EPA-AA-AC ^ .75-02
Technical Support Report for Regulatory Action
Analysis of Aircraft Emission Control Parameters
October 1975
Notice
Technical support reports for regulatory action do not necessarily
represent the final EPA decision on regulatory issues. They are intended
to present a technical analysis of an issue and recommendations resulting
from the assumptions and constraints of that analysis. Agency policy
constraints or data received subsequent to the date of release of this
report may alter the recommendations reached. Readers are cautioned to
seek the latest analysis from EPA before using the information contained
herein.
Standards Development and Support Branch
Emission Control Technology Division
Office of Mobile Source Air Pollution Control
Office of Air and Waste Management
U.S. Environmental Protection Agency
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Abstract
In comparing the relative merits and disadvantages of the two
contending parameters (thrust normalized and fuel normalized emissions)
for regulating aircraft engine emissions, it can be concluded that there
are many similar features. The fundamental difference is that the thrust
based parameter encompases two essential features lacking in the fuel
based parameter. The first is the inclusion of all variables which
contribute to the quantities of pollutants being emitted by the engine.
The second is the consideration which is given to the usefulness of the
pollutant source, in that the limiting parameter allows pollution in
proportion to services rendered by the source.
Prepared by - Project Manager
Aircraft
Approved - Branch^Chief SDSB
oved - Division Dire
or
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BACKGROUND
The impetus for this current analysis of the controlling parameter for
aircraft emissions is the ongoing development of international emission
regulations through the International Civil Aviation Organization (ICAO).
The May 1975 meeting of the Aircraft Engine Emission Study Group (AEESG) of
ICAO discussed various approaches to specifying the parameter for controlling
aircraft emissions. Appendix A is the record of the meeting pertinent to
the controlling parameter. During the May meeting the viable parameters
were narrowed to two basic approaches. Namely, the use of a measure of
pollutants normalized by fuel flow and pollutants normalized by thrust or
impulse. This report provides an analysis of the merits and disadvantages
of these two different approaches.
DISCUSSION
It was determined by the ICAO study group that the basic parameters for
characterization of emission performance for ICAO regulations should have
the following characteristics:
a) express effectively the significance of the pollution emitted
by a given engine and be usable for planning purposes by
providing a reliable indication of the amount of pollutants
emitted by a given level of traffic under particular operational
conditions;
b) measure the level of technology by the manufacturers and allow
a comparison between engines;
c) be measured with enough accuracy by the best techniques and
equipment available at the time without involving necessary
sophistication; and
d) be expressed in International System of Units (SI Units) to
the greatest extent practicable in all calculations involving
reduction of emission data and the final reporting of test
results.
To determine the extent that the significant parameters comply to these
characteristics a detailed evaluation is required.
Characteristics (a) should be given the highest priority in that the
parameter not only gives an indication of the amount of pollutant emitted
by the engine for planning purposes but is more importantly a limiting
parameter. If effective control is to be realized from a regulation, the
controlling parameter must indicate the total amount or rate of pollutants
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which are permitted, when the aircraft is operated in the critical environ-
ment. To achieve this, the parameter should have in part the dimensions of
mass pollutant per operation or mass pollutant per time with time related
to operation.
a *
To illustrate how a thrust based parameter achieves this, the EPAP
parameter can be examined.
All
Aircraft
Types
fr^t = Y^ (Engine Emission Engines) X (Operations)
Pollution Operation ' 'TypE
The number of engines on a particular type of aircraft can be obtained
from many sources, a popular one being Janes, "All the Worlds Aircraft."
The operations for that particular aircraft at a given airport is accurately
obtained from the "Official Airline Guide" supplemented with other statisti-
cal compilations for unscheduled traffic. This leaves only the engine
emissions per operation to be determined. This can be obtained from the EPAP
by multiplying the thrust impulse for the particular engine.
Engine Emissions = (EpAp) x (TQTAL mpULSE)
Operation ' ' .
