CA1200166A

CA1200166A - Internal combustion engine having manifold and combustion surfaces coated with a foam

Info

Publication number: CA1200166A
Application number: CA000383141A
Authority: CA
Inventors: Jay D. Rynbrandt
Original assignee: Chevron Research and Technology Co
Current assignee: Chevron USA Inc
Priority date: 1980-08-22
Filing date: 1981-08-04
Publication date: 1986-02-04
Also published as: FR2489416A1; GB2086470A; DE3133223C2; DE3133223A1; BR8105356A; IT8123581A0; FR2500002A1; IT1168020B; FR2489416B1; ES504877A0; ES8205941A1; GB2086470B; FR2500002B1

Abstract

ABSTRACT OF THE DISCLOSURE
For the more efficient operation of both spark-and compression-ignited internal combustion engines, and/or control of octane requirement increase in spark-ignited internal combustion engines, at least a portion of the surfaces of the intake manifold exposed to the fuel-air mixture, and/or the combustion chamber exposed to combustion, are fabricated from coated with a material having the combination of thermal conductance and thermal penetration which permits the temperature of said surface during the combustion process to be in excess of the tem-perature at which deposits form and said surface storing insufficient heat to substantially raise the temperature of the incoming air-fuel charge during the engine intake and compression stroke.

Description

INTERNAL CO~IBUSTION ENGINE HAVING MANIFOLD
~ND COMBUSTION SURF~CES COATED WITH A FOAM

BACI~GROUND OF THE INVENTION
This invention relates to internal combustion engines, both spark ignition (SI) internal combustion (IC) engines, and compression ignition (CI) internal combustion engines such as diesel engines. It also relates to the coating of the internal surfaces of internal combustion engines, in particular, it relates to the coating of the unwiped surEace of a combustion chamber e~posed to combus-tion gases, the intake valves, the surface of the intake manifold exposed to fuel-air mixtures, and the sur~ace of the exhaust manifol-l e~posecl to exhaust gases~
It has been recognized that the tllermal efi~i-ciency oE the internal combustion engine could be improved by coating the aforementioned combustion chamber surfaces with a thermally insulating coating to recluce heat losses to the coolant during the compression and power cycles (fully 30-~096 of the total heat generated in an IC engine is lost to the coolant). ~.S. Patents 4,074,671 and 3,820,523 provide a thin ceramic coating oE the combustion chamber sur:~ace ~or this purpose. U.S. Patents 3,91l,39:L
and 3,552,370 provide Eor the deposition o~ l.ayers of cert:ain materials or t:ile same purpose.
U.S. Patents 3,066,663 and 3,019,277 teach the coating of certain combustion chamber surfaces with a ceramic insulation of appropriate thickness and thermal conductivity purporteclly to avoid the formation of sur-face-ignition and rumble-causing deposits when a fuel or lubricant containing phosphorous is used in a high com-pression engine.
3 The deposition of substances on combustion cham-b~r surfaces is believed to lead to an increase in the octane requirement of new spark ignited engines as they operate on unleaded fuels due to increased compression ratio and heat regenerated to the fresh ?ir-Euel charge.
~0 ~3~ tD

Octane requirement inerease, ORI, in such a ease can be as much as six or more octane numbers. The "octane require-ment" is the minimum octane necessary to avoid noticeable knock. ~voidance of ORI would permit the use oE higher compression ratios for greater efficiency and/or the use of lower octane unleaded fuel.
A thermal insulating coatinc3 on the piston top surface would also tend to reduce the heat loading of the piston. This in turn would reduce the rate of deposit formation in the ring bel-t æone thereby reducing ring-sticking.
Insulated intake manifolds and intake valves decrease heating o the intake air-uel charge in SI
engines, whieh reduees their octane ~equirement, and decreases intake air heating in CI engines, which improvc.~s their volumetric efEiciency.
:[nsulated exhaust malliEolds increase the heat

2~ available to turbo-charge CI and SI engines and increase the temperature of exhaust ~ases at the catalyst in SI
engines.
A low heat capacity, insulatin~ coating on the piStOIl top sur~ace and combustion ch~mber surlaces reduces the thickness o the non-burninc~ quellch layer near these surfaces and improves volatiliæation clurincl combustion of ally hydroearbon liquicls whicll are on the sur~aces. These two processes promote hydroearbon burnillc~ and reduce hydrocarbon exhaust emissions.
SUMMARY OF T~IE INVENTION
Internal combustion engines in general can be operated more efficiently, and spark-ignited en~ines, in particular, can be operated on unleaded fuel of lower octane without knoeking, if in an internal combustion engine eomprising a combustion ehamber ha~ling a surface exposed to combustion. At least a portion of said surface has a combination of thermal conductance and thermal penetration which permits the tempera-ture of said surface .Pl~i~

