Variably tuned Helmholtz resonator with linear response controller

A variably tuned Helmholtz resonator which has a connection establishing fluid communication between fixed volume chamber and a duct of an induction system for an internal combustion engine. The tubular connection has a special configuration which affects changes in open area and length of the tubular connection so as to create a linear relationship between the resonant frequency and the angular position of the tuning plate. The tuning plate is positioned correspondingly to engine speed to provide noise attenuation over a wide range of engine speeds.

BACKGROUND OF THE INVENTION 
Helmholtz resonators have been employed in internal combustion engine 
induction systems to reduce engine noise. Such resonators consist of a 
fixed volume chamber connected to an induction system duct by a tubular 
connection or neck. The frequency associated with the primary order of 
engine noise is directly proportional to engine speed, but a fixed 
geometry Helmholtz resonator is only effective at attenuating noise in a 
narrow frequency range, such that the resonator would be ineffective in 
attenuating primary order noise over much of the complete range of engine 
speeds encountered during normal operation of a vehicle powered by the 
engine. 
It has heretofore been proposed that a Helmholtz resonator be variably 
tuned in accordance with engine speed in order to increase the range of 
engine speeds over which the resonator will be effective to suppress 
primary order engine noise. This approach is described in U.S. Pat. No. 
4,539,947 which shows a movable element mounted within the tubular 
connection or neck between the duct and the Helmholtz chamber. The 
position of the movable element is varied in accordance with engine speed 
to vary the effective cross sectional area and/or length of the tubular 
connection. This has the effect of changing the resonant frequency of the 
Helmholtz resonator so as to be effective over a wider range of engine 
speeds. 
However, the effect of change in cross sectional area and length of the 
tubular connection on the resonant frequency is markedly non linear, such 
that the design and performance of controls to execute proper movement of 
the movable element in correspondence with engine speed is rendered 
problematic. 
It is the object of the present invention to provide a variably tuned 
Helmholtz resonator in which a linear response to the control variable is 
achieved. 
SUMMARY OF THE INVENTION 
The above-recited object of the present invention is achieved by providing 
a tuning plate pivoted to sweep across the cross section of a tubular 
connection between the resonator chamber and a duct with which the 
resonator is associated. The tubular connection has a particular curved 
roughly triangular cross sectional shape produced by mapping the bisector 
of a triangle onto the radius of a circle, such that incremental angular 
movements of the plate produce a proportionate change in the open area of 
the tubular connection. 
The tubular connection extends down into the resonator chamber and is 
truncated such that end corrected effective length remains effectively 
constant as the tuning plate is swept across the width of the tubular 
connection. 
The end result is a linear relationship between the angular position of the 
tuning plate and the resonant frequency of the Helmholtz resonator. 
Thus, by positioning the timing plate in correspondence to an engine speed 
signal, noise suppression across most of the engine operating speed range 
can be achieved.

