Shock tube studies of ethanol preignition

Abstract

CitationFigueroa-Labastida M, Badra J, Elbaz AM, Farooq A (2018) Shock tube studies of ethanol preignition. . Available: http://dx. AbstractUnderstanding premature ignition or preignition is of great importance as this phenomenon influences the design and operation of internal combustion engines. Preignition leading to super-knock restricts the efficiency of downsized boosted engines. To gain a fundamental understanding of preignition and how it affects an otherwise homogeneous ignition process, a shock tube may be used to decipher the influence of fuel chemical structure, temperature, pressure, equivalence ratio and bath gas on preignition. In a previous work by Javed et al.[1], ignition delay time measurements of nheptane showed significantly expedited reactivity compared to well-validated chemical kinetic models in the intermediate-temperature regime. In the current work, ethanol is chosen as a representative fuel that, unlike n-heptane, does not exhibit negative temperature coefficient (NTC) behaviour. Reactive mixtures containing 2.9% and 5% of ethanol at equivalence ratios of 0.5 and 1 were used for the measurement of ignition delay times behind reflected shock waves at 2 and 4 bar. Effect of bath gas was studied with mixtures containing either Ar or N2. In addition to conventional side-wall pressure and OH* measurements, a high-speed imaging setup was utilized to visualize the shock tube crosssection through a transparent quartz end-wall. The results suggest that preignition events are more likely to happen in mixtures containing higher ethanol concentration and that preignition energy release is more pronounced at lower temperatures. High-speed imaging shows that low-temperature ignition process is usually initiated from an individual hot spot that grows gradually, while hightemperatures ignition starts from many spots simultaneously which consume the reactive mixture almost homogeneously.

Dual-camera high-speed imaging of the ignition modes of ethanol, methanol and n-hexane in a shock tube

“…A correlation between the appearance of ignition non-idealities and thermal diffusivity of mixtures was first introduced by Walton et al [25] in their study of iso-octane ignition. In our recent work on ethanol [7], we could not find a conclusive evidence that thermal diffusivity is the primary factor to describe the preignition tendency of ethanol. A good correlation was found with the laminar flame thickness (δ) of various mixtures, based on the criterion proposed by Kalghatgi et al [20], wherein a mixture with smaller δ would be more prone to sustain ignition in the presence of a hot spot.…”

“…The dual-camera imaging experiments were performed to study ignition modes of various fuels over a range of experimental conditions. Two alcohol fuels, methanol and ethanol, have recently been studied in the context of non-homogenous ignition [3,4,7,13,[20][21][22][23][24]. Here, we have investigated the ignition of these two alcohols with the help of our unique imaging diagnostic.…”

“…The driver and driven sections of this shock tube are each 9.1 m long, separated by a polycarbonate diaphragm, with an inner diameter of 14.2 cm. Facility and instrumentation details are described previously [7]. Methanol (99.8%), ethanol (> 99.8%) and n-hexane (99%) were obtained from Sigma-Aldrich; oxygen and argon (99.999%) were supplied by Air Liquide.…”

“…Ethanol has been identified as a fuel with high tendency to undergo preignition due to its high burning velocity, thus low laminar flame thickness [13,20] and this has been observed in shock tube [7,13,21] and rapid compression machine experiments [22,23] in the form of expedited IDTs and the visualization of non-homogeneous ignition. A correlation between the appearance of ignition non-idealities and thermal diffusivity of mixtures was first introduced by Walton et al [25] in their study of iso-octane ignition.…”

“…In the light of observed non-idealities in ignition measurements using shock tubes, high-speed imaging has been applied to identify non-ideal events and determine possible mechanisms of such sightings. Our previous work on ethanol mixtures [7] utilized high-speed imaging from a transparent endwall to understand ignition modes of ethanol at different conditions, where inhomogeneous ignition was observed at lower temperatures and higher ethanol concentration. Other shock tube endwall imaging studies are found in literature.…”