U.S. patent number RE34,143 [Application Number 07/756,258] was granted by the patent office on 1992-12-15 for oilless internal combustion engine having gas phase lubrication.
This patent grant is currently assigned to Ford Motor Company. Invention is credited to Vemulapalli D. N. Rao, Wallace R. Wade.
United States Patent |
RE34,143 |
Rao , et al. |
December 15, 1992 |
Oilless internal combustion engine having gas phase lubrication
Abstract
An oilless engine having a piston reciprocal within a cylinder
and along an axis of such cylinder while providing an annular gap
therebetween to receive combustion gases and a gas phase blow-by
control system for such engine. The system comprises (a) an annular
receptacle in at least one of said piston and cylinder; (b) an
annular body of graphite carrying high temperature solid lubricant
disposed in said annular receptacle, said body presenting a face
projecting out of said receptacle: (c) an elastomer material
retentive of elasticity at the maximum operating temperature to be
experienced by said body, interposed between said body and
receptacle to urge said body to close said gap under all operating
conditions of said engine; and (d) axially directed grooves in said
body face sized to substantially trap cylinders of combustion gases
therein by viscosity under low pressure gradients and to limit the
passage of a combustion gases through said grooves under high
pressure gradients to one percent or less of the cylinder gas
charge volume, the gas cylinders functioning as bearings to ride
the piston during reciprocation.
Inventors: |
Rao; Vemulapalli D. N.
(Bloomfield Township, Oakland County, MI), Wade; Wallace R.
(Farmington Hills, MI) |
Assignee: |
Ford Motor Company (Dearborn,
MI)
|
Family
ID: |
26856123 |
Appl.
No.: |
07/756,258 |
Filed: |
September 6, 1991 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
159615 |
Feb 23, 1988 |
04872432 |
Oct 10, 1989 |
|
|
Current U.S.
Class: |
123/193.4;
123/193.2 |
Current CPC
Class: |
F02B
23/0672 (20130101); F16N 15/00 (20130101); F02F
7/0085 (20130101); F01M 9/00 (20130101); F02F
3/00 (20130101); Y02T 10/125 (20130101); F05C
2253/16 (20130101); F01M 2001/083 (20130101); F05C
2203/0882 (20130101); Y02T 10/12 (20130101) |
Current International
Class: |
F02B
23/02 (20060101); F02F 7/00 (20060101); F02B
23/06 (20060101); F16N 15/00 (20060101); F02F
3/00 (20060101); F01M 9/00 (20060101); F01M
1/08 (20060101); F01M 1/00 (20060101); F02F
023/00 () |
Field of
Search: |
;123/193P,193CP,193C
;51/319 ;92/155,153 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
"A Low Friction, Unlubricated Silicon Carbide Diesel Engine", SAE
paper No. 830313, by Timoney et al..
|
Primary Examiner: Cross; E. Rollins
Assistant Examiner: Macy; M.
Attorney, Agent or Firm: Malleck; Joseph W. May; Roger
L.
Claims
We claim:
1. In an oilless engine having a piston reciprocal within a
cylinder and along an axis of such cylinder while providing an
annular gap therebetween to receive combustion gases, a gas phase
blow-by control system comprising:
(a) an annular receptacle in at least one of said piston and
cylinder;
(b) an annular body of graphite carrying high temperatures solid
lubricant disposed in said annular receptacle, said body presenting
a face projecting out of said receptacle;
(c) an elastomer material retentive of elasticity at the maximum
operating temperature to be experienced by said body, interposed
between said body and receptacle to urge said body to close said
gap under all operating conditions of said engine; and
(d) axially directed grooves in said body face sized to
substantially trap cylinders of combustion gases therein by
viscosity under low pressure gradients and to limit the passage of
a combustion gases through said grooves under high pressure
gradients to one percent or less of the cylinder gas flow charge
volume, the gas cylinders functioning as bearings to ride the
piston during reciprocation and provide predominantly gas-phase,
lubrication.
