Abstract:
The invention relates to improved cam contacting devices for use in internal combustion engines and preferably for use in internal combustion engines having variable valve timing. In particular, the use of ceramics including silicon nitride and silicon carbide have been demonstrated as providing effective cam contacting surfaces allowing an axially displaceable cam shaft having a variable profile cam to be run with resulting improvements in idle speed and volumetric efficiency.

Description:
RELATED APPLICATION  
         [0001]    This application is related to Canadian Patent application 2,257,437 which is incorporated herein by reference.  
         FIELD OF THE INVENTION  
         [0002]    The invention relates to improved cam contacting devices for use in internal combustion engines and preferably for use in internal combustion engines having variable valve timing. In particular, the use of ceramics including silicon nitride and silicon carbide have been demonstrated as providing effective cam contacting surfaces allowing an axially displaceable cam shaft having a variable profile cam to be run with resulting improvements in idle speed and volumetric efficiency.  
         BACKGROUND OF THE INVENTION  
         [0003]    The design of an internal combustion engine requires numerous trade-offs between conflicting design or performance parameters and particularly with respect to 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 such conflicting parameters leading the designer to compromise with the design to achieve a balance between the parameters. As such, designers will often 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). Such compromises are often 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.  
           [0005]    The breathability of an engine is primarily determined by the physical structure of the cam shaft, cam lobes, valve lifters (and the associated push-rods, rocker arms, if applicable). In particular, the physical shapes or profiles of the cams and their relative orientation with respect to one another determine the timing of the intake and exhaust valve opening, the duration of opening, and the timing of valve closure which, along with the orientation of respective intake and exhaust valves about the camshaft, determine the power map of the cylinder.  
           [0006]    As a result of the high-temperature, high-pressure and mechanical speed of the working environment as well as the physical complexity of these components, adjustment of valves during operation of the engine is difficult and accordingly, most engines utilize a fixed cam timing system wherein the relative timing between valve opening and closure does not vary with engine speed. As a result, fixed cam timing engines require trade-offs between the performance parameters of the engine.  
           [0007]    More specifically, the camshaft function is to open and close valves at the proper time, to fill the cylinders before combustion and to empty them after combustion. The cams are mounted on the camshaft and have a profile which determines the timing of valve opening, the duration of opening and the timing of valve closing. The lifters are in intimate contact with the cam surface and ride the cam surface in order to impart opening/closing forces to the valves. The opening and closing of valves is thereby timed to the rotation of the camshaft which in turn is controlled by the crankshaft.  
           [0008]    Accordingly, the physical dimensions or shapes of the cams, lifters and the orientation of the cams with respect to one another are parameters which can be varied in order to obtain desired engine performance.  
           [0009]    With respect to the physical dimensions or design of a cam, the following terms are generally used to describe the shape of a cam and the physical movements of a valve. For example, the base circle of the cam defines the period that the valve is closed, the clearance ramp defines the time of transition between closure and measurable valve lifting, the flank or ramp provides the time for and characteristics of valve opening, the nose defines the time of full valve opening and maximum opening displacement and the duration defines the time that the valve is off its seat.  
           [0010]    Each of these parameters of a cam cannot be independently controlled during engine operation and therefore require compromises between what the physical dimensions of a cam will allow in relation to the other parameters. For example, duration is a compromise between opening the valves long enough to fill and/or evacuate the cylinders to the loss of dynamic compression by opening the valves too long and increasing lift increases power but is limited by lifter diameter.  
           [0011]    With respect to the design of lifters (or tappets), the technology of lifters is variable between engines. Generally, the primary goals of the design of a lifter is to maintain contact between the lifter surface and cam surface while minimizing noise during operation. There are two main classes of lifters, solid and hydraulic with each class providing variable contact ends including flat, mushrooms and rollers. The use of hydraulic lifters generally reduces valve lash and noise. A flat tappet-cam normally has a slight taper across its surface whereas the corresponding tappet end surface is normally marginally convex in order to compensate for mis-aligned lifter bores.  
