A numerical methodology based on CFD and plasma chemical kinetics simulations: a focus on the cyano radical

Anticipating the effect of implementing new fuel blends and combustion strategies is crucial in order to deal with the short-medium term green transition envisaged for passenger car fleets. In spark ignition engines, the use of alternative low-carbon fuel blends including biofuels, e-fuels and hydrogen, together with the stabilization of ultra-lean and highly diluted combustion conditions may accomplish both fuel economy and pollutant emission targets. In this framework, studying the very early ignition phase in terms of produced radicals is fundamental in order to optimize operating strategies. To this aim, optical analysis in dedicated engine configurations is a powerful device to extensively characterize those phenomena. The use of numerical tools further enhances investigative possibilities, as they provide insight into plasma-fluid interactions as well as related production of radicals and allow significant extensions of operative conditions along with the reduction of costs and efforts associated with experimental campaigns. This work is focused on the presentation of a numerical methodology supported by UV-visible emission spectroscopy on an optically accessible spark ignition direct injection engine. Focusing on the spark plug zone, the numerical tool traces the cyano radical (CN), which can be a key marker for ignition processes if considering its high sensitivity to the presence of CO2 and N2, namely the main components of charge diluent (e.g., when applying exhaust gas recirculation). The core of the methodology is a white-box zero-dimensional model for the simulation of non-equilibrium plasma chemical kinetics, accounting for the collisions with electrons and reaction schemes. The local mixture composition of the gaseous phase to be provided to the code is determined by means of three-dimensional CFD engine simulations performed with AVL-FIRE. Besides evaluating the effect of air-fuel ratio on CN, which is largely demonstrated in the literature (leaner mixtures feature lower CN production due to the smaller amount of carbon available for the CN-paths), the effect of the injection timing was investigated. Two different values of injection timings were tested for both stoichiometric and lean conditions at the same spark timing. The CN analysis showed that experimental data and numerical results are well correlated. The key role of the mixture local stratification on the CN production as a result of the injection timing was identified and discussed.