The demand of powertrain technologies able to efficiently employ several non-fossil fuels types, as bio or synthetic ones, compels to develop simplified strategies to accelerate the engines industrial optimization. High-tumble ultra-lean SI engines are currently an attractive option, thanks to their fuel flexibility and the potential of extending the lean combustion limit with novel ignition strategies. This work presents a methodology to initialize tumble flow fields at closed-valves inside pent-roof SI engines, without the need of simulating the gas exchange process. First, assuming an elliptical-shaped vortex, a velocity field is initialized into the cylinder according to the target tumble ratio at inlet valve closing (IVC). Then, the flow field is evolved at fixed piston position for a time duration proportional to the integral time scale of turbulence. This process is essential to adapt the vortex morphology to the surrounding geometrical details, despite some kinetic energy is lost due to frictions. To compensate this last effect, the velocity field is re-scaled at the end of the process to match the initial tumble ratio. Finally, the parameters affecting the tumble initialization are calibrated by comparing the development of the flow-field during the compression stroke with the results from a full-cycle simulation in a pent-roof SI engine. The methodology is validated on the Darmstadt optical engine. First, the numerical velocity fields are compared against PIV measurements at different crank-angles. Then, the combustion prediction is also evaluated, to assess the quality of the initialized tumble flow field. The achieved results are satisfactory, demonstrating the industrial applicability of the presented methodology.

<div class="section abstract"><div class="htmlview paragraph">SI engines fueled with hydrogen represent a promising powertrain solution to meet the ambitious target of carbon-free emissions at the tailpipe. Therefore, fast and reliable numerical tools can significantly support the automotive industry in the optimization of such technology. In this work, a 1D-3D methodology is presented to simulate in detail the combustion process with minimal computational effort. First, a 1D analysis of the complete engine cycle is carried out on the user-defined powertrain configuration. The purpose is to achieve reliable boundary conditions for the combustion chamber, based on realistic engine parameters. Then, a 3D simulation of the power-cycle is performed to mimic the combustion process. The flow velocity and turbulence distributions are initialized without the need of simulating the gas exchange process, according to a validated technique. However, coupled 1D-3D simulations of the engine scavenging can be carried out as well to increase the accuracy of the predicted intake valve closing (IVC) flow fields. The proposed methodology was validated against experimental measurements from a pent-roof single-cylinder spark-ignition (SI) engine, in which different values of hydrogen-air dilution were investigated. The achieved results were able to capture the measured pressure and heat release trends, demonstrating the industrial applicability of the presented methodology.</div></div>