CA2102575C

CA2102575C - Low friction valve train

Info

Publication number: CA2102575C
Application number: CA002102575A
Authority: CA
Inventors: V. Durga Nageswar Rao; Daniel Michael Kabat; Harry Arthur Cikanek
Original assignee: Ford Motor Company of Canada Ltd
Current assignee: Ford Motor Company of Canada Ltd
Priority date: 1992-11-12
Filing date: 1993-11-05
Publication date: 1998-08-11
Anticipated expiration: 2013-11-05
Also published as: DE4336920A1; GB2273139B; JPH06212914A; DE4336920C2; GB2273139A; CA2102575A1; GB9321515D0

Abstract

A low friction valve train is provided for use with a high speed internal combustion engine, The valve train includes a tappet formed of a lightweight metal preferably aluminum or magnesium alloy. The outer surface of said tappet is impregnated with a solid film lubricant to provide reduced friction during operation.
The valve train further includes a cam shaft having cams, the wear/bearing surfaces of the cams are impregnated with a solid film lubricant to further reduce friction during operation.

Description

Ford DisclosureNo. 91-762 LOW FRICTION VALVE TRAIN
.
BACKGROUND OF ~IE lNV~N'l'lON ~ ;

1. Field Of the Xnvention The present invention relates generally to internal c~mbustion engines and, more particularly to, a low friction valve train for an internal combustion engine.

2. Descri~tion Of the Related Art It is known to construct ~al~e ~rains fox opening and closin~ valves in engines such as internal combustion engines. Such a valve train may be a direct acting hydraulic bucket tappet valve train for an overhead cam type lnternal combustion engine.
Generally, the valve train includes a tappet which contacts a cam on a cam shaft which is used to translate rotational motion of the cam shaft into axial motion of the valve. The valve is closed by a valve spring which biases the valve in a closed position.
;The valve train includes a hydraulic lash adjuster which compensates for a change in valve length . 25 due to thermal expansion caused by temperature changes : as well as valve seat wear. This type of valve train is a high pressure system which, through hydraulic pressure generated by the lubrication system, keeps the ~alve lifter in proper contact with the cam to perfor~ the ,' ~, c~ 5 valve opening/closing function. The constant hydraulic pressure continuously applied to the valve to maint~in proper contact with the cam, in addition to the forces induced by the cam, results in increased friction losses and significant wear to the components o~ the valve train.
However, the hydraulic pressure is expected to provide hydrodynamic film lubrication between a journal - of the cam and bearing surfaces of the cam shaft, and the tappet surface and the cam surfac~s. Because of high unit loads, the valve train operates in a predo-;n~tely boundary/mix~d lubrication regime, particularly in the 70-2000 engin2 speed range. '~'~is speed range represents more than 80% of the driving cycle for passenger vehicle operation. Because the operation is in the pre~o-;n~ntly boundary/mixed lubrication regime, the contacting components are subject to significant wear, as much as 30 to 150 microns on the cam during the li~e of the engine.
Additionally, engine speed is limited by the incidence ~f "valve toss!' which is due to the reciprocating mass of the valve train. Reducing the valve train mass decreases the forces due to inertia and, as a result, permits higher engine operating speeds which, in turn, result in greater engin~ output.
Further, reducing the friction between the moving comp~nents significantly reduces the wear and eliminates ':

, 2~'j7 ~ ~

the need ~or a heavy, complex and expensive hydraulic system and enables the engine to operate at normal hydraulic pressures without the friction losses and corresponding wear encountered in standard hydraulic systems. The reduction in friction, in turn, results in ~ fuel economy impL~v. ?nt and the reduction in wear improves component durability and, as a consequence, engine life. Thus, there is a need in the art to reduce the mass of the valve train and friction between moving components of the valve train.

