[0001] The present invention relates to a compressor and in particular to a compressor having
an inlet structure the characteristics of which are such that noise levels external
to the structure are reduced as compared with conventional inlet structures.
[0002] Turbochargers have been designed which incorporate a compressor inlet structure that
has become known as a "map width enhanced" (MWE) structure. Such an MWE structure
is described in for example US patent number 4930979. In such arrangements, the compressor
inlet comprises two coaxial tubular inlet sections, the inner inlet section being
shorter than the outer section and having an inner surface which is an extension of
a surface of an inner wall of the compressor housing which faces vanes defined by
an impeller wheel mounted within the housing. An annular flow path is defined between
the two tubular inlet sections, the annular flow path being open at the upstream end
and opening at the downstream end through apertures communicating with the inner surface
of the housing which faces the impeller wheel.
[0003] With an MWE inlet structure, when the flow rate through the compressor is high, air
passes axially along the flow path defined between the two tubular sections towards
the compressor wheel. When the flow through the compressor is low, the direction of
air flow through the flow path is reversed so that air passes from the apertures adjacent
the impeller wheel to the upstream end of the inner tubular section of the inlet structure.
As is well known, the provision of such a flow path stabilises the performance of
the compressor.
[0004] It is well known that compressors incorporating MWE inlet structures tend to exhibit
high levels of noise as compared with conventional structures in which an inlet is
defined by a single tubular member. This problem is addressed in British patent number
2256460 which discloses an MWE inlet which incorporates a noise-reduction baffle located
upstream of the inner tubular section of the structure and retained within the upstream
end of the outer tubular section of the structure. The baffle thus closes off the
otherwise open axial end of the annular flow path defined between the inner and outer
tubular sections of the inlet structure, the flow path communicating with the inlet
through slots defined between the baffle and the upstream end of the inner tubular
section of the inlet structure. The baffle may incorporate a conical section expanding
outwards from the slots adjacent the upstream end of the inner tubular section of
the structure.
[0005] The provision of a cone shaped baffle of the form illustrated in British patent 2256460
does reduce the noise emitted from the annular flow path defined between the two tubular
sections of the structure and generally results in a reduction in the overall noise
level. In some operational circumstances however the noise level within the main inlet
flow passage is increased.
[0006] It is an object of the present invention to provide an improved MWE structure which
addresses the noise problems referred to above.
[0007] According to the present invention, there is provided a compressor comprising a housing
defining an inlet and an outlet, and an impeller wheel rotatably mounted in the housing
such that on rotation of the wheel gas within the inlet is moved to the outlet, the
housing having an inner wall defining a surface located in close proximity to radially
outer edges of vanes supported by the wheel, wherein the inlet is defined by a first
tubular portion an inner surface of which is an extension of the said surface of the
inner wall of the housing, a second tubular portion located radially outside the first
portion to define an annular passage between the first and second portions, and an
wall extending across the annular passage between the first and second tubular portions,
the wall being located between upstream and downstream ends of the first tubular portion,
sections of the passage on opposite sides of the wall communicating through at least
one aperture, and at least one aperture being defined adjacent the wheel in the said
surface of the inner wall of the housing to communicate with the annular passage
[0008] The wall which extends across the annular passage suppresses the propagation of noise
along the annular passage. Preferably the wall is located at or adjacent the position
of an anti-node of a noise wave which may be expected to propagate along the annular
passage during normal use of the compressor. The wall may be in the form of a simple
radially extending flange, or alternatively may extend in a direction inclined to
the radial direction, and may be shaped to define a helix or other configuration with
an axial component.
[0009] The inlet may comprise a wall defining an annular surface facing the annular passage
and extending outwards from adjacent the upstream end of the first tubular portion
to the upstream end of the second tubular portion, an aperture being defined between
the upstream end of the first tubular portion and the radially inner edge of the annular
surface. The annular surface may be frusto-conical, and may extend in the radially
outwards and upstream direction from adjacent the upstream end of the first tubular
portion.
[0010] Preferably the inlet comprises a wall defining a tubular surface extending in the
upstream direction from adjacent the upstream end of the first tubular portion. Such
a structure ensures that noise propagating in the upstream direction along the inlet
is subjected to a rapid expansion at the upstream end of the tubular surface. This
further reduces the noise output.
[0011] The wall extending across the annular passage may be in the form of flange extending
radially outwards from the first tubular portion, at least one aperture being defined
in radially outer portions of the flange adjacent the second tubular portion.
[0012] At least the first tubular portion and the wall extending across the annular passage
may be defined by a sub-assembly which is received within the second tubular portion.
