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Figure I-2. Histogram of the emissions arising from
the idle portion of the ICAO LTO for all engines.
operating at 15°C (59°F). The trace denoted by the dashed line core turbine. A hollow shaft connects the core turbine to
is the estimate based on the emissions measurement observed the core compressor, and a second shaft passes through the
in this work, assuming the entire idle phase is characterized core and is connected to the outer fan. This is the scheme for
by ground idle with a nominal bleed air demand (rather than a two-spool engine. There are variants that use an additional
the ICAO certification value of 7% thrust). For both cases the stage of compression and are called three-spool engines. The
area under the particular mode is proportional to the total reason these engines are generally referred to as high-bypass-
emissions. This figure demonstrates (for this engine) that ratio turbofans is that the majority of the air flow (and hence
the idle phase of airport activity in the standard LTO cycle is the thrust) does not go through the combustor region, but
responsible for most of the hydrocarbon emissions from aircraft rather bypasses the engine core.
engines. The UHC emission rate for the auxiliary power unit The generalized schematic depicted in Figure I-3 labels
(APU) has been taken from Table 6-1 of Wade (Wade 2002) the major pieces of a turbofan engine, where pressure (P)
and assumes a GTCP331-200 APU type. Other APU types that and temperature (T) are noted with a number. For example,
are typically installed on a narrow-body aircraft (Gerstle et al. in this project, the effect of ambient temperature on the emis-
1999), such as that presumed by the CFM56-7B24 engine type sions of various HAP species would schematically manifest
in the example depicted in Figure I-1, have similar emission itself as a perturbation to T2, the temperature of the gas coming
levels. The choice of time for APU use during the LTO is into the engine.
arbitrary and is included to provide a sense of how the APU
emissions compare with those from the main engines.
UHC emissions arising from the idle phase in the example
in Figure I-1 account for 94% of total UHC emissions (for the
CFM56-7B22). This is fairly typical of the aircraft engines
tabulated in the ICAO databank. Figure I-2 depicts a histogram
of this calculation (fraction of total UHC emissions from the
idle phase during a standard LTO cycle), which shows that the
idle phase dominates total UHC emissions for the vast majority
of engine types. This project has focused on characterizing
selected HAPs, total UHC, and other specific VOCs emitted
by aircraft engines during the idle phase.
Figure I-3. Schematic of turbofan engine.
The component pieces (labeled on the lower
I.2Anatomy of the High-Bypass-Ratio portion of the figure) of a turbofan engine
Turbofan Engine are depicted with air flow proceeding
from left to right. The station numbers
Modern commercial aircraft are powered by engines T2, P3, T3, and T4 are taken from Aircraft
typically classified as high-bypass-ratio turbofan engines. Engines and Gas Turbines (Kerrebrock
The combustor uses a turbine-driven fan to draw air into 1977) and are referred to in subsequent
the combustor, and the exhaust outflow goes through the sections of this report.
