DE102014219819A1 - Method for the thermal insulation of a combustion chamber and / or an exhaust system of an internal combustion engine - Google Patents

Abstract

In a method for thermal insulation of a combustion chamber (22) and / or an exhaust system of an internal combustion engine, a surface layer (46) is formed on at least one surface bounding the combustion chamber (20) and / or the exhaust system by means of anodic oxidation with spark discharge.

Description

The invention relates to a method for the thermal insulation of a combustion chamber and / or an exhaust system of an internal combustion engine.

In the operation of an internal combustion engine, a large part of the heat energy generated during the combustion of fuel and oxygen is released as heat loss to the environment. The ongoing efforts to increase the efficiency of internal combustion engines, therefore, also put in the reduction of this heat loss.

Heat loss arises in particular in a discharge of the still-warm exhaust gas to the environment and as a result of a transition of heat energy from the combustion chambers and the exhaust system to the combustion chambers and the exhaust line forming components of the engine and of these directly or indirectly to the ambient air.

In modern internal combustion engines in which the combustion chambers limiting components (cylinder crankcase, cylinder head and piston) and at least some components of the exhaust line for weight reasons and to achieve the least possible moving masses (in particular the piston) are often made of light metal and aluminum in particular, is formed The problem of heat transfer in a special degree, because aluminum, in addition to its relatively low density, in particular also characterized by a relatively high thermal conductivity.

Attempts have been made to minimize heat losses from the combustion chambers of internal combustion engines through the use of pistons made wholly or partly of a ceramic material. However, these pistons have not yet been used for mass production of internal combustion engines due to low durability and high costs.

Of internal combustion engines, which are provided for the drive of commercial vehicles, the use of steel pistons is also known, since with steel better fatigue strength can be achieved. In addition, steel also has a lower thermal conductivity than aluminum and therefore acts better thermally insulating. The higher density of steel compared to aluminum and the associated higher masses of the pistons is less critical in internal combustion engines for commercial vehicles because of the relatively low maximum speeds provided for them.

From the EP 2 420 658 A1 Furthermore, it is known to coat the combustion chamber defining surfaces of an internal combustion engine including the corresponding surfaces of the piston by means of an electrolytic process with an anodic oxide layer having a thickness between 20 microns and 500 microns and a porosity of at least 20%. In this case, the components having the surfaces to be coated, each coated as an anode in an electrolyte bath by applying a relatively low DC voltage between 10 V and 190 V. By means of the anodic oxide layer, the thermal efficiency of the internal combustion engine is to be increased, which is attributed to the low conductivity of this oxide layer. The from the EP 2 420 658 A1 Known coating has been found to be detrimental to the durability and the surface texture, as in this increases with increasing layer thickness, the roughness.

From the US 2003/0150419 A1 Furthermore, a piston for an internal combustion engine is known, in which the provided in the shell of the piston and provided for receiving piston rings grooves and small, on both sides of the grooves adjacent portions of the shell are provided with ceramic wear protection layers. The wear protection layers do not extend to the top of the piston bounding the combustion chamber. To form the wear protection layers, a process called "plasma electrolytic oxidation" (PEO) is used. This is a process for anodic oxidation, in which a specially modulated AC voltage is used, resulting temporarily and locally limited to a spark discharge due to plasma discharge. The PEO process is therefore also referred to as "anodic oxidation under spark discharge" (ANOF). The resulting from the spark discharge local melting of the surface to be coated should lead to a particularly wear-resistant coating.

Based on this prior art, the present invention seeks to provide a way to reduce heat transfer from the combustion chambers and the exhaust gas leading channels on this forming components of an internal combustion engine, without adversely affecting the durability and operation of the internal combustion engine effect.

This object is achieved by a method according to claim 1. Advantageous embodiments of the method according to the invention are subject matter of the dependent claims and will become apparent from the following description of the invention.

