6.2.3 The procedure for checking the performance of each analyser 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 (FSD) concentration; adjust instrument gain accordingly and record its setting;
c) introduce approximately 30 per cent, 60 per cent, and 90 per cent range FSD concentration and record analyser 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 CO 2 analyser used in their basic form without linearization of output, a least squares curve of appropriate mathematical formulation shall be fitted using additional calibration points if jud ged necessary. If any point deviates by more than 2 per cent of the full scale value (or ±l ppm*, whichever is greater) then a calibration curve shall be prepared for operational use.
* Except for the CO2 analyser, for which the value shall be ±100 ppm.
** A more comprehensive and precise alternative methodology which is acceptable is presented in Attachment E to this appendix.
Supplemental Information :
For most, if not all modern analyzers, the adjustment and recording of settings is performed by an internal computer. When the computer stores the calibration coefficients and generates a least square curve for each operational range, this should be considered equivalent to those system controls performed manually in older systems as described in 6.2.3.
6.3 Operation
6.3.1 No measurements shall be made until all inst ruments 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 tests the system shall be checked for leakage by isolating the probe and the analysers, 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: isolat e 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 t he hydrocarbon analyser reading. The reading shall not exceed 1 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.
6.3.2 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 r equisite operating mode, continue to run it and observe pollutant concentrations until a st abilized reading is obtained, which shall be recorded;
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 mo re than ±2 per cent of range FSD, the test shall be repeated after restoration of the instrument to within its specification.
6.4 Carbon balance check Each test shall include a check that the air/fuel rati o as estimated from the integrated sample total carbon concentration exclusive of smoke, agrees with the es timate based on engine air/fuel ratio within ±15 per cent for the taxi/ground idle mode, and within 10 per cent for all other modes (see 7.1.2).
7. CALCULATIONS
7.1 Gaseous emissions
7.1.1 General The analytical measurements made shall be the concentr ations of the various classes of pollutant, as detected at their respective analysers for the seve ral engine operation modes, and these values shall be reported. In addition, other parameters shall be computed and reported, as follows**.
** A more comprehensive and precise alternative methodology which is acceptable is presented in Attachment E to this appendix.
7.1.2 Basic parameters ⎛ [CO] ⎞ ⎛ 103 MCO ⎞ EI(CO) = ⎜ ⎟ ⎜ ⎟ (1+ TP( 0 m))⎝ [CO2] + [CO] + [HC ] ⎠ ⎝MC + (n m M ⎠) H ⎛ ⎞ ⎛ 3 ⎞[HC] 10 MHC EI(HC) = ⎜ ⎟ ⎜ ⎟ (1+ TP( 0 m))⎝ [CO2] + [CO] + [HC ] ⎠ ⎝MC + (n m M ⎠) H ⎛ [NOX] ⎞ ⎛ 103 MNOX ⎞ X = ⎟ ⎜ ⎟ (1+ TP( 0EI(NO ) ⎜ m))⎝ [CO2] + [CO] + [HC ] ⎠ ⎝MC + (n m M ⎠) H Supplemental information:
The basic descriptive parameter for gas eous 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.
MAIR / fuel ratio = ( P / ) 0 ⎡⎢ ⎤⎥Air m C + (/ ) HM nm M ⎣ ⎦ where:
2Z-( / )nmP/ m= ( - T Z / 2 ⎤⎦) o 41 + h ⎡⎣ and:
2- CO - 2/ x - y/2x HC + NO
[CO ] [ ] [ ] Z= [ ] ([ ] [ ])[ ] [ 2 ] 2 + CO + HC MAIR Molecular mass of dry air = 28.966 g or, where appropriate, = (32 R + 28.156 4 S +
44.011 T) g MHC molecular mass of exhaust hydrocarbon, taken as CH4 = 16.043 g MCO molecular mass of CO = 28.011 g MNO2 molecular mass of NO2 = 46.008 g MC molecular mass of carbon = 12.011 g MH molecular mass of hydrogen = 1.008 g R 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 [NO2] mean concentration of NO2 in exhaust sample, vol/vol, wet ([NO ]c ) −[NO]x = η [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 number of C atoms in characteristic exhaust hydrocarbon molecule y number of H atoms in characteristic exhaust hydrocarbon molecule Page 51 x
The value of n/m, the ratio of the 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 characterization (x, y) of the exhaust hydrocarbons, the values x= 1, y=4 are to be used.1f dry or semi-dry CO and CO 2 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.
