Impact of CO ₂ Dilution on Ignition Delay Times of Full Blend Gasolines in a Rapid Compression Machine 2021-01-1199

Autoignition delay times of two full blend gasoline fuels (high and low RON) were explored in a rapid compression machine. CO₂ dilution by mass was introduced at 0%, 15%, and 30% levels with the O₂:N₂ mole ratio fixed at 1:3.76. This dilution strategy is used to represent exhaust gas recirculation (EGR) substitution in spark ignition (SI) engines by using CO₂ as a surrogate for major EGR constituents(N₂, CO₂, H₂O). Experiments were conducted over the temperature range of 650K-900K and at 10 bar and 20 bar compressed pressure conditions for equivalence ratios of (Φ =) 0.6-1.3. The full blend fuels were admitted directly into the combustion chamber for mixture preparation using the direct test chamber (DTC) approach. CO₂ addition retarded the autoignition times for the fuels studied here. The retarding effect of the CO₂ dilution was more pronounced in the NTC region when compared to the lower and higher temperature range. The effect of dilution was more pronounced for the higher RON fuel in comparison to the lower RON fuel. With CO₂ dilution, ignition delay times of the low RON fuel matched the undiluted high RON fuel delay times showcasing octane relaxation potential. The ignition delay times of the gasoline fuels were compared with that of iso-octane, a major component in gasoline surrogates, at both diluted and undiluted conditions. The CO₂ dilution did not affect the ignition delay curve of the fuels or iso-octane but rather shifted it horizontally, hence no significant chemical reactivity due to CO₂ dilution was observed in this study.