           [0012]    Another class of lifters are roller lifters which include a wheel or roller in contact with the cam. Roller lifters allow for highly aggressive ramp profiles and, as a result, require high valve spring tensions to keep the roller in contact with the cam. Roller lifters also reduce frictional losses between the lifter and cam and thereby will increase the overall power or efficiency of the engine.  
           [0013]    Mushroom lifters have a bulge at the end and are used to provide more lift per duration.  
           [0014]    The relative orientation of the intake and exhaust cams with respect to one another contributes to defining the power map of the engine. Specifically, the lobe separation angle or overlap determines the time during which the intake and exhaust valves are opened simultaneously, wherein a wider lobe separation angle generally improves idle quality, idle vacuum and top-end power whereas a narrower lobe separation angle decreases idle quality but provides better mid-range torque.  
           [0015]    Degreeing a cam is also a parameter which can be used to affect engine performance and refers to altering the point where the cam activates the valves in relation to the crankshaft. Specifically, retarding the cam shaft, that is, opening a valve later relative to the crankshaft moves the power up the rpm band and can increase horsepower while decreasing lower end torque. In contrast, advancing the cam shaft (opening the valves earlier) has the opposite effect.  
           [0016]    In order to address some of the problems associated with fixed cam timing, variable cam timing systems have been designed. Generally, such systems provide a cam lobe having a three-dimensional surface and a lifter which is allowed to move axially over the three-dimensional cam surface. Accordingly, the axial position of the camshaft will determine the specific cam profile which controls valve timing. Variable valve timing thereby permits the alteration of valve timing during the operation of the engine allowing engine performance to be modified to match operating conditions. Variations in the relative shapes of a cam within a variable cam system can enable any one of independently phasing the intake cams, independently phasing the exhaust cams, phasing the intake and exhaust equally or phasing the exhaust and intake cams independently of one another.  
           [0017]    For example, by diluting the in-cylinder mixture by reducing fuel intake characteristics by providing shorter intake times increases fuel economy but decreases the torque response of the engine. In contrast, by enriching the in-cylinder mixture by increasing fuel intake times by providing more lift and duration leads to an increase in horsepower. A variable valve timing system can accommodate such conflicting objectives by providing different timing profiles depending on the rpm of the engine thereby contributing to improving the breathability of the engine and increasing the manifold pressure.  
           [0018]    In high performance applications, the current state-of the-art recognizes the single axis roller or wheel based lifter as the optimal performance enhancing device for valve train operation. However, as the desire for higher engine rpm has grown, it has been found that wheel based lifters will fail under the higher tension springs utilized in the higher rpm engines. Typically, failure occurs in two ways; roller bearing failure in the wheel itself and/or the catastrophic failure of the lifter, both a result of wheel “flat spotting” which produce valve lifter and valve train vibration.  
           [0019]    Furthermore, existing wheel-based lifter designs do not provide direct delivery of lubrication to the roller bearing but rather lubrication occurs indirectly which decreases the ability to dissipate heat from the bearing surfaces. Accordingly, bearing life may be reduced as the wheel may be in direct contact with the bearing race with minimal oil film between the two surfaces.  
           [0020]    To achieve maximum bearing life in a single axle based system, the designer must balance three parameters given that the wheel diameter is maximized within the confines of the lifter body. These three factors are roller bearing diameter, axle diameter and wheel thickness. Each of these parameters must be varied to minimize the compressive and contact stresses on the bearing surfaces, minimize the stresses in the axle and minimize the deflection of the axle which directly affects the contact stresses within the roller bearings.  
           [0021]    While past variable valve timing systems have been disclosed, for example in U.S. Pat. No. 2,969,051, German publication DE 197 55 937, Swiss publication DE 304494 and U.S. Pat. No. 2,307,926, the lifter/cam contacting systems have experienced neither widespread implementation or success. The reason for this lack of success is postulated to be a result of failures experienced in the actual implementation of such systems. That is, within the harsh operating conditions of an internal combustion engine, it is speculated that previous variable valve timing systems experience bearing failure within the bearings/races of these systems.  