A method for thermal insulation of a combustion chamber and / or exhaust system of an internal combustion engine, wherein at least one of Combustion chamber and / or the exhaust gas guide limiting, preferably made of a light metal and in particular aluminum, magnesium and / or titanium surface is applied a surface layer by means of anodic oxidation, according to the invention is characterized in that the anodization takes place under spark discharge, ie as ANOF process ,

An ANOF process is a combined process from the fields of plasma technology and electrochemistry, by which surfaces of components which are formed of so-called valve metals can be provided with a surface layer of an oxide ceramic. In particular, native barrier layer formers such as aluminum, magnesium or titanium come into the selection as valve metals. The generation of the surface layer can be carried out in particular in aqueous electrolytes. The component to be oxidized is poled anodically and immersed in the electrolyte together with a counter electrode (cathode). The component initially forms a purely chemically induced passive layer. The growth of this passive layer can be achieved by applying a potential between the anodically poled component and the cathode. In this case, the oxide layer of the component to be coated will penetrate locally, wherein plasma-chemical solid-state reactions, the spark discharges, are triggered. This process does not take place nationwide but at those points where the thickness of the oxide layer and thus the local electrical resistance is lowest. Since the plasma reactions thus always take place at those locations of the passive layer which locally have the lowest layer thickness, and there ensure a layer thickness growth, the surface is coated with a very uniform surface layer. In order to permanently break the increasing dielectric property of the growing oxide layer with a breakdown voltage, the applied electric potential is increased so long that the desired layer thickness of the surface layer is reached.

The inventively provided application of anodic oxidation with spark discharge, a surface layer can be generated, which is characterized both by a relatively low thermal conductivity and by a relatively low heat capacity. Both properties lead to a good thermal insulation of the combustion chamber, which can have an advantageous effect on the efficiency of the internal combustion engine. The surface layer formed according to the invention can also be characterized by high wear resistance, good thermal resistance and good adhesion to the coated surface. This can result in a good durability of the surface layer in an operation of the internal combustion engine. Furthermore, the surface layer is a corrosion protection layer for the coated surface.

The improved by the inventive method thermal insulation of a combustion chamber can in particular lead to an increase in efficiency for the internal combustion engine. The improved thermal insulation of an exhaust gas guide can in particular lead to an increase in efficiency for an exhaust gas aftertreatment device integrated in an exhaust line (comprising the exhaust system) of the internal combustion engine.

In addition to components made of light metal, the combustion chamber or the exhaust gas guide limiting components made of other materials, in particular metallic materials such as steel or cast iron, according to the invention can be coated.

In a preferred embodiment of the method according to the invention can be provided that the surface layer is applied using an AC voltage. As a result, the spark discharge characteristic of the method can be advantageously controlled in a known manner. However, the method according to the invention can also be implemented using a DC voltage.

In a particularly preferred embodiment of the method according to the invention may also be provided that in the surface layer particles are provided from a deviating from a base or matrix material of the surface layer material, compared to the base or matrix material of the surface layer, a relatively high or low thermal conductivity exhibit. It can be particularly preferably provided that both those particles are provided which have a relatively high thermal conductivity compared to the base or matrix material of the surface layer, as well as those which have a relatively low thermal conductivity.

This aspect of the invention is based, on the one hand, on the finding that the surface layer produced in the context of the method according to the invention represents an advantageous compromise with regard in particular to thermal insulation and durability, but alternative materials are present which are characterized by an even lower thermal conductivity and thus a lower thermal conductivity further distinguished improved thermal insulation. However, these can not be used for complete formation of a surface layer for various reasons. By introducing particles from one or more of these alternatives Materials in the surface layer produced according to the invention can be further lowered their average thermal conductivity and thus the thermal insulating properties can be further improved, without this having a relevant effect on the further advantageous properties of the surface layer according to the invention, ie in particular a good durability and a low surface roughness, negative , Therefore, it can be provided particularly advantageously that particles having a relatively low thermal conductivity are provided in the entire surface layer (based on the area and optionally also the layer thickness).

As material for the particles with relatively low thermal conductivity, for example, Y-stabilized zirconia (Zr (Y) O ₂ ), alumina (Al ₂ O ₃ ), spinel (Al ₂ O ₃ / MgO), mullite (Al ₂ O ₃ / SiO ₂ ), zirconium corundum (Al ₂ O ₃ / ZrO ₂ ), titanium oxide (TiO ₂ ) or silicon oxide (SiO ₂ ) and mixed ceramics with essential constituents of said oxides into consideration.

Even if the thermal conductivity of the introduced particles in their pure bulk state is not lower than that of the matrix, the thermal conductivity of the composite material of the surface layer formed from both can nevertheless be lower overall, since the introduced particles act as impurities for the propagation of the crystal oscillations (phonons). In this respect, the concretization "with relatively low thermal conductivity" according to the invention is not limited exclusively to an actual material property of the particles, but should also include a heat conductivity reducing effect within the matrix.

