AN INTERNAL COMBUSTION ENGINE HAVING LOW OCTANE NUMBER REQUIREMENTS

Abstract: The invention relates to a spark-ignition internal combustion engine having a combustion chamber which, at least in part, presents surface areas (10) which have been treated to reduce the octane number require­ ment of the engine. The surface area to be treated may, for example, be either blasted or etched to produce surface depres­ sions, and is then flame sprayed with copper particles to form heat-reflective surfaces. At least this lat­ ter method results in solely low-temperature heating of the walls of the combustion chamber, which can be advantageous in the case of diesel engines, but not in the case of Otto-cycle engines. The improvement in such engines afforded by the in­ vention is that the surface areas (10) are instead provided with at least one layer (12) which presents to the combustion chamber (1) a defining surface (9) which exhibits an extremely low reflectivity to electromagnetic waves in the wavelength range of about 0·2-7·0 microns, and which also results in diffuse reflection of non-absorbed incident waves (14).

Description: Technical Field The present invention relates to a spark-ignition in­ ternal combustion engine of the kind having a combus­ tion chamber which, at least in part, presents surface areas that are treated to reduce the octane number re­ quirement of the engine, said surfaces preferably cov­ ering a total combustion chamber area equal to at least 10% of the top face of the piston. Background Prior Art. An internal combustion engine of this kind is known, for example for US Patent Specification 4 508 070. According to this Patent Specification, the surfaces of the combustion chamber are given a fine surface structure in the form of recesses comprising a multi­ plicity of grooves or pits, each having a width of up to about 3 microns and a depth of up to about 1 mi­ cron. Surfaces treated in this way are said to im­ prove the octane number requirement of an engine by about 5, i.e. an engine requiring e.g., a 98 octane fuel can be driven on a 93 octane fuel. Tests have shown, however, that for reasons not yet fully under­ stood this improvement is not always achieved. As is well known, subsequent to ignition the flame-­ front propagates in the combustion chamber at the same time as the un-combusted fuel/air mixture is compress­ ed and heated. If the temperature and the pressure of the non-combusted gas mixture reaches critical values before the characteristic flame-front has passed through the whole of the gas mixture, the non-combust­ ed gas mixture will combust spontaneously and result in combustion knock. Apparently the surface structure is able to influence energy conditions in the same combus­ tion chamber in some instances, so as to prevent com­ bustion knock. It is also possible to increase engine efficiency, and therewith reduce fuel consumption, and to reduce heat losses in a similar manner, as recognised in US Patent Specification 3 459 167. This Specification teaches internal combustion engines in which combustion cham­ ber surfaces have been coated with a material which is highly reflective to infrared radiation and which also forms a thermal barrier, so as to keep the walls of the combustion chamber at a relatively low temperature. The coating may have the form of a layer of cuprous oxide, and is applied by first cleaning the surface or surfaces to be treated and then blasting said surface (surfaces) to create irregularities therein. The cuprous oxide layer is then applied, e.g., by flame spraying copper particles to a thickness of at least 0·15 mm. In this way there is provided a coating which is at least 75% reflective to thermal radiation within a wave length of 0·7-10 microns at prevailing combustion chamber temperatures. Summary of the Invention The object of the present invention is to provide an internal combustion engine of the aforesaid kind in which the treated surfaces of the combustion chamber are effective in influencing the combustion process in a manner to reduce the octane number requirement of the engine without resulting in unreasonable thermal losses. To this end there is provided in accordance with the invention a spark-ignition internal combustion engine of the kind in which the engine combustion chamber presents, at least in part, surface areas which have been treated in a manner to reduce the octane number requirement of the engine, and in which the treated surfaces with regard to each combustion chamber pre­ sent a total area which is equal to at least 10% of the piston head area. The engine is characterized in that each treated surface area presents to the combustion chamber at least one layer having extremely low reflective properties to electromagnetic waves in the wave length band of