автореферат диссертации по машиностроению и машиноведению, 05.02.01, диссертация на тему:Исследование локального разупрочнения деформированной аустенитной Cr-Ni стали под воздействием лазерного облучения и повышение долговечности прокладок цилиндров автомобильных двигателей

МОСКОВСКИЙ ГОСУДАРСТВЕННЫЙ ИНСТИТУТ СТАЛИ И СПЛАВОВ (ТЕХНОЛОГИЧЕСКИЙ УНИВЕРСИТЕТ)

Исследование локального разупрочнения деформированной аустенитной Сг-№ стали под воздействием лазерного облучения и повышение долговечности прокладок цилиндров автомобильных двигателей

Специальность - 05.02.01 - " Материаловедение в металлургии"

На правах рукописи

Вилфрид Юлиус Эрб

Диссертация на соискание ученой степени кандидата технических наук

Научный консультант: доктор физико-математических наук, профессор Ю.Х. Векилов

Москва, 2003

MOSCOW STATE INSTITUTE OF STEEL AND ALLOYS (TECHNOLOGICAL UNIVERSITY)

AS MANUSCRIPT

Studies of Local Softening of a Deformed Austenitic Cr-Ni-Steel under LASER Heat Treatment and an Improvement of Lifetime of Cylinder Head Gaskets in Automobile Engines

Speciality: 05.02.01 "Material Science in Metallurgy"

DISSERTATION FOR CANDIDATE OF TECHNICAL SCIENCES

Wilfried Julius Erb

Scientific advisor:

Doctor of physical and mathematical sciences

Prof. Yu. Kh. Vekilov

MOSCOW 2003

Studies of Local Softening of a Deformed Austenitic Cr-Ni-Steel under LASER Heat Treatment and on Improvement of Lifetime of Cylinder Head Gaskets in Automobile Engines.

The weak point at metal cylinder head gaskets is the limited life time of the beads. Beads may crack during the life time of an engine. Cracked beads do not seal any more in a proper way and as a consequence combustion gas may leak into water cooling system of the engine or into the oil circuit. These leakages will heat up the engine components and finally destroy the engine.

I had created the idea to treat the beads of steel gaskets with LASER energy to avoid cracking of the beads and to prolong lifetime of metal cylinder head gaskets. This treatment is very evident for the lifetime of engines and also for the opportunity of designing engines with even more power, running at higher temperatures and lower fuel consumption.

This new idea of mine has also been documented in a patent application at the German Patent Office, published 01. October 1997, No. DE 197 08 970 A1 and at European Patent Office, published 09. September 1998, No. EP 0 863 335 A2. [1]

Until today there is no other patent applied or published regarding the Laser heat treatment at beaded metal cylinderhead gaskets.

This dissertation is describing a method of heat treatment with Laser beam for prolonging life time of beads and finally gives recommendations for improvements on existing gasket concepts.

Preface

Some explanation is reqired to be given. Why, as a German, did I apply this dissertation for doctorate at the MISIS Institute of Technical University of Moscow?

This document has been initiated during a common project between the MISIS-lnstitute (Moscow Institute of Steel and Alloys - Technical University) and the Research and Development Institute of REINZ-Dichtungs GmbH at Neu-Ulm Germany.

I have been manager of the R&D Institute of REINZ from 1994 till 1997.

During that period the MISIS Institute had to develop a FEA calculation method for the material stress analysis of the forming process for preformed (beaded) cylinder head gaskets. The project covered the scientific analysis of the forming processes as well as the analysis of the stresses and the material flow during the forming processes.

In a further project the MISIS institute also was elaborating an advanced calculation method for the determination of the material stresses in sandwich multilayer metal cylinder head gaskets made out of rubber coated (coating thickness from 5(jrn to 50pm) steel layers whereas the layers could be made out of 0.12 mm to 1,20 mm hard stainless steel. Never before this challenge has been mastered. But the scientists of the teams of Professor Dr. Yuri Vekilov and Professor Dr. Vyacheslav Brinza could work out new methods and an accurate and close to reality stress analysis for the gasket beads when beads are formed during the production process and during the application in an engine.

At that time I had started investigating in lifetime analysis on dynamic fatigue test simulators for finding ways of improvement for multilayer steel gaskets. It turned out material selection and material treatment being of high importance to life time of the products.

