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Measurement of Gaseous HAP Emissions from Idling Aircraft as a Function of Engine and Ambient Conditions (2012)

Chapter: Section V - Emissions Model Based on Near-Idle Fuel Flow and Ambient Temperature

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Suggested Citation:"Section V - Emissions Model Based on Near-Idle Fuel Flow and Ambient Temperature." National Academies of Sciences, Engineering, and Medicine. 2012. Measurement of Gaseous HAP Emissions from Idling Aircraft as a Function of Engine and Ambient Conditions. Washington, DC: The National Academies Press. doi: 10.17226/13655.
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Suggested Citation:"Section V - Emissions Model Based on Near-Idle Fuel Flow and Ambient Temperature." National Academies of Sciences, Engineering, and Medicine. 2012. Measurement of Gaseous HAP Emissions from Idling Aircraft as a Function of Engine and Ambient Conditions. Washington, DC: The National Academies Press. doi: 10.17226/13655.
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Suggested Citation:"Section V - Emissions Model Based on Near-Idle Fuel Flow and Ambient Temperature." National Academies of Sciences, Engineering, and Medicine. 2012. Measurement of Gaseous HAP Emissions from Idling Aircraft as a Function of Engine and Ambient Conditions. Washington, DC: The National Academies Press. doi: 10.17226/13655.
×
Page 19
Page 20
Suggested Citation:"Section V - Emissions Model Based on Near-Idle Fuel Flow and Ambient Temperature." National Academies of Sciences, Engineering, and Medicine. 2012. Measurement of Gaseous HAP Emissions from Idling Aircraft as a Function of Engine and Ambient Conditions. Washington, DC: The National Academies Press. doi: 10.17226/13655.
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Page 20

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17 The essential trend observed when an engine is operating at near-idle conditions is an increase in VOC emission indices with decreasing temperature. This negative temperature dependence was demonstrated for several VOCs. Scatter is present in the observations and the apparent engine-to-engine variability will be discussed later. An empirical model of the temperature dependence of near-idle emissions can be developed and locked to the ICAO emissions performance databank values. This empirical model can also account for the effect of ambient temperature and engine operation at rotation speeds less than the 7% thrust condition. V.1 Proposed Empirical Model In the empirical model proposed here, the effect of ambient temperature and sub-7% fuel flow are treated as independent factors that influence a base reference emission index defined at 288K and 7% thrust. This is how the ICAO databank idle reference conditions are incorporated into this model approach. Near-idle emissions scaling suggests that it is plausible to rely on the FAA and EPA’s Speciate database (EPA 2008) VOC emissions profile to convert the tabulated UHC emission index to specific VOC (or specific HAP) species emission indices for a reference emission index. The datasets and the measurement conditions they span are used to develop the empirical emissions index model shown in Figure V-1. The APEX1 datasets, which first showed a strong dependence of emissions on ambient temperature at the ground idle engine condition, were conducted at temperatures at or greater than the ICAO reference temperature (288K). The two idle conditions defined in that test matrix were ground idle (with no bleed air demand) and idle defined by N1 = 25%. Similarly, the AAFEX tests were conducted at the same engine conditions; however, the ambient temperatures encountered during that testing were essentially at or lower than the ICAO reference temperatures. Note that while the study goals of AAFEX were to study alternative fuel emissions, only the Jet A (proxy fuel for JP-8) data have been considered here. The JETS/APEX2 testing, also conducted at zero bleed air demand and N1 = 25% fuel flows, was performed on the CFM56-7B combustor, but only modest ambient temperature variations were encountered during the test. The testing protocol included tests at the ground idle (zero bleed) and N1 = 25% fuel flow points, but also probed intermediate fuel flows. These latter tests were conducted at both cold weather (MDW 2009/2010) and warm weather (DAL 2009) venues. 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 the fuel flow and ambient temperature space are available. The essential functional forms are a linear representation of the dependence of the emission index on fuel flow and a semilinear (weakly quadratic) dependence of the emission index on ambient temperature. A smooth interpolation has been applied between the temperature dependence observed for the ground idle fuel flow data and the N1 = 25% data. An abbreviated encapsulation of the model developed for CFM56-7B22 engines is tabulated in Table V-1. At the reference fuel flow rate of 0.105 kg s-1 (7% thrust fuel flow) and temperature (288K) the emission index is by definition 1. If an application of this model were directed to estimate the effect of ambient temperature and non-7% fuel flow on an emission rate, the emission index could be multiplied by the factor in Table V-1. For example, the model predicts that emission rate for any VOC or HAP compound at 278 K and a fuel flow of 0.095 kg s-1 would be estimated by the following equation: Emission Rate g s kg s Referen − −( ) = × ( ) × 1 12 1 0 095. . ce Emission Index g kg−( )1 The data tabulated in Table V-1 are graphically depicted in Figure V-2. S e c t i o n V Emissions Model Based on Near-Idle Fuel Flow and Ambient Temperature

