- Email: [email protected]
Investigations on surrogate fuels for high-octane oxygenated gasolines
Fuel 90 (2011) 640–646
Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Investigations on surrogate fuels for high-octane oxygenated gasolines Guilherme B. Machado a,⇑, José E.M. Barros b, Sérgio L. Braga c, Carlos Valois M. Braga c, Edimilson J. de Oliveira a, Antonio H.M. da F.T. da Silva a, Leonardo de O. Carvalho a a
Petróleo Brasileiro S.A. – Petrobras/Research and Development Center, Av. Horácio Macedo, 950, Cidade Universitária – Ilha do Fundão, CEP: 21941-915, Rio de Janeiro – RJ, Brazil Department of Mechanical Engineering, Universidade Federal de Minas Gerais – UFMG, Av. Antonio Carlos, 6627, Campos Universitário – Pampulha, CEP: 31270901, Belo Horizonte – MG, Brazil c Department of Mechanical Engineering, Pontifícia Universidade Católica do Rio de Janeiro – PUC-RJ, Rua Marquês de São Vicente, 225, Gávea, CEP: 22453-900, Rio de Janeiro – RJ, Brazil b
a r t i c l e
i n f o
Article history: Received 13 April 2010 Received in revised form 27 September 2010 Accepted 13 October 2010 Available online 23 October 2010 Keywords: Surrogate fuel Gasoline Ethanol Engine test
a b s t r a c t Gasoline is a complex mixture that possesses a quasi-continuous spectrum of hydrocarbon constituents. Surrogate fuels that decrease the chemical and/or physical complexity of gasoline are used to enhance the understanding of fundamental processes involved in internal combustion engines (ICEs). Computational tools are largely used in ICE development and in performance optimization; however, it is not possible to model full gasoline in kinetic studies because the interactions among the chemical constituents are not fully understood and the kinetics of all gasoline components are not known. Modeling full gasoline with computer simulations is also cost prohibitive. Thus, surrogate mixtures are studied to produce improved models that represent fuel combustion in practical devices such as homogeneous charge compression ignition (HCCI) and spark ignition (SI) engines. Simplified mixtures that represent gasoline performance in commercial engines can be used in investigations on the behavior of fuel components, as well as in fuel development studies. In this study, experimental design was used to investigate surrogate fuels. To this end, SI engine dynamometer tests were conducted, and the performance of a high-octane, oxygenated gasoline was reproduced. This study revealed that mixtures of iso-octane, toluene, n-heptane and ethanol could be used as surrogate fuels for oxygenated gasolines. These mixtures can be used to investigate the effect of individual components on fuel properties and commercial engines performance. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction The simplest surrogate fuels for gasoline consist of single components. For instance, iso-octane is often used to represent gasoline, including when computational fluid dynamics CFD software is used for ICE development. Binary blends of n-heptane and iso-octane, which are primary reference fuels (PRF), are used to represent gasoline with variable octane numbers. Pitz et al. [1] has presented an extensive review about surrogate fuels, describing all possible components and classifying its importance and the availability of physical and thermochemical data. Their studies indicate that the three necessary components of any gasoline
Abbreviations: AKI, anti-knock index; ANOVA, analysis of variance; CFD, computational fluid dynamics; CFR, Cooperative Fuel Research; ECU, electronic control unit; FC, fuel consumption; HCCI, homogeneous charge compression ignition; ICE, internal combustion engine; MBT, maximum break torque; PRF, primary reference fuels; PROÁLCOOL, Brazilian Ethanol Program; SI, spark ignition; SFC, specific fuel consumption; WOT, wide open throttle. ⇑ Corresponding author. Tel.: +55 21 3865 3546/92429084. E-mail addresses: [email protected], [email protected] (G.B. Machado). 0016-2361/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2010.10.024
surrogate are n-heptane, iso-octane and toluene, as the latter is typically the most abundant aromatic compound in gasoline. There are several studies in the literature on the kinetic models of isooctane [2–4], n-heptane [5–9] and toluene [3,10–14]. For example, Andrae [15] presented a detailed kinetic model to describe the autoignition of surrogate fuels consisting of iso-octane, n-heptane, toluene, diisobutylene and ethanol. These studies were based on experiments conducted at different pressures and temperatures, as well as in different environments, including shock tubes, rapid compression machines and constant volume chambers. The experiments were conducted to reproduce various thermodynamic conditions, which represent different engine operating conditions. Due to the recent increase in the use of ethanol as a renewable gasoline additive in countries such as Brazil, USA and Europe, ethanol may be considered for use in surrogate fuels. Since the 1970s, Brazil has operated vehicles on 100% hydrated ethanol derived from sugar cane. At this time, the Brazilian Ethanol Program (PROÁLCOOL) was established; PROÁLCOOL is a government program that promotes the incorporation of ethanol into fuel matrices. Regular Brazilian gasoline contains 20–25% anhydrous ethanol by volume, which is required by law and varies depending on ethanol production and international market conditions [16].
641
G.B. Machado et al. / Fuel 90 (2011) 640–646
In the literature, the authors found few direct correlations between surrogate fuels behavior in more controlled experiments using shock tube and rapid compression machine and surrogate fuels ability to reproduce gasoline performance in commercial SI engines. Andrea [15] has presented how such a correlation could be done. However, the behavior of surrogate fuels in a modern ICE is not well understood. Thus, the objective of this study was to evaluate the ability of surrogate fuels to reproduce the performance of full gasoline in commercial engines and to determine the optimal amount of each component in a surrogate fuel for high-octane oxygenated gasolines.
