On Iso-octane Combustion with Ozone Addition under HCCI Engine-Like Conditions

Numerical simulations were carried out to study the influence of ozone on the Ignition Delay Time (IDT) of iso-octane/air mixtures under typical operating conditions of HCCI engines. 0-D and 2-D CFD simulations were carried out to compute IDT and to characterize compression, combustion, and expansion in an HCCI engine, respectively. A kinetic model was developed by merging a mechanism for iso-octane, a sub-mechanism for nitrogen oxides, and a sub-mechanism for ozone. The model was used to investigate iso-octane/air/ozone mixtures under typical operating conditions of HCCI engines running with very lean mixtures (equivalence ratio equal to 0.3). Parametric analyses were carried out considering different values of temperature (500 to 1200 K), pressure (15 to 40 bar) and ozone concentration (0 to 50 ppm). The results show that as ozone concentration increases IDT decreases, with a greater impact at low temperatures, and the NTC (Negative Temperature Coefficient) effect decreases. However, the reduction of IDT with ozone addition is less as ozone concentration increases, especially at low temperatures. An increased pressure, on the other hand, generally promotes faster ozone decomposition and enhances ozone effect on IDT reduction for all temperatures except in the range 760-840 K, where the opposite effect occurs due to NTC phenomenon. Finally, when temperature is very high, i.e. 1200 K, both ozone and pressure have little influence on the percentage reduction of IDT.