Review articleAn overview on the light alcohol fuels in diesel engines
Introduction
Biofuels are gaining attention as promising sustainable sources to replace conventional fuels while legislation restrictions have been made on the usage of fossil fuels to diminish greenhouse gas effects and preserve natural resources. Although, it is well known that biofuels can be alternative for the petroleum-based fuels, the use of conventional fuels has been rising every year. It was reported that more than 84% of carbon emissions have been released to the atmosphere from fossil based fuels since the 1980’s [1].
Biofuels meet a considerable part of energy demand while making a remarkable reduction impact on greenhouse gases, air pollution and cost of energy [2], [3]. The renewable fuels can be benefited as pure or blend with fossil fuels to satisfy many of the energy needs for transport systems, heating and industrial processes [4], [5].
Diesel engines are commonly used in various areas like industry, agriculture and transportation owing to their convenient properties such as high thermal efficiency, reliability, adaptability and low costs. Meanwhile, it is thought that diesel vehicles are among environmental pollutants and play a major role in development of energy shortage. So as to abate energy shortage and meet the new emission regulations, alternative fuels are among the most effective solutions [6], [7].
Alternative fuels such as alcohols have been widely used as additives in CI engines. Although alcohols have economic benefits in diesel engines in comparison with conventional diesel and can directly be used as a single-fuel or additive in diesel engine without any engine modifications [8], there are some difficulties needed to be eliminated regarding utilization of alcohols, blending with diesel fuel, low lubricity, difficulty of vaporization and high auto-ignition temperature. These difficulties can be overcome by several methods such as using additives or increasing intake air temperature [9], [10]. Mixing alcohols with diesel fuels is another approach of removing lubricity and vaporization problems. But alcohols and diesel fuel are not miscible and require co-solvents or emulsifiers in order to blend these liquids. However, these methods are expensive and the mixing process could become troublesome due to blending, separation, splashing, heating or other steps [11]. Even though alcohols are immiscible with diesel fuel, they are miscible with biodiesel. In certain limitation of alcohols, diesel/biodiesel/alcohols can be used in the same mixture without any miscibility problems [12].
Ethanol (ethyl alcohol) and methanol (methyl alcohol) are two types of light alcohols [13]. Methanol (CH3OH) fuel is being considered as one of the optimal fuels for internal combustion (IC) engines [14], [15]. Furthermore, ethanol (C2H5OH) is considered to be one of the most important components of biodiesel fuel and promising alternative fuel in IC engines [16], [17], [18], [19]. Oxygenates such as methanol and ethanol have been widely used in IC engines because of their improved volatility and higher latent heating properties [20]. Many researchers have been concentrated on improving the blends of diesel fuel, biodiesel and alcohols as an alternative fuel in CI engines. But there are some complications such as lowered heating value, phase separation, pour point and unsafety conditions for storage and transportation of the ternary blends [21].
Biofuels can directly or indirectly be obtained from biomass sources. Biomass resources can be any form of solid, liquid or gaseous [22]. Liquid biofuels from biomass can be produced both from edible and inedible oils [23]. Fuels from bio-origin (directly from vegetable oils) are renewable and sustainable which also have similar properties to conventional diesel fuel and considered as a solution of the IC engine-based problems. On the other hand, the fuels obtained from vegetable oils have high viscosity and lower boiling point, cetane number and volatility values which are very important features for compression ignition (CI) engines. These drawbacks result in engine oil contamination, incomplete combustion, engine deposits and higher exhaust emissions which make these fuels unfavorable for direct use in diesel engines [24], [25].
More studies have so far been made on the developing of ethanol and gasoline blends in SI engines in the literature. However, ethanol can be blended with diesel fuel up to 95% of ethanol in CI engines. High concentration of ethanol in CI engines can be used with several changes like increasing compression ratio and additives in order to improve cetane number [26]. Generally, alcohols are used as additives in CI engines and described as to be very helpful for reducing viscosity and density of pure biodiesel fuels because they have lower viscosity and density values in comparison to conventional diesel fuel. Moreover, these additive fuels increase the combustion efficiency of engine and emit lower smoke emissions to the earth atmosphere when blended with diesel duel. In addition, ethanol and methanol have approximately 35% and 30% more oxygen molecules in their molecular structure, respectively, which led more complete combustion and produce lesser exhaust gas emissions [27], [28], [29].