. .
~: SUM2~RY OF ~HE lNVl~;N'l'lON
Acc~rdingly, the~resent invention is a unique lightweight and low friction valve train for an engine such as an internal combustion engine. In general, the valve train includes a cam shaft having at least one ~ cam, the outer surfaces thereof treated such that the ~ treated surface has an open porosity. A solid film lubricant is impregnated on the treated surfaces. The - 20 ~alve train further includes a lightweight tappet ha~ing ~~ a p~ripheral surface treated such that the treated surface has an open porosity. The treated surface is impregnated with a solid ~ilm lubricant. The tappet includes an insert which contacts the cam. The insert of the tappet includes a wear resistant contact surface.
In addition, a valve guide has an inner surface treated .
~ to create an open porosity and impregnated with a solid : :

" ~ .

:
'?~ 7 5 - film lubricant to reduce the ~riction at khe valve/valve guide inter~ace may also be incorporated into the valve train. ~he solid film lubricant has an affinity for oil and promotPs rapid formation of a stable oil film to reduce ~riction between the components.
one advantage of the present invention is that a low friction valve train is provided for an internal combustion engine. Another advantage of the present invention is that a solid ~ilm lubricant is applied to the contacting surfaces of the valve train, thereby reducing contact pressures which c~rrPspondingly reduces ~riction and wear. Yet another advantage of the present i~ invention is that thi~ ~alYe train inc~rp~rates a s~lid film lubricant to avoid the frictional losses occurring 1~ as a result of hydraulic loading of the tappet against the cam. A further advantage of the present invention is that the solid film lubricant applied to components of the valve train results in the f~icti~nal losses and corresponding wear being significantly reduced, thereby obviating the need for a heavy, complex and expensive hydr~ulic system. Additionally, such,a low frictio~
valve train will reduce or eliminate wear during oil starved conditions such as cold start and, thus, increase component life and engine life significantlyO
Other objects, features and advantages of the present invention will be readily appreciated as the same becomes better understood after reading the ' ~ ~~57~

' following description in conjunction with the - accompanying drawings.

'~ BRIEF DESCRIPTION OF THE DRAWINGS
. 5 FIG. 1 is a partial fragmentary view of a '' valve train, according to the present invention, illustrated in operational relationship to an engine.
. FIG. 2 is an enlarged view of a tappet assem~ly for the valve train o~ FIG. 1.
FIG. 3 is an exploded view of a portion of the tappet assembly of FIG. 2.
.~ FIG. 4 is an enlarged view of the portion of the tappet assembly o~ FIG. 3 as assembled.
FIG. 5 is an enlarged view of a portion in circle 5 of FIG. 4.
FIG. 6 is an enlarged view of a cam for the valve train of FIG. 1.
FIG. 7 is an Pnlarged view of a valve and valve guide for the valve train of FIG. 1.
FIG. 8 is an enlarged view o~ a valve and~
valve seat for the valve train of FIG. 1.
FIG. 9 is an enlarged view of a portion of the valve train of FIG. 1 prior to break-in. :
FIG. 10 is a YieW similar to FIG. 9 after break-in.

