Design Of System For Interpolating The Surface Of Allotypic Piston Skirt

In recent years, with the increasing requirements of the engine performance, reliability, longevity and emissions,the design and manufacturing of piston become more complex. Now,the middle-bulged varying ellipse piston is widely used and is recognized the fine pison skirt surface. The piston skirt of the cross section is oval, the vertical cross section through the axis is an asymmetry in the middle-bulged shape, and the ellipticity of cross section along the axis is changing. This piston becomes a similar cylindrical due to work pressure in the cylinder wall and the deformation. So the piston skirt and cylinder wall can be a good fit and cylinder gap greatly reduced. This can improve the lubrication conditions and reduce the channeling gas, noise and prevent cylinder cavitation. Therefore, the piston has been widely applied. But,the drawing of piston generally given only some discrete points of data of the longitudinal profile and the standard cross section of the pison skirt. that is not enough to process the piston. So the profile of the pison must be fitted and interpolated for obtaining sufficient data.First,this paper analyses the abnormal surface characteristics of piston skirt, and further analyses the variation of the cross-section skirt, the variation of longitudinal section and the variation of the ellipticity of the piston along the axis.This provided the basis for selecting the piston skirt-line fitting and interpolation techniques and the design of the piston skirt surface numerical interpolation system.Second,this paper describes the piston skirt surface mathematically and establish a mathematical model for the skirt surface. Then this paper introduces the cubic spline interpolation fitting for the piston skirt surface of the longitudinal profile and the periodic spline interpolation method for the non-standard elliptical cross-section interpolation.Finally, It is proved through an example that the fitting method achieves higher interpolation accuracy, precision and roughness of the piston to meet the requirements.Finally, the system for interpolating the surface of allotypic piston skirt is designed using Visual C++6.0. First, based on the analysis for the characteristics of shaped piston surface we determine the interface design of the interpolation system which primarily allow users to enter processing parameters according to different piston skirt. The system includes the module of standard oval numerical interpolation system,the module of dual oval numerical interpolation system, the module of non-standard oval numerical interpolation system and the modified module of numerical interpolation system. In these modules, based on the cross section parameters of the piston skirt,the longitudinal profile is fitted according to the long axis of the cross section, and then according to the fitted longitudinal profile, we interpolate the cross section by regular intervals, then we obtain enough processing data. As the error exists in the piston cylindrical processing, or the initial fitted profile of the piston skirt below standard, or the piston data needs to change, so the modified module of numerical interpolation system is designed. Based on these data by inputed again, we re-fitting curves and calculates data points, and modify the database.It is proved through an example that the system has a friendly interface and high-precision interpolation.The system can satisfy the interpolation requestments of various types of piston and can provide enough processing data for the NC machining of pison.