycle using a non-proprietary Rich-Quench-Lean (RQL) combustor rig, following prior parallel calibration of identical nvPM number and mass instrument technologies. Additional laboratory experiments using a range of aerosol sources were conducted to evaluate instrumentation and calibration uncertainty independently.
With calibration and instrument-technology uncertainty minimised and enhanced cleanliness protocols applied, the nvPM Emission Index (EI) number showed <1 % systematic difference with 3 % variability between the EUR and CH systems, and the nvPM EI mass showed 3 % bias with 8 % variability. No annual instrument drift was observed in either the nvPM mass or number instruments. However, when directly comparing nvPM measurements across different calibration and instrument technologies, variability increased noticeably: Condensation Particle Counter (CPC) counting efficiency was impacted by particle type and morphology, even within the same CPC models, and a variability of 12 % (with biases up to 33 %) was observed across a range of nvPM mass instruments. In addition, particle shedding from the prescribed 1-μm cyclone was observed during RQL combustor rig exhaust sampling and was found to significantly impact nvPM mass measurements at low concentrations, which are typical of modern engine technologies and sustainable aviation fuels. Yet, with improved cleanliness protocols, accurate nvPM mass quantification down to 3 μg/m3 was achieved. These findings provide critical insight into current regulatory practices and identify areas for improvement towards more accurate characterisation and future reduction of harmful nvPM emissions from aircraft engines. |
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