Abstract:
The invention describes a rocker arm displacement system for variable valve timing in internal combustion engines. More specifically, the system provides a rocker arm displacement system including a rocker arm having a pivot, a cam-contacting portion and a valve stem contacting portion and a linear displacement system for linear displacement of the cam-contacting portion with respect to a variable profile cam.

Description:
RELATED APPLICATIONS  
         [0001]    This application is related to Canadian Patent Applications 2,257,437 and 2,315,595, and to Applicant&#39;s co-pending PCT application to “Improvements in Cam-Contacting Devices” filed Aug. 7, 2001, all of which are incorporated herein by reference.  
         FIELD OF THE INVENTION  
         [0002]    The invention describes a rocker arm displacement system for variable valve timing in internal combustion engines. More specifically, the system provides a rocker arm displacement system including a rocker arm having a pivot, a cam-contacting portion and a valve stem contacting portion and a linear displacement system for linear displacement of the cam-contacting portion with respect to a variable profile cam.  
         BACKGROUND OF THE INVENTION  
         [0003]    The design of an internal combustion engine requires numerous trade-offs between conflicting design or performance parameters particularly with respect to the design of cam profiles and their effect on valve timing.  
           [0004]    For example, in the design of an engine, a designer may wish to minimize exhaust emissions and provide increased fuel economy without compromising satisfactory engine performance. In the past, the design of such an engine would be limited by these conflicting parameters leading the designer to compromise with the design to achieve a balance between the parameters. As a result, designers have been forced to focus on a primary performance goal (such as lower emissions) which may be to the detriment of desired engine performance (such as torque or idle stability). These compromises are essentially caused by the lack of the designer&#39;s ability to incorporate breathability into the engine-that is, an optimal intake of fuel and air and the exhaust of spent gases after combustion at different operating speeds.  
           [0005]    The timing of air intake and exhaust (breathing) is controlled by the shape and phase angle of cams. To optimize breathing, engines require different valve timing at different speeds. When rpm increases, the duration of intake and exhaust stroke decreases with the result that combustion air cannot enter the combustion chamber fast enough and exhaust gases similarly cannot leave the combustion chamber fast enough. The best solution to this problem is to open the inlet valves earlier and close the exhaust valves later. That is, the overlapping between intake period and exhaust period should be increased as rpm increases.  
           [0006]    Furthermore, lift and duration of valve opening also affects breathability. At high speed, higher lift quickens air intake and exhaust, However, at lower speed such lift will generate counter effects like deteriorating the fuel and air mixing process resulting in a decrease in output or even misfire. Therefore the lift and duration should also be variable according to engine speed.  
           [0007]    With fixed cam engines, a single cam profile is used with the result that engineers choose a single timing sequence (or cam profile) which is usually a compromise based on the desired general characteristics of the vehicle. For example, a van may adopt less overlapping for the benefits of low speed output whereas a racing engine may adopt considerable overlapping for high-speed power. An ordinary sedan may adopt valve timing optimised for mid-range rpm so that both the low speed drivability and high-speed output will not be sacrificed to a significant extent. Similarly, lift and duration is determined for a limited rpm range. In other words, the valve timing is optimized for a limited speed range.  
           [0008]    Variable valve timing provides a solution to the above problems by enabling the adjustment of valve timing as rpm changes with the result that power and torque are optimized across a wide rpm band.  
           [0009]    Several major manufacturers have designed and implemented various forms of variable valve timing systems, For example, Honda has successfully implemented its VTEC system (Valve Timing Electronic Control) which essentially uses two (or three) different set of cams which are specifically optimized for different rpm ranges and mechanically actuated based on engine speed. The VTEC system does not allow a continuous change of timing but does provide two (or three) distinct phases of performance. The cam-changing system of the VTEC engine is mechanically complex.  
           [0010]    Another system is the Toyota VVTL-I which provides both variable camphasing and lift and duration. This system does not provide continuous variation of lift and duration but instead utilizes a 2-stage variable lift design.  
           [0011]    Other proposed designs include the use of continuously variable cam profiles in which a camshaft having a three-dimensional cam profile is linearly displaced across a cam follower. Such systems allow lift, duration and degreeing to be addressed with a 3 dimensional cam profile. However, such systems have not been successful as a result of the failure of components of cam followers brought on by contact stresses at the cam follower/cam interface. That is, the cam followers of a fixed profile cam minimize the compressive loads of the cam follower on the cam by distributing the compressive load across a wider area. In a continuously variable system, a fine contact point is required in order to optimize the resolution of the cam profile being used. This fine contact point, for a given valve spring pressure, results in higher interface pressures as compared to the standard contacting devices. These higher pressures will cause cam followers, implemented using conventional materials, to fail.  
