Numerical analysis of the impact of water injection on combustion and thermodynamics in a gasoline engine using detailed chemistry

Water injection is a promising technology to improve the fuel efficiency of turbocharged gasoline engines due to the possibility to suppress engine knock. Additionally, this technology is believed to enable the efficient operation of the three-way catalyst also at high-load conditions, through limiting the exhaust temperature. In this numerical study, we investigate the effect of water on the chemical and thermodynamic processes using 3D computational fluid dynamics (CFD) Reynolds-averaged Navier–Stokes (RANS) with detailed chemistry. In the first step, the influence of different amounts of water vapor on ignition delay time, laminar flame speed, and heat capacity is investigated. In the second step, the impact of water vaporization is analyzed for port and direct injection. For this purpose, the water mass flow and the injection pressure are varied. A steady-state, medium-speed, high-load engine operating point is investigated with focus on the effect of water injection on knock tendency and exhaust temperature. The impact of water injection on oxidation chemistry and auto-ignition is investigated using a detailed ethanol toluene reference fuel (ETRF) (ethanol, iso-octane, n-heptane, and toluene) reaction scheme. The combustion is predicted using the level-set method for flame propagation and a well-stirred reactor model in the unburned zone to predict auto-ignition. The laminar flame speed is retrieved from precompiled look-up tables calculated for each specific composition (surrogate, diluents, and oxidizer). Engine knock is evaluated using Bradley’s detonation diagram (Bradley et al. 2002, Gu et al. 2003). With numerical models, we are able to separate the influence of chemical and thermodynamic properties by using different flame speed tables, thermodynamic properties, and third body efficiencies for pressure-dependent reactions. This allows to quantify and rank the impact of the investigated properties. The impact on the knock limit spark advance in descending order of importance is found to be laminar flame speed, heat of vaporization, chemical equilibrium, water vapor heat capacity, third body efficiency, and ignition delay time.