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9 S e c t i o n III Key Project Findings Despite the complexities of the idle settings, an inverse the combustor pressure and fuel-to-air ratio do not vary relationship between emissions and fuel flow rate was linearly when bleed air demand is introduced. The overall observed. carbon dioxide (CO2) mixing ratio by volume at the engine Figure III-1 shows the fuel flow rate as a function of test exhaust plane is greater for the high bleed air demand con- point conditions during the MDW 2009 tests (with a CFM56-7 dition than when N1 = 25%, which indicates a substantial engine). shift in fuel-to-air ratio and thus combustion performance. The operational categories in Figure III-1 are drawn from The geometry of the turbomachinery is designed for maxi- the nominal name given to each test point in the test matrix. mum performance at continuous-cruise state, where the The data points have been slightly offset horizontally within majority of fuel is consumed, and the use of variable bleed each category in order to better distinguish points that are air during ground idle operation further exacerbates any very close in fuel flow rate. combustion inefficiency. In addition to fuel flow rate, two other engine The incorporation of variable bleed air demand into the characteristics--fraction of maximum rotational fan speed test matrix employed in this study has required consideration (N1) and exhaust gas temperature (EGT)--were varied as of additional engine state parameters. The relationship that a consequence of the test point conditions. N1 is related is depicted in Figure III-2 between fuel flow, N1, and EGT to the rotational speed of the fan in the turbofan engine highlights the problem with assuming N1 (or thrust) varies and is reported in units normalized by the maximum rota- linearly with fuel flow at idle. At this stage, we might be tional speed for that engine and reported as a percentage. tempted to abandon this study in favor of more controlled N1 = 25% defines the 7% idle condition for the CFM56 fam- combustion rig-based measurements. To do so, however, ily of engines. Figure III-2 shows the relationship between would fail to produce a meaningful capability to estimate HAP N1 and fuel flow and the effect of test conditions on exhaust emissions for airport operations. As designed, this project was gas temperature. able to test 11 engines of the same model that were pulled from At first glance the measurements suggest there are two operational service. A mixture of newer and older engines was disparate data groups that do not correlate well with fuel present in the test pool. We do not have detailed knowledge flow or EGT. Setting the data points with N1 greater than of each engine's clearances, tolerances, or overall efficiency. 23% aside, EGT generally increases with fuel flow (dark red at For some engines, documented maintenance histories were lowest fuel flow, orange and yellow at higher fuel flow). Because unavailable. The tally of potential system inefficiencies is the engine bleed air demand is responsible for increasing further complicated by the fact that bleed air demand involved fuel flow rate and there is no concomitant increase in the systems that are not part of the engine (e.g., cabin packs). engine intake fan speed, relatively more combustion heat is There were no a priori assumptions made as to which engine available than at lower fuel flows, which increases the EGT. parameters would be needed for describing the emissions When the engine is set to produce a specified rotational performance near idle. After examining the data tabulated speed (e.g., 25%), the outer fan speed ensures that the fuel- in Appendix A and the detailed engine data recorded in the to-air ratio and the pressure in the combustor are scaled data recorder, fuel flow seems to best collapse the observed accordingly. The EGT is cooled with the increase in bypass variability in emission indices for the speciated VOCs of air flow induced by N1. interest. Several HAP compounds were quantified in this This underscores the complex relationships in the near-idle study and tended to exhibit highly correlated behavior, as will on-wing regime between fuel flow, N1, and EGT. Similarly, be shown later. Observed correlations among the measured