The sub-assembly may be retained in position within the second tubular portion by
engagement between radially outer sections of the wall defining an annular surface
and indentations defined within the second tubular portion.
[0013] The invention also provides a compressor comprising a housing defining an inlet and
outlet, and an impeller wheel rotatably mounted in the housing such that on rotation
of the wheel gas within the inlet is moved to the outlet, the housing having an inner
wall defining a surface located in close proximity to radially outer edges of vanes
supported by the wheel, wherein the inlet is defined by a first tubular portion an
inner surface of which is an extension of the said surface of the inner wall of the
housing, a second tubular portion located radially outside the first portion to define
an annular passage between the first and second portions, a wall defining a surface
facing the annular passage and extending from adjacent the upstream end of the first
tubular portion to the upstream end of the second tubular portion, and a wall defining
a tubular surface extending axially in the upstream direction from the upstream end
of the first tubular portion, at least one first aperture being defined between the
downstream end of the wall defining the tubular surface and the upstream end of the
first tubular portion to communicate with the annular passage, at least one second
aperture being defined adjacent the wheel in the said surface of the inner wall of
the housing to communicate with the annular passage, and the surface facing the annular
passage being inclined to the radial direction.
[0014] An embodiment of the present invention will now be described, by way of example,
with reference to the accompanying drawings, in which:
Figure 1 is a schematic sectional view through a conventional inlet section of a turbocharger
compressor;
Figure 2 is a schematic sectional view of an inlet section of a known compressor provided
with a map width enhanced inlet:
Figure 3 is a schematic part-sectional illustration of a known compressor inlet section
incorporating a noise-reducing baffle;
Figure 4 is a part-sectional illustration of a compressor housing in accordance with
the present invention;
Figures 5 and 6 are perspective views of a baffle structure incorporated in the housing
illustrated in Figure 4;
Figure 7 is a section through the baffle illustrated in Figures 5 and 6;
Figure 8 illustrated the noise output obtained with an inlet structure as illustrated
in Figure 3, an inlet structure as illustrated in Figure 4, and an inlet illustrated
in Figure 3, an inlet structure as illustrated in Figure 4, and an inlet structure
of the type illustrated in Figure 4 after removal of a tubular portion of the structure
shown in Figure 4;
Figure 9 is a section through an alternative baffle structure which may be incorporated
in an embodiment of the present invention;
Figure 10 illustrates the noise output which results from using a baffle of the type
shown in Figure 9;
Figure 11 is a section through a baffle of the type shown in Figure 9 after removal
of an annular portion defining a conical surface; and
Figure 12 illustrates the noise output from the compressor inlet incorporating the
baffle of Figure 11.
[0015] Referring to Figure 1, the illustrated conventional inlet section of a compressor
is not provided with a map width enhanced structure. The illustrated structure comprises
a housing 1 a tubular inlet portion 2 of which defines an inlet passage 3 which tapers
in the downstream direction. The inlet communicates with a cavity defined within the
housing 1 within which an impeller wheel 4 is mounted to rotate about an axis indicated
by broken line 5. The wheel 4 supports vanes 6 the radially outer edges of which sweep
across an inner surface 7 defined by the housing 1.
[0016] As is well known, the conventional structure illustrated in Figure 1 is unstable
in certain operating conditions and in particular only operates satisfactorily over
a relatively limited range of impeller wheel flows. It is known to overcome this problem
by providing an MWE inlet structure of the type shown in Figure 2.
[0017] Referring to Figure 2, the same reference numerals are used as in Figure 1 where
appropriate. The inlet structure illustrated in Figure 2 comprises a tubular first
portion 8 an inner surface of which is an extension of the inner housing surface 7
and a tubular second portion 9 which is located radially outside the first portion
8 to defined an annular passage 10 between the first and second portions. Apertures
11 are formed through the housing at the downstream end of the tubular first portion
8, the apertures opening into the surface 7 defined by the housing. The radially outer
edges of the vanes 6 sweep across the surface 7 in which the apertures 11 are formed.
[0018] When the wheel 4 rotates, air is drawn in through the inlet passage 3 and delivered
to a volute 12. If the wheel 4 rotates at a high speed and flow condition, air is
drawn into the housing through the tubular first inlet portion 8 and through the annular
passage 10 and aperture 11. As the mass flow through the impeller wheel 4 falls however
the pressure drop across the apertures 11 falls and eventually reverses, at which
time the air flow direction in the annular passage 10 also reverses such that some
of the air entering the housing though the tubular first inlet portion 8 is recirculated
via the annular passage 10. In a well known manner this stabilises the operation of
the input stage of the compressor.