Engine Engine
The impulse in a fixed percentage of the rated thrust for a given
engine class, except for idle. Rated thrust for any given engine is well
known. For the EPAP, idle emissions are determined at the engine manufacturers
recommended thrust level. The uncertainty of the idle thrust is the only
variable that is not well documented and readily available. For the EPA T2
engine class the impulse can be written as
Modes
Impulse = (Percent Thrust in Mode) X (Rated Thrust) X (Time in Mode)
where the present thrust and time in mode are fixed in the EPAP as:
Thrust Time
Idle/Taxi Manufacturer spec. 26.0Min
Approach 30% 4.0
Climb Out 85% 2.2
Take Off 100% 0.7
These specified thrust levels are considered representative of a typical
operation. As well, the times in mode are considered typical of a major U.S.
airport during periods of heavy operations, stimulating concern for air
pollutant levels. -'
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From the previous discussion it should be clear that the EPAP parameter
or a similar parameter reflecting the total pollutants emitted by an engine
over a typical operational cycle can reflect the airport pollution burden.
Consideration should now be given to normalizing the pollutant rate with
respect to the usefulness or services rendered. Initially one may consider
the pollution allowed should be in proportion to the passenger-distance
traveled. However, since the problem is related to the airport environment
and not the path which the aircraft has come or is departing for, passenger
traffic is more representative of the usefulness. To eliminate the variable
of load factors, let us consider the passenger traffic to be indicated by
the aircraft capacity. Figure 1 clearly illustrates that the total emissions
over an operation is proportional to the capacity. Figure 2 and 3 further
indicate that the capacity of the aircraft can be represented by the total
rated thrust. Thus it is apparent normalizing the aircraft engine emissions
by thrust allows the engine to emit pollutants in proportion to its services
rendered.
To review the potential for a fuel based parameter (specifically the
El) to achieve the goals of the characteristics (a) the required steps are
outlined. The airport pollutant level may be determined from the same
initial equation as the thrust based parameter. The deviation from the thrust
based computations is in the determination of the engine emissions per
operation.
The engine emissions per operation can be determined from the Emission
Index in the following manner.
All Modes
(El) X (SFC) X (Impulse)
Mode Mode Mode
The computational method for the above relationship has all the same dis-
advantages as the thrust based parameter but adds the very significant
requirement of knowing the El and SFC values at each of the operational
modes of idle-taxi, approach, climb out, and take off. This information
is extremely difficult to obtain for a large population of aircraft and
would make airport pollutant burden assessments impossible for all but the
highly skilled analysist.
Since the El for a given engine varies with thrust it would be
necessary to either specify the limiting value as a function of thrust or
limit only the critical modes. The latter approach is understood to be the
popular choice. For present day aircraft the critical operational mode for
hydrocarbon and carbon monoxide emissions is the taxi-idle cycle. Unfor-
tunately this is not always true of engines utilizing low emission combustors.
Figures 4 and 5 illustrate how the El varies as a function of thrust for
a particular developmental emission control concept. Controlling NO at
takeoff power and HC and CO at idle only will not result in adequate control
over the operational cycle.
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Thus it is concluded that the El parameter does not comply with
requirement (a). While it may, with proper manipulation, allow the
computation of the significance of the pollution it requires an extensive
engine data file accessible only to a limited few (major government agencies
which have the cooperative support of the various engine manufacturers).
Even though the El can be used to calculate the engine emissions
(assuming other parameters are known) it does not provide a means of limit-
ing the engine emissions. Proponents of the El parameters argue that
emissions can be controlled by limiting El only, without consideration to
engine SFC. The competitive market provides the necessary incentives and
the best SFC will be realized. Unfortunately this is not considered to
be a valid arguement nor is it expected to materialize. Discussions with
a U.S. engine designer revealed that low SFC will continue to be a design
goal for cruise conditions only and without the pressures of emission
regulations they would not commit significant development funds for reducing
idle SFC. However, it is hoped that some improvements in SFC will be
realized throughout the operating range with efforts to improve cruise SFC.
