Recent progress in gasoline surrogate fuels

TL;DR: A comprehensive review of the available experimental and chemical kinetic studies which have been performed to better understand the combustion properties of gasoline fuels and their surrogates can be found in this paper, where a detailed analysis is presented for the various classes of compounds used in formulating gasoline surrogate fuels, including n-paraffins, isoparaffin, olefins, naphthenes and aromatics.

About: This article is published in Progress in Energy and Combustion Science.The article was published on 2018-03-01 and is currently open access. It has received 270 citations till now. The article focuses on the topics: Gasoline & Combustion.

Summary

1. Introduction

• TaggedP ransport is an essential part of modern society, accounting for around 20% of the world's energy use [1].
• TaggedP he outlook for transport energy, the prime movers for change in transport technology, alternatives to conventional engines and fuels [4 6], and implications for combustion science in these developments [7,8] have been discussed.
• There is also increasing pressure to minimize tailpipe emissions of unburned hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOx) and particulate matter (PM).
• Nanoparticles are of particular concern for public health.
• Once surrogates are formulated, fundamental combustion experiments comparing them with their target gasoline fuels are useful for refining the formulation and studying relationships between fuel composition and combustion performance.

2. Surrogate fuel formulation

• 1. Defining target properties TaggedPAn appropriate surrogate fuel mixture can accurately emulate specific target properties of a gasoline fuel under investigation.
• Table 1 provides an overview of surrogate fuels, Progress in Energy and Combustion Science (2017), Fig. 1. Design process for developing optimal gasoline fuel-engine technologies with high efficiency and low environmental impact.
• Because each property impacts the combustion process, the relationship between the target property and combustion performance must be considered.
• Target properties are classified into physical and Table 1 Summary of relevant ASTM standards for gasoline fuels.

2.2. Physical target properties

• TaggedP he physical properties of gasoline determine important enginerelated performance measures.
• Overview of physical properties TaggedP he volatile characteristics of gasoline describe how easily the fuel vaporizes under various conditions.
• TaggedPA major drawback of the ASTM D86 methodology is that it does not measure the temperature under thermodynamically vapor liquid equilibrium (VLE) conditions.
• In principle, the high volatility of gasoline-like fuels are desirable in HCCI and other compression ignition engine concepts, where sufficient premixing of fuel and air is required prior to combustion [41 47].
• They showed that the higher volatility components in multi-component fuel mixtures evaporate quickly when fuel temperature is high, resulting in disruption of the spray structure and formation of a vapor core within the spray core.

2.3. Chemical target properties

• TaggedPChemical properties of gasoline affect the combustion process because fuel/air oxidation is controlled by the fuel's molecular structure.
• This relationship between molecular structure and LHV is shown in Fig. 5 [54], in which the LHV is linearly proportional to the H/C ratio of a complex real fuel mixture.
• TaggedPIn addition to hydrocarbon components, gasoline fuels may also contain various oxygenated additives.
• It is well accepted that oxygenated additives, surrogate fuels, Progress in Energy and Combustion Science (2017), TaggedPsuch as ethers and alcohols [60,84], improve combustion efficiency, thereby reducing particulate matter (i.e., soot), HC, CO emissions.
• TaggedP he effect of fuel composition on HCCI engine combustion was investigated by Truedsson et al. [95,96] by investigating various PRF mixtures and mixtures of n-heptane, isooctane, toluene, and ethanol.

2.4. Surrogate formulation strategies/methodologies

• TaggedP o this point, discussion has focused on important physical and chemical properties of gasoline fuels and their impact on engine combustion processes.
• Puduppakkam et al. [110] outlined a computational methodology (Fig. 7), to formulate a surrogate that matched defined target properties.
• The target properties of interest included physical properties, such as density, LHV, volatility, and viscosity, and chemical properties, such as H/C ratio and ignition quality (i.e., RON).
• This requires predicting, without actually measuring, the properties of various candidate surrogate mixtures to find one that best matches the properties of the target fuel.
• Subsequent sections will discuss experimental and computational studies performed on gasoline surrogate mixtures and relate them to investigations performed on real gasoline fuels.

3. Fundamental combustion experiments on gasoline and surrogate fuels

• TaggedPGasoline fuels are primarily used in spark ignition engines and in advanced low-temperature combustion counterparts.
• Targeted engine experiments may be carried out on real gasoline and proposed surrogate mixtures to assess how well the surrogate captures real fuel behavior, such as heat release rate, combustion phasing and emissions.
• Lastly, engine experimental data cannot be directly used to develop and validate detailed chemical and physical models for the surrogate mixtures.
• TaggedPFundamental combustion experiments carried out in welldesigned chemical reactors and/or canonical flames may provide valuable data that can be used to (i) understand trends and comparisons among various fuels and fuel surrogates, (ii) build detailed chemical models to propose optimal surrogates, and (iii) describe fuel behavior over wide ranges of engine operating conditions.
• Secondly, the ignition process in HCCI engines, for example, is indeed dominated by fuel chemistry [127].

