U.S. patent application number 13/886107 was filed with the patent office on 2013-11-07 for using valve timing to improve engine acoustics.
This patent application is currently assigned to TULA TECHNOLOGY, INC.. The applicant listed for this patent is TULA TECHNOLOGY, INC.. Invention is credited to Louis J. SERRANO, Joshua P. SWITKES.
Application Number | 20130291816 13/886107 |
Document ID | / |
Family ID | 49511580 |
Filed Date | 2013-11-07 |
United States Patent
Application |
20130291816 |
Kind Code |
A1 |
SERRANO; Louis J. ; et
al. |
November 7, 2013 |
USING VALVE TIMING TO IMPROVE ENGINE ACOUSTICS
Abstract
A method for improving the operation of an internal combustion
engine implementing cylinder deactivation is described. Generally,
the pattern of combustion events that are fired and skipped
together with the geometry of the exhaust and/or intake system can
create unpleasant acoustic issues. By slightly altering the timing
of the cylinder intake and exhaust valves, these acoustic issues
can be mitigated. The valve timing can be altered on a combustion
event by combustion event basis. Alternatively, valve timing for
different groups of cylinders can be modified together.
Inventors: |
SERRANO; Louis J.; (Los
Gatos, CA) ; SWITKES; Joshua P.; (Menlo Park,
CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
TULA TECHNOLOGY, INC. |
San Jose |
CA |
US |
|
|
Assignee: |
TULA TECHNOLOGY, INC.
San Jose
CA
|
Family ID: |
49511580 |
Appl. No.: |
13/886107 |
Filed: |
May 2, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61641765 |
May 2, 2012 |
|
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Current U.S.
Class: |
123/90.17 ;
123/90.1 |
Current CPC
Class: |
F02D 13/0215 20130101;
F01L 1/344 20130101; Y02T 10/18 20130101; Y02T 10/12 20130101 |
Class at
Publication: |
123/90.17 ;
123/90.1 |
International
Class: |
F01L 1/344 20060101
F01L001/344 |
Claims
1. A method of controlling sound of an engine having a crankshaft,
a camshaft and a plurality of cylinders, each cylinder having an
associated piston that is driven by the crankshaft, an associated
exhaust valve, and an associated exhaust path length, wherein the
respective exhaust path lengths associated with at least some of
the plurality of cylinders differ and wherein at least some of the
exhaust valves are driven by the camshaft, the method comprising:
operating an engine in a skip fire mode; and dynamically varying a
phase of the camshaft between different exhaust events during a
sequence of skip fire operation in a manner that at least partially
compensates for differences in the exhaust path lengths associated
with fired cylinders to thereby help suppress audible beats that
would otherwise occur in the event that the camshaft phase was held
substantially constant during the sequence of skip fire
operation.
2. A method as recited in claim 1 wherein the camshaft phase is
further based at least in part on the current engine speed.
3. A method as recited in claim 2 wherein the camshaft phase is
further based at least in part on estimated exhaust gas
temperature.
4. A method of controlling sound of an engine having a crankshaft
and a plurality of cylinders, each cylinder having an associated
piston that is driven by the crankshaft, an associated exhaust
valve and an associated exhaust path length, wherein the respective
exhaust path lengths associated with at least some of the plurality
of cylinders differ, the method comprising: operating the engine in
a skip fire mode; and dynamically varying the relative timing of
the exhaust valve opening events relative to crankshaft angle
between different cylinder exhaust events during skip fire
operation.
5. A method as recited in claim 4 wherein the dynamic varying of
the relative timing of the exhaust valve opening events is arranged
to at least partially compensate for acoustic delays caused by the
different effective exhaust path lengths.
6. A method as recited in claim 4 wherein the dynamic varying of
the relative timing of the exhaust valve opening events is arranged
to shape the exhaust noises in a desired manner.
7. A method as recited in claim 6 wherein the engine acoustics are
shaped in a manner that suppresses audible beats that would
otherwise occur in the event that all cylinders fired during a
sequence of skip fire operation were fired at a similar relative
exhaust valve opening timing.
8. A method as recited in claim 4 wherein the dynamic varying of
the relative timing of the exhaust valve opening events occurs
while the engine is operated at a firing fraction or firing pattern
selected from a predetermined group of firing fractions or firing
patterns.
9. A method as recited in claim 4 wherein the engine has eight
cylinders arranged in two banks; and the dynamic varying of the
relative timing of the exhaust valve opening events occurs while
the engine is operated at a firing fraction selected from the group
consisting of 1/3, 2/3, 1/5, , 3/5, 4/5 and 1/2.
10. A method as recited in claim 4 further comprising: determining
a desired baseline valve timing based at least in part on current
engine operating conditions; determining an exhaust timing
adjustment for a particular exhaust event relative to the baseline
valve timing; and adjusting the exhaust valve timing for the
particular exhaust event in accordance with the determined
adjustment.
11. A method as recited in claim 10 wherein the exhaust valve
timing adjustment is based, at least in part, on the relative
effective exhaust path length of the cylinder associated with the
exhaust event.
12. A method as recited in claim 11 wherein the exhaust valve
timing adjustment is further based at least in part on the current
engine speed.
13. A method as recited in claim 11 wherein the exhaust valve
timing adjustment is further based at least in part on estimated
exhaust gas temperature.
14. A method as recited in claim 6 wherein the engine acoustics are
shaped in a manner that produces a desired engine rumble.
15. A method as recited in claim 4 further comprising altering at
least one additional engine parameter such that the torque
generated by each fired cylinder is substantially constant, wherein
the at least one additional engine parameter includes an engine
parameter selected from the group consisting of: intake valve
timing, throttle position, fuel injection parameters and spark
timing.
16. A method of controlling acoustic characteristics of an engine
having a plurality of cylinders, a crankshaft, a camshaft and a cam
phaser, the method comprising applying an periodic signal to the
cam phaser to adjust the phase of the camshaft relative to the
crankshaft, wherein the periodic signal has a frequency that
substantially corresponds to a characteristic acoustic frequency
associated with a current operational condition of the engine and
wherein the periodic signal is arranged to help suppress such
acoustic frequency.
17. A method as recited in claim 16 wherein the periodic signal is
selected from the group consisting of: a sinusoidal signal; a
triangular signal; and a rectangular signal.