2. The system as in claim 1, in which a coating of solid film
lubricant is disposed on the other of said piston or cylinder not
selected for said receptacle in step (a).
3. The system as in claim 1, in which a solid film lubricant
coating is placed on the face of said annular body, said coating
having a coefficient of friction at high temperatures of less than
the coefficient of friction of said body.
4. The system as in claim 1, in which said gap is in the range of
0.0002-0.04 inches.
5. The system as in claim 2 or 3, in which said coating is
comprised of a polymer based molybdenum disulfide and graphite
mixture.
6. The system as in claim 1, in which said elastomer is comprised
of silanes compounded with glass fibers, zinc oxide and carbon
black and processed to provide a resilient high temperature
resisting material.
7. The system as in claim 1, in which said piston has a cylindrical
side wall interrupted by a transversely extending piston pin, the
axial extent of said receptacle and body extends from adjacent the
bottom of said piston side wall to adjacent the bottom of said
piston pin.
8. The system as in claim 7, in which said body is comprised of a
material that maintains its antifriction characteristics up to a
temperature of 550.degree. F. or less.
9. The system us as in claim 1, in which said grooves make an angle
with respect to said cylinder axis of 2.degree.-15.degree..
10. The system as in claim 1, in which said piston has a crown and
a side wall interrupted by a transversely extending piston pin, and
in which said annular receptacle and body have a axial extent
proceeding from the bottom of said piston side wall to above the
piston pin but below the crown of said piston.
11. The system as in claim 10, in which the material for said
annular body is selected to retain antifriction characteristics up
to 880.degree. F.
12. The system as in claim 1, in which receptacles are provided in
both said piston side wall and cylinder wall for receiving an
independent annular body and elastomer therein.
13. The system as in claim 1, in which a receptacle and body is
provided at each of two separated locations, one adjacent the top
of the piston and another adjacent the bottom of the piston.
14. A gas phase lubricating system for an internal combustion
engine piston-cylinder arrangement substantially devoid of liquids,
comprising:
(a) a dry lubricant ring, mounted around said piston;
(b) means for mechanically resiliently biasing said dry lubrication
ring, toward engagement with said cylinder; and
(c) grooves in either or both of said cylinder or ring to form
small cylinders of gases attempting to migrate between said piston
and cylinder, and which gas phase small cylinders function to
separate and lubricate the relative movement between said cylinder
and piston.
15. The lubricating system as in claim 14, in which said grooves
are generally axially directed with respect to the axis of said
piston and cylinder, but make an angle with respect to the cylinder
axis of about 2.degree.-15.degree..
16. The lubrication system as in claim 14, in which said grooves
have a depth of 0.003-0.01 inches.
17. The lubrication system as in claim 14, in which the total
cross-sectional volume of all of the grooves together provide
blow-by of said gases under a high pressure gradient that is
limited to one percent or less of the gas charge volume attempting
to migrate between said piston and cylinder. .Iadd.
18. A fluid lubricating system for a piston-cylinder arrangement,
comprising:
(a) a dry lubricant ring mounted around said piston;
(b) means for mechanically resiliently biasing said dry lubrication
ring toward engagement with said cylinder; and
(c) grooves in either or both of said cylinder or ring to form
small cylinders of fluid attempting to migrate between said piston
and cylinder, and which fluid small cylinders function to separate
and lubricate the relative movement between said cylinder and
piston. .Iaddend. .Iadd.