           [0012]    Thus, while various variable timing systems have been proposed, there remain several problems with respect to the implementation of the technology to its theoretical potential. That is, the current state-of-the-art as implemented by Honda, Toyota and other major engine manufacturers provides variable valve timing systems which are mechanically complex and which do not provide true continuously variable timing with respect to lift, duration and phasing. Furthermore, variable valve timing systems which propose continuously variable lift and duration have been unsuccessful as a result of cam follower failure.  
           [0013]    Accordingly, there has been a need for variable valve timing systems which provide the ability to adjust lift, duration and degreeing with a relatively simple mechanical system which does not require the complex mechanical interplay of many mechanical components. In particular, there has been a need for a system which provides for a fine cam follower/cam interface and which can survive the high compressive loads at this interface. Further still, there has been a need for a variable valve timing system which can be retrofit to existing vehicles by replacing a rocker arm/fixed cam system with a linear displaceable rocker arm/variable profile cam system or factory installed into new engines. Further still, there has been a need for systems which will significantly reduce fuel consumption and emissions particularly during idling without affecting vehicle performance.  
           [0014]    In particular, and with reference to applicant&#39;s copending Canadian application 2,315,595, a solution to cam follower failure is provided in which ceramic materials are used as cam-contacting devices where ceramic materials have been demonstrated to overcome the problems of cam follower failure. More particularly, the use of silicon nitride has been shown to be particularly effective as a material for use as a cam-contacting surface in an internal combustion engine.  
         SUMMARY OF THE INVENTION  
         [0015]    In accordance with one embodiment of the invention, there is provided a rocker arm displacement system for linear displacement of a rocker arm having a cam contacting portion with respect to a rotating, variable-profile cam comprising a linear displacement system operatively connected to a rocker arm.  
           [0016]    In accordance with another embodiment, there is provided a rocker arm displacement system comprising:  
           [0017]    a rocker arm having a pivot, a cam contacting portion and a valve stem contacting portion;  
           [0018]    a linear displacement system for linear displacement of the cam contacting portion with respect to a variable profile cam  
           [0019]    In further embodiments, the cam-contacting device has improved thermal dissipation properties including a coefficient of thermal expansion less than 3×10 −6 /degree Celsius.  
           [0020]    In other embodiments, the cam contacting system is a ball bearing and a bearing race and support and the coefficient of thermal expansion of the ball bearing is less than the coefficient of thermal expansion of the bearing race and support.  
           [0021]    Further still, in preferred embodiments, the cam contacting system is ceramic selected from any one of silicon nitride (including CERALLOY 147-31E, 147-31N, 147-1E, or 147-1) or silicon carbide.  
           [0022]    In other embodiments, it is preferred that the cam contacting devices is selected from any one of a radiused wheel, a ball bearing or a semi-spherical surface and wherein the cam contacting surface is selected from any one of CERALLOY 147-31E, 147-31N, 147-1E, or 147-1. The valve stem contacting portion may also be selected from these materials.  
           [0023]    In a further still embodiment, the linear displacement system includes a sliding block operatively connected to a linear displacement cylinder and the sliding block pivotally retains the rocker arm.  
           [0024]    It is also preferred that a lash adjustment system is provided within the sliding block, rocker arm or valve stem. In a more specific embodiment, the lash adjustment system is hydraulic and includes a piston within the sliding block for setting the range of motion of the rocker arm within the sliding block.  
           [0025]    In a further embodiment, the rocker arm is pivotally connected to the valve stem.  
           [0026]    In a more specific embodiment, the invention provides a rocker arm displacement system comprising:  
           [0027]    a rocker arm having a pivot, a cam contacting system and a valve stem contacting system, the cam contacting system a silicon nitride ball bearing rotatably retained within the rocker arm;  
           [0028]    a silicon nitride valve stem tip for operative engagement with the valve stem contacting system; and  
           [0029]    a sliding block operatively connected to a linear displacement cylinder for linear displacement of the cam contacting portion with respect to the variable profile cam wherein the sliding block pivotally retains the rocker arm and wherein the sliding block includes a hydraulic lash adjustment system.  
           [0030]    In another embodiment, the invention provides a method of retrofitting an internal combustion engine with a rocker arm displacement system as in claim 1, the internal combustion engine having a valve cover, valve stems, a rocker arm assembly and a camshaft comprising the steps of:  
           [0031]    a) removing the valve cover to expose the valve stems, rocker arm assembly and camshaft;  
           [0032]    b) removing the camshaft and installing a variable profile camshaft;  
           [0033]    c) installing a rocker arm displacement system having a rocker arm, a cam contacting portion, a valve stem contacting portion and a linear displacement system.  
