Internal combustion engines are the primary transportation mover for today society and they will likely continue to be for decades to come. Hybridization is the most common solution to reduce the petrol-fuels consumption and to respect the new raw emission limits. The gasoline engines designed for running together with an electric motor need to have a very high thermal efficiency because they must work at high loads, where engine thermal efficiency is close to the maximum one. Therefore, the technical solutions bringing to thermal efficiency enhancement were adopted on HVs (Hybrid Vehicles) prior to conventional vehicles. In these days, these solutions are going to be adopted on conventional vehicles too.
The purpose of this work was to trace development guidelines useful for engine designers, based on the target power and focused on the maximization of the engine thermal efficiency, following the engine rightsizing concept. The originality of the present work stands in the comparison of the effectiveness of the most common strategies adopted today between two types of engine. The chosen engines for this study were modern boosted GDI engines, in line with the current automotive market, designed by CAD at the University of Bologna. 3D CFD computations of non-reacting flows were carried out by means Fire Code 2020 by AVL. The paper aimed to numerically investigate the rightsizing concept depending on the target level of power: two levels of power were chosen, 290 kW and 120 kW respectively, typical the former one of a high-power engine, the last one of an engine more devoted to efficiency purposes. The two engine bores were selected based on the common automotive solutions depending on the target power: 84 mm for the high-power engine, 75 mm for the other engine. Once fixed the bore value and pursuing the maximization of the thermal efficiency, a study on the possible geometries was performed, searching for the best stroke-to-bore ratio S/B: the long stroke engine design finds its limit in the maximum average piston speed, depending on the engine regime at maximum power. Then, the study was moved to the compression ratio increase and the adoption of over-expanded cycles, both aimed to increase the thermal efficiency. For solving the knocking issues arising from the adoption of increased compression ratio, the water injection strategy was analyzed too. Finally, some considerations were deduced on the effectiveness in applying the over-expanded cycles to the two different types of engine: the critical point to be solved is if the applicability and thus the effectiveness of the over-expanded cycles depend on the engine type.