An overview of engine durability and compatibility

Energy Conversion and Management 128 (2016) 66–81

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Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman

An overview of engine durability and compatibility using biodiesel–bioethanol–diesel blends in compression-ignition engines S. Dharma a,b, Hwai Chyuan Ong a,⇑, H.H. Masjuki a, A.H. Sebayang a,b, A.S. Silitonga a,b a b

Department of Mechanical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia Department of Mechanical Engineering, Medan State Polytechnic, 20155 Medan, Indonesia

a r t i c l e

i n f o

Article history: Received 24 March 2016 Received in revised form 17 July 2016 Accepted 25 August 2016

Keywords: Biodiesel Bioethanol Performance Emissions Durability Alternative fuels

a b s t r a c t The realization of declining fossil fuel supplies and the adverse impact of fossil fuels on the environment has accelerated research and development activities in renewable energy sources and technologies. Biofuels are renewable fuels made from edible, non-edible or waste oils, as well as animal fats and algae, and these fuels have been proven to be good substitutes for fossil fuels in the transportation sector. Bioethanol and biodiesels have gained worldwide attention in order to address environmental issues associated with fossil fuels, provide energy security, reduce imports and rural employment, as well as improve agricultural economy. Bioethanol has high oxygen content and octane content up to 35% and 108, respectively and hence, it increases oxygenation and improves combustion of fuel. In addition, bioethanol has lower vaporization pressure, which reduces the risks associated with evaporative emissions. In contrast, biodiesel has good lubricity, which helps protect the surface of engine components from wear and friction. The use of biodiesel–bioethanol–petroleum diesel blends poses a greater challenge with regards to improving the compatibility of the materials with the fuel system in compression ignition (CI) and spark ignition (SI) engines. In this work, the technical conditions of an engine (i.e. engine deposits, wear of the engine components and quality of the lubrication oil) are assessed by the application of with biodiesel–bioethanol–petroleum diesel blends. It is deemed important to evaluate the effects of using bioethanol and biodiesels in diesel engines. This paper provides insight on the feasibility of biodiesel and bioethanol feedstocks, the compatibility of biodiesels, bioethanol and their blends with diesel engines as well as the physicochemical properties of these fuels. Ó 2016 Published by Elsevier Ltd.

Contents 1. 2. 3.

4.

5.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Feasibility of biodiesel and bioethanol feedstocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Physicochemical properties of biodiesels, bioethanol and biodiesel–bioethanol–petroleum diesel blends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Properties of biodiesel and petroleum diesel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Properties of bioethanol and gasoline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Properties of biodiesel–bioethanol–petroleum diesel blends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Engine durability, performance and emission characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Biodiesel–petroleum diesel blends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Bioethanol–gasoline blends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Biodiesel–bioethanol–petroleum diesel blends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Issues related to the compatibility of engine materials with biofuels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. Wear and friction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.1. Wear and friction characteristics of engine components in which biodiesels are used as fuels . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.2. Wear and friction characteristics of engine components in which ethanol/bioethanol are used as fuels . . . . . . . . . . . . . . . . . . . 5.1.3. Wear and friction characteristics of engine components in which biodiesel–bioethanol–petroleum diesel blends are used as fuels 5.2. Corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

⇑ Corresponding author at: Department of Mechanical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia (H.C. Ong). E-mail addresses: [email protected], [email protected] (H.C. Ong). http://dx.doi.org/10.1016/j.enconman.2016.08.072 0196-8904/Ó 2016 Published by Elsevier Ltd.

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6. 7.

5.2.1. Corrosion behaviour of metals and their alloys immersed in biodiesels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.2. Corrosion behaviour of metals and their alloys immersed in ethanol/bioethanol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3. Lubricity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.1. Lubricity of biodiesel–petroleum diesel blends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.2. Lubricity of bioethanol–gasoline blends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.3. Lubricity of biodiesel–bioethanol–petroleum diesel blends. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Life cycle greenhouse gas emissions of biofuels. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction Energy crisis has become an important issue in recent years and is the focus of investigation in many countries throughout the world. The on-going economic growth coupled with the increasing standards of living have made us rely heavily on energy in our lives, especially after the Industrial Revolution in the last few centuries [1]. However, energy is typically generated from the combustion of fossil fuels in various sectors including agricultural, transportation and industrial sectors. This has an undesirable consequence since the use of fossil fuels is detrimental to the environment such as climate change and global warming [2]. The high demand for fossil fuels has led researchers to search for sustainable and environmental-friendly sources of energy [3]. Our dependency on fossil fuels is one of the reasons which lead researchers to develop alternative fuels such as biodiesel or bioethanol. Biodiesel is an alternative fuel which is environmental-friendly compared to diesel [4,5]. Biodiesel is biodegradable and non-toxic, and it has low carbon content, high lubricity and higher flash point compared to diesel. These properties make biodiesel an attractive substitute for diesel fuels [6]. Nowadays, researchers throughout the globe are working actively on producing biodiesels from various sources as well as blending biodiesels with diesel since this produces fuels with desirable physicochemical properties. The amount biodiesel added into diesel is typically within a range of 2–20% [7]. Besides, the combustion of biodiesels also produces nitrogen oxides (NOx), which is highly undesirable since it pollutes the environment. Furthermore, the combustion of biodiesels generates less engine power compared to diesel and the production of biodiesel is more costly compared to conventional fossil fuels [8]. Biodiesel can be produced from a variety of sources which include edible oils (e.g. canola, soybean, sunflower and palm oils), non-edible oils (e.g. Jatropha curcas, Calophyllum inophyllum, and Croton megalocarpus oils), waste oils as well as animal fats (e.g. chicken fat, beef tallow and poultry fat) [9,10]. Bioethanol is a type of biofuel and it is generally perceived that bioethanol is one of the solutions to address pollution issues resulting from the burning of fossil fuels [11]. More importantly, bioethanol can be mixed with diesel fuel without the need to modify existing engines. The amount of bioethanol added into diesel is typically within a range of 5–10% [12]. Bioethanol serves as an oxygenation additive when it is blended with gasoline, and it helps increase the octane number of the fuel while simultaneously reduce carbon monoxide (CO) emissions and air pollution [13]. Bioethanol produced from corn, sugar cane, cassava, and barley sugar are also known as first-generation bioethanol [14]. Owing to the increasing use of land mass for the cultivation of crops for bioethanol feedstocks as well as growing concern over food shortages, bioethanol is also produced from lignocellulosic materials. Such bioethanol is known as second-generation bioethanol and they are cheaper and environmental-friendly compared to

