EP3054136B1 - Combustion engine for a motor vehicle - Google Patents

Description

The invention relates to an internal combustion engine for a motor vehicle with the features according to the preamble of claim 1.

From the DE 10 2009 002 183 A1 an internal combustion engine with a combustion chamber is known. A surface of a component of the combustion chamber that comes into contact with an air-fuel mixture or an exhaust gas flow, or of a component of the internal combustion engine that is close to the combustion chamber and guides the flow of exhaust gas, now has a catalytic coating. The coating is applied in particular to a piston head, a cylinder inner surface and a cylinder head inner surface. The catalytic coating has a nanostructure made up of elongated structural elements, with one end of the structural elements adhering to the coated surface and the second end ending in the combustion chamber or the exhaust gas flow-guiding chamber. Such a coating is preferably carried out using the methods of gas flow sputtering or magnetron sputtering. These sputtering methods are among the methods of physical vapor deposition. The catalytic coating is intended to support the oxidation of carbon deposits from the combustion process. A high conversion rate should be ensured by a high catalytic activity. The layer thickness should be between 0.1 µm and 50 µm.

From the DE 195 24 015 A1 an internal combustion engine with a piston is known. The piston has a piston body, the piston body having a piston crown at the gas combustion zone. The gas combustion zone is enclosed by a combustor surface. A coating, namely a thermally conductive coating of low thermal conductivity, is now applied to the piston crown and to the combustion chamber surface with a thickness suitable for use as a thermal diode. The thermally conductive coating is intended to limit heat transfer to the piston body and combustion chamber. The thermally conductive coating may be thoria, zirconia, titanium alloy, or stainless steel with 22 wt% chromium. The thermally conductive coating has a thermal conductivity in the range of 0 to 70*10 ^-6 metric units and correspondingly 93*10 ^-6 metric units for aluminum pistons. The thickness of the thermally conductive coating should be in the range of 0.5 mm to 1.8 mm. The piston also has a thermally conductive abrasive layer in the area of a peripheral surface of a top land which wears away during initial operation of the internal combustion engine to conform the piston to the cylinder bore wall with as little clearance as possible. The thermal conductivity of the abrasive layer is achieved by conductive particles or flakes of copper or aluminum in a layer matrix. The layer matrix has a mixture of at least two Elements, wherein the two elements are selected from a group consisting of graphite, molybdenum disulfite and boron nitride.

From the WO 02/18770 A1 an internal combustion engine with a piston is known. The piston has a piston head and a piston skirt. The piston crown is connected to the piston skirt by a retaining spring. The retaining spring is intended to allow micro-movements to be compensated for as a result of different thermal expansions of the piston head and the piston skirt. The heat transfer between the piston crown and the piston skirt is impeded by a coating on the piston crown. The coating is applied to at least one seat surface of the piston crown, with which the piston crown rests against the piston skirt. The piston crown can therefore assume significantly higher temperatures than the piston skirt without transferring too much heat to the running surfaces. The coating can also be formed on the side of the piston head facing the combustion chamber. The coating may include titanium or titanium compounds, ceramic, hardcoat, or other materials applied by chemical or physical vapor deposition.

From the DE 198 15 988 C1 an internal combustion engine with a piston-cylinder arrangement is known. A corresponding metal or ceramic cylinder has an inner bore which forms a piston guide surface. The piston guide surface is equipped with a hard coating. This coating is a few tenths of a millimeter or microns thick. Hard materials in particular can be used as a coating, ie carbides, oxides, nitrides and diamond coatings. The coating can consist of polycrystalline diamond or can be in the form of a diamond-like carbon coating (DLC—“diamond-like carbon”). For example, the coating can have a metal-containing hydrocarbon (Me:CH), with metal doping for example titanium, tungsten, boron or their particularly hard carbide phase, which at the same time has a favorable affinity for carbon, being considered. Furthermore, the use of amorphous hydrocarbon (a:CH) or tetragonally coordinated carbon (a:C) is conceivable. The piston has two sliding sections, each in the form of a ring, whose closed cylindrical circumference each defines a sliding section of the piston. The material of the sliding portions is mesophase graphite or ultrafine-grain graphite. The rings can be made from mesophase graphite by primary forming.