7.1.3 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 re ference 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 (d | TBref − TB | / c)× exp (d | h-0.00629| )Bref B ref B 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 PB TB FARB h Pref Tref PBref 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 a,b,c,d 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).
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.
7.1.4 Using the recommended curve fitting technique to relate emission indices to combustor inlet temperature effectively eliminates the exp ((T Bref - TB)/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 Page 52
EI(NOX) corrected = EI derived from EI(NOX) (PBref/PB)0.5 exp (19 |h - 0.006 29|) v. TB curve If this recommended method for the CO and HC emissions index correction does not provide a satisfactory correlation, an alternative method us ing parameters derived from component tests may be used.
Supplemental Information In the generalized correction, for K the term (PB/PBref), (in the EI(CO) and EI(HC) expressions) should be (PBref/PB) 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 (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 the 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 correcte d 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 F n,, (corrected to ISA sea level conditions) and T B, (corrected to ISA sea level conditions);
Note. — These are illustrated, for example, by Figure 3-2 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 t he description of the engine to be certificated and accepted by the certificat ing authority to be representative of the engine type for which certification is sought.
The manufacturer shall also supply to the certificat ing authority all of the necessary engine performance data to substantiate these relationships and for ISA sea level ambient conditions:
d) maximum rated thrust (F oo); and
e) engine pressure ratio (π) at maximum rated thrust.
Note.— These are illustrated by Figure A3-2 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 Wfn 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 procedu res which utilize mathematical expressions representing the curves illustrated if the expression have been derived using an accepted curve fitting technique .
Тв
a) El v TB b) Wf v TB
(700) " CI F VTE d) FvT
SA SEA LEVEL ISA SEA LEVEL EI = EMISSION INDEX TB = COMBUSTOR INLET TEMPERATURE Wf = ENGINE FUEL MASS FLOW RATE F = ENGINE THRUST = ENGINE PRESSURE RATIO Figure A3-2 ICAO 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 certificating authority, after receiving satisfactory technical evidence of equivalent results obtained by an alternative procedure, may approve an alternative procedure.
ATTACHMENT A TO ICAO MANUAL APPENDIX 3.
SPECIFICATION FOR HC ANALYSER Note 1.- As outlined in 5.2 of Appendix 3, the measur ing element in this analyser is the flame ionization detector (FID) in which the whole or a representativ e portion of the sample flow is admitted into a hydrogen-fueled flame. With suitably positioned electrodes an ionisation current can be established which is a function of the mass rate of hydrocarbon entering the flame. It is this current which, referred to an appropriate zero, is amplified and ranged to prov ide the output response as a measure of the hydrocarbon concentration expressed as ppmC equivalent.
Note 2. — See Attachment D for information on calibration and test gases.
1. GENERAL Precautions: The performance specifications indicated are generally for analyser full scale. Errors at part scale may be a significantly greater percentage of reading. The relevance and importance of such increases shall be considered when preparing to make measurements. If better performance is necessary, then appropriate precautions shall be taken.
Supplemental information The performance specifications for these analyzers, given in terms of full scale response, can have a significant and adverse impact on part scale measurement s. In extreme instances, concentrations of HC at high power, such as take-off, can differ from concentrations at idle, by orders of magnitude e.g., 3,500 ppmC at idle and 10 ppmC at take-off. The FID specificat ion suggests the high level can be 5,000 ppmC. For most modern engines the idle concentrations are considerably lower but still of concern when setting up the FID to maintain suitable accuracy of measurement. In general, where concentrations of species to be measured are known to vary this way, it is alwa ys good practice to use a multi-range instrument and to adjust ranges such as to keep the measurement in the upper 30% of the instrument response range. A measurement made at 20% of full scale could result in an error 5 times the er ror specified as a percent of full scale. Calibrations should be performed on each range used as required.
