If you have ever owned a car, you are probably familiar with the mandatory (bi-) annual emissions test, which were introduced to ensure that wear and tear on the engines do not have a negative impact on the exhaust gas quality. And even if you have never owned a car, chances are that this connection between old cars and dirty exhaust comes intuitively, thanks to numerous pop-culture references. But did you ever ask yourself if this is also a concern with aircraft engines? We sure did!
In December 2023, we successfully concluded the Ageing Aircrafts II project (AGEAIR II), which investigated the effects of engine deterioration with use on the emission performance of the engines. Within the AGEAIR projects, we measured engine emissions in the SR Technics’s test cell alongside engine break-in runs after maintenance had been conducted (piggyback set-up). Our set-up enabled extensive testing with minimal interruption to SR Technics’s operations . During the five-year combined projects (AGEAIR I & II), we measured 101 in-service engines, including CFM56 7B, -5B, -5C engines and PW4000-94” and 100” engines. We measured gaseous and particulate emissions (focusing on the soot particles (non-volatile particulate matter, nvPM)) using the Swiss Mobile Aircraft Emission Measurement System (SMARTEMIS). All engines were run with Jet A1 fuel.
The final report of the AGEAIR projects has been published on the FOCA website. Visit this link [AGEAIR REPORT] to read the full report. For the data analyses within this project, we worked with emission measurements dating back to 2013, when SMARTEMIS was operated under EMPA by our former colleague and emissions expert, Dr. Lukas Durdina (now at green-let).
Here are some of our findings: Engine maintenance and nvPM emissions
The piggyback tests were only possible after maintenance had been done on the engines. Maintenance improves the efficiency of the engines, which in turn can reduce the engine’s emissions. However, by comparing the emissions from different engines of the same model but with different levels of maintenance (or maintenance work scopes), we get some insight on the degree to which different maintenance work scopes improve emissions. Here we present the soot (nvPM) emissions as mass and number per kg of fuel used (emission indices or EI) from CFM56 7B engines. The emissions have been corrected for losses in the sampling system.
Maintenance work scopes on the engines were:
- Repair (RE), involving the repair or replacement of damaged components in the engine;
- Performance improvement (EP), involving repair or replacement of some key components of the engines and limited life parts that were due for change;
- Overhaul (OV), involving extensive maintenance and overhaul of most components in the engine, restoring engine’s exhaust gas temperature margin.
The results showed that OV’d engines had significantly lower nvPM mass and number emissions compared to engines with EP or RE work scopes. This difference was significant for thrusts greater than landing approach thrust (APP, 30% of maximum rated thrust in the International Civil Aviation Organization Landing-takeoff Cycle (ICAO LTO cycle)). nvPM number was also significantly lower at idle/taxi (idle) thrust for overhauled engines than for other work scopes.
As part of the global effort to reduce aircraft emissions and environmental impact, engine manufacturers are now required to report their nvPM emissions (mass and number) for in-production engines as part of their engine emission certification process (ICAO 2019[1]). These reported data are publicly available in the ICAO engine emissions databank (EEDB[2]). For the certification process, engine manufacturers generally use “new” engines. So, we can compare the in-service engine emissions that we measured to the reported emissions in the EEDB.
Mass emissions from our measured in-service engines were higher than those reported in the EEDB at all power settings in the ICAO LTO cycle (Idle/Taxi (idle), Approach (App), climb out (C/O) and take-off (T/O)). The difference was smaller at low thrusts (idle to approach), while at higher thrusts, only overhauled engines had similar emissions to the EEDB-reported emissions. For nvPM number emissions, at idle, measured in-service engine emissions were much higher than reported in the EEDB. At higher thrust settings, only the repaired engines had higher emissions than reported in the EEDB, while EP’d and OV’d engines had lower nvPM number emissions than in the EEDB.
Implications
Our observations show that just like car engines, engine wear with use leads to lower quality exhausts. Also, emissions reported in the EEDB, which are used for estimating aviation’s impact on air quality, may not be accurate. Our results show that wear in in-service engines leads to lower performance than the “new” engines in the EEDB. This deterioration can however be reversed or reduced through maintenance. This is good news, with benefits to the environment and health of airport staff and neighboring communities, as well as to the airlines as reduced emissions implies higher engine efficiency. Therefore, airlines and engine owners looking to reduce their nvPM emissions and improve engine efficiency could target their maintenance schedule.
A follow-up question from this insight is: what steps in the maintenance profile are most relevant for reducing emissions?
Acknowledgements
BAZL for funding the projects; SR Technics; our former colleagues and backbone of the AGEAIR project: Dr Julien Anet, Dr Lukas Durdina, Curdin Spirig; collaborators during the AGEAIR projects – PSI, ETH Zurich, Cardiff University
[1] International Civil Aivation Organization, 2019 Environmental Report. Aviation and Environment, 2019.
[2] ICAO Aircraft Engine Emissions Databank v30, 2024.