US20130291816A1 - Using valve timing to improve engine acoustics - Google Patents

Using valve timing to improve engine acoustics Download PDF

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Publication number
US20130291816A1
US20130291816A1 US13886107 US201313886107A US2013291816A1 US 20130291816 A1 US20130291816 A1 US 20130291816A1 US 13886107 US13886107 US 13886107 US 201313886107 A US201313886107 A US 201313886107A US 2013291816 A1 US2013291816 A1 US 2013291816A1
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Prior art keywords
exhaust
engine
cam
phase
timing
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Abandoned
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US13886107
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Louis J. Serrano
Joshua P. Switkes
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Tula Technology Inc
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Tula Technology Inc
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    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F01MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
    • F01LCYCLICALLY OPERATING VALVES FOR MACHINES OR ENGINES
    • F01L1/00Valve-gear or valve arrangements, e.g. lift-valve gear
    • F01L1/34Valve-gear or valve arrangements, e.g. lift-valve gear characterised by the provision of means for changing the timing of the valves without changing the duration of opening and without affecting the magnitude of the valve lift
    • F01L1/344Valve-gear or valve arrangements, e.g. lift-valve gear characterised by the provision of means for changing the timing of the valves without changing the duration of opening and without affecting the magnitude of the valve lift changing the angular relationship between crankshaft and camshaft, e.g. using helicoidal gear
    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F02COMBUSTION ENGINES; HOT-GAS OR COMBUSTION-PRODUCT ENGINE PLANTS
    • F02DCONTROLLING COMBUSTION ENGINES
    • F02D13/00Controlling the engine output power by varying inlet or exhaust valve operating characteristics, e.g. timing
    • F02D13/02Controlling the engine output power by varying inlet or exhaust valve operating characteristics, e.g. timing during engine operation
    • F02D13/0203Variable control of intake and exhaust valves
    • F02D13/0215Variable control of intake and exhaust valves changing the valve timing only
    • YGENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02TCLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO TRANSPORTATION
    • Y02T10/00Road transport of goods or passengers
    • Y02T10/10Internal combustion engine [ICE] based vehicles
    • Y02T10/18Varying inlet or exhaust valve operating characteristics

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.

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 ⅓rd 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 ⅓, ⅔, ⅕, ⅖, ⅗, ⅘ and ½. 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 ⅓ 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 ⅓.
  • [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 ⅓ 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° 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:
  • [0000]

    L=½*75+1.5*15+D
  • [0000]
    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
  • [0000]
    d 1 = 75 + 3 * 15 + D - L = 1 / 2 * 75 + 1.5 * 15 = 600 mm
  • [0038]
    For a given exhaust gas temperature, the speed of sound (S) can be approximated with the equation:
  • [0000]

    S=331+0.6*T
  • [0000]
    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° 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 ⅓. One pattern to realize a firing fraction of ⅓ 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 ⅓. The particular firing pattern to realize the ⅓ 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 ⅓ 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 ⅓ 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 ⅓, ⅕, ⅔, ⅖, ⅗, ⅘, 1/11 and 1/13 tend to be among the least pleasing. For different reasons, a firing faction of ½ 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 ⅓ and ⅕ (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° 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 ⅓, ⅕ 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.

Claims (20)

    What is claimed is:
  1. 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. 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. 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. 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. 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. 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. 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. 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. 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 ⅓, ⅔, ⅕, ⅖, ⅗, ⅘ and ½.
  10. 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. 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. 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. 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. 14. A method as recited in claim 6 wherein the engine acoustics are shaped in a manner that produces a desired engine rumble.
  15. 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. 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. 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. 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. 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. 20. An engine as recited in claim 19 wherein the camshaft offset includes a static offset.
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