A dynamically tuned exhaust system

Abstract: A method and apparatus for muffling motor vehicle engine noise comprises conventional noise attenuation apparatus and techniques as well as a tuning circuit for delivering separate signal outputs to at least two transducers (30,32) separated along the length of the exhaust conduit (12). A spacing between the transducers (30,32) corresponds to the range of tuned exhaust lengths required at various engine speeds within the operating range of the motor vehicle engine. The tuning circuit (22,24) varies the strength of the signal delivered to each of the transducers to vary the cancellation point within the conduit (12) so as to vary the effective length of the conduit (12) in a manner that maintains a minimum or negative pressure pulse at the valve exhaust port and aids in the exhaustion of combustion gases from the cylinder. As a result, engine performance is improved throughout the operating range of the motor vehicle engine.

Description: The invention relates to a dynamically tuned exhaust system and a method of dynamically tuning a motor vehicle exhaust conduit. In the operation of internal combustion engines, the opening of exhaust valves releases the expanding combustion products from the engine cylinder into an exhaust conduit. Since the combustion generates a substantial amount of noise, the repeated opening and closing of the valves induces a sound pressure pulse train through the conduit. Conventionally, the noise generated is suppressed by a passive muffler system in which the sound waves are broken up by baffles, passageways and the like or absorbed in a sound-deadening material. However, such devices and techniques for reducing the sound level also obstruct the free flow of exhaust gases through the exhaust conduits. As a result, such mufflers interfere with efficient operation of the vehicle engine. Even where such mufflers have not been used for the sake of performance, the length of the conduit has been found to play an important role in engine operating efficiency. For example, in a multi-cylinder engine, an exhaust conduit for each of the cylinders may be coupled to a common collector. The length of each of the exhaust pipes leading to the collector will be determined so that pulses reaching the collector do not induce a reactive pulse of improper phase within a conduit for another cylinder which could interfere with the release of the exhaust gases from that conduit. If positive pressure pulses arrive at the valve port for the other cylinder when the valve is open, they can impede efficient release of the exhaust gases. Conversely, a negative pressure pulse at the valve port when the valve is opened aids in the extraction of exhaust gases from the cylinder. The timed arrival of the pulses is related to the frequency of the pulse train and the length of the conduit. Consequently, a fixed length of pipe will provide optimum performance at substantially a single engine rpm and the advantage diminishes within a small range of engine rpm. Thus, the fixed length is often selected to provide optimum performance at an engine speed providing maximum torque. For example, the engine speed chosen may be related to the shift point for changing gears in the transmission. In exhaust systems where a single exhaust conduit is provided for each exhaust port, the release of the pulse train at the end of the conduit can induce a negative standing wave reaction which must be timed to arrive at the valve port so that it enhances rather than obstructs the release of combustion gases from the cylinder through the exhaust port. Again, since the arrival time of the pulse is dependent upon the frequency of the pulse and the length of the conduit through which it travels, it will be understood that the optimum performance of a fixed length conduit is limited to a particular engine rpm, and the efficiency decreases outside of a small range. It is expected that similar phenomena will be encountered in the application of noise attenuation techniques in active muffler systems. In particular, the introduction of cancellation pulses to counteract generated pulses within an exhaust conduit does not actually eliminate the energy of the pulses which cancel each other. Rather, a significant portion of the energy not removed as heat forms a standing wave transmitted toward the source of the sound pressure pulses at the valve port. Accordingly, proper timing of the reaction pulses so that the reaction waves produce a negative pressure when the exhaust valve is opened would enhance extraction of the combustion gases from the cylinder. Proper timing of this effect depends upon the position at which the cancellation signals are introduced. However, the transducers of previously known noise cancellation systems are fixedly positioned with respect to the conduits in which they act. Although the previously known electronic controls include adaptive filters for adjusting to the changing sound pressure pulse frequencies, they do not affect the reaction wave phenomena discussed above. Moreover, since the exhaust pipes have a fixed length, any reactive waves induced would provide enhanced cylinder evacuation results only at or very close to a single predetermined frequency. U.S. Patent No. 4,473,906 to Wanaka et al discloses numerous prior art sound attenuation system embodiments. The patent discusses the inclusion of additional transducers and electronic