Fundamental Minimum Emissions and Climate and Air Quality Societal Costs of Conceptual Turbofan Cycles

For decades, increasingly stringent aviation emissions regulations have focused on improving local air quality by limiting nitrogen oxides (NOx), carbon monoxide (CO), unburned hydrocarbons (UHC), and particulate matter (PM). However, these pollutants alone do not capture aviation’s full climate impact. A climate-based assessment requires consideration of the long-term warming effects of carbon dioxide (CO2) and nitrous oxide (N2O), as well as the short-lived climate forcers produced during cruise. Similarly, evaluating aviation’s impact on human health requires accounting for the health-related damages associated with this broader set of emissions. To design combustors that minimize aviation’s overall societal impact, this paper combines reduced-order reactor network modeling with the Climate and Air Quality Societal Cost (CAQSC) metric to evaluate unstaged and staged combustors operating within conventional Brayton cycles and Water-Enhanced Turbofan (WET) cycles. These architectures are optimized for both minimum NOx emissions and minimum societal cost, and this work highlights the factors that influence each optimization goal. The sensitivity of emissions and societal cost to high-level design parameters is also explored. Results show that improvements in fuel efficiency, represented by reductions in thrust-specific fuel consumption (TSFC), lead to the largest decreases in CAQSC because of their effect on CO2 and contrail-related damages. Although the modeled architectures exhibit substantial variation in their theoretical minimum NOx emissions, these differences translate to very small changes in CAQSC due to the dominant contribution from CO2 at cruise. However, real combustors emit NOx at levels far above these theoretical minimums, and NOx therefore remains a major driver of aviation’s air quality impacts. These findings highlight the importance of modeling both TSFC and NOx emissions with sufficient fidelity when evaluating future combustor designs.