Abstract:
A tappet assembly includes a first follower engaging a cam lobe profile, and a second follower engaging an engine poppet valve. A latch member is operable to fix the first follower in an extended position, to provide a high lift condition. The first and second followers define first and second stop surfaces, respectively, such that when the latch member is retracted, the lift portion engages the first follower and moves it toward the poppet valve until the stop surfaces engage, and thereafter, further downward movement of the first follower moves the second follower to provide a low lift. One benefit of this feature is to prevent accumulation of unburned fuel in a port fuel injection type of gasoline engine.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority, under 35 U.S.C. 119, of earlier-filed EPO Application No. 05257265.8, filed Nov. 25, 2005.  
       BACKGROUND OF THE DISCLOSURE  
       [0002]     The present invention relates to tappets for use in internal combustion engines, to transmit motion directly from a cam lobe profile of an engine cam shaft to an engine poppet valve. Thus, the present invention relates to engine valvetrain of the “direct acting” type.  
         [0003]     Although the improved tappet of the present invention could be utilized in various types of engines, in terms of the type of fuel utilized by the engine, the present invention is especially advantageous when used in a gasoline engine with Port Fuel Injection of the type utilizing intake valve deactivation for one of a pair of intake poppet valves. The invention is even more advantageous in an engine valve control system of the type described above which is utilized for “swirl” control, as that term is now well understood by those skilled in the engine art.  
         [0004]     In terms of the type of lift imparted to the engine poppet valve in a direct acting valve train, there are two general categories of such tappets. The first is the conventional mechanical or hydraulic tappet (“bucket tappet”) which receives its input from a single cam lobe profile and therefore, imparts only a single “valve event” to the engine poppet valve. The second category comprises “dual lift” tappets of the general type illustrated and described in U.S. Pat. No. 5,193,496. In dual lift tappets of the type taught in the &#39;496 patent, the tappet includes a central portion and an outer portion with the central portion engaging a low lift cam, to produce a low lift valve event, and the outer portion of the tappet engaging a pair of high lift cam lobe profiles to provide a high lift valve event. Thus, the known, prior art dual lift direct acting tappet typically has associated therewith three separate cam lobe profiles (one low lift, and two high lift), making such an arrangement extremely expensive to manufacture and difficult to package.  
         [0005]     The improved tappet, and improved valve control system of the present invention was developed in connection with an effort to improve what is referred to as the “charge motion” (i.e., the flow pattern of the air-fuel mixture after it flows past the intake poppet valve). Specifically, the effort was to increase the charge motion at low to medium engine speeds, on gasoline engines utilizing port fuel injection. It was believed that a dual lift tappet arrangement was needed for this particular application, although for the reasons discussed previously, it was clearly not acceptable to require three, or even two, separate cam lobe profiles for each intake poppet valve, merely to achieve the desired dual lift valve event for each intake poppet valve.  
         [0006]     It was also determined during the course of development of the present invention that for this particular type of engine application, utilizing port fuel injection, it would not be acceptable for the dual lift tappet to provide, selectively, either a normal lift (“high lift”) valve event, or a deactivated valve event. During the low speed operation of the engine, with one of the two intake poppet valves deactivated, it was observed that because of the fuel being injected directly into the intake port, a quantity of fuel would accumulate behind the deactivated valve, over a period of time. Then, once that particular intake poppet valve would again be operated in the normal lift mode, the quantity of fuel which had accumulated would be drawn into the combustion chamber, and could result in an uncontrolled combustion condition. Such an uncontrolled combustion condition could lead to various operating problems of the engine, such as extra, undesirable emissions and NVH (“noise-vibration-harshness”) type conditions.  
       BRIEF SUMMARY OF THE INVENTION  
       [0007]     Accordingly, it is an object of the present invention to provide an improved tappet and an improved valve control system for use on intake engine poppet valves, wherein the improved tappet and valve control system overcome the above-described problems of the prior art.  
         [0008]     It is a more specific object of the present invention to provide such an improved tappet and improved valve control system such that the intake poppet valve can operate in either a normal lift mode or in another mode which is at least able to prevent the accumulation resulting from the operation of a port fuel injection system.  
         [0009]     It is a related object of the present invention to provide an improved tappet and improved valve control system which accomplishes the above-stated objects, but without the need for multiple cam lobe profiles to achieve the multiple lift conditions of each intake poppet valve.  