controls to improve the performance of the active acoustic attenuator, by reducing the effect of the feedback of the cancellation signal which arrives at the sensor. U.S. Patent No. 4,677,677 to Erickson further improves attenuation by including an adaptive filter with on-line modelling of the error path and the cancelling speaker by using a recursive algorithm without dedicated off-line pretraining. U.S. Patent No. 4,677,676 adds a low amplitude, uncorrelated random noise source to a system to improve performance. Likewise, U.S. Patent Nos. 4,876,722 to Decker et al discloses transducer placement at the transverse centre of a duct. U.S. Patent No. 4,783,817 to Hamada et al discloses transfer functions for adaptive filters where the cancellation transducers are positioned between sensing microphones spaced along a duct. These patents do not disclose adaptations of active attenuator noise control systems to motor vehicles, nor do they teach that such noise control can improve motor vehicle engine performance. The present invention overcomes the above mentioned disadvantages by providing an active muffler using a noise attenuation system which improves the performance of the engine as well as reducing the noise emanated from the engine without imposing passive restrictions within the exhaust conduit. In general, the muffler comprises an active noise cancellation system with at least two transducers which are longitudinally spaced apart from each other at predetermined locations along the length of the conduit. An electronic control generates a signal in a conventional manner for driving each transducer and generating a pulse train which cancels the pulse train passing through the conduit at a respective predetermined location. In addition to the conventional adaptive filter circuity, the electronic control includes means for variably driving the spaced apart transducers so that the effective cancellation point varies upstream of the first and second predetermined locations of the transducers as a function of the frequency of the pulse passing through the conduit. As a result, the effective length of the conduit for the purpose of tuning the conduit changes, assuring that negative pressure reaction pulses arrive at the valve port when the valve is opened to aid in the extraction of combustion gases from the engine cylinder as in previously known tuned exhaust systems, but throughout the entire range of engine speeds. In the preferred embodiment, the electronic control generates signals to each of the cancellation transducers where each signal amplitude is adjusted with respect to the other so that addition of the pulses generated by the transducers cancels out the sound pressure pulses travelling through the conduit downstream of the cancelling transducers. In addition, a minimal or negative pressure is timed to arrive at the upstream valve port as the valve opens. The position of the transducers is spaced from the valve a distance as large as possible to avoid harsh temperature conditions, and is spaced from the input sensor to provide a sufficient amount of processing time for the transducers' driver. The distance between the transducers is then related to the wavelengths of the highest and lowest frequencies to be encountered. Preferably, the distance between the transducers is greater than one-sixteenth of the wavelength of the highest frequency, i.e. the shortest wavelength, and less than one-fourth the wavelength of the lowest frequency, i.e. the longest wavelength, so that the effective, tuned length of the conduit remains upstream of the transducers. The effective length of the conduit is adjusted as necessary by a tuning circuit to provide a minimal or negative standing wave pulse at the valved port throughout the range of engine speeds. At a first selected frequency related to engine speed, such as the frequency of valve opening at normal idle engine speed can be determined by the conventional ranges of engine speeds and related frequencies of valve operation. The second predetermined frequency is related to operation of the valve at a second predetermined engine speed, such as the engine speed at which gear shifts will occur, to assure that the reaction pulse arrives at the valve to minimise interference with or cooperate with exhaustion of the combustion gases from the cylinder when the valve is open. In such an embodiment, the first and second transducers will be proportionally activated to cancel the sound pressure pulses downstream when the engine is operating. At a low engine speed, for example, idle speed, the cancellation signal generated in the conduit by adding the output of the transducers will generate an upstream standing wave acting as though the conduit has an effective length approaching toward the transducer locations. At higher engine speeds, the sum of the transducer outputs and the input pulse train generates an upstream standing wave acting as though the conduit has an effective length closer to the sound pressure source. At intermediate engine speeds, the electronic control varies the amplitude of the signal delivered to each of the transducers so that they cooperate in cancelling sound pressure pulses related to intermediate engine speeds. Such co-operation provides an effective conduit length upstream of the transducer positions that correspond to the pulse frequency, and thus the engine speed. Therefore, the system dynamically tunes