Chemical Ignition Characteristics of Ethanol Blending with Primary Reference Fuels

Abstract

The synergistic octane blending behavior of ethanol with gasoline and its surrogates has been observed by many researchers. The non-linear octane boosting tendency is observed at mid and high molar blends of ethanol in primary references fuels. The present work aims to provide chemical insight into this non-linear blending behavior of ignition processes when ethanol is blended with primary reference fuels. To this end, ignition delay time calculations, using a wellvalidated mechanism, were performed for several fuel blends of iso-octane, n-heptane, and ethanol. Temperature and pressure values were found, correlating experimentally measured octane numbers and simulated homogenous batch reactor ignition delay times. The temperature and pressure conditions obtained, were then used to study the evolution of heat release and reactivity before the onset of auto-ignition in a homogeneous premixed reactor. Markers of low and high temperature reactivity (OH and HO2) were analyzed for various molar blends of n-heptane with ethanol/iso-octane. Ethanol was observed to be better at radical scavenging than iso-octane at higher mole fraction. A computational singular perturbation analysis was conducted for a selection of blends to clarify the reactions responsible for the synergistic blending behavior of ethanol in nheptane. The role of the H-abstraction reactions was highlighted during the first ignition stage; reactions related to n-heptane were found to compete with the H-abstraction reactions of iso-octane or ethanol. Notably, the H-abstraction path of ethanol was more favored than that of the iso-octane, as a result of the smaller activation energies of the related reactions in the ethanol. The competition of the H-abstraction paths resulted in a smaller radical pool in the n-heptane-iso-octane-air case, and an even smaller pool in the n-heptane-ethanol-air. In all the cases considered, the second stage was dominated mainly by hydrogen-related reactions, regardless of the initial mixture, with the H2O2 (+ M) → 2OH (+ M) and H + O2 → O + OH playing the most important roles. This work employed a novel approach to examine specific reactions responsible for auto-ignition in ethanol blends, which can be used for fuel design, primarily around the generation/consumption of radical pool intermediates by interaction with fuel components.

Impact of ethanol on oxidation of iso-octane at low and intermediate temperatures

“…The kinetic mechanism causing the inhibition effect, however, has not been fully explored. It is noted that similar inhibition effects have been reported for ethanol/n-heptane and ethanol/PRF mixtures [18][19][20][21][22][23], and the effects have been attributed to the reduced OH formation due to ethanol blending. This mechanism needs to be examined for ethanol/iso-octane mixtures since both iso-octane and ethanol have much lower reactivity than n-heptane and neither produces a significant amount of OH during low temperature oxidation.…”

Autoignition behavior of gasoline/ethanol blends at engine-relevant conditions

“…Although it has been recently found that computational singular perturbation can be utilized to enable systematic analysis of the specific reactions responsible for autoignition suppression with ethanol addition [13], the autoignition chemistry behind various blending effects is still not well understood.…”

Investigating Water Injection in Single-Cylinder Gasoline Spark-Ignited Engines at Fixed Speed

“…Ignition delay times calculated using homogeneous batch reactor simulations. Water is “added” to the premixed air-fuel quantity in varying ratio of water–fuel at initial pressure of 25 bar and initial temperature varied from 600 to 1000 K. Temperatures corresponding to RON and MON condition according to refs and are marked in dashed lines. Water injection impacts the reactivity in the NTC region.…”