during the combustion process to be in excess of the temperature at which deposits form and said portion of said surface stores insufficient heat to substantially raise the temperature of the incoming air-fuel charge during the engine intake and compression stroke.
Thus, according to the present invention, there is provided in an internal combustion engine comprising a combustion chamber having a surface exposed to combustion, the improvement which comprises at least a portion of said surface, having a combination of a thermal conductance and a thermal penetration which permits the temperature of said surface during the combustion process to be in excess of the temperature at which deposi-ts form, said surface storing insufficient heat to substan-tially raise the temperature of the incoming air-fuel charge during the engine intake stroke and compression stroke, wherein said surface portion has a thermal penetration expressed as ~ of less than about 600 J/m2 I~secl/2, and a thermal conductance expressed as K/d of at least about 2000 J/m2 l~sec, K representing thermal conductivity, p representing density, C represen-ti.ng heat capacity and d represen-ting thic]cness, and the product of the thermal penetration times thermal conductance is less than about 3X106 J2/m4 K2sec3/2, said surface portion being a thermally stable closed-cell foam having at least about 40~ volume microspheric voids in a matrix selected from the group consisting of a polyimide resin, a polyimide resin composite including carbon and/or silica, and a ceramic of -the group consisting of the oxides, nitrides, and carbides of Si, Ti, Cr, Ta, Nb, and Zn.
In another aspect, the invention provides a method for inhibiting octane requirement increase in an internal combustion engine having a combustion chamber surface exposed to _ combustion gases comprising: fabricating at least a portion of said surface exposed to said combustion gases from a material having a combination of thermal conductance and thermal penetra--tion, permits the temperature of said surface during the combustion process to be in excess of the tempera-ture which deposits form, said surface storing insufficient heat to substantially raise the temperature of the incoming air-fuel charge during the engine intake and compression strokes, wherein said surface portion has a thermal penetration expressed as ~ of less than about 600 J/m2 Ksecl/2, and a thermal conductance expressed as K/d of at least about 2000 ~/m2 .l~sec, K representing thermal conductivity, p representing density, C representing hea-t capacity and d representing thickness, and the product of the thermal penetration times thermal conductance is less than about 3X106 J2/m4 K2sec3/2, said surface portion being a thermally stable closed-cell foam having at least about 40~ volume microspheric voids in a matrix selected from the group consisting of a polyimide resin, a polyimide resin composite including carbon and/or silica, and a ceramic of -the group consisting of the oxides, nitrides, and carbides of Si, Ti, Cr, Ta, Nb, and Zn.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a fragmentary vertical sectional view showing a portion of an engine employing some coatings of the present invention.
Figure 2 is a cross-sectional schematic drawing of a coating made from a scanning electron micrograph of the coating of this invention after cutting said coating in a direction perpendicular to the surface and viewing same in a direction parallel to said surface.
DESCRIPTION OF THE PREFERRED EMBODII~ENTS
The present invention may be used in any internal ~- 3a -combus-tion engine and particularly the common reciprocating~
piston internal combustion engine. Figure 1 illustrates an embodiment of the invention wherein a coating of a material having the required thermal penetration and thermal conductance is applied to the surfaces of a conventional combustion chamber 1 and intake manifold 2 of a 4-cycle engine. The piston 3 and intake valve 4 are shown in approximate relative position early in the intake-downstroke of the piston. The fuel-air mixture (not shown) passes through the intake manifold 2, past the opened intake valve 4 and into the combustion chamber 1 where it will eventually be ignited by the spark plug 5. In accordance with the present invention, at least a portioll of one or more of the combustion chamber surfaces, the intake valve, the exhaust and intake manifold surfaces are coated with a material having the recited thermal parameters such as a thermally-stable, resinous closed-cell foam of about 0.02 mm to about 1 mm thickness and preferably, 0.04 mm to 0.2 1~l. In the illustration of Figure 1, at least a pOî tion of the unwiped surface of the combustion chamber 6-~ is coated, in particular, the unwiped portion of the cylinder head 6, the face of the ~ - 3b -01 --~-intake valve 7, and the piStOIl top surface ~ (i.e., the portion exposed to combustion); and at least a portion of 05 the intake manifold surface 9-10 is also coated, in parti-cular, the intake manifold chamber 9 and the tulip portion of the intake valve 10, which, for convenience, is con-sidered part of the intake manifold surface.
FIG. 2 schematically illustrates a particular embodiment of the coated surface of the present invention in close-up cross-sectional view. FIG. 2 is drawn from an actual scanning electron micrograph of a resinous closed-cell foam coating oi- this invention having the recluired the~rmal parameters. The coating was cut in a direction perpendicular to the surface and is viewed para:Llel to that surfaee. In FIG. 2, hollow microspheres 11 in a res-inous matrix 12 provicle a closed-cell foam coating 13 on the surEace 14. The dimenc;ion of 100 microns is inclicated 15. 'rhe microspheres 11 contain a yas (not sllown~ which may be air.
The closed-cell material of the present inven-tion is a unique material of low heat capacity. q'he material can be an integral part o~ the combLIstion chamber surface fa~ricated during the manuEactul. illCJ process or applied as a sur~ace coating to finisllecl comhu~tion chc~ ber parts as illustrclted by ~`l(.. 1.
'~`he recluired theLIllal corlductivity ancl heat capacity closed-cell materials of the invention can be defined by the mathematical models for thermally oscil-lating systerns such as the combustion chamber surEace of an internal combustion engine.
The requirements for the materials suitable foruse in the invention are expressed in terms of therrnal penetration and thermal conductance. A combustiorl chamber Witil a coating of low thermal penetration and high thermal conductanee regenerates less heat to the -fresh air-fuel eharge than one with conventional deposi-ts, formed from unleaded or leaded gasoline; and thus, it has a lower octane requirement than a chamber with these conventional deposits. During at least part of the eombustion proeess, p~

