Figure A5-2. Sample transfer and analysis system, schematic Supplemental information In contrast to non-afterburning jet engine exhaust, lo cal temperatures in the plume of an afterburning engine can be extremely high (in excess of 2,000 ° C) and very reactive. Measuring emissions in the hot, reactive part of the exhaust plume places a heavy burden on probe design and provides no assurance that the species measured are those whic h eventually contribute to atmospheric pollution.
It is possible to calculate the final combustion products from species measured in the reacting flow.
The methods however are complex, error prone, and the results subject to question.
Additionally, the concentration profiles in the plane of the exhaust plume immediately downstream of the engine exhaust nozzle are highly variable and make obtaining a representative sample difficult, time consuming and costly. Sampling at some di stance downstream of the nozzle exit takes advantage of the turbulent mixing in the plume whic h serves to cool the gases to the point where chemical reactions cease, and to smooth the concentration profiles, making sampling easier. This occurs through mixing of the exhaust gases with ambient air. This dilution must be accounted for when the data are analyzed and calculations are made.
Depending on the amount of dilution, special care may be required in having suitable concentration ranges for the analytical instruments. This will be true of the calibration and standard gases as well.
Because the products of combustion are the same for non-afterburning and afterburning engines, the same cautions apply to both.
The following diagram, Figure A5-3, from AFAPL-TR- 75-52, illustrates the degree of severity of the problem:
Figure A5-3 Exhaust Concentration Profile
5. DESCRIPTION OF COMPONENT PARTS Note.— A general description and specification of the principal elements in the engine exhaust emissions measurement system follows. Greater detail, where necessary, will be found in Attachments A, B and C to this appendix.
5.1 Sampling system
5.1.1 Sampling probe
a) The probe shall be constructed so that individual samples can be withdrawn at various locations across a diameter of the plume. Mixed samples shall not be permitted.
b) The material with which the sample is in contact shall be stainless steel and its temperature shall be maintained at a value not less than 600C.
c) The sampling plane shall be perpendicula r to the projected engine nozzle centre line, and shall be situated as close as poss ible to a position 18 nozzle diameters from the nozzle exit plane, consistent with 7.1.2, but in no case greater than 25 nozzle diameters. The nozzle exit diameter shall be for the maximum engine power condition. Between and including exit and sampling planes there shall be an unobstructed region of at least 4 nozzle exit diameters in radial distance about the project engine nozzle centre line.
d) The minimum number of sampling points shall be equal to 11. The measurement plane, located at a distance X from the engine shall be divided into three sections demarcated by circles centred around the exhaust stream axis with radii R1 = 0.05X R2 = 0.09X and a minimum of 3 samples shall be taken from each section. The difference between the number of samples in each section must be less than 3. The sample taken at the most remote distance from the axis shall be from a point located at a radius of between 0.11X and 0.16X.
5.1.2 Sampling lines The sample shall be transferred from the pr obe to the analysers via a line of 4.0 to 8.5 mm inside diameter, taking the shortest route practicable and using a flow rate such that the transport time is less than 10 seconds. The line shall be maintained at a temperature of 1600C ±150C (with a stability of ±100C). When sampling to measure HC, CO, CO2 and NOx components, the line shall be constructe d in stainless steel or carbon-loaded grounded PTFE.
Supplemental information To be consistent with appendix 5, Figure 5-2, “Sam ple transfer and analysis system, schematic”, this paragraph should be changed to read:
“The sample shall be transferred from the probe to the analysers via a line of 4.0 to 8.5 mm inside diameter, taking the shortest route practicable and usi ng a flow rate such that the transport time is less than 10 seconds. The line shall be maintained at a temperature of 160 0C ±150C (with a stability of ±100C) except for the branch which supplies samples to the CO, CO 2, and NO X analysers. This branch line shall be maintained at a temperature of 60 °C ±150C (with a stability of ±100C). When sampling to measure HC, CO, CO 2 and NOx components, the line shall be constructed in stainless steel or carbon-loaded grounded PTFE.”
