Analysis of Ethanol as a Gasoline Compliment

Introduction
Commercial grade gasoline is a complex organic matrix of up to 1000 unique compounds that is obtained by the fractional distillation of petroleum [1]. There are a large number of variables that influence the outcome of the refinement process. Therefore, there is often a fair degree of variability observed in the composition of different batches of gasoline [2]. In addition to the random variation, there are also intentional differences due to the additives added to the matrix in order to optimize the product for specific needs. Together, these differences determine the physical and chemical nature of the blend, including the reactivity, stored energy, volatility, and combustion characteristics [3]. These properties are what determine its performance in its typical application, powering an internal combustion engine.

The internal combustion engine makes use of the chemical energy stored in the bonds of the hydrocarbons during combustion. Since the engines are sensitive to the composition and the properties of its fuel, it is important that some method of keeping track of these properties [4]. Enter the octane rating. The octane rating has a few manifestations. However, all of the methods of determining the octane rating have the same purpose, to quantitatively report a value that is reflective of its behavior during controlled combustion inside the engine. More specifically, it is a measure of its ability to resist premature ignition within the cylinder [1, 5]. Two of the most common methods of reporting an octane rating are the research octane number (RON) and the motor octane number (MON). The RON and MON are measured values that are determined by doing a set of controlled tests involving an internal combustion engine. These terms are defined by the reference compounds n-heptane (ON=0) and iso-octane (ON=100) [2, 5, 6]. The other, perhaps more mainstream value used in the United States, is anti-knock index (AKI). The AKI is simply calculated as (RON+MON)/2. This is the value seen as used by the gas station [7]. Since the physical properties differ from engine configuration to engine configuration, the octane rating at which a given engine operates most efficiently also varies [4]. Therefore, for an engine with well documented behavior, it is possible for an individual to quickly identify the optimum octane rating for their purpose.

It follows that since the octane rating of a given fuel is a measure of its ability to avoid premature combustion, it could also serve as a hint as to what may be present in the matrix. This is because the matrix consists of diverse collection of hydrocarbons in unpredictable ratios, with C5 – C13 compounds contributing to most of the overall composition [3]. Each component, having its own unique set of properties, will make a contribution to the properties of the matrix. The magnitude of the contribution can be estimated by the magnitude of the component property weighted by its fraction of the entire mixture. Furthermore, a guess as to the properties of the entire matrix can be made by considering the weighted contribution of each unique species and their interactions with the other matrix components. Considering the quantity of unique compounds that could be present, this would not make a practical approach to an analysis of the whole matrix. However, it does demonstrate an important point, that the value of the property of an individual component can influence the value of the same property for the entire matrix in a reasonably predictable fashion [6, 8]. There are many compounds present that have similar octane numbers so chromatographic techniques can be used in the determination of the presence and/or quantity of each ambiguous species [1, 9].

Factors Influencing Blend Decisions
The gasoline that is obtained from the fractional distillation can sometimes exhibit a degree of variation from one batch to the next that is high enough to make the prediction of properties of the output difficult [1, 10, 11]. However, commercial gasoline is most commonly purchased in discreet non-varying increments. This gives rise for a need to be able to manipulate the octane rating of the product. Thus, by capitalizing on the partial contribution exhibited by the components, the octane of the gasoline can be modified to a more desirable value by addition of a variety of individual components with known properties in strategic ratios [8]. But there are other factors that are considered when determining the most effective way to blend the gasoline.

The hydrocarbons in the gasoline that were obtained from the fractional distillation account for the release of energy that makes gasoline such an exceptional fuel source. However, incomplete combustion of oxygenates and olefin molecules causes some suspended hydrocarbon content in the exhaust [12]. These are hydrocarbons, hydrocarbon radicals and impurities from the gasoline that are produced in small quantities when a small fraction of the hydrocarbons or impurities are not able to react completely with the air inside the cylinder [2]. This is typically not a short term concern as most of these particles will leave the engine while still suspended in the exhaust gasses. But the particles themselves are not gaseous and tend to behave as a solid. Although the combustion within most spark ignition engines that are regularly tuned approaches complete conversion to products, the reaction never quite reaches one hundred percent conversion to products. Buildup within engines tends to increase with extended or strenuous use. This accumulation is mostly harmless until it begins to hinder the physical motions that are essential to the function of the engine [13]. And there is not an easy way to remove these deposits manually aside from taking the engine apart. Of course regular manual cleanings would be an expensive and time consuming method of removing deposits. Fortunately, it turns out that certain compounds called detergents when added to the gasoline are capable of loosening these deposits. Once the particles are mobile they are able to either exit with effluent exhaust gasses or through the oil if particles become entrapped after contact with the oil. These detergents are present in low concentrations in most commercially produced gasoline. This can prevent the buildup when used consistently. In addition, there are also concentrated detergent formulas that are produced for manual addition to a tank of gasoline with the intention of accelerating deposit removal [12]. And although the detergent additives are a component of the combustible mixture, it is not well documented whether or not these compounds undergo any reaction that would possibly contribute to the antiknock properties during the combustion of the hydrocarbon components. Therefore, any octane influences will be overlooked and assumed to be negligible due to low concentrations [12].