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first-generation bioethanol [15]. However, the third-generation bioethanol appears to be a more viable alternative compared other forms of bioethanol, whereby macroalgae, microalgae or seaweed are used as feedstocks [16]. These feedstocks offer a number of advantages over edible and non-edible feedstocks since they do not compete with other crops for arable land and water. In addition, algae fuels can produce energy per hectare up to 30– 100 times more compared with terrestrial plants such as corn and soybean [17]. A large number of studies have been carried out to investigate the effects of using biodiesel or bioethanol on engine performance and emissions. Despite the abundance of articles related to biodiesels and bioethanol available in the literature, there are only a few studies which are focused on biodiesel–bioethanol–petroleum diesel blends. Hence, this review is focused on the potential feedstocks used for biodiesel and bioethanol production, the physicochemical properties of biodiesels, bioethanol and biodie sel–bioethanol–petroleum diesel blends, and the use of biodie sel–bioethanol–petroleum diesel blends on the performance and emissions of compression-ignition (CI) and spark ignition (SI) engines. The compatibility of biodiesels, bioethanol and their blends with diesel engines is also discussed in this paper.

2. Feasibility of biodiesel and bioethanol feedstocks The steady growth of the world population over the years coupled with the increasing use of energy derived from fossil fuels leads to a critical need for renewable and sustainable sources of energy. Biofuels one of the alternative sources of energy to fulfil this need [18]. Biofuels are alternative fuels with great potential, providing energy security and bringing benefits to the economy and environment [19,20]. A number of countries have already put policies related to biofuel production and large-scale acquisition of lands into force [21]. For instance, India has been actively exploring biofuels since 2001 and a policy was issued in 2009 whereby 20% of the national diesel fuel requirements will be fulfilled by biofuels in 2017 [22]. The production of biodiesels from non-edible feedstocks such as Jatropha and Pongamia oils is expected to increase the energy security in India [23]. An energy policy has also been implemented in China with the issuance of the Renewable Energy Law in January 2006 along with a series of regulations [24]. China has set a target in which 15% of the energy use is derived from renewable sources by year 2020 [25]. Energy policies are also enforced in the USA, which is evidenced by the strong support given by the US government through the promulgation of the 2005 Energy Policy Act. In addition, the USA is rich in raw materials for biofuel production and therefore, it is likely that the USA will be the world’s largest biodiesel producer. It is projected that the USA will supply a total of 36 billion gallons (136 billion litres) of biofuels to the international market in 2022 [26].

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In 1999, Malaysia introduced a renewable energy programme known as the Five-Fuel Diversification Strategy, whereby palm oil is chosen as the biodiesel feedstock [27]. Malaysia is one of the leading producers of palm oil in the world, making up 42.3% of global palm oil production. The Government of Malaysia established agencies such as the Malaysian Palm Oil Board (MPOB) in order to ensure the sustainability of palm oil [28]. The Five-Fuel Diversification Strategy continued until 2006 when the Ministry of Plantation Industries and Commodities Malaysia implemented the ‘National Biofuel Policy’ in anticipation of the rising demand for fuels in the transportation sector, which involved encouraging the use of diesel fuel blended with 5% palm biodiesel [27]. Energy policies are also enforced in Indonesia, whereby the Government of Indonesia aims to substitute transportation fuels with 10% biofuels by year 2010. In response to this policy, 5.52 million hectares of unused lands were developed for energy production crops [27,29]. The Organisation for Economic Co-operation and Development (OECD) released a list of bioethanol and biodiesel-producing countries from 2012 to 2015, as shown in Tables 1 and 2 [30]. It can be seen that there is a growth in bioethanol and biodiesel production in recent years for all countries. It is evident that the USA is consistent in the development of bioethanol and biodiesel, producing 66,763.06 million litres of bioethanol and 4,986.91 million litres of biodiesel in 2015. 3. Physicochemical properties of biodiesels, bioethanol and biodiesel–bioethanol–petroleum diesel blends 3.1. Properties of biodiesel and petroleum diesel The fatty acid composition is the main chemical property which influences the engine injection, combustion and emission characteristics of the biodiesel [31]. In addition, the high saturated fatty acid content and low unsaturated fatty acid content of various feedstocks help improve the physicochemical properties of the

