Cylinder Head Sealing Systems

Sealing Systems. Many different types of seals and gaskets, and almost as many different materials, are found in internal combustion engines. One normally becomes aware of these inconspicuous engineering elements only when they fail. In such cases, however, functioning of the entire system is endangered.

The great importance of seals and gaskets is clear right from the early stages of engine development. A lack of properly functioning seals makes it virtually impossible to undertake component testing.

Modem sealing systems are extremely reliable. Great development effort has been devoted to devising solutions that ensure long and dependable service life even under critical conditions such as aggressive media, high pressures, and extreme temperatures.

This section is intended to provide the reader with an overview of the various types of seals, their uses, and basic information on how they function.

Cylinder Head Sealing Systems. The head gasket is becoming more important in modem engines. In addition to sealing off the combustion chamber, the cooling system, and the oil passages, the head gasket also serves to transmit forces between the cylinder head and the engine block. Thus, it exerts considerable influence on force distribution within the entire assembly system and the associated deformations in elastic components.

More stringent requirements for fuel consumption and emissions have given rise to engine designs with optimized weights and, particularly in diesel engines, higher ignition pressures. The use of aluminum and the reduction of wall thicknesses in castings are reasons to anticipate further reductions in component stiffness. In order to further reduce cylinder warping, which is detrimental to exhaust gas composition, engineers are striving to reduce bolt forces.

These efforts result in a considerably greater load on the head gasket in the form of dynamic fluctuations at the sealing gap. The combustion chamber seal must be able to ensure the minimum sealing force, permanently and at all operating states. This causes very high demands on the durability of the sealing system selected for use here.

Ferrolastic Elastomer Head Gaskets. The head gasket made of asbestos-free ferrolastic elastomers, Fig. 7-288, is the system most widely used after converting to materials containing no asbestos at the end of the 1980s. The structure consists of a notched metal substrate with elastomers rolled onto both sides.

Fig. 7-288. Ferrolastic elastomer head gasket

The sealing effect is distributed over the entire surface area, and that requires high bolt forces. The disadvantages of this system are found in the relatively low elastic resilience. Great dynamic fluctuations in sealing gap width or changes in pressure due to thermal effects cannot be compensated and can be neutralized only in part by greater bolt forces. Engines with high thermal loads, narrow web widths, and wide oscillations in the sealing gap mark this system’s limits, triggering the development of higher- performance systems.

Metal-Elastomer Head Gaskets. Metal-elastomer head gaskets, Fig. 7-289, are used today primarily in heavy-duty utility vehicle engines. The principle behind this design (Fig. 7-290) is distinguished by the separation of functions (separate sealing for the combustion chamber and the liquid circuits) and the system’s great potential in each case. Not only are bead concepts with purely plastic properties used to seal the combustion chamber, but elastic systems as well.

Fig. 7-289. Metal-elastomer head gasket

Fig. 7-290. Cross section of combustion chamber, through a metal-elastomer head gasket

The passageways for liquids sealed with elastomer sealing lips exhibit great adaptability and elastic resilience. Selecting a suitable elastomer material ensures suitable aging resistance when exposed to fuel, coolant, and oil. Depending on the overall concept for the gasket, the elastomer lips may be injected onto the end of the sealing plate or on the surface. As an alternative, so-called inserts, i.e., metal substrates with a sealing lip vulcanized in place, may be used.

To avoid component warping and to achieve closely defined introduction of pressure into adjacent components, support elements may optionally be provided at the outer edge of the gasket.

Since the elastomer elements require only insignificant sealing forces in relationship to the bolt force, almost all the bolt force can be devoted to combustion chamber sealing and, if indicated, to supporting the components. In this way, the available bolt force is very efficiently utilized.