One requirement of piston pin is must be light, in addition to having sufficient stiffness and strength. Stiffness relative to bending can be increased greatly, as the fourth power of the increase in diameter. Deflection also increases approximately as the third power of the support span of the piston pin, i.e., with the piston pin boss spacing. A reduction in this value thus causes a severe reduction in bending and thus increases stiffness. If a shorter piston pin can be used, then mass reduction is also possible. An increase in stiffness relative to ovalization can be achieved only with a greater wall thickness and thus always increases mass. The stiffness of the piston pin has a significant effect on the loads on the piston pin boss, support, and bowl rim, as shown in Figure 1.
The susceptibility of the piston to pin boss cracks is shown in Figure 2 as a function of the piston pin geometry, as a result of engine testing. Owing to higher peak cylinder pressures, diesel engines require stiffer piston pins in comparison to gasoline engines. The limit of maximum allowable contact pressure in the piston pin boss also demands larger pin diameters. Nevertheless, because of greater peak cylinder pressures in turbocharged engines, for example, piston pin bosses can be overloaded.
If potential piston design measures for reducing the critical stresses in the area of the piston pin boss have been exhausted, such as by increasing the piston pin outer diameter, reducing the pin boss spacing, and so forth, then a solution can be found with the use of shaped pin bores in the piston pin boss or profiled pins. These significantly reduce the stresses in the piston pin boss by means of a softer fit between the piston pin and boss. The diameter of the pin bore is slightly retracted in the area of the inner or top edges, according to the load.