[0019] Referring to Figure 3, the illustrated inlet structure is as described in Figure
14 of published British patent specification number 2256460. The structure of Figure
3 is generally similar to that of Figure 2 except for the addition of a baffle located
upstream of the tubular first portion 8 within the tubular second portion 9. The baffle
is a frusto-conical annular structure defining a conical surface 13 and a tubular
portion 14 which is a tight fit within the tubular second portion 9 of the inlet structure.
A slot 15 is defined between the downstream end of the tubular surface 13 and the
upstream end of the tubular first portion 8 of the inlet structure.
[0020] Given the arrangement illustrated in Figure 3, pressure wave fronts propagating through
the apertures 11 in the annular passage 10 break out through the slot 15 into the
relatively high velocity air stream entering the tubular first portion 8 of the inlet
structure. As a result the overall output of noise from the assembly is reduced. Noise
output is also reduced due to the changes in direction of movement of the air stream
passing through the annular passage 10. It has been found however that with the known
structure of Figure 3, although the noise output is less than that with the conventional
MWE structure as illustrated in Figure 2, it is still greater than the noise output
of the conventional non-MWE structure illustrated in Figure 1.
[0021] Referring now to Figures 4,5,6 and 7, the structure of a first embodiment of the
present invention will be described. The illustrated embodiment comprises a tubular
first portion 16 within which a moulded plastics assembly is received, that assembly
incorporating elements which make up second, third, fourth and fifth portions of the
of the overall assembly. The second portion is in the form of a tubular portion 17
extending in the upstream direction from adjacent a slot 18, the functional purpose
of the slot 18 being the same as that of the slot 11 as described above with reference
to Figures 2 and 3. An annular passage 19 is defined between the tubular first portion
16 and the tubular second portion 17. The third portion is in the form of a wall 20
which extends radially outwards from the tubular second portion 17 across the passage
19. The fourth portion is in the form of a frusto-conical wall 21 which extends in
the radially outwards and upstream directions from the upstream end of the tubular
second portion to an inner surface of the tubular first portion 16. The angle of inclination
of the wall 21 relative to the radial direction could be reversed such that the surface
extends in the radially outwards and downstream directions. In both cases, the frusto-conical
surface suppresses noise across a range of frequencies. If the wall was radial, noise
suppression would occur only at one frequency. The fifth portion is in the form of
a tubular extension 22 of the tubular second portion 17. Slots 23 are formed between
the tubular second and fifth portions, the slots 23 performing the function of the
slot 15 as described with reference to Figure 3 above.
[0022] The wall 20 extends only part way across the annular passageway 17 but supports four
lugs 24 which bear the inner surface of the tubular first portion 16. Thus the tubular
passageway 19 is divided into two separate sections located on opposite sides of the
wall 20, the wall being in effect apertured as a result of the four slots defined
between each adjacent pair of lugs 24. Thus air flows through the annular passageway
19 between the slots 18 and 23 via the apertures defined in the wall 20. The direction
of flow of air through the annular passageway 19 is a function of the flow rate through
the inlet structure as a whole as is the case with any conventional MWE inlet structure.
[0023] The radially outer end of the conical fourth portion 21 supports four lugs 25 which
define radially projecting ribs that are received in an annular groove formed within
the tubular first portion 16.
[0024] Referring to Figure 8, this illustrates the performance in terms of output noise
for three different inlet structures. The upper full line trace represents the weighted
sound pressure level resulting from the operation of a turbocharger compressor having
an inlet structure as illustrated in Figure 3. The lower broken-line trace shows the
result of replacing the inlet structure of Figure 3 with the inlet structure as shown
in Figures 4 to 7. The intermediate full line trace represents the noise level recorded
using an inlet structure of the type illustrated in Figures 4 to 7 but modified by
removal of the fifth portion, that is the tubular extension 22. It will be noted that
structures as illustrated in both the modified and unmodified forms result in a substantial
reduction in output noise, particularly at the higher frequencies. The best performance
is obtained using the unmodified inlet structure as illustrated in Figures 4 to 7,
but significant improvements are also obtainable using the modified form of that inlet
structure, that is without the tubular extension 22.
