Aviation and the Global Atmosphere

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In a report addressing the effects of aviation on the global atmosphere, the link between emissions and the technological status of aircraft now and in the future is clearly a central issue. The subject is complex. Our approach here, therefore, has been to identify a number of key questions, the answers to which provide an assessment of technical issues, problems, and the prospects of solving them.

The questions and their corresponding answers are as follows:

Answer-The overriding technological consideration in the design of aircraft today is safety. Given that prerequisite, aircraft are designed to provide an efficient and environmentally acceptable system of transport from ground level to the demanding conditions associated with high-speed flight at high altitudes. To achieve high efficiency, fuel consumption must be minimized by reducing the weight and drag of the aircraft. This requirement also ensures that there is a constant drive toward the highest levels of energy conversion efficiency from the engine. Together, these factors ensure that carbon dioxide CO2) and water outputs are minimized.

The most fuel-efficient engines for today's aircraft are high bypass, high pressure ratio gas turbine engines. No known alternatives are in sight. These engines have high combustion pressures and temperatures; although these features are consistent with fuel efficiency, they increase NOx formation rates-especially at high power take-off and at altitude cruise conditions.

Current low-sulfur fuels minimize SOxO emissions. Small amounts of fuel-bound sulfur (400-600 ppm) and associated organic acids provide important lubricity properties for critical fuel system components. Processing to remove all traces of sulfur would remove important organic acids, so sulfur-free fuels are unlikely to be adopted in the short term. Sulfur removal would also result in a small net rise in CO2.

At present there is only limited knowledge about the formation and behavior of minor, trace species and aerosols found in the exhaust plumes of engines. Even less is known about how they are influenced by engine features and characteristics. . Question-What progress has been made to date in reducing emissions, and how may new advances in aircraft and engine technology help reduce them further in the future?

Answer-In the past 40 years, aircraft fuel efficiency has improved by 70% through improvements in airframe design, engine technology, and rising load factors. More than half of this improvement has come from advances in engine technology. These trends are expected to continue, with airframe improvements expected to play a larger role through improvements in aerodynamic efficiency, new materials, and advances in control and handling systems. New, larger aircraft with, for example, a blended-wing body or double-deck cabin offer prospects of further benefits by relaxing some of the design constraints attached to today's large conventional aircraft. Because of the very long total lifetimes of today's aircraft (up to 50 years), however, replacement rates are low, and the fuel efficiency of the whole fleet will improve slowly. Rising market demand will ensure that this trend is maintained, however.

The intrinsic link between lower CO and rising levels of NOx is being successfully countered with relatively simple strategies in state-of-the-art combustors. These combustors have achieved 20-40% reductions in NOx. Consolidation of these improvements to broaden their applicability to newer, even more fuel-efficient engines demands further improvements in combustor technology. Major research programs are underway to do so.

Although the use of hydrogen as a fuel offers a way to eliminate CO2 and further reduce NOx from aircraft, widespread use of hydrogen fuel presents major design problems for aircraft and would entail global changes in supply, ground handling, and storage. Hydrogen would also substantially increase water vapor emissions from aircraft. Thus, kerosene-type fuels are considered to be the only viable option for aircraft within the next 50 years (to 2050).

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