         SUMMARY OF THE INVENTION  
         [0022]    In accordance with the invention, there is provided a cam contacting device having improved thermal properties for use in an internal combustion engine. Various properties of the cam contacting material which enable its use are disclosed. These include any one or a combination of a density less than 6 g/cc (preferably about 3.1-3.2 g/cc), a Young&#39;s modulus greater than 310 Gpa (preferably about 310450 Gpa), a Vickers hardness greater than 1150 (Hv) (preferably 1650-2850 (Hv)), a coefficient of thermal expansion less than 10.5×10 −6 /degree Celsius (preferably about 3.1-5.5×10 −6 /degree Celsius), a thermal conductivity less than 50 W/m K (preferably about 22-26 W/m K), a Weibull modulus greater than 12 (preferably about 12-18), and an abrasive wear resistance greater than 900 (preferably about 947-1263)  
           [0023]    In more specific embodiments, the cam contacting device is a ball bearing within a bearing race and support wherein the coefficient of thermal expansion of the ball bearing is less than the coefficient of thermal expansion of the bearing race and support and more specifically where the cam contacting device is a ceramic ball bearing within a steel bearing race and support.  
           [0024]    In one embodiment, the cam contacting device is a silicon nitride or silicon carbide ball bearing.  
           [0025]    Specific cam contacting material may include any one of CERALLOY 147-31E, 147-31N, 147-1E, or 147-1.  
           [0026]    In a further embodiment, the cam contacting device includes a lubrication system for providing lubrication to the cam contacting device/cam interface.  
           [0027]    In a further still embodiment, the invention relates to the use of a cam contacting device as described above in an internal combustion engine having variable profile cams.  
           [0028]    Further still, the invention provides an internal combustion engine having cylinders, a rotating camshaft, valves and valve lifters in operative communication between the valves and rotating camshaft, further comprising a variable profile camshaft and valve lifters linearly displaceable with respect to one another wherein the valve lifters include a silicon nitride bearing for operative contact with the rotating camshaft and which may include a valve seat having fuel injection ports within the valve seat for injecting fuel into the cylinders; 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0029]    These and other features of the invention will be more apparent from the following description in which reference is made to the appended drawings wherein:  
         [0030]    [0030]FIG. 1A is a schematic diagram of roller lifter in accordance with the invention;  
         [0031]    [0031]FIG. 1B is a schematic diagram of an assembled and unassembled ball bearing lifter in accordance with the invention;  
         [0032]    [0032]FIG. 1C is a schematic diagram of a semi-spherical solid lifter in accordance with the invention;  
         [0033]    [0033]FIG. 1D shows an end view of a variable valve timing camshaft with both a fixed centreline and with a variable centreline;  
         [0034]    [0034]FIG. 2 is a schematic diagram of a combined overhead camshaft system with a spherical bearing valve depressor and valve seat fuel injector;  
         [0035]    [0035]FIG. 3 is a photograph showing wear patterns of a steel ball-bearing lifter which has been run in an engine in comparison to a steel ball-bearing lifter which has not been run in an engine;  
         [0036]    [0036]FIG. 4 is a photograph showing the wear patterns of a ceramic bearing lifter and steel bearing lifter; and,  
         [0037]    [0037]FIG. 5 is a photograph showing the wear patterns of a steel radiused wheel lifter and ceramic bearing lifter.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0038]    General Overview  
         [0039]    The Figures show various designs of cam contacting devices for use in internal combustion engines and specifically adapted for a variable valve timing engine as well as test results from the use of cam contacting devices in accordance with the invention within a variable valve timing engine.  
         [0040]    [0040]FIGS. 1A, 1B and  1 C shows three designs for cam contacting devices in accordance with the invention including a radiused wheel lifter, a spherical bearing lifter and a solid semi-spherical lifter. FIG. 1D shows an end view of a variable valve timing camshaft with both a fixed centreline  1  and with a variable centreline  2 . Within the context of this description, cam contacting device is intended to mean any device within an internal combustion engine which contacts or follows the outer surface of a rotating cam so as to directly or indirectly affect valve opening and closing. Accordingly, cam contacting device may include valve lifters, rocker arms or cam-followers directly configured to a valve stem.  
