A numerical investigation of the influence of ozone on combustion to improve the performance of spark ignition engines

The aim of this work is to exploit the influence of using ozonized air to achieve stable and efficient combustion of lean mixtures in a gasoline-fuelled Spark-Ignition (SI) engine. The influence of ozone on the combustion of near-stoichiometric mixtures, which are typical of SI engines, has also been assessed. A Computational Fluid Dynamics (CFD) model has been employed to simulate compression, combustion, and expansion of a spark ignition, axisymmetric engine fuelled with iso-octane/air/ozone mixtures. The aim is to assess how ozone improves the engine performance under different engine speeds, ignition timings and equivalence ratios. The model has been validated against experimental data available in the literature. Parametric analyses have been carried out by considering three values of engine speeds (800, 1000 and 1200 rpm), three different ozone concentrations at Intake Valve Closure (IVC) (0, 100 and 200 ppm) and two equivalence ratios (0.9 and 0.7). The results show that ozone enables reactions in the Low Temperature Combustion (LTC) regime, modifies the mixture chemical composition and the auto-ignition tendency. Specifically, for all the cases under examination, the addition of ozone to the air/fuel mixture reduces the combustion duration, leading to an increase in terms of work output and a reduction of the specific fuel consumption. Moreover, the advantage of using ozone is greater for lean mixtures than for near-stoichiometric mixtures. Finally, for the near-stoichiometric cases, when the residence time of the mixture is high enough, auto-ignition occurs in the end gases.