Vladimir Balepin and Chris Ossello

MSE Technology Applications, Inc., Butte, Montana

Chris Snyder

Glenn Research Center, Cleveland, Ohio

NOx Emission Reduction in Commercial Jets

Through Water Injection

NASA/TM—2002-211978

October 2002

AIAA–2002–3623
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Vladimir Balepin and Chris Ossello

MSE Technology Applications, Inc., Butte, Montana

Chris Snyder

Glenn Research Center, Cleveland, Ohio

NOx Emission Reduction in Commercial Jets

Through Water Injection

NASA/TM—2002-211978

October 2002

National Aeronautics and

Space Administration

Glenn Research Center

Prepared for the

38th Joint Propulsion Conference and Exhibit

cosponsored by the AIAA, ASME, SAE, and ASEE

Indianapolis, Indiana, July 7–10, 2002

AIAA–2002–3623
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NASA/TMó2002-211978 1 

NOX EMISSION REDUCTION IN COMMERCIAL JETS THROUGH WATER INJECTION 

Vladimir Balepin*

 and Chris OsselloÜ 

MSE Technology Applications, Inc. 

Butte, Montana 59702 

Chris Snyderá

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Abstract

This paper discusses a method of the nitrogen oxides 

(NOx) emission reduction through the injection of water 

in commercial turbofan engines during the takeoff and 

climbout cycles. In addition to emission reduction, this 

method can significantly reduce turbine temperature 

during the most demanding operational modes (takeoff 

and climbout) and increase engine reliability and life. 

Introduction

The National Aeronautics and Space Administration 

(NASA) has set aggressive goals for reducing 

emissions of future aircraft by a factor of three within 

10 years and by a factor of five within 20 years. Thus, 

the Ultra-Efficient Engine Technology (UEET) 

Program addresses local air quality concerns by 

developing technologies to reduce both takeoff and 

landing NOx emissions by 70% from the 1996 

International Civil Aviation Organization (ICAO) 

standards. Advanced concepts research for reducing 

carbon dioxide and NOx, as well as analytical/ 

experimental research in control (through component 

design, control, and/or fuel additives) of aircraft 

exhaust emissions is sought. 

In support of this effort, the authors have investigated 

using water injection-at-takeoff (WIT) as a method of 

reducing NOx emissions from commercial turbofan 

engines. This method employs combustor temperature 

reduction through water injection in the inlet of the 

low-pressure compressor (LPC) or high-pressure 

compressor (HPC) and mixture ratio control to the level 

where a substantially lower amount of NOx is generated 

and emitted. Water injection substantially reduces the 

required compressor work and significantly improves 

reliability. Potential applications for the WIT method 

include virtually any commercial jet currently in service 

* Staff Scientist, AIAA Senior Member 

Ü

 Analytical Engineer, AIAA Member 

á

 Aerospace Engineer 

or projected in the future. The WIT method can also be 

applied to ground applications. A similar method has 

proven to be the lowest cost technology for reducing 

NOx emissions by 30% and increasing power output by 

15% to 30% in the gas turbine power generation 

industry. MSE Technology Applications, Inc. (MSE) 

and the State of Montana are currently considering the 

use of this technology in power generation and natural 

gas distribution facilities in Montana. 

This innovative idea will provide commercial aviation 

with a low cost, low risk technology to reduce NOx

production and extend the cycle/service life of 

commercial aircraft engines. 

WIT Concept

Water injection is a relatively simple and 

straightforward technique for improving engine 

performance. It has been extensively studied and 

applied as a means to increase thrust, maximum 

altitude, and/or flight speed of turbine engines.1ñ3

The WIT concept has the potential to reduce NOx

production by 50% to 70% and also extend the service 

life in current technology turbofan engines. In the WIT 

concept, the combustor temperature is reduced by water 

injection at the inlet of the LPC or HPC and by control 

of the fuel mixture ratio, which substantially reduces 

NOx production. Water injection significantly reduces 

air temperature of the compressor inlet air and 

combustion products, resulting in one or a combination 

of the following advantages: 

ñ increased compressor pressure ratio; 

ñ increased air flow through the engine; 

ñ decreased turbine expansion ratio; 

ñ decreased turbine gas temperature; and 

ñ decreased turbine cooling bleed flow. 