Due to the close and successful cooperation of the both institutes, Prof. Vekilov and Prof. Brinza have asked me to extend my investigations and to document it in an application for a doctorate at the MISIS Institute. This report therefore is also a proof for the good relationship and scientific cooperation on an international level proceeding in knowledge and science.

I confirm the theme of the dissertation has been raised by myself. All of the experiments have been designed, initiated, done or supervised by myself. All results have been elaborated by myself. Therefore I confirm being fully responsible for the results of the investigations and also for this entire dissertation.

Neu-Ulm / Moscow, 22. June 2002

Wilfried Erb

Professor Dr. Tschitschenev, Head of Department of Machine and Equipment for Metallurgical Enterprises and Professor Dr. Leonid M. Gluchov, Head of Metal Plastic Deformation & Strain Hardening Research Laboratory, have accepted the work in this existing edition as dissertation at the Moscow Institute of Steel and Alloys.

Prof. Dr. Leonid Gluchov

Prof. Dr. Nicolay Tschitschinev

Acknowledgements

All of my respect and my very special thanks go to Professor Dr. Leonid Gluchov. He did really dig deep and did challenge me with the thesis. He was discussing all the technical details and methods of testing and stress analysis. Professor Dr. Gluchov was teaching me the methods and forms for consistancy of the work and the logic of the results and resumes. He has kept me on track in the work for the doctorate. He also has sacrificed himself and has spent innumerable hours for detailed advice and involvement.

My special thanks go to Prof. Dr. Tschitschenev. He took some burden with me to support the specialized part of my work on the ground of metal forming and Laser application. He also gave me insight into their laboratories and their work.

There is Professor Dr. Dmitry Livanov behind all international relations of the MISIS Institute. He was also behind me and supporting so much the process of promotion. Thank you very much!

Professor Dr. Yuri Vekilov and Dr. Edgar Schmid, two very extraordinary and generous mentors with their farsighted attitudes, have made possible the long and very fruitful cooperation of the MISIS Institute at Moscow, Russia and the REINZ R&D department at Neu-Ulm, Germany. To both, my special heartful thanks.

Dr. Edgar Schmid gave the approval, the support and encouragement for going across the national borders for various projects with the Moscow Institute MISIS. He also was the never resting fighter for increasing the theoretical knowledge package of sealing and gasketing. Therefore he, as General Manager of REINZ and Vice President of DANA Corp. Europe, had extended the REINZ laboratory to a highly recognized research and development institute in the automotive industry. He gave me the

challenge to grow and the opportunity to develop new gasketing concepts, materials and processes, especially on the field of rubber coated steel gaskets (SLS/MLS) and soft gasket materials.

Professor Dr. Yuri Vekilov, Head of Department of Theoretical Physics at the Moscow State University, MISIS Institute, has the merit of bringing the capabilities of the MISIS Institute and its employees to the market of Western Europe. He has created the relations and had evaluated the Russian team members for all the common projects. He is also an excellent teacher and successful 'ambassador'. He brought in the theoretical fundaments and the 'cultural lessons'. He has achieved to keep the cooperation highly efficient and has made outstanding Russian scientists joining the teams. A thank you to Dr. Eyvaz Isaev and Prof. Dr. Aleksandr Aleksandrovic, members of the MISIS - REINZ -Team.

Professor Dr. Dr. Brinza has been the organizational talent with best contacts to all sections of the MISIS Institute and to the Ministries. He took care of the resources and allowances to make the projects finally happen. He had delivered new ideas and approaches to solve scientific problems as well as to declare all the other obstacles to be 'Zero'.

A heartful thanks to Frank Popielas, Fred Weiss and Dr. Dieter Grafl, outstanding scientists and the real best sealing specialists. And to all other sixty employees of my R&D team at REINZ. They have contributed their competent support, ideas, preparations and work in the years from 1993 to 1998.

All together we had a winning partnership. And we could achieve a significant progress in the sealing science and technology. Above all, the cooperation with the MISIS Institute brought a better understanding of the Russian culture and academic work. I have learnt to know and to appreciate the excellent niveau of the involved Russian scientists.

