7.7. Engine Emissions Database and Correlation

Since the early 1970s, when engine manufacturers started measuring emissions to demonstrate compliance with regulations developed for the airport vicinity, there has been a steady increase in data available for the development and refinement of databases associated with subsonic aircraft engine emissions. This section provides a brief overview of current ICAO engine standards and the status of the emissions database. It also discusses methods used to correlate sea-level emissions measurements with in-flight levels. Special emphasis is placed on simplified methods of NOx prediction and their validation for inventory purposes, as well as emissions variability.

7.7.1. ICAO Engine Standards and Emission Database

To control pollutants from aircraft in the vicinity of airports, ICAO established emissions measurement procedures and compliance standards for soot (measured as smoke number-SN), unburned hydrocarbons, carbon moNOxide, and oxides of nitrogen. A landing and take-off cycle was defined to characterize the operational conditions of an aircraft engine within the environs of an airport; this LTO cycle is illustrated in Figure 7-28. The standards are applied to all newly manufactured turbojet and turbofan engines that exceed 26.7 kN rated thrust output at International Standard Atmosphere (ISA) sea level static (SLS) conditions. The smoke standards took effect in 1983, and those for gaseous emissions took effect in 1986. Measurements of the exhaust emissions of a single engine are performed at the manufacturer's test facilities as part of the certification process, in compliance with the requirements of ICAO international standards and recommended practices of Annex 16 to the convention on international aviation (ICAO, 1993).

The data are published in an ICAO exhaust emissions data bank (ICAO 1995b). Engine emissions are given for the standardized LTO cycle represented by an engine power setting of 7 (taxiing), 30 (approach), 85 (climb-out), and 100% (take-off) of rated output and given times in mode (see Figure 7-28 and Table 7-8). Together with fuel flow, emission indices of HC, CO, and NOx in g per kg of fuel burned and maximum SN are reported. For a variety of engines, the measured SNs for all power settings are provided. Except for smoke, the emissions of each LTO cycle mode (EI x fuel flow x time in mode) are summed (Dp) and expressed in the form Dp/F00 (g kN) where (F00) is the maximum thrust of the engine at take-off under ISA SLS conditions.

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 NOx, 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/F00) 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 NOx, +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.

The HC, CO, and SN standards have remained unchanged within the CAEP process. For NOx, the approach was to tighten NOx stringency in accordance with technology gains. The baseline (see line CAEP/1 in Figure 7-29) had been introduced to allow NOx to rise with maximum engine 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 produce many different emissions. However, only emissions that have been derived for use within the ICAO LTO cycle certification process-such as HC, CO, NOx, and smoke-or are easily correlated using simple conversion factors related to fuel burn (such as CO2, H2O, and SOxO) have been included in significant numbers in today's inventory studies. Apart from the major exhaust emissions, a large variety of minor species are produced. HC emissions, which are typically reported as equivalent mass of CH4, can be broken down into numerous complex compounds. There has not been sufficient characterization to date for global inventory purposes, with most of the available data acquired from a selection of military engines using kerosene-type fuel with different specifications. Additionally, nitrous oxides (N2O) have not been rigorously characterized. Other emissions are not currently modeled in emissions databases because of very small quantity or the fact that little data exists. This subject is further discussed in Section 7.6.

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