The particles with a relatively high thermal conductivity, on the other hand, can advantageously be used to avoid or reduce localized peaks in the wall temperature of the surface provided with the surface layer, by a relatively high local transition of heat energy from the combustion chamber or the exhaust gas passage through these particles as well as possible larger area of the surface layer is distributed. As a result, the formation of locally high wall temperatures, which may have a negative effect on the ignition delay (i.e., the time period between the injection of fuel into the combustion chamber and the ignition of the fuel), can be avoided. This may be sufficient if the particles with relatively high thermal conductivity in only one or more sections, but not in the entire surface layer (based on the area and preferably also the layer thickness) are provided. Such a localized provision of particles with relatively high thermal conductivity does not therefore have to be associated with a relevant deterioration of the mean thermal conductivity of the entire surface layer.

A relatively large ignition delay achieved by avoiding locally high wall temperatures is particularly important for self-igniting internal combustion engines, i. diesel engines, in particular, so that the method according to the invention can be used particularly advantageously in the improvement of such a self-igniting internal combustion engine. However, there may also be advantages in the application of the method for improving spark-ignited internal combustion engines, in particular gasoline engines.

As a material for the particles having a relatively high thermal conductivity, for example, copper, iron, beryllium, aluminum, copper, silver, silicon, molybdenum, tungsten, carbon, beryllium, beryllium nitrite, silicon nitrite and / or silicon carbide and mixtures and / or alloys thereof come into consideration.

If both relatively low thermal conductivity particles and relatively high thermal conductivity particles are to be provided, their distribution in the surface layer should be such that the mean thermal conductivity of the surface layer locally increased by the relatively high thermal conductivity particles does not result in a significantly higher heat transfer the arranged below the surface layer region of the coated, the combustion chamber and / or the exhaust gas guide limiting component leads. This can advantageously be achieved in that the particles with relatively high thermal conductivity exclusively in a first, adjacent to the combustion chamber and / or the exhaust gas guide sub-layer of the surface layer and the particles with relatively low thermal conductivity in a second, from the combustion chamber and / or the exhaust system provided by the first sub-layer separated sub-layer. The particles with a relatively high thermal conductivity can then ensure the most uniform possible distribution of the heat energy transferred into the surface layer within the first sub-layer, while the second sub-layer with the particles with relatively low thermal conductivity is particularly thermally insulated and consequently a heat transfer from the first sub-layer reduced lying below the surface layer region of the component.

Anodic oxidation under spark discharge makes it possible to arrange particles in the surface layer in a relatively simple manner. This applies in particular to an application of anodic oxidation with spark discharge by means of an alternating voltage, in which either the positive or negative voltage phases can be used alternately to accumulate the particles contained in the electrolyte at the growing surface layer, while the corresponding other voltage phases are used for the growing formation of the surface layer.

A particular advantage of the use according to the invention of anodic oxidation with spark discharge for the formation of a surface layer may lie in the fact that, by immersing the component to be coated in the electrolyte, it is also possible to easily coat areas of the components which are difficult to access. This may be a significant advantage over other coating methods, such as thermal spraying. Thus, it can also be provided, in particular or only such a combustion chamber and / or an exhaust system of an internal combustion engine limiting components having poorly accessible surfaces to be coated to provide by means of anodic oxidation under spark discharge with a corresponding surface layer, while other components or surfaces by alternative coating measures be provided with a (also) thermally insulating surface layer.

The indefinite articles ("a", "an", "an" and "an"), in particular in the patent claims and in the description which generally explains the claims, are to be understood as such and not as number words. Corresponding to this concretized components are thus to be understood that they are present at least once and may be present more than once.

The present invention will be explained in more detail with reference to an embodiment shown in the drawings. In the drawings shows:

1 : an internal combustion engine in a schematic representation;

2 a cross section through an internal combustion engine of the internal combustion engine; and

3 : an area of 2 in an enlarged view.

The in the 1 shown internal combustion engine includes an example operating on the diesel principle internal combustion engine 10 , which is formed, for example, as a four-cylinder reciprocating internal combustion engine. The internal combustion engine 10 is about a fresh gas train 12 supplied with fresh gas (ambient air). For this purpose, the fresh gas after sucking from the environment by means of a compressor 14 compacted. The compressed fresh gas is then passed through a charge air cooler 16 guided, in which the heated as a result of compression fresh gas until reaching the desired temperature for entry into the engine 10 is cooled. About a suction pipe 18 the fresh gas enters combustion chambers 20 of the internal combustion engine 10 in which this or the oxygen contained therein in a known manner with directly into the combustion chambers 20 injected fuel is burned.