           [0034]    Further still, the invention provides an internal combustion engine characterized by a rocker arm displacement system for linear displacement of a rocker arm with respect to a camshaft. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0035]    The invention is described with reference to the following drawings in which:  
         [0036]    [0036]FIGS. 1A, 1B and  1 C are schematic drawings of the geometric design of three embodiments of a rocker arm displacement system showing the relative positions of the valve stem/spring, pivot and cams with respect to a rocker arm;  
         [0037]    [0037]FIGS. 2A and 2B are forward and rear perspective views of a rocker arm displacement system for a variable valve timing system for an overhead cam engine in accordance with one embodiment of the invention;  
         [0038]    [0038]FIG. 2C is a partial cross-sectional view of a rocker arm displacement system for a variable valve timing system for an overhead cam engine in accordance with one embodiment of the invention showing details of a linear displacement cylinder;  
         [0039]    [0039]FIGS. 3A and 3B are forward and end perspective views of a rocker arm displacement system for a variable valve timing system for an overhead cam engine in accordance with another embodiment of the invention;  
         [0040]    [0040]FIG. 3C is a partial cross-sectional view of a rocker arm displacement system for a variable valve timing system for an overhead cam engine in accordance with another embodiment of the invention showing details of a linear displacement cylinder and sliding block; and,  
         [0041]    [0041]FIG. 4 is a schematic diagram of a further embodiment of a rocker arm displacement system for variable valve timing system for an overhead cam engine in accordance with a further embodiment of the invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0042]    System Overview  
         [0043]    With reference to the Figures, a rocker arm displacement system  10  for variable valve timing is described.  
         [0044]    The system generally includes a rocker arm (R) in operative connection with a valve stem (V)/spring (Vs), a cam contacting device (CCD) contacting a variable profile cam shaft (C) and a linear displacement system (LDS) for linear displacement of the rocker arm with respect to the cam shaft.  
         [0045]    Different embodiments of the system are schematically illustrated in FIGS. 1A, 1B and  1 C wherein options for the geometric layout of the rocker arm (R), camshaft (C), valve stem (V) /spring (Vs, with ↑ showing the direction of spring force) are shown with respect to a rocker arm pivot (P). FIG. 1A shows a rocker arm having a centrally located pivot with both the valve stem/spring and camshaft on the same side of the rocker arm. FIG. 1B shows a rocker arm with the pivot at one end with a centrally positioned camshaft and with the valve stem/spring on the opposite side and end of the rocker arm. FIG. 1C shows an embodiment with the pivot at one end with a centrally positioned valve stem/spring and with the camshaft on the opposite side and end of the rocker arm.  
         [0046]    [0046]FIGS. 2A, 2B and  2 C show perspective and cross-sectional views of the embodiment of FIG. 1A. The camshaft includes variable profile lobes  20  on a rotating camshaft C. Rocker arms R for each cam are mounted on a rocker shaft  22  at the rocker arm pivot and include cam followers or cam-contacting devices  24  biased against each respective variable profile lobes by valve springs Vs acting on the opposite end of the rocker arm. A linear displacement system (LDS) is operatively connected to the rocker shaft and includes a hydraulic cylinder  26  for linear displacement of the rocker shaft  22  and, hence, linear displacement of the cam followers  24  with respect to the variable profile lobes  20 .  
         [0047]    As shown in FIG. 2A, the cam followers  24  are spherical ball bearings rotatably retained within bearing races within the rocker arm. As described in applicant&#39;s copending application, Canadian application 2,315,595, the spherical bearing provides a fine contact point with respect to the cam lobe thereby permitting continuous setting of the valve timing between the end limits of the cam  20 .  
         [0048]    With reference to FIG. 2B, the opposite ends of the rocker arms contact the valve stems and include a valve stem contacting device such as valve stem bearing  28  rotatably retained within a tip ball retainer  30  (as shown in FIG. 2 b ). Alternatively, valve stem bearing may be rotatably retained within the rocker arm (not shown). In addition, the rocker arm will preferably include a hydraulic lash adjustment system  32  enabling adjustment of the clearance between the rocker arm and valve stem.  
         [0049]    As with the cam follower  24  and camshaft, the valve stem end of the rocker arm is displaced with respect to the valve stem when actuated by the LDS.  