Table 1 Fuel properties.
a
2. Surrogate fuels selection methodology
Fuel
Density (kg/m3)
LHV (kJ/kg)
Stoic. mass A/F ratio
AKI
A B C D E F G H I J Reference
758.3 770.2 768.7 735.9 747.8 746.3 780.6 779.1 713.6 782.1 733.1
38,712 38,506 38,531 39,117 38,898 38,924 38,331 38,354 39,546 38,307 38,913
13.04 12.95 12.95 13.22 13.12 13.13 12.87 12.87 13.41 12.86 13.13
90.6 96.9a 86.7 85.8 94.2 80.8 94.2a 82.7 79.7 102.0a 98.2a
RON estimated.
2.1. Component and concentration selections
2.2. Fuel properties
The significance of toluene as a component of gasoline has been confirmed by gas chromatography analyses of 10 different types of gasoline produced at different Brazilian refineries. Toluene appeared in all types of gasoline in concentrations above 1% by mass. All analyzed gasoline samples contained more than four hundred components, with only 20–25 components at concentrations greater than 1% by mass [17]. Similarly, Assis [18] determined that toluene was the main aromatic component in fifty gasoline samples from northeast Brazil. Furthermore, gas chromatography analyses conducted on high-octane Brazilian gasolines revealed that iso-octane and toluene were present in high concentrations. In addition to toluene and iso-octane, n-heptane is also an important component for controlling the octane rating. Therefore, 10 gasoline surrogate mixtures were prepared with different concentrations of iso-octane, n-heptane and toluene. Anhydrous ethanol was incorporated in all mixtures at a fixed concentration of 25% by volume. The concentration of each component was defined based on experimental design. Fig. 1 displays the volumetric fraction of each component in the surrogate fuels, normalized with respect to the surrogate fuel in the absence of ethanol. The surrogate fuels are represented by points in the triangular domain of possible mixtures.
The stoichiometric air/fuel ratio, density and lower heating value were calculated based on the C:H:O ratio of each mixture. The fuels were individually tested in a Cooperative Fuel Research CFR engine to determine the values of MON and RON, and to calculate the anti-knock index (AKI), which is the average of both measurements. The properties of surrogate fuels are presented in Table 1 and are within the typical range of ethanol oxygenated gasoline blends. However, the boiling curves of surrogate fuels are different when compared to gasoline, due to the reduced number of components in surrogate fuels. The reference fuel, a high-octane oxygenated gasoline, was composed of 18.9% aromatics, 52.3% paraffins, 2.2% olefins, 0.3% naphthenics, 25% ethanol and 1.3% unidentified hydrocarbons, by volume. 3. Engine test 3.1. Test cell preparation The engine utilized in this study was a Fiat Fire, 1.4 l, tetrafuel engine; this engine can run on pure gasoline, Brazilian gasoline (25% anhydrous ethanol by volume), various mixtures of Brazilian gasoline and hydrated ethanol (E0–E100), or natural gas. The main engine specifications are presented in Table 2. The original engine electronic control unit (ECU) was replaced with a MoTeC M800 programmable unit. The original narrow-band lambda sensor was replaced with a linear, wide-band Bosch LSU 4.0, which provided lambda information during the tests. The test cell was equipped with a Schenck W130 dynamometer, a model 735S AVL fuel mass flow meter, a model 753C AVL fuel temperature controller and an AVL PUMA OPEN automated system to record engine performance. The test cell provided ambient and engine conditions, including ambient pressure, temperature and humidity, engine oil, water, intake and exhaust gas temperature and pressure. Cylinder 1 also included an AVL GU 1 3Z-24 piezoelectric combustion pressure transducer and an AVL 365 angle encoder, which provided the combustion pressure at a specified crank-angle position, as well as knocking observations. An
Table 2 Specifications of Fiat Fire tetrafuel engine.
Fig. 1. The experimental design employed in this study, including the volume fraction of surrogate fuel components (normalized with respect to the surrogate fuel in the absence of ethanol).
Swept volume Number of cylinders Cylinder diameter Stroke Piston bore Compression ratio Valves per cylinder Camshaft
1368 cm3 4 in line 72 mm 84 mm 71.9 mm 10.35:1 2 1 (overhead)
642
G.B. Machado et al. / Fuel 90 (2011) 640–646
AVL IndiModul 621 data acquisition system was used to record engine combustion observations. The occurrence of knocking was further monitored by an audio system. 3.2. Test methodology The fuel mixtures were prepared by determining the mass of each component required to achieve the desired volumetric proportion at a temperature of 20 °C. Sixty liters of each mixture were prepared for fuel-line cleaning and test execution. Gas chromatography analyses were conducted on all mixtures, revealing a maximum difference of 0.4% between the desired and stated concentrations of each component present in the final mixture. Considering the amount of fuel necessary to perform engine tests and the logistics involved in the preparation of mixtures, the observed differences between the calculated and the
actual concentrations of the mixture components were considered acceptable. To cover a wide range of engine operating conditions, the tests were performed at six different operating points (speed and throttle position), as shown in Table 3.
Table 3 Engine operating conditions evaluated. Speed (rpm)
Load (% throttle position)
Lambda
5500 3875 3875 2250 2250 1500
100 100 16 100 16 16
0.9 0.9 1.0 0.9 1.0 1.0
Fig. 2. Engine torque results under wide open throttle (WOT) and partial-load conditions, presented as the percent difference with respect to the reference fuel. Error bars are related to measurement uncertainties (a–f).
