PWI and DWI Systems in Modern GDI Engines: Optimization and Comparison Part II: Reacting Flow Analysis

The water injection is one of the recognized technologies capable of helping the future engines to work at full load conditions with stoichiometric mixture. In the present work, a methodology for the CFD simulation of reacting flow conditions using AVL Fire code v. 2020 is applied for the assessment of the water injection effect on modern GDI engines. Both Port Water Injection and Direct Water Injection have been tested for the same baseline engine configuration under reacting flow conditions. The ECFM-3Z model adopted for combustion and knock simulations have been performed by adopting correlations for laminar flame speed, flame thickness and ignition delay times prediction, to consider the modified chemical behavior of the mixture due to the added water vapor. This improved methodological approach allows considering both the fluid-dynamics aspects (in terms of turbulence level close to ignition time and average in the combustion chamber), the mixture quality and the chemical properties of the mixture (first of all the laminar flame speed) related to the water injection. The main result for the design process is the need of finding the best trade-off between the cooling of the unburnt mixture (which reduces the knock risk), the need to preserve both the turbulence and the laminar flame speed (which is already penalized by the cooling of the charge).