19. A machine having a piston and cylinder reciprocating relative
to one another while providing an annular gap therebetween to
receive pressurized gases, a gas phase blow-by control system
comprising:
(a) an annular receptacle in at least one of said piston and
cylinder;
(b) an annular body of graphite carrying high temperature solid
lubricant disposed in said annular receptacle, said body presenting
a face projecting out of said receptacle;
(c) an elastomer material retentive of elasticity at the maximum
operating temperature to be experienced by said body, interposed
between said body and receptacle to urge said body to close said
gap under all operating conditions of said engine; and
(d) axially directed grooves in said body face sized to
substantially trap cylinders of pressurized gases therein by
viscosity under low pressure gradients and to limit the passage of
pressurized gases through said grooves, the gas cylinders
functioning as bearings to ride the piston during reciprocation and
provide predominantly gas-phase lubrication. .Iaddend. .Iadd.20. A
gas phase lubricating system for a reciprocating piston-cylinder
arrangement substantially devoid of liquids, comprising:
(a) a dry lubricant ring mounted around said piston;
(b) means for mechanically resiliently biasing said dry lubrication
ring toward engagement with said cylinder; and
(c) grooves in either or both of said cylinder or ring to form
small cylinders of gases attempting to migrate between said piston
and cylinder, and which gas phase small cylinders function to
separate and lubricate the relative movement between said cylinder
and piston. .Iaddend.
Description
BACKGROUND OF THE INVENTION
1. Technical Field
This invention relates to the art of engine lubrication and, more
particularly, to oil-less lubrication for the piston-cylinder
chamber.
2. Description of the Prior Art
A low heat rejection engine, particularly for a diesel engine, has
the potential to provide significant improvement in fuel economy.
Heat rejection can be reduced by eliminating liquid cooling
normally incorporated in the block of a diesel engine and replacing
all or a portion of the combustion chamber components with
materials that can operate at uncooled combustion temperatures,
such as ceramics. This is sometimes referred to as an adiabatic
diesel engine.
The temperature gradient in such low heat rejection engine will
range up to 1600.degree. F. (871.degree. C.). At such temperatures,
conventional oil, used as a piston lubricant, will pyrolyze.
Therefore, some means must be provided to create an antifriction
relationship between the cylinder wall and piston which is devoid
of fossil lubricants.
One approach, suggested in 1983 by S. Timoney and G. Flynn in an
article entitled "A Low Friction, Unlubricated Silicon Carbide
Diesel Engine", SAE Paper #830313, was to install a close-fitting
SiC piston in a SiC cylinder, the piston having no ring grooves.
Blowing of gases past the pistons could not be detected; the
authors concluded that the piston must be riding on a gas film due
to the reduction in friction horsepower. However, much of their
test work was carried out without the engine firing, so a
pressurized gas film was not the total reason for nonscuffing but
was also due to the low interfacial friction of SiC on SiC. The
structure of the Timoney and Flynn piston and cylinder had made no
accommodation for thermal growth and assumed uniform dimensions;
oil lubrication was fed to the piston pin area which assured little
dimensional change and, in fact, contributed to oil lubrication
notwithstanding the authors' label of an unlubricated engine. This
reference merely defined the problem without providing a specific
solution as how to provide a reliable gas phase lubrication while
encountering thermal growth, wide variations in the fit, and
without oil lubrication. This reference did suggest that if
clearances could somehow be controlled, a gas film would function
to lubricate the sliding piston in such cylinder.
Thus, it is an object of this invention to be able to control the
dimensional clearances between the piston and cylinder of an
internal combustion engine, which uncontrolled clearances may be
wide-ranging due to thermal variations of the materials used for
the piston and cylinder and mechanical variations resulting from
connecting rod loads.
It is also an object to move gas phase lubrication theory from the
experimental laboratory stage into the commercial environment
involving imperfect dimensional clearances.
Still another object of this invention is to provide an oilless
uncooled internal combustion engine that has improved gap control
and reduced friction with considerably reduced gas blow-by.
