New Computational and Experimental Stress Analysis Method for the Design Decision on Optimum Piston Configuration of Production Engine 920065

In response to new customer needs in recent years, development efforts related to car engines are now being directed toward higher loads, speeds and output through the adoption of turbo chargers and other types of superchargers. Requirements for a lower Emission (EM) and better fuel Economy (FE) must also be satisfied in order to cope with the environmental problems such as the greenhouse effect and acid rain. Under such circumstances where so many factors must be taken into consideration, pistons, which are a central component of car engines, must have higher performance. They must also be lightweight and have low friction to produce a high FE, which should be much improved over that in a conventional design. Thus, it seems that research and development on pistons has become increasingly complicated, sophisticated and time-consuming. Problems encountered in the development of pistons include cracks on the ceiling and the boss caused by explosions and inertia loads, slap noise, fatigue deformation of the skirt due to the piston motion, and seizures resulting from overheating.

The cracks that form on pistons constitute the majority of the trouble, and are the result of severe operational conditions, including the requirements for higher engine performance and reduced weight in order to achieve longer operation time at extremely high temperatures which decrease the allowable stress or strength of the material.

Under these circumstances, we have been using the finite element method (FEM) as a tool for stress analysis in an effort to solve the cracking problem. However, the finite element method has various disadvantages as a tool for design verification. It takes much time to make a model for stress calculation and to change the model configuration, (e.g., to change the curvature of the corner where the ceiling intersects the rib).

To eliminate these drawbacks inherent to the finite element method, we have improved the boundary element method (BEM), which can calculate stress on the piston merely by meshing the piston surface shape as we reported in FISITA 90526*1. We have also developed methods for unit tests, and thermal load fatigue tests used to verify the results of stress analysis and have designed optimized piston configurations to solve the crack problem as quickly as possible. Specific features of these methods are explained below.

As the result of the remarkable improvement of the boundary element method (BEM), it is now possible to design every aspect of piston configuration and to conduct stress analysis on detailed areas where modeling has been impossible in the past, such as at the curved ceiling corner. At the same time, improvements on the link-type signal pick-out device have extended piston life and enabled temperature measurements to be taken on ordinary roads. We can now estimate what the durability of the piston will be under actual condiitons by comparing the calculated stresses with those estimated on the basis of the temperatures of aluminum on the S/N curve.

By setting various temperature conditions for high-frequency hydraulic vibration test equipment with a newly developed, heating device, we can now conduct the thermal load fatigue in 1/10 the time required by the conventional bench test method.

These results provide us with a faster and more precise method to optimize piston configuration and to test piston performance.