01 -5~

the surface temperature of the combustion chamber becomes high enough to prevent formation of conventional deposits.
05 Computer models suggest that thin coatings or a surface region with high closed-cell void volumes on combustion chambers may approach the low regenerated heat of a clean metal combustion chamber and still attain sufEicient sur-face temperature during combustion to prevent conventional deposits from forming. Reduction of regenerated heat is observed in computer models upon reduction of thermal penetration and upon increase of thermal conduction.
Simultaneou~ reduction of thermal penetration and increase of thermal conduction is more bencficial than changlllg of either property separately. Thermal penetration has no lower llmit of beneficial affect. The upper limit to tllermal conductance usefulness is that point at whicll the surface no longer becomes hot enough during combustion to prevent the build-up of deposits which eliminate the ORI
control advantage of the coating. This upper limit is lower for higher values of thermal penetration. Thermal penetration is defined as ~ , where K is tllermal con-ductivity, p is density, and C is heat capacity. Thermal conductance is cleEinecl as K/cl where cl is thickness.
Closed-cell materials of the present invention have a therma] penetration, in Sl units, o~ less tllan abollt 600 J/m2Ksec~ and a thermal conductance of at least about 2000 J/'m2Ksec. Suitable materials will have thermal penetration times thermal conductance of less than about 30 3X106 J2/m4K2sec3/2. In addition, the heat storages of the surface should be insufficient to raise the -tempera-ture of the incoming air-fuel charge more than about 33C
above the temperature increase of clean en~ine. Preferred coatings, such as a closed-cell polyimide resinous foam containing carbon blac~ and microspheres, have a thermal penetration oE about 380 J/m2Ksec~2 and a thermal conductance of 3000 J/m2Ksec.
The material can be any solid inorganic or organic material or modified materials thereof havina ~0 sufficient void volume therein to fall within the recited 0l -6-parameters. 5uitable organic materials are high temper-ature polyimide resins ancl resinous foams incorporating 05 microspheres. Suitable inorganic materials are materials such as the oxides, nitrides, and carbides of Si, Ti, Cr, Ta, Nb, Zn, and the like~ which have been modified by the incorporation of a sufficient void volume, i.e., micro~
spheres, blowing agents, or gases, and the like, to come ~ithin the recited thermal conductance and thermal pene-tration parameters. The thermal properties of a foam with a given non-void volume ~NV) can be estimated from the follot~ing relationships:
1~ ~ Rs * NV/(2 - NV) P = Ps * NV
C - Cs where the subscript s designates the thermal property of the solid.
One can quickly determine a desired value for NV
by substitutiny the K, p an~ C expressions for the foam into the formula for thermal penetration and adjustin~ the value of NV until the desired thermal penetration value is attained. The thickness oÇ the coatiny, d, may be adjusted to briny the thermal conductance within the desirecl limits. ~'or exam~le, sllicoll nitL-ide has IC of l$
W/mk, p of 32~10 ky/m3 and C o~ l0~0 J/kyk. ~ foam oE
silicon nitride with NV of 0.0~ would have an estimated K
of 0.75 W~mk and an estimated thermal penetration of 45 J/m2ksec~2. ~ 0.2 mm tllick coatiny oE this foam ~7Ould 30 have a thermal conductance of 3750 W/m2k.
The modified inorganic composites can be formed by mixing organic or inorganic microspheres ~i-th the inorganic material and hot pressing or scintering the material to a solid or by other methods known in the artO
The surface coating functions in the combustion chamber to prevent heat loss, to prevent the permanent deposition of substances having higher heat capacity than the coatiny, and to promote combustion of hydrocarbons in the quench zone adjacent to the coatin~'s free surface.