Neu-Ulm / Moscow, 22. June 2002

Content

Chapter Page

Thesis (appreviated) 2

Preface 3

Acknowledgements 5

Content 7

1. Introduction and problem analysis 8

2. Thesis (detailed) 12

3. The Single Layer Steel and Multy Layer Steel (SLS/MLS) Cylinder 18 Head Gasket (CHG); CHG function and state of the art CHG

4. Status of the applied technology for avoiding bead cracking, 30 literature analysis

5. LASER-technology; critical LASER parameters; 43 literature analysis

6. Conclusions after literature studies about state of the art MLS 53 cylinder head gasketing, state of the art 'how to avoid bead cracking' and Laser application technology

7. Application of LASER energy to the critical beaded areas of 56 SLS/MLS beads

8. Influence of Laser heat treatment on mechanical properties of 87 Beads; prove of thesis

8.1 Static and elastic/plastic properties of MLS/SLS beads 87

8.2 Dynamic and plastic/elastic properties of beads 100

8.3 Simulation of 'close to engine' conditions for beads; 107 Bead cracking tests; Prove of thesis

9. Recommendations for improving gasket lifetime 129

10. Bibliography 136

1. Introduction and problem analysis

oLS/MLS cylinder head gaskets (SLS/MLS, the acronyms for singe! layer stee! and multi layer steel cylinder head gaskets) represent the dominating sealing concept for Otto and Diesel motors since from the early 1990s. They have almost abolished the impregnated paper-metal compound cylinder head gaskets and graphite gaskets. [2]

CHG SEALING CONCEPTS

1.1 sealing concepts for cylinder head gaskets

Modern engines are characterized by:

• Light weight

• High ratio of power to weight

• High dynamic of components

• Short development lead time

• Simultaneous development for all engine components supported by finite element analysis (FEA) for all engine components

• Modern, proved, tested and predictable materials and components

• Almost safe and no service required during entire lifetime of engine and car. By this the gaskets have to be 100 % safe, computable and predictable. The MLS sealing concept is almost fulfilling this requirements, because of FEA-

supported design and its over 90 % metal content. The beaviour of the metal is much more predictable than composite materials. Sealing companies have Invested tremendous amounts of money during the last ten years to create more and more better performing products. The results, realized in the MLS-concept, satisfy most of the requirements. Gasket failures in the field are really rare but can base on various reasons like

• Leakage

• Bead cracks

• Delamination of rubber coating from the MLS sheets

• Deterioration of rubber coating structure

• Bolt force losses followed by leakages

• Fatigue stressed engine components with bolt force losses followed by higher sealing gap dynamic and leakages.

Of course the rubber characteristics are of high importance for the lifetime sealability of a gasket. But in this dissertation it is not reflected furthermore. Its influence on gasket and sealability is also not discussed. If we consider the some hundred million of cycles of dynamic clamping stress for any cylinder head gasket, we easily accept the importance of an accurate working spring rate and of a lifetime enduring elasticity for a beaded metal gasket. The beads have to bridge sealing gap dynamics up to 8 pm. Cracked beads absolutely limit the lifetime of MLS gaskets as they result in a collapsing elasticity of MLS gaskets, leakages and finally in a destroyed engine. [2] For the durability of MLS gasket the beading process is of high importance as well as the material of the bead. Involved in the beading process are the beading tools. They define

• the form of the beads,

• the hardness increase during the forming process,

• elasticity of the bead under load when installed in the engine,

• spring rate of beads,

• specific sealing pressure along the bead lines,

• recovery sealing pressure under varying sealing gap movement speeds.

BEAD CRACKS

1.2 bead crack at top layer of MLS gasket, created on a testing device

Together with the conditions, given by the bead surrounding engine components (bolts, cylinder head, engine block), the forming process and the bead materials together influence the relaxation of beads and longterm residual resilience forces, Least are mainly responsible for the sealing function around engine combustion chambers.

The hardness increase during the forming process of the sealing beads is on the one side improving the spring behavior of the bead and on the other side limiting lifetime of a bead. Bead cracking is immediately destroying the proper bead characteristics. Therefore avoiding bead cracks is of very high priority. Once a crack has started, the fracture progress can be rapid and often unpredictable.

Crack propagation depends on

• ratio of yield strength to tensile strenghth

• basic hardness of the material

• increase of hardness and increase of stress of the material during forming the beads and during installation into the engine

» stress concentration due to surfaces of clamping components

• stress concentration due to inhomogenity of material

• grain size of material

• load distribution

• resulting sealing gap movements (amplitude, load level)

Experiences out of engine endurance testing are showing us once bead cracking has started, the cracking process is continuing till gasket and its sealing capability completely fail with final destruction of entire engine function.

