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Measured and Predicted Vapor Liquid Equilibrium

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Measured and Predicted Vapor Liquid Equilibrium NREL/CP-5400-71336. Posted with permission. Presented at WCX18: SAE World Congress Experience, 10-12 April 2018, Detroit, Michigan. 2018-01-0361 Published 03 Apr 2018 Measured and Predicted Vapor Liquid Equilibrium of Ethanol-Gasoline Fuels with Insight on the Influence of Azeotrope Interactions on Aromatic Species Enrichment and Particulate Matter Formation in Spark Ignition Engines Stephen Burke Colorado State University Robert Rhoads University of Colorado Matthew Ratcliff and Robert McCormick National Renewable Energy Laboratory Bret Windom Colorado State University Citation: Burke, S., Rhoads, R., Ratcliff, M., McCormick, R. et al., “Measured and Predicted Vapor Liquid Equilibrium of Ethanol-Gasoline Fuels with Insight on the Influence of Azeotrope Interactions on Aromatic Species Enrichment and Particulate Matter Formation in Spark Ignition Engines,” SAE Technical Paper 2018-01-0361, 2018, doi:10.4271/2018-01-0361. Abstract the azeotrope interactions on the vapor/liquid composition relationship has been observed between increasing evolution of the fuel, distillations were performed using the ethanol content in gasoline and increased particulate Advanced Distillation Curve apparatus on carefully selected Amatter (PM) emissions from direct injection spark samples consisting of gasoline blended with ethanol and heavy ignition (DISI) vehicles. The fundamental cause of this obser- aromatic and oxygenated compounds with varying vapor pres- vation is not well understood. One potential explanation is sures, including cumene, p-cymene, 4-tertbutyl toluene, that increased evaporative cooling as a result of ethanol’s high anisole, and 4-methyl anisole. Samples collected during the HOV may slow evaporation and prevent sufficient reactant distillation indicate an enrichment of the heavy aromatic or mixing resulting in the combustion of localized fuel rich oxygenated additive with an increase in initial ethanol concen- regions within the cylinder. In addition, it is well known that tration from E0 to E30. A recently developed distillation and ethanol when blended in gasoline forms positive azeotropes droplet evaporation model is used to explore the influence of which can alter the liquid/vapor composition during the dilution effects versus azeotrope interactions on the aromatic vaporization process. In fact, it was shown recently through a species enrichment. The results suggest that HOV-cooling numerical study that these interactions can retain the aromatic effects as well as aromatic species enrichment behaviors should species within the liquid phase impeding the in-cylinder be considered in future development of predictive indices to mixing of these compounds, which would accentuate PM forecast the PM potential of fuels containing oxygenated formation upon combustion. To better understand the role of compounds with comparatively high HOV. Introduction A uniform mixture is less likely to auto-ignite (i.e. engine n most internal combustion engines (ICEs) the complex knock) and results in minimized emissions of soot, NOx, and energy conversion process begins with the injection of a unburned hydrocarbons (UHC) [2, 3]. In direct injection Iliquid fuel followed by the atomization and vaporization spark ignition (DISI) engines, the fuel is injected directly into of the fuel, which then must mix with air before finally an air filled combustion chamber rather than in an intake reacting via combustion converting the chemical energy of port. DISI has advantages stemming from evaporative cooling the reactants into heat and then work. Each step in this ther- effects that can provide increased resistance to knock and modynamic process plays an important role in ensuring that allow for higher mass loading of reactants into the cylinder the chemical energy is converted to work efficiently with creating additional boost and fuel efficiency. However, to minimal emission of harmful pollutants. In spark ignition ensure a high degree of mixing between the reactants prior (SI) ICEs it is critical for the fuel and air to mix prior to the to combustion, the spray must vaporize quickly and completely initiation of the spark which commences combustion. before the mixture is ignited. As such, in modern engines that © 2018 SAE International; National Renewable Energy Laboratory. 2 MEASURED AND PREDICTED VAPOR LIQUID EQUILIBRIUM OF ETHANOL-GASOLINE FUELS employ these injection strategies, the spray and evaporation Ethanol is by far the most prolific biofuel, constituting 79.8% process can play an enormous role on the engine performance of the U.S. biofuel energy consumption, and currently provides and emissions, especially particulate matter (PM) emissions. 6.4% of the energy consumed by gasoline vehicles [15]. Some Motor vehicles emit fine particulate matter (PM) as studies have shown that ethanol can reduce lifecycle green- products of incomplete combustion. PM emissions are house gas emissions compared to gasoline [16, 17], and that primarily composed of soot and soluble organic compounds increasing ethanol content in gasoline blends would provide which can be toxic and contribute to global warming [4, 5, 6, a decrease in greenhouse gas emissions. 