Lecture 8 - Alternative Fuels Flashcards
What is the contribution of aircraft to world C02 emissions
2%
What is a big worry with aviation emissions
Big error bounds on what they emit, could be much worse than predicted
Developments in aircraft have led to
82% reduction in aircraft fuel burn per seat
and 49% reduction in fuel consumption
But need to reduce further as more people want to travel
What is the emission trade off
small range of temperature and ratios that can be used
increase temp NOx increases
decrease temp CO increases
What is UHC
unburnt hydro carbon
What is the stoichiometrix mixture
complete reaction, convert all methane to carbon - usually you want to provide a bit extra air to completely combust fuel
What causes the increase in NOx
Increasing temperature
What happens to smoke generated in gas turbines
Gets consumed at later stages of combustion
Where is smoke and NOx generated in a gas turbine
Smoke initilly produced at start of flame and peaks at the centre before decreasing to nothing at the end of the flame, it is later consumed
NOx follows the temperature and spikes just after the centre of the flame
What are other major sources of NOx
road and general background sources - aviation isnt major cause of NOx levels approximately 25-30% aviation
What is the issue with biofuels for jet engines
when take into account whole life cycle can actually be more polluting especially if needed to be transported long distance
What is the issue with comparing fuels
Very difficult to compare as dont always have the right data
What is the issue with current regulations
Only takeoff and land off are regulated, therefore emissions reductions focus on this often increasing overall emissions
Where does the majority of NOx come from
Jet and alternative fuels do not contain NOx, comes from thermal
How is NOx generation affected by use of alternative fuels
generally it is slightly reduced
Why do alternative fuels produce less NOx
Lower flame temperature due to fuels altered H/C ratio
tests with controlled temperature see no NOx change
Why is it difficult to reduce NOx production
Aim for particular exhaust gas temperature to give particular amount of thrust
To get less NOx have to reduce temperature and therefore reduce amount of thrust
What does CO2 emissions normally follow
fuel burn
What has resulted in smaller CO2 emissions
higher calorific values
altering hydrogen carbon ratio - more hydrogen, fuel volume goes up, increased weight and reduced flight distance
What will reduced C02 emissions come with
Increase in H2O and possible increases in contrail
How much change can be made with CO2 emissions with fuels
very little 1% change in emissions
Can UHC emissions be changed with fuel
A little bit, but generally property of engine not fuel - with error margin little difference
Can alternative fuels be used to alter sound and vibration levels
Yes different fuels can reduce the noise emissions and vibrations within aircraft
This would affect engine life
What type of emissions cannot be changed very much by alternative fuels
NOx CO2 UHC
What is PME
Particulate Matter Emissions
Small particles of carbon, soot etc
How is PME affected by alternative fuels
Already with in spec fuels can reduce emissions by 40%
What is the reason that alternative fuels are not currently used
Despite reduce PME, jet fuel is so much cheaper
What prevents new alternative fuels being used
complex legislation process that they have to go through to get passed off and accepted for use - checking they work with valves etc
Conclusion on NOx and Alternative fuels
unaffected, more engine control system and conventional combustor - changes to hardware may allow alternative fuels to reduce emissions
How does energy content of a fuel affect emissions
It will affect CO2 emissions and range of the aircraft
How do biofuels affect aircraft peformance
little CO2 savings from flight itself, but over entire life cycle may see reductions
What is the other possible advantage of alternative fuels aside from sustainability
security of supply
What are other options for reducing emissions
Direct and shorter routes using GPS
Continuous descent approach
Don’t use aircraft engines for taxi
Hydrogen
What are the issues with hydrogen as a fuel
flying lower due to lower GWP at high altitude Burning leaner to reduce NOx Public Perception Issues Needs infrastructure Expensive