SUMMARY OF THE INVENTION
The invention is a gas phase lubrication system which operates
effectively within an oilless engine. Such engine has a piston
reciprocal within a cylinder and along an axis of such cylinder
while providing an annular gap therebetween to receive combustion
gas. Such gas phase lubrication system comprises: (a) an annular
receptacle in at least one of the piston and cylinder; (b) an
annular body of graphite carrying high temperature solid lubricant
disposed in such receptacle, the body presenting a face projecting
beyond the receptacle; (c) an elastomer material, retentive of
elasticity at the maximum operating temperature to be experienced
by the body, interposed between the body and receptacle to urge the
body to close the gap under substantially all operating conditions
of the engine; and (d) axially directed grooves in the body face
sized to substantially trap cylinders of combustion gases therein
by viscosity under low pressure gradients and to limit the passage
of the combustion gases through such grooves or from such trapped
gas cylinders under high pressure gradients to one percent or less
of the cylinder gas charge volume, said gas cylinders functioning
as bearings to ride the piston during reciprocation.
The receptacles for receiving the annular solid lubricant may be
(i) restricted to the lower region of the piston side wall as a
shallow ring, which region will oppose the cylinder wall and
experience a maximum operating temperature of 550.degree. F., (ii)
receptacles in the piston side wall extending substantially upward
from adjacent the bottom of the piston to above the piston pin, as
a sleeve, which region will oppose the cylinder wall having a
maximum operating temperature of up to 800.degree. F., (iii)
shallow annular ring receptacles separated at two locations, one
adjacent the top and one adjacent the bottom of the piston side
wall, (iv) reversed in position from any of the above by
positioning the receptacles in the cylinder wall rather than the
piston side wall, and (v) include the addition of similar
receptacles as in (i)-(iii) to the cylinder wall thereby providing
complementary receptacles in both the cylinder wall and piston side
wall.
The chemistry for the elastomer and solid lubricant may be varied
depending upon the maximum operating temperature to be experienced,
and a coating of solid film lubricant with a reduced coefficient of
friction may be disposed on the opposing cylinder wall or piston
wall not containing the first receptacle.
The grooves are sized (cross-section) to insure gas therewithin
under low gradients as well as under high pressure gradients. Such
grooves preferably have a radial depth of 0.003-0.005 inches and
preferably are semicylindrical so that the width of such grooves
will be in the range of 0.006-0.01 inches. Advantageously, the
alignment of the grooves will make an angle with the cylinder axis
of 2-5% to facilitate rotation of the trapped gas phase cylinder as
the piston reciprocates.
SUMMARY OF THE DRAWINGS
FIG. 1 is a partially sectional and partially schematic view of a
four-stroke uncooled oilless engine within which the invention
herein is incorporated;
FIG. 2 is a thermal gradient map superimposed on each of the piston
and cylinder wall;
FIG. 3 is an enlarged central sectional view of a piston and
cylinder assembly presenting a first embodiment of this invention
using a gas phase control restricted to the lower region of the
piston side wall;
FIG. 4 is an enlarged view of a portion of FIG. 3;
FIG. 5 is an enlarged sectional view taken substantially along line
5--5 of FIG. 4;
FIG. 6 is an enlarged sectional view taken along line 6--6 of FIG.
1;
FIG. 7 is a graphical illustration plotting the coefficient of
friction with time for various matched materials useful in an
adiabatic engine;
FIG. 8 is a graphical illustration of time plotted against wear for
the various combinations of materials used in FIG. 7;
FIG. 9 is a graphical illustration of friction and wear under
different conditions for a solid film lubricant coating of this
invention against graphite;
FIG. 10 is an enlarged schematic view of the piston and cylinder
and gas phase control of FIG. 3 at top dead center and bottom dead
center, both in the cold and the hot operating conditions;
FIG. 11 is an enlarged central sectional view of a piston and
cylinder representing a second embodiment of this invention;
FIG. 12 is an enlarged sectional view taken substantially along a
vertical radial plane of FIG. 5;
FIG. 13 is a schematic view of the piston and cylinder arrangement
of FIG. 11 showing the change in thermal expansion of such
components from hot to cold;
FIG. 14 is an enlarged central sectional view of still a third
embodiment of this invention;
FIG. 15 is an enlarged sectional view taken substantially along
line 15--15 of FIG. 17; and
FIG. 16 is an enlarged central sectional view of a fourth
embodiment of this invention.