The second characteristic (b) requires the emission parameters to be
a measure of the level of technology used by the manufacturers and allow
a comparison between engines. It is debatable whether either the thrust
based or fuel based parameter meets this directly. Assessing the level
of technology is a highly complex process and no one parameter does this
well for all pollutants.
The emission index is generally accepted as a parameter for evaluating
combustion emission performance. However, the El can not describe the
level of technology with a single value nor is it independent of the engine
operating cycle. For example, figures 6 and 7 illustrate how the El for
HC, CO and NO varies with engine pressure ratio. The El parameter does
have the advantage over the EPAP for comparing combustor performance
in that the effects of engine bypass ratio are eliminated.
It should be kept in mind that the level of technology relevant to
the engine emissions is that of the engine, and not the combustor alone.
For example, high pressure ratio and high bypass ratio engines are
characteristically the more advanced technology (for high speed subsonic
aircraft). Since engine emissions are dependent upon combustor technology
and engine cycle technology it is only reasonable that a parameter which
incorporates both aspects would be desired for indicating the level of
technology.
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In the short term the significance of the incorporation of the engine
operating cycle into the controlling parameters has limited value because it
is highly impractical to modify the operating cycle for an existing engine.
The importance of the cycle dependent emission parameter is to influence
the design of future engines, assuring proper consideration to environmental
needs. For the existing engines, the reduction in emissions are primarily
the responsibility of the combustor design. With the EPAP as a controlling
parameter there will result a range of sophistication for combustors meeting
the needs of a variety of engines. The responsibility for determining the
technical feasibility of reducing emissions and establishing equitable limits
belongs to the controlling organization. These limits can be established
equally well with the two contending parameters.
The third characteristic (c) requires the parameter "be measured with
enough accuracy by the best techniques and equipment available at the time
without involving unnecessary sophistication." By comparing the thrust
based parameter (e.g. EPAP) computations and the fuel based parameter (e.g.
El) computations, it will be realized that the measured parameters are
identical for the two approaches.
The EPAP is computed from the emission test data as follows:
. Pounds pollutant v _ ..Sum of pollutant mass/mode of each mode.
\000 pounds-thrust hours Sum of work output of each mode
The pollutant mass and work output per mode are obtained from:
pollutant mass/mode = pollutant emission rate X time in mode
work output of each mode = power (in 1000 pounds thrust) X time in mode
The emission rates for each mode are determined from measurements of con-
centration of the pollutants in the exhaust and a computation method which
requires the knowledge of fuel flow rates. For example the hydrocarbon
emission rate for each mode is determined from:
HC emission rate
where HC mass/mode = total mass of hydrocarbon emissions in pounds
emitted during an operational mode.
HC emission rate - pounds/hour of exhaust hydrocarbons emitted
in an operational mode.
M _ = molecular weight of methane, M^ = 16.04.
Mp = atomic weight of carbon.
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atomic weight of hydrogen, >L. = 1.008.
a = atomic hydrogen-carbon ratio of fuel.
For each operating mode
(HC) = concentration of hydrocarbons in the exhaust sample in
parts per million carbon equivalent, i.e., equivalent propane
x 3.
(CO) = concentration of carbon monoxide in the exhaust sample in
parts per million by volume.
(C0_) = concentration of CO in the exhaust sample in volume percent.
F = mass rate of fuel flow in pounds per hour.
TIM = time in mode as specified in paragraph (d) of this section,
divided by 60 to yield time in mode in hours.
Similar procedures are used for the carbon monoxide and oxide of nitrogen
emissions.
The Emission Index El is computed from the same equation as the pollutant
emission rate but the fuel flow term is transposed. For example for HC
emissions:
El =
HC rate
F
MHC
(CO)
3MH) IO4
(HC)
io4
+ (co2) -
(HC)
"io4
Thus, the measurement methods are identical for both parameters.
