The valve spring setting has the same importance as the cam design for the performance of the engine system. The function of valve spring includes preventing valve from jumping out of valve seat under pressure load, and controlling valve movement to avoid valve train separation. The design of valve spring affects cam stress, valve train friction and spring chatter. The valve spring of an engine is usually a closed spiral compression spring at both ends. Most engines use fixed stiffness springs, although some use variable stiffness springs. For diesel engines with low speed, single spring design is usually enough to meet the requirements, but sometimes double spring design with a damper spring or inner spring is needed to reduce the severity of valve spring flutter. Valve spring design is a very complicated task. It can be used as an example to illustrate the principles of engine system design, for two or three reasons. Firstly, the analytical spring design method shows the link between the component parameters and the system design parameters. Secondly, the analytical spring design method shows that there are two different mathematical construction methods for the same design problem: one is to deal with as a deterministic solution, the other is to solve as an optimization problem. In the mathematical construction of optimization problems, both objective functions and constraint functions are listed as examples of explicit functions. It should be noted that in other areas of engine system design (such as cycle performance, cam design, valve train dynamics). Functions used to optimize construction are usually more complex implicit functions. Thirdly, the analytical spring design method gives an example of using graphic design method to construct parameter sweep design diagram. These typical parameter diagrams can be used to deal with the multi-dimensional design problems often encountered in the design of diesel engine systems.

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