Abstract
The high cycle fatigue (HCF) and thermomechanical fatigue (TMF) problems faced in durability testing programs of cylinder heads are more pronounced in the last decade due to the drastic increase in thermal and mechanical loads (higher specific power and peak firing pressure). To avoid such problems rather than to solve them, modern development processes use computer aided engineering tools with the aim of “validation in simulation”. However, ineffective virtual engineering may produce late results, which has less influence on the end product.
This paper aims to present an integrated simulation approach covering the engineering needs in terms of turnaround times, accuracy and reliability during the different phases of cylinder head development process. An overview on optimization possibilities including production process, material and design is given for passenger car cylinder heads. A through understanding of fatigue mechanisms is provided along with mathematical formulation of the design optimization problem and cylinder head optimization roadmap. The TMF problem is addressed within the concept development phase, where tools such as empirical/analytical temperature estimation method and optimization database are utilized. For the local HCF problem zones, automatic structural optimization is used in conjunction with know-how criteria/optimization database. The focus is therefore given to target-oriented abstraction and analysis to remove structural weaknesses on cylinder heads.
The result is an integrated cylinder head design optimization for cost effective engine development, where the calculation methods add value to the product. The proposed methodology is applied to a high end diesel passenger car engine cylinder head with 100kW/L specific power and 200 bar peak firing pressure. The initial dimensioning of the valve bridge proved to be safe in terms of TMF, where the calculated peak temperatures are less than the specified material limits and the degree of restraining in the valve bridge is on an acceptable level. Furthermore, for the most critical locations in terms of HCF, approximately 50% improvement in safety factors is achieved within five iterations of an automatic overnight calculation. The results of the application example prove the effectiveness of the proposed methodology.
Keywords: cylinder head, thermomechanical fatigue, high cycle fatigue, structure optimization, fatigue design