Progress in the development of combustor technology to meet the standards for the allowable pollutant emission levels of aircraft gas turbine engines is reported. The high-bypass-ratio turbofan engines which power the large commercial aircraft were emphasized along with efforts to reduce emission for near term applications. Recommendations for continuing research to reduce emissions to meet far term needs are given

Diehl, L. A.

Petrash, D. A.

English

NASA Technical Reports Server

GAS TURBINE ENGINE EMISSION REDUCTION TECHNOLOGY PROGRAM 
Donald A. Petrash and Larry A. Diehl 
Lewis Research Center 
SUMMARY 
'Ihe Clean Air Act of 1970 empowered the Environmental Protection Agency to 
establish standards for the allowable pollutant emission levels of aircraft gas 
turbine engines. The standards, first issued in July 1973, established allow- 
able levels for three gaseous pollutants and smoke. The gaseous pollutants 
were hydrocarbons, carbon monoxide, and the oxides of nitrogen. These emission 
standards were sufficiently stringent that combustor technology existing at the 
time was not sufficient to permit the design of the advanced combustors that 
would be needed to meet the standards. NASA therefore began a major program in 
emission reduction technology. The program consisted of in-house experimental 
research on low-emission advanced combustor concepts and a contracted research 
program with the major aircraft engine manufacturers. The purpose of this pre- 
sentation is to review the results of the contracted research program with em- 
phasis on the high-bypass-ratio turbofan engines which power the large corraner- 
cial aircraft. 
INTRODUCTION 
The Clean Air Act of 1970 empowered the Environmental Protection Agency to 
establish standards for the allowable emission levels of aircraft gas turbine 
engines. The standards were first issued in July 1973. Earlier, in mid-1971, 
NASA began a major program in emission reduction technology, which would consist 
of a continuing in-house effort on low-emission combustor concepts and of con- 
tracted research programs with the major aircraft engine manufacturers. This 
paper gives an overview of the contracted emission reduction technology pro- 
gra-, which were begun with two firm objectives. 
First, it was essential to investigate new combustor concepts that had the 
potential to significantly lower the emission levels. Considerable research 
with existing combustors had already shown that present concepts would not meet 
all of the EPA standards. The new concepts would have to be developed not only 
from an emissions standpoint but also from a conventional performance goals 
standpoint. Second, it was necessary to measure the combustor emissions in an 
engine test. The engine test would show whether the combustor concept could be 
installed in an engine and meet the engine operating requirements while produc- 
ing the desired low emissions. Engine testing was also required to achieve the 
needed pressure levels and to avoid extrapolation of emission levels from lower 
pressure tests. And finally, engine testing would reveal those areas of the 
combustor that needed further development. 
205 
https://ntrs.nasa.gov/search.jsp?R=19780019115 2020-03-20T13:36:48+00:00Z
Multiphase contracts were awarded to the engine manufacturers. These pha- 
ses consisted of screening, refining, and engine testing. In the first phase 
many combustor concepts would be screened to determine those having the most 
potential for low emissions. The best concepts would be further developed dur- 
ing the refinement phase, where combustor performance and emission reduction 
would be emphasized. Finally the best, or most engine ready, combustor would 
be installed and tested in an engine. 
PRCGRAMPLAN 
As conceived, the emission reduction technology program would develop 
technology for representative engines in each of the EPA engine classes. With 
the exception of the T4 class, which consists solely of the JT8D family of en- 
gines, competitive contracts were awarded in each class. Table I shows the EPA 
classes, the engines, and the manufacturers that participated in the program. 
The Tl class consists of engines withthrustsless than 36 kN (8000 lb). The T2 
class consists of engines withthrustsgreater than 36 kN (8000 lb), and the P2 
class consis,ts of turboprop engines. Engines in the remaining two EPA gas tur- 
bine engine classes, T3 and T5, were not studied as a part of this program. 
The T3 class consists solely of the JT3D family of engines, and the T5 class 
consists of engines for supersonic aircraft, at present only the Olympus engine 
in the Concorde SST. 
The goal of these programs was to meet the 1979 EPA Aircraft Engine Emis- 
sion Standards. Table II shows the 1979 standards for the three gaseous pollu- 
tants and smoke for each of the engines in the program. The EPA standards are 
