The present invention relates to a low-noise integrated air-filtering
As is known, noise reduction in reciprocating-type engines, especially
internal-combustion engines for vehicles for non-military use, is a requirement
that is assuming an ever-increasing importance. The said noise is mainly caused
by pressure waves that are generated on account of the reciprocating motion of the
pistons in the cylinders and that propagate along the air-intake and exhaust-gas
Consequently, in order to achieve the aim, silencer devices, such
as candle-like perforated elements, are currently employed, which enable conversion
of part of the energy associated to the pressure waves into heat. Normally, the
silencer devices are set along the exhaust pipe and contribute to reducing considerably
the overall noise level of the engine (see for example GB 1 507 247, JP 6 048 386,
US 2 553 326).
Frequently, however, this is not sufficient, and it necessary to adopt
additional solutions. In particular, it is possible to add further silencer elements
also along the air-intake pipe, for example upstream of the air-intake filter.
The known solutions, however, present a number of drawbacks, which
are linked mainly to the bulk, in so far as the silencer elements currently available
must be inserted externally to the air-intake filter, and which are linked to the
characteristics of noise-deadening of the silencer device, which can be made by
assembling distinct elements together.
As is known to a person skilled in the art, in fact, the performance
of a silencer device is markedly affected by the geometry both of the device itself
and of the flow of air that conveys the noise that is to be attenuated. On the other
hand, the use of distinct silencer elements that are assembled along the air-intake
pipe does not enable an optimal geometry, and hence noise is reduced only partially.
Furthermore, it is not possible to modify either the dimensions or
the noise-attenuation characteristics of the individual elements that form the silencer
device, which consequently is not suitable for being used on engines that are different,
for example, in terms of displacement or in terms of other constructional features.
It is thus necessary to provide different elements according to the type of engine
on which the said elements are to be used, and this entails high production costs.
The purpose of the present invention is to provide a low-noise integrated
air-filtering device which enables the above-mentioned drawbacks to be overcome
and which, moreover, is of simple and inexpensive implementation.
Provided in accordance with the present invention is a low-noise integrated
air-filtering device according to claim 1.
In this way, the device is not only compact and of reduced overall
dimensions, but can also be built with an optimal geometry which enables noise abatement
in an extremely efficient way. In particular, the use of a resonator element and
a damping element, which substantially operate in contiguous frequency bands, makes
it possible to achieve a high damping effect over a wide spectrum of frequencies.
In addition, the device forms a single body which can be conveniently
mounted on different engines as a replacement for the traditional air-intake filter.
The linear geometry and the symmetry of the pipe with respect to a
barycentric axis of symmetry enable effective reduction of the undesired effects
of resonance due to the transverse modes of propagation of the pressure waves, and
thus enable a further improvement in noise deadening.
For a better understanding of the present invention, an embodiment
thereof will be described hereinafter, purely by way of non-limiting example and
with reference to the attached drawings, in which:
- Figure 1 is a simplified diagram of an integrated device according to the present
invention, in a longitudinal cross-sectional view; and
- Figure 2 presents plots of quantities regarding the device of Figure 1.
With reference to Figure 1, a low-noise integrated air-filtering device,
designated as a whole by 1, comprises a casing 2, which has a longitudinal axis
A of symmetry, an inlet pipe 3 and an outlet pipe 4, which are coaxial to the longitudinal
axis A. Housed inside the casing 2 are a filter cartridge 5, of a type in itself
known, and a silencer device 7, which includes at least one resonator element 8
and one first damping element 10. In detail, the resonator element 8, the first
damping element 10 and the filter cartridge 5 are set in series together and form
an axial sequence, in which the resonator element 8 and the first damping element
10 are set upstream of the filter cartridge 5.
According to the present invention, the longitudinal axis A of symmetry
is also a barycentric axis of the device 1. In addition, the resonator element 8,
the first damping element 10 and the filtering cartridge 5 define a linear conduit
11 coaxial to the longitudinal axis A of symmetry. In particular, the linear conduit
11 is traversed by a flow of air sucked in towards the engine (not shown). The said
flow of air conveys pressure waves which are generated by the engine itself during
its normal operation and which are the source of the noise that is to be attenuated.
Preferably, the resonator 8 is an in-line Helmholtz resonator and
has a neck 12, which has an adjustable length L, and a volume V. In this way, the
resonator element 8 is particularly suited for attenuating noise in a medium-to-low
frequency band, up to approximately 300 Hz. In addition, the frequency of maximum
damping can be adjusted, as will be explained hereinafter.
