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OCR for page 21
21 Section VI Additional Findings VI.1 Engine Warm-Up Emissions times. The first, initial rapid emissions change takes place within 20 to 60 seconds, where the emission index shows a During engine ground start there are three basic tempo- rapid decay. The second change in emission index is longer ral regimes to consider: pre-ignition, post-ignition/pre-idle and is characterized by a time constant of approximately two acceleration, and post-ignition at ground idle. The pre-ignition to three minutes. VOC emissions will simply be the result of evaporated fuel. The data suggest that following cold start, the VOC emission Around the moment of ignition there will be a mixture of index is approximately doubled compared with that for warm fuel-like hydrocarbons and partially burned hydrocarbons. operation for less than a minute. A simplistic approach to The post-ignition period will be characterized by increasing gauge the effect this has on inventory modeling of a 13-minute combustion efficiency, and VOC emissions will be accompa- idle time (engine-on; taxi-out) is to add approximately one nied by concomitant CO2 emissions. minute worth of additional emissions due to post-ignition In the winter MDW 2009 test, an effort was made to char- warm-up. acterize the post-ignition emissions to address the question The argument's weaknesses are that no study has system- of "warm-up." Whenever possible, the mobile laboratory was atically looked at the reproducibility of this observation or positioned about 40 m downwind of the engine during engine the time needed to return to cold start conditions. Further- start. This was only done when the engines were known not more, the initial ten seconds following ignition are not well to have been operated for at least two hours prior to the start. represented in this dataset. The warm-up aspect of the study The order of the test matrix initially precluded measurement was secondary to the overall test goals, but these results could of the engine starts for engines one and two. We found we be used to further refine assessments of warm-up emissions, had sufficient flexibility with the sampling scheme, however, as well as to design better tests for future work. to attempt two additional near-start observations. In Figure VI-1, a subset of the warm-up data offers insight into the time required to establish an equilibrium exhaust VI.2 Near-Idle VOC Scaling gas temperature. This result suggests that the exhaust gas The relationships among exhaust concentrations of numer- temperature parameter requires three minutes to reach 90% ous VOCs (e.g., formaldehyde, benzene, acetaldehyde, ethene) of its steady-state value. have been discussed in several archival publications (Herndon The formaldehyde emission index as a function of time et al. 2008, Herndon et al. 2009, Knighton et al. 2007, Yelvington following ignition is depicted in Figure VI-2. The time offset et al. 2007). Briefly, it has been observed that VOC emissions has been computed from the flight data recorder information all scale together (i.e., when the formaldehyde emission index and the time-coded notes taken in the mobile laboratory. doubles because of a decrease in temperature and/or fuel flow The chemical information in the emissions profile at other rate, so does the ethene emission index). These data and the engine-state changes has been used to refine the estimate of the initial detailed profile of Spicer and coworkers (Spicer et al. time offset between the flight data recorder and mobile labora- 1994, Spicer et al. 1992) have been used recently to refine the tory time. Conservatively, the absolute time since engine start Speciate database (EPA 2008) VOC profile for commercial is accurate within five seconds. aircraft emissions (FAA Office of Environment and Energy Figure VI-2 suggests that the CFM56-7B24 engines at the and EPA Office of Transportation and Air Quality 2009). temperatures of this test (-7C, -2C) have two characteristic While this near-idle VOC scaling observation has proven

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22 Figure VI-1. Measured exhaust gas temperature as a function of the approximate elapsed time since engine start. Triangles, squares, and circles denote different aircraft tests. Dark blue points reflect lower N1 rotational speeds than the lighter blue points for two of the starts. Figure VI-2. Formaldehyde emission indices are plotted as a function of the post-ignition warm-up time. Although the relative times are precise, the absolute time following ignition for any of these curves is uncertain by five seconds. The solid lines represent fits of the data to an exponential decay. The approximate time constant associated with each curve is noted in the legend.