         [0050]    With reference to FIG. 2C, a partial cutaway of the actuation cylinder of the linear displacement system is shown having dual hydraulic oil ports  34 . In this embodiment, the actuation cylinder  26  may be positively positioned at a full range of positions across the width of the cam as may be determined by an appropriate control algorithm, such as one where the linear position of the rocker arms is proportional to engine rpm.  
         [0051]    With reference to FIGS. 3A, 3B and  3 C, a second embodiment of the system corresponding to that shown schematically in FIG. 1B is described. In this embodiment, the pivot is located at one end of the rocker arm with the cam followers located in the middle of the rocker arm and valve stem/spring located at the opposite end.  
         [0052]    As shown in FIG. 3A, the linear displacement system includes a sliding block  40  which pivotally retains the rocker arm. In this embodiment, the sliding block permits pivotal up and down motion of the rocker arm but prevents side-to-side motion of the rocker arm within the sliding block. As shown, the sliding block is retained within a retaining system  42  allowing linear displacement of the sliding block under the control of the linear displacement cylinder  26 .  
         [0053]    The sliding block may also contain a lash adjustment system  32   a  permitting adjustment of the clearance between the valve stem and rocker arm. The lash adjustment system may be hydraulically controlled wherein the sliding block includes hydraulic pistons  32   b  (as shown in cutaway in FIG. 3C) for setting the range of motion of the rocker arm.  
         [0054]    As in the FIG. 2 embodiment, the valve stem includes a valve stem contacting device such as a valve stem bearing  28  rotatably retained within a tip ball retainer  30  (as shown in FIG. 3B).  
         [0055]    In another embodiment, as shown schematically in FIG. 4 (plan view), the rocker arm is pivoted about a second axis P 2  orthogonal to the main pivot axis P of the rocker arm. That is, the rocker arm does not include a tip bearing or roller but rather is pivotally connected to the valve stem and is pivotally connected to the LDS thereby allowing rotational movement of the rocker arm with respect to the valve stem.  
         [0056]    Cam Contacting Device  
         [0057]    The cam-contacting device may be embodied in different forms including a ball bearing, a roller, a tapered wheel and a half sphere as described in applicant&#39;s copending applications. The cam-contacting device will preferably provide a fine contact point between the cam-contacting device and cam so as to provide for continuous variability of the cam profile between the different ends of the cam. Furthermore, the cam-contacting device is preferably a ceramic material such as silicon nitride, which provides improved thermal properties between the cam contacting device and the cam to accommodate the high contact stresses.  
         [0058]    Valve Stem Contacting System  
         [0059]    The valve stem contacting system may be embodied in different forms to achieve the result of valve actuation while permitting linear or pivotal movement of the rocker arm with respect to the valve stem. Accordingly, the valve stem contacting system may include a bearing system or other contact system located on either the rocker arm or valve stem or between the rocker arm and valve stem. The bearing system may be a ball bearing or roller bearing. Other contact systems may include flat or curved surfaces engageable with one another and linearly displaceable with respect to one another. The valve stem contacting system preferably utilizes a ceramic material such as silicon nitride, which provides improved thermal properties between the rocker arm and the valve stem to accommodate the high contact stresses.  
         [0060]    As shown in FIG. 4, the rocker arm may also be pivotally connected to the valve stem.  
         [0061]    Linear Displacement System  
         [0062]    The linear displacement system may be actuated by a hydraulic cylinder as described above or other linear motion systems as are known in the art including electromechanical systems.  
         [0063]    Retrofitability  
         [0064]    The rocker arm displacement system as described is readily retrofit to existing overhead cam engines. In order to retrofit the system to an existing overhead cam engine, the following general procedure would be followed. Initially, the valve cover would be removed to expose the existing rocker arm and camshaft assembly. The existing fixed profile camshaft would be removed and replaced with a variable profile camshaft. The existing rocker arm assembly would similarly be removed. Any required modification to the valve stem tips would be completed for the particular design of the valve stem contacting system. A new rocker arm displacement assembly would be installed between the camshaft and valve stems, likely requiring the addition of an adaptive plate to permit linear movement of the new rocker arm assembly. Modification to the valve cover will likely be required for the linear displacement system to extend through the valve cover, if required. Alternatively, a specifically designed valve cover assembly may be utilized to accommodate the linear displacement system. The linear displacement system would be connected to an appropriate control system such as an electro-hydraulic, electromechanical or electrical system.  
         [0065]    It is also envisaged that specific cylinder heads be designed to incorporate a rocker arm displacement system such that retrofitting is not required but rather is factory installed.