Published: 31 December 2011

Nonlinear vibrational analysis of diesel valve gear

J. Avsec1
Z. Praunseis2
M. Marcic3
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A valve gear system is currently more or less a classical mechanical system in a majority of diesel engines. In our case, diesel engine durability test was carried out incorporating a conventional valve gear. After the 3000-hour engine test, the manifestations of an intensive wear detected on the exhaust side of the valve gear. The wear of the exhaust cam was particularly intensive right under the top. As evident from dynamical analysis and experimental data of the valve gear the loads are excessive mainly on the exhaust cam. The assessment of the minimum oil film thickness at the top of the exhaust cam does not provide any favorable results. As the largest loads are exerted at the top of the cam, where the highest wear was measured, it is necessary to reduce the normal stresses and improve the lubrication properties. Dynamic valve gear analysis was performed in order to estimate cam wear intensity and to compare the new cam designs with the existing ones. The model of the actual valve gear design can be represented by means of the partial differential equations. We used a vibrational system with three degrees of freedom. Partial differential equations were solved by means of finite differences and Runge-Kutta methods. Vibrational analysis allows studying kinematic and dynamic behavior of the nonlinear spring and nonlinearity of the damping forces in the valve gear system. The presented theory will be applied in the future research works for determining the conditions for chaotic behavior of the valve gear system. The dynamic model of the valve gear was used to analyze the causes of the excessive wear. Since the cam, designed in accordance with the polysine curve, offered too few possibilities for an optimum cam profile, we wanted to manufacture a completely new type of cam with more possibilities for an optimum adjustment. At the same time, we attempted to summarize some findings on the dependence of the cam design on the fuel consumption and valve gear noise. Thereby a new MULTICAM cam was developed. Contrary to the conventional theory of polysine cam, the motion in the MULTICAM cam can be written by means of seven curves. The analysis of Hertz pressures provides more favorable results for the MULTICAM cam profile. By using the new cam profile the Hertz pressures were substantially reduced. The top of the cam is subjected to the lowest loads with the MULTICAM curve shape. In spite of a higher contact force the normal stresses are lower mainly due to the higher radius of cam curvature. Dynamic analysis demonstrates that both newly designed cams exhibit lower stresses at the top of the cam and better lubrication properties, whereas the flow geometrical cross-sections and the other control values remain similar for all three cam versions.

About this article

10 September 2011
04 December 2011
31 December 2011
diesel valve gear vibration
cam design
valve gear dynamics