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Figure III-3 presents all measured formaldehyde emission
indices, from every test conducted when the ambient air was
lower than 0°C in 2009 and 2010 at MDW, plotted versus fuel
flow for 11 different CFM56-7B24 combustors. Of all param-
eters recorded in the digital flight data records, fuel flow best
linearizes the trend in emission index. Similar patterns are
observed in the other specific hydrocarbon measurements,
including ethene, acetylene, and 1,3-butadiene.
III.1Dependence of VOC
on Fuel Flow Near Idle
The ICAO engine certification databank (ICAO 2006)
defines the idle condition as 7% of rated thrust. An ICAO
engine entry specifies the engine combustor and the thrust
rating for each particular model certified for use on commer-
cial aircraft. In some cases, the combustor technology is not
drastically different for entries in the databank with different
Figure III-1. Fuel flows resulting from the named thrust ratings. Because the ICAO definition of idle is relative
engine condition in the near-idle test matrix to maximum thrust, if the engines using the same combustor
(GI = ground idle). These tests were conducted at designs can be identified there may be preexisting data that
ambient temperatures between 265K and 271K. can address the dependence of VOC emissions on fuel flow
for near-idle operations (DuBois and Paynter 2006).
emission indices for several VOC species are actually more Examination of the tabulated total UHC emission indices
precise than the capability to reproduce low-fuel-flow engine should provide a qualitative guide for the UHC emissions-
states. Although some important differences in the VOC related performance to compare with the trend observed in
profile have been uncovered in this study, the same trend Figure III-3.
will generally be present in other HAP species when data for Figure III-4 shows a selected portion of the ICAO emissions
a particular HAP or VOC are evaluated. databank for total hydrocarbons. The blue data entries in the
Figure III-2. N1 fan speed versus fuel flow rate. The data
points are colored by exhaust gas temperature. The small data
points are the warm-up test points. The data circled in blue
represent points where de-icing technologies were enabled.