biodiesel [32]. The kinematic viscosity, density, cetane number, calorific value, flash point, oxidation stability, iodine value, cloud point and pour point are the important physicochemical properties which need to be considered when producing biodiesels for use in diesel engines [33,34]. In general, a high kinematic viscosity is undesirable because it reduces the intake stroke, which delays the mixing of air with fuel in the combustion chamber [33]. The kinematic viscosity is determined according to the method given in the ASTM D445 standard [35]. According to Cernoch., [36] the kinematic viscosity depends on the amount of free glycerol, free fatty acids and glycerides present in the biodiesel as well as temperature [37]. There is an increase in the oxidation of the fuel at high temperatures which in turn increases the fuel viscosity resulting from the Diels-Alder reaction, forming dimers and polymers [38]. Other biodiesel properties is density, which is an important physical property for biodiesels because it is used to determine the precise volume of fuel which needs to be injected into the combustion chamber [39]. The density affects the fuel injection process and it is related to the cetane number, calorific value and kinematic viscosity of the fuel [40]. The density is dependent on the fatty acid composition and purity of the biodiesel [41]. In general, decreasing the degree of unsaturated fatty acids improves the density of the biodiesel [41]. The density of the biodiesel is determined according to the method given in the ASTM D1298 standard [42]. According to Agarwal et al., [43] the fuel density is an important property since it influences the injection characteristics of the engine such as the total mass of fuel injected as well as pressure waves. It shall be highlighted here that the fuel density may also affect the production, transportation and distribution processes that take place in the internal combustion engine [44]. Flash point is the temperature at which the fuel will ignite when it is exposed to either spark or flame. Even though the flash point does not have a direct impact on the combustion characteristics, increasing the flash point ensures safe storage and

Table 1 World bioethanol production from 2012 to 2015 [30]. Country

United States of America Brazil China India Canada Thailand Pakistan Argentina Ukraine Republic of South Africa

Millions of litres 2012

2013

2014

2015

56,552.46 25,755.84 9361.44 2580.77 1732.04 967.98 634.95 564.15 440.79 459.01

58,571.11 28,370.26 9455.04 2774.36 1827.61 1093.39 631.73 595.29 513.61 513.95

61,851.90 31,393.59 9516.79 2927.53 1908.22 1218.08 671.53 646.38 578.55 568.37

66,763.06 34,485.14 9600.72 3085.56 2001.72 1342.68 704.07 673.03 643.03 622.85

Table 2 World biodiesel production from 2012 to 2015 [30]. Country

United States of America Argentina Brazil Indonesia Thailand India Australia Colombia Malaysia Canada

Millions of litres 2012

2013

2014

2015

4782.05 3173.94 2521.36 526.74 748.59 471.05 657.00 536.94 282.23 230.90

5001.45 3282.36 2589.43 633.56 809.08 559.86 665.15 575.15 430.53 267.35

4999.23 3400.22 2659.35 736.89 880.04 652.09 673.04 620.78 527.88 300.35

4986.91 3544.98 2731.15 1033.83 945.35 742.91 680.75 662.90 598.45 331.34


transportation of the fuel [45]. The flash point is defined as the minimum temperature at which a fuel produces sufficient vapour to burn momentarily, resulting in the first flash [42]. The flash point is measured according to the method given in the ASTM D93 standard and in general, the flash point is inversely proportional to the volatility of the fuel [45,46]. According to Mittelbach et al., [47] the decrease in flash point of the biodiesel results from the presence of alcohol residue and other low boiling-point solvents. The cloud point is the temperature at which wax crystals first become visible when the fuel is cooled. The cloud point is measured when wax crystals begin to form upon cooling the fuel [7,48]. The presence of solidified wax thickens the oil, which clogs the fuel filters and injectors in internal combustion engines. Furthermore, the solidified wax accumulates on the cold surface of the engine parts, forming an emulsion with water [49]. For this reason, the cloud point is also an indicator of the tendency of the oil to plug filters or small orifices at cold operating temperatures [49]. In contrast, the pour point is the lowest temperature at which the fuel is still able to flow. In general, biodiesels with higher saturated fatty acid content will have a higher pour point. One of the main issues concerning the use of biodiesels is their poor physicochemical properties at low temperatures, particularly biodiesels produced from palm oil containing stearic and palmitic acids. It has been shown that the higher the unsaturated fatty acid content, the poorer the physicochemical properties of the biodiesel at low temperatures [48]. According to Imahara et al., [50] the cloud point of a biodiesel can be determined from the amount of saturated chains that are separated from unsaturated chains. To date, blending biodiesels with diesel fuel is the most common method used to improve the cold flow properties of the fuel since the diesel fuel acts as a solvent when crystals, waxes or gels form in the fuel [48]. Oxidation stability is the tendency of the fuel to react with oxygen at ambient temperature, and it reflects the relative vulnerability of the fuel degraded by oxidation [51]. According to Dunn et al., [52] biodiesels have lower oxidation stability compared to petroleum diesel. The FAME content and the existence of natural antioxidants in the feedstocks used for biodiesel production are the factors which affect the oxidation stability of biodiesels [53]. The auto-oxidation of FAMEs is a chain reaction which involves three basic steps: initiation, propagation and termination [54]. Oxidation is characterized by a free radical mechanism which generates hydroperoxides, short-chain carboxylic aldehydes, ketones and acids [55]. These radical peroxides generate new radicals in the esters which bind oxygen in the air, and the hydroperoxides grow rapidly to the propagation stage. The formation of decomposed byproducts occurs at an exponential rate during the auto-oxidation phase [31]. Cetane number is a relative measure of the time delay between the injection and auto-ignition of the fuel [56]. Cetane number is used as an indicator of the quality of fuel combustion during the ignition process [57]. Cetane number may be represented by the elapsed time between the start of fuel injection and the onset of ignition [58]. In general, it is desirable if the fuel has a high cetane number for CI engines [59]. Cetane number can be controlled by the addition of 0.25 vol% of additives or by changing the composition of the hydrocarbons [60]. One of the main advantages of using biodiesels is that they are biodegradable, whereby they can be decomposed by living organisms. In addition, biodiesels can be used in existing diesel engines without the need for engine modifications and they produce less harmful gas emissions such as sulphur oxide [61]. By using biodiesel, the release of carbon monoxide and sulphur content can be reduced by 10 and 30%, respectively [62]. The sulphur content of biodiesels produced from rapeseed (glucosinolates) or animalderived feedstocks is lower due to the contact between the

