Muffler with variable sound-absorbing characteristics

Details Muffler with variable sound-absorbing characteristics Muffler with variable sound-absorbing characteristics

2000-06-29

2002-12-31

Nappi, Robert E. (Department: 2837)

Acoustics

Sound-modifying means

Muffler, fluid conducting type

C181S254000, C181S272000, C181S275000

Reexamination Certificate

active

06499562

ABSTRACT:

CROSS REFERENCE TO RELATED APPLICATIONS
None.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
None.
REFERENCE TO MICROFICHE APPENDIX
None.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a muffler and more particularly to mufflers for pulsating gases, especially exhaust gases of internal combustion engines with variable sound absorbing characteristics!
2. Description of Related Art including information disclosed under 37 CFR 1.97 and 1.98.
The invention relates to a muffler according to the preamble of claim 1. It therefore relates to mufflers for pulsating gases, especially exhaust gases of internal combustion engines, with variable sound-absorbing characteristic.
Legal noise reduction requirements are being introduced increasingly in all walks of life. This is particularly true for motor vehicles powered by internal combustion engines. The considerable improvement of sound absorption required for this application, even in the low speed range, is leading to longer flow paths inside the muffler system and thus to greater flow resistances. The greater energy expenditure must be supplied by the engine. To counter these drawbacks, the flow cross sections available to the pulsating gas must be increased, but the costs for the exhaust-gas system are greater for larger muffler volume. These circumstances are inconsistent with the challenge to the automobile manufacturers, which is to lower costs and decrease the size of the installation space.
One approach to resolving this conflict of objectives was proposed in the paper entitled “Active sound absorption—Possibilities for variable modulation of discharge noise” [in German] (MTZ Motortechnische Zeitschrift 53 (1992) No. 7/8 p. 3). It relates to a rear muffler with two tailpipes. A closure element in the form of a throttle valve is disposed in one of the two tailpipes. This valve is closed when the engine is running at low speed. The exhaust gas flows along a longer flow path. Part of the muffler volume acts as a Helmholtz chamber. When the closure element is open, the exhaust gas flows through both tailpipes. The ratio of the pressure losses, frequently also known as backpressures, with the closure element closed and open corresponds approximately to the square of the reciprocal of the ratio of the free flow areas. The closure element is connected via a crank mechanism and a rod to a pressure cell as the actuating element. This negative-pressure cell is actuated via a control system comprising solenoid valve, vacuum accumulator, nonreturn valve and connecting lines by the intake section of an internal combustion engine, where negative pressure prevails. A control unit which evaluates the engine speed and the throttle-valve position acts on the solenoid valve.
Numerous further proposed solutions for mufflers with variable sound-absorbing characteristic have been formulated. They can be divided into two main groups, which are referred to as externally controlled and autonomous solutions depending on whether the signal for controlling the closure element originates from outside or from inside the muffler.
The example described in the introduction and those of DE-OS 4416739 and DE-OS 3835079 correspond to externally controlled solutions. Such solutions are also found as shutoff valves in other portions of an exhaust-gas system (see, for example, DE 19630164 A1 or DE 9413493 U1). These solutions have the advantage that switching processes can be initiated highly selectively and operate relatively independently of the flow processes in the muffler. A disadvantage of externally controlled solutions is that the switching element can be disposed for the most part only on one tailpipe, outside the muffler. In view of its heat resistance, the pneumatically operated actuating element has only a limited service range. A further critical disadvantage is found in the large number of necessary components: 1. The control system comprises solenoid valve, vacuum accumulator, nonreturn valve and connecting lines. 2. The actuating element is assembled from a diaphragm, housing and spring. 3. A gear mechanism converts the reciprocating motion into pivoting motion. 4. The closure element comprises a shaft, bearing arrangement and gasket ring. In addition, a secondary energy source such as a pneumatic system is needed.
To overcome these disadvantages, autonomous solutions have been proposed. These use selected flow parameters inside a muffler in order to initiate the switching process.
Directly controlled systems rely on the concept, known from reciprocating pump and compressor design, of directly activating valves by fluid flow. Thus they directly utilize the gas stream to be influenced for actuation of the closure element; the actuating element and closure element are combined. Pertinent prior art can be found in particular in DE 19729666 A1, WO 95/13460, DE 19520157 A1, DE 19720410 A1, DE 19540716 C1, DE 19503322 A1, U.S. Pat. Nos. 5,821,474, 5,801,343, 5,739,483, 5,723,827, 5,709,241, 5,708,237, 5,614,699, 4,971,166, 4,484,569, EP 0902171 A1, DE 9207838.9 U1, DE 9405771.0 U1, DE 9406200.5 U1, DE 29803183 U1. Usually spring elements (WO 95/13460) or/and magnets (DE 19520157 A1) are used to generate the restoring forces. The proposed solutions have a critical disadvantage: in general they do not permit stable operating behavior.
When the closure element in one of the parallel flow paths is closed, a relatively large pressure drop develops, depending on the resistance of the free flow path. If the force from the pressure difference is larger than the force of the spring holding the closure element closed, the closure element opens the second flow path. The differential pressure decreases immediately the further the second flow path is released. The opening force vanishes. The spring opposing the force of pressure causes the previously released flow path to close. The useful life of the system is adversely influenced by these unstable switching conditions.
Even systems with all components involved favorably matched, such that the closure elements can occupy intermediate positions depending on flow condition, are not free of disadvantages. In contrast to the externally controlled systems, they must satisfy the prerequisite of a well defined pressure drop in order that a switching process can be initiated. This also explains their poorer acoustic effect, however, compared with externally controlled systems. Bistable switching positions are difficult to adjust. The variability of the acoustic system is limited. In addition, some of the proposed solutions are technically complex.
A more advantageous solution appears to be one proposed in DE 19619173 C1. The incoming flow to the muffler takes place via a venturi nozzle. A negative pressure is generated in the narrowest cross section and, via connecting lines, acts on one side of a pneumatic switching element. The pressure in the inlet conduit upstream from the venturi nozzle acts on the other side. The pressure difference causes opening of the closure element.
Since on the one hand the total mass flow of the incoming flow is used to establish the negative pressure in the venturi nozzle and on the other hand the positive pressure in the inlet conduit upstream from the venturi nozzle is used to open the closure element, although the closure element releases precisely this positive pressure region as the flow path, this solution can be classified among the semi-directly acting systems. This classification will become clearer upon closer examination of the mechanism of action. Upon release of the second flow path upstream from the venturi nozzle, the total pressure difference vanishes, as is necessary to accelerate the exhaust gas to a velocity sufficient to generate negative pressure in the venturi nozzle.
Even in this solution the instability explained hereinabove is not eliminated. Furthermore, extremely high velocities must be achieved in the venturi nozzle in order to establish the necessary negative pressure. As is known, high velocities lead to

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