~2~166 01 _7_ It functions in the intake manifold, including the tulip portion of the intake valve, to prevent the excessive OS heating of the fuel-air charge (i.e., heating over that necessary for volatilization). Within the combustion chamber the deposit precursors are volatilized by the high temperature at the free surface of the coating during the power cycle. High surface temperatures of the coating are achieved by the selection of a coating having low heat capacity.
The coating of the present invention finds use in internal combustion en~ines of the spark ignition and compression ignitlon type, such as two- or Eour-cycle lS engines, as well as rotary piston engines commonly called "Wankel" en~ines. The surfaces which may be coated in the internal combustion engine consist of at least a portion of the combustion chamber surface, meaning the unwiped surface which is in contact with combustion gases and including the piston top surface (i.e., the portion exposed to combustion~, valve face and cylinder head;
and/or intake manifold surface, meaning the surface which contacts the incoming fuel-air charge between the carbure-tor and the combustion chamber and including the intake 2S valve tulip. The coating is applied by any suitable method to form a substantially uniorm layer whlch adheres to the surace, or to a suitably prepared surface, and has a thickness of about 0.02 mm to 1 mm, preferably about 0.2 mm to 0.0~ mm, and most preferably about 0.03 mm to about .15 mm, for surfaces exposed to combustion. The thermal properties of the coating are such that it is durable, has lo~ heat capacity, and is harmless to the engine, as well as, having substantial thermal stability.
At least a portion of some of the aforementioned surfaces are coated with the organic or inorganic foam.
Preferably, a portion of the combustion chamber, more particularly, the cylinder head, which is in contact with - the combustion gases, is coated. The piston top surface ~1.

(i.e., the combustion face of the plston) is also a pre-ferred surEace for coating. The valve faces which are 05 exposed to combustion may be coated, especially the intake valve. The surface o the intake manifold which contacts the incoming fuel-air mixture between the carburetor and the combustion chamber may be coated with resinous foam, preferably those portions of the intake manifold surface which have the highest temperature due to proximity to the combustion chamber, and more particularly, the tulip por-tion of the intake valve. Either, or both, the combustion chamber and the intake manifold surfaces are coated, at least in part, in the aforementioned manner, but for lS different reasons. While the combustion chamber is coated to provide more adiabatic-like operation of the engine, i.e., reducing heat~loss to the coolant at the end oE the compression stroke and throughout the power stroke, as well as to reduce deposition and heat regeneration; the intake manifold is coated to avoid excessive heating of the fuel-air charge over that necessary for volatilization of the mixture, as well as, to reduce deposit build-up around the tulip of the intake valve. Manifold coating is more applicable to carbureted engines than to those using in-cylinder fuel injection.
In another embodimetlt of this invention at least a portion of the exhaust port surEace area is coated with said organic or inorganic oam to insulate the exhaust gases, thereby helping to keep the exhaust temperature higher for use by a turbocharger, or for improved cata-lytic emissions control.
The thermally-stable, closed-cell material coating of the present invention resists oxidation and decomposition even at the high surface temperatures to which it is intermittently exposed in internal combustion engines (e.g., about 400C and above). It owes its pro-perties both to the materials from which it is constructed and to its manner of construction. The coating consists of a large number of voids embedded in a resinous matrix 4~ or inorganic material previously recited. In particular, - - \

_9_ ()1 the void spaces comprise about ~0 or more volume percent of the coating. The coating is accurately termed a "foam"
o5 because of the large number of voids (actually gas-containing cells) contained therein. The voids are sub-stantially sealed, i.e., the foam is a closed-cell foam, so that the pressure within a given closed-cell does not fluctuate with engine pressure cycles. A relatively simple manner of constructing the closed-cell foam coating of the present invention is to embed a large number of preformed hollow spheres, heretofore and hereinafter called "microsph~re~," in a resinous matrix, such a foam ~ ~s~n ~
-~' is called a "~r~tiG oam" (~lodern ~lastics Encyc:lopedia 1978-7~, ~IcGraw-Hill, page 1~5~.
'rile microspheres are mixed into the resin solution ~hich is ~ater curec] to a rigid matri~ on the surface. Commercially available microspheres can be made from a wide variety of inor~anic and orc~anic materials, and mixtures thereof. Suitable inorganic microspheres are selected from the group consisting oE glass, ceramic, and quartz microspheres, and mixtures tl~ereof. Suitable organic microspheres are phenol-formaldehyde plastic microspheres, and like materlclls. Tlle inorganic micro-spheres are of about 0.01 mm to 0.2 mm average diameter aild are preEera~ly present in all amount oE ~om about ~10 to a~out ~0 volume percent of cured Eoam, and preferably from about 50 to about 65 vo1uMe percent. Organic micro-spheres are preferably of about 0.01 mm to 0.1 mm average diameter and are preferably present in an amount of from about 50 to about 70 volume percent oE cured foam. The plastic microspheres are classified according to pressure tolerance by pressurizing the spheres to approximately 2,750 kilo Pascals, (k Pa), in li~uid followed by flota-tion of the desired fraction. Inorganic microspheres are used to add strength in addition to void volume to the closed-cell foam. The inorganic microspheres are present in an amount of from about 40 to 70 volume percent of the cured foam. The microspheres are pressurized to about 12,000 k Pa prior to flotation.