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biodiesels and animal meat or bones. However, biodiesels generally have low sulphur content which fulfils the specification given in the ASTM D6751 biodiesel standard, which is 0.05% by mass [63,64]. The quality of biodiesel is assessed by its measuring its physicochemical properties in accordance with the methods outlined in the ASTM D6751 and EN14214 standards [65]. The physicochemical properties of biodiesels produced from edible feedstocks, non-edible feedstocks, waste cooking oils and animal fats are summarized in Tables 3 and 4 [1,66–79]. 3.2. Properties of bioethanol and gasoline Bioethanol and ethanol are essentially the same substance since they have the same molecular formula and structure. However, bioethanol is produced from sugar whereas ethanol is produced from plants [80]. Bioethanol is an oxygenated fuel which contains 35% oxygen and this reduces particulate matter and NOx emissions from combustion [81]. In general, bioethanol has higher octane number, higher enthalpy (i.e. heat of evaporation), higher flame speed and a wider flammable range compared to gasoline [82]. These properties result in a higher compression ratio, shorter burning time and leaner combustion engines [83]. These are some of the advantages of using bioethanol in internal combustion engines [82]. In addition, ethyl tertiary butyl ether is produced synthetically from bioethanol, and it serves as an octane enhancer which reduces exhaust emissions [84]. Blending gasoline with 10% of bioethanol improves the physicochemical properties of the fuel for SI engines in accordance with the EN 228 standard [85]. Several studies have shown that blending ethanol with gasoline improves engine torque, engine power and thermal efficiency braking [86,87]. However bioethanol has its own disadvantages. The energy content of bioethanol is only 66% of the energy content for gasoline, which indicates that bioethanol has lower energy density. In addition, bioethanol is corrosive, miscible with water and toxic to the ecosystem. Bioethanol also has low flame luminosity and low vapour pressure, making cold starts difficult [88]. However, Masum et al., [80] highlighted that bioethanol has high octane number and therefore, the fuel is able to withstand high compression before detonation/ignition. It is known that premature ignition of the fuel can cause damage to the engine. The physicochemical properties of methanol, ethanol, gasoline and bioethanol are summarized in Table 5 [83,84,89]. 3.3. Properties of biodiesel–bioethanol–petroleum diesel blends In general, biodiesel–bioethanol–petroleum diesel blends produce lower carbon monoxide (CO), carbon dioxide (CO2) and particulate matter up to 40% [90]. The solubility of bioethanol– biodiesel–petroleum diesel blends is influenced by two factors: temperature and water content. The components of these blends can be mixed easily at ambient temperatures but they will separate at temperatures below 10 °C [91]. An emulsifier can be added to prevent separation of the different fuels in the blend, which will suspend small droplets of ethanol in the petroleum diesel. Alternatively, a co-solvent can be added into the blend, which acts as a bridging agent and produces a homogeneous mixture through molecular compatibility and bonding [14]. Fig. 1 shows the phase behaviour of an ethanol–biodiesel–petroleum diesel blend [90]. It can be seen from Fig. 1 that bioethanol, biodiesel and petroleum diesel can be blended at room temperature, provided that the percentage volume of petroleum diesel is less than 20%, since the three components of the blend fall within the one-phase region [90,91]. It is known that the temperature and purity of bioethanol affects the phase formed in biodiesel–bioethanol–petroleum diesel

848.5 4.42 53.59 67.45 40.1 0.28 8 166 0 – – 62.1 – – 0 – 2 –

4. Engine durability, performance and emission characteristics

931 6.13 55 – 43.42 0.42 3 95 7 – – – – – – – – – 879.5 4.8 51.6 104 39.23 0.4 2 135 2.7 0 1a – – – 1.2 0.009 3.2 – 913.8 4.039 37.9 128–143 39.76 0.266 2 76 9 11 1b – – – 0.8 0.005 2.1 – 880 4.439 49 – – 0.027 – 160 3.4 3 1a – – – 0.2 0.005 0.9 – 864.42 4.5 54.6 54 – 0.24 15 135 16 12 1a – – – 0.003 0.002 10.3 172 860–900 3.5–5.0 Min. 51 Max. 120 Min. 35 Max. 0.5 – >120 – Max. 5 – – – – – 0.02 Min. 6 – Max. 880 1.9–6.0 Min. 47 – – Max. 0.5 15 to 16 Min. 100–170 3 to 12 19 Max. 3 Min. 77 Min. 12 Min. 11 Max. 0.05 0.02 Min. 3 314 839 2.91 49.7 – 45.825 0.17 1.0 71.5 2.0 – 1 88.5 13.5 0 – – 23.7 – kg/m3 mm2/s – mg I2/g MJ/kg mg KOH/g °C °C °C °C – wt% wt% wt% % (m/m) % (w/w) h – Density at 15 °C Kinematic viscosity at 40 °C Cetane number Iodine value Calorific value Acid number Pour point Flash point Cloud point Cold filter plugging point Copper strip corrosion (3 h at 50 °C) Carbon Hydrogen Oxygen Sulphur content Sulphate ash content Oxidation stability at 110 °C Lubricity

