Oil leakage paths within compressors of jet engines and oil concentration in aircraft cabin air

Abstract

Purpose - Investigation of oil leakage paths from the engine seals into the gas path of jet engine compressors and the estimation of oil concentration in aircraft cabin air based on fundamental aircraft and engine parameters. --- Approach - Real jet engine illustrations were collected and used for a detailed investigation of possible paths of oil-contaminated air into the main gas stream of the jet engine compressor before the extraction of bleed air. Furthermore, an equation was analyzed to calculate the oil concentration in the cabin air. Required aircraft and engine and parameters were collected for five engines: CF34, V2500, CFM56, PW4000, and GE90 on typical aircraft. --- Findings - Based on the illustrations it is evident that the oil finds a path into the compressor airstream and into the cabin, once it has left an oil sump. Regarding the oil concentration in normal operation, it could be shown that larger engines tend to produce lower oil concentration in the cabin air. One reason is that they have a lower oil consumption per thrust. The initially unknown parameter, x_seal (ratio of oil out of all seals versus the total oil consumption) was later identified based on the concentration of total volatile organic compounds (TVOC) measured inside aircraft cabins. The difference of TVOC measured in cruise in 31 conventional aircraft and the TVOC measured in four bleed-free B787 is 285 µg/m³ (based on the mean values over all aircraft in a group and all flights). For the identification, parameters of the Airbus A320 with CFM56 engines were used and an oil consumption of 0.28 qt/h per engine. Based on this, x_seal = 28%. The number of engines, n_eng has no effect and cancels out of the equation, when oil consumption is given per engine. The flow of oil into the cabin can be calculated from the oil consumption and a ratio of compressor air for air conditioning to total core flow. If this is assumed to be x_CA = 3%, the oil into the cabin would be 3 l per year or 60 l in 20 year (as for an older aircraft). The oil concentration in the cabin in a cabin air contamination event (CACE) is given. It can be estimated from the visibility in the cabin, when the visibility is reduced by smoke. --- Research Limitations - All investigations are based on common sense. The oil concentration in the cabin was calculated based on the assumption that 1 % of oil consumption goes into the cabin. --- Practical Implications - The influence of fundamental aircraft and engine parameters on oil concentration is better understood. These parameters are number of engines, engine air intake diameter, by-pass ratio, total number of seals and their number upstream of the bleed port, oil consumption, cruise Mach number, and cruise altitude. --- Social Implications - People concerned about aircraft cabin air contamination can better argue their case with the information in this project. --- Originality - It is an important step in the argument to show how the oil can find its way into the compressed air path. Before, it was just assumed that leaking oil from the engine seals will "somehow" make its way. The usefulness of the equation for oil concentration in the cabin air is shown now with an increased number of five engines instead of just one engine as before. This gives increased credibility to the equation.