Impact of CO2 dilution on ignition delay times of iso-octane at 15% and 30% dilution levels in a rapid compression machine 2019-01-0569

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 charge (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.