The instrument to be used shall be such as to maintain the temperature of the detector and samplehandling components at a set point temperature within the range 155ºC to 165ºC to a stability of ±2 °C. The leading specification points shall be as follo ws, the detector response having been optimized and the instrument generally having stabilized:
Supplemental information Stability is usually defined in terms of a time interval which, because this is a temperature control set point, can be taken as the duration of the test.
a) Total range: 0 to 5 000 ppmC in appropriate ranges.
Supplemental information A total range of 0 to 5,000 ppmC, while appropriate fo r the engines in use when Annex 16 was published in 1981, is broader than needed for today's engines. Given pr actical limitations on instrument design the result could restrict the number of appropriat e instrument ranges available in any single instrument. Restricting the number of ranges available makes it difficult to follow good measurement practices by restricting measurements to the upper 30% of range.
b) Resolution: better than 0.5 per cent of full sc ale of range used or 0.5 ppmC, whichever is greater.
c) Repeatability: better than ±1 per cent of full scale of range used. or ±0.5 ppmC. whichever is greater.
d) Stability: better than ±2 per cent of full sca le of range used or ±l.0 ppmC, whichever is greater, in a period of 1 hour.
e) Zero drift: less than ±1 pe r cent of full scale of range used or ±0.5 ppmC, whichever is greater, in a period of 1 hour.
f) Noise: 0.5 Hz and greater, less than ±1 per c ent of full scale of range used or ±0.5 ppmC, whichever is greater.
Supplemental information The FID requires fuel and oxi dant gases for operation. T he fuel gas is typically either a mixture of hydrogen and nitrogen or hydrogen and helium. If the noise specification cannot be met and a hydrogen/nitrogen mixture is being used as the fuel gas, it sometimes helps to change to a hydrogen/helium mixture.
g) Response time: shall not exceed 10 seconds fr om inlet of the sample to the analysis system, to the achievement of 90 per cent of the final reading.
h) Linearity: response with propane in air shall be linear for each range within ±2 per cent of full scale, otherwise calibration corrections shall be used.
Supplemental information The performance specifications for the HC analyzer are typical of those analyzers offered by major instrument manufacturers.
2. SYNERGISTIC EFFECTS Note.- In application there are two aspects of performance which can affect the accuracy of measurement:
a) the oxygen effect (whereby differing proporti ons of oxygen present in the sample give differing indicated hydrocarbon concentration for constant actual HC concentrations); and
b) the relative hydrocarbon response (whereby ther e is a different response to the same sample hydrocarbon concentrations expressed as equivalent ppmC, dependent on the class or admixture of classes of hydrocarbon compounds).
The magnitude of the effects noted above shall be determined as follows and limited accordingly.
Oxygen response: measure the response with two blends of propane, at approximately 500 ppmC concentration known to a relative accuracy of ±l per cent, as follows:
1) propane in 10 ± 1 per cent O2, balance N2
2) propane in 21 ± 1 per cent O2, balance N2 If R1 and R2 are the respective normalized responses then (R1 - R2) shall be less than 3 per cent of R1.
Supplemental information The typical range of O 2 concentrations in the core exhaust gas fr om idle to take-off, is approximately 18% at idle, to 15% (or somewhat less) at take-off. The specification for a response of <3% between samples of 10% and 21% is conservative, effectively limiting the differentia l response to <1% over the range of interest. If needed the O2 response can be minimized by adjusting the FID burner fuel/air ratio.
Differential hydrocarbon response: measure the response with four blends of different hydrocarbons in air, at concentrations of approximately 500 ppmC, known to a relative accuracy of ±l per cent, as follows:
a) propane in zero air
b) propylene in zero air
c) toluene in zero air
d) n-hexane in zero air.