about 0·2-7·0 microns, and which also reflects incident non-absorbed waves in a diffuse manner. Further characteristic features of the invention are set forth in the depending claims. The invention is based on the concept of selectively preventing electomagnetic radiation having a wave length of up to about 7 microns and originating from the process of combustion from being reflected to any appreciable extent from the defining surfaces of the combustion chamber, back into the combustion space and heating and igniting the residual gas mixture before said mixture is combusted. This can be achieved by treating the surface areas in a manner which renders said surfaces highly absorptive to wave lengths lying in the aforesaid range and for diffuse reflection of the non-absorbed incident waves. Thus, the nature of these surfaces is the opposite to that of the sur­ faces according to the last mentioned US Patent Spec­ ification, according to which the surfaces described therein are intended to absorb as little radiation as possible, in order to prevent the surfaces from being heated to high temperatures, resulting in heat losses and therewith poor efficiency and possible ignition of the non-combusted gas mixture before the flame front reaches said mixture. However, this reflected radia­ tion, which is rich in energy tends to enhance com­ bustion knock, and therefore has a pronounced negative effect on any improvement in efficiency that might otherwise be achieved. In accordance with one embodiment of the invention, the treatment of said surfaces involves the applica­ tion of one or more layers on the top of the afore­ mentioned low reflective layer, wherewith an underly­ ing defining surface may be given properties such as to reflect radiation within said wavelength range in a diffuse manner. One of said layers may comprise, for example, a cermet material, i.e. a combination of metal particles and an oxide. In accordance with a preferred embodiment of the in­ vention the treated surfaces are made highly reflec­ tive to wave lengths in the wave length range above about 7 microns. The purpose hereby is to reduce, to the greatest possible extent, the generation of heat as a result of subjecting the surfaces of the com­ bustion chamber to prolonged thermal radiation, by reflecting the thermal radiation containing the least energy back to the interior of the combustion chamber, where it is unable to cause ignition, and to maintain total efficiency at the highest possible level. Brief Description of the Drawings. The invention will now be described in more detail with reference to the accompanying drawings, in which Figure 1 is a part sectional view of a combustion chamber of an internal combustion engine; Figure 2 is a curve illustrating the reflective pro­ perties of a treated surface in the internal combus­ tion engine, said properties being given as a function of the wavelength of the incident electromagnetic radiation; Figures 3 and 4 are sectional views of two different surfaces, selected for illustration purposes, compris­ ing respectively one and two layers formed in accor­ dance with the invention. Description of Preferred Embodiments. Figure 1 illustrates a combustion chamber 1 above a piston 2 located in a cylinder 3 of an internal com­ bustion engine 4. The combustion chamber is defined upwardly by a cylinder block 5, which incorporates valves, such as valve 6, and a spark plug 7. The walls of the combustion chamber are formed essen­ tially by the top face of the piston and the undersur­ face 8 of the cylinder block 5 facing said chamber 1, and only to a minor part by the wall of the cylinder 3 located above the piston 2. Subsequent to introducing an air/fuel mixture into the combustion chamber and compression of the mixture therein, combustion is initiated with the aid of one or more sparks delivered by the spark plug, during continued compression of the mixture. Normally, the process of combustion continues until the flame front has propagated through the whole body of the mixture, in the absence of spontaneous com­ bustion of non-combusted mixture residues. As the mixture is compressed it is heated both as a result of compression and as a result of being irra­ diated with electromagnetic waves emitted from the flame front, and from the hot gases and any particles of soot present therein, and also from the defining surfaces of the combustion chamber, these waves being characteristically reflected towards the chamber walls, therewith creating a serious risk that the walls will become heated and that ignition of non-­ combusted gas mixture will take place, resulting in deleterious combustion knock. In order to prevent the occurence of such events, a surface, e.g. the part-surface 10, located on the upper face of the piston 2 and having a