In addition, water injection supplements engine thrust at 

the reduced turbine temperatures through increased 

mass flow. 
NASA/TMó2002-211978 2 

The NASA Engine Performance Program (NEPP) was 

used for preliminary analyses of the high bypass 

turbofan engine, which is referred to hereafter as the 

ìcurrent technology baseline engine.î4

 Engine 

performance and the NOx emission index are similar to 

the General Electric GE90 and Pratt & Whitney 

PW4084 engines, both of which are employed on the 

Boeing 777 commercial jet. 

Five different cases of water injection were examined. 

Water was injected in three different areas of the 

turbofan engine, including the LPC, HPC, and 

combustor. The water injection model used in this 

study assumed that the water was totally evaporated 

before entering these components. These cases used 

the same bleed air flow as the baseline engine. Two 

additional cases with water injection in front of the LPC 

and HPC were examined at reduced bleed air flow. 

Reduced air bleed was assumed to be possible due to 

significant turbine temperature reduction. Each case 

consisted of optimizing the fuel mixture ratio to 

maintain the desired combustion temperature drop 

(up to 300 K) and injecting water to maintain 85,000 

pounds of thrust at takeoff. 

Figure 1 is a schematic of the turbofan engine and the 

examined water injection points. 

Results are summarized in Figures 2 through 4 and in 

Table 1. Figure 2 shows water fraction as a percentage 

of the core air flow versus NOx emission reduction for 

all five examined cases. It can be seen that injection in 

the LPC with reduced bleed provides the most 

economical means of NOx reduction in terms of the 

required amount of water. The worst case corresponds 

to water injection in the combustor, as it requires 5 to 6 

times more water for the same NOx reduction effect 

compared to the LPC with a reduced bleed case. Other 

cases are closer to the best case, meaning that some 

flexibility exists in injection point selection. 

Table 1. Engine performance corresponding to 50% 

NOx reduction. 

Fuel saving, % of total fuel 0.046 

Water fraction, % of total fuel 0.42 

Water fraction, % of GTOW 0.19 

High-pressure turbine inlet temperature drop, K 233 

Low-pressure turbine inlet temperature drop, K 127 

LPC exit temperature drop, K 27 

HPC exit temperature drop, K 43 

High-pressure rpm ratio 0.966 

Low-pressure rpm ratio 0.998 

LPC

HPC combustor

Figure 1. Turbofan engine and examined water 

injection points. 

0

2

4

6

0 20 40 60

1

Water Fraction in the Core Airflow, %

NOx Reduction, %

2

3

4

5

Figure 2. Water fraction in the core air flow versus 

NOx reduction when water is injected at the inlet of: 

1) LPC, reduced cooling air bleed; 2) HPC, reduced 

cooling air bleed; 3) LPC, normal cooling air bleed; 

4) HPC, normal cooling air bleed; and 5) combustor. 

-300

-200

-100

0

0.95

1

1.05

1.1

0 10 20 30 40 50 60 70

Temperature Drop, K

Relative Rotational

Speed

NOx Reduction, %

THPT

TLPT

THPC

TLPC

nLP

nHP

Figure 3. Change of temperatures and rotor speed 

versus NOx reduction when water is injected at the 

inlet of the LPC with reduced cooling air bleed.
NASA/TMó2002-211978 3 

0

0.1

0.2

0.3

0.4

0.5

0.6

0 10 20 30 40 50 60 70

Additional Mass Fraction, %

NOx Reduction, %

Figure 4. LPC additional water mass as a fraction 

of total fuel mass versus NOx reduction. 

It should be noted from Case 1 of Figure 2 that the 

amount of water injected (1% to 1.6% of the core air 

flow) is comparable with natural air humidity levels. 

Figure 3 displays the drop of key temperatures and 

rotor speeds versus NOx reduction at constant engine 

thrust. The most important high-pressure turbine 

temperature, THPT, can be reduced by as much as 300 K. 

The maximum obtained temperature drop behind HPC 

THPC is as much as 60 K. Low-pressure turbine speed is 

only slightly lower; however, high-pressure turbine 

speed is nearly 5% lower than that of the baseline 

engine. 

In terms of specific impulse (Isp) [inverse thrust specific 

fuel consumption (TSFC)] to reduce NOx by 1%, 1.5% 

of the Isp should be sacrificed in the case of water 

injection in the combustor and only 0.6% in the case of 

water injection in front of the LPC at reduced bleed. 

