Variation in aircraft engine exhaust emissions in relation to flight altitude and degraded engine performance
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This work is a study to quantify the exhaust emissions from a short-haul commercial jet flight and the way wngine fouling and aging influence these quantities. Focus was set on the distribution of the released gases along the altitude scale, and the particular relevance to subArtic regions is pointed out. An introductory study shows the continuously increasing trend of commercial air traffic with the steepest rate of growth occurring in what are traditionally considered low technology regions. A 500 km Boeing 737-400 domestic flight in Norway was chosen as the representative case flight, and the range from sea level to cruise altitude at 11 278 m was studied. Engine cold end fouling was simulated by shifting the characteristics of the compressor sections, realisitic increments were chosen in accordance with the literature recommendations. Carbon dioxide, water vapor, nitric oxides, carbon monoxide and unburned hydrocarbon emmissions are subjects of the study. Two independent computer-modeling systems were used to simulate the flight and calculate the emission quantities. A simple model is based on a parametric study of the engines and a rough representation of the airplane. This model is much smaller and far less laborious then the second code, the Turbomatch, ehich is developed at Cranfield University in England for gas turbine analyses. Turbomatch was used in the work to generate engine data for all operational and engine conditions, and a somewhat more detailed and comprehensive representation of the aircraft was set up. Emission indices were established to suit the CFM56-3C1 engine, and is used similarly in the two models. The predictions came quite close for the carbon dioxide, water vapor and nitric oxide emissions. Carbon monoxide and unburned hydrocarbons emissions are only studied in the larger model. The distribution of the exhaust emissions over the altitude range of operation for a one hour jet transport flight is found. It is evident that in the extreme low and the cruise altitude levels of the atmosphere, emissions per altitude increment are 30 - 50% higher than in the intermediate levels. The unburned hydrocarbons are the exception as most of these emissions end up close to cruise altitude. An engine degradation parameter that is primarily related to cold end fouling, was defined, and simulations demonstrate how the emissions of carbon dioxide, water vapor and nitric oxide increase almost linearly with this parameter, the nitric oxides three to four times faster than the other two. The relative increase with engine degradation for these emission species is slightly steeper for the higher flight altitudes. Carbon monoxide and unburned hydrocarbon emissions are predictied to drop considerably with engine performance degradation. A literature review is offered to document the current status and practice regarding gas turbine compressor fouling and cleaning for aircraft engines and other applications. Smaller engines, such as helicopter engines and turboprops, are plagued by fouling is not so much in focus possibly becouse it is a less dominant reason for engine performance degradation.