5.2 HC analyzer The measurement of total hydrocarbon sample content shall be made by an analyzer using the heated flame ionization detector (FID), between the electrodes of which passes an ionization current proportional to the mass rate of hydrocarbon entering a hydrogen flame. The analyzer shall be deemed to include components arranged to control temperature and flow rates of sample, sample bypass, fuel and diluent gases, and to enable effective span and zero calibration checks.
Note.- An over-all specification is given in Attachment A to this appendix.
5.3 CO and CO2 analysers Non-dispersive infra-red analysers shall be us ed for the measurement of these components, and shall be of the design which utilizes differentia l energy absorption in parallel reference and sample gas cells, the cell or group of cells fo r each of these gas constituents being sensitized appropriately. This analysis sub-system shall in clude all necessary functions for the control and handling of sample, zero and span gas flows. Temperature control shall be that appropriate to whichever basis of measurement, wet or dry, is chosen.
Note - An over-all specification is given in Attachment B to this appendix.
5.4 NOX analyzer The measurement of NO concentration shall be by the c hemiluminescent method in which the measure of the radiation intensity emitted during the reaction of the NO in the sample with added O3 is the measure of the NO concentration. The NO2 component shall be converted to NO in a converter of the requisite efficiency prior to measurement. The resultant NOX measurement system shall include all necessary flow, temper ature and other controls and provide for routine zero and span calibration as well as for converter efficiency checks.
Note.- An over-all specification is given in Attachment C to this appendix.
6. GENERAL TEST PROCEDURES
6.1 Engine operation The engine shall be operated on an open air static test facility which is suitable and properly equipped for high accuracy performance testing, and which conforms to the requirements for sampling probe installation as specified in 5.1. The emissions tests shall be made at the power settings prescribed by the certificating authority. The engine shall be stabilized at each setting.
6.2 Ambient air conditions
6.2.1 A check shall be made on t he ambient concentrations of CO, HC, CO2 and NO, with the engine under test running at the test condition. Unusually high concentrations indicate abnormal conditions such as exhaust gas recirculation, fuel spillage or some other source of unwanted emissions in the test area and such situations shall be rectified or avoided as appropriate.
Note.- For guidance, the normal ambient concentration of CO2 is 0.03 per cent, and ambient concentration levels for CO and HC of 5 ppm and NOX of 0.5 ppm are unlikely to be exceeded under normal conditions.
6.2.2 Extreme climatic conditions, such a those involving precipitation or excessive wind speed shall also be avoided.
6.3 Major instrument calibration Note.- The general objective of this calibration is to confirm stability and linearity.
6.3.1 The applicant shall satisfy t he certificating authority that t he calibration of the analytical system is valid at the time of the test.
6.3.2 For the hydrocarbon analyzer this calibration shall include checks that the detector oxygen and differential hydrocarbon responses are within the limits specified in Attachment A to this appendix. The efficiency of the NO2/NO converter shall also be checked and verified to meet the requirements in Attachment C to this appendix.
6.3.3 The procedure for checking the performance of each analyzer shall be as follows (using the calibration and test gases as specified in Attachment D to this appendix):
a) introduce zero gas and adjust instrument zero, recording setting as appropriate;
b) for each range to be used operationally, introduce calibration gas of (nominally) 90 per cent range full-scale deflection (F SD) concentration; adjust instrument gain accordingly and record its setting;
c) introduce approximately 30 per cent, 60 per cent, and 90 per cent range FSD concentrations and record analyzer readings;
d) fit a least squares straight line to the zero, 30 per cent, 60 per cent and 90 per cent concentration points. For the CO and/or CO2 analyzer used in its basic form without linearization of output, a least s quares curve of appropriate mathematical formulation shall be fitted using additional calibration points if judged necessary.
If any point deviates by more than 2 per cent of the full scale value (or ±1 ppm*, whichever is greater) then a calibratio n curve shall be prepared for operational use * Except for the CO 2 analyzer. for which the value shall be ±lOO ppm
6.4 Operation
6.4.1 No measurements shall be made until a ll instruments and sample transfer lines are warmed up and stable and the following checks have been carried out:
a) leakage check: prior to a series of te sts the system shall be checked for leakage by isolating the probe and the analyse rs, connecting and operating a vacuum pump of equivalent performance to that used in the smoke measurement system to verify that the system leakage flow rate is less than 0.4 L/min referred to normal temperature and pressure;
b) cleanliness check: isol ate the gas sampling system from the probe and connect the end of the sampling line to a source of zero gas. Warm the system up to the operational temperature needed to perform hydrocarbon measurements. Operate the sample flow pump and set the flow rate to that used during engine emission testing. Record the hydrocarbon analyzer reading. The reading shall not exceed I per cent of the engine idle emission level or 1 ppm (both expressed as methane), whichever is the greater.
