Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
The emissions measurements are taken at the exit plane of the engine's exhaust nozzle (within
0.5 nozzle diameter). Kerosene-type fuel complying with specified properties-density, heat value,
boiling points, aromatics (15-23% volume), sulfur (less 0.3% mass), hydrogen (13.4-14.1%
mass)-is used. No additives for smoke suppression are allowed. A set of correction procedures,
approved by ICAO bodies, has been developed for gaseous emissions to ensure that the
observed emission indices can be compared at reference day conditions [ISA SLS pressure
(101.325 kPa) and temperature (288.15 K)]. For NO x , an additional correction is made to take
account of ambient humidity, using as a reference an absolute humidity value of 0.00629 kg
water per kg dry air (about 60% relative humidity). For economic reasons, only a small sample of
engines of any type is tested. For regulatory purposes, therefore, a statistically based correction
is used to account for engine-to-engine variability resulting from manufacturing tolerances. To
ensure that the mean value of a population of an engine type will meet the given limits of
standards within a confidence level of 90%, an additional factor is applied to the measured mean
value of the LTO cycle emission (Dp/F 00 ) to give the so-called characteristic value, which must
be in compliance with the regulatory level. The additional statistical factor-derived from a variety
of engines measured by all engine manufacturers-depends on the emission species and the
number of engines tested. For two engines to be regarded as representative of a type, the
increases applied to the mean measured emissions value for each species are as follows:
+10.0% for NO x , +13.9% for CO, +30.1% for HC, and +17.3% for SN. The relative amount to be
added to the measured value decreases with the number of engines tested. Table 7-8 shows an
example of engine emission data extracted from an ICAO data sheet submitted by the
manufacturer with two engines tested.
Ongoing revision of the regulatory level (see last line of Table 7-8), as well as the entire emission
certification process, is one of the objectives to be followed by CAEP.
Figure 7-33: Comparison of all available in situ
measured NO x emission index values with
The HC, CO, and SN standards have remained unchanged within the CAEP process. For NO corresponding predicted values: (A) Schulte et al.
x ,
(1997); (B) Schlager et al. (1997); (C) Schulte and
the approach was to tighten NOx stringency in accordance with technology gains. The baseline
Schlager (1996); (D) Fahey et al. (1995a); (E) Fahey
(see line CAEP/1 in Figure 7-29) had been introduced to allow NOx to rise with maximum engine et al. (1995b); and (F) Haschberger
pressure ratio and associated temperature, a parameter that strongly influences the rate of NOx production (see also Section 7.4.3). In a second stage, the regulatory NOx level was decreased
by 20%. This level is often referred to as the CAEP/2 standard (Figure 7-29); it has been effective for new engine types since 1996 and will apply to newly
manufactured engines from the year 2000. From 2004 onward, a further reduction in the LTO regulatory values of NOx was agreed at a fourth meeting of the CAEP
(see line CAEP/4 in Figure 7-29). This value is 16.25% below the CAEP/2 standard at an engine pressure ratio of 30, with some allowances for engines with higher
pressure ratios.
Present regulatory procedures based on the LTO cycle were designed to address airport air quality problems, but the CAEP is now pursuing new certification
methodologies that take account of the flight mode as well. Although there are links between trends in LTO and flight exhaust emissions, new correlation procedures
(see below) are needed to enable ground test measurements to be used to provide quantitative methods for predicting altitude emissions from aircraft. Aircraft engines
http://www.ipcc.ch/ipccreports/sres/aviation/107.htm (2 von 3)08.05.2008 02:43:43