Specific impulse includes jet fuel and water. 

The sacrifice of Isp for NOx reduction is a disadvantage; 

however, the reduction in NOx emissions is a critical 

issue only during the two shortest modes of the four 

modes of the landing and takeoff cycle (LTO) and not 

at all during the remainder of a typical flight profile. 

Then, the time in the takeoff mode would be used to 

estimate the total NOx emissions. The Federal Aviation 

Administration (FAA) breaks down an LTO cycle into 

four modes: 

1) Takeoff ñ full-throttle operation until the aircraft 

reaches 500 to 1,000 feet altitude (power setting 

100%, time 0.7 min); 

2) Climbout ñ time following takeoff that ends when 

the aircraft leaves the mixing zone (power setting 

85%, time 2.2 min); 

3) Approach ñ time the aircraft enters the mixing zone 

when it lands (power setting 30%, time 4 min); and 

4) Idle ñ time the aircraft is taxiing before takeoff and 

after landing (power setting 7%, time 26 min). 

The above numbers, which correspond to turbofan 

engines of the GE90 or PW4084 type, are taken from 

the FAA Engine Emission Database (FAEED), Version 

2.1, 1995. 

Water injection would only be needed until the aircraft 

reaches the mixing zone height, which encompasses 

two of the four modes, takeoff and climbout. The 

mixing zone height is the height at which the aircraft 

emissions can have an effect on the ground-level 

pollutants at the airport being considered. The 

maximum height is variable and is dependent on the 

environmental conditions, location, and time of year. 

In this study, the FAEED was used to evaluate the LTO 

of a Boeing 777-200 aircraft equipped with two 

PW4084 turbofan engines. The default mixing height 

in this program is 3,000 feet. A gross takeoff weight 

(GTOW) of 545,000 pounds and a fuel capacity of 

31,600 gallons were assumed. The resulting FAEED 

Boeing 777-200 analysis estimated the time in the 

takeoff and climbout modes to be 0.7 and 2.2 minutes, 

respectively. The above data (GTOW and fuel 

capacity) were combined with the results from the 

NEPP WIT system analysis (LPC water injection with 

reduced bleed flow), which resulted in the summary of 

the engine performance and system characteristics. 

Several Important Observations

Data for the WIT method with 50% NOx reduction are 

given in Table 1. The following observations can be 

drawn from Table 1: 

1. The total amount of water required to reduce NOx

emissions using the WIT method is insignificant 

(0.42% of total fuel storage on a mass basis). 

Additionally, little fuel saving (in the amount of 

one-tenth of required water) can be obtained. 

2. Several important temperatures defining engine 

durability are significantly reduced compared to 

the baseline engine. This observation shows the 

opportunity to significantly reduce NOx emissions 

along with engine durability extension through 

strength increase and stress reduction due to the 

lower temperatures in all the engine components 

and the lower high-pressure shaft speed. 

3. The turbine temperature in the ìcurrent technology 

baseline engineî (similar to the GE90 or PW4084) 

drops from 1,715 K during takeoff operation to 
NASA/TMó2002-211978 4 

 1,482 K during the cruise segment of the flight. 

If the WIT technology were applied, the turbine 

temperature could be kept constant or nearly 

constant (on the order of 1,482 K) during both the 

take off and cruise modes. 

4. The reduced temperature and reduced shaft speed 

(See Figure 3 and Table 1) will also translate into a 

smaller thermal and mechanical expansion of the 

rotor blades during the takeoff cycle; therefore, the 

possibility exists to reduce the turbine tip clearance 

and increase the turbine efficiency during the 

cruise mode. 

 

Figure 4 shows water mass fraction (additional water 

mass as a fraction of total fuel mass) versus NOx

reduction with the assumption that all the water is 

evaporated before entering the LPC. From this figure, 

it can be clearly seen that increased water injection 

further reduces NOx emissions. However, large 

amounts of liquid water entering the compressor are not 

desirable since this may cool and shrink the compressor 

case and promote rotor blade tip rub and blade erosion. 

Hardware demonstration of the WIT technology is 

required to identify the injection limits. 

The WIT method can also provide the aircraft with the 

ability to take advantage of mass-augmented thrust 

through water injection as described in References 1ñ3. 