Note 1.-It is good practice to back-purge the sampling lines during engine running, while the probe is in the engine exhaust but emissions are not being measured, to ensure that no significant contamination occurs.
Note. 2-It is also good practice to monitor the inlet air quality at the start and end of testing and at least once per hour during a test. If levels are considered significant, then they should be taken into account.
6 4.2 The following procedure shall be adopted for operational measurements:
a) apply appropriate zero gas and make any necessary instrument adjustments;
b) apply appropriate calibration gas at a nominal 90 per cent FSD concentration for the ranges to be used, adjust and record gain settings accordingly;
c) when the engine has been stabilized at the requisite operating conditions and sampling location, continue to run it and observe pollutant concentrations until a stabilized reading is obtained, which shall be recorded At the same engine operating condition repeat the measurement procedure for each of the remaining sampling locations;
d) recheck zero and calibration points at the end of the test and also at intervals not greater than 1 hour during tests. If either has changed by more than ±2 per cent Page 89
of full scale of range, the test shall be repeated after restoration of the instrument to within its specification.
Supplemental information Note 2: For significant levels in the inlet air, say 1% of full scale of appropriate range in use acceptable practice would allow subtracting out the inlet concentration from that measured in the exhaust.
One objective of this sampling protocol is to take advantage of the relatively low spatial variability of the exhaust species by sampling in the fully turbulent free jet zone. Doing this requires a method of data analysis which accounts for dilution air which is well mixed with the core jet by calculating emission indices using a total carbon balance and ensuring – through a cross correlation calculation – that the CO2 correlates well with each of the other species, acceptable accuracy is achieved.
7. CALCULATIONS
7.1 Gaseous emissions
7.1.1 General The analytical measurements made shall be the concentrations of the various classes of pollutant, at the relevant afterburning mode(s) of the engine, at the various locations in the sampling plane. In addition to the recording of these basic parameters, other parameters shall be computed and reported, as follows.
7.1.2 Analysis and validation of measurements
a) At each engine setting, the concentrati ons measured at different probe sampling positions must be averaged as follows:
n j= ∑ 1 Cim o y = Ci j where:
n ∑ 1=j Cij Concentration of species i measured at the j th sampling position.
Summation of the total number n of sampling positions used.
Cim o y average or mean concentration of species i.
All dry concentration measurements shall be converted into real wet concentrations. (See Attachment E to this appendix).
b) The quality of the measurements for each pollutant will be determined through a comparison with measurements of CO2, using the correlation coefficient:
Values of ri which are near to 1 indicate that measurements taken over the entire sampling period are sufficiently stable and that the curves are Gaussian. In the event that r i i s le s s tha n 0 .95, measurements must be repeated in a sampling plane located at a more remote distance from the aircraft engine. The measurement process, per se, is then followed by the same calculations and the same demonstration as previously.
7.1.3 Basic parameters ⎛ 3 ⎞⎛ [] ⎞ 10 MCO CO ⎜ ⎟ ⎜ ) H ⎠ 1 TPEI(CO) = ⎟ ( + ( 0 m))⎝ [CO2] + [ ] [] CO + HC ⎠ ⎝MC + (n m M ⎛ [] HC ⎞ ⎛ 103 MHC ⎞ EI(HC) = ⎟ ⎟ (1+ ( 0 m))⎜ ⎜ TP⎝ [CO2] + [ ] [] CO + HC ⎠ ⎝M + (n m M ) ⎠C H ⎛ [NOX] ⎞ ⎛ 103 MNOX ⎞ EI(NO ) = ⎜ ⎟ ⎜ ⎟ (1+TP( 0 m))X 2] [ ] [] ⎝ C⎝ [CO + CO + HC ⎠ M + (n m M Supplemental information The basic descriptive parameter for gaseo us emissions is the emission index (EI) , defined as the mass of a product of combustion, in grams, divided by the mass of fuel used in kilograms, i.e., grams per kilogram.