expressed in EPA parameter values for the specified landing-takeoff cycle. The 
production engine values are given as a percentage of the EPA standard values; 
In general, the production values exceed the standards by several hundred per- 
cent. Therefore, to meet the EPA standards, combustor technology had to be de- 
veloped with the potential for significantly lower emission levels. Noteworthy 
are a few instances where the standards were already achieved - the oxides of 
nitrogen (NO,) level for the P2 class and the smoke for the T2 class. 
CHARACTERISTICS OF ENGINE EMISSIONS 
Emission characteristics common to all engine classes are shown in fig- 
ure 1. This figure is a plot of typical production engine emissions as a func- 
tion of takeoff thrust level. The landing-takeoff cycle points are identified 
on the abscissa with their associated thrust levels. The ordinate values were 
obtained by summing the species emission index values over the landing-takeoff 
cycle and are shown as the percentage contribution at each cycle point. Emis- 
sions from all engine classes conform very well to this trend. 
Virtually all hydrocarbon and carbon monoxide (CO) emissions are generated 
at low power , primarily at engine idle. These emissions are significantly re- 
duced at approach power levels and virtually disappear at high power levels. 
Typical production aircraft engines have combustion inefficiencies at idle of 
4 to 12 percent. This accounts for 'the high hydrocarbon and CO emissions at 
206 
idle. To reduce these emissions, combustor research must be directed toward 
increasing combustion efficient at idle. In practice, relatively large reduc- 
tions in hydrocarbon and CO emissions have been achieved with relatively minor 
combustor modifications. 
Oxides of nitrogen emissions, on the other hand, are at a minimum at en- 
gine idle and increase as engine power increases. To reduce NO, emissions, 
combustor research must be directed toward the high power operating conditions. 
In general, NO, reduction requires the lowering of the flame temperature and 
reduction of the residence time of gases at high temperatures. In practice, 
significant reduction in NO, emissions require relatively major combustor modi- 
fications which are difficult to implement. 
EARLY EMISSIONS REDUCTION RESEARCH 
Before the contracted emission reduction technology program had even been 
planned, advanced combustor research at Lewis had identified several approaches 
to obtain low emissions. The research had indicated two promosing multiple- 
burning-zone combustors - specifically, the double-annular and swirl-can modu- 
lar combustors. Both air-assist and air-blast fuel-injection techniques were 
studied to evaluate their potential for reducing emissions. Controlled combus- 
tion was also studied by varying the fuel and air schedules to the advanced 
combustors. 
Some of the results obtained in 1972 tests are shown in figure 2, which 
shows the NO, emissions for conventional combustors and the two advanced com- 
bustor concepts tested at Lewis. Note that the NO, emissions are a strong 
function of combustor inlet-air temperature. This is because the inlet-air 
temperature is directly related to the combustion flame temperature and NO, 
formation. This figure also indicates that NO, control will be more difficult 
with high-pressure-ratio engines since an increase in engine compressor pres- 
sure ratio results in an associated increase in combustor inlet-air temperature. 
The advanced combustors did demonstrate significant reductions, and these re- 
sults showed that substantial NO, emission reductions were possible. 
Improvements in fuel atomization may have a dramatic effect on pollutant 
emission levels. During engine idle, the fuel is sprayed from the duplex noz- 
zle through the small-flow primary nozzle. This results in a spray that is 
generally coarse, consisting of large, sometimes poorly distributed drops. 
This combination of large drops and poor distribution results in high levels of 
hydrocarbon and CO emissions. An air-assist nozzle uses a small amount of air 
bled from the engine compressor and injects it through the unused secondary 
fuel nozzle. This air-assist reduces droplet size, improves the uniformity of 
the spray, and thereby reduces engine idle emissions. Typical results obtained 
from the application of this technique are shown in figure 3. The tests were 
conducted on a JT8D combustor and a production fuel nozzle. Emissions of hy- 
drocarbons and CO are given on the ordinate and the injected air differential 
pressure on the abscissa. The use of air-assist considerably lowers the emis- 
sions: hydrocarbons were decreased by a factor of 8 and CO by nearly a factor 
of 4. The amount of air injected was quite small, being less than 0.5 percent 
of the combustor airflow at the maximum differential pressure. 
207 
RESULTS AND DISCUSSION 