3.1. Challenges of gasoline measurements

• Ignition measurements in shock tubes TaggedPFuel autoignition quality is critical in determining the operating regime of SI engines.
• TaggedP he challenges encountered in RCM measurements of gasoline ignition delay times are very similar to those mentioned for shock tubes.
• This behavior may be detected by analyzing the pressure traces carefully for evidence of pre-ignition and flame propagation prior to the bulk homogeneous ignition.
• Similar to other fundamental combustion experiments on gasoline fuels, adequate vaporization and avoidance of fuel condensation is very important, particularly for gasolines with a relatively high boiling point.
• These non-idealities and uncertainties were addressed previously [136,141,142], and must be taken into consideration when using speciation data for model development and validation.

3.2. Fundamental combustion experiment literature

• Autoignition studies in shock tubes and rapid compression machines TaggedPDespite the importance and practical implications of gasoline fuels, ignition delay time measurements of distillate gasolines are scarce in the literature.
• It was shown that both gasolines exhibited comparable ignition delay times over the wide range of experimental conditions (715 1500 K, 10 40 bar, f D 0.5 and 1), and a PRF surrogate adequately captured the ignition requirements of these gasolines.
• TaggedPSarathy et al. [58] recently studied the autoignition and surrogate formulation for two high octane FACE gasolines, F and G. However, the PRF surrogates were more reactive at low temperature conditions, the differences being greater for higher fuel concentration cases (f D 1 and 2).

Ignition Delay Time Studies

• Table 3 Reference Fuels Experimental device T, P, mixture fraction Lenhert et al. [195] 1-pentene/toluene/n-heptane/isooctane.
• Gasoline, gasoline/ethanol blends Coflow diffusion flame 1 bar Lemaire et al. [200].
• Figure adopted from [182] with permission from Elsevier Publishing.
• The measured volume fractions in Fig. 24 show decreasing amounts of soot with increasing ethanol fraction.

4. Chemical kinetic modeling of gasoline surrogate fuel combustion

• TaggedP he chemical kinetics of gasoline was previously discussed in the context of experimental work performed on surrogate mixtures and full boiling range refinery gasoline fuels.
• Numerical simulations can also aid understanding the effects of gasoline fuel composition on combustion properties and engine performance.
• Chemical kinetic models are capable of simulating reactions in the flame zone that result in heat release, the ignition characteristics of fuel/air mixtures, and the formation of major and minor pollutant species.
• TaggedPGasoline fuels contain hundreds of different molecules, and developing such kinetic models for each individual component is unmanageable.
• Kinetic models are only available for surrogate fuel mixtures comprising a limited number of components.

4.1. Chemical kinetic models for pure components

• TaggedPAs described previously, gasoline fuel components can be divided into five main hydrocarbon classes (paraffins, isoparaffins, olefins, naphthenes, and aromatics) and oxygenates.
• An important implication of this finding is that in replicating the high temperature combustion characteristics of a target gasoline fuel, any n-alkane (n-pentane or larger) could be a suitable surrogate to represent all the n-alkanes in the gasoline.
• It is clear that well validated kinetic models exist for all the important n-alkanes, i.e., n-butane, npentane, n-hexane, n-heptane, and n-octane.
• It is clear that high temperature (above»950 K) ignition delay times of n-alkanes are similar.
• H-atom abstraction form normal secondary carbon sites is also possible, and the resulting allylic radicals react like those derived from n-alkanes.

4.2. Detailed chemical kinetic models for gasoline surrogate mixtures

• TaggedPChemical kinetic models for gasoline surrogate mixtures are simply combinations of various pure component models.
• TaggedPAn example of the predictive capabilities of detailed chemical kinetic models for PRFs is given here in the context of HCCI engine simulations.
• The authors showed that ignition simulations with simpler PRF and TPRF surrogate for FACE gasolines can reproduce experimentally measured ignition delay times of the real gasoline at high temperatures.
• The 10-component model has also been used to simulate surrogate mixtures of high octane oxygenated gasolines containing ethanol [154] and low octane gasolines produced from light and heavy naphtha [152,153a].
• These mixtures still include a significant amount of n-heptane, which undergoes radical chain branching reactions at low temperatures below »750 K.