G.B. Machado et al. / Fuel 90 (2011) 640–646
To reproduce common engine operating conditions, full and partial-load conditions were established for intermediate speeds, while partial- and full-load conditions were used with lower and higher speeds, respectively. Engine water cooling temperature was maintained at 90 ± 2 °C. Oil temperature remained stable at the maximum value for each engine operating point. As shown in Table 3, lambda was fixed at a value of 0.9 for full-load conditions (wide open throttle – WOT) and 1.0 for partial-load conditions. This is justified by the calibration carried out by engine manufacturers, which is conducted to optimize power at WOT and to reduce emissions under partial-load conditions. The lambda value for gasoline to achieve maximum power is typically within the range of 0.88–0.92, while lambda to achieve maximum catalyst efficiency and minimal emissions at partial-load is 1.0. To maintain
643
a constant lambda at each engine operating point, the tests were performed in a closed loop. Spark timing was varied according to fuel and engine operating condition to attain the maximum break torque (MBT), which was limited by the occurrence of knocking and a maximum exhaust gas temperature of 900 °C. A high-octane oxygenated gasoline was utilized as a reference fuel. It was not possible to test all fuels in the same day due to the logistics of fuel preparation and fuel-line cleaning requirements. Furthermore, the work required to evaluate the engine operating conditions investigated and to adequately calibrate them was time consuming. The reference fuel was tested under identical engine operating points and was repeated daily. Surrogate fuel results were compared to reference results obtained on the same day.
Fig. 3. Engine fuel consumption (FC) results under wide open throttle (WOT) and partial-load conditions, presented as the percent difference with respect to the reference fuel. Error bars are related to measurement uncertainties (a–f).
644
G.B. Machado et al. / Fuel 90 (2011) 640–646
At least three measurements of torque and fuel consumption were performed for each fuel after at least one minute of engine stabilization per engine condition. The observed torque was corrected to a reference condition of 25 °C and 99 kPa dry pressure, with a total pressure of 100 kPa and a vapor pressure of 1 kPa, in accordance with ISO 1585 [19]. Fuel temperature was maintained between 20 and 30 °C. Before tests were conducted, the effect of the position of the fuel injection point in relation to the position of the intake valve on engine performance was investigated. The tests were conducted with the reference fuel and an effect on the injection point was not observed. Thus, effects on engine performance due to differences in the boiling curves of surrogate fuels were minimal. The injection point was fixed at a crankshaft angle of 26° before the intake valve was opened.
4. Results and discussion Figs. 2–4 show the torque, fuel consumption and specific fuel consumption, respectively, of each surrogate fuel under all six engine conditions. Results are shown as the percent difference with respect to the reference fuel, and these percent differences were calculated from average values. Figs. 2–4 also show error bars related to the standard error of the percent difference. Error was associated with uncertainties related to measurement, equipment and uncontrolled variables associated with ambient and test cell conditions. An analysis of variance (ANOVA) was conducted to identify surrogate fuels that yielded results similar to those of the reference fuel. Table 4 displays the statistical equalities observed between surrogate fuels and the reference fuel, at a significance level of 0.05.
Fig. 4. Engine specific fuel consumption (SFC) results under wide open throttle (WOT) and partial-load conditions, presented as the percent difference with respect to the reference fuel. Error bars are related to measurement uncertainties (a–f).
G.B. Machado et al. / Fuel 90 (2011) 640–646
The results summarized in Table 4 were obtained from the statistical analysis of raw data. However, the data presented in Figs. 2–4 are related to the percent difference with respect to the reference fuel. Thus, direct comparisons between the data displayed in Figs. 2–4 and in Table 4 should be made with caution because the data provided in Figs. 2–4 show general trends, while Table 4 displays the statistical significance of comparisons between datasets. If tests with small differences were repeated, the results may be slightly different due to the extensive number of factors affecting this type of engine test. Nevertheless, general trends can be obtained from the data in Figs. 2–4 and Table 4. The absence of columns and error bars for certain fuels and conditions in Figs. 2–4 represent virtually equal performance to the reference fuel and insignificant standard errors, that could not be adequately displayed on the graphs. The engine has operated successfully using all the surrogate fuels tested. As shown in Figs. 2–4 and Table 4, fuels B and E performed most similarly to the reference fuel. Fuel B has an AKI that is most similar to the AKI of the reference fuel; fuels E and G have AKI values that are also similar to that of the reference fuel. Fuel J presented the highest AKI. The observed AKI similarities reflect the similarities in engine torque that is produced when these fuels (B, E, G, J and reference) are used. Further analysis of the data revealed that correlations between AKI and torque were prevalent in engine operating conditions where knocking often occurs, such as when the engine is under full-load conditions. This result revealed that torque reached an upper limit at elevated octane ratings. Alternatively, correlations were not observed under partial-load conditions, where knocking typically does not occur. Thus, other properties must be evaluated to characterize torque under partial-load conditions.