Because of the time required for a detailed discussion of all the pro- 
grams and because of the thrust of the present conference, the T2 class engine 
will be emphasized in this discussion of the emission reduction technology 
program. The two engines studied in this part of the program were the Pratt & 
Whitney JT9D-7 and the General Electric (X6-50, both high-bypass-ratio turbofan 
engines which power current large aircraft. 
The Pratt & Whitney JT9D-7 (fig. 4) engine has a maximum thrust of 205 kN 
(46 150 lb), a pressure ratio of 22:1, and an annular combustor. The production 
engine emission values (also shown in the figure) indicate that all emissions 
with the exception of smoke require large reductions. Figure 5 shows the pro- 
duction JT9D-7 combustor and the advanced low-emissions combustor that was used 
in the engine tests. The advanced-technology, Vorbix combustor is an axially 
staged design. The pilot zone has been optimized to reduce hydrocarbon and CO 
emissions at engine idle. And the main zone was optimized to reduce high-power 
NOx emissions. In all, the pollution reduction concepts used to design the 
Vorbix combustor included multiple burning zones, air-blast fuel injectors, en- 
hanced mixing, and fuel staging. Clearly, this combustor modification is more 
than minor and will require further development before it can be put into ser- 
vice. 
The data obtained from the full-scale engine test of the Vorbix combustor 
are shown in figure 6. The CO, hydrocarbons, and NO, emissions were 74, 25, 
and 90 percent of the EPA standard values, respectively; smoke levels exceeded 
the EPA standard value. The high smoke level appears to be result of fuel- 
rich zones at the main combustor inlet. It is felt that smoke levels can be 
reduced to acceptable levels. 
The General Electric CF6-50 ,(fig. 7) engine has a maximum thrust of 224 kN 
(50 000 lb), a pressure ratio of 30:1, and an annular combustor. The produc- 
tion-engine emission levels shown in the figure also indicate that large reduc- 
tions in all gaseous emissions are required to meet the emission standards. 
Figure 8 shows the production CF6-50 combustor and the advanced low-pollutant 
combustor used in the engine tests. This advanced-technology, double-annular 
combustor is a radially staged design. Again, the pilot stage was optimized to 
reduce hydrocarbon and CO emissions at engine idle, and the main zone was opti- 
mized to reduce the high-power NO, emissions. As with the Vorbix, the double- 
annular combustor used the low-pollutant concepts of multizone burning, air- 
blast fuel injectors, enhanced mixing, and fuel staging. 
The data that were obtained from the full-scale engine test of the double- 
annular combustor are shown in figure 9. Also shown are the best values ob- 
tained during the combustor refinement phase of the program aonducted in the 
combustion test facility. Preliminary analysis of the engine-test data yields 
values for CO, hydrocarbons, and NO, of 147, 38, and 187 percent of the stand- 
ard value. The smoke level is also considerably above the EPA standard value. 
These disappointing results had not been anticipated. As can be seen, rig test 
values for CO and hydrocarbons were below the EPA standard values. The com- 
bustor tested in the engine had been substantially altered from the version 
tested in the previous phase. Most of the modifications involved ."upgrading" 
208 
the combustor to an "engine ready" status. Additional rig testing, conducted 
in an attempt to restore the lost emissions performance, was only partially 
successful. However, the results of the earlier phase of the program encourage 
our belief that engine emission levels of CO, unburned hydrocarbons, and smoke 
can be reduced to acceptable levels. 
Table III is a summary of all the emission results obtained in the pro- 
gram, with the engines ordered by increasing engine pressure-ratio. The un- 
burned hydrocarbon standards were achieved for all engines. Carbon monoxide 
standards were essentially achieved for all but the JT8D and the (X6-50. The 
difficulty of achieving the NO, standard increased directly with increasing 
engine pressure-ratio. The 501-D22A engine, with a 9.7:1 pressure ratio, 
easily met the NO, standard with only a minor combustor modification. The 
TFE-731 engine, with a 13:l pressure-ratio, barely met the NO, standard and 
required a relatively major combustor modification. The JT8D, which has a 17:l 
engine pressure-ratio, did not achieve the standard, in large part because of 
the high specific fuel consumption of the engine. For the T2 class engines, 
major modifications to the combustor of the JT9D, with an engine pressure-ratio 
of 22:1, did result in achievement of the NO, standard, but the (X6-50 with an 
engine pressure ratio of 3O:l did not meet the standards. 
The results presented have been compared with the 1970 EPA standards. 