DETAILED DESCRIPTION AND BEST MODE
An uncooled oilless four-stroke engine 10 is shown in FIG. 1,
having solid structural ceramic components (head 11, cylinder walls
12, piston 13 and valves 14) in the vicinity of the combustion
chamber 15; metal components are eliminated in the high temperature
areas of the engine. Uncooled is used herein to mean an engine that
is devoid of conventional cooling such as a water jacket or fins
for air cooling. The resulting higher operating temperatures can be
projected to provide at least a 9% improvement in the indicated
specific fuel consumption relative to a water cooled, base line
engine at part load operating conditions (i.e., 1200 rpm at 38 psi
BMEP). Since conventional oil lubrication cannot be used at the
higher operating temperatures because such oils will pyrolyze, gas
phase lubrication is used. Oil is also eliminated in the crankcase;
without crankcase oil, a sealing system to separate the oil from
the hot upper cylinder area, where coking can occur, is not
required. Oilless ceramic roller bearings 17 and 16 for the
crankshaft and connecting rod respectively eliminate this need for
oil in the crankcase. With ceramic roller bearings for the valve
train finger followers and camshaft (19 and 18), as well as
suitable dry lubrication, the engine is further simplified
eliminating the need for oil, the oil pump, oil filter and oil
gallery drilling. Thus, oilless is used herein to mean devoid of
conventional piston rings between the piston and cylinder wall that
are designed to ride on a fluid film.
Sintered silicon nitride was used as the material for the
structural cylinder wall and piston. Sintered silicon nitride has
coefficient of thermal expansion of about 3.6.times.10.sup.6
/.degree.C., a modulus of rupture of about 85 ksi which is stable
up through the temperature range of 1600.degree. F. and has a
thermal conductivity which is about 50% of the value of cast iron.
However, this invention provides a gas phase lubrication control
for any material of which the piston and cylinder wall may be
constructed. This comprises cooled engines as well as uncooled
engines; it is to be recognized that the benefits of this invention
will accrue to a greater degree with an uncooled engine.
Finite element analysis was used to calculate the maximum operating
temperatures and thereby the stresses that would occur in the
ceramic cylinder wall 12 and piston 13. The results were used to
provide a thermal map shown in FIGS. 2 and 3 for the cylinder wall
and for the piston respectively.
Gas phase lubrication between a piston and cylinder wall is
dependent on maintaining a tight clearance or annular gap effective
in triggering viscous drag to hold a gas phase film therein.
Unfortunately, it is very difficult to achieve and maintain a tight
and uniform annular gap throughout all aspects of engine operation.
The gas phase changes in viscosity and pressure during each of the
strokes of the engine operation, and concentricity of the piston
within the cylinder bore changes due to major and minor mechanical
side thrust loads of the connecting rod which is articulating from
side to side. Moreover, there is considerable thermal growth of
some regions of the piston (i.e., crown) and side wall (upper
region) due to combustion temperatures, which change the gap
fundamentally from cold to hot.
Embodiment One
FIGS. 3-8 illustrate a first embodiment of a gas phase blow-by
control useful in an oilless engine. This invention recognizes that
it is very difficult to design a consistently tight gap h between
the piston 13 and cylinder wall 12 under all operating conditions.
Instead, this invention closes the available annular gap h by use
of an antifriction annular body 20 that is radially biased. As
shown in FIG. 5, gas phase cylinders 21 are trapped in grooves 22
in the face 23 of such body 20 to act as bearings during
reciprocation of the piston 13.
As shown in FIG. 3, this embodiment restricts the receptacle 24 for
receiving the annular gap closing body 20 to the lower region 25 of
the piston side wall 26, which region 25 will oppose the cylinder
wall to experience a maximum operating temperature of about
550.degree. F. The region 25 preferably extends from adjacent the
bottom 26a of piston side wall to below but adjacent the piston pin
opening 27. The annular receptacle or groove 22 may be dovetailed
or under-cut in cross-sectional configuration to facilitate holding
the annular body 20 therein. The undercut may form a negative angle
of 5.degree.-15.degree. with the cylinder wall surface 26.