Table 4 Statistical equalities observed between surrogate fuels and the reference gasoline – ANOVA, a = 0.05. Surrogate fuel
A
B
C
D
E
F
G
H
I
J
Load
WOT
Speed (rpm)
2250
Torque FC SFC Torque FC SFC Torque FC SFC Torque FC SFC Torque FC SFC Torque FC SFC Torque FC SFC Torque FC SFC Torque FC SFC Torque FC SFC
Part-load 3875
5500
4
1500
2250
4
4
3875 4 4
4
4 4
645
Fuel E was the most similar to the reference fuel with respect to fuel consumption, which was associated with the stoichiometric air/fuel ratio. Correlations between stoichiometric air/fuel ratio and fuel consumption were observed under all engine conditions. As a general trend expected, a higher stoichiometric air/fuel ratio provided a lower fuel consumption. Specific fuel consumption was similar among fuels B, E and reference. The raw experimental data indicated a correlation between stoichiometric air/fuel ratio and specific fuel consumption in regimes where knocking typically does not occur. Thus, the stoichiometric air/fuel ratio can be a superior parameter for fuel classification under partial-load conditions, where knocking is not a limitation. An apparent correlation between lower heating value and torque was not observed. Thus, further analysis is necessary to identify a possible correlation between lower heating value and engine performance. 5. Conclusions There is considerable interest in identifying representative surrogate fuels for oxygenated gasolines used in commercial engines. Surrogates decrease the chemical and/or physical complexity of fuel and can be used to enhance the understanding of fundamental processes involved in commercial gasoline-powered ICEs. In the literature, the authors found few direct correlations between surrogate fuels behavior in more controlled experiments using shock tube and rapid compression machine and surrogate fuels ability to reproduce gasoline performance in commercial SI engines. The contribution of this paper is to start to fulfill this gap, identifying surrogate fuels that could represent gasoline performance in SI commercial engine to be further investigated. To identify suitable surrogates, SI engine tests were conducted under a typical range of engine operating conditions. Ten surrogate fuel formulations, based on iso-octane, n-heptane and toluene mixtures, were selected, and 25% ethanol by volume was added to each formulation. The results of this study revealed that fuels B (31.25% iso-octane, 31.25% toluene, 12.5% n-heptane and 25% ethanol) and E (37.5% iso-octane, 18.75% toluene, 18.75% n-heptane and 25% ethanol) could be used as surrogate fuels for high-octane oxygenated gasolines. Results suggested that the surrogate fuels studied can represent the performance of oxygenated gasolines in commercial engines. They could be used for further investigations on the combustion behavior of oxygenated gasolines, including the development of flame propagation models. Combustion pressure measurements can be utilized to develop a methodology for estimating fuel flame speed inside combustion chamber. Other statistically significant correlations among component concentration, fuel properties and engine performance should be further investigated to improve gasoline formulations. The results obtained can also help researchers in this area to select mixtures of components, which have more representative performance in SI commercial engine, to further investigate surrogate fuels. This can be done using more controlled experiments in shock tube and rapid compression machine to try to better understand the chemistry controlling ethanol blended hydrocarbons. The author’s intention is to perform further tests in a planar burner and in a shock tube that is under construction using the mixtures evaluated in this paper.
Torque – , fuel consumption (FC) – , specific fuel consumption (SFC) – 4 (marks mean statistical equality).
References [1] Pitz WJ, Cernansky NP, Dryer FL, Egolfopoulos FN, Farrell JT, Friend DG. et al. Development of an experimental database and chemical kinetic models for surrogate gasoline fuels. SAE Paper 2007-01-0175; 2007.
646
G.B. Machado et al. / Fuel 90 (2011) 640–646
[2] Curran HJ, Gaffuri P, Pitz WJ, Westbrook CK. A comprehensive modeling study of iso-octane oxidation. Combust Flame 2002;129:253–80. [3] Davidson DF, Gauthier BM, Hanson RK. Shock tube ignition measurements of iso-octane/air and toluene/air at high pressures. Proc Combust Inst 2005;30(1):1175–82. [4] He X, Donovan MT, Zigler BT, Palmer TR, Walton SM, Wooldridge MS, et al. An experimental and modeling study of iso-octane ignition delay times under homogeneous charge compression ignition conditions. Combust Flame 2005;142(3):266–75. [5] Curran HJ, Gaffuri P, Pitz WJ, Westbrook CK. A comprehensive modeling study of n-heptane oxidation. Combust Flame 1998;114(1–2):149–77. [6] Herzler J, Jerig L, Roth P. Shock tube study of the ignition of lean n-heptane/air mixtures at intermediate temperatures and high pressures. Proc Combust Inst 2005;30(1):1147–53. [7] Berta P, Aggarwal SK, Puri IK. An experimental and numerical investigation of n-heptane/air counterflow partially premixed flames and emission of NOx and PAH species. Combust Flame 2006;145(4):740–64. [8] Gauthier BM, Davidson DF, Hanson RK. Shock tube determination of ignition delay times in full blend and surrogate fuel mixtures. Combust Flame 2004;139(4):300–11. [9] Silke EJ, Curran HJ, Simmie JM. The influence of fuel structure on combustion as demonstrated by the isomers of heptane: a rapid compression machine study. Proc Combust Inst 2005;30(2):2639–47. [10] Bounaceur R, Costa ID, Fournet R, Billaud F, Battin-Leclerc F. Experimental and modeling study of the oxidation of toluene. Int J Chem Kinet 2005;37(1): 25–49.
[11] Dagaut P, Pengloan G, Ristori A. Oxidation, ignition and combustion of toluene: experimental and detailed chemical kinetic modeling. Phys Chem Chem Phys 2002;4(10):1846–54. [12] Emdee JL, Brezinsky K, Glassman I. A kinetic-model for the oxidation of toluene near 1200-K. J Phys Chem 1992;96(5):2151–61. [13] Pitz WJ, Seiser R, Bozzelli JW, Seshadri K, Chen CJ, Costa ID, et al. Chemical kinetic study of toluene oxidation under premixed and nonpremixed conditions. Lawrence Livermore National Laboratory, UCRL-CONF-201575; 2003. [14] Sivaramakrishnan R, Tranter RS, Brezinsky K. A high pressure model for the oxidation of toluene. Proc Combust Inst 2005;30(1):1165–73. [15] Andrae JCG. Development of a detailed kinetic model for gasoline surrogate fuels. Fuel 2008;87:2013–22. [16] ANP Resolution # 309. Brazil: ANP – Brazilian National Petroleum, Natural Gas and Biofuels Agency; 2001. [17] Machado GB. Simulação da Vaporização de Monocomponentes Presentes na Gasolina. Petrobras/CENPES Technical Rep DPM 007/07. Brazil: Petrobras/ Research and Development Center; 2007. [18] Assis JCR, Jaeger HV, Almeida SQ, Teixeira LSG, Pontes LAM, Sobrinho EV, et al. Caracterização dos Principais Componentes da Gasolina Automotiva através de Cromatografia Gasosa. Brazil: II Congresso Brasileiro de P&D em Petróleo e Gás; 2003. [19] ISO 1585. Road vehicles – engine test code – net power. Switzerland: International Organization for Standardization; 1992.
Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Investigations on surrogate fuels for high-octane oxygenated gasolines Guilherme B. Machado a,⇑, José E.M. Barros b, Sérgio L. Braga c, Carlos Valois M. Braga c, Edimilson J. de Oliveira a, Antonio H.M. da F.T. da Silva a, Leonardo de O. Carvalho a a
Petróleo Brasileiro S.A. – Petrobras/Research and Development Center, Av. Horácio Macedo, 950, Cidade Universitária – Ilha do Fundão, CEP: 21941-915, Rio de Janeiro – RJ, Brazil Department of Mechanical Engineering, Universidade Federal de Minas Gerais – UFMG, Av. Antonio Carlos, 6627, Campos Universitário – Pampulha, CEP: 31270901, Belo Horizonte – MG, Brazil c Department of Mechanical Engineering, Pontifícia Universidade Católica do Rio de Janeiro – PUC-RJ, Rua Marquês de São Vicente, 225, Gávea, CEP: 22453-900, Rio de Janeiro – RJ, Brazil b
a r t i c l e
i n f o
Article history: Received 13 April 2010 Received in revised form 27 September 2010 Accepted 13 October 2010 Available online 23 October 2010 Keywords: Surrogate fuel Gasoline Ethanol Engine test
a b s t r a c t Gasoline is a complex mixture that possesses a quasi-continuous spectrum of hydrocarbon constituents. Surrogate fuels that decrease the chemical and/or physical complexity of gasoline are used to enhance the understanding of fundamental processes involved in internal combustion engines (ICEs). Computational tools are largely used in ICE development and in performance optimization; however, it is not possible to model full gasoline in kinetic studies because the interactions among the chemical constituents are not fully understood and the kinetics of all gasoline components are not known. Modeling full gasoline with computer simulations is also cost prohibitive. Thus, surrogate mixtures are studied to produce improved models that represent fuel combustion in practical devices such as homogeneous charge compression ignition (HCCI) and spark ignition (SI) engines. Simplified mixtures that represent gasoline performance in commercial engines can be used in investigations on the behavior of fuel components, as well as in fuel development studies. In this study, experimental design was used to investigate surrogate fuels. To this end, SI engine dynamometer tests were conducted, and the performance of a high-octane, oxygenated gasoline was reproduced. This study revealed that mixtures of iso-octane, toluene, n-heptane and ethanol could be used as surrogate fuels for oxygenated gasolines. These mixtures can be used to investigate the effect of individual components on fuel properties and commercial engines performance. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction The simplest surrogate fuels for gasoline consist of single components. For instance, iso-octane is often used to represent gasoline, including when computational fluid dynamics CFD software is used for ICE development. Binary blends of n-heptane and iso-octane, which are primary reference fuels (PRF), are used to represent gasoline with variable octane numbers. Pitz et al. [1] has presented an extensive review about surrogate fuels, describing all possible components and classifying its importance and the availability of physical and thermochemical data. Their studies indicate that the three necessary components of any gasoline
Abbreviations: AKI, anti-knock index; ANOVA, analysis of variance; CFD, computational fluid dynamics; CFR, Cooperative Fuel Research; ECU, electronic control unit; FC, fuel consumption; HCCI, homogeneous charge compression ignition; ICE, internal combustion engine; MBT, maximum break torque; PRF, primary reference fuels; PROÁLCOOL, Brazilian Ethanol Program; SI, spark ignition; SFC, specific fuel consumption; WOT, wide open throttle. ⇑ Corresponding author. Tel.: +55 21 3865 3546/92429084. E-mail addresses: [email protected], [email protected] (G.B. Machado). 0016-2361/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2010.10.024
surrogate are n-heptane, iso-octane and toluene, as the latter is typically the most abundant aromatic compound in gasoline. There are several studies in the literature on the kinetic models of isooctane [2–4], n-heptane [5–9] and toluene [3,10–14]. For example, Andrae [15] presented a detailed kinetic model to describe the autoignition of surrogate fuels consisting of iso-octane, n-heptane, toluene, diisobutylene and ethanol. These studies were based on experiments conducted at different pressures and temperatures, as well as in different environments, including shock tubes, rapid compression machines and constant volume chambers. The experiments were conducted to reproduce various thermodynamic conditions, which represent different engine operating conditions. Due to the recent increase in the use of ethanol as a renewable gasoline additive in countries such as Brazil, USA and Europe, ethanol may be considered for use in surrogate fuels. Since the 1970s, Brazil has operated vehicles on 100% hydrated ethanol derived from sugar cane. At this time, the Brazilian Ethanol Program (PROÁLCOOL) was established; PROÁLCOOL is a government program that promotes the incorporation of ethanol into fuel matrices. Regular Brazilian gasoline contains 20–25% anhydrous ethanol by volume, which is required by law and varies depending on ethanol production and international market conditions [16].
641
G.B. Machado et al. / Fuel 90 (2011) 640–646
In the literature, the authors found few direct correlations between surrogate fuels behavior in more controlled experiments using shock tube and rapid compression machine and surrogate fuels ability to reproduce gasoline performance in commercial SI engines. Andrea [15] has presented how such a correlation could be done. However, the behavior of surrogate fuels in a modern ICE is not well understood. Thus, the objective of this study was to evaluate the ability of surrogate fuels to reproduce the performance of full gasoline in commercial engines and to determine the optimal amount of each component in a surrogate fuel for high-octane oxygenated gasolines.
