Engine Exhaust Emissions

Currently, the civil aviation sector burns about 12% of the fossil fuels consumed by the worldwide transportation industry. It is responsible for an approximate 3% annual addition to greenhouse gases and pollutant oxide gases. The environmental debate has become intense on issues such as climate change and depletion of the ozone layer, leading to the debate on long-term effects of global pollution. Smog consists of nitrogen oxide, which affects the pulmonary and respiratory health of humans. The success of the automobile industry in controlling engine emissions is evident by dramatic improvements achieved in many cities.

The U. S. Environment Protection Agency (EPA) recognized these problems decades ago. In the 1980s, the need for government agencies to tackle the engine emissions issue was emphasized. The early 1990s brought a formal declaration (i. e., the Kyoto agreement) to limit pollution (specifically around airports). Currently, there are no regulations for an aircraft’s cruise segment. In the United States, FAR Part 35, and internationally, the ICAO (i. e., Annexure 16, Volume II), outline the emissions requirements. EPA has worked closely with both the FAA and the ICAO to standardize the requirements. Although military aircraft emissions standards are exempt, they are increasingly being scrutinized for MILSPECS standards. Emissions are measured by an emission index (EI).

Combustion of air (i. e., oxygen plus nitrogen) and fuel (i. e., hydrocarbon plus a small amount of sulphur) ideally produces carbon dioxide (i. e., CO2), water (i. e., H2O), residual oxygen (i. e., O2), and traces of sulphur particles. In practice, the combustion product consists of all of these plus an undesirable amount of pollu­tants, such as carbon monoxide (i. e., CO, which is toxic), unburned hydrocarbons (UHC), carbon soot (i. e., smoke, which affects visibility), oxide of sulphur, and var­ious oxides of nitrogen (i. e., NOX, which affects the ozone concentration). The reg­ulations aim to reduce the level of undesirable pollutants by improving combustion technology. The sustainability of air travel and growth of the industry depend on how technology keeps up with the demands for human-health preservation.

Lower and slower flying reduces the EI; however, this conflicts with the market demand for flying higher and faster. Designers must make compromises. Reduction of the EI is the obligation of the engine manufacturer; therefore, details of the air­worthiness EI requirements are not provided herein (refer to the respective FAA and ICAO publications). Aircraft designers must depend on engine designers to supply certified engines that comply with regulatory standards.

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