Krzysztof Siczek

The researches on the friction properties of nc-WC/a-C : H coating on the lightweight valve stem

The lightweight valves, especially made of TiAl alloys, are more and more often used in modern combustion engines with cam and camless valvetrain. To increase the wear resistance and to decrease the coefficient of friction between valve steam and valve guide, different kinds of the coating are used. The researched valve has been outlet one made of TiAl6Zr4Sn2Mo2 alloy with the original chrome coating on the valve stem. The nc-WC/a-C:H nanocoating has been deposited on the chromed stem of researched valve, which has mated with the guide made of cast iron. The aim of the present paper is to investigate friction properties of such nc-WC/a-C:H deposited onto the chromed and polished surface of the valve stem. The scheme of research stand has been presented in the paper. On the stand it has been possible to measure values of valve lift and of valve acceleration, the impact force between valve and its seat insert, the friction force between valve stem and its guide, the temperature values for valve guide and for seat insert and a sound level during impacting. Researches have been performed in conditions of room temperature and without oil lubrication in the valve stem – guide contact zone. Basing on the measured values of friction force between valve stem and its guide vs. time and loading frequency, the coefficient of friction has been estimated. Obtained curves of friction force and of friction coefficient vs. time, for different frequency values of valve loading have been presented in the article.

Decreasing of movable masses in valve timing using roller tappet

The method for reduction of valve timing movable masses, by the use of composite cams for camshaft, which mate with roller tappet, is presented. It has been carried out an analysis of stresses under the surface of the cam using the methodology and computer programs allowing the synthesis and analysis of the cam mechanism, and to estimate the fatigue life, as well as using the finite element method. Mating roller tappets with a light camshaft, forged from TiAl alloy, can be done if the camshaft has steel caps set under the pressure on the surfaces their cams. The problem, however, is the formation of high stress in the Bielaiev point, when a roller tappet mates with segment cam. Because of the certainty for the cap fixing, it is assumed that the depth for the Bielaiev point should not exceed half the thickness of the cap. The stress analysis shows, that exceeding of the material yield strength is controlled by the interference value between the cam and its cap and by the apex radius of the cam. Effect of contact stress is at least an order of magnitude smaller. The maximum depth of the Bielaiev point does not exceed 0.09 mm, which is about half the thickness of the proposed cap. There is a need not for too small radii between the cam and the apical cap. Optimal selection of proper geometry of the cam will be easier after a proper modification of the program used for the synthesis and analysis of the cam mechanism, such that it is possible to ask any operational surface of the cam profile.