This could give the aircraft the ability to achieve a 

cruise altitude in less time or to increase the GTOW; 

however, the NOx reduction would be sacrificed in 

these situations. The possibility to taking off 200 K to 

300 K of high-pressure turbine inlet temperature 

and 40 K to 60 K of HPC exit temperature during 

the most demanding mode (i.e., takeoff) can change 

commercial jet design philosophy and increase 

engine durability.

Supersonic transport aircraft could also benefit from the 

WIT method. This type of aircraft could use the WIT 

method to not only reduce NOx emissions during the 

takeoff and climbout modes but also to increase 

acceleration through the transonic region with massaugmented thrust. Supersonic transport designers 

would also have the ability to size the aircraft 

propulsion system more for the cruise mode than for the 

transonic mode of operation, which would make the 

aircraft more economical to operate. 

It was also found in this study that the jet velocity is 

reduced when injecting water and could help noise 

levels in the surrounding area of the airport. Further 

investigation of the possible noise reduction with the 

WIT method is required. 

Technology Issues

The WIT method of NOx reduction would add a small 

amount of complexity to the existing system, since 

storage space would be required for the water. Airports 

would need to have a supply of demineralized water to 

replenish aircraft water supplies; therefore, a cost 

analysis is required to define the economical merits of 

the WIT technology. 

Technology Benefits

A summary of the unique benefits of the WIT method 

include: 

ñ NOx emissions reduced by 50% to 70%. To reduce 

LTO NOx emissions of the ìcurrent technology 

baseline engineî by 50%, a small amount of water 

(0.4% of the total fuel for the aircraft of the Boeing 

777 type) is required; 

ñ the WIT method could be combined with other 

methods of NOx reduction; 

ñ a minimum amount of water is required for the same 

emission reduction effect when water is injected in 

front of the LPC rather than the HPC or combustor; 

ñ a flexible thrust profile including thrust increase 

during the takeoff and climbout modes; 

ñ increased fuel efficiency; 

ñ increased engine reliability due to maximum turbine 

temperature reduction (by 200 K to 300 K) during 

demanding takeoff operations (maximum 

compressor temperature can be reduced by 40 K to 

60 K); and 

ñ the WIT is a promising technology for economical 

transonic operation for supersonic jets. 

References

1) Henneberry, H. and C. Snyder, Analyses of Gas 

Turbine Engines Using Water and Oxygen 

Injection to Achieve High Mach Numbers and High 

Thrust, NASA/TMó1993-106270, NASA Glenn 

Research Center, 1993. 

2) Walker, J.; S. Thomas; and R. Sorge, 

Demonstration of J85-21 Turbojet Operation at 

Mach 3 Using Pre-Compressor Water Injection, 

NASA/TMó1999-209397, NASA Glenn Research 

Center, 1999. 

3) Balepin, V. and G. Liston, The SteamJetTM: Mach 

6+ Turbine Engine with Inlet Air Conditioning, 

AIAAñ2001ñ3238, 2001. 

4) Klann, J.L. and C.A. Snyder, NEPP Programmers 

Manual (NASA Engine Performance Program) 

Volume: Technical Description, NASA Technical 

Memorandum 106575, NASA Glenn Research 

Center, September 1994.
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10

NOx Emission Reduction in Commercial Jets Through Water Injection

Vladimir Balepin, Chris Ossello, and Chris Snyder

Gas turbines; Thrust augmentation; Water injection; Nitrous oxides

Unclassified -Unlimited

Subject Category: 07 Distribution: Nonstandard

Prepared for the 38th Joint Propulsion Conference and Exhibit cosponsored by the AIAA, ASME, SAE, and ASEE,

Indianapolis, Indiana, July 7–10, 2002. Vladimir Balepin and Chris Ossello, MSE Technology Applications, Inc., Butte,

Montana; and Chris Snyder, NASA Glenn Research Center. Responsible person, Chris Snyder, organization code 2400,

216–977–7018.

This paper discusses a method of the nitrogen oxides (NOx) emission reduction through the injection of water in

commercial turbofan engines during the takeoff and climbout cycles. In addition to emission reduction, this method can

significantly reduce turbine temperature during the most demanding operational modes (takeoff and climbout) and increase

engine reliability and life.