5. Summary and outlook

• TaggedPGrowing global demand for transportation is increasing the need for gasoline fuels.
• TaggedPDespite recent advances in their understanding of gasoline surrogate fuel formulation, experimental measurements and chemical kinetic modeling, this review identifies areas where significant research advances are required to overcome challenges facing the development of high efficiency and low emission gasoline engines.
• The general design process presented in Fig. 1 shows how fuels and engines can be optimized to maximize efficiency and decrease negative environmental impacts.
• These experiments should use the same surrogate mixtures as those used in fundamental devices (e. g., ST, RCM, flames), so a link can be established between fundamental and applied combustion systems.
• Furthermore, kinetic models capable of simulations multicomponent mixtures (e.g., up to ten components) will be useful for determining if and when complex surrogate mixtures are needed.

Acknowledgments

• TaggedPFig. 1 was produced by Heno Hwang, scientific illustrator at King Abdullah University of Science and Technology .
• The research reported in this publication was supported by Saudi Aramco and KAUST under the FUELCOM program.

Figures

Fig. 5. Correlation between LHV and H/C ratio. Reproduced from Narayanaswamy et al. [54], with permission from Elsevier Publishing.

Fig. 7. Flow chart for surrogate fuel formulation from Puduppakkam et al. [110]. Reproduced with permission from SAE International.

Table 4 Comprehensive chemical kinetic models for various gasoline surrogate components published since 2000.

Table 4 Comprehensive chemical kinetic models for various gasoline surrogate components published since 2000.

Fig. 12. Temperature measurement in a rapid compression machine using chirpedlaser absorption spectroscopy. Simulated temperature is obtained by running a zerodimensional reactive simulation. Pc D 9.21 bar, Tc D 742 K, 2.56% n-pentane/20.46% O2/75.98% N2/1%CO. Figure adopted from Nasir and Farooq [134] with permission from Elsevier Publishing.

Fig. 21. Comparison of gasoline burning velocities reported in various studies.

Fig. 19. Unstretched laminar flame speed at initial temperature of 323 K and initial pressure of 1 bar. Fig. adopted from [171] with permission from The American Chemical Society.

Fig. 20. Laminar burning velocities of non-oxygenated Exxon gasoline and TRF (toluene/isooctane/n-heptane) at initial temperatures of 298, 318, 328, 338 and 358 K. Predictions of the energy fraction mixing rule are also shown. Figure adopted from Sileghem et al. [137] with permission from Elsevier Publishing.

Fig. 29. Comparison of experimental (left) and simulated (right) ignition delay times at 20 n-hexane [249], and cyclohexane [276]. Figure adapted from Al Rashidi et al. [272] with per

Fig. 6. (Top) Typical PIONA range for U.S. market gasoline fuels. Reproduced from Pitz et structures in gasoline fuels. Reproduced from Sarathy et al. [58] with permission from Elsev

Fig. 28. The nine isomers of heptane and their measured RON/MON values.

Fig. 23. Laser-based measurements of species’ profiles during the oxidation of FACE A, FAC initial pressure is 2 bar. Figure adopted from [186] with permission from Elsevier Publishing

Table 6 Composition, RON, and S of selected gasoline surrogate mixtures stu

Fig. 33. Simulated ignition delay times for various gasoline surrogate mixtures at 25 bar and stoichiometric fuel/air mixtures. Legend denotes the mixtures RON and sensitivity (S). Figure adapted from Singh et al. [344] with permission from The American Chemical Society.

Fig. 11. Effect of compression stroke on the first stage and overall ignition delay time. Open symbols represent reactive calculations performed considering the RCM compression stroke. Solid line simulation is obtained by initializing the reactive calculations at the end of compression stroke. Figure adopted from Sung and Curran [133] with permission from Elsevier Publishing.

Fig. 32. Measured (symbols) and simulated (lines) ignition delay times for various TPRF mixtures near 20 bar and stoichiometric fuel/air mixtures. Each mixture is identified by its RON (97.5, 91, 80, 70). Figure taken from Javed et al. [158] and reproduced with permission from Elsevier Publishing.

Fig. 24. Measured soot volume fraction distributions in the E0, E20, E50, and E85 flames. Peak soot volume fraction in each flame is indicated. Figure adopted from [199] with permission from Elsevier Publishing.

Fig. 27. Simulated homogeneous gas-phase ignition delay times for various n-alkanes at 25 bar, stoichiometric fuel/air mixtures.

Table 2 Typical properties of gasoline fuels.

Fig. 2. Distillation curves for FACE gasolines measured by the ADC method of Burger et al. [32]. Reproduced with permission from The American Chemical Society.

Fig. 16. Measured (symbols) and simulated (lines) ignition delay times of light naphtha. Solid lines represent simulations performed using a five-component surrogate. Dashed lines represent simulations performed with a two-component PRF 64.5 surrogate. Figure adopted from [152] with permission from Elsevier Publishing.

