Note: Descriptions are shown in the official language in which they were submitted.
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In the internal combustion engine art and particularly
that portion dealing with diesel engines, the insulated
combustion chamber has been proposed to increase efficiency
and minimize pollutants. Efficiency is increased because
the amount of combustion heat rejected to the engine cooling
system is minimized. Furthermore, the existing temperature
available to drive the turbine of a turbocharged engine is
greatly increased.
Past attempts at insulating the combustion chamber
may be separated into a number of broad categories. The
first is where surfaces of the combustion chamber are sprayed
with a ceramic insulating material. These surfaces would
include the piston crown, the cylinder head and possibly the
walls of the cylinder. The difficulty with this approach
is maintaining the integrity of the layer because of the
t stresses of the combustion cycle and the problems of thermal
expansion.
Another approach has been to make one or more of
the components which define the combustion chamber out of
insulating material. Alternatively one may utilize a ceramic
insert of substantial thickness in one of these components.
Problems arise with devices of this type because
they are exposed on one side to the hot combustion gases and
on the other side by the cooler supporting structure around
the combustion chamber. As such, rather substantial temperature
gradients (as high as 1200F) may exist across these materials.
Excessive thermal stresses and distortion will result if the
part is made from material other than ultra low thermal
expansion material.
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The above problems are solved by a piston having
an annular crown of temperature resistant material positioned
~! over a corresponding piston body. The crown and piston body
- are secured together and the interface between them is
, provided with means having a relatively low overall conductivity
so that the temperature gradient across the crown is minimized.
According to the invention there is provided a
piston for an internal combustion engine having a generally
, annular crown portion of temperature resistant material with
the crown portion having a circular periphery. The crown
portion is located so that both it and the generally annular
piston body having a circular periphery have opposed planar
surfaces. A plurality of stacked discs having a relatively low
conductivity planar interface between adjacent discs are
positioned between the opposed planar surfaces of the crown and
, piston body for forming a low effective thermal conductivity
t~ interface. The planar surface of the crown extends radially
' outward at least as far as the periphery of the discs. Further
included is a means for fastening the crown portion to the
piston body so that the temperature gradient across the crown
is minimized.
Advantageously the crown and piston body have aligned
holes with the one in the piston body terminating in an interior
recess thereof. The fastening means is comprised of a bolt
extending through the latter mentioned holes having a head
positioned on the crown and a threaded portion extending through
the piston body to the interior recess. A nut threaded onto
the end of the bolt acts on a means positioned between the
nut and the piston body for yieldably urging the piston body
and crown towards one another. The latter urging means may
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comprise a spring means positioned around the bolt and acting
against the nut and the piston body, the spring means
comprising a plurality of belleville washers.
Preferably the discs do not extend to the periphery
of the interface between the crown and the piston body being
of an overall thickness so that the crown and piston body are
maintained out of contact with one another by utilizing an
annular recess in the piston body so that compressive loads
on the crown are transmitted to the latter across an annular
area with the radius of the annular recess being approximately
equal to the difference in radius between the crown and the
outer periphery of the discs, a uniform bending load being
imposed upon the crown by compressive forces.
The above and other features of the invention will
be apparent from a reading of the following description of
the disclosure shown in the accompanying drawings and the
novelty thereof pointed out in the appended claims.
Figure 1 is a longitudinal view, partially sectioned,
illustrating a piston which embodies the present invention.
,~ 20 Figure 2 is a fragmentary longitudinal view of a
piston incorporating an alternate embodiment of the present
invention.
Referring to figure 1 there is shown a piston 10
comprising a generally annular body portion 12 having a ring band
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section 14 formed around its circular periphery. The ring band
, section 14 includes a plurality of circumferential grooves
16 which receive compression and oil rings 15, 17 respectively.
The piston rings 15, 17 seat against a bore 19 which terminates
to form a combustion chamber 21.
' 30 The piston 10 is reciprocable in the bore 19. The
piston body has wrist pin bosses 18 with aligned bores 23
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adapted to receive a wrist pin which permits the piston body
to be connected to a connecting rod (both elements are not
shown). In an internal combustion engine the connecting
. rod is journaled on a crankshaft to provide a rotary output.
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The piston 10 is shown as a cross head type piston
jlwhich incorporat~s a separate skirt assembly connected to
i,the piston body through the wrist pin. Although a skirt is
, l¦necessary to successfully utili~e the piston in an internal
51icombustion engine, a specific description will be omitted in
order to simplify the description of the present invention.
I It should be apparent to those skilled in the art that skirt
j~sections may also be formed integrally with the piston body
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10 I The piston also includes a crown portion 20 positioned
~ ~over the upper surfaoe 22 of piston body 12. Crown 20 has a
s - ¦circular periphery and an upper face 2~ exposed to the combustion
l ¦~ chamber 21 which has a "Mexican hat" configuration for
I~efficient combustion. The configuration of upper face 24,
15!l however, need not be limited to this specific type. The
Icrown 20 may be formed from any one of a number of heat
,~ resistant materials. Examples of such materials are ceramics
, such as silicon nitride (Si3N4), lithia alumina sillca
i! (LAS), fused silica (SiO2) and silicon carbide (SiC), reaction
20~l sintered silicon carbide (RSS:C) sintered silicon carbide
(SSic), and reaction bonded silicon nitride (RBSN). Any one
I of these materials can withstand temperature levels experienced
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! in the combus-tion chamber. However, the construction shown
Il below permits all of the above to be used successfully in
251l spite of some oth~r properties such as high thermal coefficient
il and thermal expansion.
