Environmental Engineering Reference
In-Depth Information
F
UTURE
EVOLUTIONARY
OPTIONS
The continued development of both engine and airframe technologies will produce
more efficient, cleaner and quieter aircraft; however, it is unlikely that these evolu-
tionary developments will provide the large reductions in environmental impact that
have been seen during the last 30 years. This can be seen in Figures 10.1 and 10.3,
where the levelling out of the trends would suggest a technology floor is being reached.
Perhaps more importantly, predictions by both the IPCC report and the Royal Com-
mission for Environmental Pollution (RCEP) have indicated that to become sustain-
able, CO
2
emissions need to be reduced by 60 per cent from current levels (RCEP,
2000, p2).
Engine
Specific fuel consumption
The overall efficiency of an engine is the product of thermal and propulsive effi-
ciency. Figure 10.4 shows how the thermal efficiency of an engine can only be sig-
nificantly improved if both OPR and TET are raised together.
To date, the increases in OPR and TET have been achieved through advances in
technology with the development of smaller high-speed cores, materials with improved
temperature properties, and turbine components with better cooling characteristics.
For thermal efficiency to increase further by this route, these associated improve-
ments must also continue. If not, then any gains in thermal efficiency will be eroded
through increased cooling requirements and component scale effects. An estimated
3 per cent gain in thermal efficiency can be achieved over the next 20 years, pro-
vided advances in materials and cooling technology continue.
The most practical option available to increase the propulsive efficiency of the
engine is to reduce the jet velocity. This has been achieved through the introduction
of progressively larger by-pass ratios. Modern engines have by-pass ratios in the
range of five to nine; however, increases much beyond this will lead to the need for a
gearbox between the power turbine and the fan, increasing weight and reducing the
benefits in efficiency gained through having the larger by-pass ratio. Above a by-pass
ratio of about 15, the duct must be removed otherwise drag and weight penalties
become severe. Very large by-pass ratio engines cannot be accommodated on existing
subsonic aircraft, as there is insufficient space to hang the engine below the wing.
A possible development to increase the propulsive efficiency of an engine is to
improve the mixing of the propulsor and main flows in the duct before the exhaust
nozzle. It has been estimated that efficiency gains in the order of 1 per cent could be
achieved in this way (IPCC, 1999, p233).
Noise
At take-off, engine noise dominates, but at landing the noise from the engine and
the airframe is similar. The main noise sources in the engine are the fan and the front
compressor stages, the low-pressure turbine stages and the exhaust flow. While the
most significant reductions in noise have been achieved through the introduction of
