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Section V
Emissions Model Based on Near-Idle Fuel Flow
and Ambient Temperature
The essential trend observed when an engine is operating Jet A (proxy fuel for JP-8) data have been considered here.
at near-idle conditions is an increase in VOC emission indices The JETS/APEX2 testing, also conducted at zero bleed air
with decreasing temperature. This negative temperature demand and N1 = 25% fuel flows, was performed on the
dependence was demonstrated for several VOCs. Scatter is CFM56-7B combustor, but only modest ambient temperature
present in the observations and the apparent engine-to-engine variations were encountered during the test. The testing
variability will be discussed later. An empirical model of protocol included tests at the ground idle (zero bleed) and
the temperature dependence of near-idle emissions can be N1 = 25% fuel flow points, but also probed intermediate
developed and locked to the ICAO emissions performance fuel flows. These latter tests were conducted at both cold
databank values. This empirical model can also account for weather (MDW 2009/2010) and warm weather (DAL 2009)
the effect of ambient temperature and engine operation at venues.
rotation speeds less than the 7% thrust condition. Figure V-1 indicates that, to the extent that the normal-
ization of datasets can allow comparisons between different
CFM56 combustors, empirical data from all quadrants of
V.1 Proposed Empirical Model
the fuel flow and ambient temperature space are available.
In the empirical model proposed here, the effect of ambient The essential functional forms are a linear representation
temperature and sub-7% fuel flow are treated as independent of the dependence of the emission index on fuel flow and
factors that influence a base reference emission index defined a semilinear (weakly quadratic) dependence of the emission
at 288K and 7% thrust. This is how the ICAO databank index on ambient temperature. A smooth interpolation has
idle reference conditions are incorporated into this model been applied between the temperature dependence observed
approach. Near-idle emissions scaling suggests that it is for the ground idle fuel flow data and the N1 = 25% data.
plausible to rely on the FAA and EPA's Speciate database An abbreviated encapsulation of the model developed
(EPA 2008) VOC emissions profile to convert the tabulated for CFM56-7B22 engines is tabulated in Table V-1. At the
UHC emission index to specific VOC (or specific HAP) species reference fuel flow rate of 0.105 kg s-1 (7% thrust fuel flow)
emission indices for a reference emission index. and temperature (288K) the emission index is by definition 1.
The datasets and the measurement conditions they span are If an application of this model were directed to estimate the
used to develop the empirical emissions index model shown effect of ambient temperature and non-7% fuel flow on an
in Figure V-1. The APEX1 datasets, which first showed a emission rate, the emission index could be multiplied by the
strong dependence of emissions on ambient temperature at the factor in Table V-1. For example, the model predicts that
ground idle engine condition, were conducted at temperatures emission rate for any VOC or HAP compound at 278 K and
at or greater than the ICAO reference temperature (288K). The a fuel flow of 0.095 kg s-1 would be estimated by the following
two idle conditions defined in that test matrix were ground equation:
idle (with no bleed air demand) and idle defined by N1 = 25%.
Similarly, the AAFEX tests were conducted at the same engine Emission Rate ( g s -1 ) = 2.1 × 0.095 ( kg s -1 )
conditions; however, the ambient temperatures encountered × Reference Emission Index ( g kg -1 )
during that testing were essentially at or lower than the ICAO
reference temperatures. Note that while the study goals of The data tabulated in Table V-1 are graphically depicted
AAFEX were to study alternative fuel emissions, only the in Figure V-2.
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Figure V-1. Datasets available to develop fuel flow/ambient
temperature model for predicting near-idle emissions. The red
datasets were collected for the CFM56-2C combustor on the NASA
DC-8. The blue datasets for the CFM56-7B24 combustors were
collected as part of this project.
The emission index (grams of HAP per kilogram of The multiplicative factor is applied to the emissions index,
fuel) multiplicative factor for the CFM56-7B22 depicted in with the total emission rate as the product of EI and fuel
Figure V-2 and tabulated in Table V-1 should not be inter- flow rate. Thus, at lower fuel flow rates, the emission rate
preted as a direct multiplicative factor on the total emission is linearly reduced with the reduced fuel burn. This effect is
rate (grams of HAP per second) when a fuel flow rate besides accounted for in Figure V-3, which depicts the direct factor
the reference fuel flow rate is considered. for the temperature dependent emission rate that accounts for
Table V-1. Emissions index correction factor for near-idle
CFM56-7B22 operation.
° ° ° ° ° °
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Figure V-2. Relative emission index as a function of
ambient temperature and fuel flow. The gray-scale shading
and labeled contours are the multiplicative factor for the
reference emission index.
Figure V-3. Near-idle emission rates (not emission index)
as a function of ambient temperature and fuel flow. The
contours and shading in the figure are derived from the
data in Figure V-2. The effect of reduced fuel flow rate has
been applied.