The organic resinous matrix material finding use within the scope of the present invention is any resin 05 which sets to a rigid matrix havillg the properties of durability and thermal stability heretofore described.
Resinous composites containing resin, carbon, and/or silica are preferred. Many such resins are high-temper-ature polymers containing aromatic rings (Advances in Macromolecular Chemistry, Vol. 2, Academic Press, New York, 1970, pages 175-236, M. M. Koton), fluorinated polymers or organo-silicon polymers; e.g., polyaromatics, polyphenylene oxides; aromatic polyesters, polyamides, polyanhydrides and polyureas havin~J meltin~ points c~reater than 300C are known. Pre~erably, the resin is a thermo~-set or thermoplastic polyimide or polyamide resin, .5uch as the aromatic polyme~s that cure to form poly(amide-imides), U.~. Patent 3,190,856, e.~., polymers o~
trimellitic anhydride and aromatic diamines disclos~d in U.S. Patents 4,136,085 and 3,347,80S. Polyimide resins are widely available (Modern Plastics Encyclopedia 1979-80, McGraw-Hill, pages 76-78). Thermoset polyimides exhibit no distinct softening point below their thermal degradation temperature which can be as high as 260C, or higher. Thermoplastic polyimide has a melting point oE
about 31n-365C. Polyimicl~s are ~ellerally procluce(l by thc reaction of anhydrides or clianhydricles wi~h di(primary)-amines (Polyimides a New Class o~ Thermally-stable Polymers, Technomic Publ. Co., Stamford, Conn., 1970, N. A. Adrova et al.), e.g~, the reaction oE

O O

/ >~ X ~~<C/
ll ll O O

iG

o o ,. ..
~ C ~ ,C\o with El2N ~ NE12, or El2N - ~ Y ~ ~ 2' ,. ..
O O

wherein -X- and -Y- are selected from -C~CF3)2, -0-, -C-, etc.
In a preferred embodiment, the resinous matrix is a composite of polyimide and carbon black or graphite, preferably, also containing other fillers such as the powdered forms of boron, ZnO, Al, ancl amorphous fumed silica. Preferably, a dense carbon black of low-surface nrea to vol~lmc?
ratio is used. Such composites are str~lcturally more rigicl an(l are more rcfractory, in that they retain their shape and clurability even .Ibove the softeniilg point of the resill. A preferred composite resinous matrix com-prises about 30 to 85 weight percent of polyimidc rcsin, 70 to 15 weight per-cent carbon black, and 0 to 7 weight percent of fumed silica. ~lore prefer-ably, with plastic microspheres, the composite resinous matrix comprises about 35 to 50 weight percent of polyimide, about 65 to 50 weigllt pcrcent carbon black, ancl 0 to 4 weight percent fulllecl sil;ca. A prcferrecl closed-cell foam compositioll with inorgallic micros~ eres is frolll about 60 to abo~lt 80 ~e:ight percent poly;mi(le resill and about 40 to about 20 welgllt percent carbon black.
Tile foam coating may be formed on the metallic surface ~it is recogni~ed that plastic surfaces may be substituted, in certain instances, for metallic, if the internal combustion engine is constructed in part from plastic parts) by mixing polyimide with a solvent ~i.e., l-methyl-2-pyrrclidone) and optionally a diluent such as p-xylene, a filler, such as carbon black, graphite and/or fumed silica and/or boron powder~ and micro-spheres. This liquid mixture is coated or sprayed onto the clean surface to the desired thickness. The surface may be heated 12~6~

before spraying. The diluent and solvent are carefully removed from the coating to control or prevent solvent 05 gradients in the coating. The coating is cured by heating from about 140C to about 260 to 370C in an 3-24 hour curing process. The maximum practical curing temperature may be limited by the nature of the microspheres, since the phenolic microspheres are weakened at the highest tem-perature. The coating is generally applied to the surfacein the thickness which is desired for continual use in the engine. Ilowever, while the coating need not be an abla-tive coating, if it is too thick, it may ablate back to a more preferred thlckness in the range of about 0.04 mm to 0.2 mm for surfaces exposed to combustion.
The invention will be ~urtller illustrated by the followin~ examples, but it is to be understood that the invelltion is not meant to be limited solely to the details described therein. Modifications which would be obvious to one of ordinary skill in the art are contemplated to be within the scope of the invention.
Example 1 In this example a CFR r~head (single cylinder IC
engine of 37.3 cu. in. displacement and 7.0:1 compression ~25 ratio) combustion chamber surface ~as coated with a mix-ture prepared by blendillc~ 1 volume part ~moco ~I~l(~ amide-imide polymer (a condensation polymer contaillin~ aromatic imide linkages and made from trimellitic anilydride and diamine), 3 volume parts N-methyl-2-pyrrolidone (NMP) and 1 volume part Cabot Sterling MT (medium thermal) carbon black in a high-speed blender until smooth; 3 dry volumes of Union Carbide BJO-0930 (phenolic resin) microballoons were added and stirred until smooth. The mixture was painted onto the combustion surface, heated with a hot air gun until set, then oven dried overnight at 60C and cured at 200C, 230C, 260C, 290C and 315C for 2 hours each, respectively. This coating finished a 65-hour CFR engine test with unleaded gasoline (alkylate) at 1200 rpm, 4 in.

