Otimização dos parâmetros operacionais de um motor linear via simulações multidimensionais

Abstract

Numerical models capable of simulating a linear engine and an electric generator in a coupled manner generally employ a representation of combustion inside the engine cylinder based on a quasi-dimensional thermodynamic model. In this sense, the space dependent physical and chemical phenomena are represented in a simplified way. Due to the simplifying assumptions associated with the model, all phenomena related to combustion chemical kinetics, fluid dynamics, and heat transfer are summarized in the universal gas law and in a combustion heat release function. In such models, the calculation is extremely simple from a computational point of view and allows the execution of an optimization process for the selection of optimal characteristics and operational parameters of the linear engine, leading to the sizing and geometric design of the combustion chamber used in this work. The aim of this work is to validate and propose modifications to the optimized design by using multidimensional simulations of the designed engine, seeking performance improvements. Thus, throughout the development of this study, the objective was to maximize parameters such as thermal efficiency and gross indicated work per cycle. Three distinct configurations were investigated. The first concerns the original configuration of the engine, in which a stoichiometric air-fuel mixture enters the cylinder through ports. It was found, however, that such configuration caused reverse mass flow through the ports and autoignition of hydrocarbons in the pyrolysis gas. In the second configuration, the air-fuel mixture was admitted through valves, and it was observed that the reverse mass flow through the valves can be mitigated provided that an appropriate displacement profile is applied. Even in this configuration, C3H6 present in the pyrolysis gas undergoes autoignition, and the performance remains below expectations. For this reason, the third and final configuration was tested, with fresh air intake through the ports and direct injection of pyrolysis gas into the cylinder. This configuration, with a pyrolysis gas injection pressure of 8 bar, proved to be the most promising, achieving a thermal efficiency of 54.20% and a gross indicated work per cycle of 160 J.