The fourth and final characteristic (d) is that the parameters be
expressed in International System of Units (SI units). This requirement
causes no difficulties with either of the contending parameters.
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Figure G Combustion Efficiency, CxHy Emissions and CO
Emissions Characteristics of Various General
Electric Aircraft Turbine Engines at Ground
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Figure 7 NOX Emissions Characteristics of Var-
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Engines at Take-off Power.
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3.3.5 Parameters for characterization of emission performance
3.3.5.1 The basic parameters for characterization of emission performance
should have the following characteristics:
a) express effectively the significance of the pollution emitted by
a given engine and be usable for planning purposes by providing
a reliable indication of the amount of pollutants emitted by a given
level of traffic under particular operational conditions;
b) measure the level of technology used by the manufacturers and allow
a comparison between engines;
c) be measured with enough accuracy by the best techniques and
equipment available at the time without involving unnecessary
sophistication; and
d) be expressed in International System of Units (SI Units) to the
greatest extent practicable in all calculations involving
reduction of emission data and the final reporting of test results.
3.3.5«2 The gaseous emission data could be expressed as parameters in the
following form: 9
a) concentration (parts per million by volume);
b) pollutant mass flow;
c) pollutant mass flow/fuel mass flow (emission index);
d) mass pollutant per reference LTO cycle;
e) mass pollutant per reference LTO cycle/characteristic thrust; and
f) mass pollutant per reference LTO cycle/characteristic fuel pass.
3.3.5«3 These parameters were described as follows:
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a) Concentration;- It was generally agreed that the determination of
concentration was a first step in the measurement process, but that
it is not a useful parameter to describe the emission performance
of an engine.
b) Pollutant mass flow;- This parameter is of great use in actual
emission impact calculations and is therefore required from engine
manufacturers irrespective of the unit used for certification
purposes.
c) PoUutart m&ss flow/fael mass flow;- Otherwise known as the
emission. \~^°\ (El) for the mode; this parameter is a simple measure
of emission performance especially in comparison with other emission
sources and provides information on the technological level of the
combustor design.
d) Mass pollutant per reference LTO cycle;- This gives a measure of the
total nuisance created by an engine during the reference LTO cycle.
e) Mass pollutant per reference LTO cycle/characteristic thrust;- This
parameter relates the mass pollutant emitted during the LTO cycle
to the characteristic thrust. It combines in a single number the
effects of:
l) the choice of an engine cycle; and
2) the use of a given combustor technology level.
f) Mass pollutant per reference LTO cycle/characteristic fuel mass
cycle;- This parameter is similar to the El but integrated over the
whole LTO cycle, thus reducing the index to a single figure. It
characterizes mainly the combustor technology and removes the effect
,j of specific fuel consumption.
3.3.5.1* After examining the relative merits of the above parameters, it vas
concluded that the results of engine emission tests at each specified power setting
should be reported by the manufacturer in terms of mass pollutant per unit time and
nass pollutant per unit fuel burned. These were considered essential for the
preparation of environmental impact data for diflererr,- aerodromes and for the purpose
of computation of the various parameters mentioned in >3«5-2 above. It was agreed tt
the following engine data should be supplied by the manufacturer in each case:
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Pover setting Idle Approach Climb Take-off
Thrust Rating X X X X
< •
Fuel Flow X ' X X X
Emission rate X X X X
g/sec l
Enission index X X X X
g/kg fuel
Gross Emission per
reference LTO cycle X
3.3.5.5 The Group considered that all the possible regulatory parameters (see
para 3.3.5.2) could be divided into two different categories as follows:
a) the first category which comprises the parameters c) and f) is
considered to be an indicator of the combustor cleanliness; and
b) the second category which comprises the parameters b), d) and e)
characterizes the resulting impact of the pollutant emitted.
The main characteristics and properties of these two categories of parameters are
presented and commented on hereunder.