defining sur­ face 9 which faces the chamber 1 is treated in ac­ cordance with the invention in a manner which will result in the absorption of the major part of all energy-rich waves in the wavelength up to about 7 mi­ crons, while energy-lean waves having a wavelength above 7 microns will mostly be reflected, as illus­ trated in Figure 2, and with which incident non-ab­ sorbed waves will be reflected in a diffuse fashion, thereby to reduce the risk of heating the chamber walls and igniting the non-combusted mixture. This treatment can be effected by first blasting or etching the piston surface, to create irregularities 11 therein, such that the surface has a degree of roughness which results in diffuse reflection of incident waves of wavelength above 2·5 microns, and then applying to the thus roughened surface a layer of cermet material comprising a metal oxide 12 and metal particles 14 to a thickness of about 0·25 mi­ crons. The amount of metal particles applied in­ creases with increasing layer depths, so that the refractive index n for the electromagnet radiation at a location adjacent the defining surface 9 facing the combustion chamber 1 is close to 1 (one) i.e. the refractive index of air, and at a location ad­ jacent the bottom of the layer is close to the refractive index of metal, n m , as illustrated in Figure 3. The major part of the thermal radiation, here illlustrated in the form of an electromagnetic wave 14, incident on the layer 12, 13, will pass into the oxide layer 12, while a small part 15 of said radiation will be reflected, since the refractive index is close to 1. The penetrating part 16 of this radiation impinges on the metal particles 13, and ultimately on the undulating surface 11, while being repeatedly reflected, wherewith the electro­ magnetic radiation is progressively converted into heat, whereas that part 17 of the radiation which is not absorbed is reflected in a diffuse fashion back to the combustion chamber. The aforesaid selectivity is achieved through suitable choice of dimensions and materials, as will be readily understandable to those skilled in this art. As illustrated in Figure 4, the upper surface of the piston 2 may be provided with several layers of cermet or other materials 12, 13. In order to enhance the absorption of thermal radiation still further, small air-filled cavities 18 may be provided in the close proximity of the defining surface 9, of at least the upper layer 12, 13, said surface facing towards the combustion chamber 1. It will be understood that other parts of the combus­ tion chamber defining surfaces can be treated in ac­ cordance with the aforegoing, for example, selected surfaces on the cylinder block 5, the valves 6, etc.

Claims: 1. A spark-ignition internal combustion engine having a combustion chamber which, at least in part, presents surface areas that are treated to reduce the octane number requirement of the engine, the total area of the surfaces thus treated for each combustion chamber being equal to at least 10% of the area of the top surface of the piston, characterized in that the thus treated surface area (10) comprises at least one layer (12) which presents to the combustion chamber (1) a defining surface (9) that exhibits very low reflectivity to electro magnetic waves in the wave­ length range of about 0·2-7·0 microns, and which said at least one layer (12) is constructed such as to re­ flect incident non-absorbed waves in a diffuse fashion. 2. An engine according to Claim 1, character­ ized in that at least one layer (12) or at least one of said layers has a defining surface (11) which faces away from said combustion chamber and which re­ flects in diffuse fashion substantially all incoming elctromagnetic waves (14) having a wavelength above 2·5 microns. 3. An engine according to Claim 1 or 2, charac­ terized in that the at least one layer (12) or at least one of said layers incorporates cermet material, i.e. an oxide matrix with metal particles (13). 4. An engine according to Claim 3, character­ ized in that the cermet layer (12, 13) has a refrac­ tive index which varies with layer thickness, where­ with that part of the layer located nearest the com­ bustion chamber (1) has a refractive index close to 1, i.e. approximately equal to that of air, whereas that part of the layer distal from the combustion chamber (1) has a higher refractive index, i.e. approximately equal to that of metal. 5. An engine according to any of Claims 1-4, characterized in that the treated surface area (10) is highly reflective to electromagnetic waves in the wavelength range above about 7·0 microns.

Inventors: FORSLUND, LENNART, NIKLASSON, RAOUL, WICEN, JAN
Applicants: OKTAN AKTIEBOLAG
Application year: 1986
Main classification: F02F3
IPC codes: F02B1, F02B77, F02F3, F02B3, F02B51