Year by year the MLS gaskets have been improved by varying the number of layers, material of layers and design of the different layers of a MLS gasket. Many patents have been applied describing different forms of gaskets, forms of beads and their combinations. Also coatings are developped for better dynamic damping characteristics of a gasket. In addition to this the method of application and distribution of any coating on gasket surfaces are of importance to the tendency of beadcracking as the coating is influencing dynamic and friction conditions of engine components.

All this form and material variations do not create the same impact as Laser treatment to the beads. Laser treatment is offering new dimensions of dressing the material. Therefore the thesis of this dissertation is reflecting on limited heat treatment of beads to prolong lifetime of beads and gasket functions.

2. Thesis

Heat treatment applied to SLS/fi/ILS gasket sealing beads, limited to specific surface areas and specific depth into material, is reducing bead cracking tendency and is prolonging bead and gasket lifetime.

The treatment of beads, applied through LASER energy, should reduce surface hardness of beads to a certain depth of the material. This means, the hardness reduction should not happen to the entire cross section of the bead. The hardness reduction is necessary to reduce the risk of cracking. A limitation of depth of hardness reduction of the material is necessary to avoid significant weakening of the entire bead structure and spring rate. Therefore hardness reduced zone has been limited during the testing for this dissertatin to a limited depth of round about 20% of material thickness.

2.1 The bead deformation

Real beads are made in a forming process [3] which is

• stretching

• bending

• coining and squeezing

the metal in the beaded area and the small zone aside of the bead.

Aside of the regular wanted deformation (defined through the tool design), additional, unwanted tool imprints into the bead material are caused by

• micro and macro form irregularities of the tools,

• form tolerances

• surface roughness of tools

The microstructure of the bead surface and its defects depend on the initial surface roughness of tools and wear of tools during a tool lifetime. This imprints (coining) are potential starting spots for fractures as they also create a spot like

microfocal stress increase. They represent some of the microstructure reasons for bead cracking.

The macrostructure of a bead defines load and stress distribution at bead and entire gasket. It delivers the 'macro' reasons for bead cracking. These macro reasons mostly result in fatigue cracking.

The bead material itself is defined through

• basic hardness of material

• hardness increase due to forming process

• yield strength, tensile strength

• ratio of tensile strength to yield strength, which defines remaining deformation capacity

• alloy and grain structure of material

• cleanlyness and purity of alloy

• surface (roughness, coating, lubricants, etc)

• material production process (deformation ratio, heat treatments, etc)

Bead Forming Process

2.1 Bead forming as a 3-D process; critical spots with high residual stresses

The material's macro and micro structure together are responsible for how and when cracks occur, where they start and how they proceed. The task of this dissertation is to evaluate and describe the method of LASER heat treatment for avoiding bead cracks and prolonging bead lifetime. It's not discussed how cracks start and how propagation is supported. Fracture mechanic science has already answered most of this questions long ago. [6]

2.2 Stress situation in a bead

2.2.1 Stress situation in a half-sinus bead during bead forming process, at fully closed tool; tool width of male part: 0,7 mm; tool width of female part: 2,3mm

The graph above is showing the stress calculation, as main stress von Mises, for the beading process, when the beading tools are in the position of full closure. Areas A-B, C-D and E-F have highest stress up to yield strength. Remaining deformations at these spots are defining the final form of the bead after production process. Also in the above mentioned areas of the beads we can expect a hardening of the material.

The residual stress after deformation through production process and the residual stress in a bead after assembling in an engine should indicate the areas in a bead, where a reduction of stress is of influence on bead life time.

VS3 - 2.17 - 8 elements

2.2.2 Stress situation after production of the bead; tool opened; tool width of male part: 0,7 mm; tool width of female part: 2,3mm

We can see that the stresses are less compared to the situation when the tool is closed. But in the middle of the bead (section C-D), bottom side at D, the main stress is still about 1200 N/mm2 to 1400 N/mm2, just close to the yield strength. At the sections A-B and E-F max. stress is reduced to 800 N/mm2 to 1000 N/mm2

We like to know now the conditions when the gasket is installed. Installation is causing an additional deformation of the gasket bead. Therefore we can expect remaining main stress close to the yield strength as under production conditions.

