FR2877391A1 - RESONATOR OF HELMHOLTZ AND EXHAUST LINE COMPRISING IT - Google Patents

Abstract

The Helmholtz resonator (18) comprises a closed chamber (20) opening only through a neck (22) connecting to a pipe (14). It comprises: - a transducer (24) placed in the closed chamber (20) and suitable for converting acoustic energy into electrical energy; and- an electrical load impedance (26) electrically connected to the terminals of the transducer (24). Application to an exhaust line

Description

2877391 1

  The present invention relates to a Helmholtz resonator for an exhaust line, of the type comprising a closed chamber opening only by a connecting neck to a line of the exhaust line.

  Motor vehicles with a combustion engine cause, due to the operation of the engine, noise pollution. The engines are, for this purpose, equipped with exhaust lines comprising exhaust silencers through which the exhaust gas flows, as well as Helmholtz resonators consisting of a closed chamber stitched on the exhaust pipe in which circulate the gases via a reduced section connecting neck through which the closed chamber of the resonator opens in the exhaust pipe.

  The Helmholtz resonator constitutes an acoustic filter which is tuned to a particular frequency given by its dimensions.

  Thus, the tuning frequency Fo is given by Fo = c 27z s while lV its bandwidth is given by B, = Q with: c = sound velocity (mis) s = section of the resonator neck (m2) 1 = length of the resonator neck (m) V = volume of the resonator (m3) Q = quality coefficient.

  The resonance frequency Fo is given by the limit at infinity of the resonator background impedance brought back to the mouth of the neck.

  Such a resonator is effective but it is only granted on a frequency initially chosen and its bandwidth is narrow. Since the engine speed is variable, the frequencies to be filtered extend over a very wide range of frequencies, which a known Helmholtz resonator is not likely to attenuate throughout its range.

  The object of the invention is to propose a Helmholtz resonator and an exhaust line comprising it making it possible to attenuate the noise pollution produced by a heat engine over a wide range of frequencies.

  To this end, the subject of the invention is a Helmholtz resonator of the aforementioned type, characterized in that it comprises: a transducer placed in the closed chamber and suitable for converting an acoustic energy into an electrical energy; and an electric charge impedance electrically connected to the terminals of the transducer.

  According to particular embodiments, the Helmholtz resonator comprises one or more of the following characteristics: it comprises means for collecting the engine speed and means for modifying the electric load impedance as a function of the engine speed; the load impedance comprises at least two elementary impedances, and it comprises means for modifying the load impedance which comprise switching means adapted to ensure a selective connection of the elementary impedances across the transducer, and means controlling switching means; the at least two elementary impedances are chosen from the group consisting of a negative resistance, a wire and a parallel mounting of an inductance and a capacitor; the modifying means are capable of modifying the impedance according to a piecewise constant function as a function of the engine speed; the transducer is chosen from the group consisting of an electro-acoustic loudspeaker comprising a membrane and an electromagnet and a piezoelectric membrane element; the load impedance comprises an inductance whose value is greater than 10 Henry; the carrying capacity has a capacity whose value is greater than 10 Farads; and the load impedance comprises a negative resistance of value equal to the opposite of the value of the resistance of the transducer.

  The invention also relates to an exhaust line on which is stitched a Helmholtz resonator as described above.

  The invention will be better understood on reading the description which will follow, given solely by way of example, and with reference to the drawings, in which: FIG. 1 is a schematic view of a line of FIG. escapement 5 equipped with a Helmholtz resonator according to the invention; FIG. 2 is an electrical diagram of the variable electrical impedance implemented in the Helmholtz resonator of FIG. 1; and FIG. 3 is a graph showing the attenuation in decibels of noise as a function of frequency by a resonator according to the invention.

  In FIG. 1 is illustrated a motor 10 connected to an exhaust line 12. This exhaust line comprises, as known per se, an exhaust duct 14 connected to the engine output and the length of which is provided a muffler 16 traversed through the exhaust gases from an inlet 16A to an outlet 16B. The exhaust line further comprises a Helmholtz resonator 18 connected to the pipe 14 through a stitch formed thereon.

  The Helmholtz resonator comprises a closed chamber 20 formed, for example, of a metal box. This closed chamber opens only by a single neck 22 connecting to the exhaust pipe 14. The chamber 20 has, for example, a cylindrical side wall closed on one side by a solid bottom and, the other side, by a solid wall through which opens the neck 22. The neck 22 is formed of a section of conduit much smaller than that of the chamber 20 and of predetermined length.