18. An engine comprising: a crankshaft; at least one camshaft; a
plurality of cylinders, each cylinder having an associated piston
that is driven by the crankshaft and an associated exhaust valve
driven by the at least one camshaft; at least one exhaust manifold;
and an exhaust pipe that receives exhaust gases expelled from the
cylinders through the at least one exhaust manifold, each cylinder
having an associated exhaust path length from the cylinder through
the at least one exhaust manifold to the exhaust pipe and wherein
the respective exhaust path lengths associated with at least some
of the plurality of cylinders differ; and wherein the at least one
camshaft includes a plurality of exhaust valve cam lobes, each
exhaust valve cam lobe being arranged to actuate an associated
exhaust valve, wherein the phases of the exhaust valve cam lobes
relative to their associated pistons vary in accordance with the
exhaust path length associated with their respective cylinders such
that the sound associated with each exhaust event exits the exhaust
path in a more evenly spaced manner than would occur in the event
that the phases of the exhaust valve cam lobes were consistent
relative to their associated pistons.
19. An engine as recited in claim 18 wherein the plurality of
cylinders are arranged into first and second cylinder banks; the at
least one camshaft consists of a first camshaft and a second
camshaft, the first camshaft being associated with the first
cylinder bank and the second camshaft being associated with the
second cylinder bank; the at least one exhaust manifold consists of
a first exhaust manifold and a second exhaust manifold, the first
exhaust manifold being associated with the first cylinder bank and
the second exhaust manifold being associated with the second
cylinder bank; a Y-pipe intermediate between the exhaust manifolds
and the exhaust pipe, wherein a first exhaust path length between
the first exhaust manifold and the exhaust pipe is greater than a
second exhaust path length between the second exhaust manifold and
the exhaust pipe; and wherein respective phases of the first and
second camshafts are offset from one another.
20. An engine as recited in claim 19 wherein the camshaft offset
includes a static offset.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority of U.S. Provisional
Application No. 61/641,765 filed May 2, 2012, which is incorporated
herein by reference.
FIELD OF THE INVENTION
[0002] The present invention relates generally to the management of
acoustics in engines and is particularly applicable to controlling
acoustics during skip fire operation of an engine.
BACKGROUND
[0003] Fuel efficiency of internal combustion engines can be
substantially improved by varying the displacement of the engine.
This allows for the full torque to be available when required, yet
can significantly reduce pumping losses and improve thermal
efficiency by using a smaller displacement when full torque is not
required. The most common method today of implementing a variable
displacement engine is to deactivate a group of cylinders
substantially simultaneously. In this approach the intake and
exhaust valves associated with the deactivated cylinders are kept
closed and no fuel is injected when it is desired to skip a
combustion event. For example, an 8 cylinder variable displacement
engine may deactivate half of the cylinders (i.e. 4 cylinders) so
that it is operating using only the remaining 4 cylinders.
Commercially available variable displacement engines available
today typically support only two or at most three
displacements.
[0004] Another engine control approach that varies the effective
displacement of an engine is referred to as "skip fire" engine
control. In general, skip fire engine control contemplates
selectively skipping the firing of certain cylinders during
selected firing opportunities. Thus, a particular cylinder may be
fired during one engine cycle and then may be skipped during the
next engine cycle and then selectively skipped or fired during the
next. In this manner, even finer control of the effective engine
displacement is possible. For example, firing every third cylinder
in a 4 cylinder engine would provide an effective displacement of
1/3.sup.rd of the full engine displacement, which is a fractional
displacement that is not obtainable by simply deactivating a set of
cylinders.
[0005] U.S. Pat. No. 8,131,445 (which is incorporated herein by
reference) teaches a continuously variable displacement engine
using a skip-fire operational approach, which allows any fraction
of the cylinders to be fired on average using individual cylinder
deactivation. In a continuously variable displacement mode operated
in skip-fire, the amount of torque delivered generally depends
heavily on the firing fraction, or fraction of combustion events
that are not skipped. In other skip fire approaches a particular
firing pattern or firing fraction may be selected from a set of
available firing patterns or fractions.
[0006] In addition to fuel efficiency, another desirable attribute
of modern vehicles is a low level of noise, vibration, and
harshness, termed NVH in the auto industry. Unfortunately, efforts
to increase fuel efficiency using cylinder deactivation can result
in the engine generating different noises than when the engine is
firing at all combustion opportunities. One source of noise arising
from operating the engine is emitted at the tailpipe outlet. Other
sounds can be generated at the air intake.
[0007] The exhaust systems of commercial vehicles are designed to
produce a generally pleasing sound. Engineers have a number of
tools at hand to accomplish this task. For example, intake runners
can alter the sound from the intake manifold. Variable exhaust has
a valve to change the flow of the exhaust to a separate muffler.
Muffler size can be changed to increase exhaust noise suppression.
Quarter wave tubes can cancel sound of a certain frequency.
However, the resulting design usually assumes that all cylinders
are firing at each combustion opportunity. Even engines that use
cylinder deactivation typically only have two or three modes (two
or three displacements) in which the engine is run, and so the
exhaust system design need only consider these few operational
modes. In contrast, an engine using cylinder deactivation to
support continually variable displacement will have a wide range of
possible firing sequences. This makes tuning the exhaust system to
produce an acceptable sound difficult.
[0008] One of the aspects of the exhaust system that can affect the
sound is the relative path length from the exhaust valve of a
cylinder to the tailpipe outlet. If this distance is different from
cylinder to cylinder, the sound generated by the exhaust flow from
the cylinders will arrive at the tailpipe at a relatively different
time. A regular or repeated variation in the timing of the pulses,
as can occur with certain firing fractions, can result in
unpleasant or unexpected fluctuation in the sound of the exhaust
which might be perceived as beats.
SUMMARY
[0009] A variety of methods and devices for controlling engine
acoustics are described. In many embodiments, the relative timing
of the exhaust (and/or intake) valve opening events are dynamically
varied relative to crankshaft angle between different cylinder
exhaust (and/or intake) events. Some of the described approaches
are particularly useful in controlling engine acoustics during skip
fire operation. In some preferred embodiments, the dynamic varying
of the relative timing of the exhaust valve opening events is
arranged to at least partially compensate for acoustic delays
caused by the different effective exhaust path lengths associated
with different cylinders.
