EPA-AA-SDSB 79-25
Technical Report
Evaluation of Aircraft Smoke Standards
for the Criterion of Invisibility
by
Richard W. Hunt
August 1979
NOTICE
Technical Reports do not necessarily represent final EPA decisions
or positions. They are intended to present technical analysis of
issues using data which are currently available. The purpose in
the release of such reports is to facilitate the exchange of
technical information and to inform the public of technical devel-
opments which may form the basis for a final EPA decision, position
or regulatory action.
Standards Development and Support Branch
Emission Control Technology Division
Office of Mobile Source Air Pollution Control
Office of Air, Noise and Radiation
U.S. Environmental Protection Agency
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I. Introduction^
In 1973, the EPA promulgated regulations for the control of
HC, CO, NOx, and smoke from aircraft engines._!_/ Then in 1978, the
EPA proposed certain amendments to those regulations.^/ Commenting
on these proposed amendments, General Motors (Allison Division)
asserted that the smoke standard for turboprop engines was unduly
stringent and cited for comparison the requirement imposed by the
DOD on military engines.V This report investigates the GM allega-
tion and compares the EPA and DOD requirements against the cri-
terion of invisibility. An adjustment to the smoke standard for
turboprop engines is proposed.
II. Summary
This brief study concludes that the military specification and
the EPA standard for turbojets and turbofans are based upon a
slightly different criterion for invisibility, but are otherwise
consistent. The EPA standard for turboprop engines, however, does
not appear to be consistent with the invisibility criterion pre-
sumed for jets and is thus inconsistent with the rest of the
regulatory package. This inconsistency is most evident for the
larger engines and consequently, a modification to the turboprop
smoke standard is proposed:
1. For rated output (RO) less than 1,000 kilowatts:
SN". 277 (RO)°-28°
(which is unchanged from that promulgated in 1973);
2. For RO greater than or equal to 1,000 kilowatts:
SN = 15.02 x log1Q [2.158 x 10~6RO]
For RO > 1,000 kW, this smoke standard is somewhat less
restrictive. A comparison is shown on Figure 1.
III. Discussion
A. Smoke Number
The smoke number, as defined in the EPA test procedure, is a
measure of the relative reflectance of a sample of exhaust particu-
late properly gathered on a filter paper. It has the limits of 0
I/ 40 CFR Pt 87, see FR _38_, N. 136, p. 19088.
21 FR 43_, N. 58, p. 12615.
3/ General Motors Response to "Proposed Standards for Control of
Air Pollution from Aircraft and Aircraft Engines, FR 43, No.
58, March 24, 1978," December 2, 1978.
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(no change from the clean filter and, therefore, presumably no
particulates gathered) up to 100 (zero reflectance on the spot,
implying the presence of a considerable amount of flat black
material). The scale is nonlinear with the mass of particulate
gathered: the initial coating of particulates will substantially
lower the reflectance (increase SN), but a considerable increase in
the particulate deposition beyond that will have a lesser effect on
SN (see Figure 2).
B. Obscuration and Invisibility
The visibility of an exhaust plume depends upon the opacity of
the plume and the contrast between the plume color and the back-
ground. The latter influence is primarily a meteorological one
over which control cannot be exercised. The former influence is
partially controllable. The opacity of the plume is due to the
linear rate at which light is absorbed or scattered by the particu-
late in the plume and the path length of the light through the
plume. The path length depends on the orientation of the observer
to the plume (over which no control can be exercised) and the basic
size (e.g., diameter) of the plume (which is dependent in turn upon
the size of the engine). It is because of the dependence of the
opacity on the engine size that the smoke standard (i.e., the
invisibility threshold) is thrust or power dependent. This is
discussed further in Part C.
The only aspect of the opacity that remains is the linear rate
of light absorption or scattering. This in turn is dependent upon
the size and. number density of the particles. Absorption is the
dominant mechanism of light attenuation when the by particle size
is much less than that of the wave length of the incident light. In
this case, smoke, particles are typically ^ 0.1 micrometers, com-
pared with the wavelight of sunlight averaging 0.55 micrometers.
For larger particles, roughly the same size as the light wave-
length, scattering would dominate. It is the absorption and the
subsequent opacity that is controllable to a degree by combustor
design. Proper design will change the size or the number density
of particles in the exhaust and hence reduce the opacity by reduc-
ing the light absorption.
Such a change will also influence the smoke number. Fewer
particles will obviously coat the filter paper less as.will much
smaller particles which are more likely to pass completely through.
It is therefore possible to correlate SN and plume size to visi-
bility. Such a curve is given in Figure 3, after Blazowski and
Henderson.4/ A visible plume represents anything less than a 95
percent transmission of light (i.e., 5 percent has been absorbed in
4/ "Aircraft Exhaust Pollution and Its Effect on the U.S. Air
Force," W.S. Blazowski and R.E. Henderson, Air Force Aero
Propulsion Laboratory, AFAPL-TR-74-64, August, 1974.
