Author: Lukas Durdina

Paper published: First reported nonvolatile particulate matter emissions of a business jet aircraft measured according to a new international emissions standard

Particle emissions of small turbine engines are unknown and unregulated

Jet engines for commercial airliners in production now and in the future are certified for particle emissions from a virtual landing and take-off cycle. However, one potentially large group of engines is left out. Small jet engines (<26.7 kN rated thrust), found on business jets, as well as other types of turbine engines used in helicopters and propeller-powered aircraft have only to pass an old-fashioned exhaust smoke visibility test. Although these unregulated engines burn a small fraction of the world’s jet fuel, there are concerns about their contribution to local air pollution. Small turbine engines have therefore appeared in the spotlight of regulatory agencies and researchers. Unlike the grounded commercial fleet, private jet traffic has been resilient and increasing in many places in the second half of 2020.

Swiss “Air Force One” underwent an emissions test using a unique measurement system

To fill the knowledge gap in particle emissions of business jets, together with my colleagues from Empa at the time, we performed an emissions test on the Dassault Falcon 900EX of the Swiss Air Force, a VIP transport plane of the federal government. This modern plane is equipped with engines, which are widely used around the world. The test campaign was possible thanks to the Swiss Air Force’s support and coordination by Mr. Theo Rindlisbacher from the Swiss Federal Office of Civil Aviation (FOCA). This campaign marked the first deployment of the Swiss Mobile Aircraft Emissions Measurement System (SMARTEMIS). SMARTEMIS is one of the three reference systems in the world for measuring nonvolatile particulate matter emissions. For the emission tests, a forklift held a custom-built exhaust probe right behind the center engine. With the brakes applied, the pilot put the engine through a test cycle from maximum power down to idle several times. The exhaust sample was transported via heated lines to the instruments placed more than 25 meters away inside a hangar.

Sunrise behind the Falcon 900 with the exhaust sampling and measurement equipment installed.

Small plane – low emissions?

The jet engine on the Falcon 900 burns approximately 20% of the fuel needed by a Boeing 737 engine; thus, one can expect the overall pollutant emissions to be proportionally lower. However, for the standard LTO cycle and per aircraft, the small Falcon 900 emitted more particles, both in terms of mass and number, than a Boeing 737. Most importantly, for health effects, the highest number of particles was emitted at low power applied for ground movements. Since aircraft are designed to spend most of the time in the air and not on the ground, I developed an engine performance model to correct the ground test data to estimate the particle emissions during the flight. The estimated particle emissions per flight hour at cruise altitude were found to be in the range found previously for large commercial aircraft.

More work underway to get the bigger picture

This study reports the first nvPM emissions for a business jet engine measured using the standardized methodology. Since only one aircraft type was measured, one should not extrapolate the results to the entire fleet (as some modelers are often tempted to do so). Since the Falcon 900 tests, SMARTEMIS has been deployed behind another business jet, and more tests are planned in Switzerland and abroad. We have been also improving our models for correcting ground measurements to cruise.

Reference

Durdina, L., Brem, B. T., Schönenberger, D., Siegerist, F., Anet, J. G., & Rindlisbacher, T. (2019). Nonvolatile Particulate Matter Emissions of a Business Jet Measured at Ground Level and Estimated for Cruising Altitudes. Environmental Science and Technology, 53(21), 12865–12872. https://doi.org/10.1021/acs.est.9b02513

SAE E-31 Committee Meeting in Cardiff

On January 13-17, 2020, Julien and I attended the meeting of the SAE E-31 Committee, which took place in the historic Glamorgan Building of Cardiff University (little did I know that it would be the only face-to-face meeting the committee would have this year…). As usual, the first day of the meeting was devoted to the contributions of the bleed air subcommittee, which develops practices and standards for cabin air quality testing. The remaining days were taken by the gaseous and particulate subcommittee, where I am a member.

Entrance of the Glamorgan Building of Cardiff University.