[0010] The described techniques can be used to shape the exhaust
noises in any desired manner. For example, in some embodiments, the
engine acoustics may be shaped in a manner that suppresses audible
beats that would otherwise occur in the event that all cylinders
fired during a sequence of skip fire operation were fired at a
similar relative exhaust valve opening timing. In various
implementations, the dynamic varying of the relative timing of the
exhaust valve opening events is performed while the engine is
operated at one of a group of preselected firing fractions or
firing pattern. By way of example, the exhaust systems associated
with some V-8 engines produce audible beats where operating at some
desirable firing fractions such as 1/3, 2/3, 1/5, , 3/5, 4/5 and
1/2. In such engines, the described techniques can be used when the
engine is operated at those (and other) firing fractions of
concern. The desired timing adjustments will often vary in
accordance with a number of parameters including, for example, the
relative effective exhaust path lengths associated with specific
exhaust events, the current engine speed (RPM) and the exhaust gas
temperature.
[0011] When valve operation is controlled by a camshaft (or a
plurality of camshafts), the phase of at least one camshaft can be
varied in a controlled way to suppress undesirable engine sounds.
This can be accomplished, for example, by controlling a cam phaser
in a manner that changes the relative phase of the camshaft in a
controlled way between exhausts events to mitigate the undesirable
sounds.
[0012] When the engine is operated in a region which generates an
undesirable characteristic acoustic frequency, such sounds can
sometimes be suppressed by applying a counteracting periodic signal
to the cam phaser to adjust the phase of the camshaft relative to
the crankshaft in an offsetting manner. By way of example suitable
periodic signals might include any of: a sinusoidal signal, a
triangular signal; a rectangular signal or others.
[0013] Various controllers capable of providing such acoustic
shaping are also described.
[0014] In still other described arrangement, static phase shifts
may be incorporated into a camshaft design or other valve actuation
hardware. Such static phase shifts may be used alone or in
combination with dynamic phase shifting to further enhance the
engine acoustics.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] The invention and the advantages thereof, may best be
understood by reference to the following description taken in
conjunction with the accompanying drawings in which:
[0016] FIG. 1A is a schematic of an exemplary V8 engine together
with its exhaust system.
[0017] FIG. 1B is a diagrammatic perspective view of an exemplary
exhaust system.
[0018] FIG. 2 is a graph of relative distance from the exhaust port
through the exhaust system to the tailpipe outlet.
[0019] FIG. 3 is the same graph as in FIG. 2 with the delay
expressed in crank angle degrees, at 1500 RPM.
[0020] FIG. 4 is a graph of the relative pulse delays for a firing
fraction of 1/3 at 1500 RPM. Skipped combustion events are
omitted.
[0021] FIG. 5 is a graph of the experimental data from a firing
pattern realizing a firing fraction of 1/3.
[0022] FIG. 6 is a frequency spectrum of the data shown in FIG.
5.
[0023] FIG. 7 is a diagram showing how a cam phase controller may
be integrated into the engine and engine control in accordance with
one described embodiment.
[0024] FIG. 8 is a functional block diagram that diagrammatically
indicates a control structure that is suitable for operating the
engine in accordance with one embodiment of the present
invention.
[0025] FIG. 9 is a graph of exemplary cam phasing adjustments that
may be generated by a representative cam phase controller in
response to the conditions of FIG. 4.
[0026] FIG. 10 is a graph of the resultant relative pulse delays
for a firing fraction of 1/3 at 1500 RPM with the cam phasing
adjustments of FIG. 9 implemented. Skipped combustion events are
omitted.
[0027] FIG. 11 is a functional block diagram of an alternative
embodiment using sinusoids as cam phase control signals.
[0028] FIG. 12 is a functional diagram of an alternate embodiment
of the invention showing a control structure responsive to which
bank the combustion events occur on.
[0029] FIG. 13 is cam phase interpreter used on a dual cam engine
in conjunction with the controller in FIG. 12.
DETAILED DESCRIPTION
[0030] The present invention relates generally to improving the
operation of a variable displacement engine by modifying the
fluctuation of exhaust or intake sound produced by the engine. A
method to reduce the amount of fluctuation in the sound created by
an internal combustion engine being operated in skip fire type
variable displacement mode is described. Generally, a rate and
amount of fluctuation is determined from knowledge of the firing
fraction and engine speed, and the exhaust/intake valve timing is
altered using this knowledge to mitigate the expected sound
fluctuation.
[0031] In most commercially available engines one or more camshafts
are used to drive the operation of intake and exhaust valves. The
camshaft is typically coupled to the engine crankshaft by a
synchronizing mechanism such as a timing belt, a timing chain or a
geared connection. Such mechanisms ensure that the timing of the
opening and closing of the intake and exhaust valves are
synchronized with the movements of pistons that are mechanically
coupled to the crankshaft via connecting rods. Traditionally, the
connection between the crankshaft and camshaft was preset and
unchangeable, and therefore the valve timing was fixed relative to
the crankshaft position. Although such fixed valve timing works
well, it is well understood that the engine's performance and fuel
efficiency can be enhanced by varying the valve timing as a
function of certain current operating conditions such as engine
speed and load. Therefore, over the years, a number of devices have
been developed which are designed to facilitate the adjustment of
the timing of the intake and/or exhaust valves during operation of
an internal combustion engine. Some of these devices are arranged
to adjust the rotational angle (i.e. phase) of a camshaft (and
therefore any cam lobes carried thereon) relative to a crankshaft.
Changing the phase of a camshaft relative to the crankshaft
inherently adjusts the timing of the valves controlled by that
camshaft.
[0032] One valve timing adjustment device that is popular today is
known as a cam phaser. Although their designs vary, cam phasers are
generally hydraulic or electric based devices. Hydraulic cam
phasers tend to utilize two concentric parts with a hydraulic fluid
(typically engine oil) introduced into a phaser cavity therebetween
in order to control the phase of the camshaft relative to the
crankshaft. The cam phaser typically includes electronically
controlled hydraulic valves that direct high-pressure engine oil
into the phaser cavity. Most often, a pulse width modulation (PWM)
controlled solenoid is arranged to move a spool valve that
regulates the flow of oil into the phaser cavity. Changing the
hydraulic pressure within the phaser cavity causes a slight
rotation of the camshaft relative to the cam phaser housing (and
thus the crankshaft), which results in the valve timing being
advanced or retarded in accordance with the rotation (phase shift)
of the camshaft. A powertrain control module or engine control unit
(ECU) can be arranged to adjust the camshaft timing based on
factors such as the engine load and engine speed (i.e. RPM). This
allows for more optimum engine performance, reduced emissions and
increased fuel efficiency compared to engines with fixed
camshafts.