From the DE 36 059 18 A1 and EP 0 258 330 A1 a piston-cylinder arrangement is known, the piston being made of pressed graphite in the form of hard-burned carbon or electrographite. A base of the piston is designed to be anti-combustion on the surface on the combustion chamber side, with silicon powder being applied in a thin layer and siliconized in the autoclave, with the powder having a homogeneous connection and the coating layer created in this way represents protection against erosion and also contributes to the strengthening of the structure of the impact surface. The piston is fitted with piston rings. The piston rings can be made of ceramic-coated ferrous iron, ceramics such as zirconium oxide or of tempered carbon fiber material. A running surface of the cylinder is formed from the same material, but one that is hardened on the running surface, or a similar material that has approximately the same thermal expansion behavior as the material of the piston. The wall of the cylinder is correspondingly hardened. Siliconized carbon, silicon carbide, silicon nitrite or Invar steel with a ceramic sprayed coating are mentioned as materials for the running surfaces of the cylinder.

From the U.S. 4,446/399A a cylinder arrangement is known, wherein a cylinder liner is surrounded by a bandage consisting of a fiber composite material, the thermal expansion of which in the radial direction is less than that of the cylinder liner. The piston has two components, namely a cap exposed to the hot combustion gases and a force introduction core which is connected to the cap via rotationally symmetrical parts and is not acted upon by the hot combustion gases. The cap is made of a ceramic material containing graphite. The areas of the cylinder liner and the cap that are in direct contact with the hot combustion gases and where no relative displacement occurs have a protective layer of MoSi ₂ , Al ₂ O ₃ or TaC. This protective layer is therefore formed on the combustion chamber side and is not opposite the cylinder running surface. The cap consists of a ceramic material, for example silicon nitrite. An intermediate layer is arranged between the cap and the force introduction core, which is used for thermal insulation and pressure transmission between the core and the cap. This intermediate layer consists of aluminum titanate or cordierite. This intermediate layer is on the inside of the cap and does not face the cylinder running surface.

From the generic DE 10 2008 011 921 A1 it is known to apply a DLC coating (DLC - "diamond like carbon") to thermally and tribologically highly stressed components of an internal combustion engine such as bolts, piston rings, connecting rod bearings, piston running surfaces and the like, so that friction, wear and adhesion tendency are reduced. A first DLC coating is applied to the component by means of plasma-enhanced chemical vapor deposition, with this first DLC coating taking on the function of an adhesion promoter layer. A further DLC coating is then applied, which compared to the first DLC coating has reduced rigidity, hardness and lower internal stress. A DLC coating, namely a coating made of diamond-like carbon, should be understood to mean all the layers listed in Group 2 of VDI 2840. DLC coatings consist essentially of carbon bound in sp2 and/or sp3 bonds, with the bond character essentially determining the properties of the layer. An amorphous, hydrogen-containing, metal-free DLC coating in which the two types of bonds are present in a ratio of 60 to 80 to 20 to 40 is specified as being particularly advantageous.

The generic internal combustion engine is not yet optimally designed.

Increased efficiency and performance of internal combustion engines can lead to higher thermal loads. For reasons of temperature protection, components close to the combustion chamber, in particular pistons of the internal combustion engine, are cooled. Injection cooling is known as an active cooling option for the pistons. Furthermore, pistons with cooling channels are known. The cooling ducts and the spray nozzles can result in design restrictions and the expenditure of energy for circulating the cooling medium oil is disadvantageous. Oil coking in cooling channels must be avoided.

The object of the invention is to avoid excessively high component temperatures during operation of the internal combustion engine. In particular, the object of the invention is to provide cooling of a cylinder during operation of an internal combustion engine.

This object on which the invention is based is now achieved by an internal combustion engine, in particular of a motor vehicle, having the features of the characterizing part of claim 1 . Advantageous developments of the invention are characterized in the dependent claims.

An internal combustion engine according to the invention, in particular for a motor vehicle, comprises a piston, which has a coating with an emissivity of ε > 0.8 for thermal radiation, and a cylinder running surface opposite the coating of the piston, which has a coating with an emissivity of ε > 0 .8 which is substantially equal to the emissivity of the coating of the bulb.

The emissivity is in particular the total emissivity, in particular the directional total emissivity, preferably the total emissivity in the direction of a surface normal. The piston and the cylinder running surface belong to a cylinder. The internal combustion engine is preferably a reciprocating piston engine.