Table 1 Fuel properties.
a
2. Surrogate fuels selection methodology
Fuel
Density (kg/m3)
LHV (kJ/kg)
Stoic. mass A/F ratio
AKI
A B C D E F G H I J Reference
758.3 770.2 768.7 735.9 747.8 746.3 780.6 779.1 713.6 782.1 733.1
38,712 38,506 38,531 39,117 38,898 38,924 38,331 38,354 39,546 38,307 38,913
13.04 12.95 12.95 13.22 13.12 13.13 12.87 12.87 13.41 12.86 13.13
90.6 96.9a 86.7 85.8 94.2 80.8 94.2a 82.7 79.7 102.0a 98.2a
RON estimated.
2.1. Component and concentration selections
2.2. Fuel properties
The significance of toluene as a component of gasoline has been confirmed by gas chromatography analyses of 10 different types of gasoline produced at different Brazilian refineries. Toluene appeared in all types of gasoline in concentrations above 1% by mass. All analyzed gasoline samples contained more than four hundred components, with only 20–25 components at concentrations greater than 1% by mass [17]. Similarly, Assis [18] determined that toluene was the main aromatic component in fifty gasoline samples from northeast Brazil. Furthermore, gas chromatography analyses conducted on high-octane Brazilian gasolines revealed that iso-octane and toluene were present in high concentrations. In addition to toluene and iso-octane, n-heptane is also an important component for controlling the octane rating. Therefore, 10 gasoline surrogate mixtures were prepared with different concentrations of iso-octane, n-heptane and toluene. Anhydrous ethanol was incorporated in all mixtures at a fixed concentration of 25% by volume. The concentration of each component was defined based on experimental design. Fig. 1 displays the volumetric fraction of each component in the surrogate fuels, normalized with respect to the surrogate fuel in the absence of ethanol. The surrogate fuels are represented by points in the triangular domain of possible mixtures.
The stoichiometric air/fuel ratio, density and lower heating value were calculated based on the C:H:O ratio of each mixture. The fuels were individually tested in a Cooperative Fuel Research CFR engine to determine the values of MON and RON, and to calculate the anti-knock index (AKI), which is the average of both measurements. The properties of surrogate fuels are presented in Table 1 and are within the typical range of ethanol oxygenated gasoline blends. However, the boiling curves of surrogate fuels are different when compared to gasoline, due to the reduced number of components in surrogate fuels. The reference fuel, a high-octane oxygenated gasoline, was composed of 18.9% aromatics, 52.3% paraffins, 2.2% olefins, 0.3% naphthenics, 25% ethanol and 1.3% unidentified hydrocarbons, by volume. 3. Engine test 3.1. Test cell preparation The engine utilized in this study was a Fiat Fire, 1.4 l, tetrafuel engine; this engine can run on pure gasoline, Brazilian gasoline (25% anhydrous ethanol by volume), various mixtures of Brazilian gasoline and hydrated ethanol (E0–E100), or natural gas. The main engine specifications are presented in Table 2. The original engine electronic control unit (ECU) was replaced with a MoTeC M800 programmable unit. The original narrow-band lambda sensor was replaced with a linear, wide-band Bosch LSU 4.0, which provided lambda information during the tests. The test cell was equipped with a Schenck W130 dynamometer, a model 735S AVL fuel mass flow meter, a model 753C AVL fuel temperature controller and an AVL PUMA OPEN automated system to record engine performance. The test cell provided ambient and engine conditions, including ambient pressure, temperature and humidity, engine oil, water, intake and exhaust gas temperature and pressure. Cylinder 1 also included an AVL GU 1 3Z-24 piezoelectric combustion pressure transducer and an AVL 365 angle encoder, which provided the combustion pressure at a specified crank-angle position, as well as knocking observations. An
Table 2 Specifications of Fiat Fire tetrafuel engine.
Fig. 1. The experimental design employed in this study, including the volume fraction of surrogate fuel components (normalized with respect to the surrogate fuel in the absence of ethanol).
Swept volume Number of cylinders Cylinder diameter Stroke Piston bore Compression ratio Valves per cylinder Camshaft
1368 cm3 4 in line 72 mm 84 mm 71.9 mm 10.35:1 2 1 (overhead)
642
G.B. Machado et al. / Fuel 90 (2011) 640–646
AVL IndiModul 621 data acquisition system was used to record engine combustion observations. The occurrence of knocking was further monitored by an audio system. 3.2. Test methodology The fuel mixtures were prepared by determining the mass of each component required to achieve the desired volumetric proportion at a temperature of 20 °C. Sixty liters of each mixture were prepared for fuel-line cleaning and test execution. Gas chromatography analyses were conducted on all mixtures, revealing a maximum difference of 0.4% between the desired and stated concentrations of each component present in the final mixture. Considering the amount of fuel necessary to perform engine tests and the logistics involved in the preparation of mixtures, the observed differences between the calculated and the
actual concentrations of the mixture components were considered acceptable. To cover a wide range of engine operating conditions, the tests were performed at six different operating points (speed and throttle position), as shown in Table 3.
Table 3 Engine operating conditions evaluated. Speed (rpm)
Load (% throttle position)
Lambda
5500 3875 3875 2250 2250 1500
100 100 16 100 16 16
0.9 0.9 1.0 0.9 1.0 1.0
Fig. 2. Engine torque results under wide open throttle (WOT) and partial-load conditions, presented as the percent difference with respect to the reference fuel. Error bars are related to measurement uncertainties (a–f).