         [0041]    With reference to FIG. 1A, a radiused wheel lifter  3  is shown with side and front views. In this design, a roller wheel is fixed in the end of the lifter with bearings allowing the roller wheel to rotate about a fixed axis in the lifter base.  
         [0042]    With reference to FIG. 1B, a ball bearing valve lifter  4  is shown in an assembled  10  and disassembled showing a hydraulic damping system  5 , bearing retainer  6  and spherical bearing  7 . 35  shows a lubrication port and  36  shows the inner receiving surface of the lifter.  
         [0043]    With reference to FIG. 1C, a solid semi-spherical lifter  8  is shown having a semi-spherical end with a lubrication port  37  for delivery of lubrication to the cam-contacting device/cam interface.  
         [0044]    [0044]FIG. 2 shows a schematic diagram of a combined overhead camshaft system with a spherical bearing valve depressor  16  and valve seat fuel injector  20 . The system includes a varable profile cam  15  in an overhead cam layout, an overhead valve depressor  16  with spherical bearing (or a radiused wheel or half-sphere as described above), a cylinder head  17 , a valve  18  and valve spring  19 . The valve seat  20  may include fuel injector nozzles  21  with fuel delivery line  22 . The intake port  23  delivers air to the cylinder through valve  18 . The valve depressor  16  may include spherical bearing race  24  seated on a bearing housing  25  and valve spring retainer  26 .  
         [0045]    Cam Contacting Device Materials  
         [0046]    Specific designs of the lifter including ceramic bearings selected from silicon nitride and silicon carbide were tested within an internal combustion engine (ICE) having a fixed profile camshaft and within an ICE having a variable profile camshaft. Other ceramic materials and their properties are shown in comparison Table 1.  
                                             TABLE 1                           Bearing Steel/Ceramic Comparison Table                    Silicon                               Nitride               Bearing               Si 3 N 4     Silicon           Steel       Product       Ceralloy   Carbide   Alumina   Zirconia   AISI       Alloy   Units   147-31N   SiC   Al 2 O 3     ZrO 2  Y-PSZ   52100               Reference (see       (1)   (1) (2) (3)   (1) (2) (3)   (3) (6)   (2) (3) (4)       below)               (6)       Density   (g/cc)   3.2   3.14-3.20   3.70-3.99   5.9   7.8       Young&#39;s   (Gpa)   310   390-450   350-460   205-210   208-210       Modulus       Compressive   (Mpa)   2500 (6)       3900   3000       Strength       Poissons Ratio       0.27   0.14-0.17   0.22-0.23   0.31   0.3       Fracture   (Mpa ·   5.8   2.5-6.7   3.8-5.2   7.5-12    18       Toughness   M){circumflex over ( )}½       Vickers   (Hv)   1800   1650-2850   2000   1150-1400   700       Hardness       Thermal   (10{circumflex over ( )}−6/° C.)   3.1   3.4-5.5    5.5-10.2     10-10.5   12.5       Expansion       Coefficient       Thermal   (W/m · K)   26    22-200   28-35   &gt;2.0-3.1     50       Conductivity       Flexural   (Mpa)   800   375-634   350-460   1100-1300   2500       Strength       Thermal Shock   (° C.)   610   157-500   200-280   280-300       Resistance       Weibull       &gt;15   12-18   12-13   25   0.24-7.87       Modulus       Abrasive Wear   (7)   1110    947-1263    610       689       Resistance                  
 
       REFERENCES  
       [0047]    (1) Ceradyne Inc. Silicon Nitride Properties  
         [0048]    (2) Research and Development of Bearings for Special Environments Hiroyuki Ito, Basic Research and Development Center Shigeki Matsuiiaga, Precision Bearing Technology Department NSK Corporate Website  
         [0049]    (3) Ceramic Materials for Special Environments Shin Niizeki, Bearing Technology Center NSK Corporate Website  
         [0050]    (4) Effects of extreme pressure additives in lubricants on beating fatigue life. H. P. Nixon, The Timken Co. Canton Ohio Corporate website  
         [0051]    (5) Thermal Conductivity of Ceramic and Ceramic Coatings Andrew Slifka http://www.boulder.nist.gov/div853/Annual%20Report%202000%20HTML/P23.html  
         [0052]    (6) http://www.doceram.com/e_mat3.htm  
         [0053]    (7) Awr=FT{circumflex over ( )}½×H{circumflex over ( )}1.43×Em{circumflex over ( )}−0.8  
         [0054]    An initial evaluation of the ceramic ball bearing lifter design was tested at Isky Cams Inc. in the Los Angeles county area. The lifter (hereinafter lifer  1 ) used a ceramic bearing obtained from Ceradyne Inc. was made of CERALLOY 147-3 IN material.  
