Development of a mono-dimensional numerical model for an Opposed Free-Piston Engine Linear Generator (OFPLG)

The challenge of de-carbonization is one of the hot topics of today's society. The automotive field, being one of the most affected sectors by this policy, has always been involved in the pursuit of higher efficiency solutions, especially for what concerns the energy production, and the development of new technologies has been a key point in the last years. The free-piston engine linear generator (FPELG) technology has caught the attention of the research community thanks to its potential in terms of efficiency, emissions and versatility. Compared to traditional rotational internal combustion engines, it lacks the complexity of the crankshaft and connecting rod mechanism, which is also responsible for high friction losses due to the thrust forces pushing the piston against the cylinder liner. In fact in this typology of engine, the piston motion is not constrained by mechanical devices, instead it is regulated by thermodynamical laws alone and, possibly, the implementation of controllers. This peculiarity implies a larger versatility in the operation of the engine in terms of compression ratio, which can be adjusted to allow the adoption of different fuels or combustion strategies. Another key point of this technology is the intended application in series hybrid electric vehicles (HEVs), for which it will provide the electrical energy needed to recharge the batteries. This particular use is characterized by the possibility of optimization around a fixed load operation of the engine, with consequent lower emissions and higher efficiency with respect to the wide operational range required by a traditional internal combustion powered vehicle (ICEV). The aim of the presented work is to investigate the main characteristics of this typology of engine with particular focus on the relationship between independent operational variables and output performance parameters and to propose a numerical modeling approach describing its working principles both mechanically and thermodynamically. After a brief introduction on the topic, a Design of Experiment based on Sobol's sequence approach is presented, analyzing the influence of intake conditions, linear generator characteristics and the compression ratio on the performance parameters of the engine. Subsequently the results of a correlation analysis carried out on these parameters are shown and predictive models are proposed. Lastly, the numerical modeling of the engine is thoroughly described and its results and possible future development commented.