) H ⎠ where:
2Z n m− ( ) m =P0 ( +− [41 hT Z 2]) and 2C O −[ ]− ([2 ] −[x y x N O 2 ])[H C ]+ [ 2 ] CO2 + CO + HC Z = [ ] [] [] Page 91
molecular mass of dry air = 28.966 g or, where appropriate, = (32 R + 28.156 4 S + 44.011 T) g molecular mass of exhaust hydrocarbon, taken as CH4 = 16.043 g molecular mass of CO = 28.011 g molecular mass of NO2 = 46.008 g molecular mass of carbon = 12.011 g molecular mass of hydrogen = 1.008 g concentration of O2 in dry air, by volume = 0.209 5 normally S concentration of N2 + rare gases in dry air, by volume = 0.709 2 normally T concentration of CO2 in dry air, by volume = 0.000 3 normally [HC] mean concentration of exhaust hydrocarbons vol/vol, expressed as carbon [CO] mean concentration of CO vol/vol, wet [CO2] mean concentration of CO2 vol/vol, wet [NOX] mean concentration of NOX vol/vol, wet = [NO + NO2} [NO] mean concentration of NO in exhaust sample, vol/vol, wet MAIR MHC MCO MNO2 MC MH R ([ NOx ] ) −[ NO ]c = η [NO2] mean concentration of NO2 in exhaust sample, vol/vol, wet [NOX]c mean concentration of NO in exhaust sample after passing through the NO2/NO converter, vol/vol, wet η efficiency of NO2/NO converter h humidity of ambient air, vol water/vol dry air m number of C atoms in characteristic fuel molecule n number of H atoms in characteristic fuel molecule x number of C atoms in characteristic exhaust hydrocarbon molecule y number of H atoms in characteristic exhaust hydrocarbon molecule The value of n/m, the ratio of t he atomic hydrogen to atomic carbon of the fuel used, is evaluated by fuel type analysis. The ambient air humidity, h, shall be measured at each set condition. In the absence of contrary evidence as to the characte rization (x, y) of the ex haust hydrocarbons, the values x= 1, y=4 are to be used. If dry or semi-dry CO and CO2 measurements are to be used then these shall first be converted to the equivalent wet concentration as shown in Attachment E to this appendix, which also contains interference correction formulas for use as required.
7.1.4 Correction of emission indices to reference conditions Corrections shall be made to the measured engine emission indices for all pollutants in all relevant engine operating modes to account for deviations from the reference conditions (ISA at sea level) of the actual test inlet air conditions of temperature and pressure. The reference value for humidity shall be 0.00629 kg water/kg dry air.
Thus, EI corrected = K x EI measured, where the generalized expression for K is:
K = ( P / P ) a × ( FAR / FAR ) b × exp | ( T T | / c )× exp (d | h − 0.00629 | )Bref B ref B Bref − B Page 92
PB TB FARB h Pref Tref PBref Combustor inlet pressure, measured Combustor inlet temperature, measured Fuel/air ratio in the combustor Ambient air humidity ISA sea level pressure ISA sea level temperature Pressure at the combustor inlet of the engine tested (or the reference engine if the data is corrected to a reference engine) associated with TB under ISA sea level conditions TBref Temperature at the combustor inlet under ISA sea level conditions for the engine tested (or the reference engine if the data is corrected to a reference engine). This temperature is the temperature associated with each thrust level specified for each mode.
FARref Fuel/air ratio in the combustor under ISA sea level conditions for the engine tested (or the reference engine if the data is to be corrected to a reference engine).
a,b,c,d Specific constants which may vary for each pollutant and each engine type The combustor inlet parameters shall preferably be measured but may be calculated from ambient conditions by appropriate formulas.
