This report reviews current turbofan engine noise and engine noise reduction technology, specifically focusing on the engine technology of larger passenger jetliners which have entered into service within the last ten (10) years. Important factors in turbofan engine design from a community noise perspective and the sources of noise along with their relative importance are also presented. A review of different engine noise reduction technologies is presented, as well as an estimate of the technology’s readiness level. Finally, potential trade-offs, challenges, and future technology directions are outlined.

1. CURRENT TURBOFAN
ENGINE TECHNOLOGY
Presented by Sammy Jamar
Presented to Dr. Kamel Shorbagy

2. Introduction
 This report reviews current turbofan engine noise and engine noise reduction
technology, specifically focusing on the engine technology of larger passenger
jetliners which have entered into service within the last ten (10) years.
 Important factors in turbofan engine design from a community noise
perspective and the sources of noise along with their relative importance are
also presented. A review of different engine noise reduction technologies is
presented, as well as an estimate of the technology’s readiness level.
 Finally, potential trade-offs, challenges, and future technology directions are
outlined.

3. CONT..
 Turbofan engines are commonly used on commercial transports due to their
advantage for higher performance and lower noise.
 The noise reduction comes from combinations of changes to the engine cycle
parameters and low-noise design features.
 Turbofan engine noise technology has seen significant advances, generally
dominated by improvements in the engine cycle with the bypass ratio (BPR)
increasing and the fan pressure ratio (FPR) decreasing significantly.
 Details in engine design like the detailed fan/OGV design play a significant role
and, in some cases, can even overwhelm the general trend of lower noise with
higher BPR and lower FPR.
 Looking ahead, it is unclear if the trend of increasing overall propulsive efficiency
with the corresponding fuel burn reduction and lower community noise will
continue.


4. 1.CURRENT ENGINE TECHNOLOGY
 Fig. 1 Range of BPR and FPR of current engines

5. CONT..
 Figure 1 shows the cycle range and specific thrust trend of current engines.
For a given amount of thrust, to increase the BPR and drop the FPR at
maximum takeoff power, the specific thrust (thrust/mass flow) is reduced and
the fan diameter is generally increased.

6. CONT…
 Fig. 2 Noise and fuel burn trend with fan diameter

7. CONT..
 Fig 2: Even on new airplanes, as the fan diameter increases, the overall
wetted area increases and integration issues increase, which can increase
weight and drag and lead to a fuel burn minimum and, at some value of fan
diameter, the fuel burn benefit goes away and becomes a penalty.

8. CONT..
 Fig. 3 Typical sources of noise from a turbofan engine

9. CONT..
 Fig. 4 Typical source breakdown of a current (~ BPR10) engine

10. CONT..
 The amount of community noise which a given engine makes is determined
not only by the engine’s FPR and BPR but also;
• Detailed fan and fan OGV design,
• The details of the LPT and low-pressure compressor (LPC) design,
• How oil cooling is done
 As the fan diameter increases, the weight and integration issues increase and
the impact on things like the fan/OGV spacing become significant. Reduced
fan/OGV spacing leads to an increase in noise.
 Increasing BPR and lowering FPR results to an increase in engine heat and
thus cooling is required through the introduction of bleeds which are a great
source of noise again.

11. CONT..
 Recently, combustors have been designed for high efficiency and low nitrogen
oxide (NO2) emissions. To achieve these requirements, the engines tend to
operate at higher temperatures and at, or close to the fuel-lean limit. This
makes the combustion process unsteady and increases the combustion noise.
 The design of the low-pressure system including the compressor and turbine
greatly depends on whether a gearbox is used to drive the fan. In the case of
a geared fan, both the low-pressure compressor (LPC) and turbine (LPT) are
designed to run at a higher speed, generally closer to their peak efficiency.
 For the geared fan engine, unless low blade counts are used, the LPT noise
will be close to the high end of the audible range.

12. CONT..
 Fig. 5 Cumulative noise relative to Sect. 3 of the recently certified first of
model twin airplanes

13. CONT..
 Figure 5 shows the ICAO-certified noise levels [8] of twin turbofan powered
first of model airplanes recently delivered as a function of the published
bypass ratio. Even though the published BPR is notoriously inaccurate and is
usually quoted at the cruise condition, it does show that there is more than
one example where the noise delta between the engines is opposite of the
expected trend

14. 2.REVIEW OF NOISE REDUCTION
TECHNOLOGIES
 Fan and jet noise are still the most dominant noise sources and this section
will focus on technologies that reduce these sources.

