Impact of CO ₂ Dilution on Ignition Delay Times of Iso-Octane at 15% and 30% Dilution Levels in a Rapid Compression Machine

Iso-Octane (2,2,4-trimethlypentane) is an important gasoline primary reference fuel (PRF) surrogate. Auto ignition of iso-octane was examined using a rapid compression machine (RCM) with iso-octane, air and carbon dioxide (CO₂) mixtures. Experiments were conducted over a temperature range of 650K-900K at 20bar and 10 bar compressed conditions for equivalence ratios (Φ =) 0.6, 0.8, 1.0 and 1.3. CO₂ dilution by mass was introduced at 0%, 15% and 30% levels with the O₂:N₂ mole ratio fixed at 1:3.76 emulating the exhaust gas recirculation (EGR) substitution in spark ignition (SI) engines. In this study the direct test chamber (DTC) approach is used for introducing iso-octane directly into the RCM test chamber via a direct injector. The results using this approach are compared with other RCM data available in the literature at undiluted Φ = 1.0 and 20 bar compressed pressure and show good agreement. For a given equivalence ratio, the negative temperature coefficient (NTC) region was fixed irrespective of the dilution levels confirming the fact that CO₂ does not participate in the chemistry of the base fuel but rather reduces the reactivity leading to increased ignition delay times. At 30% dilution levels the increase in ignition delay times is more than twice that of the 15% dilution levels for the same compressed conditions and stoichiometry.