[0033] Modern engines utilize a variety of different camshaft
assembly architectures to drive the intake and exhaust valves. For
example, some engines have a single camshaft that controls both the
intake and exhaust valves, and the relative timing between the
intake and exhaust valves cannot be altered. In other engines,
separate camshafts are provided for the intake and exhaust valves
so that the intake valve timing may be adjusted independently of
the exhaust valve timing. In multi-bank engines, each bank may have
one or two independent camshafts such that the intake and exhaust
timing on one bank is independently adjustable from the timing of
the other bank or banks. For example, each cylinder bank may have
an associated camshaft that drives both the intake and exhaust
valves in that bank of cylinders. In such arrangements it is
sometimes possible to adjust the valve timing independently by
bank. In other implementations, each bank may have two associated
camshafts--a first that drives the intake valves and as second that
drives the exhaust valves. In such arrangements it may be possible
to individually adjust the intake valves by bank and to
individually adjust the exhaust valves by bank. Still other types
of cam assemblies have more complex mechanical linkages that allow
control of the timing of individual valves. While the details of
the control algorithms used to control the cam may vary depending
on the exact cam design, the basic concepts described herein may be
applied to all types of cam operated valves. The concepts may also
be applied to non-cam actuated valves, such as electronically
controlled valves.
[0034] In one aspect of the invention, a cam phase controller is
used to determine changes to the valve timing to improve the engine
sound quality. The cam phase controller may use inputs such as
firing fraction, engine speed, and other measurements of the engine
state to determine the changes to the valve timing. Control of the
valve timing over a modest range, say .+-.10.degree. of crank
angle, may have a negligible effect on the engine efficiency, while
having a significant effect on the generation of undesirable
noise.
[0035] FIG. 1A is a schematic diagram of an exemplary V8 engine 100
and its accompanying exhaust system. The engine 100 includes two
banks 102, 103 of cylinders 101 in an engine block 104. Each bank
has an associated exhaust manifold (105 and 106 respectively) which
in the embodiment illustrated takes the form of in-line log style
exhaust manifolds. The locations of the cylinders 101 within the
engine block 104 are shown, as well as the exhaust system that
transmits the exhausted gas from each cylinder to the tailpipe
outlet. Of course, it should be appreciated that other elements,
such as a catalytic converter 121, a resonator 123, a muffler 125,
and other components which are not shown, would typically also be
present in the exhaust system. In the illustrated embodiment, the
exhaust manifolds 105 and 106 are coupled to the remainder of the
exhaust system via a segment of exhaust pipe commonly referred to
as a "Y-pipe" 108. A common characteristic of a Y-pipe used in a
V-style engine is that the length of piping between the respective
exhaust manifolds and the junction of the exhaust flows is quite
different. By way of example, consider a design in which the
spacing from one set of cylinder exhaust ports to its neighbor is
15 centimeters, and the difference in length of the branches of the
Y-pipes is 75 centimeters. These values are exemplary only and are
used to aid in clearly explaining the invention.
[0036] FIG. 1B is a diagrammatic perspective view of an alternative
representative exhaust system alone. In FIG. 1B, fan style exhaust
manifolds are shown together with the relative locations of a
Y-pipe 108, a catalytic converter 121, a resonator 123, the muffler
125 and a tailpipe 127. Of course, other exhausts systems may
incorporate more or different components and their relative
positions may vary.
[0037] As shown in FIG. 2, the amount of delay of the exhaust sound
pulses relative to the mean acoustic transit time through the
exhaust system can be determined using the assumptions and geometry
in FIG. 1A and the speed of sound through the exhaust system. In
the illustrative example, the average distance from cylinder to
tailpipe outlet is given by the formula:
L=1/2*75+1.5*15+D
where D is the unspecified length of the exhaust system common to
all cylinders. Then the distance (d1) from cylinder 1 to the
tailpipe outlet relative to an average distance is given by the
formula
d 1 = 75 + 3 * 15 + D - L = 1 / 2 * 75 + 1.5 * 15 = 600 mm
##EQU00001##
[0038] For a given exhaust gas temperature, the speed of sound (S)
can be approximated with the equation:
S=331+0.6*T
where T is the average exhaust gas temperature in degrees Celsius
and S is the sound speed in meters per second. If the exhaust gas
temperature is 500.degree. C., then S is approximately 631 meters
per second. So the exhaust pulse from cylinder 1 will appear at the
tailpipe outlet 0.60 m/631 mps=0.95 milliseconds later than the
average pulse. The assumptions used to generate the values in FIG.
2 are those given above: cylinder to cylinder spacing of 15
centimeters, and exhaust pipe length difference of 75 cm between
the cylinder banks. Other models to estimate the acoustic speed in
the exhaust system may be used.
[0039] If we further assume a particular engine speed, a speed of
sound of 631 m/s, and that the engine is a 4-stroke type, the
relative delays of FIG. 2 can be expressed in terms of crank angle
degrees. FIG. 3 shows the delays in terms of crank angle degrees
when the engine RPM is 1500. The crank angle degree scale with
engine speed so for an engine speed of 3000 RPM, the crank angles
corresponding to the differential Y-pipe length delay would be
twice the values shown in FIG. 3.
[0040] Although in-line log style exhaust manifolds are shown in
FIG. 1A for the purposes of the present explanation, it should be
appreciated that most commercially available exhaust manifolds
connect to the Y-pipe at a more central location in the manifold as
diagrammatically illustrated in FIG. 1B so that the exhaust path
lengths of the manifold runners are much closer together than in
the illustrated embodiment. This, of course, reduces the phase
shift that would be required to compensate for such
differences.
[0041] In commercially available V-8 engines, the cylinder firing
order is somewhat different than cylinder number sequence shown in
FIG. 1A and the firing order can vary somewhat from manufacturer to
manufacturer. For example, in some commercial V8 engines when all
cylinders fire, the order of the combustion events in an engine
cycle is 1-8-7-2-6-5-4-3. FIGS. 2 and 3 are based on this type of a
firing pattern and therefore show the respective delays in a time
ordered combustion event sequence.
[0042] If the engine is operating in a skip-fire mode, the pattern
of the relative delay between the sounds generated from consecutive
cylinder firings leaving the tailpipe outlet will change. To
illustrate this effect consider a firing fraction of 1/3. One
pattern to realize a firing fraction of 1/3 is to skip two
combustion opportunities and then fire at the next opportunity.