18 The emission index (grams of HAP per kilogram of fuel) multiplicative factor for the CFM56-7B22 depicted in Figure V-2 and tabulated in Table V-1 should not be inter- preted as a direct multiplicative factor on the total emission rate (grams of HAP per second) when a fuel flow rate besides the reference fuel flow rate is considered. The multiplicative factor is applied to the emissions index, with the total emission rate as the product of EI and fuel flow rate. Thus, at lower fuel flow rates, the emission rate is linearly reduced with the reduced fuel burn. This effect is accounted for in Figure V-3, which depicts the direct factor for the temperature dependent emission rate that accounts for 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. ° ° ° ° ° ° Table V-1. Emissions index correction factor for near-idle CFM56-7B22 operation.

19 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.

20 the effects of both ambient temperature and fuel flow rate. The contours reflect that some of the dependence on fuel flow has been softened, relative to the dependence in Figure V-2. V.2 Example Application of the Model The overall approach adopted here is to convert the recorded fuel flow rates to instantaneous emission rates using both the temperature and fuel flow dependent emission index. The steps associated with the fuel flow conversion only are outlined here for a single operation for an A320 aircraft being operated at Zurich Airport. The ambient tempera- ture during the operation was within 2°C of the reference temperature (15°C) and thus the temperature correction is very small. In Figure V-4, shortly after engine start a sub-7% fuel flow is established. Periodically, the fuel flow actually increased above the 7% thrust, presumably to achieve “breakaway” thrust— the point where the aircraft begins to move. The units of the emissions index correction factor are grams UHC/grams UHC7% kg fuel7%/kg fuel. This is a multiplicative factor to the 7% thrust emission index; however, the potential difference in fuel flow needs to be treated explicitly. Figure V-5 depicts the estimated adjusted emission index factor for the example fuel flow time series depicted in Figure V-4. Recorded fuel flow from an A320’s taxiway phase is shown as a function of time since engine start. The reference ICAO 7% thrust fuel flow is depicted in the horizontal line, and the trace is derived from the digital flight data record. The actual reported fuel flow has been divided by two since the original data source represents the sum from two engines. Figure V-5. Relative emission index adjusted for fuel flow rate for the example calculation described in the text. Figure V-1. In this example, the extrapolation is assumed to be linear above the 7% thrust value. HC HC Fuel Flow FF s kg Fuel Flow ( ) ( ) = + −    7 1 52% −[ ]FF7% In Figure V-6 the UHC emission rate is calculated for the time-dependent fuel flow rate from the example operation. As noted earlier, the fuel flow dependent model is probably not very accurate for the breakaway accelerations, but this example at least estimates this portion of the fuel flow only dependence. The ratio of the integrated emission for the example operation to the ICAO 7% emission of equivalent duration is 1.34 for this portion of the LTO. Figure V-6. Fuel dependent adjusted emission rate is depicted for two cases. The brown trace is the example taxiway operation, and the light red trace is the ICAO emission rate for idle phase.

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TRB’s Airport Cooperative Research Program (ACRP) Report 63: Measurement of Gaseous HAP Emissions from Idling Aircraft as a Function of Engine and Ambient Conditions is designed to help improve the assessment of hazardous air pollutants (HAP) emissions at airports based on specific aircraft operating parameters and changes in ambient conditions.

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