[0025] It is believed that the presence of the apertured wall 20 (the third portion of the
inlet structure) significantly reduces the output noise as pressure waves travelling
along the annular passage 19 from the slot 18 encounter a reduction in cross-sectional
area in the passageway at the wall and then a sudden expansion in that cross-sectional
area. Ideally the wall 20 should be at the position of an antinode of noise wave passing
along the annular passageway 19, but the position of antinodes is a function of the
frequency of the noise in most applications. An antinode will be located at a distance
of one quarter of the wavelength of the noise wave as measured from the slot 18. This
frequency varies over a wide range during normal operation of most devices. Experiments
have shown that in applications where wide impeller speed (and hence frequency) variations
are expected the wall should be positioned approximately midway between the slot 18
and 23. In applications where sustained operation at a predetermined speed is expected,
the wall 20 is ideally placed at an antinode of the noise wave to be expected given
that operating speed.
[0026] As illustrated in Figure 8, the provision of the wall 20 in the otherwise conventional
structure results in a substantial reduction in noise output. A further improvement
is achieved by providing the tubular extension 22. It is believed that the inclusion
of such an extension is effective because a noise wave passing in the upstream direction
encounters a sudden expansion in the cross-sectional area of the passageway along
which it is transmitted when it reaches the upstream end of the extension 22. Although
not illustrated in Figure 8, providing the tubular extension 22 even in the absence
of the wall 20 provides some reduction in the noise output.
[0027] The inlet structure illustrated in Figures 5, 6 and 7 may be a single piece moulding
or may be an assembly of separately moulded pieces. Generally the assembly will be
moulded from plastics material although a metal structure could be used.
[0028] The lugs 24 provided on the wall 20 served the purpose of locating the integrally
moulded components within the compressor housing. The lugs do not have an aerodynamic
or noise reduction function however and can be omitted if alternative arrangements
are made to ensure the correct relative location of the various components. Tests
have been conducted after removal of the lugs 24 with no measurable increase in output
noise.
[0029] The inner diameter of the tubular extension 22 is shown to be slightly larger than
the inner tubular section 17. Differences between these diameters may affect noise
output and aerodynamic performance and selection of the appropriate diameters for
these components may be determined experimentally for specific applications. Similarly,
the outside diameter of the wall 20, that is the wall of the wall 20 without the lugs
24, may be optimised best by experimentation for specific applications.
[0030] It will be appreciated that the structure illustrated in Figures 5 to 7 could be
formed as an assembly of individual moulded components or cast components. For example
the wall 20 could be a separate component fitted onto the tubular portion 17. Similarly,
the tubular portions 16 and 17 could form part of an integral casting defining an
annular passageway into which an annular member defining the wall 20 could be inserted.
The conical wall 21 and tubular extension 22 could be formed as a single integral
casting or moulding.
[0031] Tests have been conducted to assess the importance of providing a conical surface
at the end of the annular bypass passageway remote from the impeller wheel. These
tests are described with reference to Figures 9 to 12.
[0032] Referring to Figure 9, the illustrated sub-assembly was mounted within a tubular
inlet to a compressor such that a radially outer surface 26 was engaged against the
radially inner surface of a tubular portion of the inlet, an end surface 27 formed
one side of a slot which was functionally equivalent to the slot 18 in the arrangement
of Figures 4 to 7, a conical wall 28 was functionally equivalent to the conical portion
21 of the structure shown in Figures 4 to 7, and a radial wall 29 was functionally
equivalent to the wall 20 of the arrangement of Figures 4 to 7. The assembly also
incorporated slots 30 which were functionally equivalent to the slots 23 of the arrangement
of Figures 4 to 7. In contrast the to the arrangement of Figures 4 to 7, the fifth
portion of the assembly which is upstream of the slots 30 is not tubular but rather
flares outwards towards the surface 26.
[0033] Figure 10 illustrates in full line the noise output from a conventional MWE compressor
of the type generally illustrated in Figure 2. It will be noted that the noise output
peaks significantly in the 4000 to 8000 hertz range. Figure 10 also shows in broken
line the performance of an MWE input structure incorporating the assembly illustrated
in Figure 9. It will be noted that across the frequency range the two traces overlap
but there is a significant reduction in noise output in the 4000 to 8000 frequency
range.
[0034] The assembly of Figure 9 was formed from three components, that is a flanged tube
defining the surfaces 26 and 27 and the slots 30, an annular ring of triangular cross-section
defining the conical surface 28, and an annular ring of rectangular cross-section
defining the wall 29. Tests were also conducted with a structure identical to that
of Figure 9 except for removal of the annular ring defining the conical surface 28.
Such a structure is shown in Figure 11 and the noise output from that structure is
shown in Figure 12.
