Combustion Valve Wear: A Tribological Study of Combustion Valve Sealing Interfaces
The exhaust valve system of combustion engines experiences a very complex contact situation of frequent impact involving micro sliding, high and varying temperatures, complex exhaust gas chemistry and possible particulates, etc. In addition, the tribological situation in the exhaust valve system is expected to become even worse due to strict future emission regulations, which will require enhanced combustion and cleaner fuels. This will substantially reduce the formation of combustion products that might ease the contact conditions by forming tribofilms on the contacting surfaces. The lack of protective films is expected to result in increased wear of the contact surfaces.
The aim of the work presented in this thesis has been to increase the tribological understanding of the valves. The wear that takes place in the valve sealing interface and how the change in operating conditions affects it have been studied. Such understanding will facilitate the development of future valve designs.
A test rig has been developed. It has a unique design with the ability to insert ppm amounts of media into a hot air flow, in order to simulate different environmental changes, e.g. varying amount and composition of combustion residue particles.
PVD coated valves were evaluated in a dry atmosphere. It was concluded that although some of the coatings showed potential, the substrate could not support the thin, hard coatings.
Investigations with an addition of different oils have been performed. Fully formulated oils proved to build up a protective oil residue tribofilm. This tribofilm has been in-depth analysed and proved to have similar composition and appearance as tribofilms found on low wear field tested valves. With a non-additivated oil, wear particles from the valve seat insert formed a wear particle tribofilm on top of the valve sealing surface. Without any oil the surfaces showed severe wear with wear particles spread over the surfaces.
The results presented give a hint about what to be expected in the future, when the engine oils are replaced with ash less oils with reduced amount of additives and the consumed amount of oil within the cylinders are reduced.