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.
10 Exhaust Emissions from In-Use General Aviation Aircraft sampling lines going back to the AML and trailer. The tripod is weighted with sandbags before any measurement. A test matrix was constructed to guide the measurements. This test matrix, reproduced in Appendix B, was used by a cockpit observer to direct the engine test and note relevant cockpit parameters. A key development in this testing procedure was the addition of âreturn to idleâ points between each high-power state. The aircraft engine can be idled for long periods on the ground, allowing the measurement team the time to gain an understanding of the âidle signa- tureâ and allowing for the cockpit observer to collect information on the aircraft and engine and describe the next test point. Idle then provided a chemical marker defining the beginning and end of the higher engine states, which can only be accessed for short periods on the ground without overheating. Live, preliminary analysis of the emission ratios was performed by a scientist sitting in the pas- senger seat in the AML. This scientist could then determine whether data was of sufficient qual- ity to proceed with the next test point. Communication with the cockpit observer was achieved through radio communication or SMS messaging. These test procedures allowed an experienced scientist team and a pilot without any previous ground-testing experience to complete a full engine test in less than 15 minutes. Figure 2-4. Ideal geometry for engine testing. The aircraft test is performed next to an unused taxiway. The exhaust and propeller wash are directed into an open field with no buildings or aircraft. The probe is placed just off the taxiway. The measurement equipment (the white truck) is out of the way.
11 Trends in Emission Indices By design, gas turbine engines installed in turboprop and turbofan (jet) aircraft operate in a pre- scribed manner. The combustion in these engines is well controlled by aircraft computers, and there is a strong link between the power produced by an engine and the resulting emissions. In contrast, piston engines, which drive small propeller planes, operate in a much more flexible manner. Piston engines are rugged and imprecise and pilots can operate them in various ways with simple levers (e.g., the throttle and mixer) in the cockpit. Power and emissions are weakly linked, particularly in low-power states like idle and taxi. The nature of piston engines means that there is also a great deal of variability in their emissions, even for the same pilot operating the same airplane. Gas Turbine Engines Gas turbine engines operate in a very controlled manner. Engine operation is always lean (excess air) and combustion efficiency is high throughout the range of operational states. Fig- ure 3-1 shows the expected trends in emission indices. This schematic was constructed based on trends observed during a 2006 field study of a General Electric CFM56-2-C1 jet engine (Anderson et al. 2006). HC and CO drop off precipitously above taxi, while NOx emissions increase steadily with power. For ACRP Project 02-54, the researchers measured gaseous and particulate emissions from four gas turbine aircraft engines: ⢠A TPE331-6-252B turboprop engine from Garrett AiResearch, ⢠A PT6A-60A turboprop engine from Pratt & Whitney, ⢠A FJ44-1AP turbofan engine from Williams International, and ⢠A CF34-3A1 turbofan engine from General Electric. Among these measurements, the CF34-3A1 and TPE331 engine tests were performed with a cockpit observer and well-defined engine states (the other two jets were fortuitous measure- ments), so the trends in emissions from these two engines were investigated as a function of engine state. Figure 3-2 shows the emission indices of gas-phase species. The CO EIs and HC EIs decrease with engine thrust for both the CF34 jet and the TPE331 turboprop, while NOx EIs increase with engine thrust. This inverse-correlation of NOx to CO and HC is primarily due to incomplete combustion at lower combustor temperatures, leading to the appreciable production of CO and unburned hydrocarbons. As the combustor temperature increases at high power, the CO and HC emissions are minimized and the NOx emission index increases. Figure 3-2 shows that both the emissions magnitude and trends for CO and NOx EIs are similar among the very different gas turbines. However, although the trends are similar for the C H A P T E R 3
12 Exhaust Emissions from In-Use General Aviation Aircraft HC emissions, the Garrett AiResearch TPE331 turboprop engine has systematically higher HC emissions than the GE CF34 jet by about 8 g/kg fuel. Data for the General Electric CF34 jet is available in the ICAO database (ICAO 2013). Fig- ure 3-3 shows a plot of these certified EI values for the CF34 engine along with the research teamâs data. Except for the approach power condition, the ACRP Project 02-54 research teamâs results are in relatively good agreement with the ICAO-certified CF34 EIs. For instance, at idle, EIs of CO, HC, and NOx from the ICAO emission data bank are 42.6, 3.95, and 3.85 g/kg fuel, respectively. The research team obtained EIs of CO, HC, and NOx of 67, 2.8, and 3.5 g/kg fuel, respectively. The ICAO values are averaged over several minutes of engine operation and use engine testing rigs (not in-use aircraft) and multiple sampling probes. All of these sampling differences may explain why the research teamâs instantaneous approach readings with a single sampling probe differ from ICAOâs published values. Figure 3-1. The trends in emissions for gas turbine engines. 0.1 1 10 100 E Is [g /k g F ue l] 100806040200 percent maximum fuel flow [%] HC, CO, NOx General Electric CF34 Garrett AiResearch TPE331 Figure 3-2. Emission indices of CO, HC, and NOx for jet (CF34) and turboprop (TPE331) engines.