850 2.0–4.5 40–55 – 42–46 – 15 to 5 60–80 35 to 15 25 1 84–87 12–16 0–0.31 0.05 0.01 – 685

Pongamia FAME

Biodiesel produced from non-edible

Jatropha FAME Soybean FAME Petroleum diesel

ASTM D6751 limit

EN 14214 limit

Sunflower FAME

Biodiesel produced from edible

Palm FAME

Petroleum diesel

ASTM D975 limit

Unit Property

Table 3 Physicochemical properties of biodiesels produced from edible and non-edible feedstocks compared to petroleum diesel [1,66–79].

876.7 4.11 55 – 40.43 0.19 6 153 7 1 1a – – – 1.9 0.005 1.85 139.5 888.6 7.724 51.9 85 – 0.76 – 151 38 – 1b – – – 16 – – –

blends [92]. In general, the intersolubility of the components in these blends decreases with a decrease in temperature [93]. It has been shown that blends composed of 15% biodiesel, 5% bioethanol and 85% diesel have a calorific value close to that of petroleum diesel [94]. According to Lapuerta et al., [95] blending biodiesel, bioethanol and petroleum diesel results in an unstable formation of two liquid phases which may be a gelatinous interphase or gelatinous phase at the bottom of the glass cell. Chotwichien et al., [92] investigated the effect of blending ethanol and butanol with palm biodiesel and diesel at various temperatures. They observed that blending bioethanol (up to 10%) with diesel at 10 °C results in good solubility but decreases the density, kinematic viscosity and cetane number of the fuel. However, this can be compensated by the addition of biodiesel into the blend. The physicochemical properties of biodiesel–bioethanol–petro leum diesel blends are summarized in Table 6 [14,92,96–102].

Cottonseed FAME Calophyllum FAME


Peanut FAME

70

4.1. Biodiesel–petroleum diesel blends One of the main advantages of biodiesels is their high lubricity, which helps reduce friction losses and improve brake power. In addition, biodiesels have high cetane number, low sulphur content and oxygen content within a range of 10–11 wt% [103]. However, a high percentage volume of biodiesel in biodiesel–petroleum diesel blends can decrease engine performance due to the increase in kinematic viscosity and density as well as decrease in calorific value [104,105]. In terms of durability, biodiesels are considered superior to petroleum diesel since they have lower soot formation and higher lubricity, which will reduce particulate emissions [106]. However, according to Mofijur et al., [77] the burning of biodiesels over long hours will degrade engine components such as hoses, gaskets, O-rings, elastomer seals, adhesives and plastics since the rubber, adhesives and plastics will begin to break down. The addition of biodiesel into diesel fuel also increases the tendency of fuel filter blockage [107]. This is due to the formation of wax crystals in the diesel fuel and the amount of wax crystals increases with a further decrease in temperature, which will eventually clog fuel filters and injectors. The accumulation of the wax crystals results in fuel gelation and the fuel eventually stops flowing [108]. Bari et al., [109] studied the performance of a four-stroke diesel engine powered metrobus fuelled by ultra-low sulphur diesel (ULSD) blended with 20% canola biodiesel. The results showed that the engine torque and engine power of the 20% canola biodiesel–diesel blend are similar to those for ULSD. In addition, the UHC and CO emissions are reduced for this blend, however, the NOx emissions increases by 4.4%. Ozener et al., [110] observed the performance, emission and combustion characteristics of soybean biodiesels (i.e. SB10, SB20 and SB50) in direct injection (DI) diesel engine with an engine speed from 1200 to 1300 rpm. The results showed that a decline in engine torque by 1–4% results in a significant reduction of CO emissions. The reduction of CO emissions and significant increase of NOx emissions is due to the high oxygen content of the biodiesel, which leads to advanced injection process [111]. Moreover, biodiesels lead to lower ignition delay and lower premixed combustion phase, and they can be used readily in diesel engines without any engine modifications. Kumar et al., [112] reported that the following Jatropha biodiesel–diesel blend (J40) can be used successfully in a modified engine during an endurance test up to 512 h. Furthermore, the increase in the premixed combustion phase and fuel spray can reduce carbon deposition. In general, the heat release rate is one of the indicators of the fuel combustion characteristic and it depends on the ignition delay

71

S. Dharma et al. / Energy Conversion and Management 128 (2016) 66–81 Table 4 Physicochemical properties of biodiesels produced from waste cooking oils compared to animal fats [66,69,71,72,74–76]. Property

Density at 15 °C (kg/m3) Kinematic viscosity at 40 °C (mm2/s) Calorific value (MJ/kg) Cetane number Flash point (°C) Pour point (°C) Cloud point (°C) Cold filter plugging point (°C) a b c d e f g h i j k l m