\

E~g OL intake vacuum and 1.5 ppm of exhaust CO and 20 degrees of spark advanee, with no evidence of deteriora-05 tion Example 2 In this example another CFR l~head combustion ehamber was eoated with a mixture prepared by blending 24 grams of Monsanto Skybond 700~polyimide a heat reactive aromatic resin which is thermally eured to a erosslinked polyimide, 24 grams of Cities Service Columbian Raven MT
(medium thermal) carbon blaek, 12 grams N-methyl-2-pyrrol-idone, and lO grams p-xylene in a high-speed blender.
Seventy cc of Union Carbicle BJO-O~O~(phenolic resin) microballoons were added to 62 grams of the above blencl along with 5 c3rams p-xylene cliluent. The microballoons were prepared by pressuril1g them to 3no psi in isooctane then floating off the uncruslled fraction. The cliluted coating was sprayed onto a cylinder head preheated to 50C
throu~h an air-atomizing spray nozzle, air dried overnight to remove the bulk of the solvent, thell oven dried at 50C, 75C, 100C, and l~nC for 2 hours each and cured at 200C, 220e, 240C, and ~.60C for 2 hours eaeh. ~`his coating completecl a lO0-hour CL;~R engine t:est under the conditions of Example l wit~ no eviderlce oE deter.ioratioll.
Example 3 ~ CFR L-hecld combustion chamber was eoated with a mixture prepared by blending 25 grams Monsanto Skybond 700 heat reactive aromatie resin system which can be thermally cured to a crosslin~ed polyimide, 13 grams N~IP, 15 grams Cities Service Columbian Raven MT thermal process carbon black beads until smooth in a high-speed blender.
2.6 grams of Union Carbide BJO 0930 microballoons, classified by discarding the floaters in a downward flo-~-ing column of ethanol, were added to 41 grams of the polyimide-carbon black matrix blend. This coating was dried and cured in the manner of Example 20 This eoating completed a 65-hour CFR engine test with unleaded gasoline and the test conditions of Example l virtually unchanged.

0l -l4-The coated engine showed only 2.3 numbers of octane requirement increase (ORI) at 30 degrees of spark advance 05 compared with 6.3 numbers of ORI for a test with the same head wlthout the coatin~.
Example 4 A CFR L-head combustion chamber was spray-coated with a mixture prepared by blending 24 grams of Monsanto l~ Skybond 700, 24 grams of Cancarb grade N-907 thermal carbon black, 24 grams of a mixture 26.5% NMP and 73.5%
p-xylene in a high-speed blender. Seventy cc of treated ~nion Carbide BJO-0840 microballoons were added to 64 grams of the above blend along with 6 grams of the above NMP-p-xylene mixture. The microballoons were t.reat:ed by heating them to 170C for 2 hours under 25 in. Ilg of vacuum with a slow N2 purge, then pressuri~in~ the said microballoons to 400 psi under isooctane, floating off the uncrushed fraction, and drying the residual isooctane.
The final blend was air-atomizer sprayed to a c~red thick-ness of 0.005 inch. The coating was air-dried overnight, then oven-dried and cured for 2 hours at 50C, 80C, 120C, 1~0C, 180C, 200C, 220C and 240C. The coating was unchanged after 100 hours of en~ine test with unleadecl fuel, and the test conditions oE Example 1.
Example 5 For this example, a CFR ~~head was mil:led to 9.5 compression ratio before coating. The coating mixture was prepared by ball milling: 31 grams of Monsanto Skybond 700, 23 grams of Monsanto Skybond 705, 21 grams of N-methyl-2-pyrrolidone, l.~ grams of 95% pure, 0.3-1.5 ~l amorphous boron powder from Atomergic Chemetals, and 10 grams of ~o. 907 stainless-medium-thermal carbon black from Cancarb Limited. After milling was completed, 9.7 grams of Emerson and Cumings FTD 202~insoluble glass microspheres were added. The microspheres were classified by pressuring to 14,000 k Pa under water and retaining the floating fraction. The cylinder head was preheated to 70C for spraying. The sprayed head was cured in a ~ f~