Let's have a closer look to the stress situation after the installation of a gasket. Another deformation of the bead is taking place. This deformation is reducing the hight of the bead to the working thickness of the gasket bead.

VS3 -2.17 -8elements-continued vwth 2.19

StressJI INMm2|

1400 <■ ■■■ ». 1418.3

t200 <. on < 14Q0

1000 < 1200

800 « S22I5S « 1000

600 « SOD

400 « eoo

200 « 400

0 « 200

2.2.3 Stress situation after installation in an engine

(tool width of male part: 0,7 mm; tool width of female part: 2,3mm)

We still can see the areas A-B, C-D and E-F being the critical zones of stress. The Mises-stress is at a level of about 1000 N/mm2 to 1200 N/mm2. This slightly below the yield strength.

2.3 Evaluation method and design of experiments

• Lifetime long elasticity is the key parameter of a gasket bead. State of the art gasket functions and designs are described. The forming process of gasket beads and materials is of importance and is included as well.

• In a second step it is explained how traditionally bead cracking could be avoided.

• Then the basic understanding is given about LASER energy application.

• Literature analysis.

• Experimental LASER energy application to the critical bead areas according the thesis. This chapter includes C02 and ND: YAG LASER experiments.

• Test of LASER treated specimen (static, dynamic and close to engine condition tests), influence of treatment.

• Conclusions and hints for how to improve beads and gaskets.

3. The Single Layer Steel and Multi Layer Steel (SLS/MLS) Cylinder Head Gasket (CHG)

3.1 CHG function and state of the art concept

The cylinder head gasket (CHG) is an engine component, designed for sealing the most critical sealing gap of an engine. Because at this gap the engine is cut into two pieces :

- engine block with the pistons, cylinders and crankcase

- the cylinder head with the combustion chambers and valves.

Cylinder Head Gasket Installation

♦ Block and Head

Corrosion

♦ Dynamic load ♦Closed Deck ♦Heat ♦Open Deck

3.1.1 Cross section through engine block and head at combustion chamber [4]

All the combustion power of an engine occurs between the crankcase and the cylinder head. By this, all high dynamic and temperature changes happen in this 'hot' zone of an engine. The flat CHG is clamped between the block and head and therefore is undergoing all the stresses and aging processes which go along

with the combustion firing, heating and cooling and relative movements of the engine components (cylinder head and cylinder block). The gasket has to sea' combustion gas pressures up to 160 bar, oil pressure up to 10 bar and water pressure up to 1,5 bar. Wear occurs as there are forced movements (into direction of piston movement) of head and block of up to 8 pm, so-called vertical dynamic sealing gap swinging. And there are also some tenth of a millimeter movements straight to the sealing gap, so-called shift of head and block [2].

SEALING GAP VARIANCE

MLS-7KD (VW TOI) Fs = 70!<N

Pi ,150 „ „120,

Yo 11.7 8.6 5.2

Pi .t150^,J2p ^ SO

Yo 12 2 6 9 5.1

Pi -1

: YO 53 4.4 2.8

Pi hydraulic, static test pressure in combustion chamber (bar) YD cylinder head lift at gap sensor (micro rn) / »

3.1.2 Sealing gap variances through static internal combustion chamber pressure

3.2 Speed of movements, dynamic cylinder head lifting

The vertical dynamic movements of the sealing gap follow the frequency of the ignition of a cylinder. E. g. an engine running at 6000rpm fires 3000 times per minute and cylinder. The sections between two neighboring cylinders, depending on the engine design, are often very narrow. A width of 5 mm to 6mm is usual. These narrow bridges have the dynamic of both cylinders. With 6000 engine revolutions per minute, the bridging gap is also swinging with a frequency of 6000

per minute. Therefor this area is of high dynamic and very difficult to be sealed. The graph below is showing the dynamic cylinder head displacement after specific testing conditions with two different type of iviLS gaskets [2].

DYNAMIC CYLINDER HEAD LIFTING

3.2 Sealing gap variances through engine dynamic

The shift movement between head and block and gasket mainly follows the warming up and cooling down phases of an engine. Once an engine is warm, the thermostatical regulation of coolant liquid delivers a pretty stabile coolant temperature of the 'water jacket' of the engine. So frequency out of this is very low. During a fast warm up of an engine, e. a. when an engine is running immediately after the cold start with full load and speed, we can watch a significant temperature difference between engine head and block and also temperature differences inside the head and inside the block. This results in different elongation of head and block as well as different elongation of different sections in head and block. Due to this temperature differences the components try to bend, expand and slide relatively to each other. At bigger engines this slide

can reach 3/10 mm. The engine components are rubbing the gasket surfaces and induce shearing forces to the MLS beads and the rubber coating of the gasket.