         [0055]    Lifter  1  was initially tested within the engine without engine combustion using a SPINTRON control system as this machine provided the best opportunity for lifter evaluation without the danger of engine damage in the event of lifter failure. The first evaluation was conducted with low pressure valve springs; 200 lbs/inch, to initially determine if the higher contact loads of the lifter/cam surface interface would result in camshaft scoring. The test cycle was completed in 8 hours and no damage to the lifter or camshaft was evident. A second evaluation was then conducted using a NASCAR (North American Stock Car) specification valve spring with a higher pressure of 800 lbs/inch. This test was also conducted over 8 hours and no damage was observed in either the lifter or the cam lobe.  
         [0056]    A second phase of evaluation was conducted on a Chevrolet V8 engine; ZZ4 P/N 24502609, under actual running conditions. This test used two different lifter designs, the ball bearing and the radiused wheel and also to evaluate a material variation in the ball bearing design. FIGS. 3, 4 and  5  show the results of these tests and Table 2 summarizes the lifter/test design.  
                           TABLE 2                       Lifter #   Design   Material   Supplier                   1   Ball Bearing lifter with   Ceralloy 147-31N   Lifter body -           lubrication to race   silicon nitride   Shaver Engines                   Ball Bearing-                   Ceradyne       2   Ball bearing lifter with   #52-100 Alloy steel   Lifter body -           lubrication to race       Shaver Engines                   Ball Bearing -                   Timken       3   Radiused lifter   #52-100 Alloy steel   Lifer body and                   wheel - Shaver                   Engines                  
 
         [0057]    The test was conducted at the Shaver Engine facility in Torrance Calif. The three different designs were placed in the test engine using high performance springs (p/n 10134358 rated at 356 lbs/inch). The engine was started and under load the rpm was controlled and set at 2000 rpm. After 2 minutes, a noticeable miss was detected and the engine operation was suspended. The engine was immediately disassembled and the components inspected.  
         [0058]    [0058]FIG. 3 shows the wear on the steel ball lifter (lifter  2 ) in comparison with the original steel ball. FIG. 4 shows lifter  1  (left) and lifter  2  (right). As shown, lifter  1  has no material loss and has not degraded the camshaft lobe in any way. Lifter  2  has suffered extensive material loss and has further degraded the cam lobe appreciably.  
         [0059]    [0059]FIG. 5 shows lifter  3  (left and centre) and lifter  1  (right). In this case, we see no damage to either the lifter or the camshaft lobe from the test.  
         [0060]    Subsequently, the engine was reassembled with lifters  1  and  3  and a new camshaft with the same specification was installed and the testing was continued for 6 hours at various rpm&#39;s (idle to 6000) and loads. The tested engine produced the same horsepower and torque levels specified and no problems in engine operation were detected. Upon completion, the engine was disassembled and lifters and camshaft were measured for wear. There was no appreciable wear on either the lifters or camshaft lobes.  
