PWI and DWI Systems in Modern GDI Engines: Optimization and Comparison Part I: Non-Reacting Flow Analysis

Currently engine designers are focusing their attention on the improvement of the engine efficiency, led by the reduction of in-cylinder temperature and the adoption of stoichiometric combustion in the full range of the engine operation map. The most demanding points are those close to full power: water injection is thought to help in fulfilling this goal, thus contributing towards more efficient engines. To perform a rapid optimization of the main parameters involved by the water injection process, it is necessary to have reliable CFD methodologies capable of capturing the most important phenomena. In the present work, a methodological approach based on the CFD simulation of non-reacting flows of S.I. GDI turbocharged engines under water injection operation is pursued using AVL Fire code v. 2020. Port Water Injection (PWI) and Direct Water Injection (DWI) have been tested for the same baseline engine configuration and they have been run at full power condition, at the same rated power engine speed by varying: i) the injection pressure; ii) the injection timing (water injection phasing has significant effect on the water evaporation rate and on its impact on walls); iii) the normalized water injected mass on the stoichiometric fuel mass. The main results have been checked in terms of evaporation rate, cooling temperature, and efficiency, also considering the mixture quality and the fluid-dynamics aspects, in particular the possible degeneration of the turbulence level during the water injection process. The main aim of these simulations is to maximize water injection benefits and minimize possible disadvantage, such as primarily oil dilution and incomplete water evaporation to reduce water tank volume and refilling frequency. Water injection has demonstrated to allow to adopt higher compression ratio with limited penalties on performance. Therefore, for pursuing the target of improving the engine efficiency over the whole engine map and maintaining good performance level, the geometric compression ratio of the baseline engine has been increased. The adopted CFD methodology has shown to be able to capture the thermodynamic effects of water injection.