GB2169362A - Cylinder-head gasket - Google Patents

Abstract

A cylinder-head gasket comprises a base sheet 22 which is coated on both sides with a layer 24 of elastomer material with metallic sealing element 20 which define the combustion chamber apertures being secured directly to the base sheet and the elastomer layers being provided with material displacement accommodation spaces 26 in order to achieve adequate compressibility. <IMAGE>

Description

SPECIFICATION Cylinder-head gasket The invention relates to a cylinder-head gasket with at least one combustion chamber aperture, the gasket comprising a base sheet coated on both sides with layers of soft material and a metallic sealing element in the combustion chamber aperture, with eccommodation spaces for displaced soft material being provided in order to provide adequate compressibility for the soft material coatings.

A cylinder-head gasket of this kind having a comparatively thin base sheet with anchorage points pressed out of the sheet metal to both sides thereof and, by comparison with this, relatively thick, soft material layers of compressivle material, namely synthetic-resin-bonded asbestos fibres, is already known from DE-PS 26 46832. In order to enable a comparatively good adaption of the layers of soft material to the superficial structure of the areas on the engine block and the cylinder head which are to be sealed, the free surfaces of the soft material layers are provided with shallow, dish-like impressions leaving between them a regular grid of interconnecting web areas. When the cylinder-head nuts are tightened down, these relatively easily deformable web regions are flattened out.

Conventional soft material layers consisting of synthetic resin-bonded asbestos fibre not only demand a relatively great outlay in plant to produce the mixed compound of bonding agent and asbestos fibre as well as to apply this compound to the base sheet, but also require subsequent processing stages or after treatment with a view to improving the sealing ability. Moreover, the soft material must be prevented from sticking to the faces of the cylinder head and the engine block so that the cylinder head gasket may be readily stripped. Such after treatment consists in completely or partially impregnating the soft material with impregnating agents and applying a top surface coating.

It is much easier to produce soft material coatings consisting of an elastomer such as rubber.

Such soft coatings also adapt optimally to the surface structure of the areas which are to be sealed, but they raise a problem which heretofore has not been solved in cylinder-head gaskets of the kind specified. With today's high performance engines and with engines for commercial vehicles very high demands are made in respect of the mechanical strength of a cylinder-head gasket for which reason it is desirable to secure the metallic sealing elements surrounding the combustion chamber apertures directly to the base sheet, like the known flanged edge-trim rings which are made from sheet metal.However, if, when the cylinder-head nuts are tightened these edge trim rings cannot be pressed into the material of the soft material layers overlapped thereby, as is the case with conventional cylinder-head gaskets and also with the known cylinder-head gasket according to DE-PS 26 46 832, incompressible soft layers of an elastomer material show insufficient or even totally lacking, height adaptability between the metallic sealing elements surrounding the combustion chamber apertures and the incompressible soft layers so that it may happen, when the cylinder-head nuts are tightened, that the required sealing area compression is obtained only in the region of the sealing elements surrounding the combustion chamber apertures or only in the region of the soft material layers.In other words, if an incompressible elastomer soft material layer is made too thick by merely a few hundredths of a millimetre the soft material coatings will carry the major part of the clamping forces when the cylinder-head nuts are tightened and the distribution of sealing area compression between the metallic sealing elements surrounding the combustion chamber apertures on the one hand and the soft material layers is precisely opposite to operative requirements in as much as the pressures to be sealed by the cylinder-head gasket are very much higher in the combustion chamber region than in the region of the apertures for coolant and lubricant.In other words, the degree of precision required in the production of a cyliner-head gasket when using incompressible soft coatings made from an elastomer material would be far higher than any economically acceptable manufacturing tolerances.

The aim of the present invention therefore resides in providing a cylinder-head gasket which is suitable for application to modern high performance engines and particularly for the combustion engines of commercial vehicles, which can be produced relatively cheaply and affords a definable, functionally correct and safe distribution of sealing compression across the gasket, that is to say a relatively high sealing compression in the region of the combustion chamber apertures and a lesser sealing area compression in the remaining regions of the gasket.

According to the invention a cylinder-head gasket of the kind specified is characterised in that the sealing element is secured directly to the base sheet and is incompressible, that the layers of soft material which project vertically of the sealing plane above the sealing element consist at least substantially of an elastomer, the thickness of the base sheet amounting to from 2 to 8 times the thickness of a soft material layer and the material displacement accommodating clearance volume is approximately from 50 to 80% of the soft material volume.

In order to create such clearances for the accommodation of displaced elastomer the soft layers may be made from a foamed elastomer material or at least on one of their boundary faces be provided with depressions, alternatively the base sheet may be a perforated sheet so that the elastomer material of the soft layers can be displaced into the holes of the base sheet when the cylinder-head nuts are tightened or a combination of the above may be used.

