Does a flat gasket really seal?
This month's article will deal with gaskets and seals. It will describe that sealing a certain structure is not just about putting a seal or gasket between the two components, but that there is more to think about. In this case we will look at one of the oldest sealing solutions, perhaps the simplest one - the flat gasket.
As we all know, an engine cannot be made seal-less, although that would make our engineering lives a lot easier. And I'm sure we can agree that sealing the combustion chamber, including the surrounding passages - coolant jacket, oil drillings, oil return cavities and so on - is one of the most complex tasks in high-performance engine design. Because of the efforts being spent in this area, we have learned to understand the specifics of head gaskets. In addition to the head gasket though, there are many other locations in and on our engines that need sealing, such as intake and exhaust, coolant, oil and fuel system interfaces.
Looking at the installation of the engine in the vehicle, quick connectors are often used in order to enable easy exchange of the engine. These connectors are designed in such a way that, as soon as they are opened, the individual connector ends shut themselves, enabling clean disassembly. All further connections that do not require opening or closing during engine installation can be separated roughly into two groups - flat gaskets and compressible seals, mostly elastomers (O-rings, for example). Flat gaskets are considered to be one of the simpler sealing concepts, but when there is a leak it is often with these rather simple seals that they occur.
A flat gasket is meant to seal two components where flat mating surfaces are being brought together. Remember, a gasket is only required when one or both surfaces shows some irregularity, such as scratches or pitting for example, whereas perfectly flat surfaces can easily be sealed without the need for a gasket. Reality, however, has shown that such a perfect sealing surface cannot be achieved easily. So, when designing the engine, the design engineers need to make sure the gaskets are taken into account, both in terms of their material spec and their dimensions. Flat gaskets can be made from a broad range of sealing materials, depending on the sealing medium and (non-) operating conditions, and can be made in various thicknesses.
Flat gaskets are typically found on connections such as those between coolant pump and engine block (at the least complex end of the spectrum) and between exhaust manifold and turbocharger(s), the latter being one of the more complex gaskets, given the severe environment in which they need to function. The cylinder head gasket is a third example of a flat gasket, in this particular case a gasket typically sealing more than one medium.
Apart from the gasket itself, in order to establish a proper sealing function, the relevant components need to provide sufficient stiffness to achieve an even contact pressure over the gasket. Looking at the cylinder head gasket again, the sealing of the combustion chamber is achieved between the very stiff cylinder liner and cylinder head structure. The areas outside this region have far lower stiffness due to internal differences in stiffness near the coolant and oil cavities. Since it is very difficult to achieve a robust seal in these areas, elastomer seals or local shape adaptations (beads/pressings) are often integrated into the head gasket.
Another well-known critical area where flat seals are used is in the exhaust system. Turbocharger flanges in particular (with their non-ideal rectangular shape and non-uniform bolt distances) are known for their multi-layer steel gaskets with integrated beads. These beads, which can be single or double beads, are designed to achieve and maintain sufficient local pressure between gasket and components in these kinds of harsh environments. Specific attention needs to be paid to high stiffness gradients over the sealing interface, which might lead not only to a leaking seal but to temperature-related cracks in the surrounding structure. A wise lesson I've learned over many years is that it is not maximum stiffness that is required, but useful stiffness. This means one should try to achieve sufficient stiffness with moderate stiffness gradients over the total sealing structure, including the flat gasket - and, absolutely not to forget, proper specification bolts, maintaining pre-tension when cycling between maximum and minimum temperatures.
In summary, sealing interfaces come in two flavours - complex and less complex. Simple ones do not exist.
Fig. 1 - Gasket detail
Fig. 2 - Gasket pressure graph
Written by Dieter van der Put