Background
Increasing global concern due to air pollution and to the limited oil reserves has generated much interest in the environmental friendly fuels alternative to petroleum-based fuels, in particular for the transport sector in which the energy consumption depends almost exclusively on fossil fuels. Several countries aim to use sustainable biofuels, which generate a clear and net GHG saving and have no negative impact on biodiversity and land use. In this scenario, butanol has strong potential as a biofuel. Like ethanol, butanol can be produced both by petrochemical and fermentative processes. The production of biobutanol by fermentation for use as a biofuel is generating considerable interest as it offers certain advantages in comparison with bioethanol. These include higher energy content, lower water adsorption and corrosive properties, better blending abilities and the ability to be used in conventional internal combustion engines without the need for modification. Biobutanol can be produced from starch or sugar-based substrates by fermentation (acetone-butanol-ethanol named ABE fermentation process).
However, cost issues, the relatively low yield and sluggish fermentations, as well as problems caused by end product inhibition and phage infections, meant that ABE butanol could not compete on a commercial scale with butanol produced synthetically, and almost all production ceased as the petrochemical industry evolved. However, the increasing interest in the use of biobutanol as a transport fuel induces a number of companies to explore novel alternatives to traditional ABE fermentation, which would enable biobutanol to be produced on an industrial scale.
Regarding the automotive use of biobutanol, the technology to make biobutanol, a nonfood-based biofuel, cost-competitive with fossil fuels isn't here yet, but several companies are working with this target. With respect to gasoline, butanol (or biobutanol) has a number of advantages over other common alcohol fuels such as ethanol and methanol. The energy density of gasoline is about 32 MJ/L, while butanol shows 29.2 MJ/L compared to ethanol, 19.6 MJ/L, and methanol, 16 MJ/L. This makes butanol so close to gasoline that it can allow a straight-across replacement in terms of energy [1]. Butanol is far less hygroscopic than methanol, ethanol and propanol. These lower alcohols are fully miscible with water, whereas butanol has only a modest water solubility. This allows a low-energy intermediate purification step [2]. Butanol is less corrosive than ethanol, can be transported in existing pipelines and is much safer to work with than lower alcohols based on its relatively high boiling point and flashpoint. In comparison with ethanol, the adding of butanol to conventional hydrocarbon fuels for use in a spark-ignition engine can increase fuel octane rating and power for a given engine displacement and compression ratio, thereby reducing fossil fuel consumption and CO2 emissions [3–5]. Ethanol use has been widely investigated for spark-ignition engines, while few studies have been performed on butanol-gasoline combustion and on butanol-fueled engines [6, 7]. Literature is particularly poor with respect to boosted spark-ignition (SI) engine experimental data. Almost all of the studies about butanol-gasoline blends consisted of the evaluation of performance, fuel consumption and exhaust emissions for different engine-operating conditions [6–10]. The in-cylinder process characterizations were principally realized through pressure measurements. These research activities demonstrated that the concentrations of 20% to 40% butanol in gasoline enabled to run the engine at a leaner mixture than gasoline for a fixed performance. These blends offered UHC emissions similar to gasoline, and they increased at higher butanol concentrations. The blends decreased the NOx emissions to a lower level than with pure gasoline at its leanest mixture. The slight increase in specific fuel consumption (SFC) with the butanol addition was related to the blend's reduced combustion enthalpy. For example, B40 has a 10% lower combustion enthalpy than gasoline, which increases SFC of 10% for stoichiometric and slightly lean mixtures. It was measured that, by adding butanol, the coefficient of variation of indicated mean effective pressure (COVIMEP) was reduced, particularly with lean mixtures, and the fully turbulent combustion phase (10% to 90% MFB) was similar in duration for all blends and pure gasoline. This latter finding showed that butanol has a similar or higher laminar flame speed than gasoline [6, 11].
In recent works, the performance of a gasoline engine fuelled with gasoline-butanol blends of different mixing fractions was analyzed. It was demonstrated that butanol is a very promising alternative fuel with great potential for saving energy; a reduction of 14% in brake-specific energy consumption and emissions was observed [12].
Recent experimental investigations conducted using a single-cylinder spark-ignition research engine allowed comparing the performance and emissions of neat n-butanol fuel to that of gasoline and ethanol. It was found that gasoline and butanol are closest in engine performance, with butanol producing slightly less brake torque. Exhaust gas temperature and nitrogen oxide measurements show that butanol combusts at a lower peak temperature. Of particular interest were the emissions of unburned hydrocarbons, which were between two and three times to those of gasoline, suggesting that butanol is not atomizing as effectively as gasoline and ethanol [13].
At the same time, fundamental biobutanol combustion work was carried out; the oxidation of butanol-gasoline surrogate mixtures (85 to 15 vol.%) was studied using a jet-stirred reactor in the work by Dagaut and Togbé [14]. The aim of this paper is better comprehension of in-cylinder phenomena correlated with butanol-gasoline combustion in a SI engine. To this goal, cycle-resolved visualization was performed to follow the flame propagation from the spark ignition to the late combustion phase. The experiments were realized in a single-cylinder ported fuel injection (PFI) SI-boosted engine. The optically accessible engine was equipped with the cylinder head of a commercial SI turbocharged engine with the same geometrical specifications (bore, stroke and compression ratio) of the research engine. Butanol-gasoline blend was tested for several engine operating conditions. Changes in spark timing and fuel injection phasing were considered. Comparison between the parameters related to flame luminosity and to pressure signals were performed, and in-cylinder investigations were correlated to engine out emissions.