7.1.5 Using the recommended curve fitting technique to relate emission indices to combustor inlet temperature effectively eliminates the exp ((T Bref - T B)/c) term from the generalized equation and for most cases the (FAR ref/FAR,) term may be considered unity. For the emissions indices of CO and HC many testing facilities have determined that the humidity term is sufficiently close to unity to be eliminated from the expression and that the exponent of the (PBref /PB) term is close to unity.
Thus, EI(CO) corrected = EI derived from (PB /PBref) EI(CO) v. TB, curve EI(HC) corrected = EI derived from (PB /PBref) EI(HC) v. TB, curve EI(NOX) corrected = EI derived from EI(NOX) (PBref/PB)0.5 exp (19 |h - 0.006 29|) v. TB curve Supplemental information From the generalized correction for K , the term (P B/PBref) – in the EI(CO) and EI(HC) expressions – should be (PBref/PB).
If this recommended method for the CO and HC em issions index correction does not provide a satisfactory correlation, an alte rnative method using parameters derived from component tests may be used.
Any other methods used for making corrections to CO, HC and NO, emission indices shall have the approval of the certificating authority.
7.2 Control parameter functions Page 93
(Dp , Foo , π)
7.2.1 Definitions Dp The mass of any gaseous pollutant emitted during the reference emissions landing and take-off cycle.
Foo The maximum thrust available for take -off under normal operating conditions at ISA sea level static conditions, without the use of water injection, as approved by the applicable certificating authority.
The ratio of the mean total pressure at t he last compressor discharge plane of the π compressor to the mean total pressure at the compressor entry plane when the engine is developing take-off thrust rating at ISA sea level static conditions.
7.2.2 The emission indices (EI) for each pollutant, corrected for pressure and humidity (as appropriate) to the reference ambient atmospheric conditions as indicated in 7.1.4 and if necessary to the reference engine, shall be obtained for the required LTO engine operating mode settings (n) of idle, approach. climb-out and take-off, at each of the equivalent corrected thrust conditions. A minimum of three test points shall be required to define the idle mode. The following relationships shall be determined for each pollutant:
a) between EI and TB; and
b) between Wf (engine fuel mass flow rate) and TB; and
c) between Fn,, (corrected to ISA sea level conditions) and TB, (corrected to ISA sea level conditions);
Note. — These are illustrated, for example, by Figure 5-3 a), b) and c).
When the engine being tested is not a "reference" engine, the data may be corrected to "reference" engine conditions using the relationships b) and c) obtained from a reference engine.
A reference engine is defined as an engine substantially configured to the description of the engine to be certificated and accepted by the cert ificating authority to be representative of the engine type for which certification is sought.
The manufacturer shall also supply to the certif icating authority all of the necessary engine performance data to substantiate these relationships and for ISA sea level ambient conditions:
d) maximum rated thrust (Foo); and
e) engine pressure ratio (%) at maximum rated thrust.
Note.— These are illustrated by Figure A5-4 d).
7.2.3 The estimation of EI for each pollutant at each of the required engine mode settings, corrected to the reference ambient conditions, shall comply with the following general procedure:
a) at each mode ISA thrust condition F n determine the equivalent combustor inlet temperature (TB) (Figure 3-2 c));
b) from the EI/TB characteristic (Figure 3-2 a)), determine the EIn value corresponding to TB;
c) from the W f/TB characteristic (Figure 3-2 b)), determine the W fn value corresponding to TB;
note the ISA maximum rated thrust and pressure ratio values. These are F oo and π respectively (Figure 3-2 d));
calculate, for each pollutant Dp = Σ (EIn)(Wfn)(t) t time in LTO mode (minutes) Wfn fuel mass flow rate (kg/min) Σ is the summation for the set of modes comprising the reference LTO cycle.
7.2.4 While the methodology described above is the recommended method, the certificating authority may accept equivalent mathematical procedures which utilize mathematical expressions representing the curves illustrated if the expression have been derived using an accepted curve fitting technique.
Figure A5-4 Calculation Procedure
7.3 Exceptions to the proposed procedures In those cases where the configuration of the engine or other extenuating conditions exist which would prohibit the use of this procedure, the cert ificating authority, after receiving satisfactory technical evidence of equivalent results obt ained by an alternative procedure, may approve an alternative procedure.