         [0010]     The above and other objects of the invention are accomplished by the provision of a tappet for use in an internal combustion engine including an engine poppet valve and a camshaft having a cam lobe profile including a base circle portion and a lift portion. The tappet is operably disposed between the cam lobe profile and the engine poppet valve. The tappet comprises an inverted, cup-shaped first follower adapted for engagement with the cam lobe profile, and an upright, cup-shaped second follower disposed for reciprocable movement within the first follower, and adapted for engagement with the engine poppet valve. A lost motion spring is operably associated with the first and second followers, and biases the first follower toward an extended position, relative to the second follower and into engagement with the base circle portion of the cam lobe profile.  
         [0011]     The improved tappet is characterized by a latching mechanism operably associated with the second follower and including a latch member moveable between a retracted, disengaged position and an extended, engaged position, engaging the first follower to fix the first follower in the extended position, relative to the second follower, and to provide a high lift of the engine poppet valve. The first and second followers define aligned first and second stop surfaces, respectively, disposed such that when the latch member is in the retracted, disengaged position, engagement of the lift portion of the cam lobe profile with the first follower moves the first follower toward the engine poppet valve. This movement of the first follower compresses the lost motion spring until the first stop surface engages the second stop surface, and thereafter, further movement of the first follower moves the second follower to provide a low lift of the engine poppet valve. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  is a perspective view of a portion of a valve control system utilizing the tappet of the present invention.  
         [0013]      FIG. 2  is a partially broken-away, exploded, perspective view of the improved tappet of the present invention.  
         [0014]      FIG. 3  is a partially broken-away, assembled perspective view of the improved tappet of the present invention.  
         [0015]      FIGS. 4 and 5  are graphs of Lift and of cam profile velocity, respectively, as a function of Cam Angle (in degrees), illustrating the operation of the improved tappet and the improved valve control system of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0016]     Referring now to the drawings, which are not intended to limit the invention,  FIG. 1  is a simplified perspective view of a valve control system of the type to which the present invention relates, and which is typically referred to as being of the “direct acting” type. In the valve control system shown in  FIG. 1 , there is an engine poppet valve generally designated  11  including a head portion  13  and a valve stem  15 . Received within the cylinder head (not shown) is a valve seat insert  17  such that, when the engine poppet valve  11  is in the closed position, the head portion  13  is seated against the valve seat insert  17  in a manner well known to those skilled in the art of internal combustion engines. Thus, in  FIG. 1 , the engine poppet valve  11  is illustrated in a fully open condition (to be referred to subsequently as a “high lift” condition).  
         [0017]     Opening and closing motion is transmitted to the engine poppet valve  11  by means of a camshaft  19  on which is formed a cam lobe  21  having a cam lobe profile (which will also hereinafter bear the reference numeral “ 21 ”), including a base circle portion  23  and a lift portion  25 . Disposed between the cam lobe profile  21  and the engine poppet valve  11  is a tappet assembly, generally designated  27 .  
         [0018]     Referring now primarily to  FIG. 2 , but also to  FIG. 3 , the tappet assembly  27  comprises an outer follower  29  which, in the subject embodiment, and by way of example only, comprises an inverted (i.e., opening “downward” in its normal orientation), cup-shaped element. The outer follower  29  includes an “upper” wall portion  30  providing an upper follower surface  31 , adapted to be in substantially constant engagement with the cam lobe profile  21 .  
         [0019]     The tappet assembly  27  also includes an inner follower  33  which is preferably disposed for reciprocable movement within the outer follower  29 . As may best be seen in  FIG. 3 , the inner follower  33  includes a lower wall portion  34  which defines, on its underside, a valve tip surface  35 . Preferably, the inner follower  33  is also generally cup-shaped, but unlike the outer follower  29 , the inner follower  33  preferably opens upwardly as is shown in  FIG. 2 . It will be understood that, as used herein, the terms “upper” and “lower”, and words of similar import should not be construed as limitations on the invention, but instead, as merely explanatory, assuming the tappet assembly is in its normal operating position, as shown in  FIG. 1 .  
         [0020]     The cylindrical wall of the outer follower  29  defines, on the inside surface thereof, an annular groove  37  and disposed therein, when the tappet assembly  27  is fully assembled, is a stopping retainer  39 , which may be in the general form of a C-clip, as is also visible in  FIG. 3 . Disposed axially between the upper wall portion  30  of the outer follower  29 , and the lower wall portion  34  of the inner follower  33 , is a coiled compression spring  41 , the function of which is to bias the outer follower  29  away from the inner follower  33  to an extended position as shown in  FIG. 3 . This extended position shown in  FIG. 3  would correspond to the condition when the upper follower surface  31  is in engagement with the base circle portion  23  of the cam lobe  21 . The extended position of the outer follower  29 , relative to the inner follower  33 , is determined by the location of the stopping retainer  39 .  
