The aviation industry represents approximately 3% of global greenhouse gas emissions, however with significant growth expected over the coming decades this proportion is expected to increase. Continued governmental and social pressure to reduce global emissions is posing a challenging question to the industry; how to improve environmental efficiency and reduce emissions with increasing industry growth. The environmental impact of aviation globally is discussed, examining the significant emissions and protocols that exist and their relative impacts both environmentally and economically. The viability of alternative biofuels is discussed, determining the life cycle environmental impact of future replacements to kerosene based jet fuel. This thesis therefore aims to provide an understanding of the fundamentals of aviation emissions but also most importantly provide possible solutions to assist the industry in reducing its emissions ‘footprint’. An important factor in determining efficiency improvements is to understand the impact of particular stages of an aircraft life and the impact they have individually. This was achieved using an established methodology called Life Cycle Assessment (LCA), which is an efficient tool for the analytical consideration of the environmental impact of manufacturing, operation and decommissioning. The results of a comprehensive LCA study of an Airbus A320 are documented considering all phases of the service life. The study draws useful conclusions, indicating the significance of special materials such as carbon fibre reinforced plastic (CFRP) on the total manufacturing emissions of the aircraft and indicating its operational phase as the one contributing most in its environmental performance breakdown. The thesis also examines short-term efficiencies for emissions reduction in commercial aviation, focussing on improvements in aircraft routing. The initiation of the EU emissions trading system (ETS) within European aviation willincentivise airlines to reduce their annual CO2 emissions. An alternative routing strategy is proposed for selected long haul routes, which introduces multiple stages into the route utilising two aircraft and is shown to reduce total CO2 emissions by up to 13.7%. Combined with blended biofuel, this reduction was estimated to increase to 16.6% with a reduction in ticket fares estimated to be as high as $19 per passenger per flight.

Biofuels in civil aviation is actively studied in recent years to identify potential alternative jet fuels to meet stringent environmental regulations imposed to tackle degraded air quality caused by fossil fuel combustion. In this context, the aviation industry prefers to develop ‘drop-in’ fuels which may not require substantial modifications in existing jet engine technologies. The thesis aims at evaluating the atomization characteristics of camelina- and jatropha-derived drop-in biofuels discharging from simplex swirl atomizer used in aircraft gas turbine engines. The test fuels are characterized in detail and all fuels meet current ASTM D7566 specifications. The experiments are conducted by discharging fuel spray into quiescent atmospheric air in a fuel spray booth to obtain spray characteristics such as fuel discharge behaviour, spray cone angle, breakup behaviour of swirling fuel sheet and spray drop size distribution. The characteristics of sheet breakup are deduced from the captured images of biofuel sprays and the measurements of spray droplet size distribution are obtained using Spraytec (laser-diffraction instrument). A systematic comparison is made between the biofuel sprays and the 100% Jet A-1 (conventional aviation kerosene) sprays to evaluate the drop-in feature of the biofuels. All the measured spray characteristics of the biofuel sprays follow the Jet A-1 both in qualitative and quantitative terms which ensure the drop-in nature of the tested biofuels. The minor differences observed in the comparison of the quantitative spray measurements are attributed to the variation in the fuel properties. This claim is supported using the predictions obtained from the liquid film breakup model and the empirical correlation reported in the literature for the determination of sheet breakup characteristics and mean drop size for sprays discharging from simplex swirl atomizers.