Ignition delay measurements of four component model gasolines exploring the impacts of biofuels and aromatics

Abstract

This study explores the impacts of combinations of biofuel (ethanol, isobutanol and 2-methyl furan) and aromatic (toluene) compounds in a four component fuel blend, at fixed research octane number (RON) on ignition delay measured in an advanced fuel ignition delay analyzer (AFIDA 2805). Ignition delay measurements were performed over a range of temperatures from 400 to 725 °C (673 to 998 K) and two chamber pressures of 10 and 20 bar. The four component mixtures are compared to primary reference fuels at RON values of 90 and 100. The ignition delay measurements show that as the aromatic and biofuel concentrations increased, two stage ignition behavior was suppressed, at both initial chamber pressures. But both RON 100 (isooctane) and RON 90 reference fuels showed two stage ignition behavior, as did fuel mixtures with low biofuel and aromatic content. RON 90 fuels showed stronger two stage ignition behavior than RON 100 fuels, as expected. Depending on the type of biofuel in the mixture, the ignition delay at low chamber temperatures could be far greater than for the reference fuels. In particular, for the RON 100 mixtures at either 10 or 20 bar initial chamber pressure, the ignition delay at 400 °C (673 K) for the high level blend of 2-methyl furan and toluene (30 vol% of each) exhibited an ignition delay that was 10 times longer than for neat isooctane. The results show the strong non-linear octane blending response of these three biofuel compounds, especially in concert with the kinetic antagonism that toluene is known to display in mixtures with isooctane. These results have implications for the formulation of biofuel mixtures for spark ignition and advanced compression ignition engines, where this non-linear octane blending response could be exploited to improve knock resistance, or modulate the autoignition process.

Introduction

Coupled development of fuel formulation and combustion processes in internal combustion engines has become a topic of worldwide interest, with the intention of achieving higher thermal efficiency and lower greenhouse gas emissions [1,2]. In parallel, there has been an emerging interest in high octane fuels, which can also lead to higher efficiency spark ignition engines, by enhancing knock resistance [3]. Some current and emerging biofuel compounds can provide significant enhancement of octane number or octane index, thereby yielding fuel conversion efficiency improvements in optimized engines [4,5]. Ethanol is the most widely used biofuel and is used to fulfill oxygenate mandates and to boost octane number. Other compounds show promise as next generation biofuels, such as biobutanol, which can be produced from cellulosic biomass and possesses many advantages over ethanol, such as lower water solubility [4]. For more than a decade [6], bioderived furan fuels have been examined, because they have the potential to be produced from cellulosic feedstocks with relatively low process energy required. Among the furans, 2-methyl furan (2-MF) is commercially available and shows promise as a biofuel, as indicated by recent interest in the oxidation behavior of 2-MF as outlined below.

While the impacts of ethanol and butanol on knock resistance and autoignition have been studied extensively, studies of emerging compounds such as furanic species like 2-MF are far less extensive. Sirjean et al. reported shock tube studies of 2,5-dimethyl furan and published a detailed reaction mechanism for its oxidation [7]. Somers et al. reported ignition delay times, laminar burning velocity and a detailed kinetic mechanism for the oxidation of 2-MF [8]. Akih-Kumgeh and co-workers have reported on oxidation of furans and alkyl furans in shock tube studies to measure ignition delay [9,10].

While a potential fuel may exhibit desirable combustion properties in neat form, the behavior in the presence of other representative fuel compounds must also be assessed, since blending response may be nonlinear [4]. Kalaskar et al. used three component gasoline mixtures to explore ignition behavior of ternary blends of PRF fuel mixtures (n-heptane and isooctane) with addition of toluene, ethanol or isobutanol [11]. These surrogate fuel mixtures were examined to better understand gasoline compression ignition combustion, with an interest in defining surrogate fuel mixtures to represent gasoline-biofuel blends. However, Pitz et al. [12] have suggested that there are three necessary components required to adequately define a gasoline surrogate: n-heptane (representing n-alkanes), isooctane (representing isoalkanes), and toluene (representing aromatics). The autoignition behavior of these reference fuels has been studied in depth, and detailed kinetic models are available to describe their ignition behavior (n-heptane [13], isooctane [14] and toluene [15]).

The present work extends upon Kalaskar et al., by using four component surrogate mixtures, to compare ignition behavior of model gasolines containing toluene and several biofuel compounds: ethanol, isobutanol and 2-methyl furan. These studies were undertaken using a new instrument for spray ignition studies, which has extensive improvements in testing capabilities relative to existing ASTM standard instruments for measurement of ignition delay and derived cetane number [16,17]. As such, these experimental results are novel and because recent work has demonstrated good agreement between 0-D modeling and ignition delay measurements from the new instrument, these data offer an opportunity for mechanism validation and generation of valuable combustion insights. Specifically, these experimental results demonstrate how the autoignition of these biofuel compounds compares with primary reference fuels, and PRFs plus toluene, and how they interact with PRF compounds. These results have direct relevance to work on advanced compression ignition combustion, as well as, boosted direction injection spark ignition combustion.