         [0061]    Variable Cam Lobe Profile Test  
         [0062]    A full test of an engine with a variable profile camshaft was tested as follows:  
         [0063]    Engine Tested: Chevrolet LS-15.7 L Overhead Valve Pushrod Engine  
         [0064]    Compression: 10:1  
         [0065]    Bore×Stroke: 99.00×92.00 mm  
         [0066]    Sequential Injection  
         [0067]    Prior to modification the engine had a horsepower rating of 345 @5400 rpm and an idle speed of 700 rpm.  
         [0068]    The engine was modified to include a variable profile camshaft and hydraulic actuation system for linear displacement of the camshaft. The variable profile lobes of the camshaft varied lift, duration and degreeing (7 degrees). The cam contacting devices for all 16 valves of the engine were modified to include a silicon nitride ball bearing (Ceralloy 147-31N). Spring pressures of 350 pounds were utilized.  
         [0069]    The engine was run initially for 5 minutes, shut-down, and run again for 45 minutes during which the camshaft was axially displaced between two extreme ends of the lobes as the engine was run from low rpm to high rpm.  
         [0070]    With the modified camshaft, the engine horsepower was measured at 420 hp @5400 rpm and the lowest idle speed obtained was 400 rpm. The overall increase in volumetric efficiency (VE) was calculated to be 25%  
         [0071]    Attempts to reduce the idle below 400 rpm were unsuccessful as the Electronic Control Module (ECM) configured to the engine consistently overrode the tuning being applied. That is, as attempts to reduce idle speed below 400 were made, the idle air control module (ICM) of the controller would apply fuel to increase idle speed. Based on the cam profile utilized for idle, it is envisaged that idle speeds as low as 200 rpm can be achieved.  
         [0072]    Following the engine run, the engine was disassembled and examined. The camshaft and cam contacting devices showed no evidence of wear.  
         [0073]    Discussion:  
         [0074]    The original lifter tests demonstrate that a cam contacting device having a fine point of contact with a camshaft can survive very high point pressures while operating within an ICE. Specifically, the use of ceramic silicon nitride bearings provide effective cam-contacting devices with fixed profile camshafts. The original lifter tests further demonstrate that the use of a lubricated steel bearing as a cam contacting device is ineffective and will quickly lead to the cam contacting device failure.  
         [0075]    The variable profile camshaft tests demonstrate that a cam contacting device having a fine point of contact can effectively enable the operation of variable valve timing system having a continuously variable cam profile. The practical results of this test demonstrated that idle speed can be significantly reduced and overall engine volumetric efficiently significantly increased as compared to a fixed cam profile engine.  
         [0076]    Discussion of Steel and Ceramic Bearings  
         [0077]    The failure of the steel ball appears to be from galling (i.e. localized welding) of the steel ball to the steel valve body. Once galling started, the ball would intermittently slide and roll both in the pocket and on the camshaft. This galling and sliding action of the ball would account for its uniform wear (0.042″). This unintended sliding action of the ball against the camshaft resulted in the severe damage (i.e. groove) to the camshaft that was seen.  
         [0078]    The success of the ceramic (Silicon Nitride) ball appears to be a result of the lower coefficient of friction and superior heat dissipation properties of the ceramic. Since the ceramic ball did not gall, it would continue to roll in its pocket and rolling contact with the camshaft would be maintained. This would account for the minimum damage/wear seen on the camshaft. When the ball rolls on the camshaft, it must slide in the pocket of the lifter body. There is consequently some friction, and heat generation inherent in this design. However, with the lower coefficient of friction of the ceramic, the heat generation as compared to steel is less. Moreover, the oil supplied through the lifter to the sliding surface between the ball and the lifter body would further reduce this friction as well as cool the ball.  
         [0079]    Still further, since the ceramic ball is more rigid than the metal ball, it would not deform as much underload. Consequently, the heat generated internally in the ball would also be less in the ceramic ball.  