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its course through the plume). Similarly, an invisible plume
represents anything greater than a 98 percent transmission of light
(i.e., only 2 percent of the light has been reflected). The band
in between the 95 percent and 98 percent transmission bands
represents a transition wherein the visibility of the plume is
faint and is dependent largely upon the background against which
the plume is placed.
In Figure 3, the abscissa is the length over which the light
travels through the plume and although this depends upon the
orientation of the observer to the plume (looking up the tailpipe
of a departing jet will obviously aid in seeing the smoke), for
simplicity's sale, the diameter only will be considered. Even here
the aircraft configuration is relevant inasmuch as a tight grouping
of engines makes for a larger overall plume of reduced transmis-
sivity (compare, e.g., the DC-9 vs. the B737, both using JT8D
engines, but the former with twin tail engines and the latter with
twin wing engines). Again, simplicity requires the examination of
individual engines and not the aircraft configuration. Therefore,
the abscissa, shall be taken to be the plume diameter, roughly
equal to the diameter of the engine core flow at the exit plane.
Figure 3 thus shows that for a given path length of light
(plume diameter), as the smoke number (SN) decreases from 100,
eventually 95 percent or more transmissivity is reached and the
plume visibility becomes marginal; at a still lower SN (i.e.,
lower number density and/or particle size), 98% transmissivity is
reached and the plume is invisible regardless of the background.
Furthermore,. given a particular SN, e.g., 30, for very small path
lengths, the plume is invisible (transmissivity > 98 percent)
because the absorption rate at the SN is insufficient over that
small distance to absorb more than 2 percent of the light. For
longer path lengths (exhaust diameters), more and more light is
absorbed and the plume is unequivocally visible).
C. Relationship Between Exhaust Diameter (Plume Size) and
Engine Size
The disadvantage of a smoke standard based upon Figure 3 is
that by accounting for the physical geometry of the engine and not
its useful output (considered to be maximum power), the format is
incompatible with the gaseous emissions standards which are based
on the philosophy of allowing more pollution if more useful output
is performed. There is not, unfortunately, a perfect correlation
between exhaust diameter and thrust or power because the engine
cycle characteristics, especially bypass ratio, strongly influence
the size of the exhaust for a given maximum output. Nonetheless,
such a correlation can be made, especially among a group of similar
engines such as all modern high bypass turbofan engines, or all
turboprop engines. Figures 4 and 5 show such correlations and the
supporting data for them. The data are also tabulated in Appendix
A.
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There are uncertainties and ambiguities which affect confi-
dence in these correlations. First of all, the exhaust diameters
were largely obtained from available pictures, diagrams, and
published dimensions and may, therefore, be somewhat in error.
Secondly, the existence of tail cones (see Figure 6) in the exhaust
plane of some engines lead to an ambiguity, namely, whether to
take the overall diameter as the relevant dimension, or to take an
effective diameter based upon the area of the exhaust annulus. For
simplicity, and because the error incurred would not usually be
large, the outside diameter, uncorrected for an exhaust plug or
cone was taken. The third ambiguity arises from the existence of
mixed flow exhaust engines, notably the JT8D. In this case, the
fan air, which is clean, is mixed v/ith the core exhaust before the
exit plane, thus diluting the combustion products and reducing the
SN. The question is which diameter is then relevant: the overall
diameter which includes the fan air, or the effective diameter of
the core air? It might be surmised that the overall diameter is
relevant, for while the core air containing the smoke particles is
diluted (number density reduced) the effective path length is
increased to compensate. This is probably not the case, however,
as the dilution would be proportional to the square of the diameter
ratio (i.e., the area ratio), but the path length of absorption
would only be proportional to the diameter ratio to the first
power. Hence, the product of the number density times the path
'length (to give the overall attenuation) would decrease in propor-
tion to the reciprocal of , the diameter ratio. The question is
still open.
For the examination of the relationship between exhaust
diameter and power output for large turboprops, there is a paucity
of data, and in fact, for the sizes of the future "propfan" en-
gines, there is a total absence of aircraft data. Hence, it was
necessary to project a relationship utilizing anticipated cycle
configurations. The details are presented in Appendix B.
D. Visibility Criterion Selected
Figures 7 and 8 present a comparison between the EPA standards
for classes TF and TP (jets or fans and props, respectively) and
the military specification, MIL-E-8593A. The latter is given in
terms of exhaust diameter, whereas the EPA standards are given in
terms of rated output. Hence, for purposes of comparison, the EPA
standards have been converted to terms of equivalent exhaust
diameter using the correlations of Figures 4 and 5. Presented also
on Figures 7 and 8 are the 95 percent and 98 percent light trans-
missivity lines from Blazowski and Henderson. It is readily
ascertainable that the EPA standard is based upon a criterion of
about 98 percent transmissivity (total invisibility) whereas the
military specification is based upon about 97 percent transmis-
sivity (thus allowing a faint plume to be seen).