There are several applications on the use of alcohols in diesel engines such as direct injection, blending, emulsification and port injection (fumigation) [6], [30]. The limit of alcohol in the direct injection method cannot be over 20% due to poor properties of alcohols such as miscibility, heat value and high auto ignition temperature. Therefore, it is difficult to apply direct injection method on the engine. There are many studies that investigated the effect of alcohol usage in IC engines with port injection method as fumigated additive. In the port injection method, diesel is directly injected while alcohol is injected into the intake ports. The blend method is applied as additives to satisfy homogeneity of the mixture. Emulsion method is based on using emulsifier to mix the fuel blend to prevent phase separation which is able to displace up to 25% diesel fuel [30], [31], [32], [33]. But the port injection method is allowed to use higher concentration of alcohols in diesel engine in comparison with other methods [34].
As oil price are increased along with the rapid accumulation of greenhouse gas emissions and strict emission legislative regulations, it is necessary to put forward some new, renewable and environmentally friendly fuels instead of conventional fuels to solve these issues. Because of their abundant resources, relatively lower emissions and improved mixing and burning characteristics, the light alcohols, namely methanol and ethanol, have the potential to bring about some solutions to the addressed issues. This paper aimed to deliver comprehensive information including the productions, economic benefits, applications, demand and supply, environmental and human impacts of light alcohols and the emissions and performance effects of these alcohols in diesel engines to lead and contribute researchers in their investigations hereafter.
Section snippets
Production of light alcohols
Thermochemical and biochemical (fermentative) productions are two main methods for the production of biofuels [35].
Production methods of methanol have undergone changes for over a century [36]. Nowadays, there are several ways to obtain methanol from various feedstocks as an alternative fuel. These resources are biomass, natural gas, coke oven gas and CO2 [36], [37], [38], [39], [40]. On the other hand, ethanol is mainly produced from two feedstocks which are starch-based feedstock including
Worlds light alcohol supplies and demands
Methanol is produced the most commonly from natural gas all around the world. 85% of installed global capacity is located in Europe and North America. But other regions such as the Middle East, Africa and South America have been increasing the share of methanol production owing to the access to low cost natural gas. In Chine, with the growing demand for methanol and the existence of rich coal sources have been provided a sharp rise in coal-based methanol production in the beginning of 21th
Economic benefits of light alcohols
Currently, ethanol and methanol have been drawn attention mostly because of their presence into market and their potential to improve security and decrease the adverse effect of climate change. Biofuel usage has been mandated around the world. According to these legislations, biofuel usage will be obliged to consume 10% by 2020 in the EU and 36 billion gallons of biofuels by 2022 in the USA [90], [91].
Lin et al. [92] created a model to evaluate the ternary blend fuels consisting of ethanol,
Impacts of light alcohols on the environment and human health
Ethanol is widely known as ethyl alcohol and colorless liquid with an odor. It is commonly used as a raw synthesis material for production of ethylene, ethyl acrylate, ethylamine, glycol ethers, acetic acid, ethyl acetate, ether oxides as a solvent for extraction processes in laboratories and the production of inks, perfumes, varnishes, plastics, pharmaceuticals, adhesives, explosives cosmetics, paints) and as alternative fuel and additive in internal combustion engines [93], [94]. However,
Light alcohol applications as a fuel
Alcohol use was widespread even when the refined petroleum fuels were first developed. The usage of light alcohols in internal combustion engines as a fuel goes back to Nikolaus Otto who found the spark ignition engine in 1860. It is known Otto used and recommended alcohols and biodiesel fuels in his invention [108]. Since that time, the alcohols have been attempted to be integrated to internal combustion engines as an alternative fuel throughout history. With facing the oil crises of the 1970s
Light alcohols in CI engines
Light alcohols including methanol and ethanol have been considered as promising oxygenated additives in reduction of CO and NOx emissions [156], [157]. In addition, many esters converted from methanol and ethanol have been used as additives with blending with diesel fuel in CI engine [141], [142], [143], [144], [145], [146].
Physical and chemical properties give clues regarding behaviors of fuel in the combustion. Furthermore, all the parameters including engine performance, combustion quality
Conclusions and recommendations
Light alcohols (methanol, ethanol) are among the most promising alternative resources in terms of their low emissions and low costs to replace the conventional fuels. The production of light alcohols is mostly made through renewable sources so it makes more valuable and easier to meet the demand in case of its excessive usage. In the current paper, a comprehensive overview was made to display favorable and adverse outcomes of light alcohols. Overall, conclusions are summarized as follows:
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Light
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