Effect of Exhaust Gas Recirculation and NO on Ignition Delay Times of Iso-octane in a Rapid Compression Machine

The
ignition delay times (IDTs) of iso-octane with or without exhaust
gas recirculation (EGR) in the 600–950 K temperature range
and at pressures of 20 and 30 bar were measured under various equivalence
ratios (Φ = 0.6–1.4) with the aid of a rapid compression
machine. As for neat iso-octane IDTs measurements at 20 bar, it was
found that the IDTs obtained in this work were consistent with the
ones measured by other groups. Notable prolonging effects can be seen
at the negative temperature coefficient region with EGR, which is
independent of equivalence ratios. Moreover, the IDTs with EGR under
fuel-rich and stoichiometric conditions showed a similar trend at
both 20 and 30 bar. In the presence of NO in EGR, at a temperature
of 632 K and temperatures above 732 K, the promoting effect of NO
was observed, resulting in reductions of the IDTs. In parallel, the
influence of NO was negligible in the 667–732 K temperature
range. A comprehensive detailed kinetic model was employed to interpret
the experimental data. The model performance was improved by adopting
a high-level theoretical calculation of the 2HO2 = H2O2 + O2 reaction. Merging the N submechanism
into the core model by considering the interactions between NOx and C0–C2 species
can generally capture the influence of NO on the IDTs. Reaction path
analysis indicated that the NO + HO2 = NO2 +
OH reaction dominated in the presence of NO and its reaction rate
increased as a function of temperature, which can explain the different
IDTs behaviors with NO addition.