Università degli studi di Modena e Reggio Emilia

In recent years, increasing demands for higher power density, lower emission and lower fuel consumption drive the research on internal combustion engines towards the adoption of innovative types of combustion like Homogeneous Charge Compression Ignition, Spark Controlled Compression Ignition and Gasoline Direct Injection Compression Ignition. All these systems exhibit aggressive combustion strategies and, as a consequence, a high-pressure gradient can be encountered in the combustion chamber at the early beginning of the combustion process. On one hand, the specific value of the pressure gradient marginally affects the kinematic analysis of the engine piston and its primary axial translational motion. On the other hand, it can possibly affect its secondary motions, namely piston transversal motion and piston tilting. Therefore, to investigate the influence of the pressure gradient, a numerical approach capable to capture the whole piston dynamic behaviour is mandatory. In this contribution, different pressure profiles are taken into account and then employed in different Multibody analyses to understand the influence of the pressure gradient on the piston secondary motions. In particular, three different pressure profiles exhibiting an increasing gradient, but with the same total heat released, are considered. In addition, three different radial clearances between the piston and the cylinder liner are simulated for each pressure profile. So that, a total of nine Multibody analyses are performed. The results are then post-processed and the Dang Van high cycle fatigue safety factor contour plot on the piston is obtained for each configuration. The results show a dependency of the fatigue life of the piston on the pressure gradient, albeit no detrimental structural issues are detected for the different configurations analysed. The presented methodology represents a useful tool for the structural assessment of pistons when a first original safe combustion strategy is modified towards more efficient ones characterized by pressure profiles presenting a higher gradient.

This contribution describes a methodology to evaluate the influence of bellmouth profiles on the elastohydrodynamic behaviour of the contact interface between the gudgeon pin and the piston of a high-performance internal combustion engine. First, a thermo-mechanical simulation is performed to evaluate the piston bosses thermal strains. Consequently, a Multibody elastohydrodynamic model is set up to evaluate the tribological behaviour of this lubricated interface considering a perfectly cylindrical shape of the piston bosses and their thermal deformation. Then, a preliminary bellmouth profile is adopted and a second Multibody model is performed. Nevertheless, the results, together with issues encountered analysing some tested pistons, suggest that modifications of piston bosses have to be introduced. Finally, an optimised bellmouth geometry is proposed and analysed.

The piston slap strongly affects the structural integrity of engine pistons. This phenomenon is caused by the dynamic effects of the piston secondary motions, both horizontal translation and tilting. This occurrence amplifies the contact forces between the piston and the cylinder liner if compared to the ones calculated by using simple kinematic analysis. A numerical approach is therefore mandatory. This paper presents a numerical methodology to predict the influences of piston secondary motions on the piston fatigue life. A combined Multy-Body/FEM strategy is developed to obtain truthful forecasts saving computational effort. First, Multi-Body simulations are performed to evaluate the piston secondary motions and the loadings involved. The most critical instants are retrieved and equivalent lateral accelerations are derived in order to prepare simplified quasi-static Finite Element models. Then, the stress field and the resulting fatigue safety factor distribution of the piston are obtained. The methodology reveals itself to be a useful tool to predict the fatigue life of pistons capable of limiting the computational effort and supporting the dimensioning of engine components during the early stages of the design process.

Advances in modern engines are becoming more and more challenging. The intense increase of thermal and mechanical loads, as a consequence of a higher power density, requires the improvement of the main couplings encountered between moving engine components. In this scenario, the cylinder liner/piston coupling plays a crucial role in terms of engine performance and durability, especially with regards to pollution emission and friction reduction. In this paper a numerical methodology is proposed, which aims at simplifying the Finite Element evaluation of the cylinder liner bore distortion in an eight-cylinder V-type four stroke turbocharged engine. Finite Element simulations are performed to obtain a virtual approval of the component geometry, in advance with respect to the component manufacturing. In particular, preliminary Finite Element analyses are developed which accurately follow the experimental procedure, where a single engine bank is coupled with a simplified test engine head. The Finite Element model is properly tuned in order to obtain the same cylinder liner distortion registered by experimental measurements. Further Finite Element analyses, both thermal and thermo-mechanical, are then performed to evaluate the cylinder liner distortion considering the actual engine head. In order to speed up the analyses, the engine head, the gasket, and the bolt tightening are subsequently substituted by pressure distributions mimicking the actual contact interactions. The methodology reveals itself to be well correlated with the experimental evidences and with the complete Finite Element model of the engine bank thus consisting in a useful tool for reducing the time necessary for the component approval. © 2019 SAE International and © 2019 SAE Naples Section. All rights reserved.