Efforts to reduce carbon dioxide (CO2) emissions appear at the top of the engineering agenda at every major aircraft engine maker. The air transport industry also confronts the need to mitigate the environmental damage from other emissions from engines, including contrails formed by water vapor, nitrogen oxides (NOx), sulfur oxides, carbon monoxide, soot, unburned hydrocarbons, aerosols, and traces of hydroxyl compounds.
The complexity around how OEMs can reduce and mitigate the various non-CO2 emissions has meant that, for now, they are not covered by legally binding mandates. Still, they remain on regulators' list of priorities. While increased use of sustainable aviation fuel (SAF) forms part of the solution, manufacturers continue to focus on design changes to engines and possible adjustments to the operational profile of flights, including altitude, that could drive down emissions.
In March, the FAA announced plans to work with Pratt & Whitney, the Missouri University of Science and Technology, Aerodyne Research, and the Environmental Protection Agency to better understand and reduce the environmental impact of such emissions as part of the agency’s Ascent program. The work will measure emissions from a Geared Turbofan (GTF) combustor rig test stand using conventional jet-A fuel and a 100% blend of SAF based on hydroprocessed esters and fatty acids synthetic paraffinic kerosene.
In June, results from the ECLIF3 study, involving Airbus, Rolls-Royce, Germany’s DLR aerospace research agency, and energy group Neste, showed a 56% reduction in contrail-forming ice crystals from an A350 aircraft with Trent XWB turbofans powered by unblended SAF. Simulations conducted by the DLR projected a 26% reduction in the climate impact from contrails from engines running on SAF instead of jet-A.
According to Timothy Snyder, Pratt & Whitney’s aerothermal chief engineer, the scientific community now seeks more accurate data from flight evaluations to assess factors such as contrail formation, rather than depending on data generated purely from ground-based evaluations. The studies have involved the deployment of chase aircraft to witness contrail formation at different altitudes.
“Now we are looking at the contributors to contrail formation, which are mainly non-volatile and volatile particulate matter (nVPM and VPM),” Snyder explained. “These come from sulfur in fuel, incomplete combustion, and lubrication oils. What happens when these contributors are combined with the water-rich exhaust plume is what we are trying to understand. So we measure these emissions and the number of ice crystals to determine if, for example, sulfur is reduced to zero and how much have ice crystal numbers reduced.”
Combustor Improvements Reduce Non-CO2 Emissions
Even before scientists completely understand the mechanism behind the reductions, clearly improvements in combustor technology such as lean staged and advanced rich quench lean combustors can minimize temperatures and so reduce both NOx and VPM emissions. According to Pratt & Whitney, the GTF engines already in service have reduced NOx emissions by 50% and nVPM by more than 100-fold.
“When it comes to future plans, we have a full annular combustor rig that we plan [to use] to take emissions measurements at inflight conditions to quantify the benefits of low-sulfur SAF,” Synder told AIN. “We will use this emissions data to estimate the reduction in contrails and then validate them once we acquire inflight engine emissions data. Based on this data, we can then make further changes to the engine and/or combustor to minimize both ground and cruise emissions.”
Meanwhile, Pratt & Whitney’s RTX group sister company Collins Aerospace is working to detect so-called ice super-saturation regions from data collected during flights to provide guidance to airlines on how they might change altitudes to avoid forming contrails. That said, Snyder acknowledged that, in some circumstances, such changes might increase fuel burn and CO2 emissions.
For now, non-CO2 emissions are not assessed by the European Union’s emissions trading scheme (ETS), but some in the industry expect that to change. Airline industry group IATA has focused more intently on the issue and has issued a policy document, while member carriers including Delta, Air France, and KLM have participated in pilot studies on emissions avoidance.
Business Jets Could Lead the Way
In May, 4Air, a U.S.-based company that helps business aircraft operators fulfill their sustainability objectives and obligations, recently released a study assessing the projected non-CO2 emission footprint from that part of the industry. Its research concluded that containing those emissions cannot purely be assessed in terms of fuel burn.
“Non CO2 emissions are down to where operators emit and in what conditions, so it is very location dependent,” 4Air president Kennedy Ricci told AIN. “We don’t know yet how the ETS will deal with these emissions, but we believe the EU will be monitoring them from next year and currently it uses average emission rates for offsetting CO2 emissions.”
The study assessed more than 16,000 business aviation flights and 27,000 flight hours. It concluded that with operational adjustments of just 50 out of 16,888 flights, the non-CO2 impact from the overall sample would have been cut in half.