2 ~ 7 ~

DESCRIPTION OF THE PR~n~ EMB~DIMENT~S) Re~erring to the d:rawings and in particular FIG. 1 thereof, a valve train 12, accordingly to the present in~ention, is illustrated in operational relationship to an engine, generally indicated at 14, ; such as an internal combustion engine. The engine 14 includes a cylinder or engine block 15 having at least : one, pre~erably a plurality of hollow cylinders 16 - th~rein. The engine 14 also include~ a cylinder or engine head 18 secured to the cylinder block 15 by suitable means such as fasteners (not shown). The cylinder head 18 has an intake passaqeway 20 and an exhaust p~gci~geway 22 communicating with the cylinders 16.
15The valve train 12 includes at lea~t one, ;~ p*eferably a plurality of valve assemblies, generally indicated at 24 for opening and closin$ the intake passageway 20 and exhaust passageway 22. Preferably, separate valve assemblies 24 are used ~or the intake passageway 20 and the exhaust passageway 22. The valve train 12 also includes at least one, preferably a plurality of cam shafts 26 for opening and closing the valve assemblies 240 The cam shaft 26 includes a shaft member 27 rotatably supported within the cylinder head 18 as is known in the art. The cam shaft 26 has at least one, preferably a plurality of cams 28 which ' , contact and move the valve assemblies 24. The cams 28 have a base circle portion 30 and a lobe portion 320 Each valve assembly 24 includes a val~e 34 having a head portion 35 and a stem portion 36 slidably 5 disposed in a valve guide 37. The valve guide 37 is disposed in an aperture 38 of the cylinder head 18 as is known in the art. The valve assembly 24 also includes a tappet assembly 39 contacting one end of the stem portion 35 of the valve 34 and engaging a cam 28 of the 10 cam shaft 26. The tappet assembly 39 is slidably disposed in a tappet guide aperture 40 o~ the cylinder head 18 as is known in the art. The valve assembly 24 further includes a valve spring 41 dispose~ about the stem portion 35 of the valve 34 and having one end 15 contacting the cylinder head 18 and .the other end contacting a valve spring retainer 42 disposed about the stem portion 35. The valve spring 41 urges the head portion of the valve 34 into engagement with a valve seat 43 to close a corresponding intake or exhaust .
passageway 20, 22. The valve seat 43 is disposed in a recess 44 of the cylinder head 18 at the end of the intake or exhaust passageway 20~ 22 adjacent the cylinder 16.
Referring now to FIG. 2, a tappet assembly 39, : 25 according to the present invention, is illustrated. The tappet assembly 39 includes a tappet body 46 which is generally cylindri~al in shape and having a hollow 2~2~ ~
-~ 8 . .
interior 47 to receive the stem portlo~ 35 o~ the valve 34. Pre~erably, the tappet body 46 is made from a metal ,~ material such as a die cast high stren~th aluminum or , .
magnesium alloy. The outer periphery or surface o~ the tappet body 46 is hard anodized. The anodizing process results in a coating which is submicroscopically porous, e.g., a pore size of approximately 3-10 microns, for allowing a solid film lubricant 50 to be impregnated within the tappet body 46 prior to finish grinding. It is important that the depth of the anodized layer be ~ adequate, approximately 30~40 microns, to support the '' bearing loads. Also, the anodizing proces~ should j. .
produce a s~itable anodized layer o~ sufficient depth and integrity that it does not crumble under fatigue ~- lS loading. The solid film lubricant 50 must be 7 impregnated to a depth of at least a few microns greater ~''! than the expected wear, e.g., if expected wear is around 30 microns then a solid film lubricant impregnation to i. approximately 35-40 microns is satisfactory.
'~jî 20 The solid film lubricant 50, as used herein, is a solid lubricant that has a coe~ficient of friction of .02-.1 at 600~F. The solid film lubricant 50 is ; pre~erably a composite, by volume of 40% graphite, 20%
;' MoS2 and the remainder a thermally stable (does not decompose up to 375~C or 700~F) polymer such as polyarylsufone or a high temperature ~poxy such as bisphenol A and vinyl butoryl co~bined with P:
.. ' .

~ ~ ~2~7~.~
g dicyandianide. The solid film lubricant 50 of the type described here promotes rapid stable oil film formation due to its affinity ~or conventional lubricating oils.
The solid film lubricant 50 may also be a metal matrix composite having about 40~ graphite and the L~ ; n~r aluminum or cast iron. Such metal matrix composite may be formed by powder metallurgy or other suitable means to provide a porus material that can expose graphite for intermittent or supplementary lubricati~n purposes. Up to 13% of the graphite may be substituted with boron nitride. The solid lubricant may also include up to 10%
copper and ~ne of ~iF, NaF, and CaF as a substitute for the MoS2. It should be appreciated th~ other compositions suitable as solid film lubricants may also be used. ~ -As illustrated in FIGS. 2 through 5, the tappet assembly 39 also includes a cavity 51 at an upper end thereof. The cavity 51 is generally cylindrical in shape. The tappet assembly 39 also includes a wear resistant insert 52 having a contacting surface 54 which contacts a cam 28 or a cam shaft 26. Preferably, the insert 52 is made of ceramic material but may also be manufactured from a high strength steel, toughened alumina or silicon nitride sintered. The insert 52 is machined to fit in the cavity 51 of the tappet body 46.
The insert 52 and cavity 51 are matched for a smooth fit. Pxef~erably, the sides o~ khe insert 5~ and the '"' '2'~ r3~