Figure 1
Combustion_Example.jpg[2]
Fig. 1 - an example of plausible reaction of hydrocarbons with OH radicals

Though a key goal in the blending of gasoline is performance, they still participate in the free market economy, and therefore, are subject to societal forces. For example, the growing concern that emissions from internal combustion engines could be causing significant environmental disturbances has received a considerable amount of attention from the mainstream media. These concerns commenced a push for methods of lowering emissions [4]. Likewise, it was also recognized that another criterion that should be met when seeking alternative fuels should be renewability. The gasoline markets were not exempt from the pressure to deliver lower-emission renewable fuels. Consequently, the final blends of commercial gasolines are heavily influenced by these societal pressures [14].

Common Additives

Ethanol
Initiated by pressure from a growing environmentally minded subset of the population, major gasoline suppliers began incorporating significant quantities of ethanol (EtOH) into their formulations. Abbreviations, defined by the capital letter E coupled with a number (the percent ethanol), allow for the identification of the ethanol ratio of the mixture. For example, E85 refers to a mixture that consists of 85% EtOH and 15% gasoline components. These shorthand adaptations were popularized as it was phased into widespread use. Ethanol is unique in the sense that it can be combined in quite significant portions with the naturally occurring gasoline hydrocarbons and the performance of the engine is not compromised. That is, it combusts in a manner akin to that of gasoline with no additional ethanol. When compared to the octane range of typical gasoline grades, the ethanol octane value score at the high end of scales. Where a typical range for RON of gasoline is from 88 to 100, the ethanol RON is measured at about 108.6. Likewise, the common MON of gasoline is range from 80 to 90. EtOH MON is reported to be about 89.7 [14].

Clearly, the compound demonstrates its ability to resist knocking. But it is another feature, the Heat of Vaporization (840KJ/Kg), which provides a subtle benefit when compared with that of average gasoline (305KJ/Kg). A cooling effect that takes place when the liquid fuel is atomized in the cylinder is due to the Heat of vaporization of the substance being atomized. This implies that the heat required to atomize EtOH is 840KJ/Kg is more than 2.5 times greater than the 305KJ/Kg required to atomize gasoline.(e) Being that the location of the atomization is comfortably inside the engine, the heat absorbed during this process has to come from the surrounding engine components thereby ,inducing a cooling effect. Thus, pure EtOH removes more than two and a half times the heat from the surrounding engine components than gasoline for the same process [14, 15].

The performance of pure ethanol as a fuel is quite similar to gasoline and blends of gasoline and ethanol. In terms of torque and power, E100 produces nearly identical results to E20 fuels while sweeping the engines functional speed range. The minor difference between the fuels is observed in this range and could likely be attributed to experimental error.
toque.jpg
power.jpg
Figure 2,3 [16]

However, when comparing measured thermal efficiency, ethanol has a clear advantage over E20 at nearly every speed measured. Thus, of the energy available from each fuel, ethanol is able to convert a greater percentage of its energy into useful work. Unfortunately, the thermal efficiency does not translate into fuel economy. A comparison of the volume consumed by each fuel reveals that the volume of gasoline consumed is typically only about 75% of the volume of ethanol consumed under identical conditions. This suggests that E100 would be less cost efficient if priced identically to same volume of E20. This higher consumption rate is witnessed by independent sources with different test parameters. [16, 17]
Thermal_eff.jpg
consumtion.jpg
Figure 4,5 [16]

When it comes to emissions, the observed values of unwanted hydrocarbon emissions tends to decrease with the increase of ethanol in the blend. [15,16,17,] The decrease in soot formation with an ethanol enhanced flame is evidence of a cleaner burn and more complete combustion. [18] Further evidence of the completeness of combustion is observed in the type of carbon based gasses that are emitted. The trend with increasing ethanol is that CO2 emissions will increase while simultaneously decreasing CO emissions. This suggest the complete oxidation of a higher quantity of CO into CO2 during combustion. [16]
HC.jpg
co.jpg
Figure 6,7 [16]

Ethanol has many properties that make it a viable additive or even substitute for gasoline. But it also has it's share of shortcomings. Therefore, it is necessary while considering it as a new primary fuel source to also consider the new features. For example, if gasoline with a high ratio of ethanol is being sold at the same price per gallon as a product with little or no ethanol, than a cost efficient consumer should choose the low ethanol blend. This is because in spite of it's combustion efficiency, it does less work relative to an identical quantity of low blend gas as demonstrated in Figures 4 and 5. On the other hand,