Biodiesel WFOMEa

WFOME

Waste palm oil

WMEb/WEEc

WFPOd

WCOMe/WCOEf

UCO

UVOEEg

LMEh

TMEi

CFMEj

YGMEk

SMEl

NAFm

890 4.23

888 4.32

873.7 –

888.2/854.8 4.68/4.98

877.2 6.32

887/878 5.16/4.92

– 5.18

887.2 6.13

– 4.8

– 5

– 4.3

873 5.16

885 4.3

920 –

– 54.5 171 – – 1

39.55 52 156 2.5 3 –

39.31 – 109 0 0 –

37.27/40.72 – – 3/6 1/2 6/4

39.87 62 130 10 – –

39.26/39.48 – – – – –

– 48 148 4 – –

– 47.9 130 10 10.7 –

– – 160 12 11 8.3

– 150 9 11 8

– – 150 6 4.3 1.3

– 62.6 – 12 9 –

– 51.3 169 – 6 –

– 40 – – – –

Waste frying oil methyl ester. Waste fryer grease methyl ester. Waste fryer grease ethyl ester. Waste frying palm oil. Waste cooking oil methyl ester. Waste cooking oil ethyl ester. Used vegetable oil ethyl ester. Lard methyl ester. Tallow methyl ester. Chicken fat methyl ester. Yellow grease methyl ester. Soapstock methyl ester. Neat animal fat.

Table 5 Physicochemical properties of methanol, ethanol, gasoline and bioethanol [83,84,89]. Property

Methanol (CH3OH)

Ethanol (C2H5OH)

Gasoline (C4–C12)

Bioethanol

Molecular weight (g/mol) Specific gravity Vapour density relative to air Liquid density (g/cm3 at 298 K) Boiling point (K) Melting point (K) Heat of evaporation (Btu/lb)

32 0.789 (298 K) 1.1 0.79 338 175 472

46 0.788 (298 K) 1.59 0.79 351 129 410

114 0.739 (288.5 K) 3.0–4.0 0.74 300–518 – 135

46.07 – – 0.792 351.5 – 367.13

Heating value (kBTU/gal) Lower heating value (LHV) Higher heating value (HHV) Tank design pressure (psig) Viscosity (cP) Flash point (K)

58 65 15 0.54 284

74 85 15 1.2 287

111 122 15 0.56 228

– – – – –

Flammable/explosion limits (%) Lower flammable limit (LFL) (%) Upper flammable limit (UFL) Auto-ignition temperature (K) Solubility in H2O (%) Azeotrope with H2O Peak flame temperature (K) Minimum ignition energy in air (mJ) Oxygen (wt%) Octane number (RON)

6.7 36 733 Miscib. (100%) None 2143 0.14 – –

3.3 19 636 Miscib. (100%) 95% EtOH 2193 0.23 – –

1.3 7.6 523–733 Negl. (0.01) Immiscible 2303 – 0 86–94

– – 696 – – – – 35 105–108

and fuel properties [113]. Analysis of the combustion and heat released by the biodiesel is used to reduce both energy and fuel consumption and attain acceptable engine performance parameters [114]. Teoh et al., [115] investigated the effect of Jatropha biodiesel blends on the combustion characteristics of a diesel engine with a common-rail cylinder at various operation loads. The results showed that the peak of the apparent heat release rate (AHRR) is lower for the biodiesel blends at low operation loads. However, the peak of the AHRR for the biodiesel blends is comparable to that for diesel at high operation loads. In addition, the biodiesel blends reduce ignition delay and speed up the burning time for all engine operation loads.

Mangus et al., [116] observed the combustion characteristics of coconut, Jatropha, soybean and beef tallow biodiesels in a CI engine. The results showed that increasing the percentage volume of biodiesel in the biodiesel–diesel blend reduces the heat release rate and the premixed combustion phase over a longer duration, which is due to the increase in fuel kinematic viscosity. The increase in the combustion duration is also a consequence of the flash point and low volatility due to the increase in fuel kinematic viscosity [114]. Ozcelik et al., [117] found that the maximum heat release rate (HRR) is 35 J/crank angle degree for a 1.9 multi-jet diesel engine operating at an engine speed of 2500 and 4000 rpm using Camelina biodiesel.

72


significant decrease in the maximum cylinder gas pressure and the maximum heat release rate. In addition, the combustion initiation is delayed with retarded second fuel injection timing. In another study, Turkcan et al., [126] investigated the effect of various parameters on the performance of a DI HCCI engine and the observations were focused on the start of the first and second injection. The results showed that the start of the first injection has a significant effect on the cylinder gas pressure and heat release rate. In addition, increasing the ethanol content in the bioethanol–gasoline blends decreases the maximum cylinder gas pressure compared to gasoline for the test conditions considered in their study. 4.3. Biodiesel–bioethanol–petroleum diesel blends Fig. 1. Phase behaviour of an ethanol–biodiesel–petroleum diesel blend [90].

