Fuel performances in Spark-Ignition (SI) engines: Impact of flame stretch

In a context of decreasing pollutant emissions, the transport sector has to tackle improvements to the engine concept as well as fuel diversification. The use of these different fuels often has an impact on the combustion performance itself. In the case of Spark Ignition (SI) engines, efficiency is a function of the combustion speed, i.e. the speed at which the fresh air–fuel mixture is consumed by the flame front. Every expanding flame is subject to flame curvature and strain rate, which both contribute to flame stretch. As each air–fuel mixture responds differently to flame stretch, this paper focuses on understanding the impact of flame stretch on fuel performances in SI engines. Different air–fuel mixtures (different fuels or equivalence ratios) with similar unstretched laminar burning speeds and thermodynamic properties but different responses to stretch were selected. The mixtures were studied in a turbulent spherical vessel and in an optical engine using Mie-Scattering tomography. The combustion phasing was also investigated in both optical and all-metal single cylinder engines. Results show that flame stretch sensitivity properties such as Markstein length and Lewis number, determined in laminar combustion conditions, are relevant parameters that need to be taken into consideration to predict the global performance of fuels, either experimentally or for modeling simulation.