The depth or radial extent of such receptacle is in the range of 2
mm to half the thickness of the piston wall, and may have an axial
extent 25 which is in the range of 5-15 mm if used at only the
bottom of the piston and 15-30 mm if used to hold a sleeve as in
another embodiment.
The body 20 is comprised of a graphite carrying, high temperature
solid lubricant. A high temperature solid lubricant is used herein
to mean a solid lubricant that has a coefficient of friction of
0.02-0.1 at 600.degree. F. The body presents a face 23 which
projects out of the receptacle 24 to tend to engage the opposed
cylinder wall surface 28. Such solid lubricant is preferably a
composite, by volume, of 40% graphite, 20% MoS.sub.2, and the
remainder a thermally stable (does not decompose up to 375.degree.
C. or 700.degree. F.) polymer such as polyarylsulfone; the solid
lubricant may also be a metal matrix composite having about 40%
graphite and the remainder aluminum or cast iron. Such metal matrix
composites may be formed by power metallurgy or other suitable
means to provide a porous material that can expose graphite for
intermittent or supplementary lubrication purposes. Up to 13% of
the graphite may be substituted with boron nitride. The solid
lubricant may also include up to 10% copper and one of LiF, NaF and
CaF, as a substitute for the MoS.sub.2.
An elastomer material 30 is interposed between the body 20 and the
receptacle or groove 24 to urge the body face 23 to close the gap h
under all operating conditions of the engine. The cross-sectional
configuration of the elastomer material may be similar to the shell
of an automotive rubber tire which is U-shaped with curved lips 30a
at the exposed extremity thereof.
The elastomer material 30 must be effective to retain its
elasticity up to a maximum operating temperature of about
550.degree. F. Materials that are useful for this purpose may
comprise silanes, such as Dow Corning Resin #95-077GA or Cilastic
GA. The resins are compounded with glass fibers (such as Owens
Corning #497 Fiber). The fibers are chopped to a short length (such
as 1/4 inch) and coated with Dow Corning Primer Q36-061 diluted in
trichloroethylene and dried for about 10 hours. The coated fibers
are then mixed with the silane resin as well as with zinc oxide and
some carbon black. The mixture is blended with a catalyst for about
15 minutes and then degassed for 3/4 to 1 hour and formed as an
extruded material, preferably in the tire shell shape.
Horizontal annular grooves 32 may be defined in the back of the
body 20 to provide areas where the elastomer material 30 may
interlock with the body and provide a firmer mating therebetween.
Such horizontal annular grooves may have a depth of about
0.003-0.005 inches such as shown in FIG. 4.
With the annular body 20 urged by the elastomer material 30 to
close gap h, axially directed grooves 22 are defined in the outer
face 23 of the body to trap gas phase cylinders 21 therein by
viscosity when the grooves are under a low pressure gradient, and
to limit passage of combustion gases through the grooves or from
such gas cylinders under high pressure gradients to 1% or less of
the cylinder gas flow charge volume. The gas cylinders function as
rotating bearings to ride the piston during reciprocation. Low
pressure gradient is used to mean a gradient of 4500 psi or less,
and high pressure gradient is used herein to mean a gradient from
400 psi up to 1700-2000 psi. Grooves 22 are directed axially, but
preferably deviate from perfect parallelism with piston or cylinder
axis 33 by an angle of 2.degree.-15.degree.. It is desirable that
the grooves overlap along a line-of-sight (looking along the
surface of the piston parallel to its axis) due to the skewing and
thus roll over the full interfacing surface of the cylinder wall.
Such slight skewing facilitates the rolling of the gas phase
cylinders 21 by viscous drag of the cylinder wall during piston
reciprocation.