the exhaust for maximum efficiency throughout a wide range of speeds. Thus, the present invention provides active attenuation of sound pressure pulses passing through an exhaust conduit of a motor vehicle which avoids obstruction of the exhaust gases through the passageway. More importantly, the exhaust conduit can be dynamically tuned throughout a wide range of engine speeds to enhance performance of the engine throughout the engine speed range. The invention will now be described further, by way of example, with reference to the accompanying drawings, Figure 1 is a diagrammatic representation of an active muffler constructed in accordance with the present invention; Figure 2 is a graphical illustration of the performance of the system shown in Figure 1 in comparison with previously known performance characteristics; Figure 3 is a diagrammatical representation of waveform comparisons representative of the operation of the system shown in Figure 1; Figure 4 is a graphical representation of tuned exhaust conduit performance according to the present invention at a fixed engine speed; and Figure 5 is a graphical representation similar to Figure 4 but representing performance at higher engine speed. Referring first to Figure 1, the noise attenuation system 10 is thereshown diagrammatically incorporated with an exhaust conduit 12 of a motor vehicle. One end 14 of the conduit 12 includes a port which is opened and closed by a valve 16 by appropriate means 18 such as a conventional, engine driven, valve lifter mechanism. As in previously known active cancellation systems, the noise attenuation system 10 includes a microphone 20 for sensing sound pressure pulses passing through the conduit 12 in response to repeated opening of the valve 16 during operation of the engine. The microphone 20 delivers signals to an electronic control 22 to produce signals with an appropriate wavelength for driving a transducer to cancel pulses downstream of the microphone 20. However, the system 10 also includes a tuning circuit 24 to provide separate outputs 26 and 28 to two transducers 30 and 32, respectively. The transducer 32 is positioned in a first predetermined position along the length of conduit 12 as indicated in Figure 1 at L1. This position may be dictated by packaging requirements, but preferably, this position is spaced apart from the valve to avoid the extreme environmental conditions at the valve. This location is defined in the present disclosure in terms of its distance from a desired effective exhaust conduit length LT upstream of the transducers 30 and 32. The second transducer 30 is positioned at a second predetermined position along the length of the conduit 12 as designated in Figure 1 at L2. The distance between the positions of the transducers, i.e. L1-L2 is preferably greater than one-sixteenth of the wavelength of the highest frequency and less than one-fourth the wavelength the lowest frequency. The position LT is representative of the location at which the introduction of cancellation signals creates an effective reaction wave timed to introduce a minimal or even a negative pressure pulse at the port 14 when the valve 16 is opened during engine operation. Preferably, the engine speed range will be selected to include the idle speed and maximum torque speeds of the engines, and the frequency range of valve operation is determined from those speeds. The tuning circuit 24 provides a signal through outputs 26 and 28 in response to the electronic control 22 to produce signals at transducers 30 and 32 which will cancel out the sound pressure pulses downstream of the transducers resulting in a waveform W3 shown diagrammatically in Figure 3. In addition, the tuning circuit 24 of the present invention drives the transducers 30 and 32 to generate a standing wave W2, represented in Figures 4 and 5, upstream of the transducers 30 and 32. The standing wave W2 has a wavelength that varies between wavelength L for the wavelength of the lowest frequency pulse train encountered and wavelength H for the highest frequency pulse train encountered. W2 is effective from a point LT which varies as a function of the relative amplitudes of the signals generated at transducers 30 and 32. As an example of the operation of tuning circuit 24, the input signal W1 may be represented as a signal Ae iwt-ikx , where k is equal to w/c and where w is equal to 2n f and c equals the speed of sound. In a prior art single transducer noise cancellation system, the cancellation signal would be Be iwt where B is about equal to -2A. In the present invention, signal Be iwt and Ce iwt are introduced at X=L2 and X=L1 respectively. Thus, at the conduit regions downstream of transducers 30 and 32, the sum of Be iwt-ik(LT-L2) + Ce iwt-ik(LT-L1) + Ae iwt-ikx is made equal to 0, whereby Be ik(L2) + Ce ik(L1) + A = 0. In regions upstream of transducers 30 and 32, Be iwt+ik(Lt-L2) + Ce iwt+ik(LT-L1) + Ae iet-ikx = W2, whereby Be -ik(L2) + Ce -ik(L1) + A = 0 @ x = LT. Therefore, C and B may be resolved in terms of A and it is found that C = {SIN (K(L2)}/SIN [K(L1-L2]} A, and B = {-SIN (K(L1))/SIN [K(L1-L2)]}A. Accordingly, the tuning circuit 24, such as a microprocessor programmed to function in accordance with this algorithm serves to drive transducers 30 and 32 as required to vary X=LT as necessary to maximise engine efficiency. The engine speed at a first level R1 may be a minimum engine speed, and is preferably selected