. 10 :~' cavity 51 include complementary inverse tapers 57 and ,; 58, respectively, to lock the insert 52 within the ; cavity 51. The insert 52 is secured within the ca~ity '' 51 through a shrink-fit process. The shrink-fit process includes heating the tappet body 46 to a temperature approximately 100~F higher than the engine operating temperature (approximately 310~F), and cooling th~
insert 52 to a temperature below a low end ambient temperature tapproximately -50~F) after which the insert 52 is placed in the cavity 51. When the tappet assembly 3g is brought to room temperature, the tappet body 46 shrinks around the insert 52 because of the significantly hi~ler thermal expansion of the tappet hody 46 relative to that of the insert 52. This process insures that the insert 52 remains in compression during the entire operating range of engine temperatures. It should be appreciated that the insert 52 may also be secured to th~- tappek body 46 through use of a lock ring 59 engaging corresponding annular grooves 59a and 5~b formed in both the insert 52 and the tappet body 46, respectively.
Referring to FIG. 6, a cam 28 of the cam shaft 26 is shown. The base circle portion 30 of the cam 28 includes an interior portion 60 made from a metal material o~ a soft/low carbon steel to minimize stresses occurring during rotation of the cam shaft 26. The interior portion 60 is mechanically secured to a flutsd \

' ' t ' '' ' ,: , ~ ' ' : ; '~ : .

: : ' or roughened portioned 62 o~ the shaft 27. The l~be portion 32 and the remaining portion o~ the base circle :
: portion 30 of the cam 28 are made from a metal material such as a porous medium/high carbon Ni-Cr alloy steel.
The outer periphery or surfaces of th~ base circle pvrtion 30 and lobe portion 32 are hardened to a ; normally specified hardness level ~or a cam sur~ace (usually around Rc 55) utilizing any one of the well known processes, e.g. carbo nitrating. Generally, the ; 10 porosity extends only to a depth of less than 1.0 mm. The porosity enables the outer surfaces of the cam ~8 to be impregnated with the solid film lubricant 50. The depth of the solid film lubricant 50 impregnation should be at least a few microns greater than the expected wear as previously described.
f Referring to FIG. 7, the valve guide 37 is -shown. ~he valve guide 37 has an inner surface 66 impregnated with the solid ~ilm lubricant 50 to reduce the frlction between the stem portion 35 of the valve 34 and the valve guide 37. Preferably, the inner surface 66 of the valve guide 37 includes a w~ar resistant p~rous layer formed by a suitable means to facilitate impregnation o~ the solid film lubricant 50 as previously described. - -Referring to FIG. 8, the valve seat 43 is shown. The valve seat 43 has an outer surface 68 also ' impregnated with the solid ~ilm lubricant 50 to reduce ;. ~
: .

, . ~ .

1~ .
the friction and corresponaing wear occurring between the head portion 35 and valve seat 43. Alternatively, the outer surfaae of the head porti~n 35 of the ~alve 34 may be impregnatad with th~ solid film lubricant 50 and the head portion 35 may be hollow with a wear resistant insert at the lower end thereof. It should be appreciated that the valve seat 43 is treated to form a wear resistant porous layer as preYiously descri~ed.
Referring to FIG. 9, a portion o~ the solid film lubricant 50 on a corresponding valve train component such as the tappet body 46 prior to break in is illustrated. The solid ~ilm lubricant 50 is impregnated to an ef~ective wear depth and includes a superficial layer. A~ter engine break in, the layer of solid film lubricant 50 forms a stable low friction wear resistant film as illustrated in FIG. 10.
In operation, the solid film lubricant 50 promotes the formation of a stable lubrication film.
The stable lubrication ~ilm reduces friction occurring , 20 at higher operating speeds where hydrodynamic lubrication is pre~ ;n~te. Rapid formation of a lubrication ~ilm significantly reduces cam wear by reducing the friction at lower engine speeds.
Accordingly, the solid film lubricant 50 on the valve train 10 reduces friction losses, the contact forces due to the elimination of hydraulic loading, and reduces inertia forces due to a significant reduction in .