Frequently asked questions

Q1. What contributions have the authors mentioned in the paper "Recent progress in gasoline surrogate fuels" ?

Rights NOTICE: this is the author ’ s version of a work that was accepted for publication in Progress in Energy and Combustion Science. Changes may have been made to this work since it was submitted for publication. This manuscript version is made available under the CCBY-NC-ND 4. 0 license http: //creativecommons. 

Q2. What is the effect of higher octane fuels on combustion efficiency?

Higher octane fuels enable earlier spark timing, which can improve combustion efficiency and power output at higher compression ratios. 

Q3. What is the role of ethanol in the formation of soot in gasoline engines?

Since the addition of oxygenated species (such as ethanol) to hydrocarbons is expected to reduce soot formation, a few studies have been dedicated to carry out diffusion flameTaggedP xperiments of gasoline and ethanol blends. 

Q4. How much reduction in soot agglomerates was found in the E0 flame?

the E85 flame resulted in 4 7 times reduction in nuclei mode mass and a reduction factor of about two in the size of soot agglomerates compared to the E0 flame. 

Q5. Why is it possible to differentiate different fuels in the NTC and low-temperature regions?

Due to the long test times accessible with RCMs, it is possible to differentiate various fuels in the NTC and low-temperature regions. 

Q6. What is the importance of a good ignition quality in HCCI engines?

Most notably, to obtain the desired onset of heat release and combustion phasing, an appropriate ignition quality is required in HCCI engines. 

Q7. What is the effect of ethanol blending on gasoline?

Ethanol blending is known to cause higher volatility in gasoline-ethanol mixtures, while also introducing discontinuities in the distillation profile. 

Q8. What is the importance of measuring the flame temperature as a function of the burner?

For low-pressure flames, measurement of flame temperature as a function of height above the burner is critical for the usefulness of the data. 

Q9. What is the ideal homogeneous batch reactor for measuring ignition delay times?

TaggedP he shock tube stands as the ideal homogeneous batch reactor for measuring ignition delay times as a function of temperature, pressure and mixture fraction. 

Q10. What is the main reason why CO emissions were not being oxidized to CO2?

CO emissions were not found to be directly linked to fuel composition, rather, in-cylinder temperature inhomogeneity (leading to cold spots) was found to be the primary reason why CO was not being oxidized to CO2. 

Q11. What is the importance of a robust core mechanism in gasoline surrogate fuels?

fuel mixtures behave notably different than pure components, but the cross effects are realized in species and reactions within the intermediate radical pool (and rarely with the parent fuel molecules), which highlights the importance of a robust core mechanism. 

Q12. What should be the focus of research on gasoline fuels?

Research should be directed towards acquiring liquid spray and combustion data for various gasoline fuels, surrogate mixtures, and injector geometries. 

Q13. What is the way to ensure vaporization of gasoline?

The liquid fuel should be injected into a heated chamber, such as a mixing vessel, where the temperature of the vessel is high enough to ensure vaporization of all gasoline components. 

Q14. What is the effect of blending oxygenates on combustion performance?

Di Iorio et al. [75] showed that blending oxygenates, such as ethanol, MTBE, and ETBE, increase the octane number, thereby improving combustion performance. 

Q15. What are the difficult properties to estimate?

Physical properties, such as volatility characteristics (i.e., distillation curve), are more difficult to estimate because they are not additive. 

Q16. Why are surrogate mixtures formulated to emulate the properties of the real fuel?

For this reason, surrogate mixtures are formulated to emulate the thermophysical, thermochemical, and chemical kinetic properties of the real fuel, so that fundamental experiments and predictive simulations can be conducted. 

Q17. What is the important factor in the kinetic modeling effort?

This greatly facilitates the kinetic modeling effort, as it permits complex gasoline mixtures with many isoalkanes variants to be modeled as simpler mixtures of an n-alkane and a highly branched alkane. 

Q18. What are the main goals of fundamental combustion experiments?

Such fundamental experiments may be able to decouple the physical and chemical aspects of fuel behavior and are ideal for understanding the effects of the chemical structure on fuel autoignition and emissions, for example. 

Q19. What is the simplest explanation for the lower reactivity of cyclopentane?

Al Rashidi et al. [271,272] provided a similar explanation for the lower reactivity of cylopentane compared to n-pentane; they attribute cyclopentane's lower reactivity to higher energy barriers in forming the strained bicyclic transition state, which increase the flux to concerted elimination reactions forming unreactive cyclopentene and HO2 radicals. 

Q20. What is the difference between aromatics and isoparaffins?

isoParaffins are superior to aromatics due to their higher stoichiometric fuel/ air ratio and higher H/C ratios, which improves combustion efficiency and reduces particulate matter emissions.