AA241B: Aircraft Emissions Nicolas Antoine Stanford University nantoine@stanford.edu.

AA241B: Aircraft Emissions

Nicolas AntoineStanford University

nantoine@stanford.edu

Introduction

• Commercial aircraft generate 13% of CO2 emissions by transportation sources.

• Aircraft engine emissions, just like noise, are regulated by the FAA.• Only “local emissions” (around airports) are regulated.• Low-emissions combustion is key area in engine design.

CnHm + S + N2 + O2 → CO2 + H2O + N2 + O2

+ NOx + CO + SOx + Soot + UHC

Jet Fuel Combustion

Fuel Air

Products of ideal combustion

Products of non-ideal

combustionNOx: Affects ozone (O3) concentrationCO2: Absorbs outgoing infrared radiationCO: ToxicSoot: Visible

• Only Soot, UHC, CO, and NOx are regulated.• The ICAO/FAA regulations are based on a simulated landing-

takeoff (LTO) cycle:

– Takeoff 0.7 minutes 100% Thrust– Climb 2.2 85%– Approach 4.0 30%– Taxi/Idle 26.0 7%

Emissions = EI (g emissions/kg fuel) * Fuel Flow (kg/s)* Time in Mode (s)

Note: EI is the Emissions Index

Maximum allowable values depend on thrust, overall pressure ratio (OPR)See ICAO Environmental Protection Annex 16

Emissions Regulations

Cruise Emissions

• No regulations yet, but expected in the near future (legal aspects of international airspace are difficult to resolve).

• Subsonic aircraft operate in the tropopause, a particularly vulnerable region of the atmosphere.

Contrails

• Formed as water from engine exhaust freezes.

• Contrails promote cirrus formation• May hamper atmosphere

heating/cooling schedule

Reducing Emissions

• Enormous progress since the 1960s

Convair 880 “Old Smokey” – 1960 Boeing 777 – 1995

• But continuous growth and increasing public awareness mean this is not enough…

Reducing Emissions

• International push to reduce emissions above and beyond current regulations (e.g. Kyoto Agreement).

• Cannot demand reductions without understanding:– The reduction mechanisms.– The interrelationships between various emissions.– The impact on aircraft design.

Reducing Emissions

• Detail design of the combustor is the most obvious method of decreasing emissions (by reducing the Emissions Index)

• For NOx, reducing residency time is beneficial.

• Various new combustors are under development featuring thermodynamic and chemical methods of reducing emissions.

May not be sufficient. What can be done at the aircraft and overall engine level?

Reducing Contrails

• Decrease cruise altitude (to increase ambient temperature)

A: Ambient @ 35,000 ftB: Jet Exhaust

Reducing Contrails

• Decrease cruise altitude (to increase ambient temperature)

A: Ambient @ 35,000 ftB: Jet ExhaustC: Ambient @ 25,000 ft

C

Reducing Fuel-Related Emissions

• Increasing combustion temperature and pressure promotes more complete combustion, resulting in reduced fuel flow.

Fly Slower

Higher T4,Higher OPR

Objective:Lower Fuel

ReduceDrag

Reducing NOx Emissions

Fly Slower,Lower

Lower T4,Lower OPR

Objective:Lower NOx

ReducedThrust

• Temperature and pressure are catalysts in the dissociation of N2 and O2 and the formation of NOx

Generally, changing operating conditions or combustor configurationto reduce NOx emissions increases fuel consumption and related emissions

Optimized Designs

Minimum Cost Minimum Fuel Minimum NOx

Cruise Altitude: 32,000 31,000 26,000Cruise Mach: 0.82 0.75 0.65

Diverging Requirements - Competing Objectives

Trading Emissions

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

0.95 1 1.05 1.1 1.15 1.2 1.25

Relative Fuel Carried

Rel

ativ

e L

TO

NO

x

Min Fuel

Min NOx

TRADE:

30% NOx Reduction for

6% Fuel Increase

What about noise?

INCREASE NOx PRODUCTION

INCREASE FUEL CONSUMPTION

• Recall Tuesday presentation: increasing BPR is beneficial

More Powerfrom Turbine

Higher BPRObjective:

Lower NoiseHigher T4,

Higher OPRLarger Fan

“Green” Airplane: a compromise

Airbus

Conclusions

• Emissions are only regulated during takeoff and landing.• But increasing concern will mandate cruise regulations in the near future.

• Conflicting requirements depending on emissions to be reduced (NOx vs. Fuel Consumption vs. Noise).

• While trend towards larger turbofans has improved fuel efficiency and decreased noise, it is a challenge to reduce NOx emissions.

• Open problem!

The commercial aircraft of the future might well be:

“Slower, Lower, Greener”