The axially directed grooves 22 may have a depth of about
0.003-0.01 inches and are spaced apart a distance of about 3 mm;
the total semiconductor cross-sectional volume of all the grooves
should together provide the predetermined blow-by under a high
pressure gradient that is limited to 1% or less of the gas charge
volume.
In the case of an annular body which is limited to the bottom lower
extent of the piston side wall or skirt, as shown in FIG. 3, the
annular body will not be exposed to temperatures in excess of
550.degree. F. during its normal operation in a four cycle engine.
Therefore, the elastomer and body should maintain a closure
tendency across the gap, and the graphite contained in type body
and the rotating gas cylinders in the grooves 22 will provide
sufficient antifriction lubrication therebetween. However, to
further decrease the friction between the annular body face and the
opposing cylinder wall, a solid film lubricant coating 35 (see FIG.
7) containing either BN or MoS.sub.2 with graphite may be applied
to the cylinder wall.
The coating is comprised of about 40% by weight of high temperature
thermoplastic resin such as polyarylsulfone, 40% graphite, and 20%
of either MoS.sub.2 or BN. A resin that is thermally stable up to
about 700.degree. F. is polymer 360, known as Astrel, manufactured
by Minnesota Mining and Manufacturing Company. Such resin may be
dissolved in dimethyl acetamide to make a syrupy paste to
facilitate blending of other ingredients. After the cylinder wall
surface is thoroughly cleaned to remove any oxidation, such wall
may be grit blasted to increase porosity and thereby the reception
of the coating. The blended mixture is spread over the cleansed and
porous surface and dried at about 250.degree.-300.degree. F. for at
least 15 minutes.
This polymer based coating mixture will have a coefficient of
friction which is 1/2 to 1/3 that of the metal matrix
graphite-carrying composite of body 20. In FIGS. 7, 8 and 9, a test
using such coating against Si.sub.3 N.sub.4 was compared to
uncoated Si.sub.3 N.sub.4 or graphite on Si.sub.3 N.sub.4. FIG. 7
shows a plot 36 for coefficient of friction for a coated silicon
nitride cylinder wall against which a graphite body is rubbed
thereagainst; it had the lowest overall coefficient of friction
compared to plots 37 for uncoated silicon nitride against silicon
nitride and plot 38 for a graphite body against silicon nitride.
Similarly in FIG. 8, a plot of wear 39 for coated silicon nitride
against a graphite body had a value quantity considerably less than
that of the other materials (see plots 40 and 41). As shown in FIG.
9, when a graphite body is rubbed against a coating of solid film
lubricant containing molybdenum disulfide and graphite, the
friction and wear therebetween was extremely low both in the
start-up and steady-state conditions for an engine.
BN will break down as a structural solid at about 750.degree. F.
and MoS.sub.2 will do so at about 600.degree. F. To permit such
substances to continue providing antifriction characteristics after
such breakdown, the supporting surface may be provided with
reservoirs 43 or grooves to capture or retain the solid film
lubricant coating 44, much in the manner of porosity. These
reservoirs may be grooves in the cylinder wall, which grooves are
vertically oriented, spaced apart a distance of about 3 mm, and
each may have a semicircular depth of about 0.003 inches.
In operation, and as shown in FIG. 10, the gas phase control of
this first embodiment functions to close the gap (shown as 0.06 mm)
with cylinder 12 both at top dead center and bottom dead center for
the piston 13 when the engine is cold (ambient conditions). This is
illustrated in solid full line. The piston is illustrated with an
exaggerated chamfer 46 and a piston diameter of about 80 mm. The
chamfer increases the gap at the piston shoulder to 0.10 mm. The
chamfer is needed to compensate for the mushrooming effect (see
broken line 47 of the hot piston) that takes place at the crown of
the piston due to exposure to the highest temperatures and thus the
highest thermal growth. The cylinder wall 12 will also undergo a
gradient of thermal growth with the greatest change in dimension at
the top of the cylinder wall (see broken line 48 for the hot
cylinder wall). Please note that if the piston top was not
chamfered, its hot size would interfere with the cylinder wall
during the bottom dead center position. With the chamfer the hot
size of the piston crown will always stay separated (at 49). Thus,
the control body 20 must only accommodate a change in gap from a
radial distance of 50 when cold to a radial distance of 51 when the
parts are hot, which may be a very slight change or may be very
large, depending on design.