During the meeting, I presented two works:

The first, which I prepared together with Dr. Eliot Durand, the main author, dealt with particle size distribution properties of non-volatile PM (nvPM) emissions of various commercial turbofan engines. We also analyzed the use of particle size distribution for calculating particle losses in nvPM sampling and measurement systems. For that, we combined data sets from our team (SMARTEMIS data) and the Cardiff University team who operates the European nvPM reference system. Together, we have data for a wide range of engines, ranging from small business jet engines to the largest commercial turbofans. Once we get it published, this will be a unique piece of work (and hopefully handy for modelers!).

In my second contribution, I looked at ambient temperature effects on nvPM emissions. This topic is essential both from the regulatory perspective as well as from the scientific one. When aircraft engines are certified for emissions (gaseous and particles), the engine test points are usually set using the combustor inlet temperature corresponding to a given thrust at sea level in the international standard atmosphere. However, ambient temperature (and pressure) variability affects other thermodynamic parameters in the engine. For emissions, we mainly look at the combustor inlet pressure and the total fuel-air ratio at the combustor exit. Let’s say we run the same engine at high thrust at the same combustor inlet temperature at 25 °C and at 5 °C. At 5 °C, the pressure at the combustor inlet would be approx. 30% higher, and the fuel-air ratio would be approx. 5% higher than at 25 °C. Depending on the nvPM emission characteristic of the engine, this could lead easily to a 50% higher emission index (amount emitted per kg fuel burned). Luckily, we do have such datasets from previous test campaigns done at SR Technics, and we have tried various correction methodologies developed within ICAO CAEP. The correction for ambient conditions is also very important for estimating aircraft emissions during the flight from ground test data (only a very few emission tests have been done at cruising altitudes).

The meeting gave us a chance to network with our colleagues and also to explore Cardiff and its history. We went on a tour of the Cardiff Castle, followed by a Welsh Banquet in the Undercroft (traditional food accompanied by live music and entertainment).

Dining hall in Cardiff Castle visited during the tour.

The meeting concluded with a tour of the facilities of the Gas Turbine Research Centre (GTRC). GTRC operates a high-pressure combustor rig, which will be used during our tests with SMARTEMIS there in the framework of the RAPTOR project.

Optical measurement section of the high pressure combustor rig at GTRC.

We are grateful to Dr. Andrew Crayford, Dr. Eliot Durand, and their co-workers at Cardiff University and GTRC for hosting this meeting!

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AGU Fall Meeting, 2019

If you are going to San Francisco …

It is uncommon to get such a nice view of the Golden Gate Bridge without fog (photographed from the top of Coit tower).

…make sure your poster tube fits in the overhead bin. On December 7, 2019, I was boarding a flight to San Francisco to attend the AGU Fall Meeting (American Geophysical Union). And judging by the frequent occurrence of poster tubes sticking up in the line of passengers waiting to board the plane, I was by far not the only one. My neighbor on the plane, a postdoc from the UK, an atmospheric scientist, was also going to the same place. Seeing so many scientists on just one plane made me realize the monstrous proportions of this event: around 25’000 participants from all around the world were expected to attend the 100th AGU fall meeting. AGU encompasses a wide range of subjects related to our planet and beyond. I was going there as a poster presenter and a co-chair of a session on aircraft emissions.

Keynote lecture given by the former NASA astronaut, Dr. Mae Jemison.

Back in April 2019, I was asked by a colleague from NASA Langley, Rich Moore, whom I briefly got to know a few years ago (at another conference), to co-chair a session on aircraft emissions at the AGU. I had heard about the AGU (especially about its proportions) but never attended before. Our first mission as session chairs (we were four, including Christiane Voigt from DLR and Rick Miake-Lye from Aerodyne research) was to invite people to submit contributions. We received about 30 contributions, from which we picked 8 oral presentations and the rest were assigned as poster presentations (including all session chairs). We submitted our program to the program committee by the end of August. At the beginning of October, I received a confirmation e-mail about the acceptance of my presentation and invitation to serve as a chair for the session. AGU was a go!