9
3.3.5.5.1 First category - combustor cleanliness parameters
Parameters of this type have the characteristics of "emission index"
and determine the cleanliness of the combustor technology without involving the
engine specific fuel consumption (SFC). The advocates of this approach stated that:
a) by requiring all regulated engines to employ the same level of
combustor technology without reference to SFC, they avoid imposing
unnecessary constraints on the engine cycle additional to those
already acting in favour of low emissions (through reduced fuel flo
because of the requirements for reduced noise and SFC, which are
most unlikely to be relaxed in the foreseeable future: by contrast,
parameters involving SFC require assumptions to be made about the
engine cycle when determining regulatory levels for future engines
which restrict the choice of engine cycles considerably;
b) it is not in any case self-evident that additional pressures for
reduced fuel consumption will reduce total emissions impact
beneficially, because some methods of reducing SFC, while resulting
in a reduction of HC and CO emissions, will simultaneously result
in a rapid increase in NO ;
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c) vhen considering newly manufactured engines of existing types, a
parameter involving SFC has to be fixed at a level high enough to
allow for the engines of highest SFC among existing types (which
means that the other engines of lower SFC can have dirtier combustors
than they need have), or the level has to be lower, which means
allowing a number of special cases, or the prohibition of further
manufacture of the engines of higher SFC;
d) parameters of the emission index type permit the same regulatory
levels to be used for more than one engine class, which will be
particularly helpful with respect to Classes II and V. Furthermore,
it helps to demonstrate to the general public that engines of
different designs are being re.iuz.r2d to employ the same standard of
low pollution combustor technology.
3.3.5.5.2 Second category - impact of the pollutant emitted
The parameters of this type intend to meet the requirements a) and b)
of paragraph 3.3.5*1 and.reflect the total amount of a pollutant emitted by an
aircraft in the vicinity of the aerodrome. The advocates of this approach stated
that:
a) the use of parameters of this type, which are a product of the
combustor cleanliness and engine SFC, make it possible to
guarantee directly a limit io the total pollutant production.
To reach the regulatory limits all engine design factors which could
influence the level of emissions must be taken into consideration.
If only one variable, such as combustion efficiency is controlled,
the overall engine emissions would then be controlled by the
selection of other design variables such as engine by-pass ratio or
pressure ratio, which directly influence fuel consumption. _The type
of parameters which are a product of both combustion cleanliness and
engine fuel consumption represent an incentive to the engine
designer to achieve the lowest overall engine emissions levelj
•
b) the competitive market and other pressures on designers of engines
give him strong incentives to minimize fuel consumption,
particularly at engine cruise power settings. However, available
data on a number of current engines shows a very wide range of
exhaust emission levels at idle power settings. This suggests
that an emissions parameter which includes the effects of fuel •
consumption will represent a meaningful stimulus to achieve lov
emissions and low fuel consumption at idle power setting, which
has been regarded in the past by industry as an "off-design" point;
c) with a parameter of this type, the individual manufacturer within a
regulated industry is given vide flexibility in choosing the optimum '
combination of techniques to achieve the regulatory limits, when
developing^ a new engine design. By using a regulatory parameter
which specifies the results of an engine emissions reduction
programme in terms of the performance of a complete engine or vehicle,
a more defensible and equitable (industry-wide) approach is provided,
which is consistent with normal competitive processes; and
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d) to employ an expression of engine emission "behaviour,vhich is
"based only on ccmbustor technology, can "be considered misleading
"by the general public, who vill not readily understand vhy
different engines meeting the same standard and having the same
thrust have widely different total emission "behaviour.
3.3.5*5.3 In summary, an emission index parameter is independent of the engine
SFC and regulations "based on it assume that commercial competitive pressures will
lead to reduced fuel consumption with future engines. A parameter of the second
category in effect adds a further direct incentive to reducing SFC "by involving
assumptions about the cycles to be used in future engines.