15. 2.1 FAN NOISE
 Fan noise reduction has historically been achieved through a combination of:
decreased FPR and tip speed;
 Improved acoustic treatment (such as the acoustically smooth inlet);
 Proper selection of fan and OGV blade counts
 Ample spacing between the fan and OGV rows

16. CONT..

17. CONT..

18. 2.2 JET NOISE
 Jet noise reduction has historically been achieved through cycle optimization
with generally increasing bypass ratios.

19. CONT..

20. CONT..

21. CONT..
 Looking at the above lists, it becomes significantly clear that the only
technologies that seem progress to a higher TRL and implemented are those
that have either a neutral or positive effect on fuel burn and that also do not
add a significant amount of complexity.
 It is also clear that new approaches are needed, and there needs to be
significant effort put forth into new technologies to achieve both noise
reduction and avoid negatively impacting fuel burn.

22. 3. FUTURE OPPORTUNITIES
 For every significant technology advancement, there has been a significant
movement of where the “optimum” cycle design point is.
 This technology advancement tends to increase BPR and reduce FPR, and has
traditionally depended significantly on material technology including high
temperature and lightweight materials and structure.

23. CONT..
 Fig. 6 Effects of technology on engine noise and fuel burn

24.  The ‘short and slim nacelle’ with compact nacelle reduces the overall
length/diameter (L/D) of the nacelle and, therefore, mitigates both installation
impacts and weight associated with a larger fan diameter. This unfortunately can
have a significant community noise impact and can reduce or potentially eliminate
the noise reduction that would typically be seen with the reduction in specific
thrust.
 The desire for a short engine tends to reduce the number of LPT stages, increasing
LPT speed. When there is not fan gearbox, this, in turn, can increase the fan
speed over what it would otherwise be, leading to an increase in the fan source
noise.
 Another idea that can have a fairly dramatic effect on community noise is the
open rotor. Recent studies have shown that, even with rather dramatic
compromises for noise, the best noise levels which the open rotor could achieve
are to meet chapter 14 which is significantly louder than the current generation of
turbofans.

25.  Fig. 7 Certified cumulative noise levels

26.  Figure 7 shows the noise reduction in the certified noise levels [15] seen for
the last generation versus the current one, due to the pressures just
discussed, this trend is not likely to continue without a significant increase in
community noise technology funding. High TRL technologies would need to be
available in the near future due to the time scales involved in the
development of large commercial airliners

27.  There has been a significant effort going into the Blended Wing Body concept
and other concepts that hold this potential. It would seem, however, that
these types of configurations are beyond the 2030 timeframe at least to be in
significant commercial service.
 There is another concept called distributed propulsion which has at least
some potential in the longer term. This concept reduces the specific thrust by
increasing significantly the number of propulsion units.

28.  Fig. 8 Distributed propulsion concept

29.  This is illustrated in Fig. 8. As shown, the idea would be for a single engine
core to drive several fans, thus, further reducing the specific thrust and
potentially improving fuel burn while also having the potential to reduce
community noise (depending on the integration).
 The integration for this concept would be key to whether or not it would, in
fact, result in a noise reduction. On one hand, you have increased the
potential noise sources, but, on the other, you have likely significantly
reduced the fan pressure ratio and tip speeds.
 There is also the potential to significantly improve the installation of the fans
onto the airframe and thereby help use the airplane for shielding. Finally, it
may be possible to phase the fans in such a way as to reduce the noise

30.  Although this concept may be possible with mechanical shafts and gears, to
be viable from a complexity stand point, the core would likely need to consist
of a smaller high-speed core (also significantly integrated into the airframe)
and a generator with a battery and the fans would each need an electric
motor.
 This is what is typically termed a hybrid electric concept.
 Due to the limitations in battery energy density and power limitations of a
conventional distribution system, this concept is likely limited to smaller
airplanes for the foreseeable future.

31. CONCLUSION
 This review has focused on the technology of engines on larger passenger jetliners
whose entry into service has been within the last 10 years, and showed the sources of
noise and their general relative importance.
 The noise sources of these engines, mostly dominated by fan noise with other sources
still being consequential, were shown.
 In addition, a review of different engine noise reduction technologies was shown as
well as an estimation of the technologies’ overall technology readiness level. Finally,
possible paths forward were explored and potential future technological advancements
within engine technology and engine noise were discussed.
 Looking ahead, it is unclear if the trend of increased overall propulsive efficiency with
the corresponding fuel burn reduction and lower community noise will continue. In
fact, it seems more and more likely as a significant fuel burn reduction is achieved;
lower community noise will not be and may even increase.
 Therefore, continued reduction in community noise will likely only be achieved if there
is a significant increase in overall technology funding and/or a significant change in the
overall airplane configuration.

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