• Laminar flame speed of methane–air mixtures measured in an RCEM under engine-relevant conditions. • High-speed optical diagnostics and an inverse thermodynamic model used to reconstruct flame speeds. • Stretch effects, hydrodynamic-instability and 3D CFD analyses employed to identify strictly laminar propagation regimes. • A new laminar flame speed correlation developed and validated over 15–60 bar and 550–800 K. • Comparison with literature highlights the importance of dedicated high-pressure and high-temperature measurements. Reliable laminar flame speed (LFS) data at elevated pressures and temperatures are essential for improving combustion models used in engine applications. In this work, a Rapid Compression Expansion Machine (RCEM) is employed to measure the laminar burning velocity of methane–air mixtures over 15–60 bar and 550–800 K, which are representative of engine-relevant thermodynamic conditions and only marginally addressed by existing experimental datasets. A comprehensive assessment of the flame propagation regime is performed to ensure that laminar unstretched behaviour is considered. Three-dimensional CFD simulations confirm negligible in-cylinder turbulence at spark timing, while dedicated flame stretch and hydrodynamic-stability diagnostics identify the combustion interval unaffected by cellular instability or ignition-induced disturbances. This combined experimental–numerical procedure enables a rigorous identification of the physically valid laminar flame propagation window, ensuring that the extracted flame speeds are representative of intrinsic laminar combustion properties. Burned-gas flame speeds extracted from high-speed imaging are used for the estimation of flame velocities related to the unburned gas, based on an inverse thermodynamic model. The resulting dataset is employed to develop a new LFS correlation based on a power-law formulation with equivalence-ratio-dependent coefficients. The correlation reproduces the measurements with high accuracy and demonstrates physically consistent sensitivities to temperature, pressure, and mixture composition. Comparisons with methane–air correlations available in literature show relevant discrepancies at high pressures and temperatures, highlighting the need for direct measurements in engine-relevant regimes for a reliable laminar flame speed estimation.
Gambardella et al. (Thu,) studied this question.