[1]S. V. Cherepitsa, S. M. Bychkov, S. V. Gatsikha, A. N. Kovalenko, A. L. Mazanik, D. E. Kuzmenkov, Ya. L. Luchinina, and N. N. Gremyako. GAS CHROMATOGRAPHIC ANALYSIS OF AUTOMOBILE GASOLINES. Chemistry and Technology of Fuels and Oils 2001, 37, 283-290.
http://dx.doi.org/10.1023/A:1012368107443

[2] Marco Mehl,Tiziano Faravelli, Fulvio Giavazzi, Eliseo Ranzi, Pietro Scorletti, Andrea Tardani, and Daniele Terna. Detailed Chemistry Promotes Understanding of Octane Numbers and Gasoline Sensitivity. Energy & Fuels 2006, 20, 2391-2398.
http://dx.doi.org/10.1021/ef060339s

[3] CDC. httpwww.atsdr.cdc.govtoxprofilestp72-c3.pdf (accessed November 2011).

[4] Changwei Ji, Chen Liang, Shuofeng Wang. Investigation on combustion and emissions of DME/gasoline mixtures in a spark-ignition engine. Fuel 90 2011, 1133–1138.
http://dx.doi.org/10.1016/j.fuel.2010.11.033

[5] Graham Edgar. Measurement of Knock Characteristics of Gasoline in Terms of a Standard Fuel. Industrial & Engineering Chemistry 1927, 19(1), 145-146.
http://dx.doi.org/10.1021/ie50205a049

[6] J.C.G Andrae. Development of a detailed kinetic model for gasoline surrogate fuels. Fuel 2008, 87, 2013-2022.
http://dx.doi.org/10.1016/j.fuel.2007.09.010

[7] Wikipedia. http://en.wikipedia.org/wiki/Octane_rating/ (accessed November 2011).

[8] S. A. Karpov, S. I. Kokhanov, A. V. Tsarev and V. M. Kapustin. Ashless antiknock compounds for automotive gasolines. CHEMISTRY AND TECHNOLOGY OF FUELS AND OILS 2006, 42(6), 404-411.
http://dx.doi.org/10.1007/s10553-006-0097-9

[9] N. PETROFF, A. HOSCHEITT and J. P. DURAND. AUTOMATED SIMULATED DISTILLATION BY GAS CHROMATOGRAPHY, PERFORMANCE TEST FOR PETROLEUM PRODUCT CONTROL. Journal of Chromatography A 1987, 395, 241-254.
http://dx.doi.org/10.1016/S0021-9673(01)94114-7

[10]Gasoline; Tesoro Petroleum Companies: San Antonio, TX, February 8, 2003. http://www.tsocorp.com/stellent/groups/corpcomm/documents/tsocorp_documents/msdsunleaded.pdf (accessed November, 2011).

[11] O.Yu. Begak and A.M. Syroezhko. Improvement of Standard Analytical Methods for Gasoline Quality Control. CHEMISTRY AND TECHNOLOGY OF FUELS AND OILS 2001. 37(1), 51-53.
http://dx.doi.org/10.1023/A:1014053308605

[12] E. A. Nikitina, V. E. Emel’yanov, I. F. Krylov and A. V. Fedorova. Detergent additives to automotive gasolines. CHEMISTRY AND TECHNOLOGY OF FUELS AND OILS 2006. 42(1), 30-34.
http://dx.doi.org/10.1007/s10553-006-0023-1

[13] A.M. Danilov. Fuel Additives: Evolution and Use in 1996-2000. CHEMISTRY AND TECHNOLOGY OF FUELS AND OILS 2001. 37(6), 444-455.
http://dx.doi.org/10.1023/A:1014231230570

[14] M. Bahattin Celik. Experimental determination of suitable ethanol–gasoline blend rate at high compression ratio for gasolineengine. Applied Thermal Engineering 2008. 28(5-6), 396-404.
http://dx.doi.org/10.1016/j.applthermaleng.2007.10.028

[15] K Kar, R Tharp, M Radovanovic, I Dimou, W K Cheng. Organic gas emissions from a stoichiometric direct injection spark ignition engine operating on ethanol/gasoline blends. International Journal of Engine Research 2010. 11(6), 499-513.
http://dx.doi.org/10.1243/14680874JER610

[16]Rodrigo C. Costa, José R. Sodré. Hydrous ethanol vs. gasoline-ethanol blend: Engine performance and emissions. Fuel 2010. 89, 287-293.
http://dx.doi.org/10.1016/j.fuel.2009.06.017

[17]Ahmet Necati Ozsezen, Mustafa Canakci. Performance and combustion characteristics of alcoholegasoline blends at wide-open throttle. Energy 2011. 5, 2747-2752.
http://dx.doi.org/10.1016/j.energy.2011.02.014

[18]R. Lemaire, E. Therssen, P. Desgroux. Effect of ethanol addition in gasoline and gasoline–surrogate on soot formation in turbulent spray flames. Fuel 2010. 89, 3952-3959.
http://dx.doi.org/10.1016/j.fuel.2010.06.031