Such a pattern is illustrated in FIG. 4, which shows the relative
pulse delays of the fired events for one cycle of this firing
pattern. In the illustrated pattern, the order in which the firing
combustion events occur is 2-4-8-6-3-7-5-1 if the first fired
cylinder is cylinder #2. This pattern is particularly noteworthy
because the four sequential fired combustion events occur on bank 2
followed by four sequential fired combustion events in bank 1. Thus
the second four events all have a delay relative to the first four
events corresponding to the time it takes sound to traverse the
additional exhaust pipe length (i.e., the longer Y-pipe branch). It
is noted that this pattern of combustion events takes 6 engine revs
(i.e., 3 engine cycles) to complete (for a four-stroke engine),
since sixteen combustion events will be skipped during each
repetition of the pattern.
[0043] Comparison of FIG. 4 with FIG. 3 illustrates that the
relative delays between firing events are qualitatively different
and it should be appreciated that each firing fraction may have a
unique pattern of relative delays between the fired cylinders.
However, the nature of the relative delays can be readily
determined when the firing sequence is known.
[0044] FIG. 5 shows the experimentally measured pressure at the
tailpipe outlet versus time for a firing fraction of 1/3. The
particular firing pattern to realize the 1/3 firing fraction is
fire one combustion event, then skip two events and repeat. At an
engine speed of about 1550 RPM, the potential combustion events for
a 4-stroke V8 occur about every 10 msec. With a 1/3 firing
fraction, the actual firing events occur 34.5 times per second,
making it hard to discern the time modulation induced by the path
length differences among the cylinders.
[0045] The time waveform of FIG. 5 can be displayed as a frequency
spectrum as shown in FIG. 6. The fundamental frequency of the
pattern can be seen at 34.5 Hz, as well as two sidelobes at 30 and
39 Hz. These sidelobes contribute to a low frequency fluctuation
that can be heard when the exemplary engine uses this firing
pattern. This fluctuation arises as a beat between the fundamental
and each sidelobe; the frequency offset between the fundamental and
each sidelobe is 4.5 Hz, so the beat frequency is 4.5 Hz. This beat
frequency does not manifest itself as a directly audible tone at
4.5 Hz, but as a fluctuation in the loudness of the sound
(amplitude modulation), occurring 4.5 times per second which is
audible. Audible beats of this type can become annoying to vehicle
occupants and therefore mechanism that can suppress such beats
would be desirable.
[0046] In some described embodiments these types of beats are
suppressed by varying the cam timing in a way that reduces or
changes the differences in the relative delay between the various
cylinder intake/exhaust valve openings/closings. For example, given
the uncorrected relative delays shown in FIG. 4, a cam controller
can theoretically be arranged to adjust the exhaust valve opening
timing in a way that causes the sound associated with each exhaust
event to be substantially evenly spaced at the Y-pipe junction and
therefore at locations further downstream in the exhaust system to
the tailpipe outlet.
[0047] The phase shift of the exhaust valve timing relative to the
crankshaft necessary to compensate for these distance based delays
will vary as a function of various engine operating
conditions--most notably engine speed and exhaust gas temperature.
However, engine speed is readily available and the exhaust gas
temperature can be fairly accurately estimated based on sensor data
that is available in modern engines. Therefore a cam phase
controller can readily determine the exhaust valve opening timing
(e.g. phase shift) that would compensate for the exhaust pipe
distance delays in real time during operation of the engine.
[0048] For example, the cam controller may use the input firing
fraction or pattern, the engine RPM, other engine measurements like
temperature, and synchronizing information. Given the firing
pattern, the controller can determine the next cylinder to fire,
and from that the expected delay of the exhaust gas exiting the
tailpipe outlet. Knowledge of the engine RPM and an estimate of the
speed of sound through the exhaust gases makes it possible to
translate the adjustment of the delay from a known distance to an
equivalent measurement in crank angle. This relative delay may be
compensated by adjusting the cam phasing which controls the opening
time of the exhaust valve.
[0049] For example, given knowledge of the future desired cam phase
and the time to attain it, a cam phase controller 207 can provide a
tracking signal to a phaser controller 209 as illustrated in FIG. 7
and described below. There are several ways to do this. A piecewise
constant cam target can be generated from the knowledge of the
future desired cam settings, or a piecewise linear signal using
linear interpolation can be generated. Alternatively, other
tracking signals can be generated from well-known methods. In
general the cam phase controller seeks to reduce or change the
differences in the relative delay between the various cylinder
intake/exhaust valve openings/closings.
[0050] Although the appropriate exhaust valve opening timing and/or
phase shift timing can be determined relatively easy, the response
time of the valve control hardware (e.g., cams and/or camshaft(s))
may be limited such that it is not always possible to fully
compensate for the exhaust path length differences. Full
compensation is particularly difficult at higher engine speeds and
at higher firing fractions--and especially when sequentially fired
cylinders are in different banks so that the relative distances
traveled by sequential exhaust sound pulses are great. However,
even partial delay compensation can significantly reduce the
generation of unpleasant sounds. An example of this will be
described next with reference to FIGS. 4, 9 and 10.
[0051] In the illustrated example a cam phaser is used that is not
responsive enough to track the largest phase swings shown in FIG.
4. However, given the uncorrected relative delays shown in FIG. 4,
the cam phase controller 207 may be arranged to generate the cam
adjustment pattern shown in FIG. 9 which may be attainable by the
cam hardware. When the illustrated cam phase adjustments are
combined with the unadjusted cylinder phasing, the result is as
shown in FIG. 10. More specifically, FIG. 10 is a graph showing the
resultant relative pulse delays for a firing fraction of 1/3 at
1500 RPM with utilizing the cam phasing adjustments of FIG. 9.
Comparison of the relative delays in FIG. 10 with those in FIG. 4
illustrates how significantly the differences in the relative pulse
delay have been reduced, with a corresponding reduction in the
generation of potentially unpleasant sounds.
[0052] In general, better tracking may be achieved using cams (or
other valve actuation hardware) with a faster response or
independent cylinder control.
