60228 - Low-Emissions Technology Development for Auxiliary Power Unit Combustion Systems 

The aircraft Auxiliary Power Unit (APU) is required to provide power to start the main engines, conditioned air and power when there are no facilities available and, most importantly, emergency power during flight operation. Given the primary purpose of providing backup power, APU’s have historically been designed to be extremely reliable while minimizing weight and fabrication cost. Since APU’s are operated at airports especially during taxi operations, the emissions from the APU’s contribute to local air quality.  There is clearly significant regulatory and public interest in reducing emissions from all sources at airports, including from APU’s. As such, there is a need to develop technologies that reduce criteria pollutants, namely oxides of nitrogen (NOx), unburned hydrocarbons (UHC), carbon monoxide (CO) and smoke from aircraft Auxiliary Power Units.

Honeywell has developed a Low-Emissions (Low-E) combustion system technology for the 131-9 and HGT750 family of Auxiliary Power Units to provide significant reduction in these criteria pollutants for narrowbody aircraft application. This article focuses on the combustor technology and processes that have been successfully utilized in reducing the pollutants, with an emphasis on abating NOx.

This paper describes the 131-9/HGT750 Auxiliary Power Unit family, the requirements and challenges for small gas turbine engines, and the selected strategy of Rich-Quench-Lean (RQL) combustion. Analytical and experimental results are presented for the current generation of Honeywell Auxiliary Power Unit combustion systems as well as the Low-Emissions (Low-E) system. The implementation of RQL aerodynamics is well understood within the aero-gas turbine engine industry, but the application of RQL technology in a configuration with tangential liquid fuel injection, and also required to meet altitude ignition at 41,000 ft is the novelty of this development. The Low-E combustion system has demonstrated more than 25% reduction (dependent on the cycle of operation) in NOx vs. the conventional 131-9 combustion system while meeting significant margins in other criteria pollutants. In addition, the Low-E combustion system achieved these successes as a “drop-in” configuration within the existing envelope, and without significantly impacting combustor/turbine durability, combustor pressure drop, or lean stability.