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CFD simulations of reacting flows are fundamental investigation tools used to predict combustion behaviour and pollutants formation in modern internal combustion engines. Focusing on spark-ignited units, most of the flamelet-based combustion models adopted in current simulations use the fuel/air/residual laminar flame propagation speed as a background to predict the turbulent flame speed. This, in turn, is a fundamental requirement to model the effective burn rate. A consolidated approach in engine combustion simulations relies on the adoption of empirical correlations for laminar flame speed, which are derived from fitting of combustion experiments. However, these last are conducted at pressure and temperature ranges largely different from those encountered in engines: for this reason, correlation extrapolation at engine conditions is inevitably accepted. As a consequence, relevant differences between proposed correlations emerge even for the same fuel and conditions.

The accurate representation of Internal Combustion Engine (ICE) flows via CFD is an extremely complex task: it strongly depends on a combination of highly impacting factors, such as grid resolution (both local and global), choice of the turbulence model, numeric schemes and mesh motion technique. A well-founded choice must be made in order to avoid excessive computational cost and numerical difficulties arising from the combination of fine computational grids, high-order numeric schemes and geometrical complexity typical of ICEs. The paper focuses on the comparison between different mesh motion technologies, namely layer addition and removal, morphing/remapping and overset grids. Different grid strategies for a chosen mesh motion technology are also discussed. The performance of each mesh technology and grid strategy is evaluated in terms of accuracy and computational efficiency (stability, scalability, robustness).

In recent months, the increasing debate within the European Union to review the ban on internal combustion engines has led to the pursuit of environmentally neutral solutions for ICEs, as an attempt to promote greater economic and social sustainability. Interest in internal combustion engines remains strong to uphold the principle of technological neutrality. In this perspective, the present paper proposes a numerical methodology for 3D-CFD in-cylinder simulations of hydrogen-fueled internal combustion engines. The combustion modelling relies on G-equation formulation, along with Damköhler and Verhelst turbulent and laminar flame speeds, respectively. Numerical simulations are validated with in-cylinder pressure traces and images of chemiluminescent hydrogen flames captured through the piston of a single-cylinder optical spark-ignition engine.