The resulting during the combustion of the fuel-fresh gas mixture exhaust gas is via an exhaust line 22 the internal combustion engine discharged. The exhaust system 22 includes an exhaust manifold 24 in which that from the individual combustion chambers 20 outflowing exhaust gas is merged, and one of them downstream turbine arranged 26 , The turbine 26 forms together with the compressor 14 an exhaust gas turbocharger and is by means of an adjustable bypass 28 (Wastegate) executed passable. The bypass 28 serves to, in certain leading to a large exhaust gas mass flow operating conditions of the internal combustion engine 10 , a portion of the exhaust mass flow at the turbine 26 To pass, so the boost pressure in the fresh gas train 12 to limit.

In the exhaust system 22 is downstream of the turbine 26 Furthermore integrated an exhaust aftertreatment device. The exhaust aftertreatment devices can, for example, an oxidation catalyst 30 and a particle filter 32 include.

The 2 shows a cross section through the internal combustion engine 10 in the area of a cylinder. The internal combustion engine 10 includes a cylinder housing 34 in that the individual cylinders are formed. In each of the cylinders is a piston 36 Movable up and down. Above the cylinder housing 34 closes a cylinder head 38 at. The cylinder housing 34 , the cylinder head 38 and the pistons 36 are made of aluminum alloys. In the cylinder head 38 is at least one inlet channel for each cylinder 40 and at least one outlet channel 42 integrated. The inlet channels 40 are part of the fresh gas train 12 the internal combustion engine and connect the intake manifold 18 fluid-conducting with the respective cylinders. The outlet channels 42 are part of the exhaust system 22 and connect the respective cylinders to the exhaust manifold 24 , About gas exchange valves 44 is controlled in a known manner, introducing the fresh gas into the cylinder and a discharge of the exhaust gas from the cylinders. Here are the gas exchange valves 44 for example, by means of one or more (not shown) actuated camshaft.

The combustion chambers formed by the individual cylinders 20 Each of a portion of the inner wall of the associated cylinder, from the top of the associated piston 36 . a section of the underside of the cylinder head 38 as well as from the undersides of the associated gas exchange valves 44 limited.

To the combustion chambers 20 Thermal insulation is especially on the top (of main bodies) of the piston 36 trained surfaces a surface layer 46 applied by means of anodic oxidation with spark discharge. These surface layers 46 consist essentially of alumina (Al ₂ O ₃ ), which under the anodic oxidation under spark discharge on the tops of the pistons 36 formed.

The surface layer 46 , which may have a layer thickness of, for example, about 200 microns, is already characterized in principle due to their training of alumina by a high wear resistance and good thermal resistance, whereby their use to limit the combustion chambers 20 of the internal combustion engine 46 is possible. Furthermore, the surface layer is characterized 46 also by one compared to the aluminum alloy that makes up the pistons 36 are formed, relatively low thermal conductivity and a relatively low heat capacity. This results in the desired thermal insulation of the combustion chambers and consequently a relatively low heat transfer in the combustion chambers 20 located gases on the piston 36 reached.

To transfer heat from the combustion chambers to the body of the piston 36 is to be further reduced, in the existing of alumina as a matrix material surface layer 46 particle 48 from zirconium oxide, for example, which are characterized by an even lower thermal conductivity compared to the aluminum oxide. As is clear from the 3 results are provided, the particles 48 zirconia over the entire surface of the surface layer 46 to provide in a (second) sub-layer extending between the surface of the body of the corresponding piston 36 and another, to the combustion chamber 20 adjacent (first) sub-layer is arranged.

In the first sub-layer of the surface layer 46 are not particles 48 made of zirconia, but local particles 50 of a material, for example copper, which is characterized by a relatively high thermal conductivity compared to the matrix material serving as alumina. It is planned to use the particles 50 of copper in such areas of the first sub-layer of the surface layer 46 provide, in experience, relatively high local wall temperatures may result in the operation of such an internal combustion engine. The particles 50 made of copper serve to reduce such locally high wall temperatures by forwarding the increased introduction of heat energy at these points as well as possible to the entire second partial layer. In the 3 is shown that the particles 50 made of copper, for example, at the edge transitions of a piston recess 52 as well as in the area of a central elevation of the piston recess 52 can be arranged. The 3 also shows that also the density of the distribution of particles 50 of copper, ie the number of particles per unit volume; in the formation of the surface layer 46 can be controlled by means of anodic oxidation under spark discharge (also for the particles 48 made of zirconium oxide). Thus, it is envisaged in those sections of the first sub-layer in which particles 50 are made of copper, each having a higher density of particles 50 in a central area and a decreasing to the edge of the respective section density of particles 50 provided.