~z~

01 -15~

programmed oven which heated from 9noc to 155~C in 12 minutes, from 155C to 200C in 2 hours, and from 200C to 05 370C in 13 hours. The engine with the coated head ran for 20 hours at 2 inches of manifold vacuum with alkalate fuel. At the end of the test, the coating was generally intact except for a small area of high gas velocity between the cylinder and valves; elsewhere occasionally exposed microspheres showed evidence of thermal collapse on their exposed side. The average coating thickness at the start of test was 84.8 ~m and 77.2 ~Im at the end of the test.
Example 6 This example used the same engine and run concii~
tions as Example 5. Tlle matrix to holcl the microspheres was prepared by ball milling: 21.3 grams ~lonsanto Skyboncl 705, 31.2 grams Monsanto Skybond 700, 10.9 No. 907 stainless-medium carbon black Erom Cancarb Limited, and 21.3 grams N-methyl-2-pyrrolidone. Sixty grams of this matrix was combined with 13.~ grams of Emerson and Cumings FA-A ceramic microballoons which h.ld heen sifted to remove microspheres larger than a 170 sieve and pressllrecl to 15,000 k Pa under water to relllc)ve weaker spheres. ~he coating was sL)raye~l on and cure~ in l:he Salne m;lllne~` as Example S. The coating re~maineci lntact: clurillg thc 20--hour test with aLkalate uel with some coating deterioration occurring between the valve areas and the sparlc plug.
During this run, average thickness changed from 102 ~m at start of test to ~9 llm at end of test. The used coating was run for an adclitional 20 hours with a deposit-forming mixture of unleaded gasoline with 25~ FC~ heavy component added. The coating kept the head substantially free of deposit. About of half the surface retained the white appearance due to zinc in the lube oil. All but a small area above the intake valve of the rest of the head had a tan appearance due to light deposits. (The microspheres were still clearly visible beneath it.) Normally, this fuel gives heavy black deposits over much of the head.

01 -l6-Example 7 Two cylinder heads of a four-cylinder, overhead oS cam, 2.3 liter, procluction engine were spray-coated with a mixture prepared by blending 32~4 grams of Monsanto Skybond 700, 17.5 grams of Monsanto Skybond 705, 9.5 grams of Cancarb No~ 907 ~a product of Cancarb Limited, thermal earbon blaek, and 22.2 grams of N-methyl-2-pyrrolidone, NMP, in a ball mill; then adding 70.4 grams of this blend to 8.2 grams of ~nerson and Cumings FTF 15 glass micro-balloons whieh were pressure seleeted in water at 21,000 k Pa. The cylinder head was prelleated to 100C for spray-ing. The head was then eured by the cycle of E~ample 5.
The engine operated for 230 hours on a mixed hi~hway-suburban driving cycle with a 29 mph avera~e spec-d. ~t the end of this test, the two eoated cylinders hacl arl average ORI of 3.4 compared with an average ORI of 4.6 for the uneoated eylinders; this is a l.2 ORI reduction for the coated eylinders.
Two specimells of the coatin~ in this example had thermal conductivities of 0.171 and 0.11~ J/mKsec, densities of 943 Kg/M3 and estimated heat capaelties o~
1130 J/Kc~K. Tl-us, their ~ values wer~ 42~ ancl 35G
J/m2Ksec~2, respectively. The ~specimen thicknesses were ~0xl0~~m and 50~10~6m, respectively, wllicll 9iVC ~/d va:lue~s of ~1275 alld 2360 J/m2Ksec. rrhe product oE ~he thermal penetration and thermal conductance is 1.~3x106 J2/m4K2sec3/2 and 8.4x105 J2/m4~K2sec3/2, respectively.
For comparison, a combustion chamber from unleaded gasoline deposit had a thermal conductivity of 0.25, a density of 1520 kg/m3 and a heat eapacity of 1670 which give a ~ C of 797 J/m2oKsec~2. This particular deposit was 35x10 6m thick (although deposits of above 35 100x10 6m are more common) so that its K/d was 7143 J/m2Ksec 4~ ~ 7~ n

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In an internal combustion engine comprising a combus-tion chamber having a surface exposed to combustion, the improve-ment which comprises at least a portion of said surface, having a combination of a thermal conductance and a thermal penetration which permits the temperature of said surface during the combus-tion process to be in excess of the temperature at which deposits form, said surface storing insufficient heat to substantially raise the temperature of the incoming air-fuel charge during the engine intake stroke and compression stroke, wherein said surface portion has a thermal penetratiotl expressed as of less than about 600 J/m2° Ksec1/2, and a thermal conductance expressed as K/d of at least about 2000 J/m2° Ksec, K representing thermal conductivity, p representing density, C representing heat capacity and d representing thickness, and the product of the thermal penetration times thermal conductance is less than about 3x106 J2/m4° K2sec3/2, said surface portion being a thermally stable closed-cell foam having at least about 40% volume micro-spheric voids in a matrix selected from the group consisting of a polyimide resin, a polyimide resin composite including carbon and/or silica, and a ceramic of the group consisting of the oxides, nitrides, and carbides of Si, Ti, Cr, Ta, Nb, and Zn.