The fact that the emissivity of the coating of the piston is essentially the same as the emissivity of the coating of the cylinder surface means in particular that the emissivities differ only slightly from one another, for example that the difference between the two emissivities is only 0.15, preferably only 0.1, particularly preferably is only 0.05. The emissivities are particularly preferably the same. The invention has that Advantage that a radiation cooling is possible. The emissivity ε of the coating of the piston and/or the emissivity ε of the coating of the cylinder running surface is preferably more than 0.9. This enables a higher power density due to lower component temperatures, in particular without additional weight or without additional equipment, for example without additional active cooling measures.

The coating can be used for pistons without cooling channels but also for pistons with cooling channels. The internal combustion engine can have injection cooling for cooling the piston or can be designed without injection cooling for cooling the piston. A flow of heat is supplied to the piston via the hot combustion gases. Different heat flows are emitted from the piston via piston rings, through contact with the housing air and via a piston skirt. If injection cooling is provided and/or the pistons are equipped with cooling ducts, a large part of the heat is dissipated via the corresponding oil. By choosing a coating with a correspondingly high emissivity ε, the contribution of radiation cooling to the removal of the heat introduced via the combustion gases can be significantly increased.

The coating is formed or arranged in particular on the piston peripheral surface. The coating can be formed or arranged in particular on the top land, on a ring area, on the piston skirt or on the piston skirt and/or on the underside of the piston. The coating is preferably provided both on the outer peripheral surface of the piston skirt and on an inside of the piston skirt. At the top land and at the ring land part, there is a large effective effect due to the comparatively high temperatures, which is why this configuration is advantageous. This improves the heat dissipation in the direction of the cylinder by radiation cooling.

The coating has materials whose emissivity ε for thermal radiation is more than 0.8. In particular, the emissivity ε for thermal radiation of the coating is more than 0.9. Heat transport by radiation is proportional to the surface area exposed to the environment A and to the fourth power of the surface temperature T. The heat transport is limited by the emissivity ε. The emissivity ε is 1.0 for a black body and is at its maximum. For anodized aluminum, the emissivity ε is approximately 0.85. For untreated aluminum when new, the emissivity is approximately 0.05. For aged, untreated aluminum, the emissivity is approximately 0.2.

The transmitted radiant power P between two parallel plates with the emissivities ε ₁ and ε ₂ is P = Aσ(T ₁ ⁴ -T ₂ ⁴ )/(1/ε ₁ +1/ε ₂ -1) = Aα _str (T ₁ -T2 ₎ , where the heat transfer coefficient is α _str = σ(T ₁ +T ₂ )(T ₁ ² +T ₂ ² )/(1/ε ₁ +1/ε ₂ -1). The Stefan-Boltzmann constant is σ. This relationship also applies approximately to the cylinder running surface and the opposite circumferential surface of the piston in the area of the top land, the ring area and the piston skirt. The formula given gives the following example values for the heat transfer coefficient α _str [Wm ^-2 K ^-1 ] if the temperature T ₁ of the piston is 500 Kelvin, for example, and the temperature T ₂ of the cylinder surface or the corresponding cylinder liner is 400 Kelvin.

Different heat transfer coefficients α _str depending on the degrees of emission ε ₁ and ε ₂ can now be determined with the formula mentioned. The heat transfer coefficient α _str is 2,092 Wm ^-2 K ^-1 when the emissivities are ε ₁ = 1 and ε ₂ = 1. The heat transfer coefficient α _str is 1750 Wm ^-2 K ^-1 when the emissivities are ε ₁ = 0.9 and ε ₂ = 0.92. The heat transfer coefficient α _str is 1400 Wm ^-2 K ^-1 when the emissivities are ε ₁ = 0.8 and ε ₂ = 0.8. The heat transfer coefficient α _str is 350 Wm ^-2 K ^-1 when the emissivities are ε ₁ = 0.2 and ε ₂ = 0.5. The heat transfer coefficient α _str is 100 Wm ^-2 K ^-1 if the emissivities are ε ₁ = 0.05 and ε ₂ = 0.5.

From this it can be seen that it is particularly advantageous if both the coatings of the piston and the coating of the corresponding cylinder running surface have an emissivity of ε>0.8, preferably ε>0.9. As a result, the radiant heat transfer can be increased by a factor of approximately four to twenty.