[0035] Referring to Figure 12 the output of a standard MWE input structure is again shown
in full lines. The output from the structure illustrated in Figure 11 is shown in
broken lines. It will be noted that the performance of the device in accordance with
Figure 11 is worse than the performance of the device of Figure 9, particularly in
the 5000 to 7000 hertz range. This indicates that although there is some benefit obtained
simply by providing a wall 29 in the annular passage between the two slots of the
MWE structure, further benefits are obtained if the end of the annular passage remote
from the slots adjacent the impeller wheel is closed off with a conical surface.
[0036] The term "conical" has been used in this document to describe surfaces which are
truly frusto-conical. It will be appreciated that surfaces which are not truly frusto-conical
may also be used, including surfaces which are accurate. A frusto-conical surface
is very effective at suppressing oise at a predetermined frequency, and could be used
to particular advantage in an application in which the impeller speed is expected
to be constant such that noise is propagated at that predetermined frequency. A part-spherical
or part elliptical or other curved surface might be used however to better effect
in applications where variable impeller speed operation is expected.
1. A compressor comprising a housing defining an inlet and an outlet, and an impeller
wheel rotatably mounted in the housing such that on rotation of the wheel gas within
the inlet is moved to the outlet, the housing having an inner wall defining a surface
located in close proximity to radially, outer edges of vanes supported by the wheel,
wherein the inlet is defined by a first tubular portion an inner surface of which
is an extension of the said surface of the inner wall of the housing, a second tubular
portion located radially outside the first portion to define an annular passage between
the first and second portions, and a wall extending across the annular passage between
the first and second tubular portions, the wall being located between upstream and
downstream ends of the first tubular portion, sections of the passage on opposite
sides of the wall communicating through at least one aperture, and at least one aperture
being defined adjacent the wheel in the said surface of the inner wall of the housing
to communicate with the annular passage.
2. A compressor according to claim 1, wherein the wall extending across the annular passage
is located at or adjacent the position of an anti-node of a noise wave which may be
propagated within the annular passageway during use of the compressor.
3. A compressor according to claim 1 or 2, wherein the inlet comprises a wall defining
an annular surface facing the annular passage and extending outwards from adjacent
the upstream end of the first tubular portion to the upstream end of the second tubular
portion, an aperture being defined between the upstream end of the first tubular portion
and the radially inner edge of the annular surface.
4. A compressor according to claim 3, wherein the annular surface is frusto-conical.
5. A compressor according to claim 3 or 4, wherein the surface facing the annular passage
extends in the radially outwards and upstream directions from adjacent the upstream
end of the first tubular portion.
6. A compressor according to any preceding claim, wherein the inlet comprises a wall
defining a tubular surface extending in the upstream direction from adjacent the upstream
end of the first tubular portion.
7. A compressor according to any preceding claim, wherein the wall extending across the
annular passage is in the form of a flange extending radially outwards from the first
tubular portion, at least one aperture being defined in radially outer portions of
the flange adjacent the second tubular portion.
8. A compressor according to any preceding claim, wherein at least the first tubular
portion and the wall extending across the annular passage are defined by a sub-assembly
which is received within the second tubular portion.
9. A compressor according to claim 8 as dependent upon claim 2, wherein the wall defining
an annular surface is defined by the sub-assembly and radially outer portions of the
wall defining the annular surface are received in indentations defined within the
second tubular portion to secure the sub-assembly in position.
10. A compressor comprising a housing defining an inlet and an outlet, and an impeller
wheel rotatably mounted in the housing such that on rotation of the wheel gas within
the inlet is moved to the outlet, the housing having an inner wall defining a surface
located in close proximity to radially outer edges of vanes supported by the wheel,
wherein the inlet is defined by a first tubular portion an inner surface of which
is an extension of the said surface of the inner wall of the housing, a second tubular
portion located radially outside the first portion to define an annular passage between
the first and second portions, a wall defining a surface facing the annular passage
and extending from adjacent the upstream end of the first tubular portion to the upstream
end of the second tubular portion, and a wall defining a tubular surface extending
axially in the upstream direction from the upstream end of the first tubular portion,
at least one first aperture being defined between the downstream end of the wall defining
the tubular surface and the upstream end of the first tubular portion to communicate
with the annular passage, at least one second aperture being defined adjacent the
wheel in the said surface of the inner wall of the housing to communicate with the
annular passage, an the surface facing the annular passage being inclined to the radial
direction.
11. A compressor according to claim 10, wherein the surface facing the annular passage
is frusto-conical.
12. A compressor according to claim 10 or 11, wherein the surface facing the annular passage
extends in the radially outwards and upstream directions from adjacent the upstream
end of the first tubular portion.