G.B. Machado et al. / Fuel 90 (2011) 640–646
To reproduce common engine operating conditions, full and partial-load conditions were established for intermediate speeds, while partial- and full-load conditions were used with lower and higher speeds, respectively. Engine water cooling temperature was maintained at 90 ± 2 °C. Oil temperature remained stable at the maximum value for each engine operating point. As shown in Table 3, lambda was fixed at a value of 0.9 for full-load conditions (wide open throttle – WOT) and 1.0 for partial-load conditions. This is justified by the calibration carried out by engine manufacturers, which is conducted to optimize power at WOT and to reduce emissions under partial-load conditions. The lambda value for gasoline to achieve maximum power is typically within the range of 0.88–0.92, while lambda to achieve maximum catalyst efficiency and minimal emissions at partial-load is 1.0. To maintain
643
a constant lambda at each engine operating point, the tests were performed in a closed loop. Spark timing was varied according to fuel and engine operating condition to attain the maximum break torque (MBT), which was limited by the occurrence of knocking and a maximum exhaust gas temperature of 900 °C. A high-octane oxygenated gasoline was utilized as a reference fuel. It was not possible to test all fuels in the same day due to the logistics of fuel preparation and fuel-line cleaning requirements. Furthermore, the work required to evaluate the engine operating conditions investigated and to adequately calibrate them was time consuming. The reference fuel was tested under identical engine operating points and was repeated daily. Surrogate fuel results were compared to reference results obtained on the same day.
Fig. 3. Engine fuel consumption (FC) results under wide open throttle (WOT) and partial-load conditions, presented as the percent difference with respect to the reference fuel. Error bars are related to measurement uncertainties (a–f).
644
G.B. Machado et al. / Fuel 90 (2011) 640–646
At least three measurements of torque and fuel consumption were performed for each fuel after at least one minute of engine stabilization per engine condition. The observed torque was corrected to a reference condition of 25 °C and 99 kPa dry pressure, with a total pressure of 100 kPa and a vapor pressure of 1 kPa, in accordance with ISO 1585 [19]. Fuel temperature was maintained between 20 and 30 °C. Before tests were conducted, the effect of the position of the fuel injection point in relation to the position of the intake valve on engine performance was investigated. The tests were conducted with the reference fuel and an effect on the injection point was not observed. Thus, effects on engine performance due to differences in the boiling curves of surrogate fuels were minimal. The injection point was fixed at a crankshaft angle of 26° before the intake valve was opened.
4. Results and discussion Figs. 2–4 show the torque, fuel consumption and specific fuel consumption, respectively, of each surrogate fuel under all six engine conditions. Results are shown as the percent difference with respect to the reference fuel, and these percent differences were calculated from average values. Figs. 2–4 also show error bars related to the standard error of the percent difference. Error was associated with uncertainties related to measurement, equipment and uncontrolled variables associated with ambient and test cell conditions. An analysis of variance (ANOVA) was conducted to identify surrogate fuels that yielded results similar to those of the reference fuel. Table 4 displays the statistical equalities observed between surrogate fuels and the reference fuel, at a significance level of 0.05.
Fig. 4. Engine specific fuel consumption (SFC) results under wide open throttle (WOT) and partial-load conditions, presented as the percent difference with respect to the reference fuel. Error bars are related to measurement uncertainties (a–f).
G.B. Machado et al. / Fuel 90 (2011) 640–646
The results summarized in Table 4 were obtained from the statistical analysis of raw data. However, the data presented in Figs. 2–4 are related to the percent difference with respect to the reference fuel. Thus, direct comparisons between the data displayed in Figs. 2–4 and in Table 4 should be made with caution because the data provided in Figs. 2–4 show general trends, while Table 4 displays the statistical significance of comparisons between datasets. If tests with small differences were repeated, the results may be slightly different due to the extensive number of factors affecting this type of engine test. Nevertheless, general trends can be obtained from the data in Figs. 2–4 and Table 4. The absence of columns and error bars for certain fuels and conditions in Figs. 2–4 represent virtually equal performance to the reference fuel and insignificant standard errors, that could not be adequately displayed on the graphs. The engine has operated successfully using all the surrogate fuels tested. As shown in Figs. 2–4 and Table 4, fuels B and E performed most similarly to the reference fuel. Fuel B has an AKI that is most similar to the AKI of the reference fuel; fuels E and G have AKI values that are also similar to that of the reference fuel. Fuel J presented the highest AKI. The observed AKI similarities reflect the similarities in engine torque that is produced when these fuels (B, E, G, J and reference) are used. Further analysis of the data revealed that correlations between AKI and torque were prevalent in engine operating conditions where knocking often occurs, such as when the engine is under full-load conditions. This result revealed that torque reached an upper limit at elevated octane ratings. Alternatively, correlations were not observed under partial-load conditions, where knocking typically does not occur. Thus, other properties must be evaluated to characterize torque under partial-load conditions.