DETAILED DESCRIPTION 
In the following detailed description, certain specific terminology will be 
employed for the sake of clarity and a particular embodiment described in 
accordance with the requirements of 35 USC 112, but it is to be understood 
that the same is not intended to be limiting and should not be so 
construed inasmuch as the invention is capable of taking many forms and 
variations within the scope of the appended claims. 
Referring to FIG. 1, the present invention comprises a linearly tuneable 
Helmholtz resonator 10, installed in the induction system of an engine, 
intermediate the engine air cleaner 12 and intake manifold 14. A square to 
round transition duct piece 16 enables a connection at either end to 
rounds duct connecting to the engine components. 
A solenoid actuator 18 drivingly engages a rotary tuning shaft 20 so as to 
swing a tuning plate 22 about the axis of the tuning shaft 20. 
Driver signals are applied to a controller 24 to cause the solenoid 
actuator 18 to rotate the tuner shaft 20, the driver signals generated 
from the vehicle ECU 26, which in turn receives signals from an engine 
speed transducer 28. 
The angular position of the tuning shaft 20 and plate 22 is thereby set in 
correspondence to engine speed. 
The Helmholtz resonator 10 comprises a fixed volume chamber 11, defined by 
a hollow cylindrical housing 30 closed off at its top and bottom with 
cover plates 32, 34. A roughly triangularly shaped opening 36 in the top 
cover plate 32 has a correspondingly shaped tubular connection or neck 38 
aligned therewith and affixed to the inner surface of top cover plate 32. 
Transition duct piece 16 has an opening matching the opening 36A in the top 
plate 32 and aligned therewith, the flat bottom wall 40 fixedly attached 
to the top plate 32. Thus, the chamber 11 is in fluid communication with 
the interior 42 of the duct transition piece 16 via an internal passage 
36B of the tubular connection 38 recessed into the chamber 11. 
The tubular connection 38 is supported on the bottom cover plate 34 with a 
series of posts 44 projecting upwardly and engaging respective sections of 
the bottom edges of the tubular connection 38. 
As best seen in FIG. 4, the tuning plate 22 is received in a slot 46 
extending partially through the connector 38 adjacent its upper end so as 
to be able to partially block to a varying degree the internal passage 36B 
defined within the tubular connection 38. 
The bottom of the tubular connection 38 is truncated in order to affect the 
effective length of the neck defined by the connection 38 as the tuning 
plate 22 is swung through the slot 46. 
The geometry of the internal passage 36B of tubular connection 38 is 
configured such that a linear relationship is established between the 
cross sectional area of the internal passage 36B and angular position of 
the timing plate 22 in the range of partially blocking positions. 
The resonant frequency of a Helmholtz resonator f.sub.R is given by: 
EQU f.sub.R =(c/2.pi.) .sqroot.(s/L'V)! 
where: c=speed of sound 
S=cross sectional area of neck 
L'=end corrected length of neck 
V=volume of cavity 
In order to obtain a resonator with a linear response to a tuning variable, 
we need a resonator with a variable geometry such that: 
EQU f.sub.R =.alpha..theta. 
where .alpha. is a constant and .theta. is the tuning variable. 
For the order tracking Helmholtz resonator 10, the cross sectional area of 
the tubular connection 38, S will be the geometrical component which will 
be made variable. The volume of the cavity 11 will be held fixed. 
The design for the cross sectional area is shown in FIG. 5 for the tuning 
plate angle .theta.. 
The open area of the connector internal passage 36B is given by: 
EQU S=(RC.sub.L /2) w sin .theta. 
where: RC.sub.L =70 mm 
and .theta.=tuning angle in radians 
w=maximum width of neck opening at 
tuning plate angle .theta.. 
The variable w can be expressed as: 
EQU w=W (.theta./.phi..sub.max) 
where: W=50 mm 
and .phi..sub.max =1.431 radians (i.e., 82.degree.). 
So, 
EQU S=(RC.sub.L /2) (W) (.theta./.phi..sub.max) sin .theta. 
Expanding sin .theta. in a Taylor's series, i.e.: 
EQU sin .theta.=.theta.-(.theta..sup.3 /3|)+(.theta..sup.5 /5|)-. . . 
and substituting into the express for 5 yields: 
EQU S=W(C.sub.L .backslash.2) (.theta./.phi..sub.max) .theta.-(.theta..sup.3 
/3|)+(.theta..sup.5 /5|)-. . . ! 
Retaining only the leading term for sin .theta., the open cross sectional 
area can be approximated as: 
EQU S=W(C.sub.L .backslash.2) (.theta./.phi..sub.max) (.theta.) 
Or: 
EQU S.apprxeq.(WRC.sub.L .backslash.2) (.theta..sup.2 /.phi..sub.max) 
Also, the end-corrected tubular connector length L' can be expressed as: 
EQU L'=L+1.5a 
where: L=midpoint length of the neck 
a=hydraulic radius of the neck, i.e., 
a=.sqroot.(s/.pi.)! 
So, 
EQU L'=L+1.5a.sqroot.(s/.pi.)! 
L' is to be fixed, i.e., independent of the tuning angle .theta.. So, the 
tubular connection 38 length L must compensate for the end correction, 
i.e., 
EQU L=L.sub.o -1.5.sqroot.(s/.pi.)! 
where: L.sub.o =constant (length)=15 mm 
So, the end-corrected length is: 
EQU L'=L.sub.o which is fixed. 
Note that the length L is a linear function of the tuning plate angle: 
EQU L=L.sub.o -1.5 (WRC.sub.L /2.pi..phi..sub.max).theta. 
That is, as the angle .theta. of tuning plate 22 is increased, the midpoint 
length L decreases linearly (see FIG. 7 showing the effect of the 
truncated lower end of the tubular connector 38). 
So, the tuning frequency of the order-tracking resonator is given by: 
EQU f.sub.R =(C/2.pi.) .sqroot.(s/L'V)!=.alpha..theta. 
where: .alpha.=(C/2.pi.) (1/L.sub.o V).sup.1/2 (WRC.sub.L 
/2.phi..sub.max).sup.1/2 
In practice, the tuning plate angle is established by solenoid 18 which 
will be powered by a signal from the ECU 26 proportional to the engine 
speed. The relationship between the frequency of the primary order engine 
noise and engine speed is given by: 
EQU f.sub.p =(N/2) (RPM/60) 
where: RPM=engine speed 
N=number of cylinders 
When the resonator is tuned such that the resonant frequency of the 
resonator matches the frequency of the primary order engine noise, 
EQU f.sub.R =f.sub.P 
the primary order engine noise is reflected back up the induction system 
toward the engine. The primary order engine noise is thus not allowed to 
radiate out of the induction inlet continuously for all engine speeds 
corresponding to the range of resonant frequencies of the resonator. For a 
four cylinder engine, this engine speed range for the current design is 
1800 rpm-6000 rpm. 
FIG. 6 depicts an actual geometry of the opening 36 in the top plate 32 (as 
well as the tubular connection 38). 
Accordingly, a much simpler, better performing control is enabled by the 
linear relationship between the tuning plate angle and the resonant 
frequency of the Helmholtz resonator.