Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
1 S U M M A R Y Available data on exhaust emissions of general aviation (GA) aircraft is limited, particu- larly for piston engines. For this research, the research team measured emissions and com- puted emission indices for dozens of real in-use aircraft. Forty-seven complete engine tests are reported, including 10 engines from a list of the top 20 national piston engines. The major findings described in this report are as follows: ⢠Gas turbine engines and piston engines have very different emissions, both in terms of magnitude and in trends with power. These differences can be understood based on the much higher combustion efficiencies in modern gas turbine engines versus piston engines. Piston engines emit more carbon monoxide and hydrocarbons (unburned or partially burned fuel) and less nitrogen oxides than gas turbine engines. ⢠Emission trends from piston engines agree with basic principles of combustion. Emis- sion indices exhibit a large dependence on the fuel/air mixture (see Figure S-1). ⢠Emissions from piston engines show a great deal of variability that is directly related to the nature of piston engines and the flexible way in which they are operated. ⢠Skewed distributions of emission indices are observed for piston engine emissions of hydrocarbons, oxides of nitrogen, and particulate matter (PM) (see Figure S-1). This means that the most common emission index is not equal to the average emission index. For carbon monoxide, piston engine emissions, distributions are not skewed. ⢠The variability of an average emission can be measured using 95% confidence inter- vals. A confidence interval consists of an upper limit and a lower limit such that one is 95% sure that the true average emission falls between them. Existing data is considered invalid (statistically different) if it falls outside this confidence interval. See Figure S-2 for illustration. ⢠Replicate measurements of several tested engines were used to perform a statistical vali- dation of existing data. Several invalid data points were found. The most important of these data points is the 2.3-times underestimate of the hydrocarbon emissions data for the very common Lycoming O-320 engine by the FAA-mandated software used for cal- culating airport emissions (Emission and Dispersion Modeling System [EDMS]/AEDT). ⢠A hypothetical GA airport representative of the U.S. national fleet was constructed. The effect of changing emission indices on this hypothetical airport was investigated with a sen- sitivity analysis. The effect of emission index variability was also investigated. The effect of updated emission factors is much smaller than the effect of emissions variability. ⢠Standard statistical methods combined with FAA-mandated methods yield undesirably large confidence intervals on an airportâs emissions. For example, the hypothetical airport has an average NOx emission of 2.7 megagrams per year, but it could be up to 3.9 times that amount within 95% confidence. Exhaust Emissions from In-Use General Aviation Aircraft
