Low temperature autoignition of 5-membered ring naphthenes: Effects of substitution

Abstract

The development and design of future internal combustion engines requires fundamental understanding and the capability to model the autoignition and pollutant formation behavior of petroleumbased and other fuels. Naphthenes are an important constituent of gasoline, and they can comprise larger portions of unconventionally-derived gasoline. There is a lack of data and validated models for 5-membered ring naphthenes. In this work, the autoignition characteristics of cyclopentane, and two of its substituted analogues, methylcyclopentane, and ethylcyclopentane are investigated using a twin-piston rapid compression machine. Each fuel is studied at engine-representative conditions: 20, 50 bar and 700-980 K, with mixtures containing stoichiometric fuel / oxygen ratios at various extents of dilution with inert gases.Negative temperature coefficient (NTC) behavior is observed for cyclopentane, though first-stage ignition and associated low temperature heat release behavior are only evident at temperatures below that for the transition to NTC. Pressure is found to have a larger impact on the reactivity than oxygen dilution, with both effects amplified in the NTC region. The cyclopentane experiments in this study are challenged by the sensitivity of this molecule to non-uniform, or mild ignition phenomena within the NTC region. The addition of saturated sidechains in methyl-and ethylcyclopentane significantly increases the reactivity of the molecules, especially at low temperature and NTC conditions. At the highest temperatures though, there is little difference between the three naphthenes. Typical two-stage ignition behavior is observed across a wide range of temperatures for these alkyl cyclopentanes with no mild ignition observed within the NTC region.A recently developed model for cyclopentane is extended to include reactions for methylcyclopentane, and this is used to simulate the new experiments. The simulation results indicate that low temperature reactivity of cyclopentane is dominated by HO2 elimination of the RO2 species producing cyclopentene, and this inhibits autoignition since it is a very stable molecule. When a methyl group is substituted on the ring, additional RO2 isomerization pathways are available, and these substantially increase the fuel reactivity. HO2 elimination is also important with methylcyclopentane, and this leads to significant production of cyclic olefins 3 which can further react to produce diolefins. These findings are consistent with observations that have been made in other experimental apparatuses.

An experimental and kinetic modeling study of the ignition delay and heat release characteristics of a five component gasoline surrogate and its blends with iso‐butanol within a rapid compression machine

“…There is little real‐world experimental data from fundamental experiments (such as from RCMs) available, which could help to quantify preliminary exothermicity events such as LTHR and ITHR. Heat release analyses such as those produced in this study and others 54,82,83 provide an opportunity for this and for the improvement of kinetic models by serving as an additional validation target. The current work shows that the chosen model fails to reproduce the experimentally derived heat release profiles to lesser or greater extents in different temperature regions, and that this failure may underlie further global model failures, such as the overprediction of reactivity.…”

Oxidation and pyrolysis of methyl propyl ether

“…Argonne National Laboratory's (ANL) twin‐piston rapid compression machine (tpRCM) was used to acquire autoignition data for MPE at elevated temperatures and pressures. A detailed overview of the configuration and recent modifications to the tpRCM, as well as uncertainties associated with experimental measurements can be found in, and are briefly described here 24 . The tpRCM, which can be operated as single‐, or two‐piston actuation, is pneumatically driven and hydraulically controlled.…”

“…Nonreactive mixtures are used to estimate the s. An uncertainty analysis associated with ANL's tpRCM is documented in Fridlyand et al., 24 leading to 1.0% in conservative estimate to . Heat release rates (HRRs) and integrated, or accumulated heat release (aHR) are calculated as described in Goldsborough et al., 28 where again, the nonreactive measurements are used to estimate conductive and enthalpic losses to the reaction chamber walls and piston crevices, respectively.…”

Heat release analysis for rapid compression machines: Challenges and opportunities

“…Autoignition measurements are acquired in Argonne's twin-piston RCM (tpRCM). A detailed overview, as well as experimental uncertainties can be found elsewhere [21]; it is briefly described here. The tpRCM is pneumatically-driven and hydraulically controlled.…”