In detail, the neck 12 of the resonator element 8 has an annular shape
and is defined comprised between an outlet stretch 3a of the inlet pipe 3 and a
first stretch 11a of the linear conduit 11.
In particular, the outlet stretch 3a of the inlet pipe 3 is inserted,
in an axially slidable way, inside the first stretch 11a of the linear conduit 11.
The axial position of the inlet pipe 3 with respect to the linear
conduit 11 (and hence the length L of the neck 12) can be adjusted by means of an
actuation device, comprising, for example, a rack 13, carried integrally by the
inlet pipe 3 and set longitudinally, and a gear 14, driven by a motor, of a known
type and not illustrated.
Optionally, a diaphragm 15 can be inserted inside the casing 2 in
order to reduce by a pre-set amount the volume V of the resonator element 8.
The first damping element 10 is a candle-like perforated element with
low density of perforation, for attenuation of the noise in a medium-to-high frequency
band, up to approximately 900 Hz. For example, the density of perforation is between
approximately 4% and 5%.
An annular region, which is defined between the casing 2 and the first
damping element 10 and which moreover is axially delimited by a first wall 17a and
a second wall 17b, forms an expansion chamber 17, which contributes to attenuating
the noise generated by the engine, as will be explained later on with reference
to Figure 2.
According to a preferred embodiment of the present invention, the
integrated device 1 comprises a second damping element 18, set inside the casing
2, downstream of the filter cartridge 5. In addition, the second damping element
18 is coaxial to the longitudinal axis A of symmetry and is connected to the outlet
pipe 4. In particular, the second damping element 8 is a candle-like perforated
element with high perforation density for noise damping in a high-frequency band,
substantially with frequencies higher than 600 Hz.
Figure 2 shows damping curves of the resonator element 8, of the first
damping element 10 and of the expansion chamber 17 in a frequency band of between
0 and 1000 Hz. In detail, the damping curve for the resonator element 8 is illustrated
with a solid line; the damping curve for the first damping element 10 is illustrated
with a dashed line; and the damping curve for the expansion chamber 17 (i.e., due
exclusively to a sharp variation in the section of the pipe in which the air flows)
is illustrated with a dashed-and-dotted line.
As mentioned previously, when the engine on which the integrated device
1 is operating, the inlet pipe 3, the linear conduit 11 and the outlet pipe 4 of
the integrated device 1 itself are traversed by a flow of air in which substantially
periodic pressure waves, which are a source of noise, propagate.
The noise is mainly attenuated by the resonator element 8 and by the
first damping element 10. The integrated device 1, as a whole, is particularly effective
in damping transverse modes of propagation of the pressure waves. As is known to
a person skilled in the art, the said result can be obtained when the flow of air
develops substantially about a barycentric axis of the damping device (in the case
of the integrated device 1, the flow of air develops substantially about the longitudinal
axis A of symmetry, which is a barycentric axis). In this way, in fact, it is possible
to shift secondary resonance frequencies present in the damping and air-filtering
devices towards high frequency values, namely ones that are outside the spectrum
of frequencies of the pressure waves that generate noise. The said secondary resonance
frequencies are not therefore excited, and undesired resonance effects are thus
In addition, the maximum attenuating frequency of the resonator element
8 can be adjusted. In an in-line Helmholtz resonator, such as the resonator element
8, the said maximum frequency of attenuation corresponds, in fact, to the characteristic
resonance frequency FR given by the following equation:
FR = 1 / (2π) sqrt(C2S / (LEFFV))
where C is the speed of sound, S is the area of a radial section of the neck 12
of the resonator element 8, and LEFF is the effective length of the neck
12. The said effective length LEFF is in turn defined, to a first approximation,
by the following expression:
LEFF = L + 0.8 sqrt(S)
Clearly, the possibility of varying the axial position of the inlet
pipe 3 with respect to the linear conduit 11 enables adjustment of the length L
of the neck 12 of the resonator element 8 and, consequently, also its characteristic
frequency of resonance FR.
It is moreover evident from equation (1) that the characteristic frequency
of resonance FR can be modified also by varying the volume V of the resonator
element 8. For this purpose, as mentioned previously, it is possible to insert,
inside the casing 12, the diaphragm 15, which reduces the volume V by a pre-set
amount. In this way, the integrated device 1 can be readily adapted to the noise
characteristics of various engines.
In particular, the sequence of the elements inside the casing 2 may
be different from the one illustrated. For example, the resonator element 8 and
the first damping element 10 may be set downstream of the filter cartridge 5; on
the other hand, the second damping element 18 may be set upstream of said filter