4.2. Bioethanol–gasoline blends At present, bioethanol–gasoline blends are widely used as alternative fuels in the transportation sector [118]. The high octane number and density of bioethanol–gasoline blends increases the brake thermal efficiency and compression ratio of the engine without pre-ignition [118]. In addition, it is possible to achieve complete combustion in the combustion chamber due to the high oxygen content of bioethanol (35 wt%), which reduces CO and UHC emissions [86,119]. A number of studies have been conducted over the years to determine the performance, emissions and durability of bioethanol–gasoline blends [120]. Yusecu et al., [121] discovered that E40 and E60 bioethanol–gasoline blends reduce exhaust emissions significantly, whereby the average UHC emission is reduced by 16.45%. However, ethanol fuels are not without limitations even though they are widely used in the transportation sector. Yoon et al., [87] investigated the performance of a four-cylinder SI engine fuelled with E85G15, E100 and G100 fuels and the results showed that these fuels tend to reduce CO, CO2 and UHC emissions, but not NOx emissions. In addition, they observed a 10% decrease in the brake mean effective pressure (BMEP) for the E85G15 fuel. In contrast, the decrease in the BMEP is more significant for the E100 fuel, with a value of 25%. According to Al-Hasan et al., [122] investigated the effect of bioethanol–gasoline blends on the performance of a four-cylinder SI engine (TOYOTA Tercel-3A). The results showed that the bioethanol–gasoline blend containing 20% bioethanol reduces the BSFC by 5% compared to gasoline. The addition of bioethanol into gasoline increases the engine speed and overall brake power, decreases the engine torque and results in loss of fuel economy and mileage since the energy value of bioethanol is lower than that for gasoline [123]. However, bioethanol with a high octane rating can also be used to modify the research octane number (RON) of gasoline. This gives greater thermal efficiency and/or more aggressive turbocharging to the engine and therefore, the engine can be downsized with a higher compression ratio [13]. Costagliola et al., [124] investigated the effect of bioethanol–gasoline blends (0, 10, 20, 30 and 85%) on the combustion efficiency of a SI engine. They observed that there is no significant difference in the combustion development whereas the global efficiency is slightly improved with an average value of roughly +5%. The addition of ethanol into gasoline also decreases the duration of combustion initiation due to the fact that ethanol has a faster laminar flame velocity [124]. Turkcan et al., [125] investigated the effect of second injection timing on the performance of homogeneous charge compression ignition (HCCI) engine using gasoline mixed with an alcohol (i.e. ethanol and methanol). The results showed that there is a

The steady increase in fuel consumption, the gradual depletion of fossil fuels as well as the growing concern over pollution resulting from the burning of fossil fuels have led to the development of alternative fuels that are renewable such as biodiesels and bioethanol [127]. Both of these fuels are able to fulfil the fuel specifications given in the standards and therefore, they can be used to substitute petroleum diesel in the future [91]. It is expected that the addition of biodiesel or bioethanol into petroleum diesel will enhance the physicochemical properties of the fuel [128]. According to Zhu et al., [129], biodiesel–bioetha nol–petroleum diesel blends improve the engine performance and increase the brake thermal efficiency of four-cylinder DI diesel engines. The addition of bioethanol helps reduce NOx and particulate emissions from diesel engines [129,130]. Barabbas et al., [96] highlighted that biodiesel–bioethanol–petroleum diesel blends decrease CO and UHC emissions. The decrease in particulate emissions is due to the higher oxygen content of the fuel which lowers the air/fuel stoichiometric ratio. This reduces the aromatic content of the fuel which in turn, reduces carbonaceous soot [131]. According to Labeckas et al., [130], fuel oxygenation is important to increase auto-ignition delay and combustion pressure, as well as to reduce CO emissions and smoke opacity. However, the effect of bioethanol–biodiesel–petroleum diesel blends on material durability is of primary concern. Armas et al., [132] found that the use of ethanol–biodiesel–petroleum diesel fuel affects the durability of the fuel injection pumps and injection nozzles to a similar extent as that for diesel fuel. The differences between the effective sections of the holes of the nozzles before and after the test are below of 0.1 lm for both fuels. Kannan et al., [133] tested the durability of injection pumps and nozzle injectors of a single-cylinder DI diesel engine fuelled by ethanol– biodiesel–diesel blends and the results revealed that the etha nol–biodiesel–diesel blends have the same effect on the durability of these engine components as that for diesel. The performance and emissions of engines fuelled by various biodiesel–bioethanol–petroleum diesel blends are summarized in Table 7 [96,99,109,129,130,134–140]. 5. Issues related to the compatibility of engine materials with biofuels 5.1. Wear and friction Wear of engine components is greatly influenced by the quality and lubricity of the fuel. Wear occurs due to friction and it is undesirable since it reduces the lifetime of the engine components [141]. Friction occurs when two surfaces in contact slide against each other, and severity of friction is dependent on the load, speed, temperature, type of lubricant as well as additives [142]. There are several engine components in which fuel is used to lubricate these

Table 6 Physicochemical properties of biodiesel–bioethanol–petroleum diesel blends [14,92,96–102]. Miscibility and stability of blends

Density (kg/m3)

Cold filter plugging point (°C)

Kinematic viscosity (mm2/s)

Lubricity (lm)

Surface tension (mN/m)

Flash point (°C)

Cloud point (°C)

Calorific value (kJ/kg)

Cetane number

Pour point (°C)

Carbon content (wt%)

Oxygen content (wt%)

70:10:20 JBD 99.7% 50:10:40 JBD 99.7% 85:12:3 SBD 99.7% 80:16:4 SBD 99.7% 60:30:10 WCOBD 99.9% 50:30:20 WCOBD 99.9% 90:5:5 RSOBD 99.3%

– – 3 months at NAC 3 months at NAC 1 month at 30 °C 1 month at 30 °C 30 h at 20 and 0 °C; Separated into two phases after 30 h at 8 °C 30 h at 20 and 0 °C; Separated into two phases after 30 h at 8 °C 30 h at 20 and 0 °C; Clear with sediment after 30 h at 8 °C One-phase liquid after 3 months at room temperature One-phase liquid after 3 months at room temperature One-phase liquid after 3 months at room temperature Homogeneous blend after 3 months at room temperature Homogeneous blend after 3 months at room temperature