More stringent gaseous emissions standards will apply to newly certified air- 
craft gas turbine engines in 1981. Table IV shows the levels achieved in terms 
of the 1981 standards for the advanced technology combustors tested in the 
JY9D-7 and CF6-50 engines. Although such a comparison is not completely valid, 
it is the EPA's intent that a newly certified engine be designed from the be- 
ginning with emissions control in mind and that design aspects such as pressure 
ratio, bypass ratio, allowable combustor volumes, and pressure drop and their 
influence on engine emission levels be considered. Such was not the case with 
the engines cited. The comparison does indicate where additional emission re- 
duction technology development is required. Although emission control of un- 
burned hydrocarbons appears well in hand, the same cannot be said of carbon 
monoxide. While further development of present technology may bring more CO 
reductions, it is not clear if it will be sufficient to satisfy all require- 
ments. It is clear that new technology may be necessary if high-pressure- 
ratio engines are to achieve the required NO, levels. 
CONCLUDING REMARKS 
The emission reduction technology program discussed in this report repre- 
sents NASA's most recent efforts to reduce emissions for near-term applications. 
Continuing work is addressed to the development of emission reduction concepts 
that will be required to meet far-term needs. In particular, additional re- 
search is needed to further reduce emissions of carbon monoxide and oxides of 
nitrogen. Fundamental technology programs now underway have indicated that 
further reductions by as much as an order of magnitude may be possible. The 
extent to which this fundamental technology can be converted to practical en- 
gine hardware is yet unknown and will require several more years of research 
by NASA and the engine manufacturers. 
209 
TABLE I. - SCOPE OF EMISSIONS REDUCTION TECHNOLOGY 
PROGRAM 
i EPA engine class Engine Manufacturer 
Tl - turbofans 
T2 - turbofans 
T4 - JT8D engines 
P2 - turboprops 
TFE-731-2 Garrett AiResearch 
CF6-50 General Electric 
JTgD-7 Pratt & Whitney 
JT8D-17 Pratt & Whitney 
501-D22A Detroit Diesel Allison 
Engine Engine 
class 
TABLE II. - EMISSION GOALS FOR 1979 EPA STANDARDS 
T 
Stan- 
dard 
4.9 
1.6 
.8 
.8 
.8 
THC 
I 
Produc- Stan- Produc- 
tion dard tion 
value, value, 
% of % of 
standard standard 
306 26.8 118 
331 9.4 180 
500 4.3 356 
488 4.3 198 
538 4.3 251 
T 
co Smoke 
Stan- 
dard 
12.9 
Produc- 
tion 
value, 
% of 
standard 
48 29 
Stan- 
dard 
Produc- 
tion 
value, ! % of standard 189 
3.7 162 40 118 
3.0 260 25 120 
3.0 197 20 50 
3.0 257 19 68 
210 
TABLE III. - POLLUTION SUMMARY FOR ALL ENGINE CLASSES 
EPA Engine Engine Modification THC co 
N”X 
Smoke 
class pressure required 
ratio Percent of 1979 EPA standard 
P2 501-D22A 9.7 Minor 6 17 57 59 
Tl TFE-731-2 13 Major 25 107 100 --- 
T4 JT8D-17 17 25 207 146 108 
T2 JT9D-7 22 25 74 90 150 
T2 CF6-50 30 38 77-147 147-187 132 
. 
TABLE IV. - POLLUTION SUMMARY FOR 
T2 CLASS ENGINES 
Engine THC co 
N”X 
211 
100 r 
% OF POLLUTANT 
CONTRIBUTION 
IDLE APPROACH CLIMB T/O 
5 30 a5 100 
% OF TAKEOFF THRUST 
Figure l.- Typical engine emission characteristics. 
T-- 
TYPE OF 
COMBUSTOR 
EMISSION 
INDEX, 
g NO, ___- 
kg FUEL 
INLET-AIR TEMP., K 
Figure 2.- Oxides of nitrogen emissions as function of combustor-inlet 
temperature. Combustor pressure, 6 atm. 
212 
EMISSION INDEX, 
g/kg FUEL 
Figure 3.- Effect of improving fuel atomization on gaseous emissions. 
PRODUCTION ENGINE EMISSIONS, 
% OF 1979 EPA STD.: 
. IiS! Ifll; NO, SMOKE 
198 488 197 56 
I. 'igure 4.- 1 .gure EPA class T2 jet aircraft engine JT9D-7. Thrust, 
pressure, 22:l; type of combustor, annul ar. .ar. 
205 kN; 
213 
ENGINE CONVENTIONAL 
COMBUSTOR 
VORBIX COMBUSTOR 
‘FUEL--‘/ 
Figure 5.- Low-emission, staged combustor for the JT9D-7 engine. 
36 OF 1979 
EPA STD. 
140 
l- 
120 
100 
I- 
---------------------. 
60 
40 t 
80 
t- 
20 
t 
co MC NOx SMOKE 
o- 
Figure 6.- Emissions results from Vorbix combustor tests in JT9D-7 engine. 
214 
PRODUCTION ENGINE Ml,SSIONS, 
:, 
% OF 1979 EPA STD.: ‘, _j’,; 
Q & MIX SMOKE 
251’ 538 257 68 .-- ---. - ._. .-. _-..-. 
Figure 7.- EPA class T2 jet aircraft engine CF6-50. Thrust,'224 k~; 
pressure ratio, 3O:l; type of combustor, annular. 
ENGINE CONVENTIONAL 
COMBUSTOR 
DOUBLE-ANNULAR 
COMB USTOR 
Figure 8.- Low-emission, staged combustor for the CF6-50 engine. 
215 
200 - ,:. 6. ‘0 
160 - .!,m ‘. 
12(-J - :,.; : :‘i: 
To OF 1979 1;; 
2; , ------------a ---a- .<. .$; 
EPA STD. 
i< +:; iti. :, 
40 - 
.:a :g ..y ..a. :r&- 1. 
O- 
f?.. 
co THC No, SMOKE 
Figure 9.- Emission results from double-annular combustor tests. 
Full-scale tests conducted in CF6-50 engine. 
216 


Gas turbine engine emission reduction technology program

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