The invention relates to the attenuation of dynamic pressure fluctuations in ducts, with particular but not exclusive reference to the attenuation of dynamic pressure fluctuations within the exhaust and/or inlet ducting of internal combustion engines, for the purpose of limiting or cancelling noise emitted from the ducting and/or influencing engine performance.

Much work has been done on the control of pressure fluctuations in the inlet and exhaust ducting of an internal combustion engine. One approach to the problem of reducing noise emitted from an exhaust duct has been to place a secondary source of sound in or adjacent to the exhaust duct and control the sound emitted by the secondary source such that its output is equal in amplitude but opposite in phase to the anticipated noise in the duct at the position of the secondary source. Thus following the theory of superposition, the noise is cancelled or reduced. Such systems have been described for instance in the specifications GB 2130651 and WO 89/07701. However, this technique is disadvantageous since it requires as the secondary source a transducer which must be capable of surviving the hostile environment associated with being in or adjacent to the inlet or exhaust ducting of a vehicle.

Absorption silencers, reflection silencers or combinations of the two are well known and have also been used for the sound attenuation in exhaust gas systems of internal combustion engines. However, these are passive systems which are not very efficient in attenuating dynamic pressure fluctuations at low frequencies.

An active method of attenuating dynamic pressure fluctuations in ducts has been described in specification EP 0307639. This document discloses apparatus for the attenuation of dynamic pressure fluctuations of a gas flowing in a duct comprising valve means mounted in a duct and controllable to vary the cross-sectional area through which the gas can flow, sensor means to provide an indication of at least the frequency of said dynamic pressure fluctuations to be attenuated, and control means to control the cross-section area of said valve means in response to signals from said sensor means.

The present invention provides apparatus as just described characterised in that said control means controls said valve means in response to signals from the sensor means to control the amplitude as well as the phase of the dynamic pressure fluctuations to attenuate the dynamic pressure fluctuations.

The present invention also provides a method of attenuation of dynamic pressure fluctuations in a gas flowing in a duct, the method comprising the steps of providing an indication of at least the frequency of the dynamic pressure fluctuations to be attenuated and controlling valve means to control the cross-sectional area in said duct through which the gas can flow, such that the amplitude as well as the phase of said dynamic pressure fluctuations are controlled so that said dynamic pressure fluctuations are attenuated.

In one embodiment of the present invention the valve means comprises an apertured member fixed to extend tranversely across said duct and a second apertured member movable transversely to said duct adjacent said fixed member so as to co-operate therewith and vary the cross-sectional area in said duct through which the gas can flow.

In another embodiment of the present invention the valve means comprises a first throttle flap rotatable within said duct about an axis extending across said duct and the second throttle flap rotatable within said duct about an axis extending across said duct, said throttle flaps being arranged in series along said duct.

The present invention is applicable to internal combustion engines wherein said duct can comprise the inlet and/or exhaust duct.

Examples of the present invention will now be described with reference to the drawings, in which:-

Figure 1 is a schematic illustration of a duct; Figure 2 illustrates valve means in a duct according to one embodiment of the present invention;

Figure 3 illustrates a complete control system utilising the valve means of Figure 2;

Figure 4 illustrates a purely adaptive control system utilising the valve of Figure 2;

Figure 5 shows a cut-away portion of the duct exposing a single throttle flap of valve means according to another embodiment of the present invention.

Figure 6 diagrammatically illustrates the duct with two rotatable throttle flaps constituting the valve means;

Figure 7 illustrates an adaptive control system utilising the rotatable throttle flaps of Figures 5 and 6.

Referring now to the drawings, Figure 1 illustrates a schematic duct 1 housing a valve 2. A pulsating flow arrives at the valve 2 at station B via a duct of area A . The downstream duct also has an area A. in order to maintain a steady mass flow through the valve, and hence no downstream pressure fluctuations. The valve open area A- can, to a first approximation, be modulated according to the law:

where X and p = mean density at the valve

p ~ = mean pressure at the valve

p = incident density fluctuation at the valve

II ₂ = density fluctuation at valve caused by modulation of area A by amount A "

I m = fluctuating mass flow in the absence of active control

β = 1 Ύ - 1 and = ratio of specific heats for r the gas

Thus Equation (1) gives the relationship between the variations needed in the cross-sectional area of the duct 1 provided by the valve 2 and the downstream pressure fluctuations.

In practise, there will be losses through the valve 2, so the theoretical and actual mass flow will be different. Also the Bernoulli effect will produce a sideways force on the valve 2, tending it to close. Both effects will make the valve response non-linear. Thus, as well as the control law shown in Equation (1) above, adaptive filtering can be used to optimise the performance, as will be described later.