The subdivision of the surface layer 46 in the first sub-layer and the second sub-layer results only by the different embedding of the different particles 48 . 50 and by the different functionalities for the surface layer achieved thereby 46 , A structural parting plane is not formed between the two part planes.

The particles 48 . 50 For example, they can have a size of ≦ 5 μm.

Next to the tops of the pistons 36 can also be single or all others, the combustion chambers 20 of the internal combustion engine 10 delimiting surfaces with a corresponding surface layer 46 be provided to the thermal insulation of the combustion chambers 20 continue to improve. The 2 shows by way of example that both the inner walls of the cylinder (at least in those sections that the combustion chambers 20 limit), the corresponding sections of the bottom of the cylinder head 38 and the bottoms of the gas exchange valves 44 each with a surface layer 46 , which has been formed by anodic oxidation under spark discharge, may be coated.

Also shows the 2 the possibility of serving as exhaust ducts outlet channels 42 of the internal combustion engine 10 with corresponding surface layers 46 to provide. Likewise, other surfaces of the exhaust gas line serving for exhaust gas routing may be used 22 the internal combustion engine, for example walls of an exhaust manifold and / or a turbine of an exhaust gas turbocharger, with corresponding surface layers 46 be provided.

LIST OF REFERENCE NUMBERS

10

internal combustion engine

12

Fresh gas line

14

compressor

16

Intercooler

18

suction tube

20

combustion chamber

22

exhaust gas line

24

exhaust manifold

26

turbine

28

bypass

30

oxidation catalyst

32

particulate Filter

34

cylinder housing

36

piston

38

cylinder head

40

inlet channel

42

exhaust port

44

Gas exchange valve

46

surface layer

48

Particles with relatively low thermal conductivity

50

Particles with relatively high thermal conductivity

52

piston bowl

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• EP 2420658 A1 [0007, 0007]
• US 2003/0150419 Al [0008]

Claims (10)

Method for thermal insulation of a combustion chamber ( 22 ) and / or an exhaust system of an internal combustion engine, wherein on at least one of the combustion chamber ( 20 ) and / or the exhaust gas guide limiting surface a surface layer ( 46 ) is formed by means of anodic oxidation, characterized in that the anodic oxidation takes place under spark discharge. Method according to claim 1, characterized in that the surface layer ( 46 ) is formed using an AC voltage. Method according to claim 1 or 2, characterized in that in the surface layer ( 46 ) Particles ( 48 . 50 ) are deposited, compared to a base material of the surface layer ( 46 ) have a relatively low or high thermal conductivity. Method according to claim 3, characterized in that the particles ( 50 ) with relatively low thermal conductivity in the entire surface layer ( 46 ). Method according to one of claims 3 or 4, characterized in that the particles ( 48 ) with relatively high thermal conductivity only in one or more sections of the surface layer ( 46 ). Method according to one of claims 3 to 5, characterized in that the particles ( 50 ) with relatively high thermal conductivity in a first, to the combustion chamber ( 20 ) and / or the exhaust passage adjacent partial layer of the surface layer ( 46 ) and the particles ( 50 ) with relatively low thermal conductivity in a second, from the combustion chamber ( 20 ) and / or the exhaust system are provided by the first sub-layer separated sub-layer. Method according to one of claims 3 to 6, characterized in that for the particles ( 48 ) is used with relatively low thermal conductivity zirconia. Method according to one of claims 3 to 7, characterized in that for the particles ( 50 ) with relatively high thermal conductivity copper, iron, beryllium, aluminum, silver, silicon, molybdenum, tungsten, carbon, beryllium, beryllium nitrite, silicon nitrite and / or silicon carbide and mixtures and / or alloys thereof is used. Method according to one of the preceding claims, characterized in that the surface layer ( 46 ) is formed on a surface of a light metal. Method according to claim 9, characterized in that the surface layer ( 46 ) is formed on a surface of aluminum, magnesium and / or titanium.

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