According to Ricci, modern business jets are better placed to fly higher than airliners, and so could cruise at 42,000 feet, where contrail formation becomes less prevalent. He said that many current air traffic management routes are very inefficient in terms of emission reduction and that business aircraft operators, in particular, would operate longer climb and descent profiles to get to higher altitudes.
As part of its work, 4Air teamed with private charter flight operator Flexjet to conduct a contrail reduction pilot program. The study incorporated contrail forecasts into the flight dispatch process to optimize flight paths and minimize the time aircraft spend in contrail-forming regions. Where possible, the partners adjusted flight paths to modify cruising altitudes above those regions and pilots adjusted their climbs and descents in response to the data and projections.
That said, Ricci acknowledged that operating at higher altitudes can result in higher NOx and that addressing different types of emissions can result in trade-offs between various technologies and operating procedures. “There are things that we’ve learned about business aviation that could help commercial aviation,” he concluded. “Regulations [for non-CO2 emissions] are coming so you are going to have to care about this, and it’s low-hanging fruit because reducing the footprint of flights means business less fuel and spending less money.”
Efforts to reduce carbon dioxide (CO2) emissions appear at the top of the engineering agenda at every major aircraft engine maker. The air transport industry also confronts the need to mitigate the environmental damage from other emissions from engines, including contrails formed by water vapor, nitrogen oxides (NOx), sulfur oxides, carbon monoxide, soot, unburned hydrocarbons, aerosols, and traces of hydroxyl compounds.
The complexity around how OEMs can reduce and mitigate the various non-CO2 emissions has meant that, for now, they are not covered by legally binding mandates. Still, they remain on regulators' list of priorities. While increased use of sustainable aviation fuel (SAF) forms part of the solution, manufacturers continue to focus on design changes to engines and possible adjustments to the operational profile of flights, including altitude, that could drive down emissions.
In March, the FAA announced plans to work with Pratt & Whitney, the Missouri University of Science and Technology, Aerodyne Research, and the Environmental Protection Agency to better understand and reduce the environmental impact of such emissions as part of the agency’s Ascent program. The work will measure emissions from a Geared Turbofan (GTF) combustor rig test stand using conventional jet-A fuel and a 100% blend of SAF based on hydroprocessed esters and fatty acids synthetic paraffinic kerosene.
In June, results from the ECLIF3 study, involving Airbus, Rolls-Royce, Germany’s DLR aerospace research agency, and energy group Neste, showed a 56% reduction in contrail-forming ice crystals from an A350 aircraft with Trent XWB turbofans powered by unblended SAF. Simulations conducted by the DLR projected a 26% reduction in the climate impact from contrails from engines running on SAF instead of jet-A.
According to Timothy Snyder, Pratt & Whitney’s aerothermal chief engineer, the scientific community now seeks more accurate data from flight evaluations to assess factors such as contrail formation, rather than depending on data generated purely from ground-based evaluations. The studies have involved the deployment of chase aircraft to witness contrail formation at different altitudes.
“Now we are looking at the contributors to contrail formation, which are mainly non-volatile and volatile particulate matter (nVPM and VPM),” Snyder explained. “These come from sulfur in fuel, incomplete combustion, and lubrication oils. What happens when these contributors are combined with the water-rich exhaust plume is what we are trying to understand. So we measure these emissions and the number of ice crystals to determine if, for example, sulfur is reduced to zero and how much have ice crystal numbers reduced.”
Combustor Improvements Reduce Non-CO2 Emissions
Even before scientists completely understand the mechanism behind the reductions, clearly improvements in combustor technology such as lean staged and advanced rich quench lean combustors can minimize temperatures and so reduce both NOx and VPM emissions. According to Pratt & Whitney, the GTF engines already in service have reduced NOx emissions by 50% and nVPM by more than 100-fold.
“When it comes to future plans, we have a full annular combustor rig that we plan [to use] to take emissions measurements at inflight conditions to quantify the benefits of low-sulfur SAF,” Synder told AIN. “We will use this emissions data to estimate the reduction in contrails and then validate them once we acquire inflight engine emissions data. Based on this data, we can then make further changes to the engine and/or combustor to minimize both ground and cruise emissions.”
Meanwhile, Pratt & Whitney’s RTX group sister company Collins Aerospace is working to detect so-called ice super-saturation regions from data collected during flights to provide guidance to airlines on how they might change altitudes to avoid forming contrails. That said, Snyder acknowledged that, in some circumstances, such changes might increase fuel burn and CO2 emissions.
For now, non-CO2 emissions are not assessed by the European Union’s emissions trading scheme (ETS), but some in the industry expects that to change. Airline industry group IATA has focused more intently on the issue and has issued a policy document, while member carriers including Delta, Air France, and KLM have participated in pilot studies on emissions avoidance.