[0053] It has been observed that during skip fire operation of some
V-8 engines, some of the most unpleasant sounds tend to be
associated with low denominator firing fractions that tend to
oscillate between several firings in one bank followed by several
firings in the other bank thereby creating low frequency audible
beats as described above. By way of example firing fractions such
as 1/3, 1/5, 2/3, , 3/5, 4/5, 1/11 and 1/13 tend to be among the
least pleasing. For different reasons, a firing faction of 1/2 also
can sometimes generate undesirable sounds. It has also been
observed that the unpleasant engine sounds are often more
noticeable at lower engine speed than at higher engine speeds. Both
the oscillating bank characteristic of certain problematic firing
fractions and the inverse correlation of unpleasant sounds with
engine speed make it easier to mitigate audible beats when the
valve phase adjustment hardware's response time is not fast enough
to fully track exhaust path length based sound pulse delays.
[0054] FIG. 7 is a functional block diagram of an engine 100
together with selected functional blocks of an engine control unit
(ECU) or power train control module 200. In the illustrated
embodiment, the engine control unit 200 is capable of operating the
engine 100 in a skip fire mode. Accordingly, the ECU includes a
firing fraction calculator 201, a firing decision generation block
202, a firing control block 204, a cam phase controller 207 and a
phaser controller 209. The firing fraction calculator 201, the
firing decision generation block 202 and the firing control block
204 can be constructed as taught in U.S. Pat. No. 8,131,445, an in
U.S. patent application Ser. No. 13/654,244 (both of which are
incorporated herein by reference) or in any other suitable manner
in order to operate the engine in a skip fire mode.
[0055] In general, the ECU/skip fire controller 200 is arranged to
determine a cam phase setting that is appropriate for use under the
current operating conditions which is referred to herein as the
commanded cam setting or a desired baseline cam setting. As is well
understood by those familiar with the art, the cam phase setting
controls the timing of the opening and closing of the intake and
exhaust valves and therefore has a significant impact on the amount
of air drawn into the cylinders during active working cycles and
therefore the output of each fired cylinder. The commanded cam
phase setting is typically determined by the ECU based on a variety
of factors including the desired engine output, engine speed,
manifold pressure, feedback from the exhaust gas oxygen sensors
etc. This commanded cam phase setting may be the cam setting that
would be used in the absence of the acoustic management described
herein. As such, it should be appreciated that the commanded cam
phase setting will typically vary over time.
[0056] The cam phase controller 207 converts knowledge of the
geometry of the engine and exhaust system, measurements arising
from engine operation such as the engine RPM and engine
temperature, and the firing fraction to produce a cam phase control
signal which can adjust the commanded cam setting. More
specifically, the cam phase controller 207 determines a phase shift
(relative to the commanded cam phase setting) that is appropriate
to mitigate acoustics and sends a desired camshaft phase control
signal to phaser controller 209. The phaser controller in turn
directs a cam phaser in a manner that delivers the desired camshaft
phase at any given time.
[0057] In some implementations the cam phaser controller 209 would
include a solenoid duty cycle calculator (not separately shown) and
a phaser solenoid (not separately shown). The solenoid duty cycle
calculator determines the duty cycle that is appropriate to provide
to a cam phaser solenoid to cause the cam phaser to rotate the
camshaft to or hold a desired camshaft phase angle. For the
purposes of the immediately following illustration, it is assumed
that engine 100 has a single camshaft that controls both the intake
and the exhaust valves for both banks of a V-8 engine. However, as
will be apparent, the same concepts can be used to control multiple
camshafts and/or can be used with any engine configuration (e.g.,
V-engines, in-line engines, etc.) having any number of
cylinders.
[0058] Given the firing pattern, the controller 207 may determine
the next cylinder to fire, and from that the expected delay of the
exhaust sound exiting the tailpipe outlet. Knowledge of the engine
RPM and an estimate of the speed of sound through the exhaust gases
makes it possible to translate the adjustment of the delay from a
known distance to an equivalent measurement in crank angle. This
relative delay may be compensated by adjusting the cam phasing
which controls the opening time of the exhaust valve.
[0059] FIG. 8 shows a functional block diagram of a cam phase
controller 207 suitable for use in determining the cam phase
control signal. The cam phase controller 207 may utilize a number
of inputs, including, for example, the firing fraction or pattern,
the engine speed (RPM), other engine measurements like temperature,
the commanded cam phase setting determined by the ECU or other
appropriate component and synchronizing information. The input
labeled cylinder number is synchronizing information that
synchronizes the timing of the cam control and the firing control
of the cylinders with the crankshaft position. Although a
particular architecture is shown, it should be appreciated that the
cam phase controller may be implemented in many other forms.
[0060] In the embodiment illustrated in FIG. 8, the cam phase
controller includes an expected delay calculator 241 which
correlates the firing pattern to the specific cylinders that will
be fired and indicates the relative delay associated with each
fired cylinder in terms of distance. This can be accomplished using
a simple look-up table or using a variety of other mechanism. A
speed of sound estimator 243 is arranged to estimate the speed of
sound at least through the Y-pipe junction (or other location)
where the exhaust flows meet. As described above, the speed of
sound depends most heavily on the temperature of the exhaust gases.
A target crank angle calculator 246 determines a target cam phase
shift angle that compensates for the expected exhaust path delays.
As described above, this calculation will depend on the engine
speed and the speed of sound in the exhaust gases determined by
estimator 243. The target cam phase shift is added/subtracted from
the commanded cam phase provided by the engine as appropriate by
the target crank angle calculator 246 to determine the target cam
angle. As mentioned above, it should be appreciated that the
commanded (or baseline) cam phase will typically vary over time and
that at any given point in time, the phase shift would be added or
subtracted from the then current commanded cam phase. The target
cam angle is then provided to a cam phase control signal generator
249 which creates a signal that is suitable for instructing the
phaser controller to direct the cam phaser in the desired
manner.
[0061] In the embodiment discussed above with reference to FIG. 9,
stepped phase adjustments between exhaust events are used. However,
in various alternative embodiments more continuous adjustments such
as sinusoidal or triangular cam phase adjustments may be used. For
example, when the order of potential combustion events in an engine
cycle is 1-8-7-2-6-5-4-3, and a constraint of most evenly spaced
firings is used (which is advantageous from a vibration mitigation
standpoint), firing fractions of 1/3 and 1/5 (and others)
inherently switch back and forth between sequentially firing all
four cylinders in one bank and then sequentially firing all four
cylinders in the other bank. The frequency of this oscillation can
readily be calculated based on the engine speed and firing
fraction. In such circumstances, the resultant beats can be
mitigated by applying a sinusoidal phase adjustment to a camshaft.
