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EGR (Exhaust Gas Recirculation) is adopted to reduce NOx emission in spark ignition engines. The effects of dilution with CO₂ and N₂, the main components of EGR, on laminar flame were investigated. It had been found that CO₂ and N₂ dilutions affected not only laminar burning velocity but also the response of flame to the flame stretch due to the change in the thermo-diffusive properties of the mixtures by the dilutions. Thus, turbulent flames were also investigated at fixed flame stretch factors to examine solely the thermo-diffusive effects on turbulent flame.

EGR (Exhaust gas recirculation) can reduce the pumping loss and improve the thermal efficiency of spark ignition engines. The techniques for combustion enhancement under high EGR rate condition has been required for further improvement of the thermal efficiency. In order to develop the technique of combustion enhancement by turbulence, the influences of turbulence scale on combustion properties, such as probability of flame propagation, EGR limit of flame propagation, flame quenching and combustion duration were investigated under the condition of same turbulence intensity. Experiments were carried out for stoichiometric spherically propagating turbulent i-C8H18/Air/N2 flames using a constant volume vessel. It was clarified that all of these combustion properties were affected by the turbulence scale. The development of spherically propagating turbulent flame during flame propagation was affected by the turbulence scale.