Combustion of n-butanol in a spark-ignition IC engine
Introduction
Butanol or butyl alcohol can be demonstrated to work in the IC engine designed for use with gasoline without modification. It can be produced from biomass (biobutanol) as well as fossil fuels (petrobutanol). Both biobutanol and petrobutanol have the same chemical properties. Butanol is less corrosive than ethanol and has a higher energy content than ethanol, similar energy content to gasoline. In comparison to ethanol, butanol is less prone to water contamination. As a result it could be distributed using the same infrastructure used to transport gasoline. It can be used alone in an internal combustion engine, or it can be mixed with gasoline. Four butyl alcohols can be distinguished. They all have the same chemical composition consisting of four carbon atoms, 10 hydrogens and single oxygen and examined by identical chemical pattern C4H10O, but they differ each from others with respect to their structure. They are as follows:
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1-butanol: (n-butanol) CH3–CH2–CH2–CH2OH,
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sec-butanol: CH3CH(OH)CH2CH3,
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tert-butanol: (CH3)3COH,
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iso-butanol: CH3(CH2)3OH.
In addition, each of the fuels has different thermodynamic properties and combustion characteristics. For the tests described in the paper n-butanol (1-butanol) was applied as a fuel. Characteristics of n-butanol in comparison to gasoline and other alcohol fuels are given in Table 1.
However, when taking into account the latent heat of vaporization of these fuels, butanol is less attractive than others. For port fuel injection systems, when the fuel vaporizes in the inlet port it affects a temperature decrease of the intake charge. Therefore, fuels of higher latent heat of vaporization have larger decreases in temperature of intake charge with complete vaporization in the intake port. This increases the density of combustible mixture and increases the charge mass. Furthermore, the cost of butanol production is higher in comparison with ethanol [2]. However, there are some promising circumstances for butanol production from fermentation process of agricultural feedstock by cellulosic enzymes [3] that have the potential to reduce its production cost.
Investigation of butanol usage as the engine fuel has been conducted by several research groups. It has been used both for blending gasoline and diesel fuel for IC engines [4]. Among others Alasfour [5] has conducted extensive studies. He conducted research on n-butanol combusted in an SI engine with particular focus on NOx emission and availability analysis as functions of intake temperature, air-to-fuel ratio and ignition timing. Yacoub et al. [6] performed several studies on application of alcohols C1–C5 (methanol to pentanol) as fuels blended with gasoline for the SI CFR Waukesha engine. He determined optimal spark timings and upper limits for compression ratio and knock resistance for these fuels. His studies showed that n-butanol was more prone to generate combustion knock than gasoline UTG-96. Additionally, he examined exhaust emission from these blends. Further research in this field was conducted by Gautam et al. and presented in [7], [8]. He worked with blends of 10% of the five-alcohols mixture (from methanol to pentanol) with gasoline UTG-96 with an 87 motored octane rating and tested these fuels at the stoichiometric ratios and steady state conditions. He determined that the maximum compression ratio for these blends was 10:1 with respect to combustion knock. There have also been studies in the field of butanol transport properties with respect to the application for an IC engine. For instance, Aleiferis et al. [9] conducted research on sprays of ethanol, butanol, and gasoline in a direct injected spark-ignited engine. Dagaut and Togbe [10] examined theoretical and experimental analyses of combustion mechanism of n-butanol–gasoline mixtures, conducted several combustion tests of butanol–gasoline at ratio of 85%/15%, and studied oxidation mechanism using a jet stirred reactor. They reported good agreement between experimental results and the computations of detailed chemical kinetic scheme for the n-butanol–gasoline blend. Furthermore, results of various alcohols combustion were presented by Cooney et al. [11]. They investigated the application of ethanol and butanol blends with gasoline as fuels for a series engine working without modifications to an engine control unit. They conducted these tests at partial loads, with torque 100 and 150 Nm. At lower loads, they reported engine conversion efficiencies were similar. As they presented in their work, at higher engine loads up to 150 Nm, maximum fuel conversion efficiency changed by 4% from 0.36 to 0.377 at torque of 150 Nm with n-butanol 85 and ethanol 85, respectively, but the difference in efficiency between gasoline and n-butanol 85 was small (approximately 0.002). In such insignificant changes, the error analysis for plots would have been helpful to draw proper conclusions. They also concluded that efficiency drop between n-butanol and gasoline was caused due to lower octane rating of n-butanol, even though knock combustion was not observed. On the other hand, the flame speed of n-butanol was the highest among the other applied fuels, so, theoretically faster combustion should increase the efficiency if combustion timing was adjusted.
Although there are several valuable works concerning n-butanol–gasoline-blended fuels combustion in SI engines, there is limited information of combustion characteristics over a range of blends of n-butanol and gasoline including 100% n-butanol. Therefore, the combustion studies in this work including characteristics of the mass fraction burned and heat release provide additional important insights into the application of n-butanol–gasoline blends.
Section snippets
Experimental setup
The engine used for this research is a single cylinder CFR (cooperative fuels research) engine manufactured by the Waukesha Motor Company. The engine was chosen for its versatility and robustness of construction which is important because of the inclusive studies of combustion knock. A specialized attribute of this engine is the ability to vary the compression ratio without disassembling the engine. Characteristics of the test bed and engine are shown in Fig. 1 and listed in Table 2,
Results and discussion
The study concentrated on combustion process of the fuel blends. Thus, analysis of heat release expressed by normalized mass fraction burnt (MFB) and net heat production rate examined by rate of normalized MFB were particularly of the interest. The analysis was focused on four issues, which are discussed separately in the following sections:
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Spark timing (ST) impact.
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Butanol percentage in blends.
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Compression ratio (CR) impact.
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Load impact.
Conclusions
The following conclusions can be drawn from the analysis presented above:
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At the 330 kPa NIMEP condition examined in this study, the location of 50% MFB for n-butanol is located approximately 2° earlier when compared with 50% MFB for gasoline at the same combustion conditions.
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The bulk burn combustion durations for pure n-butanol and n-butanol blends are comparable to those for gasoline. The shift in the 50% MFB location for n-butanol shifts towards TDC as a result of the shorter early burn
Acknowledgements
The authors acknowledge the support of Michigan Technological University and specifically the Mechanical Engineering and Engineering Mechanics Department and the Advanced Power Systems Research Center for support and facilities utilized in this work. This work is conducted under the Universities Wood-to-Wheels research enterprise as a subgroup of the Sustainable Futures Institute.
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