7]. Mitigating PM emissions from gasoline (SI) engines has Studies have demonstrated increased knock resistance recently become an important issue and challenge. The two for intermediate gasoline ethanol blends [18, 19, 20, 21]. key aspects of a fuel which define its propensity to form PM Chupka et al. found that research octane number (RON) in an SI engine are (1) its chemical pathways or kinetics which increases with ethanol content, but diminishing returns were lead to soot and (2) its volatility which effects its ability to mix observed for ethanol concentrations exceeding 30% by volume with the air [8, 9]. Fuels which have a high C/H ratio provide [18, 22]. This makes E30 of particular interest, as it requires a greater tendency to produce PM because of their tendency less ethanol to produce than high ethanol blends (E50 to E85), to form polycyclic aromatics hydrocarbons (PAHs), especially while achieving most of the knock-resistance benefits. These the case for molecules in the aromatic hydrocarbon moiety results are mirrored by dynamometer experiments by Leone [10]. Sooting indices, such as the threshold sooting index (TSI) et al. demonstrating that splash blended E30 could improve and the yield sooting index (YSI) have been developed to compression ratios substantially compared to E10 (from 10:1 identify the chemical potential of a particular species to form to 13:1 for the 3.5 L turbocharged DISI engine tested) [23]. soot/PM [11, 12]. However, even compounds which have a low Despite the increased compression ratio, the splash-blended TSI or YSI value can still form PM if combusted in a rich E30 allowed for higher mean effective pressure and manifold environment resulting from slowed evaporation and improper pressure across the entire engine speed range. Note that match mixing. As such, modern DISI engines, where time for the air blending for RON allowed for no benefits to compression ratio, and fuel to mix is reduced compared to port injection strate- mean effective pressure, or manifold pressure, implying that gies, tend to exhibit higher PM emissions [13]. For hydro- they are a result of the anti-knock properties of ethanol. They carbon based fuels these two behaviors can be estimated by estimated that this change would result in a 6-9% improve- relatively simple to measure properties and combined to ment to CO2 emissions with little change to MPG fuel predict the fuel’s potential to form PM in standard engines. economy, which was improved over E10 in some cases. Other Through empirical methods, the Particulate Mass Index E30 experiments with the same engine yielded similar (PMI) was derived which is used to predict the PM potential results [24]. of a fuel used in SI engines. In general, the PMI is a calculation In addition to the knock resistance effects of ethanol which uses the fuel composition and the fuel’s individual described by the RON, which is measured with a carbureted constituent’s properties such as volatility and fuel structure engine [22], further knock resistance is provided in a DISI to predict the fuel’s potential to generate PM in SI engines engine as a result of charge cooling, which is enhanced by the [14]. More specifically, PMI is a mass weighted function of the high HOV of ethanol [18, 19, 21, 25]. At 25 °C, the HOV for volatility and double bond equivalence (DBE) of each species ethanol is 924 kJ/kg while that of gasoline ranges from in the mixture (Eq. 1). The DBE, which is a marker of a mole- 350-400 kJ/kg [26]. Kasseris et al. found that the charge cule’s carbon bond saturation and C/H ratio, represents the cooling effect of direct injection was equivalent to a 14 °C chemical potential of that particular component to form PM cooler intake temperature when compared to port injection whereas the vapor pressure at 443 K, Pv (443 K), is used to for E0 [21]. In the case of E50, this effect was greater than a describe the constituent’s ability to evaporate and mix in a 30 °C difference in intake temperature. This cooling effect was timely manner. Aikawa et al. validated the PMI method of equivalent to increasing the fuel octane by 5 for E0 and by 13 predicting PM using port injected vehicles [14]. Khalek et al. for E50 (effective octane number (RON + MON)/2), demon- validated the method with DISI vehicles and three pump gaso- strating that DISI engines are uniquely able to leverage the lines, one of which contained ethanol [9]. Though PMI has properties of intermediate ethanol blends to reduce knock proven to be effective at predicting a fuel’s potential to form and increase effective compression ratios 27[ ]. PM, especially when the fuel contains no or little (less than While E30 has not been determined to be suitable for 15%) oxygenates, PMI has demonstrated less consistency to existing gasoline engines [28], it is being investigated to predict the PM emission potential of fuels which enhance the performance of purpose-built engines.
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