Aircraft engine emissions

The reduction in fuel burn and related CO2 emissions continues to be the overriding factor in engine technology developments. However, the higher temperatures and pressures needed for improved fuel efficiency often have a trade-off in the form of increased emissions of nitrogen oxides (NOX) unless mitigated through the design of the engine and control of the combustion process. The engine NOand nvPM standards26, and the aircraft noise and CO2 standards, define the design space for products to simultaneously address noise, air quality and climate change issues.

26ICAO Annex 16 Volume II contains international aircraft engine emissions standards.

The regulatory limits27 for NOX emissions from aircraft turbojet and turbofan engines have gradually been made more stringent between 1996 and 2014, and are typically referred to by the CAEP meeting in which they were agreed (CAEP/2, CAEP/4, CAEP/6 and CAEP/8).

Figure 3.4. contains certified engine NOX emissions data, including recent data from 2019-2021 [7] . It highlights that the most advanced NOX mitigation technology is not yet available across all engine types, and that engines at high Overall Pressure Ratios (OPR)28 have a smaller margin to the latest CAEP/8 limit. The 2019 ICAO Independent Experts Panel goal for leading edge NOX emissions performance in 2027 is also shown in the figure. While a number of engines with moderate OPR (<40) have almost reached this goal, it appears more challenging for engines with higher pressure ratios. A goal for 2037 was not set as more evidence was sought on the need in terms of harm to health and impact on climate.

In 2021, more than 95% of aircraft engines delivered were CAEP/8 compliant. As such we have seen a similar trend to aircraft noise (see Figure 3.3. ), where the average NOX performance of aircraft in the current European fleet that have been built over the last few years has also improved due to the entry into service of the new aircraft types and their respective engines (e.g. CFM LEAP-1x, PW 11xxG).

27NOX limits are defined as the mass (Dp) of NOX emitted during the Landing and Take-Off (LTO) test cycle and divided by the thrust of the engine (Foo). The limit also depends on the overall pressure ratio of the engine.
28Ratio of total pressure at compressor exit compared to pressure at engine inlet.

The original engine Smoke Number standard was put in place during the 1970s to control the visible pollution from aircraft emissions. It was recognised that this did not directly address health impacts and therefore non-volatile Particulate Matter (nvPM) standards have subsequently been developed to replace it. As of 1 January 2020, a mass concentration nvPM standard has been applicable to all in-production engines, while the mass and number nvPM standards will become applicable as of 1 January 2023. EASA has integrated all of the new nvPM standards into European legislation [1] , and has started to process certification applications prior to the applicability deadlines.

Figure 3.5. provides schematics of two main combustor concepts that are relevant for a reduction of NOX emissions. Rich burn-Quick quench-Lean burn (RQL) combustors are characterized by an initial rich fuel-air mixture which is quickly turned (quenched) into a lean mixture by additional air intake. Dual Annular Combustors (DAC) and fuel staged Lean Burn combustors are characterized by two separate combustion zones maintained by a complex fuel injection system. The pilot flame zone is used in low power operation only, while at higher power and in cruise the main lean burner is utilized.

Figure 3.6. and Figure 3.7. contain the new aircraft engine nvPM mass and number emissions data [7] , grouped according to different combustor technologies. The y axis in these Figures have been plotted on a logarithmic scale in order to more easily differentiate the nvPM performance of the different combustor types. It should be noted that existing engine types were designed prior to the new nvPM standards coming into force in 2020, and certain designs such as lean burn combustors perform better than others in this area. The 2019 ICAO Independent Experts Panel did not set a technology goal for nvPM improvements as further technical data was needed on the climate and air quality impacts.