If Ra, Rb, Rc and Rd are, respectively, the normalized responses (with respect to propane), then (R a - Rb), (Ra - Rc) and (Ra - Rd) shall each be less than 5 per cent of Ra.
Supplemental information Although assumed to respond as a carbon number counter, for the purposes of testing engine exhaust, the FID response is known to vary somewhat with the particular hydrocarbon or class of hydrocarbons being measured. For example thr ee molecules of methane, CH 4, will not necessarily result in the same instrument response as one molecule of propane, C 3H8. Because of this differential response it is sometimes helpful to think of the FID as responding to an “effective” carbon number. It is important to ensure the instrument responds acceptably to all of the hydrocarbons in the engine exhaust. This group of hydrocarbons – propylene, toluene, and n-hexane – with propane as a reference was chosen to represent, in terms of differential response, the range of hydrocarbons expected in the exhaust.
3. OPTIMIZATION OF DETECTOR RESPONSE AND ALIGNMENT
3.1 The manufacturer's instructions for initial setting up procedures and ancillary services and supplies required shall be implemented, and the in strument allowed to stabilize. All setting adjustments shall involve iterative zero checking, and correction as necessary. Using as sample a mixture of approximately 500 ppmC of propane in ai r, the response characteristics for variations first in fuel flow and then, near an optimum fuel flow, for variations in dilution air flow to select its optimum shall be determined. The oxygen and differ ential hydrocarbon responses shall then be determined as indicated above.
Supplemental information The FID detector response and alignment can be opti mized by adjusting the FID burner fuel and air flow while sampling a mixture containing approximately 500 ppmC propane. Care should be taken when changing fuel flow that the instrument zero does not shift. If it does the instrument zero should be reset.
Response curves, illustrating this process, shown below in Figures A3-3 and A3-4, were taken from 40 CFR Part 87, Control of Air Pollution from Aircraft and Ai rcraft Engines, Federal Register, Vol. 43, No. 58 — Friday, March 24, 1978. They were taken, in turn, from SAE ARP 1256, "Procedure fo r the Continuous Sampling and Measurement of Gaseous Emissions from Aircraft Turbine Engines".
Figure A3-3 Typical Fuel Flow Response Curve Figure A3-4 Typical Airflow Response Curve The objective of this procedure is to select operatin g flow rates which will give near maximum response with least variation for minor fuel flow variations. It may be necessary to repeat this operation in an iterative fashion:
• Adjust the fuel flow to maximize output;
• Adjust zero if necessary;
• Adjust the air flow to maximize output
• Readjust the fuel flow, if necessary, and
• Repeat until the burner output is optimized.
3.2 The linearity of each analyser range shall be checked by applying propane in air samples at concentrations of approximately 30, 60 and 90 per cent of full scale. The maximum response deviation of any of these points from a least squa res straight line (fitted to the points and zero) shall not exceed ±2 per cent of full scale value. If it does, a calibration curve shall be prepared for operational use.
Explanation
Table A3-1 summarizes the performance specifications of an HC analyzer.
Table A3-1 HC ANALYZER PERFORMANCE SPECIFICATIONS Parameter Value Total Range 0 to 5,000 ppmC Resolution better than 0.5 percent full scale range used or 0.5 ppmC whichever is greater Repeatability better than ±1 percent full scale range used or ±0.5 ppmC whichever is greater Stability better than ±2 percent full scale range used or ±1 ppmC whichever is greater in a period of 1 hour Zero drift < ±1 percent full scale range used or ±0.5 ppmC, whichever is greater in a period of 1 hour Noise 0.5 Hz and greater, < than ±1 percent full scale range used or ±0.5 ppmC, whichever is greater Response time ≤ 10 seconds from inlet of sample to achieve 90 percent of final reading Linearity ±2 percent full scale to propane in air, for each range used, or calibration correction required Page 60