3.3 The sealing function

The proper sealing is performed not only by the gasket, it is also influenced by the cylinder block, cylinder head and the bolts. The static and dynamic stiffness, the sectional spring rate of these co-working components together with the cylinder head gasket determine the specific surface pressure at specific areas of the gasket and therefore the specific sealing surface pressure at specific gasket zones. Mainly beaded areas have higher surface pressure rates than non beaded areas of a gasket. Without beads there is no sufficiant sealing function. But with beads and the clamping forces we have always additional deflections and varying surface pressures. Because of this the bead hight and stiffness is limited by the resulting deflectins and stress of the surrounding components. Load on a gasket surface is therefore of topographic nature as CHG, block and head are not of infinite stiffness. [2]

CHG COMBUSTION SEALING

Double Eyelet

Omega" (SI) Ring

Double Folded Eyelet

Metal Elastomer

Welded Wire Ring

Multi-Layer-Steel 1A

Corrugation Ring

3.3.1 Cross sections of compound and metal gaskets; combustion sealing systems

The above shown combustion sealings are designed for different engine types like Diesel- or Otto-motor and Aluminium- or cast iron-material. The MLS design concept offers a broad design variety to replace nearly all other designs.

The gasket beads have to follow the sealing gap movements through their bead spring rates. The sealing function is maintained when at highest gap opening there is enough remaining bead surface pressure to the surfaces of engine block and cylinder head to prevent the transpass of the medias like combustion gas, lubrication oil and cooling media.

Out of real engine testing experience, the static surface pressure should not be lower than 40 N/mm2 for the most critical sealing task of combustion gas sealing. To realize this surface pressure, elastic/plastic deformations of the gasket beads are necessary during installation.

This elastic/plastic adaptation of the surfaces is required to close the surface roughness and deflections of engine block and cylinder head. The plastic part of deformation is quite necessary to adopt the gasket surfaces to the engine block and cylinder head surfaces.

The elastic adaptation is given by the Young's modulus. This kind of deformation is responsible for keeping enough surface pressure during dynamic deloading of the gasket bead caused by the combustion forces.

The a, m. physical behaviour of bead is mainly determined by its width, hight and sheet thickness.

d: thickness of beaded steel sheet h: hight of bead b: width of bead

4-►

b

3.3.2 Cross section of bead; measurement system

The rubber coatings of MLS gaskets are supporting the adaptation process quite

well, especially the micro sealing of the surface roughness.

The chemical formulation of the rubber coating is very complicated and a result of

a long and intense development process. The key requirements for the coatings

are:

• Excellent adhesion to the metal sheets of the gasket

• Chemical resistance against lubricants and coolants

• High static and dynamic elasticity at -35 °C till 230 °C.

• Good adaptation to surface roughness

• Exzellent heat and pressure resistance

The materials for the coatings are blends of different Fluor elastomers, or of NBR elastomers or Silicon elastomers and are almost non compressible.

The graph below is showing a typical 4 layer design CHG with excellent sealing function. The CHG is coated with 'thick* (~24 jam) layers of FPM rubber on outer gasket surfaces and 'thin' (~5 |_im) coatings on inner gasket surfaces. [2]

MLS-CHG DESIGN

4 layer design with FPM-coating

❖ Top layer

♦ Distance layer

❖ Stopper

♦ Bottom layer

24 urn

vmiTM

3.3.4 Typical 4-layer MLS design; coating thicknesses

3.4 Load distribution

The cylinder head bolts induce the clamping forces. Through the elasticity of gasket, motor block and cylinder head the load is distributed over the entire gasket surface. Distribution is very inhomogen and has to be influenced and directed by the gasket design. The total bolt forces depend on the maximum ignition pressure and diameter of pistons. A ratio of 3.5 to 4.5 for bolt force to ignition forces is apropriate. As a minimum 50 % of the bolt forces have to be distributed to combustion sealing. Enough specific surface pressure has to be at the com