The phase of the adjustment signal may be chosen so it is
approximately 180.degree. out of phase with the unadjusted relative
delays. For the exemplary engine of FIG. 1A running the pattern
shown in FIG. 4, the maximum relative delay occurs at cylinder 1.
As can be seen for the pattern shown in FIG. 4, the relative delay
has a somewhat sinusoidal shape. As such, the cam controller may be
arranged to provide a sinusoidal timing adjustment which would have
a maximum negative value corresponding to the firing of cylinder 1
and having a period substantially equal to that of the unadjusted
relative delays. The cam phase controller would thus minimize the
difference in the relative delay between the cylinder exhaust sound
events and therefore may minimize the undesirable noise emanating
from the tailpipe outlet.
[0062] A wide variety of cam phase controller designs can be used
to generate such a sinusoidal cam phase adjustment signal. By way
of example, FIG. 11 is a functional block diagram of a particular
cam phase controller design that is arranged to generate a
sinusoidal cam phase control signal. In the illustrated embodiment,
the firing fraction is used as the index to a look-up table 251.
The look-up table 251 provides a desired amplitude and phase for
that firing fraction. The amplitude is given for a nominal RPM, say
1500 and nominal speed of sound. This nominal amplitude is scaled
by scaler 253 which uses the estimated speed of sound (provided by
speed of sound estimator 255) and measured engine RPM to get the
desired amplitude and phase (both in crank angle degrees) for the
sinusoidal control. The sinusoidal signal is generated
synchronously to the crankshaft position by sinusoid generator 257
which is synchronized with the engine speed and firing pattern.
[0063] Although a sinusoid generator 257 is used in the embodiment
illustrated in FIG. 11, it should be appreciated that other
periodic waveforms can be applied using the same approach. By way
of example periodic triangular waveforms, periodic rectangular
waveforms and/or periodic waveforms having any other desired
geometry may be created in much the same way. Although a particular
look-up table based cam phase controller architecture is shown, it
should be appreciated that a wide variety of other cam phase
controller architectures can be used to provide the same
result.
[0064] Referring next to FIG. 12 yet another noise abatement
approach will be described. In this approach, the cam phase
adjustment is based on the bank that is fired rather than the
particular cylinder being fired. This approach can be useful in
embodiments where the banks have noticeably different exhaust path
lengths as is common when Y-pipes are used to join the exhaust
flows of two separate exhaust manifolds.
[0065] In the illustrated embodiment, a bank determiner 321
utilizes the firing pattern and synchronization information to
determine the next bank where a combustion event will occur, and
from that a default adjustment to camshaft phase for the associated
exhaust valve opening event. Speed of sound estimator 255 functions
as previously described. Scaler 325 scales the default phase
adjustment appropriately for the current engine speed and the
estimated speed of sound through the exhaust. This scaled
adjustment is then inputted to a cam phase control signal generator
349 which creates a signal that is suitable for instructing the
phaser controller to direct the cam phaser in the desired
manner.
[0066] In some commercial engines, the cam timing on separate
cylinder banks can be separately controlled. Such arrangements can
make it much easier to control the exhaust valve timing when there
is a significant difference in the path lengths of the Y-pipe
branches. This is because the exhaust path length variations
between cylinders within a single bank are typically much less than
the path length variations between banks as illustrated in FIGS. 1A
and 1B. Therefore, the required motion of the camshafts between
their respective combustion events is reduced. This can be quite
useful in many applications because the ability to change the cam
quickly may be limited. A functional diagram for a cam phase
determiner that enables individual cylinder bank control is shown
in FIG. 13. Of course, other types of control structures may be
used that achieve the same level of functionality.
[0067] In yet another embodiment, a static phase shift may be built
into the respective cams to account for the different Y-pipe
exhaust path lengths. That is, the cams can be set such that they
open their respective exhaust valves at different crankshaft phases
relative to the top dead center positions of their associated
cylinder pistons. Such an arrangement is particularly useful in
engines having fixed cam timing. In such an arrangement, the static
phase shift may be based on a phase shift that would be appropriate
at a representative engine speed and temperature. In some
circumstances, the static phase shift may be incorporated between
the different lobes of a single cam. Thus the relative valve timing
between different cylinders will be slightly offset from each
other. This offset may help compensate for the acoustic delays
between cylinders in a single bank and/or (when applicable) to
compensate for exhaust path length differences between cylinder
banks having valves driven by the camshaft. In embodiments where
different cylinder banks have valves driven by different camshafts,
the relative phase of the camshafts themselves may be statically
offset from one another to compensate for the acoustic delays
between banks (e.g., the differential exhaust path distances in
their respective Y-pipe branches). In still other embodiments, both
a static phase shift between different camshafts and a static phase
shift between different lobes on one or more of the camshafts may
be provided. In some circumstances, theses types of static phase
shifts alone can attenuate the undesirable noises to an appreciable
extent.
[0068] Static phase shift can also be useful in variable phase cams
because the static shift can significantly reduce the phase shift
that must be applied to specific cylinders to compensate for
exhaust path delays.
[0069] Many firing fractions and firing patterns do not generate
particularly undesirable acoustics and for such firing
fractions/patterns there is no need to adjust the cam phasing in
the described manner. In practice, an engine/exhaust system
operated under skip fire control can be characterized and operating
regions of concern can be identified (e.g. by firing
pattern/fraction and engine speed). The cam phase controller can
then be designed to only alter the commanded cam phase setting when
the engine is operating in the regions identified as being of
concern. For example, as suggested above, in a V8 engine, some of
the least pleasant acoustics tend to be associated with firing
fractions such as 1/3, 1/5 and integer multiples thereof. Thus, the
described use of cam phase control for acoustic mitigation could be
used only when the engine is operating at such firing fractions or
at any of a predefined set of firing fractions/patterns that are
identified as being of concern. Even within such a set, the use of
the described approach can be limited to times when the engine is
operating at engine speeds of particular concerns--as for example,
below a threshold RPM associated with a particular firing
fraction.