2. The internal combustion engine of claim 1 wherein said foam has a resinous or inorganic matrix.

3. The internal combustion engine of claim 1 wherein said microspheric voids are surrounded by inorganic or organic micro-spheres.

4. The internal combustion engine according to claim 1 wherein said surface portion is fabricated from a thermally-stable resinous foam.

5. The internal combustion engine according to claim 4 wherein said resinous foam is a syntactic foam.

6. The internal combustion engine according to claim 5 wherein the microspheres are inorganic micropsheres.

7. The internal combustion engine of claim 6 wherein said microspheres are composed of glass, quartz or mixtures thereof.

8. The internal combustion engine according to claim 7 wherein said microspheres are from about 40 to 70 volume percent of the cured foam.

9. The internal combustion engine of claim 5 wherein said resinous foam comprises plastic microspheres in a resinous matrix.

10. The internal combustion engine of claim 9 wherein said plastic microspheres are composed of a phenol-formaldehyde plastic.

11. The internal combustion engine of claim 10 wherein said plastic microspheres are of about 0.01 mm to 0.1 mm average diameter and are present in said foam in an amount from about 50 to about 70 volume percent.

12. The internal combustion engine of claim 11 wherein said resinous matrix comprises a polyimide resin.

13. The internal combustion engine of claim 12 wherein said resinous matrix is a composite of polyimide resin and carbon.

14. The internal combustion engine of claim 13 wherein said resinous matrix is a composite of polyimide resin, carbon, silica,, and Al.

15. The internal combustion engine according to claim 14 wherein said foam comprises 30 to 85 weight percent of polyimide resin, about 70 to 15 weight percent of carbon black, and about 50 to 80 volume percent based on the cured foam of hollow microspheres.

16. The internal combustion engine according to claim 1 wherein said foam is a ceramic insulator foam.

17. The internal combustion engine according to claim 16 wherein said ceramic insulator foam is selected from the group consisting of the oxides, nitrides, and carbides of Si, Ti, Cr, Ta, Nb, and Zn, and further includes hollow microspheres.

18. The internal combustion engine according to claim 1 or 4 wherein said surface is a coating applied to the interior surface of said internal combustion chamber.

19~ A method for inhibitillcJ octane re~uirement increase in an internal combustion engine having a coMbustion chamber surface exposed to combustion gases comprising:
fabricating at least a portion of said surface exposed to said combustion gases from a material having a combination of -thermal conductance and thermal penetration, permits the temperature of said surface during the combustion process to be in excess of the temperature which deposits form, said surface storing insufficient heat to substantially raise the temperature of the incoming air-fuel charge during the engine intake and compression strokes, wherein said surface portion has a thermal penetration expressed as of less than about 600 J/M2°
Ksecl/2, and a thermal conductance expressed as K/d of at least about 2000 J/m2° Ksec, K representing thermal conductivity, p representing density, C representing heat capacity and d repre-senting thickness, and the product of the thermal penetration times thermal conductance is less than about 3X106 J2/m4°
K2sec3/2, said surface portion being a thermally stable closed-cell foam having at least about 40% volume micropsheric voids in a matrix selected from the group consisting of a polyimide resin, a polyimide resin composite including carbon and/or silica, and a ceramic of the group consisting of the oxides, nitrides, and carbides of Si, Ti, Cr, Ta, Nb, and Zn.

20. The method according to claim 19 wherein the material is thermally-stable resinous foam.

21. The method according to claim 20 wherein the foam is a syn-tactic foam.

22. The method according to claim 19 wherein said material is a ceramic material having uniform surface and incorporating a modifier which induces a void volume in said ceramic material.

23. The method according to claim 19 wherein said Sirius is fabricated by spray-coating said material on said combustion chamber.

24. The method according to claim 22 wherein said surface is fabricated by hot pressing an inorganic oxide, nitride, or carbide material selected from the group consisting of Si, Ti, Cr, Ta, Nb, and Zn, and hollow microspheres.

25. The internal combustion engine according to claim 1, including a piston having a surface exposed to combustion, the improvement comprising at least a portion of said surface coated with a thermally stable, resinous foam.

26. The internal combustion engine according to claim 1, including a valve, at least a portion of said valve being coated with a thermally stable, resinous foam.

27. The internal combustion engine according to claim 1, including an intake manifold having a surface exposed to intake gases, there being a coating on at least a portion of said surface, said coating consisting of a thermally stable, resinous foam of about 0.02 to about 1 mm thickness.