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E. Standards Evaluation
Figures 9 and 10 present the EPA smoke standards for the TF
and TP classes and compare them with the 95 percent and 98 percent
transraissivity lines converted from the exhaust diameter basis to
the thrust or power basis via Figures 4 and 5. Taking the criter-
ion for sufficient invisibility to be 98 percent transtnissivity,
one can see that the TF standard is correct. However, applying the
same criterion to the TP standard, one can see that the standard
diverts from the 98 percent line for engines of over 1,000 kW
rated power. Consequently, a new curve for the standard is
suggested and is shown as the dashed line in Figure 10. The
formula is:
RO > 1,000 kW:
SN = -15.02 Iog1() [2.1588 x 10~6 RO]
F. Smoke Number vs. Particulate Mass
The EPA smoke standard is based upon a cosmetic criterion -
invisibility of the exhaust. The standard as such does not control
the mass of the particulate exhaust as such. There is probably a
reasonable correlation (such as that shown in Figure 2) between SN
and mass emission rate that is met on the average for all combust-
ors of similar design, but such has not been investigated here.
IV. Conclusion
The EPA smoke standard for the TF class (jet engines and fans)
is based upon a 98 percent transmissivity (2 percent attenuation by
absorption). This assures invisibility under all conditions. A
small extra margin is also built into the standard to compensate
for the variability in the correlation between exhaust pipe dia-
meter and engine thrust.
The EPA smoke standard for the TP class (turboprops) is
inconsistent with the 98 percent transmissivity criterion for the
TF class. This inconsistency occurs for engines of greater than
1,000 kW power. A revision that is less stringent than that now in
effect is suggested; it is consistent with the 98 percent transmis-
sivity criterion.
The invisibility limit is not rigidly defined: 98 percent
transmissivity is considered totally invisible while 95 percent is
considered very noticeable. In between exists a region of a more
or less faint plume whose visibility is quite dependent upon the
background. The EPA has selected a conservative criterion of 98
percent which is justified by public, interest and the variability
which exists in the correlation between the exhaust plume size and
the engine output. In contrast, the military has evidently
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selected 97 percent transmissivity as its criterion, but has. based
its standard directly on the plume size which is the relevant
parameter (thus avoiding the uncertainty between plume size and
engine output). Neither the EPA nor the military criterion can be
considered "more correct" than the other, but each is based upon
policy and intent.
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Engine
JT9D-70
JT9D-7
RB211-524
RB211-22B
CF6-6
CF6-32
JT3D-7
JT8D-17
Spey 511
Spey 555
ALF5022
TFE731-3
CJ 610
All. 501
Dart
TPE331
PT6
All. 250
APPENDIX A
Engine Data
Exhaust
Diameter (M)
1.23
1.12
1.07
1.20
1.02
1.02
0.84
0.94
0.62
0.58
0.50
0.43
0.46
0.48
0.39
0.25
0.30
0.10
Thrust/Power
(KN/KW)
236
205
236
187
182
160
85
71
51
44
33
17
13
3490
1705
675
550
310
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APPENDIX B
Extrapolation of the Relationship Between
Power and Exhaust Diameter
As can be seen from Figure 6, there is a critical lack of data
from which to derive an accurate power-exhaust diameter curve;
in fact, for the larger engines that may possibly exist in the
future (propfans), there are no data at all. It is, therefore,
necessary to extrapolate the curve from the limited available
data. This is likely to lead to gross errors, which in turn would
be reflected in the prediction of the visibility limit (Figure 8).
Therefore, it is worthwhile to improve the extrapolation process by
analytical prediction. The simple analysis done here should not be
trusted to yield absolutely accurate values, but should provide
an acceptable indication of the trend. This should be adequate for
the purpose of extrapolation.
A simple thermodynamic analysis of a Brayton cycle (i.e., jet
engine cycle) is performed. The cycle is patterned after the
JT9D turbofan engine because any future engine will be based
upon similar modern technology. The relevant parameters are:
Pressure ratio 21.4
Turbine inlet temperature 1570K
Equivalence ration 0.33
Compressor efficiency 0.85
Turbine efficiency 0.90
Such an engine produces 400 kilojoules per kg exhaust available for
shaft or jet power extraction.
The actual distribution between shaft work and jet work will
influence the size of the exhaust passage. The greater the shaft
power extraction, the larger the exhaust. The optimal distribution
is governed by the flight regime of interest. As speed increases,
more jet energy must be available to provide acceptable propulsive
efficiency.
A simple approach shall be taken here in order to approximate
the trend of the exhaust diameter as a function of engine power.
Given the available energy per unit mass produced by a core, p,
(in this case 400 Kjoules/kg), the total power of the engine
is:
P = mp
where m is the air mass flow rate of the core.
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Because m is proportional to the exhaust area, and is also pro-
portional to the square of the exhaust diameter, then, for two
engines, A and B;
P ED 2
PB EDB
where ED = Exhaust diameter.
For the JT9D-70;
ED =< 1.2 m
m = 120 kg/sec
so P = mp = 48,000 kW.
Therefore, for any sized engine using the JT9D cycle,
(EDA)2
PA = V *' 48,000.
This curve is plotted in Figure 5, labeled "Propfan." The slope of
this curve, not its absolute location, is used to extrapolate the
data points which all lie below 3500 kW.
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