Table 4 Statistical equalities observed between surrogate fuels and the reference gasoline – ANOVA, a = 0.05. Surrogate fuel
A
B
C
D
E
F
G
H
I
J
Load
WOT
Speed (rpm)
2250
Torque FC SFC Torque FC SFC Torque FC SFC Torque FC SFC Torque FC SFC Torque FC SFC Torque FC SFC Torque FC SFC Torque FC SFC Torque FC SFC
Part-load 3875
5500
4
1500
2250
4
4
3875 4 4
4
4 4
645
Fuel E was the most similar to the reference fuel with respect to fuel consumption, which was associated with the stoichiometric air/fuel ratio. Correlations between stoichiometric air/fuel ratio and fuel consumption were observed under all engine conditions. As a general trend expected, a higher stoichiometric air/fuel ratio provided a lower fuel consumption. Specific fuel consumption was similar among fuels B, E and reference. The raw experimental data indicated a correlation between stoichiometric air/fuel ratio and specific fuel consumption in regimes where knocking typically does not occur. Thus, the stoichiometric air/fuel ratio can be a superior parameter for fuel classification under partial-load conditions, where knocking is not a limitation. An apparent correlation between lower heating value and torque was not observed. Thus, further analysis is necessary to identify a possible correlation between lower heating value and engine performance. 5. Conclusions There is considerable interest in identifying representative surrogate fuels for oxygenated gasolines used in commercial engines. Surrogates decrease the chemical and/or physical complexity of fuel and can be used to enhance the understanding of fundamental processes involved in commercial gasoline-powered ICEs. In the literature, the authors found few direct correlations between surrogate fuels behavior in more controlled experiments using shock tube and rapid compression machine and surrogate fuels ability to reproduce gasoline performance in commercial SI engines. The contribution of this paper is to start to fulfill this gap, identifying surrogate fuels that could represent gasoline performance in SI commercial engine to be further investigated. To identify suitable surrogates, SI engine tests were conducted under a typical range of engine operating conditions. Ten surrogate fuel formulations, based on iso-octane, n-heptane and toluene mixtures, were selected, and 25% ethanol by volume was added to each formulation. The results of this study revealed that fuels B (31.25% iso-octane, 31.25% toluene, 12.5% n-heptane and 25% ethanol) and E (37.5% iso-octane, 18.75% toluene, 18.75% n-heptane and 25% ethanol) could be used as surrogate fuels for high-octane oxygenated gasolines. Results suggested that the surrogate fuels studied can represent the performance of oxygenated gasolines in commercial engines. They could be used for further investigations on the combustion behavior of oxygenated gasolines, including the development of flame propagation models. Combustion pressure measurements can be utilized to develop a methodology for estimating fuel flame speed inside combustion chamber. Other statistically significant correlations among component concentration, fuel properties and engine performance should be further investigated to improve gasoline formulations. The results obtained can also help researchers in this area to select mixtures of components, which have more representative performance in SI commercial engine, to further investigate surrogate fuels. This can be done using more controlled experiments in shock tube and rapid compression machine to try to better understand the chemistry controlling ethanol blended hydrocarbons. The author’s intention is to perform further tests in a planar burner and in a shock tube that is under construction using the mixtures evaluated in this paper.
Torque – , fuel consumption (FC) – , specific fuel consumption (SFC) – 4 (marks mean statistical equality).
References [1] Pitz WJ, Cernansky NP, Dryer FL, Egolfopoulos FN, Farrell JT, Friend DG. et al. Development of an experimental database and chemical kinetic models for surrogate gasoline fuels. SAE Paper 2007-01-0175; 2007.
646
G.B. Machado et al. / Fuel 90 (2011) 640–646
[2] Curran HJ, Gaffuri P, Pitz WJ, Westbrook CK. A comprehensive modeling study of iso-octane oxidation. Combust Flame 2002;129:253–80. [3] Davidson DF, Gauthier BM, Hanson RK. Shock tube ignition measurements of iso-octane/air and toluene/air at high pressures. Proc Combust Inst 2005;30(1):1175–82. [4] He X, Donovan MT, Zigler BT, Palmer TR, Walton SM, Wooldridge MS, et al. An experimental and modeling study of iso-octane ignition delay times under homogeneous charge compression ignition conditions. Combust Flame 2005;142(3):266–75. [5] Curran HJ, Gaffuri P, Pitz WJ, Westbrook CK. A comprehensive modeling study of n-heptane oxidation. Combust Flame 1998;114(1–2):149–77. [6] Herzler J, Jerig L, Roth P. Shock tube study of the ignition of lean n-heptane/air mixtures at intermediate temperatures and high pressures. Proc Combust Inst 2005;30(1):1147–53. [7] Berta P, Aggarwal SK, Puri IK. An experimental and numerical investigation of n-heptane/air counterflow partially premixed flames and emission of NOx and PAH species. Combust Flame 2006;145(4):740–64. [8] Gauthier BM, Davidson DF, Hanson RK. Shock tube determination of ignition delay times in full blend and surrogate fuel mixtures. Combust Flame 2004;139(4):300–11. [9] Silke EJ, Curran HJ, Simmie JM. The influence of fuel structure on combustion as demonstrated by the isomers of heptane: a rapid compression machine study. Proc Combust Inst 2005;30(2):2639–47. [10] Bounaceur R, Costa ID, Fournet R, Billaud F, Battin-Leclerc F. Experimental and modeling study of the oxidation of toluene. Int J Chem Kinet 2005;37(1): 25–49.
[11] Dagaut P, Pengloan G, Ristori A. Oxidation, ignition and combustion of toluene: experimental and detailed chemical kinetic modeling. Phys Chem Chem Phys 2002;4(10):1846–54. [12] Emdee JL, Brezinsky K, Glassman I. A kinetic-model for the oxidation of toluene near 1200-K. J Phys Chem 1992;96(5):2151–61. [13] Pitz WJ, Seiser R, Bozzelli JW, Seshadri K, Chen CJ, Costa ID, et al. Chemical kinetic study of toluene oxidation under premixed and nonpremixed conditions. Lawrence Livermore National Laboratory, UCRL-CONF-201575; 2003. [14] Sivaramakrishnan R, Tranter RS, Brezinsky K. A high pressure model for the oxidation of toluene. Proc Combust Inst 2005;30(1):1165–73. [15] Andrae JCG. Development of a detailed kinetic model for gasoline surrogate fuels. Fuel 2008;87:2013–22. [16] ANP Resolution # 309. Brazil: ANP – Brazilian National Petroleum, Natural Gas and Biofuels Agency; 2001. [17] Machado GB. Simulação da Vaporização de Monocomponentes Presentes na Gasolina. Petrobras/CENPES Technical Rep DPM 007/07. Brazil: Petrobras/ Research and Development Center; 2007. [18] Assis JCR, Jaeger HV, Almeida SQ, Teixeira LSG, Pontes LAM, Sobrinho EV, et al. Caracterização dos Principais Componentes da Gasolina Automotiva através de Cromatografia Gasosa. Brazil: II Congresso Brasileiro de P&D em Petróleo e Gás; 2003. [19] ISO 1585. Road vehicles – engine test code – net power. Switzerland: International Organization for Standardization; 1992.