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2 Exhaust Emissions from In-Use General Aviation Aircraft Figure S-1. Distributions of piston engine emission indices (EIs) for hydrocarbons (HC) and carbon monoxide (CO) as a function of power state (idle, taxi, cruise, take- off). The grey arrows show how the peak of the distribution moves with increasing power. IdleHC 50 0 5 10 15 A irc ra ft C ou nt 20 25 30 100 Hydrocarbon EIs [g/kg Fuel] 150 200 250 Taxi Cruise Take Off CO Idle 0 2 4 6 A irc ra ft C ou nt 8 10 12 14 Carbon Monoxide EIs [g/kg Fuel] 400 800 1200 Taxi Cruise Take Off
Exhaust Emissions from In-Use General Aviation Aircraft 3 ⢠The research team demonstrated an alternate statistical method (Monte Carlo simula- tions) that can constrain these confidence intervals. ⢠Many factors can affect emissions. Several causes for high variability in emission indices are explored. The research team found that pilot mindset and their operation of an air- craftâs mixture setting allows for a great deal of freedom in combustion parameters. The researchers also investigated several other technical details related to GA piston engine emissions, including trends in particle size and volatility, the effect of fuel additives, and thermal production of oxides of nitrogen and its relationship to lean combustion. ⢠A full list of measured emission indices is provided in Appendix P. Figure S-2. Using confidence intervals to compare data points. Confidence intervals must not overlap for data to be considered statistically different. 210-1 -1 210 statistically different statistically âthe sameâ confidence interval upper limit lower limit average 210-1
4 C H A P T E R 1 Much work has been done in quantifying the emissions from large commercial aircraft and their engines during operation in large airports. Regional airports serving the GA community are much smaller than a typical hub airport for commercial aircraft. However, in a local community that supports a regional airport, the airport can make a significant contribution to the commu- nityâs burden of criteria pollutants like particulate matter (PM), oxides of nitrogen (NOx), and hydrocarbon emissions. Although some studies have examined the issue of lead emissions from GA aircraft burning available aviation gasoline (AVGAS) (Heiken 2015), there has been relatively less examination of these criteria pollutants from GA aircraft. Because small piston aircraft represent a small part of the overall airspace operations and are very small in terms of national gasoline fuel usage, the regulating authorities have largely given small air- craft little attention relative to large commercial aircraft and road traffic, respectively. Thus, much legacy technology is still in use in GA operations and has not been subject to regulatory emissions control. When aviation emissions regulations were first being imposed, it was reasonable to assume this small part of global aviation was a negligible part of the total pollution problem. Now, as large commercial engines have gotten cleaner and more efficient, it is important to assess the contribution that general aviation makes as a result of their operations in local communities. Because general aviation uses different engine technologies than those of large commercial aircraft, one cannot simply use emissions inventories developed for large airports and scale them for use in regional airport analysis. General aviation extensively uses piston engines burning AVGAS, and these engines have different emissions characteristics from the large commercial aircraft gas turbine engines and from ground transport piston engines. Even the smaller gas turbine engines, used in smaller business jets and similar smaller jet-propelled aircraft, can have different emissions performance than the large modern turbofans used by major commercial carriers. Thus, there is a strong need to understand the emissions performance of GA engines. The Swiss Federal Office of Civil Aviation (FOCA) made an initial study on a limited set of piston engine aircraft. ACRP Research Report 164 extends and expands this FOCA study to examine a wider range of engines and to assess multiple examples of engines and operators to evaluate the variability in the emissions performance during actual day-to-day operation. The emissions measured in this research include PM, where multiple characteristics were quantified, NOx, carbon monoxide (CO), and hydrocarbons. These are all considered criteria pollutants by the US EPA, and are emissions that are controlled for large aircraft through the EPA in concert with international standards established by the International Civil Aviation Organization (ICAO). The pollutants in this research were measured in ways different from how certification is done on new engines for regulatory purposes. The research team took advantage of cooperating oper- ators so that the engines could be measured âon-wingâ using in-service engines. This offered the Background
Background 5 advantage of acquiring data reflecting actual operation of in-use engines in the airframe, rather than idealized states of new engines in certification testing rigs. This distinction is especially important for piston engine aircraft, because there is large variability in how piston engines are operated and, thus, in the emissions performance for even the same engine model. Furthermore, testing of these in-use engines has further advantages over certification testing of new engines, given that the GA fleet has slow turnover, and many decades-old aircraft are still in-use. Throughout this report, summaries in laypersonâs terms are italicized to guide the casual reader to the main conclusions of each section. ACRP Research Report 164 has six chapters and numerous appendices. Chapters 1 and 2 pro- vide background information about aircraft emissions and describe how testing and data analy- sis is done. In Chapter 3, the main results of the emissions measurements are described and compared to previous data. Chapter 4 shows how these emission results can be used to estimate the environmental impact of a GA airport. Chapter 5 examines the details of the emissions results with an eye to understanding the effect of everything from pilot mindset to transients. Finally, Chapter 6 summarizes the main conclusions of this research and identifies topics for future research. Appendixes A through P provide supporting detail and useful data sets.
6 Research Approach Pollutants in Exhaust Four main pollutant types were measured as a part of this study: nitrogen oxides, carbon monoxide, total hydrocarbons, and PM. The sources and importance of these pollutant species are described. In this research the focus is on the main pollutant species (see Figure 2-1). Nitrogen oxides (NOx) play an important role in smog formation and can be particularly important for airports situated in ozone non-attainment zones. Such zones are areas that do not meet National Ambient Air Quality Standards (NAAQS) for ozone, a pollutant in smog. Carbon monoxide (CO) is a product of incomplete combustion. In high concentrations and in enclosed spaces, CO can be dangerous, so many households and some aircraft cockpits have CO monitors or alarms. Hydrocarbons (HC), sometimes referred to as unburned hydrocarbons (UHC), are a third pollutant of interest. HCs come from fuel that has not been completely burned in the engine. HC includes a large range of individual chemical components, including volatile organic hydro- carbons (VOCs), unchanged components from the fuel, as well as partially broken down fuel. Depending on their exact composition, HC emissions can be of concern to both human health and local air quality. PM emissions are a last category of engine pollutant. PM is fundamentally different from the previous pollutants because it is non-gaseous; any type of smoke is made up of PM. PM contains both volatile and non-volatile substances. The predominant component of non-volatile PM (nvPM) is soot, which is formed during the incomplete combustion process. Volatile PM is gen- erated by the nucleation or condensation on soot from gaseous precursors such as sulfuric acid and organic compounds. The sum of volatile and non-volatile PM is called total PM (totPM). There are two general ways to quantify PM: count them to get the number (PMn) or weigh them to get the mass (PMm). Although it is not a reported emission species, carbon dioxide (CO2) is central to all emissions measurements. Carbon dioxide is the result of complete combustion of fuel. Combined with the products of incomplete combustion (CO and HC), CO2 allows us to relate the emissions to the total amount of fuel burned. From Exhaust Pipe to Airport Emission The aircraftâs operation (power states, time spent in each mode, landing and take-off cycle at the airport) can be combined with measured emission indices and fuel flows to estimate the total emissions from an airport. C H A P T E R 2