For example, it is provided in specific embodiments that the coating is produced by an anodizing process. The emissivity ε for anodized aluminum can be around 0.85. The coating can be produced in particular by black anodizing. The coating can be formed, for example, as an oxidic protective layer, in particular as a colored, oxidic protective layer. Furthermore, black anodizing is an inexpensive, available manufacturing process. By anodizing the aluminum surface, the radiant heat transfer can be increased by a factor of four to twenty compared to a non-anodized aluminum surface.

The layer thickness should be thick enough to increase the emissivity ε accordingly and thin enough to adhere well even under thermal stress. The coatings therefore preferably have a layer thickness of 10 to 50 μm.

For further optimization, other surfaces of the piston or other cylinder surfaces can be or will be coated with materials to improve the emissivity ε. In particular, a cylinder running surface has a coating, the coating being in the form of a DLC carbon layer or a silicon carbide coating. Then the opposing cylinder running surface and outer peripheral surface of the piston have a coating with an emissivity of ε > 0.8.

The improvement in radiation cooling proposed here reduces the power requirement for the oil pump. A further increase in the specific output of the internal combustion engine is possible by combining injection cooling and/or duct cooling with radiation cooling.

The disadvantages mentioned at the outset are therefore avoided and corresponding advantages are achieved.

There are now a large number of possibilities for designing and developing the internal combustion engine according to the invention in an advantageous manner. For this purpose, reference is made to the claims subordinate to claim 1. A preferred embodiment of the invention will now be explained in more detail below with reference to the drawing in the associated description.

Fig. 1

in einer schematischen Schnittdarstellung eine Ausführungsform eines Kolbens und eines entsprechenden Zylinders einer Verbrennungskraftmaschine.

In the drawing shows:

1

in a schematic sectional view, an embodiment of a piston and a corresponding cylinder of an internal combustion engine.

In 1 shows a detail of an internal combustion engine 1, in particular a motor vehicle, namely a piston 2, a cylinder liner 3 and a cylinder head 4. The internal combustion engine 1 has a combustion chamber 5 which is delimited by the piston 2 on the one hand and by the cylinder head 4 on the other hand. Furthermore, the cylinder liner 3 delimits the combustion chamber 5. The piston 2 is designed as a reciprocating piston.

The piston 2 has a piston head 6 , the piston head 6 facing the combustion chamber 5 or at least partially delimiting the combustion chamber 5 . The piston head 6 is slightly curved in the direction of the combustion chamber 5 . The piston 2 also has a top land 7 . The top land 7 forms the topmost land and the top land 7 extends to a first groove 8. Spaced from In addition to the first groove 8, two further grooves 9, 10 are formed on the peripheral surface of the piston 2. In the grooves 8, 9, 10 a piston ring 11, 12, 13 is arranged. The area extending from the top land 7 to the lowest third groove 10 can be referred to as the ring land section 14 . The annular web section 14 is formed by the annular webs remaining between the grooves 9, 10 and 11 (not designated in more detail).

A piston skirt 16 adjoins the annular part 14 , the piston skirt 16 having an inner side 17 and an outer peripheral surface 15 . The piston skirt 16 can also be referred to as a piston skirt. The underside 18 of the piston crown adjoins the inside 17 . The piston crown 6 and the piston skirt 16 delimit a receptacle 19 , a pin eye 20 being formed in the piston skirt 16 in the region of the receptacle 19 . Not shown here is a corresponding gudgeon pin, which in the mounted state extends through the pin eye 20 and connects the piston 2 to a connecting rod (not shown).

The piston 2 now has at least one, in particular several, coatings 21, 22, 23, 24.

The disadvantages mentioned at the outset are now avoided in that the coatings 21, 22, 23, 24 have an emissivity ε of more than 0.8 for thermal radiation. The coating 21, 22, 23, 24 preferably has an emissivity ε of more than 0.9 for thermal radiation. The emissivity ε is also referred to as the emission coefficient or emissivity.

This has the advantage that radiation cooling of the piston 2 is made possible. The piston 2 can preferably also be cooled by injection cooling. Furthermore, the piston 2 preferably has at least one cooling channel (not designated in more detail) in order to further increase the cooling capacity. The radiation cooling ensures a higher power density due to lower component temperatures without additional weight, without additional equipment and without further active cooling measures.

