Lecture 8 - Alternative Fuels

Question 1

What is the contribution of aircraft to world C02 emissions

Answer 1

2%

Question 2

What is a big worry with aviation emissions

Answer 2

Big error bounds on what they emit, could be much worse than predicted

Question 3

Developments in aircraft have led to

Answer 3

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

Question 4

What is the emission trade off

Answer 4

small range of temperature and ratios that can be used
increase temp NOx increases
decrease temp CO increases

Question 5

What is UHC

Answer 5

unburnt hydro carbon

Question 6

What is the stoichiometrix mixture

Answer 6

complete reaction, convert all methane to carbon - usually you want to provide a bit extra air to completely combust fuel

Question 7

What causes the increase in NOx

Answer 7

Increasing temperature

Question 8

What happens to smoke generated in gas turbines

Answer 8

Gets consumed at later stages of combustion

Question 9

Where is smoke and NOx generated in a gas turbine

Answer 9

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

Question 10

What are other major sources of NOx

Answer 10

road and general background sources - aviation isnt major cause of NOx levels approximately 25-30% aviation

Question 11

What is the issue with biofuels for jet engines

Answer 11

when take into account whole life cycle can actually be more polluting especially if needed to be transported long distance

Question 12

What is the issue with comparing fuels

Answer 12

Very difficult to compare as dont always have the right data

Question 13

What is the issue with current regulations

Answer 13

Only takeoff and land off are regulated, therefore emissions reductions focus on this often increasing overall emissions

Question 14

Where does the majority of NOx come from

Answer 14

Jet and alternative fuels do not contain NOx, comes from thermal

Question 15

How is NOx generation affected by use of alternative fuels

Answer 15

generally it is slightly reduced

Question 16

Why do alternative fuels produce less NOx

Answer 16

Lower flame temperature due to fuels altered H/C ratio

tests with controlled temperature see no NOx change

Question 17

Why is it difficult to reduce NOx production

Answer 17

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

Question 18

What does CO2 emissions normally follow

Answer 18

fuel burn

Question 19

What has resulted in smaller CO2 emissions

Answer 19

higher calorific values

altering hydrogen carbon ratio - more hydrogen, fuel volume goes up, increased weight and reduced flight distance

Question 20

What will reduced C02 emissions come with

Answer 20

Increase in H2O and possible increases in contrail

Question 21

How much change can be made with CO2 emissions with fuels

Answer 21

very little 1% change in emissions

Question 22

Can UHC emissions be changed with fuel

Answer 22

A little bit, but generally property of engine not fuel - with error margin little difference

Question 23

Can alternative fuels be used to alter sound and vibration levels

Answer 23

Yes different fuels can reduce the noise emissions and vibrations within aircraft
This would affect engine life

Question 24

What type of emissions cannot be changed very much by alternative fuels

Answer 24

NOx CO2 UHC

Question 25

What is PME

Answer 25

Particulate Matter Emissions

Small particles of carbon, soot etc

Question 26

How is PME affected by alternative fuels

Answer 26

Already with in spec fuels can reduce emissions by 40%

Question 27

What is the reason that alternative fuels are not currently used

Answer 27

Despite reduce PME, jet fuel is so much cheaper

Question 28

What prevents new alternative fuels being used

Answer 28

complex legislation process that they have to go through to get passed off and accepted for use - checking they work with valves etc

Question 29

Conclusion on NOx and Alternative fuels

Answer 29

unaffected, more engine control system and conventional combustor - changes to hardware may allow alternative fuels to reduce emissions

Question 30

How does energy content of a fuel affect emissions

Answer 30

It will affect CO2 emissions and range of the aircraft

Question 31

How do biofuels affect aircraft peformance

Answer 31

little CO2 savings from flight itself, but over entire life cycle may see reductions

Question 32

What is the other possible advantage of alternative fuels aside from sustainability

Answer 32

security of supply

Question 33

What are other options for reducing emissions

Answer 33

Direct and shorter routes using GPS
Continuous descent approach
Don’t use aircraft engines for taxi
Hydrogen

Question 34

What are the issues with hydrogen as a fuel

Answer 34

flying lower due to lower GWP at high altitude
Burning leaner to reduce NOx
Public Perception Issues
Needs infrastructure
Expensive