  According to the invention, a transducer 24 is disposed in the closed chamber 20. This transducer is, as known per se, adapted to transform a mechanical quantity into an electrical quantity. In this case, the transducer is able to transform acoustic waves into an electric current.

  According to a first embodiment, the transducer is formed of an electro-acoustic loudspeaker comprising a membrane and an electromagnet.

  According to another embodiment, the transducer is formed of a piezoelectric membrane element, commonly designated by the acronym PVDS for "Polyvinylidene fluoride". The Helmholtz resonator further comprises an impedance 26 electrically connected across the transducer 24 to form a closed electrical circuit. This impedance 26 is preferably variable.

  Thus, the Helmholtz resonator comprises an information processing unit 28 adapted to ensure the modification of the value of the variable impedance 26. This information processing unit is connected to a motor speed sensor 30 placed, for example , on the crankshaft of the engine.

  The speed sensor 30 is formed for example of a frequency / voltage converter.

  Generally speaking, in such an installation, the transducer 24 constitutes a mechanical impedance Zm expressing itself in the form: Zn, = f + j (mw K) w where: f = the viscous damping of the membrane; m = mass of the membrane; k = stiffness; and w = pulsation.

  The electrical impedance of the loudspeaker is given by ZHP RHP + jLHPw.

  The impedance Ze resulting from the combination of the electrical impedance of the loudspeaker ZHP and the electrical impedance 26 whose value is denoted Zext is given by Ze = Zex1 + ZHP = R + jX.

  The mechanical impedance Z'm of the transducer charged with the electrical impedance Ze is given by: Zm = f + j (mw K) + B212 w Ze which then gives: B212R K B212X = f + 2 + j (mw R + X w R2 + x2).

  Depending on the values chosen for the electrical impedance 26 of value Zext, the mechanical impedance Z 'can be considerably modified.

  The following tables illustrate several cases of electrical impedance associated with the transducer.

  Cases 1 to 6 correspond to the following forms for impedance 26: Case 1: open-loop transducer; Case 2: transducer shorted; Case 3: impedance formed of a negative resistance; Case 4: impedance formed of a negative resistance in series with a Lext inductance coil; Case 5: impedance formed of a negative resistance in series with a capacity capacitor Cext; Case 6: impedance formed of a negative resistance in series with a Lext inductance coil and capacitor capacitance Cext. such

O

  3 4 5 6 Case 1 2 1 j Zext = ao Zut = 0 Z ext = R Ze = R * + JLertw z e = R Ze = R + J (Lext) is j

C W ext

  ff f f BZlzRHP f + 2 2 w2 RHP + LHP n n1 B212C 2 2 B l Cext 111 t 111 B212L HP 711 112 2 2 L C 0) 2 1 (LHP + LeYt) CextW -1 HP ex [711 2 + LHP

RHP

  K K K + B212 B2 K K K + (LHP + Lext)

LHP

  Table 1: Different possible general cases and corresponding expressions of mechanical parameters J (Lextw) CeG1 = R

O D o

CL

CD CD

CD C1 CU C /)

CD Cf) C7 C /) iv o o

C a) D cn

+ ILexr Po B212 1- (1)

RHP e = R = 0

111 B21LHP (1) RH2P ln B 212 +

LHP nl

  B212 K + (LHP + Lezr) n1 LHPCeztw2> -> - m + B212Cext LHPCextw2 - 1 B212 K +> LHPCestW 2> -5- 1, LHPCe CV 2 -e- <1 i ableau 2: Different real cases and expressions of mechanical parameters associated with K + B212 (LHP + Lext) Cex, a) 2 i5-1 (Lep + Lext) K (LHP Lam,) C, w2 - <1

K Lep

  In FIG. 2 is shown an exemplary embodiment of a variable impedance 26. This is shown in FIG. 2 (FIG. 2, FIG. It comprises a set of elementary impedances associated with switching means for the selective connection of the elementary impedances under the control of a control unit 32 connected to the information processing unit 28.

  More specifically, the impedance 26 comprises a module 34 consisting of an inductance connected in parallel with a capacitance. The value of the inductance advantageously has a value greater than 10 Henry and preferably greater than several tens of Henry. Likewise, the capacitance advantageously has a value greater than 10 Farads and, preferably, greater than several tens of farads. This module 34 is connected in series with a negative resistor 36 between the terminals of the transducer 24.

  A controlled switch 38 is connected in parallel with the module 34. Likewise, a second negative resistor 40 associated in series with a controlled switch 42 is connected in parallel with the module 34.