3^3.5.6 With regard to snioke emission measurements, the filtration method
/expressed in terrr.3 of Srrohe 5 ,~c2r (~~J}J specified in the US EPA Regulations was
considered to be generally acceptable. However, a disadvantage of this procedure
is the excessive time required at take-off thrust. It was felt that the ICAO
procedure should not require a time exceeding five minutes at maximum thrust. The
alternative method would be one using an optical technique in which measurements
are made directly by an instrument with a read-out expressed in Photo Smoke Units
(P.S.U.) or Hartridge Smoke Units (H.S.U.) depending on the specific technique used.
Work is in progress in the BIT Sub-group to determine whether a satisfactory
correlation between these methods can be obtained. If it can, the Group considered
it desirable that both methods should be allowed.
*
3.3.6 Maximum levels
The Group examined a methodology for choosing the pollution limits
which would comply with the requirements presented in paragraph 3-1 and which would
require an equitable technological effort from all manufacturers of the different
engine types described in paragraph 3.2.1. It was agreed that the limits should be
based on a complete description of the technological characteristics controlling the
exhaust emission of the engines. These characteristics are essentially the emission
indices, examples of which are given in Appendix B. The Study Group agreed that
the NO emission index may be considered, for a given level of technology, as a
function of the combustor entry temperature. It was also agreed to investigate
further the most appropriate parameter to characterize the CO and HC emission indices.
In order to prepare the final proposal, a "Methodology Sub-group" was set up. This
Sub-group will collect and analyse the available technological information and
present its results in a form suitable for direct comparison. The Study Group agreed
that the emission indices (which will allow calculation of the regulatory levels)
should be based on experimental data and on expected improvements in future
technology consistent with reasonable economical penalties.
3.3.7 , Method of measurement and analysis
The Group reviewed the Report of the First Meeting of the Instrumentation
and Measurement Technology (IMT) Sub-group which met in October 197^• It was noted
,that the Sub-group found the methods specified by the US EPA and SAE in general to
"be acceptable but there was considerable disagreement about the validity of the
specifications in some areas which could only be resolved by further research. A
number of tasks had been allocated to members of the Sub-group, but no results vere
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21
yet available.. Members of the AEESG agreed to stimulate the efforts of Sub-group
members from their States. It vas agreed that the work of the Sub-group should result
in a self-contained statement of methods of pollutant measurement and analysis vhich
vould form a part of any ICAO recommended certification scheme. It vas further noted
that the Sub-group should take due account of the methods used by engine manufacturers
for the reduction of test data tojreference conditions.
3.3.8 Applicability of the scheme
3.3.8.1 For the reasons given in 2.2 bhe Group felt it preferable at this time
not to consider an interruitions.1 certification scheme for engines in the "obsolete"
and "in use" categories l_ categories a) and b) in S.2.2/. It vas noted that at least
one State irith j.cll. ti:>r proble-s a.z particular aeroarr ncs nas already taken national
action vnich involves the codification of "in use" engines.
3.J3.8.2 With regard to the "nevly certificated" and "newly manufactured" engines
/categories c) and d) in 3.2.2/, it vas recognized by the Group that applicability
dates for the regulations should not be the same. It vas also recognized that the
levels for the tvo categories could not be the same since ibhe benefits of advanced
technologies vhich could be incorporated in nev designs could not alvays be applied
to existing designs. The Group agreed that it vas desirable to have engine emission
requirements for both categories as soon as practicable. However, the following
applicability dates for a certification scheme vere tentatively agreed upon:
fNevly Certificated Nevly Manufactured
Engines Engines
Fuel Venting Adoption date of the Adoption date of the
Specifications by ICAO Specifications by ICAO
Smoke " 1 January 1979
Carbon Monoxide and Hydrocarbons w "
Oxides of Nitrogen " "
3.3.9 Framevork for a scheme
Based on the above considerations a framework for a scheme for exhaust
emissions certification of Class I engines vas developed and this is included in.
Appendix C to this Report.
s~
J.U Certification of engines other than Class I engines
It is considered that the general approach to the certification of "
Class I engines could be used as a basis for the development of certification
requirements for other classes of engines.
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