The Laminar Flame Speed of Iso-Octane/Air/Ozone Lean Mixtures under Engine-Like Thermo-chemical Conditions

A numerical study has been carried out to evaluate the suitability of using ozone to achieve stable combustion of a very lean iso-octane/air mixture in a spark-ignition engine. A CFD model has been developed to simulate the compression stroke of the engine and the model has been validated against experimental data. Such a model is able to simulate the chemical kinetics of iso-octane/air/ozone mixtures during the compression stroke, thus predicting the composition and the thermodynamic conditions of the mixture in the chamber at the spark ignition time. These conditions have been used to compute the laminar flame speed of such a mixture by employing a 1-D solver. The accuracy of the solver has been assessed by comparing the numerical results with experimental data. As regards ozonized air, no measured laminar flame speeds for iso-octane/air/ozone mixtures are available. Hence, the model has been validated against experimental data for methane/air/ozone mixtures. The model has been used to investigate iso-octane/air/ozone mixtures, with 0, 200 and 500 ppm of ozone at IVC. The stoichiometric and a lean case with ϕ = 0.5 have been compared. The results show that, during the engine compression stroke, ozone decomposition produces oxygen atoms, which attack fuel molecules producing OH-radicals. These radicals favor the low-temperature oxidation until ignition time is reached. At the ignition time, the thermodynamic conditions of the mixture, in terms of pressure and temperature, are similar for cases with and without ozone. However, with ozone, a partially oxidized mixture is obtained, which promotes an increase of the laminar flame speed to a value comparable to the case without ozone under stoichiometric conditions.