         [0080]    The steel roller assembly has roller elements within it. Consequently, there would be rolling action of the roller against the camshaft. As was the case with the ceramic ball, the wear on the camshaft would be therefore minimized. Work hardening would occur on the camshaft as a result of the contact stress. This is most likely the cause of the narrow band that was seen on the camshaft for both the roller and ceramic ball setup. Since the steel roller assembly is dominated by rolling action and no sliding action, the friction, and consequently heat generation, would be minimal.  
         [0081]    For the silicon nitride ball:  
         [0082]    a) The total friction on the ball is less than that of a steel ball.  
         [0083]    b) The lower level of friction will generate less heat at the ball contact surfaces than a steel ball.  
         [0084]    c) Heat generated at the ball-cam and the ball-cup ( 36 ) interfaces will find its primary dissipation path through the steel interfaces rather than the ball as the thermal conductivity of the ball is significantly lower than that of the steel cup with the oil supplied to the ball/lifter providing further cooling.  
         [0085]    d) The steel contact surfaces will therefore heat up more so than the ball.  
         [0086]    e) The steel contact surfaces will therefore expand more than the ball due to the increased heat and the higher coefficient of expansion.  
         [0087]    f) Given the different expansion coefficients, the ball will always remain smaller than the surrounding cup. Therefore, the ball should not seize due to heat buildup.  
         [0088]    g) The mark on the cam was probably a result of the contact interface heat effectively work hardening the cam.  
         [0089]    For the steel ball:  
         [0090]    a) The total friction on the ball will be greater than that of silicon nitride ball.  
         [0091]    b) The higher level of friction will generate more heat at the ball contact surfaces than the silicon nitride ball.  
         [0092]    c) Heat generated at the ball-cam and the ball-cup ( 36 ) interfaces will dissipate through the ball as well as the contact surfaces with the oil supplied to the ball/lifter interface providing further cooling.  
         [0093]    d) The steel contact surfaces with therefore heat up as the ball heats up.  
         [0094]    e) The steel ball that contacts both the cam and the cup could heat up faster than the cup due to contact with the cam. If the ball heats up faster than the cup, the ball would expand faster than the cup increasing friction and possibly start to seize in the cup.  
         [0095]    f) Given the same expansion coefficients, the ball may seize in the cup if the ball heats up faster than the cup.  
         [0096]    g) The wear on the cam, ball, and cup was probably a result of the ball starting to seize (gall) in the cup effectively increasing friction and wear on all three surfaces.  
         [0097]    For the steel (radiused) wheel:  
         [0098]    a) The total friction on the steel wheel will be greater than that of silicon nitride ball.  
         [0099]    b) The higher level of friction will generate more heat at the wheel contact surfaces than the silicon nitride ball.  
         [0100]    c) Any heat generated at the wheel-cam and the wheel-roller ( 4 ) interfaces will dissipate through the wheel rim as well as the contact surfaces.  
         [0101]    d) The steel contact surfaces with therefore heat up as the ball heats up.  
         [0102]    e) The steel wheel has a smaller contact point than the silicon nitride ball and will generate more heat for a given spring load than the ball.  
         [0103]    f) The steel wheel has a large amount of clearance to the supporting lifter surfaces.  
         [0104]    g) The wheel, under thermal expansion, would not contact any of the surrounding lifter surfaces therefore the steel wheel should not seize (gall) as the steel ball did.  
         [0105]    h) Heat generated in the wheel rim can only be dissipated through the cam and the roller bearings; there is no forced oil-cooling bath.  
         [0106]    i) The steel wheel will get hotter than the silicon nitride ball due to higher levels of friction and less lubricating oil.  
         [0107]    j) The mark on the cam was probably due to the high level of heat transferred at the wheel-cam interface. The cam was being effectively work hardened by the heat of friction and contact pressure. The mark was more extensive than the silicon nitride ball due to higher temperature and pressure at the cam contact point.  
         [0108]    In conclusion, the success of the silicon nitride may be a result of a hardening of the cam lobe metal as a result of the heat generated by the contact of ceramic and metal as well as the thermal conductivity, thermal expansion coefficient as other material property differences as outlined in Table 1.