[0070] It should be appreciated that while some valve timing
control mechanisms are responsive enough such that the valve timing
for the controlled valves may be independently and fully adjusted
on a firing opportunity by firing opportunity basis, not all valve
timing control mechanisms are that flexible. However, good results
can still be obtained in many implementations even when the valve
timing control mechanism cannot guarantee such full control. For
example, when skip fire control is used, the skipping of combustion
events will often provide additional time (and therefore ability)
to adjust the valve timing in many circumstances. Further, it
should be appreciated that partial adjustments will alter the
acoustic characteristics of the exhaust. Indeed, in many
circumstances, partial adjustments of the valve timing will
adequately alter the resultant exhaust sounds so that its
unpleasant quality is no longer noticeable. More specifically,
because of the other sounds in the environment, a disturbing sound
need only be reduced and not eliminated to be unnoticeable. That
is, if the unpleasant sounds or audible aspects are diminished, the
other sounds perceived by the vehicle occupants may mask the
unpleasant sounds. Thus, even if a desired change in valve timing
cannot be fully implemented by the time the valves need to be
operated (due to mechanical latency or otherwise), the sound
characteristics may be improved by implementing a smaller
(obtainable) valve timing change. It should also be appreciated
that multiple sources of mitigation can be used to reduce an
annoying sound. For example, mitigation from mechanical sources
(e.g., equal length headers, dampers within the exhaust pipe, etc.)
can be used in conjunction with any embodiment of this
invention.
[0071] The embodiments described above have focused primarily on
controlling a cam. However, it should be apparent that for an
engine with advanced, e.g. electronically actuated, valve control
and no cam, the desired phasing information can be provided
directly to each valve controller without loss of functionality.
Many of the described embodiments contemplate using cam phasers to
control the valve timing. This is simply due to the fact that cam
phasers are currently the most popular commercially available
mechanism for varying valve timing. However, the same principles
can readily be applied to other valve timing adjustment mechanisms
including adjustable rocker arms, multi-lobe cams setups and
others.
[0072] When a cam phaser is present between the crankshaft and
camshaft, certain skip fire patterns can result in an unintentional
wiggle in the camshaft phase do to an effective elasticity of the
cam phaser system. Such unintended camshaft oscillations can be a
source of noise if not addressed. Such unintended camshaft
oscillations can be reduced or substantially eliminated using the
techniques described in co-assigned U.S. patent application Ser.
No. 13/842,234 entitled: "Cam Phaser Control", which is
incorporated herein by reference.
[0073] While the invention is very advantageous for variable
displacement engines using skip fire control, it can also be used
advantageously in engines having a number of discrete operating
modes, as for example, in 8 cylinder engines that operate on 4 or 8
cylinders or in 6 cylinder engines that operates on 2, 3, 4 or 6
cylinders. It can also be used advantageously in fixed displacement
engines that fire all cylinders all the time.
[0074] The description above focuses primarily on controlling the
exhaust valve timing in order to suppress undesirable sounds that
might be generated by the exhaust system during skip fire operation
of an engine. However, it should be appreciated that substantially
the same principles can be used to shape the sound of the exhaust
system in any desired manner. For example, some users associate a
low rumbling type engine sound with engine performance (e.g. the
type of rumble or brumble sound often heard in older muscle
cars--especially when the engine is idling). The same valve timing
adjustment principles can be used to cause the engine to replicate
that type of rumbling sound during idle (or in any other engine
state of interest).
[0075] Although the embodiments described above are particular
implementations of the invention, it should be clear that alternate
embodiments can work equally well. Generally, the timing of the
opening and closing of the exhaust valve will have the greatest
impact on the exhaust acoustics and therefore the description above
has focused primarily on the control of the exhaust valve timing.
In many implementations, alterations in the phasing of the intake
valves will correspond to alterations in the phasing of the exhaust
valves associated with the same cylinders (e.g., by design choice
or when a single cam is used control both the intake and exhaust
valves). However, this is not a requirement. When desired and
mechanically possible, the phasing of the intake valve opening
timing may be control somewhat separately from the exhaust valve
timing to accomplish specific design objectives. Thus, the
described approach can also be used to regulate intake sounds by
altering the timing of intake valve actuation in substantially the
same way.
[0076] It should be also appreciated that altering the timing of
the intake and/or exhaust valves will typically impact the mass air
charge (MAC) delivered to the cylinders (even if the intake and
exhaust timing is changed together). Therefore, if any changes to
the relative exhaust timing are made, it will often be desirable to
adjust other engine control parameters appropriately so that the
engine provides the desired output. By way of example, engine
parameters such as intake valve timing, throttle position, fuel
injection parameters and spark timing may be altered in a manner
that causes the torque generated by each fired cylinder to be
substantially constant. Alternatively, an estimate of the torque
output generated from each cylinder firing may be used in the
determination of a subsequent firing sequence. In still other
embodiments, the firing fraction may be changed slightly or
individual combustion events may be periodically added or
subtracted to help insure that the desired engine output meets the
driver's requested output.
[0077] As mentioned above, the speed of sound through the exhaust
system is heavily dependent on the exhaust gas temperature and
therefore good estimates of the speed of sound can often be based
on expected exhaust gas temperatures. However, the speed of sound
is also proportional to the density of the exhaust gases and
therefore other factors including firing fraction, manifold
absolute pressure (MAP) or mass air charge (MAC), spark timing,
engine speed and fuel composition can also impact the speed of
sound through the exhaust pipes. When desired, any of these factors
(or other factors deemed relevant by the cam phase controller
designer) can be incorporated into the sound speed calculation
model.
[0078] Similarly for engines with cams configured to independently
adjust the intake and/or exhaust valve opening and closing times on
each cylinder the controlling structure may be configured to reduce
undesirable noise from both the exhaust and intake. Both of these
valve control algorithms facilitate improved suppression of
unwanted acoustic noise, since the valve timing can be adjusted on
an individual basis. It should also be clear that the same types of
cam control can be applied to undesired sounds on the engine intake
as well as the exhaust. For the intake the timing of the intake
valves may be adjusted to minimize the relative delay between the
cylinder intake ports and the throttle.
[0079] The examples above primarily related to dual block engines
such as a V-8 because the acoustic concerns tend to be the greatest
in engines having Y-pipes with branches having significantly
different lengths. However, the same techniques can be used to
address acoustic concerns in in-line 4 and 6 cylinder engines, V-6
engines and most any other piston engine designs. As suggested
above, the described techniques are also extremely useful in
engines that can separately adjust the timing of the intake and
exhaust valves as is possible in many engines having multiple
camshaft or concentric camshafts. Therefore, the present
embodiments should be considered illustrative and not restrictive
and the invention is not to be limited to the details given herein,
but may be modified within the scope and equivalents of the
appended claims.
* * * * *