On the trade-off between aviation NOx and energy efficiency

This study aims to assess the trade-off between the ever-increasing energy efficiency of modern aero-engines and their NOx performance. The work builds on performance models previously developed to optimise the specific fuel consumption of future aero-engine designs. As part of the present work a simple and adaptable NOx emissions correlation for Rich-burn Quick-quench Lean-burn combustor designs is derived. The proposed model is computationally inexpensive and sufficiently accurate for use in aero-engine multi-disciplinary conceptual design tools. Furthermore, it is possible to adapt the correlation to model the NOx emissions of combustors designed for very aggressive future cycles. An approach to lean-burn combustor NOx emissions modelling is also presented. The simulation results show that improving engine propulsive efficiency is likely to have a benign effect on NOx emissions at high altitude; at sea-level conditions NOx emissions are particularly likely to reduce. Improving engine thermal efficiency however has a detrimental effect on NOx emissions from RQL combustors, both at high altitude and particularly at sea-level conditions. LDI combustor technology does not demonstrate such behaviour. Current legislation permits trading NOx emissions engine efficiency and hence reduce CO2 emissions. If we are to reduce the contribution of aviation to global warming, however, future certification legislation may need to become more stringent and comprehensive.