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The vehicle WLTP and RDE homologation test cycles are pushing the engine technology toward the implementation of different solutions aimed to the exhaust gases emission reduction. The tightening of the policy on the Auxiliary Emission Strategy (A.E.S.), including those for the engine component protection, faces the Spark Ignited (S.I.) engines with the need to replace the fuel enrichment as a means to cool down both unburnt mixture and exhaust gases to accomplish with the inlet temperature turbine (TiT) limit. Among the whole technology solutions conceived to make SI engine operating at lambda 1.0 on the whole operation map, the water injection is one of the valuable candidates. Despite the fact that the water injection has been exploited in the past, the renewed interest in it requires a deep investigation in order to outcome its potential as well as its limits. Many experimental campaigns have been performed while only few researches have deeply investigated the effect of the water injection on the air-fuel mixture under engine operating conditions. Since the experiments perform like a black box and they might hide some important phenomena, CFD numerical investigation steps are mandatory in order to provide a wider and deeper overview on the thermo-fluid dynamics characteristics involved in engine operations with water injection. This paper aims to provide an insight into those processes by using a CFD numerical approach to investigate the effect of the water injection on a non-reacting air-fuel mixture. The investigation has been carried out with the aid of CFD simulations by using AVL FIRE v.2017 solver. The influence of the physical, chemical and incylinder thermodynamic conditions on the effectiveness of the water cooling effect of the mixture is discussed and the application of the water injection to a spark ignition engine case at full load and high BMEP is examined. Port Water Injection (PWI) and Direct Water Injection (DWI) solutions are compared showing the high potential involved in DWI.

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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.

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A mono-dimensional code for the simulation of the effects of High Frequency Ignition systems (HFI) on the production of chemical radicals was developed and here presented. The simulations were carried out by considering the typical environmental thermodynamic conditions of a nowadays engine at full load. An electron transport model is linked with a Boltzmann solver coupled with a chemistry solver, affecting the Electron Energy Distribution Function (EEDF) in order to obtain the physical conditions leading to the production of radical components for a given fuel mixture. The transport equations for the electrons, the positive and the negative ions, and the Gauss’ law in a steady-state plasma region. Then the Boltzmann equation for the electrons, in a spatially homogeneous steady-state case, is solved in order to obtain the EEDF. Finally the chemical kinetics model is employed assuming a fuel-air mixture neglecting the fuel carbon atoms due to the assumption that electron-impact dissociation reactions, which initiate the combustion, exhibit a greater reaction rate compared to those based on hydrocarbon thermal dissociation and therefore can be neglected in this work. Results show the production of the hydrogen (H), nitrogen (N), and oxygen (O) radicals and the radius of the initial discharge under different simulated engine operating conditions characterizing the role of a plasma corona effect for the induced chemical ignition in gasoline-powered engines.