         [0021]     Surrounding the coiled compression spring  41  is an oil passage wall member  43 , which preferably comprises a thin piece of steel or other metal. The inner follower  33  defines an internal annular groove  45  (see  FIG. 2 ) which receives pressurized fluid by means of an oil feed passage  47 . Once the oil passage wall member  43  is put in place within the inner follower  33 , the internal annular groove  45  is “closed” and comprises an annular pressure chamber, receiving pressurized fluid through the oil feed passage  47  whenever it is desired to operate the tappet assembly  27  in a latched condition, to be described subsequently. Pressurized fluid enters the oil feed passage  47  in the inner follower  33  by means of a fluid port  49  formed in the cylindrical wall of the outer follower  29 , as is shown in  FIG. 2 .  
         [0022]     Referring still primarily to  FIG. 2 , the cylindrical wall of the outer follower  29  defines a plurality of latch windows  51 , each of which includes an upper arcuate latch surface  53  (best seen in  FIG. 3 ). The inner follower  33  defines a plurality (corresponding to the number of latch windows  51 ) of radial latch bores  55 , and disposed in each latch bore  55  is a cylindrical latch member  57  defining a planar latch surface  59 . As is well known to those skilled in the engine component art, the latch member  57  is normally (in the absence of pressurized fluid in the fluid port  49 ) held in a retracted, disengaged position by means of a return spring  61 , the location of which may best be seen by reference to  FIG. 3 .  
         [0023]     Referring still primarily to  FIG. 2 , the cylindrical wall of the outer follower  29  defines a vertically oriented slot  63  and the inner follower  33  defines a bore  65 . Received within the bore, and preferably, in a press-fit relationship therein, is an orientation pin  67 , the outer end of the pin  67  being received within the vertically-oriented slot  63 . Thus, the rotational position of the outer follower  29 , relative to the inner follower  33  is fixed (to be non-rotatable), while relative axial movement is permitted with the outer end of the orientation pin  67  moving vertically within the slot  63 , in a manner well known to those skilled in the art.  
         [0024]     The upper wall portion  30  of the outer follower  29  includes an annular, raised portion  71 , which is preferably formed integrally with the outer follower  29 . The annular portion  71  defines, on its underside, an annular stop surface  73 . Similarly, the inner follower  33  defines an annular, upstanding portion  75  including, on the upper side thereof, an annular stop surface  77 . Preferably, the annular portion  71  and the annular portion  75  have approximately the same inner and outer diameters, such that the annular stop surfaces  73  and  77  are, under the appropriate operating circumstances, disposed to be in a face-to-face, engaging relationship, as will be described in greater detail subsequently. Preferably, and as may best be seen in  FIG. 3 , the compression spring  41  is selected such that its outer diameter is just slightly less than the inner diameter of the annular portion  71  and of the annular portion  75 . As a result, during relative axial movement of the followers  29  and  31 , the compression spring  41  is supported by, and contained within, the annular portions  71  and  75 .  
         [0025]     When the valve control system of the present invention is operating in the base circle mode, the coiled compression spring  41  maintains the upper follower surface  31  in engagement with the base circle portion  23  while the valve tip surface  35  remains in engagement with the stem tip of the valve stem  15  of the engine poppet valve  11 , in a manner well known to those skilled in the art.  
         [0026]     When it is desired to operate the tappet assembly  27  in a normal lift (“high lift”) mode, pressurized control fluid is communicated to the fluid port  49  and from there flows through the oil feed passage  47 , filling the annular groove  45 . The annular groove  45  is in open communication with each of the radial latch bores  55 , such that the presence of control pressure in the annular groove  45  will bias the latch members  57  radially outward from their retracted, disengaged positions to their extended, engaged positions, in opposition to the biasing force of the return spring  61 . When the latch members  57  are in the latched position, with the latch surface  53  of the outer follower  29  engaged by the latch surface  59  of the latch member  57 , the outer follower  29  and the inner follower  33  are latched in a fixed axial position relative to each other as shown in  FIG. 3 . In the latched condition just described, the outer follower  29  is being maintained in its extended position, relative to the inner follower  33 , as shown in  FIG. 3 . In this extended position, when the camshaft  19  rotates such that the lift portion  25  of the cam lobe  21  engages the upper follower surface  31 , such engagement causes the tappet assembly  27  to move “downward” as a solid unit, thus causing corresponding downward movement of the engine poppet valve  11  from its normally closed position to the fully open “high lift” position (i.e., the position of the engine poppet valve  11  shown in  FIG. 1 ), in opposition to the biasing force of a valve return spring (not shown herein). The operation of the tappet assembly  27  in the latched condition, as just described, results in the “High Lift” curve shown in  FIG. 4 .  
