Exhaust Emissions from In-Use General Aviation Aircraft (2016)

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Trends in Emission Indices 13 In general, modern gas turbine aircraft engines have much higher combustion efficiencies than the conventional piston engines used in general aviation. The research team observed this phenomenon in the ACRP Project 02-54 research. For example, consider the CO emissions from the CF34 jet engine. At idle, CO constitutes about 3% of the total carbon emissions, but con- tributes <0.1% at T/O. HC emissions from this same engine are an order of magnitude smaller than CO emissions in these two states; this observation indicates that the CF34 engine has a combustion efficiency of 97% at idle and runs much more efficiently at the higher temperatures and pressures associated with high-power settings. On the contrary, for the measured piston engines discussed in the following sections, the averaged CO EI is 997 g/kg fuel at idle and 798 g/kg fuel at T/O and the averaged HC EI is 167 g/kg fuel at idle and 42 g/kg fuel at T/O. These results imply that, even at the highest engine power condition, the combustion efficiency of a piston engine is approximately 50%. Thus gas turbine engines are more than 10 times better at idle and more than two orders of magnitude better at high power than the conventional piston engines in terms of CO and HC emissions. (“CO2 Carbon Fraction as an Indicator of Combus- tion” in Chapter 5 presents more detail.) Both volatile and non-volatile PM are present in the exhaust of aircraft gas turbine engines. Emitted to the ambient air, the engine exhaust is diluted and cooled by the surrounding envi- ronment, so partitioning of the volatile species (e.g., volatile organic compounds and sulfuric acid) from gas phase to PM starts after the engine exit via condensation and new particle forma- tion. This process will continue via microphysical interactions over a distance of hundreds of meters downstream. Accurate quantification for both the non-volatile PM emissions and the volatile contributions to particle mass is necessary to estimate environmental and health impacts. The non-volatile PM number and mass emissions (nvPMn and nvPMm) as well as the total particulate matter number emissions (tPMn) are determined for the two turbine engines in question and plotted in Figure 3-4. The research team found that, in general, the total PM number EIs (tPMn, pink) are much larger than the non-volatile PM number EIs (nvPMn, red) at each engine power condition. The total particulate count contains the non-volatile count, and so this is expected. This phenomenon has been observed in plumes encountered in flight, in staged engine testing, and at airports. The difference between the two EIs is probably driven Figure 3-3. Comparison of the General Electric CF34-3A1 engine with ICAO database results. 0.1 1 10 100 E Is [g /k g Fu el ] 100806040200 percent maximum fuel flow [%] 0.0 HC, CO, NOx General Electric CF34 ICAO Database

14 Exhaust Emissions from In-Use General Aviation Aircraft by the amount of condensable organic and sulfate species present in the engine exhausts. This is true for both the CF34 jet and the TPE331 turboprop engines. As for nvPM, both the number and mass emissions of the CF34 engine are highly dependent on engine power condition. As shown in Figure 3-4, the lowest values of emissions in both nvPM mass and number occur near cruise power condition for the CF34 engine (40 – 50% of maxi- mum fuel flow, 67% thrust). This character in emission with respect to engine power results in the U-shaped EI curves, which have been observed during many previous emissions measure- ments on modern gas turbine aircraft engines. These U-shaped curves occur because PM mass emissions are minimized at the highly efficient mid-power range typical of the cruise conditions, but increase at both take-off and idle where combustion efficiency is lower. The TPE331 turbo- prop engine also shows this U-shaped character in nvPMm, although the minimum is shifted toward slightly higher powers. Conversely, the TPE331 shows continuously decreasing tPMn and nvPMn with increasing power state. Data does not exist for full tests on turboprop engines, so this is the first time such a PM signature has been characterized. For the CF34 engine, the highest EI for nvPMm is 0.106 g/kg fuel at the highest engine thrust, T/O, while the lowest nvPMm EI is 0.002 g/kg fuel at cruise (50% of maximum fuel flow, 67% thrust). This is in excellent agreement with previous measurements (Lobo et al. 2015) on a CFM56-7B24/3 engine that resulted in nvPMm EI of ~0.100 g/kg fuel at full power and ~0.001 – 0.002 g/kg fuel at cruise. Unlike PM mass, the EI in number (nvPMn) peaks at engine idle 9.3×1014 #/kg fuel, compared to 2.8×1014 #/kg fuel at full thrust. Piston Engines The large variability in piston engine emissions can be shown by plotting distributions of emission indices. Except for CO, the distributions are very skewed—the most common emission index is not equal to the average emission index. Figure 3-4. Emission indices for non-volatile particulate matter number (nvPMn), total particulate matter number (tPMn), and non- volatile particulate matter mass (nvPMm) in number and mass. 10 14 2 4 6 810 15 2 4 6 810 16 2 4 P M N um be r E Is [# /k g F ue l] 100806040200 percent maximum fuel flow [%] 0.001 2 4 6 8 0.01 2 4 6 8 0.1 2 P M M ass E Is [g/kg F uel] nvPMn, tPMn, nvPMm General Electric CF34 Garrett AiResearch TPE331