7.9. Small Aircraft, Engines, and APUs

Small aircraft engines are defined here as all turbo engines-excluding piston engines because of the very small amount of fuel burned by these engines (much less than 5% of the world's fleet fuel burn; see Section 7.2)-that are used for regional aircraft, all turbofans with less than 89 kN thrust, all turboprop and turboshaft engines, and all auxiliary power units (APUs) used in civil aircraft.

The small commercial and general aviation segment has been growing rapidly in recent years and is likely to continue to do so. This segment's impact on the environment, however, is unlikely to be significant because of low NOx emissions levels associated with the generally lower pressure ratio engines they employ and the decreasing percentage of the fleet's fuel burn they represent (see Chapter 9). Furthermore, most of aircraft in this sector fly short missions with lower cruise altitudes and reduced potential for climatic impact. Significant improvements have been made in the idle emissions of small engines in recent years, so that CO, HC, and NOx emissions from small regional and general aviation aircraft are often comparable, in terms of emissions per kilogram of fuel burned, to those from large engines (Eatock and Sampath, 1993).

This section highlights the key differences between large and small engines in terms of emissions characteristics and control technologies that might apply to small aircraft and engines. A brief overview is presented on small airframe technology, engine performance, engine emission databases, combustor technology, and unique issues related to small engine combustors.

7.9.1. Airframe Technology

The airframe technology applicable to small aircraft parallels that of large aircraft discussed in Section 7.3. Designers and manufacturers continue to strive to reduce drag and increase range/payload performance with a resultant steady improvement in overall fuel-efficiency of new small aircraft. Propeller design, of course, is of much greater importance to this sector of the world's fleets. Detailed aerodynamics research is showing some of the significant performance benefits-thus fuel savings-that can be achieved from relatively small changes in the design of small airframes.

Nacelle problems associated with higher bypass ratio engines for small aircraft are similar to those of large aircraft. Although embedded engines have been considered, there are no plans to adopt them because the small gains in nacelle efficiency are more than offset by losses in wing efficiency.

7.9.2. Aircraft Weight Reduction

Underpinning many of these issues is the emergence of a more centrally computerized product definition database introduced at an early stage in the aircraft/system definition process. This database greatly enhances the design, placement, and routing of environmental control system, hydraulic, fuel, and electrical systems and components.

7.9.3. Engine Performance

Existing small engines operate at overall peak pressure ratios from 8 to about 30, as compared with large engine values (see Section 7.4) of about 20 to > 40. Thus, NOx production in grams per kilogram of fuel is typically lower for engines designed for regional and general aviation aircraft than it is even for future staged, low NOx combustors in large engines. However, the low engine cycle pressure ratio makes efficient operation at idle intrinsically more difficult. Combustor inlet pressure at idle power may be less than half that of a modern large engine, with a resulting tendency to cause higher CO and HC emissions. However, technology advances in larger engines are reflected in the latest generation of small engines, with SFC improvements stemming from advanced combustor cooling techniques, high temperature materials, and advanced engine cycles with higher pressures and temperatures. Higher combustor inlet pressures and temperatures in modern small engine combustors now result in even lower fuel burn, which generally balances any tendency toward an increase in NOx output.

7.9.4. Engine Database

Emissions for small turbofan engines with a thrust of more than 26.7 kN thrust are measured, reported, and certified in exactly the same manner as emissions for large engines and are recorded in the ICAO database (see Section 7.7.1). Engine manufacturers usually have some emissions data on their non-certified engines (turbo-shafts, turboprops, APUs, and turbofans of less than 26.7 kN thrust), but in general these engines are not of certification quality and the data are not readily available. The ICAO LTO cycle is used for turbofan engines, but these points are not well defined for turboshaft or turboprop engines because their thrust depends on the propeller or rotor selected for each application. Shaft power is usually substituted for thrust in emissions calculations to compare turboshaft or turboprop engines, but these emissions cannot be compared directly with turbofan data (which is based on thrust).

Correlations between measured emissions at static sea level conditions and altitude operating conditions, as discussed in Section 7.7.2, are generally applicable to small engines. Exceptions may occur for very small engines in which combustor surface to volume ratios, fuel atomization quality, combustor volumes, and so forth are very different from those in large engines. Small engines may require additional correlation parameters to account for these differences (Rizk, 1994).

7.9.5. Combustor Technology

7.9.5.1. Historical Developments and Current Status

The ability of gas turbine manufacturers to design successful small engine combustors with reduced emissions has improved greatly in the past 20 years. Idle efficiency data, indicative of CO and HC levels, collected by the General Aviation Manufacturers Association (GAMA) (Eatock, 1993) and plotted in Figure 7-39 indicate combustion inefficiencies at idle power of between 5 and 15% in early combustors. The trend, however, shows that modern small engines approach and even match the idle combustion efficiency of modern large turbofan engines. Figure 7-40 shows the basic "Lipfert" correlation (ERAA, 1992; Eatock and Sampath, 1993) of EI(NOx) in grams of NOx per kilogram of fuel burned. The correlation relates emission indices of NOx to compressor discharge pressure (pressure ratio) and covers most engines operating at or near stoichiometric burning at full-power condition. Many small aircraft engines have even lower EI(NOx) than those anticipated for the future third generation of large turbofan engines.

7.9.5.2. Unique Combustors for Small Engines

The major challenge to reduce the emission level of NOx for small engines is overcoming size-related constraints. NOx reduction strategies may be restricted, for example, by the smaller passage height between the inner and outer diameters of the combustor, which limits the size and number of fuel injectors that can be used for such purposes. Small combustors are also more sensitive to minor size variations within fixed manufacturing tolerances. For these reasons, and to ensure that all such combustors will meet a given emissions goal, "nominal design" hardware must be able to demonstrate compliance with somewhat larger emissions margins than for larger engines. Many smaller engines use centrifugal compressors in combination with reverse-flow combustors. These combustors have a higher surface-to-volume ratio.Furthermore, scaling down a design has the effect of rapidly increasing combustor surface area-to-volume ratio (ICCAIA, 1993). Both factors tend to result in the need for a higher percentage of combustor airflow for cooling the combustor, leaving a smaller amount of air available for controlling emissions. Together these effects limit the direct translation of low NOx combustor technology from large engines into smaller combustion system designs. The NASA-sponsored contract for small turbofan engines (Bruce et al., 1977, 1978, 1981) demonstrated the problems very clearly. This work showed that even a modest 30% reduction in NOx could be achieved only using a design that featured variable geometry and staged combustion, with a totally impractical increase in the number of fuel injectors.

7.9.5.3. Current and Future Trends

In recent years, manufacturers of small engines have continued to develop new emissions control techniques that have minimum cost and performance impact. These techniques include combustors with optimized stoichiometry in the primary combustion zone, improved fuel/air mixing using efficient swirlers, improved fuel spray quality using piloted air-blast and aerating fuel nozzles, and combustors with optimized liner wall cooling to minimize wall quenching effects on CO and HC emissions.

Several advanced emission control concepts have also been investigated on small engines, including variable geometry and staged combustors (Bruce et al., 1977, 1978, 1981). Variable geometry concepts can reduce HC and CO emissions; however, such complexity is particularly unattractive for small engines. An axially staged combustor requires a large number of fuel nozzles, with corresponding fuel passage size reduction and added fuel control system complexity.

In summary, small engine combustor technology has progressed markedly, especially in the past 20 years. In the short term, existing combustion chambers can be modified and optimized, within the constraints defined in Section 7.9.5.2, to obtain the best NOx versus CO and HC compromise. There are no special fuel requirements for small engines and aircraft, now or in the foreseeable future.

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