One embodiment of the valve 2 is shown in Figure 2. Here, the valve takes the form of an apertured member 3 fixed to extend transversely across the duct 1. Adjacent to the fixed apertured member 3 lies an equal sized movable apertured member 4. The movable apertured member 4 is attached to a driving arm 5 and can be oscillated transversely with a force provided by an electromagnetic shaker 13 such that the vertical slots 6 provided in both the fixed and movable apertured members 3 and 4 are partially occluded as a function of time. The degree of overlap of the vertical slots 6 and hence the degree of occlusion during a single period of the oscillation gives control over the amplitude of the dynamic pressure fluctuations, whilst the timing of the translation of the movable apertured member 4 gives control over the phase. Thus the amplitude and phase of oscillation of the movable apertured member provides control of the amplitude and phase respectively of the dynamic pressure fluctuations. The oscillation of the movable apertured member 4 is performed by the electromagnetic shaker 13 under the control of signals provided on control line 18 from a controller (not shown) .

This type of valve gives control of amplitude and phase of the dynamic pressure fluctuations over a wide range of frequencies.

Since at best the valve shown in Figure 2 will even when fully open, occlude 50% of the cross-sectional area of the duct 1, the valve 2 can be placed in a position 7 in the duct 1 where the cross-sectional area is doubled. Such an arrangement is shown in Figure 3.

A full control system incorporating the valve 2 shown in Figure 2 is shown in Figure 3. An exhaust duct 1 of an internal combustion engine 12 has an enlarged region 7 which has double the cross-sectional area, in which the valve 2 shown in Figure 2 is placed. The movable apertured member 4 of the valve 2 is connected to an electromagnetic shaker 13. The shaker 13 is controlled by the controller 14 on control line 18. The controller 14 receives signals from the internal combustion engine 12 on lines 15 and 16 giving an indication of the engine speed and load respectively. Further, in order to utilise the relationship given in Equation (1) , pressure and temperature measurements are taken either side of the valve using sensors 8, 9, 10 and 11.

Assuming ideal gas laws apply and that no heat is transferred to or from the gas during its passage through the valve 2, then these measurements will suffice to implement the control law defined by Equation (1) . Compensation for losses may be brought in via a predetermined loss factor, C,, applied to the mass flow.

With the shaker 13 switched off and set to mean position A ₂ , the mean quantities p and p etc, and fluctuating quantities p _χ • , p ₂ ' , m ^« can be measured. Densities p , p ' , etc. can then be obtained by

the adiabatic and ideal gas relations:

p ¹ = c

Pi _ P2 ^p l ^ ~ p = RT c ² = γRT

After this calibration stage the instantaneous valve setting A " can then be predicted by the controller with real time measurement of p " only. A " is modulated by sliding the valve 2 with the electromagnetic shaker 13. The other quantities { . , p. etc) can be checked periodically to monitor any system changes and compensate for them.

In order to optimise the system performance, a pressure sensor 17 is located downstream of the valve 2, perhaps after the tailpipe, and its signal is fed back to the controller 14. The control system can then control the shaker 13 not only in response to the signals from the engine 12 but also in response to the degree of success in attenuating the dynamic pressure fluctuations.

The control system illustrated in Figure 3 however, while offering the potential for very fast control over a broad band of frequencies, may be too unweildy for some applications. In this case, less information can be taken regarding the pressure and temperature within the duct, and a purely adaptive system can be used as illustrated in Figure 4. In Figure 4 the engine 12 provides merely a signal on line 19 indicative of the speed of rotation of the engine 12 to the controller 14. The shaker 13 is then controlled only in response to the speed

of rotation of the engine and the degree of success of attenuation of the dynamic pressure fluctuations is monitored using a feedback microphone 17 to allow the adaption by the controller 14 in order to try to reach the best attenuation.

An example of an adaptive control system that is capable of performing such control is described in WO 88/02912, the contents of which is incorporated herein by reference.

Thus with the adaptive control system shown in Figure 4 a system can be provided for the broad band control of the amplitude and phase of dynamic pressure fluctuations in a duct.