By a gasket in accordance with the invention a cylinder-head gasket is provided comprising a hard and rigid base sheet as demanded, particularly, for the internal combustion engines of commercial vehicles, to which the incompressible metallic sealing element which defines the combustion chamber aperture is secured in a secure and firm manner because it is secured directly thereto.The soft layers of an elastomer material can be very easily produced and secured to the base sheet and the combination of not too thin soft layers and a relatively large displaced material accommodation space volume results, during the tightening of the cylinderhead nuts, in the desired thickness relationship between the sealing element and the soft layers because the latter, due to their dimensions, safeguard the volume displaceability of the elastomer material which is essential for the thickness or height coordination whereas the metallic sealing element which is secured directly to the base sheet cannot "settle" when the cylinder-head nuts are tightened thus ensuring that maximum sealing area compression is achieved around the combustion chamber apertures.By a judicious choice of the displacement accommodation volume it is possible to obtain any desired distribution of sealing area compression between the combustion chamber aperture regions and the remaining regions of the cylinder-head gasket. The sealing element defining and edging the combustion chamber aperture need not be the usual cross-sectionally C-shaped sheet metal ring element; however, if such a ring is used the present invention brings the additional advantage that flange fractures or cracking are avoided because there is no soft material beneath the circumferential flanges of the sealing element.If the cylinder-head gasket according to the invention comprises a perforated base sheet for the purpose of providing the necessary material displacement accommodation volume, this may be an evenly perforated piece of sheet metal so that no expensive stamping operation is needed for partially perforating the sheet. It is essential for the invention solution of the problem in question that appropriate constructional provisions are adopted in order to achieve the result that inherently incompressible elastomers when used for the soft material layers will behave like compressible layers.Finally it should be noted that pure elastomers layers have a high cross sectional density and sealing effect and that it is possible with a cylinder-head gasket according to the invention to produce emergency-running properties since in the event of a potential leak at the highly stressed aperture of the cylinder-head gasket in the sealing elements which are inserted in these regions the medium in respect of which the seal must be maintained can escape only across the soft layer regions surrounding these apertures which however, in a cylinder-head gasket according to the invention, are subject to application of a uniform sealing area compression.

In order to make optimum use of the strength of the base sheet it is recommended to select a base sheet thickness which amounts to approximately 2.5 to 5 times the thickness of a soft material layer. In a particularly advantageous embodiment the thickness of the base sheet is of the order of approximately 1.5 1.5+0.05 mm and the thickness of a soft material layer approxi- mately 0.3+0.1 mm. Furthermore, the sealing element is preferably a set-ring having a sheet metal thickness of about 0.1 mm.

The elastomer may be an acrylonitrile-butadiene rubber, a silicone rubber or a fluorine rubber.

Flat gaskets comprising a metal plate coated on both sides with a layer of an elastomer material, said layers being provided with a regular and even-spaced raster or pattern of moulded depresions are known from GB-PS 969 700. However, these known flat gaskets embody a base sheet which is much too thin and rubber layers which are much too thick in relation therewith.

Moreover, these known flat gaskets cylinder-head gaskets with metallically edged or bordered combustion chamber apertures are constructed so that the elastomer coatings are subject to the main clamping force and consequently these known flat gaskets are capable of taking up minor loads only.

Further characteristics, advantages and details of the invention arise from the following specific description and the accompanying drawings illustrating some preferred embodiments of a cylinerhead gasket according to the invention as follows: Figure 1 is a top view of a cylinder-head gasket according to the invention, Figure 2 is a section through this cylinder-head gasket taken on the line 2-2 of Fig. 1, Figure 3 is a section corresponding to Fig. 2, showing a second embodiment, Figure 4 is a section corresponding to Fig. 2 showing a third embodiment, and Figure 5 is a section corresponding to Fig. 2 showing a fourth embodiment similar to the embodiment according to Fig. 3.

Fig. 1 shows a cylinder-head gasket 10 for a four cylinder engine with combustion chamber apertures 12 as well as with apertures 14 and 16 for coolant and lubricant oil. The apertures for the cylinder-head studs are shown at 18.

Considered in conjunction with Fig. 2, Fig. 1 shows the sealing elements 20 surrounding and defining the combustion chamber apertures 12 which elements are conventional sheet metal edging trims or rings secured directly to a base sheet 22. The latter is coated on both sides thereof with layers 24 formed from a soft material which do not extend to the sealing elements 20 and which may, for example, be formed from a silicone rubber. In the unstressed or unloaded state the layers extend vertically relative to the plane of the gasket, above the sealing elements 20. In the outwardly facing surfaces of the layers 24 there are provided part-spherical depressions or recesses 26 which have been represented merely as dots in Fig. 1.The volume of these depressions 26 is calculated such that when the cylinder-head gasket is fitted and the cylinder-head nuts are tightened, the thickness of the soft layers 24 is reduced to such an extent as to obtain the maximum sealing area compression at the sealing elements 20 and not at the layers 24.

In the embodiments shown in Fig. 3 layers 24' consisting of a foamed elastomer are used and the foam cavities 26' of this material perform the function of the depressions 26 in the embodiment shown in Figs. 1 and 2.