The coating 21 is formed in the area of the ring land part 14 and in the area of the outer peripheral surface of the top land 7 . This is advantageous because high temperatures can occur here.

The coating 22 is formed in the area of the outer peripheral surface 15 of the piston skirt or the piston skirt 16 . The coating 24 is formed on the inside 17 of the piston skirt 16 . The temperatures in the area of the piston skirt 16 or the piston skirt are lower than in the area of the top land 7 or the piston crown 6. but through the coatings 22, 24 the contribution to the radiation cooling can also be increased.

The coating 23 is formed on the underside 18 of the piston crown. The radiation cooling is improved by the coating 23 since high temperatures can occur in particular on the piston crown 6 .

The coatings 21, 22, 23, 24 can be produced in different ways:
In one configuration, the coatings 21, 22, 23, 24 could be in the form of a colored, oxidic protective layer. The oxidic protective layer can in particular be colored black. Such an oxidic protective layer can be produced by an anodizing process. In particular, the coatings 21, 22, 23, 24 can be produced by black anodizing the piston 2 in the corresponding areas, namely the ring land area 14, the piston skirt 15, the inside 17 and the underside 18 of the piston crown.

The piston 2 is made of aluminum or an aluminum alloy. In the untreated state, the aluminum surface only has a low emissivity. Due to the high temperature prevailing in the areas mentioned, black anodizing has a great effect. Black anodizing is an inexpensive process available.

It is conceivable to coat other surfaces of the bulb 2 with materials to improve the emissivity in order to optimize the radiation cooling.

The coatings 21 and in particular 22 of the piston skirt 16 or the piston skirt 15 preferably have good tribological properties due to the friction prevailing here. These coatings 21 and/or 22 are preferably in the form of a carbon layer, in particular a DLC carbon layer, or a silicon carbide coating, ceramic coating or oxide coating.

The coatings 21, 22, 23, 24 and 26 are preferably made sufficiently thin. The coatings can have a thickness of 10 to 50 μm. The layer thickness is thus thick enough to achieve a high emissivity ε and thin enough to adhere well even under thermal stress.

The radiation cooling proposed here makes it possible to reduce the power requirement of a corresponding oil pump of the internal combustion engine. It's another Increase in specific output possible through the combination of spray cooling or channel cooling with the proposed radiation cooling.

REFERENCE LIST

1

internal combustion engine

2

Pistons

3

cylinder liner

4

cylinder head

5

combustion chamber

6

piston crown

7

fire bridge

8th

groove

9

groove

10

groove

11

piston ring

12

piston ring

13

piston ring

14

ring bridge section

15

outer peripheral surface

16

piston skirt

17

inside

18

piston crown underside

19

admission

20

bolt eye

21

coating

22

coating

23

coating

24

coating

25

cylinder surface

26

coating

Claims (4)

1. Internal combustion engine (1) for a motor vehicle, having a piston (2) which has a coating (21, 22, 23, 24) with an emissivity of ε > 0.8 for thermal radiation, wherein a cylinder liner surface (25), which faces the coating (21, 22, 23, 24) of the piston (2), has a coating (26) which has an emissivity of ε > 0.8 which is substantially equal to the emissivity of the coating (21, 22, 23, 24) of the piston (2), characterized in that the piston (2) is manufactured from aluminium or from an aluminium alloy, wherein the coating (21, 22, 23, 24) is produced by black anodizing, wherein the coating (21, 22) is formed on an outer circumferential surface of the piston (2), specifically on a top land (7), on a ring land section (14) and on an outer circumferential surface (15) of a piston skirt (16).
2. Internal combustion engine according to Claim 1, characterized in that the coating (21, 22, 23, 24) of the piston (2) has an emissivity of ε > 0.9 for thermal radiation, and/or in that the coating (26) of the cylinder liner surface (26) has an emissivity of ε > 0.9 for thermal radiation.
3. Internal combustion engine according to either one of the preceding claims, characterized in that the coating (23, 24) is formed on a piston crown underside (18) and/or on an inner side (17) of the piston skirt (16).
4. Internal combustion engine according to any one of the preceding claims, characterized in that the coating (26) of the cylinder liner surface (25) is formed as a DLC layer, as a silicon carbide coating, as a ceramic coating or as an oxide coating.

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