-~ . . . .. . .. . . . . . .

., .. the reciprocating mass. As a result, the valve tra~n 10 permits significantly higher engine operating speeds and a reduction in friction and wear which extends corresponding engine life. Because of t~e significantly reduced wear, the ~al~e t:rain 10 does not require , adjustment for life of the engine nor does it require a ; hydraulic lash adjustment and the attendant precision :"
ma~h;ning and hydraulic lu~rication requirements. A
high pressure hydraulic system is not required as normal lubrication provides satisfactory operation and avoids ~' the friction losses encountered in hydraulic systems due to hydraulic loading of the tappet against the cam.
,.~ The present invention has been described in a~
illustrative ~nnPr~ It is to be understood that the terminology which has been used is intended to be in the nature of words of description rather than of limitation.
' Many modi~ications and ~ariations of the present invention are possible in light of the abovs teachings. Therefore, within the scope of the appended claims, the present invention may be practiced otherwise than as specifically described.

.
.., .
, ~ .
, .

. , .

Claims

1. A low friction valve train for actuating at least one valve of an internal combustion engine comprising:
a cam shaft having at least one cam;
a tappet contacting said cam and the valve, said cam and tappet having outer surfaces impregnated with a solid film lubricant that has an affinity for oil and promotes rapid formation of a stable oil film to reduce friction therebetween; and a valve guide disposed about said valve, wherein said valve is slidably disposed in said valve guide, said valve guide having an inner surface adjacent said valve, said inner surface impregnated with a solid film lubricant.

2. A low friction valve train as set forth in claim 1 wherein said outer surfaces of said cam is made from a porous medium/high carbon Ni-Cr alloy steel, said solid film lubricant being impregnated within the open porosity.

3. A low friction valve train as set forth in claim 1 wherein said tappet is made from at least one material selected from the group consisting of magnesium and aluminum.

4. A low friction valve train as set forth in claim 3 wherein said outer surface of said tappet are treated to provide said outer surfaces with an open porosity.
.

5. A low friction valve train as set forth in claim 4 wherein said tappet includes means forming a cavity and a wear resistant insert disposed in said cavity to contact said cam.

6. A low friction valve train as set forth in claim 5 wherein said wear resistant insert is made from at least one material selected from the group consisting of silicon nitride, toughened alumina or hardened porous Ni-Cr alloy steel.

7. A low friction valve train as set forth in claim 5 wherein said cavity has an inwardly tapered side wall and said wear resistant insert has inwardly tapered sides cooperating with said inwardly tapered side wall to secure said insert to said tappet.

8. A low friction valve train as set forth in claim 5 including a locking ring disposed between said insert and said tappet to secure said insert to said tappet.

9. A low friction valve train as set forth in claim 1 wherein said solid film lubricant is comprised of graphite, boron nitride, molybdenum disulfide in a high temperature polymer base.

10. A low friction valve train for actuating at least one valve of an internal combustion engine comprising:
a cam shaft having at least one cam with a base portion and lobe portion, said base and lobe portions having outer surfaces treated such that the treated surfaces have an open porosity; and a tappet, having a treated outer surface such that such treated outer surface has an open porosity, said tappet further including a wear resistant insert secured to said tappet and contacting said cam, said base portion and lobe portion having outer surfaces formed of a porous medium/high carbon Ni-Cr alloy steel hardened and impregnated with a solid film lubricant.

11. A low friction valve train as set forth in claim 10 wherein said cam, said outer surfaces of said base portion and said lobe portion are impregnated with a solid film lubricant.

12. A low friction valve train as set forth in claim 11 wherein a solid film lubricant is impregnated on said treated surfaces.

13. A low friction valve train as set forth in claim wherein said solid film lubricant comprises a polymer based molybdenum disulfide, boron nitride and graphite mixture.

14. A low friction valve train as set forth in claim 10 including a valve guide surrounding said valve, said valve guide having an inner surface, said inner surface impregnated with a solid film lubricant.

15. A low friction valve train as set forth in claim 10 wherein said engine includes a cylinder head having a valve seat, said valve seat impregnated with a solid film lubricant.