Water injection is a promising technology to improve the fuel efficiency of turbo-charged gasoline engines. Additionally, this technology is believed to enable the efficient operation of the three-way-catalyst also at high load conditions. In this numerical analysis, we investigate the effect of water on the combustion chemistry and the thermodynamics using 3D CFD RANS. In the first step, the influence of different amounts of steam on ignition delay time, laminar flame speed and heat capacity is investigated. In the second step, the impact of water vaporization is analyzed for different injection strategies, such as port and direct injection. Therefore, the water mass flow and the injection pressure are varied. A steady-state, medium speed, high-load operating point is investigated with focus on the effect of water injection on fuel efficiency, knock tendency and exhaust temperature. The impact of water injection on oxidation chemistry, auto-ignition and emissions is modelled using a detailed TRF (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 pre-compiled look-up tables calculated for the individual composition (surrogate, diluents and oxidizer). Engine knock is evaluated using Bradley’s detonation diagram. For all investigations CONVERGE v2.4 is used. Through the use of a numerical model, we are able to separate the influence of the different chemical and thermodynamic properties by using different flame speed tables, thermodynamic properties and third body efficiencies for pressure dependent reactions. This enables to quantify the impact of third body efficiency, ignition delay time, laminar flame speed, heat capacity and heat of vaporization on in-cylinder combustion.