  Finally, a controlled switch 44 is connected in parallel with the negative resistor 36.

  Negative resistors are formed, as known per se, from operational amplifiers. Preferably, the value of the negative resistance is the opposite of the value of the RHF resistance of the transducer.

  The control means 32 are adapted to ensure the opening or closing of the controlled switches 38, 42, 44 as a function of the engine speed read by the sensor 30, for example according to the decoding table shown below: 38 42 44 before 1st switching Open Open Open from 1st to 2nd commutation. Closed X Closed from the 2nd to the 3rd commutation Open Closed Closed after the 3rd commutation Open Open Open Table 1: Decoding table Thus, for frequencies lower than the first switching frequency, for example equal to 20 Hz, the transducer 24 is connected from its terminals to the negative resistor 36 connected in series with the module 34 consisting of inductance and capacitance in parallel.

  From the first switching frequency to the second frequency equal, for example, to 80 Hz, the electrical load impedance of the transducer 24 is a short circuit.

  From the second switching frequency to the third switching frequency, for example equal to 100 Hz, the load impedance is formed of the negative resistance 40 connected in parallel with the module 34 constituted by an inductance and a capacitance of parallel.

  For motor speeds above the third frequency, the load impedance is formed, as for the frequencies lower than the first switching frequency, by the negative resistance 36 in series with the module 34.

  FIG. 3 represents, on the same graph, the three acoustic attenuation curves obtained as a function of frequency with the three types of load impedance implemented under the control of the control means 32.

  Thus, the curve Il corresponds to the attenuation obtained when the load impedance is the negative resistance 36 in series with the module 34, the curve 12 corresponds to the attenuation obtained when the load impedance is a short-circuit, and the curve 13 corresponds to the attenuation obtained when the load impedance is the negative resistance 40 connected in parallel with the module 34.

  Curve 14 illustrates the attenuation obtained by modifying the load impedance as a function of the engine speed, this switching taking place at the three switching frequencies indicated.

  It can be seen that the addition of variable electrical impedances switched to the desired frequencies makes it possible to raise the attenuation levels in a large proportion starting from 80 Hz. The gain is 13 dB for example at about 130 Hz.

  Such a Helmholtz resonator can be advantageously used in an exhaust line of a motor but also in an intake line of a motor.

2877391 9

Claims (9)

  1.- Helmholtz resonator (18) comprising a closed chamber (20) opening only by a neck (22) for connection to a pipe (14), characterized in that it comprises: - a transducer (24) placed in the closed chamber (20) and suitable for converting acoustic energy into electrical energy; and an electric load impedance (26) electrically connected across the transducer (24).   2.- Helmholtz resonator according to claim 1, characterized in that it comprises means (30) for collecting the engine speed and means for modifying (28) the electric load impedance (26) according to the regime of the engine (30).   3. Helmholtz resonator according to claim 1 or 2, characterized in that the load impedance (26) comprises at least two elementary impedances (34, 36, 40), and in that it comprises means (28) for changing the load impedance (26) having switching means (38, 42, 44) for selectively connecting the elementary impedances (34, 36, 40) across the transducer (24) , and means (32) for controlling the switching means (38, 42, 44).   4. Helmholtz resonator according to claim 3, characterized in that the at least two elementary impedances are selected from the group consisting of a negative resistance (36, 40), a wire and a parallel mounting (34) of an inductor and a capacitor.   5.- Helmholtz resonator according to claim 2, 3 or 4, characterized in that the modifying means (28) are adapted to modify the impedance according to a piecewise constant function according to the engine speed (30).   6. Helmholtz resonator according to any one of the preceding claims, characterized in that the transducer (24) is selected from the group consisting of an electro-acoustic loudspeaker comprising a membrane and an electromagnet and a piezoelectric element with membrane.   7. Helmholtz resonator according to any one of the preceding claims, characterized in that the load impedance (26) comprises an inductance (34) whose value is greater than 10 Henry.   8. Helmholtz resonator according to any one of the preceding claims, characterized in that the load capacitance (26) comprises a capacitance (34) whose value is greater than 10 Farads.   9. Helmholtz resonator according to any one of the preceding claims, characterized in that the load impedance (26) comprises a negative resistance (36, 40) of equal value opposite to the value of the transducer resistance.   10.- Exhaust line comprising an exhaust pipe (14) on which is stitched a Helmholtz resonator (18), characterized in that the Helmholtz resonator (18) is according to any one of the preceding claims.