~ltcrnatively crown 20 may be formed from a heat
! resistant metal. Examples of such a material are stainless
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il steel or coated more common steels. These materiais do not
¦ have as high a temperature capability as the ceramics in
Il that their temperature capability is approximately 1400-
: ¦! 16000 F (760-8700 C) compared to about 22000 F (12000 C)
, 5 ¦I for ceramics. However, their temperature resistance is
- i still high enough to permit an increase in combustion
I efficiency. In addltion they are more readily adaptable to
- mass production.
The crown 20 and the piston body 12 each have
coaxial holes 26 and 28 respectively. A bolt 30 having a
; head 32 received in recess 34 in crown 20 has a shank 36
extending through holes 26 and 28. Preferably, the head 32
of bolt 30 is shaped to form an extension of the upper face -
¦ 24 of crown 20. In addition, bolt 32 is formed from a high
lS¦1 temperature, high strength alloy, generally of a nickel or
¦1 cobalt base. Examples of such alloys are Udimet 700,
i, nlmonic 115 or 90, R-41, or Waspalloy.
The shank 36 of bolt 30 terminates in a threaded
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portion of 38 received in a recess 40 formed in the interior
20 ~ of piston body 12. Recess 40 has a shoulder 42. The flange
j! 44 of a sleeve-like spacer 46 abuts shoulder 42. A nut 48
ii is threaded over the end of bolt 30. ~ spring assembly 50
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acts against nut 48 and the inner wall of a recess 52 in
,¦ spacer 46 to hold the crown 20 and piston body 12 together
25 1I while permitting differential thermal e~pansion- As illustrated,
; li the spring assembly 50 comprises a plurality of stacked
belleville washcrs which arc wclL krlowrl compact spring
assemblies.
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prime feature of the present invention is the
: ~I provision of an interface between crown 20 and piston body
12, generally designated as 54 which has a low effective
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~~ thermal conductivity. As shown in figure 1, the interface 54 may
; 5 ¦I comprise a plurality of ~ubstantially Planar discs 56 sandwiched
jl, between the lower planar surface 58 of crown 20 and the Planar
floor of an annular recess 60 in the top of piston body 12.
~, Each of the discs 56 has a central opening 62 receiving the
shank 36 of bolt 30. The discs 56 are selected so that the
effective thermal conductivity across the interface between
¦ adjacent discs is relatively low. A low effective thermal
conductivity may be achieved with commonly available material
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il by using steel washers having roughened surfaces, for example,
" a surface roughness of 200~ in. It has been found that
, ordinary steel washers with no special roughening of their
l surface provide an overall low effective thermal conductivity.
; l, The stacked discs are extremely effective in
' producing a low thermal conductivity. Heat transfer coefficients
i!on the order of 0.5 BTU/~IR-FT- F are achievable compared to
20 ~180 BTU/HR-FT- F for a solid piston body-crown interface.
' It should be noted also that a greater or lesser number of
,'discs may be employed to achieve a greater or lesser thermal
conductivity. Since the diameter of the discs 56 does not
'extend to the peripl-ery of the crown 20 or piston body 12,
25 ',-their thickness ib sclected so that the portion of the crown
~i20 outside of these discs 56 is out of contact with the 22
' of piston body 12.
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Furthermore, the discs 56 support crown 20 over a
l' relatively broad annular planar area defined at its inner
¦, diameter by the diameter of hole 28 and at its outer diameter
I by the diameter of discs 56. This area of contact minimizes
5 ¦i the unit pressure on the discs 56 for a given pressure in
¦~ combustion chamber 21 and therefore contributes to a low ;-
¦¦ thermal conductivity across the interface between adjacent
jl discs. The annular contact area divides the radius of crown-
j 20 into approximately equal sections. It can be seen that
0~¦ the radial dimension of crown 20 between hole 26 and a point
¦i in line with the wall of hole 28 is approximately equal to
the radial dimension between the periphery of discs 56 and
the outer periphery of crown 20. The bendiny loads on the
ll top of crown 20 due to combustion pressures are therefore
lslJ equalized.
¦' The discs 56 shown in figure 1 constitute one way
ll of insulating crown pis~on body 12. Figure 2 shows an
'i alternate form of interface 54' which uses a disc 64 which
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Ij has a relatively low conduc-tivity. This disc 64 may be
20', formed of porous metal or of steel wool compressed sufficiently
to resist compressive loads. In certain instances it may be
also advantageous to use a plurality of discs 64.
With any of the interface materials described
,l above, the crown 20 is effectively insulated from the piston
25 i body 12, u5ually formed of high conductivity aluminum. This
, insulation provides a primary advantage of minimizing the
thermal gradient between the upper and lower surfaces 24 and
58 of crown 20. As such internal thermal stresses are
', substantially minimized. This permits the material selection
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jjfor crown 20 to be very flexible since the material only
must be able to withstand combustion temperatures and resist
moderate compressive loads. The temperature gradient for
¦ithe most part is taken up by the interface 54 between the
' 5l,~crown 20'and piston body 12. Since the only load on it is~a
ilcompressive one, the thermal stress caused by the temperatur.e
j~gradient is not as detrimental as it would be if it occurred ,
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'in the,crown 20. With the plurality of discs,~the gradient
is distributed over a plurality of interfaces thereby mini'mizing
101 the stresses on the individual elements.
~' i ~ Any varia~tions in thermal expansion which occur
between the piston body, interface 54 and crown 20 are
compensated for by the spring assembly 50 to assure a relatively
l!uniform compressive loading holding the parts together.
15 1l Since the shoulder 42 is larger in area than that available
on the body 12,-for receiving the spring assembly S0, spacer
' I element 46,assures a greatly improved seat for carrying the
: j;joint load on the body ].2. ,
1', The above piston achieves the objec'tives of a well
20i~insulated combustion chamber to increase efficiency and to
reduce emissions. 'Furthermore it does so with an economlcal,
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, ,.effective construction that permits ready replacement of the
: ' parts.
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