Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
reliability, durability, efficiency, and noise-all of which must be taken into account during engine development.
More fuel efficient, high bypass engines reduce not only CO 2 and H 2 O, but also HC and CO
emissions. For a given combustor design, however, NO x formation rates rise as a result of
higher air pressures and temperatures. Thus, despite the reduction in the amount of fuel that can
form NO x , the increased formation rate can result in a net rise in the mass output of NO x . This
result is shown clearly in Figure 7-17 (Rudey, 1975). Many earlier researchers have suggested
that the only way the more basic tradeoff between CO 2 and NO x could be altered was by finding
satisfactory ways of reducing fuel-rich zones without compromising stability, and reducing the
residence time of burning gases in the combustor without compromising exit temperature profiles
or pattern factors.
This important NO x -CO 2 (fuel) tradeoffs issue has been summarized as follows:
"There is no single relationship between NOx and CO2 that holds for all engine types.
However for the best current aircraft engine and combustor design technologies, there is
a direct link between the emissions of NOx and CO2 . As the temperatures and pressures
in the combustors are increased to obtain better fuel efficiency, emissions of NOx increase, unless there is also a change in combustor technology." (ICCAIA, 1997b)
Improved low-emission combustor technology can alter the precise tradeoff between CO 2 and
NO x . Theoretically, this technology can reduce NO x levels at any pressure ratio. However, for
any currently available aircraft engine combustor technology, there will still be some tradeoff
between CO 2 and NO x , although it will be at lower NO x levels. Even with very advanced
combustor technologies that minimize NO x formation by premixing fuel and air to control
combustion temperatures, there is a tradeoff between CO 2 and NO x as a result of high
combustor exit temperatures associated with advanced, highly efficient engines.
A further NOx-related issue arises from a common practice adopted by industry. In recent years,
there have been numerous occasions when operators have sought to increase the capacity
(passenger numbers or weight) or range of an existing airplane. A highly cost- and time-effective
response to such requirements is to increase the thrust level of an existing engine type. This
response has led to a type of development usually referred to as a "throttle push" or "throttle
bend" of the core engine. This approach invariably entails increasing overall engine pressure
ratio and results in higher combustor inlet pressures and temperatures, thus higher NOx levels at
high power levels. However, analytical studies (ICCAIA, 1997d) of engine growth characteristics
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Figure 7-19: Recent combustion system enhancement
for aircraft NO x reduction (ICCAIA, 1997c).