         [0027]     In accordance with an important aspect of the present invention, when it is desired to operate the valve control system of the present invention in what is nominally a “deactivated” condition, the control pressure normally communicated to the fluid port  49  is discontinued (such as by draining it to a system reservoir, or low pressure location), thus reducing the fluid pressure within the annular groove  45 . In the absence of pressurized control fluid, the return spring  61  biases the latch members  57  toward their retracted, disengaged position, such that the latch surfaces  59  are no longer in engagement with the latch surfaces  53 . When the tappet assembly  27  is operating in the above-described unlatched, disengaged condition, engagement of the base circle portion  23  with the upper follower surface  31  will result in the tappet assembly  27  being in its fully extended position shown in  FIG. 3 . However, as the camshaft  19  continues to rotate, the lift portion  25  will engage the upper follower surface  31 , and begin to move the outer follower  29  “downward” (i.e., in a direction toward the engine poppet valve  11 ).  
         [0028]     As should be well understood by those skilled in the internal combustion engine art, the biasing force of the compression spring  41  is substantially less than the biasing force of the valve return spring (not shown herein) for the engine poppet valve  11 . Therefore, as the lift portion  25  of the cam lobe  21  moves the outer follower  29  downward, the compression spring  41  will begin to be compressed, but there will be no corresponding, downward movement of the engine poppet valve  11 .  
         [0029]     As the camshaft  19  continues to rotate, with the lift portion  25  of the cam lobe  21  approaching what would normally be the “peak” of its lift, the outer follower  29  merely continues to move downward, compressing the compression spring  41 , until such time as the annular stop surface  73  engages the annular stop surface  77 . The above-described contact of the stop surfaces  73  and  77  occurs at approximately −15° of cam angle in the graph of  FIG. 4 . As the camshaft  19  continues to rotate (beyond the −15° shown in  FIG. 4 ), with the stop surfaces  73  and  77  in engagement, the engagement of the peak part of the lift portion  25  with the upper follower surface  31  will again cause the tappet assembly  27  to operate as a solid unit, but now, in a low lift condition (“blip” mode) represented by the “Low Lift” curve shown in  FIG. 4 . The term “blip” is used to indicate that the low lift condition of the present invention, when compared to the normal, high lift condition, results in a valve lift which is merely a small portion of the high lift, both in terms of lift amount (millimeters) and lift duration (degrees of cam rotation). By way of example only, in the engine on which the present invention was developed, the high lift was approximately 8.0 mm., whereas the low lift (blip) was about 0.5 mm. Also, the duration of the high lift was about 140° of cam angle, whereas the low lift was about 30° of cam angle.  
         [0030]     Once the lift portion  25  of the cam lobe  21  reaches approximately +15°, as shown in  FIG. 4 , the compression spring  41  biasing the outer follower  29  upward will cause the stop surface  73  to disengage from the stop surface  77 , and thereafter, with continued rotation of the camshaft  19 , the outer follower  29  will return to the extended position shown in  FIG. 3 . In this condition, the poppet valve  11  is permitted, under the influence of its valve return spring, to return to the fully closed position (Low Lift curve, Lift=0), as was the case just before the “blip”. As was described in the Background of the Disclosure, the purpose of this small amount (blip) of lift is to permit fuel to pass from the intake into the combustion chamber, rather then accumulating behind the intake poppet valve  11 .  
         [0031]     Referring now to  FIG. 5 , and specifically to the “Velocity” curve, it should be noted that the acceleration of the valve in the low lift (blip) mode is actually a negative quantity. However, just at the −15° of cam rotation, where the blip begins, the velocity (stop surface  77  to stop surface  73  impact velocity) is low, and acceleration is nearly zero, and then increases (in the negative direction) as the poppet valve undergoes the low lift. Then, at the +15° of cam rotation, where the blip ends, the velocity (now valve to valve seat impact velocity) is low again and acceleration is again very nearly zero. This is an important feature of the invention because, if the impact velocity (and acceleration) value were substantially higher than what is shown in  FIG. 5 , there would likely be very significant durability and NVH (noise-vibration-harshness) issues with the tappet assembly  27  of the present invention.  
         [0032]     The invention has been described in great detail in the foregoing specification, and it is believed that various alterations and modifications of the invention will become apparent to those skilled in the art from a reading and understanding of the specification. It is intended that all such alterations and modifications are included in the invention, insofar as they come within the scope of the appended claims.