Embodiment Two
As shown in FIG. 11, an alternative embodiment incorporates a
similar control body, but the modified annular body 53 is expanded
to have an axial extent 54 from the bottom of the piston side wall
to above the piston pin area, approximately three-quarters of the
way up the side wall of the piston. Such control body forms a
sleeve which will be exposed to a higher maximum operating
temperature, as high as 880.degree. F. To insure that the solid
lubricant body has a sufficiently low friction at such higher
operating temperatures, the face of the body is coated with a solid
film lubricant containing MoS.sub.2 or BN and graphite as described
earlier. Moreover, the cylinder wall 12 should contain a coating 60
of such polymer/molybdenum disulfide/graphite mixture. Both
coatings, on body face 55 and on the cylinder wall 12, should
contain reservoir grooves 61 in the supporting substrate to assist
in distributing the material at higher temperatures, as shown in
FIG. 12. The grooves on the cylinder wall may be vertically
oriented, spaced apart about a distance of 3 mm, while the grooves
on the face of the solid lubricant body may be horizontal spiral
grooves oriented to across the grooves in the cylinder wall during
piston reciprocation and promote a mutual distribution of the solid
film lubricant particularly at higher temperatures. This embodiment
is best suited to an articulated piston where the piston crown is
made separately from the piston skirt; it works well in either a
four or two stroke engine.
FIG. 13 illustrates how the sleeve type gas phase control will
function. The body 20 is exaggerated to show how it extends up a
more significant distance of the piston side wall. Elastomer
material 30 is exposed to temperatures in excess of 700.degree. F.
at the top of the body and thus a refractory coating 67 may be
deposited on the elastomer at such exposed areas 66. The body 20
must accommodate a wider fluctuation in the gap from cold to hot
(i.e., from cold radial distance 63 to hot radial distance 64 and
65).
Embodiment Three
The embodiment illustrated in FIGS. 14 and 15 provides for the
control body to be in a receptacle disposed at 70, not only in the
piston, but also at 71 on the cylinder wall. The control bodies
each will have elastomers urging the two bodies together to close
the gap between the piston and cylinder wall. One or the other of
the mating control bodies may have blow-by grooves or both may have
blow-by grooves. It is possible that the elastomer may be
eliminated from the piston in this embodiment. This is useful
because the cylinder wall runs considerably cooler than certain
regions of the piston wall and such cylinder wall will be better
able to protect the elastomer.
Embodiment Four
FIG. 16 illustrates still another embodiment which employs shallow
ring bodies 80 and 81, one adjacent the top of the piston and
another adjacent the bottom of the piston side wall. Each annular
body 80 and 81 employs the same type of elastomer and
graphite-carrying solid lubricant body, as described in Embodiment
One, to close the gaps 82 and 83 at the top and bottom of the
piston, respectively. The net effect of separated bodies is to trap
a gas film therebetween (across extent 84) to more effectively
operate as a squeeze film and thereby insure antifriction
characteristics therebetween. Both of the shallow control bodies
will have axially directed blow-by grooves to facilitate the
provision of rolling gas phase cylinders acting as bearings. This
embodiment is particularly helpful in a four-stroke engine where
any gas phase squeeze film may be particularly thin during the
intake and exhaust strokes. This trapping of the gas phase between
the annular control bodies facilitates the maintenance of such film
during such strokes.
While particular embodiments of the invention have been illustrated
and described, it will be obvious to those skilled in the art that
various changes and modifications may be made without departing
from the invention, and it is intended to cover in the appended
claims all such modifications and equivalents as fall within the
true spirit and scope of the invention.
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