With an event of such size, you need a proper venue. The conference took place in the freshly rebuilt Moscone convention center. The center has three wings (South, North, and West) and the poster sessions were held in a massive underground hall between the South and North wings with several thousand posters being presented on a given day and overall, more than 7000 presentations were given during the conference.

One of the lecture halls used for keynote speeches.

The speakers in our session “Aircraft engine emissions impacts on air quality, cloud formation and climate” gave high-quality presentations on a wide variety of subjects related to environmental impacts of air travel: ground emission measurements, airborne measurements, alternative fuel effects on emissions, contrails modeling, ambient air monitoring and novel instrumentation. Overall, the session was well attended and the audience asked intriguing questions. It went by all too quickly: only 15 minutes per talk including questions. That is why I usually get more out of a poster session. 

Program of the session on aviation emissions.

My poster dealt with a comparison of measured emissions of non-volatile particulate matter (the solid component of soot composed of light-absorbing carbon, also known as black carbon) from a wide range of aircraft engines with a method based on emissions certification data. Our results show that in-service engines can widely vary in terms of their emissions profile and often have higher particle emissions than the estimates based on their certified smoke number (a measure of exhaust smoke visibility), especially in terms of the particle number emissions (number of particles emitted per kg fuel burned). Getting the best possible answers is crucial for scientific assessments of environmental impacts. Investigating the effects of engine aging on emissions and emissions variability is the focus of our project AGEAIR.

Lukas in front of his poster.

Although big conferences like the AGU Meeting feel impersonal, they always attract big names and you never run out of things to do in smaller groups and you can always meet people that work on similar topics like you. I feel privileged to have had the opportunity to listen to and meet some of the leading scientists and policymakers who see the big picture. As the AGU put it, “as scientists and engineers, we must continue to engage with policymakers, communities, businesses, and the public to undertake solution-oriented research and analysis. Scientific institutions, including academia and governmental agencies, should expand and prioritize their support for research, application, and knowledge dissemination to address the climate crisis.”

Listen to the penguin!

Kickoff meeting of the project RAPTOR in Paris

On November 18, 2019, I attended the kickoff meeting of the project RAPTOR (Research of Aviation PM Technologies, Modeling and Regulation) organized by the coordinator ENVISA in Paris.

RAPTOR is a part of the European research program Clean Sky funded by the EU’s Horizon 2020 program. The project brings together experts from academia and industry intending to improve our understanding of the air quality and health impacts of aviation particulate matter (PM) emissions. RAPTOR includes work packages (WP) dealing with health impact, modeling review, and PM measurement. Our team is involved in the latter two.

The modeling review WP aims to improve approaches for modeling aircraft engine PM (non-volatile and volatile fractions). This requires reviewing measurement data that can be used to improve relationships and correlations used for estimating aircraft emissions and their dispersion around airports and at cruising altitudes. This WP feeds into the health impact WP, as local dispersion of pollutants around an airport can have a significant impact on health.

The measurement WP focuses on quantifying uncertainties in the current and future ICAO standards for PM. As a part of this WP, we will perform extensive experiments on a combustor rig simulating an aero-engine combustor at the Gas Turbine Research Centre (GTRC) near Cardiff in Wales, UK. There, SMARTEMIS will be reunited with the European nvPM sampling and reference system after more than 7 years (operated by the Cardiff University / GTRC team around Dr. Andrew Crayford). This will be an excellent opportunity to compare the two systems side-by-side to evaluate the variability and different sources of uncertainty. We will perform two campaigns and the first one is planned for October 2020.

The kickoff meeting was very intense, but productive. It was good to finally meet the project team members that I had only heard during teleconferences before and to gain perspective on health impact research and emissions modeling. As usual, the social activities following the meeting could not be missed.