AA241B: Aircraft Emissions

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AA241B Aircraft Emissions

• Nicolas Antoine
• Stanford University

nantoine_at_stanford.edu
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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.

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Jet Fuel Combustion

• CnHm S N2 O2 ? CO2 H2O N2 O2
• NOx CO SOx Soot UHC

Products of ideal combustion
Fuel
Air
Products of non-ideal combustion
NOx Affects ozone (O3) concentration CO2
Absorbs outgoing infrared radiation CO
Toxic Soot Visible
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Emissions Regulations

• 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

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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.

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Contrails

• Formed as water from engine exhaust freezes.
• Contrails promote cirrus formation
• May hamper atmosphere heating/cooling schedule

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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

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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.

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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?

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Reducing Contrails

• Decrease cruise altitude (to increase ambient
  temperature)

A Ambient _at_ 35,000 ft B Jet Exhaust
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Reducing Contrails

• Decrease cruise altitude (to increase ambient
  temperature)

C
A Ambient _at_ 35,000 ft B Jet Exhaust C Ambient
_at_ 25,000 ft
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Reducing Fuel-Related Emissions

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

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Reducing NOx Emissions

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

Generally, changing operating conditions or
combustor configuration to reduce NOx emissions
increases fuel consumption and related emissions
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Optimized Designs
Minimum Cost
Minimum Fuel
Minimum NOx
Cruise Altitude 32,000 31,000
26,000 Cruise Mach 0.82 0.75 0.65
Diverging Requirements - Competing Objectives
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Trading Emissions
TRADE 30 NOx Reduction for 6 Fuel Increase
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What about noise?

• Recall Tuesday presentation increasing BPR is
  beneficial

INCREASE NOx PRODUCTION INCREASE FUEL CONSUMPTION
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Green Airplane a compromise
Airbus
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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