In the embodiment shown in Fig. 4 a perforated base sheet 22" is used which is coated on both sides with layers 24" which in the unstressed state of the gasket have plane substantially parallel faces and have no cavities or hollow spaces. However, the material forming the layers is partially displaced into the holes 26" of the base sheet during the fitting of the gasket and tightening of the cylinder-head nuts.

In an example of the embodiment shown in Figs. 1 and 2 the thickness of the base sheet 22 should be 1.5 mm+0.05 mm, the thickness of the sheet material from which the sealing elements 20 are made should be 0.1 mm, the thickness of each layer 24 should be 0.3+0.1 mm, and the volume of the depressions 26 in a layer 24 should be between 60% and 80% of the volume of the layer. In the unstressed state the cylinder-head gasket will then be 2. 1 mm thick, whereas the fitted thickness, that is to say the thickness after the cylinder-head nuts have been tightened, is 1.7 mm.

With this cylinder-head gasket the thickness tolerances of the base sheet 22 relative to the thickness tolerances of the layers 24 may be neglected and with the above mentioned thickness tolerances the following relative compressions E of the soft material layers 24 are obtained when tightening the cylinder head nuts: E,omm,!v,I,, = 67% = = 75% = = 50% In other words, the difference between the maximum and minimum compression is 25%.

If no depressions 26 are provided or if neither a foamed elastomer nor a perforated base sheet are used, the layers 24 could be compressed only between 5% to 10% such displacement of the material taking place into the apertures of the cylinder-head gasket. Taking into account the requirement that in a fully fitted and tightened cylinder-head gasket the thickness of a soft material layer must not exceed the thickness of the sheet metal of the sealing element 20 (approximately 0.1 mm), it follows that a layer 24 without displacement accommodation spaces can be compressed by no more than 0.005 mm, from which it follows in turn that the manufacturing tolerances must be smaller than this degree of deformability.In such a case, however, it is also no longer possible to neglect the sheet metal thickness tolerance and with expensive thin gauge metal plates this amounts to approximately +0.02mum. Such a cylinder-head gasket without material displacement accommodation spaces would then have the following specification: thickness of base sheet: 1.5 mm thickness of sheet metal for sealing element 20: 0.1 +0.02 mm thickness of soft layer: 0.12+0.01 mm thickness of unconstrained cylinder-head gasket: 1.47+0.02 mm Design thickness of fitted and tightened cylinder-head gasket: 1.70 mm From this specification the following theoretical compressions E are calculated for the layers: E,ommai = 17% = = 38% Emm. = 0% The difference between maximum and minimum compression is therefore 38%, and the minimum compression value of 0% clearly shows that a thickness deviation of as little as 10 2 mm would give rise to total failure of the cylinder-head gasket because sealing area compression is no longer obtained. Since the scatter range of compression in the soft material layers (EmaX.Emjn.) is here greater than the actual compressibility of the soft layers it is not possible to produce at economically acceptable prices, cylinder-head gaskets wherein the base sheet, outside the sealing elements edging the combustion chamber apertures, is fully coated with incompressible elastomer soft layers.

The embodiment shown in Fig. 5 is identical to that shown in Fig. 3 except for the configuration of the combustion chamber sealing element. Instead of the cross-sectionally C-shaped sealing element 20 a solid metal ring 20', located in the combustion chamber aperture, is secured to the base sheet 22.

Claims (7)

1. A cylinder-head gasket having at least one combustion chamber aperture and comprising a base sheet coated on both sides thereof with layers of soft material, a metallic sealing element situated in the combustion chamber aperture, and material displacement accommodation spaces for the soft material for the purpose of achieving adequate compressibility of the soft material layer, characterised in that the sealing element is secured directly to the base sheet and is incompressible, that the soft material layers project above the sealing element vertically of the sealing plane consist at least substantially of an elastomer material, that the thickness of the base sheet is from 2 to 8 times the thickness of a soft material layer and that the material displacement accommodation volume is approximately 50 to 80% of the volume of the soft material.

2. A cylinder-head gasket according to Claim 1, characterised in that the thickness of the base sheet is equal to approximately 2.5 to 5 times the thickness of a soft material layer.

3. A cylinder-head gasket according to Claim 1, characterised in that the thickness of the base sheet is approximately 1.5+0.05 mm and the thickness of a soft material layer approximately 0.1 mm.

4. A cylinder-head gasket according to one or more of Claims 1 to 3, characterised in that the sealing element is a sheet metal edging trim having a sheet thickness of approximately 0.1 mm.

5. A cylinder-head gasket according to any one of the preceding claims, characterised in that the soft material layers consist at least substantially of one of the following materials: acryonitrile-butadiene rubber, silicone rubber and fluorine rubber.

6. A cylinder-head gasket according to any one of the preceding claims, characterised in that the material displacement accommodation spaces for the elastomer one or more of the following provisions are adopted: a) the soft material layers consist of a foamed elastomer; b) the soft material layers are provided with depressions in at least one of their boundary faces; c) the base sheet is a perforated sheet.

7. A cylinder-head gasket substantially as hereinbefore described with reference to Figs. 1 and 2 or Fig. 3 or Fig. 4 or Fig. 5 of the accompanying drawings.