An alternative valve arrangement is shown in Figure 5, which shows a cut-away portion of the duct 1, revealing a single rotatable throttle flap 20, of which there are two arranged in series as shown diagrammatically in Figure 6. Such a valve arrangement comprises two rotatable throttle flaps 20 and 21 which are arranged within the duct 1 and rotatable about axes that extend across the duct. The throttle flaps 20 and 21 are spun on their axes by stepper motors 22 and 23 and are connected by drive shafts 26 and 27 to the stepper motors 22 and 23 respectively. Control of the stepper motors 22 and 23 is provided by the controller 14 on lines 24 and 25.

The diameter of the duct 1 is D whilst the diameter of the throttle flap is d. In order for flow to occur down the duct even when attenuating large amplitude dynamic pressure fluctuations d must be less than D.

The two rotatable throttle valves 20 and 21 are shown arranged in series along the duct 1. However, such rotatable throttle flaps could be arranged in a parallel arrangement for instance if the duct 1 was divided into two

subducts with a single throttle flap in each subduct. The rotatable throttle flaps 20 and 21 are of the type described in EP 0307639, and are independent. Thus their combined effect is the superposition of the individual effects of a single rotatable throttle flap to a first approximation. This gives a means of controlling both the amplitude and phase of the dynamic pressure fluctuations in the duct 1.

The control law given by Equation (1) allows for the control of downstream pressure fluctuations by modulating the open area of the valve 2. For two rotatable throttle valves 20 and 21, each of area A. and located in series in a gas carrying duct 1 of area A , then the total projected open area down the duct is given by

A ₂ (t) = A _Q ^ (1-CθSΦ/ 2 CθS(ωt+α:+Φ)) (2)

where ω is the rotation frequency of both valves (which may be constant or itself a function of time) ; α is a reference angle which defines the angular position of the path relative to the instantaneous phase of the gas pressure fluctuations; and Φ is the relative angle between the two throttle flaps 20 and 21. It can be seen from Equation (2) that the open area A "(t) has a sinusoidal component of amplitude Thus the amplitude of the resulting pressure fluctuations can be adjusted by adjusting the phase Φ between the two throttle flaps 20 and 21. Equation (2) also shows that the maximum effect occurs with the valves in phase (Φ = 0) and has amplitude A while the minimum effect, at Φ = 90°, has amplitude = 0.

An adaptive control system utilising this type of valve is shown in Figure 7 and is substantially similar in function to the arrangement shown in Figure 3. This diagram shows an adaptive system for the attenuation of dynamic pressure fluctuations

utilising two rotatable throttle flaps 20 and 21. The rotatable throttle flaps 20 and 21 are driven by stepper motors 22 and 23 respectively, to give control over their phase angle o and OC+Φ respectively. In the control system the two rotatable throttle flaps 20 and 21 can be treated as a single valve 2. If a change in the absolute phase <χ is required then the stepper motors 22 and 23 are incremented equally. If a change in control amplitude is required, the desired amplitude change can be translated into a phase difference by using Equation (2) and thence to a number of angular steps. One of the rotatable throttle flaps 21 for instance, is then incremented by this number of steps relative to the other to provide the relative phase difference between the throttle flaps 20 and 21 of φ Since it is important not to . lose synchronism between the two rotatable throttle flaps 20 and 21 then the additional steps for the one throttle flap 21 must be added or subtracted in between the regular steps, so that control is retained.

As has been described for Figure 4, the system can for example be adaptively controlled using the algorithm described in WO 88/02912. Such a system provides for the control of the amplitude and phase of the dynamic pressure fluctuations at a single frequency and its harmonics.

A control system similar to that shown in Figure 3 for the first type of valve arrangement would also be used for this valve arrangement.

A method of operating the control system for both types of valve arrangement will now be described. In its simplest form a signal indicative of the frequency of the dynamic pressure fluctuations is provided on line 19 to the controller 14. Such a signal in the embodiment described is a signal representing the speed of the internal combustion engine 12. The controller 14 then controls the

frequency of rotation and hence phase and amplitude of duct occlusion performed by the valve means 2 in order to attenuate the dynamic pressure fluctuations. In this arrangement the amplitude of occlusion of the duct can be at a set predetermined level.

To improve the degree of attenuation a pressure sensor 17 can provide a signal indicative of the attenuated dynamic pressure fluctuations in the area of the duct 1 downstream of the valve 2, on line 19. The controller 14 can then compare the signal from the pressure sensor 17 with the signal at an earlier time to ascertain the degree of success in attenuating the dynamic pressure fluctuations and adapt the control signals on line 18 or 24 and 25 to improve attenuation in an adaptive manner by controlling the amplitude and phase of the dynamic pressure fluctuations.

An example of an algorithm which the controller 14 can follow to implement this adaptive technique is described in the specification of WO 88/02912.