as the idle speed of the engine. Conversely, the tuning circuit 24 also generates a signal at outputs 26 and 28 to drive the transducers 30 and 32 as required to cancel sound pressure pulses detected at microphone 20 at the upper end of the operating speed range of the vehicle engine. At intermediate speeds within the operating range, the signals sent through outputs 28 and 26 will vary relative to each other in response to the frequency detected. Thus, outputs 28 and 26 will vary the effective cancellation point LT at a position upstream of the transducers along the conduit 12. The operation of the device is best shown with reference to Figure 2 in which torque is shown as a function of engine speed. Line A represents the typical performance characteristic of an engine which is subject to normal operating inefficiencies such as those caused by exhaust restrictions. As represented by Line B, the previously known tuned exhaust system enables greater torque to be obtained over a range of engine speeds by terminating the exhaust conduit 12 at a distance LT at which the reactionary waveform delivers minimal or negative pressure to the port 14 when the valve 16 is open. It will be appreciated that the increased efficiency of the engine tapers off dramatically above and below the engine speed relating to the frequency of valve operation for which the conduit 12 is tuned. With the present invention, the combination of the tuning circuit 24 and the spaced transducers 32 and 30 provides a varying point of sound pressure cancellation within the conduit 12. Consequently, the evacuation effect of minimal or induced negative pressure at the valve 16 can be employed throughout a desired engine speed range as shown by line C. As a result, the system 10 of the present invention not only reduces noise by cancelling the sound pressure pulses travelling through the exhaust conduit 12, but at the same time, produces a reactionary pulse effect which tends to increase the efficiency of engine operation. As a result, the system optimises engine efficiency throughout its operating range.

Claims: A dynamically tuned exhaust system comprising, a sensor (20) for generating a signal representative of sound pressure pulses from a source (16) through a conduit (12), and transducer means (30,32) located down stream of the sensor (20) and means (22,24) responsive to said sensor signal for driving said transducer means to cancel said sound pulses downstream of the sensor (20) said transducer means including a first transducer (30) fixed at a first position along said conduit (12) and a second transducer (32) fixed at a second position along said conduit (12), spaced from said first position, and said means for driving said transducers including an electronic controller (22,24) for selectively driving said first and second transducers (30,32) in response to said sensor signal and producing an output pulse having a phase opposite to said sound pressure pulse at said first and second positions, characterised by means for variably driving said first transducer (30) and second transducer (32) so that the effective cancellation point varies upstream of said first position of said first transducer (30) and said second position of said second transducer (32) as a function of the frequency of the pulses passing through the conduit. A system as claimed in claim 1, wherein said means for variably driving comprises means for proportion ally varying the amplitude of the signals delivered to said first and second transducers. A method for dynamically tuning a combustion engine exhaust conduit with the system as claimed in claim 1 or 2, the method comprising, actively attenuating said sound pressure pulses by generating the cancellation signal at a first effective conduit location along said conduit at a first predeter mined frequency of said sound pressure pulses, actively attenuating said sound pressure pulses by generating a cancellation signal at a second effective conduit location at a second predetermined frequency of said sound pressure pulses, and actively attenuating said sound pressure pulses by generating a first proportional cancellation signal at said first transducer location, and a second proportional cancellation signal at said second transducer location at frequencies intermediate said first predetermined frequency and said second predetermined frequency. A method as claimed in claim 3, the method comprising, positioning said first and second transducers at a distance L2-L1 from each other, where L2-L1 is greater than one-sixteenth of the wavelength of the highest frequency and less than one- fourth the wavelength of the lowest frequency, where L2 is the distance from a variable location LT upstream of said first transducer and L1 is the distance from said location LT to said second transducer, variably driving said first and second transducers with an active attenuation signal proportional to the amplitude of the input to cancel the sound pressure pulses downstream of said transducers and tuning said signal to vary location LT to induce a minimum pressure pulse in a standing wave induced upstream of said transducers.

Inventors: GEDDES EARL RUSSELL, GEDDES, EARL RUSSELL
Applicants: FORD FRANCE, FORD MOTOR CO, FORD WERKE AG, FORD FRANCE S. A., FORD MOTOR COMPANY, FORD MOTOR COMPANY LIMITED, FORD-WERKE AKTIENGESELLSCHAFT
Application year: 1991
Main classification: F01N1
IPC codes: F01N1, F02B27