832.87a 820.42a 840b 840b 826d 821d 843.7a

– – – – – – 18

2.380c 2.018c 3.01c 3.03c 2.44d 2.14d 2.435c

– – – – – – 305

– – – – – – 30.79b

14 12 – – 18.5 15 17.5

– – – – 1 4 –

39,930 36,338 41,500 41,200 39,100 37,850 41,707

50 41 – – 47.3 47.2 51.04

3 12 – – 3 6 –

78.69 72.07 – – – – 83.22

7.77 14.53 2.3 3.1 – – 2.2

845a

17

2.421c

232

34.62b

14

–

41,560

51.2

–

82.79

2.76

847.2a

13

2.527c

276

34.66b

16

–

41,414

51.36

–

82.37

3.32

c

85:10:5 RSOBD 99.3% 80:15:5 RSOBD 99.3% 80:15:5 POBD (PME) 99.5% 80:15:5 POBD (PEE) 99.5% 80:15:5 POBD (PBE) 99.5% 82.5:12.5:4.5:0.5 (HE) POBD 99.5% 84:11:4.75:0.25 (HE) POBD 99.5%

a

–

2.63

–

–

17

–

43,800

53.2

3

–

–

837.8a

–

2.72c

–

–

15.7

–

39,300

–

3

–

–

837a

–

2.73c

–

–

15

–

43,700

–

3

–

–

–

c

–

–

–

–

44,430

56.2

–

–

–

c

–

–

–

–

44,560

57.91

–

–

–

838.3

a

829

a

828

–

2.82 2.69

NAC: Normal ambient condition; JBD: Jatropha seed biodiesel; SBD: Soybean biodiesel; WCOBD: Waste cooking oil biodiesel; RSOBD: Rapeseed oil biodiesel; POBD: Palm oil biodiesel; PME: Palm oil methyl ester; PEE: Palm oil ethyl ester; PBE: Palm oil butyl ester; RME: Rapeseed oil methyl ester; HE: Hydrous ethanol a At 15 °C. b At 20 °C. c At 40 °C. d At 27 °C.


Diesel; Biodiesel; Ethanol/Bioethanol

73

74

Table 7 Performance and emissions of engines fuelled by various biodiesel–bioethanol–petroleum diesel blends [96,99,109,129,130,134–140]. Fuel type

Engine specification

Test condition

Engine performance BSFC

BTE

NOx

Emissions UHC

CO

Smoke

–

" 4.8%

;

; 20%

–

[109]

" Min. 400 ppm;

" Min. 1250 ppm;

; Max. 2%

; Max. 8 m1

[136]

; ; ; " " ;

; Max. 3000 ppm " Min. 25 ppm; " Max. 90 ppm ; Min. 14.7%; ; Max. 43.2% –

" Min. 0.2%; " Max. 0.6%

[137]

Oil without preheating: ; Min. 3 g/kW h;

; Min. 73%; ; Max. 94% Oil without preheating: " Min. 10 g/kW h;

; Min. 8%; " Max. 78% " Min. 33.8%; " Max. 43.2% ; Min. 20%; ; Max. 80% Oil without preheating: ; Min. 5%;

" Max. 13 g/kW h Oil with preheating: ; Min. 2.8 /kW h;

" Max. 530 g/kW h Oil with preheating: " Min. 50 g/kW h;

" Max. 98%; Oil with preheating: ; Min. 5%;

4-stroke, 6-C MAN metrobus 1-C, 4-stroke, NA

2200 rpm

–

2200 rpm

B10, B20, B30, B40 (soapnut oil)

1-C, CI, WC

1500 rpm

B20, B100 (rice bran oil, Pongamia 1-C, DI, WC pinnata oil) B20, B40, B60, B80, B100 (karanja oil) 1-C, WC, CI

1500 rpm

" Min. 300 g/ " Min. 18.5%; kW h; ; Max. 460 g/kW h ; Max. 28% – " Min. 1%; ; Max. 27% Min. 3.2%, " 16.8% " Min. 4.9%; " Max. 6.8% ; Min. 0.8–7.4%; "Max. 22.71–26.79% " Max. 11–48% Oil without preheating: ; Min. 400 g/ Oil without kW h; preheating: " Max. 900 g/kW h 0–25%; Oil with Oil with preheating: preheating: ; Min. 400 g/ 0–27% kW h; ; Max. 790 g/kW h " Min. 230 g/ " Min. 16%; kW h; ; Max. 600 g/kW h ; Max. 45% " 14% " (increase is not significant) " Min. 0.075 g/kJ; – ; Max. 0.2 g/kJ " Min. 6.2%; ; Min. 0.4%; " Max. 32.4% ; Max. 1.7% " Min. 210 g/ " Min. 12.5%; kW h; ; Max. 7120 g/ ; Max. 37.5% kW h " Min. 220 g/ ; Min. 0.14%; kW h; ; Max. 680 g/kW h ; Max. 0.38%

3000 rpm

B10, B20, B50, B75, B100 (karanja oil) AC 1, 1-C, 4-stroke, WC, 1500 rpm DI, CI

BE5, BE10, BE15 (waste cooking oil and blended with ethanol)

4-C, DI, WC, NA

1800 rpm

E60

4-C, SI

E0, E5, E10, E20, E30

New Sentra GA16DE

1000– 2800 rpm 1000– 4000 rpm 1400 rpm

D100, D85B10E5, D80B10E10, 4-C, CI D70B25E5 (rapeseed oil) D70E20B10, D50E30B20, D50E40B10, Multi-C, DI D100

D85E95, D90E10, D85E15, D80E15B5 4-C, 4-stroke, NA, DI, (rapeseed oil methyl ester) WC

1600 rpm

2200 rpm

Max. 600 ppm Min. 10%; Max. 35% Min. 10.8%; Max. 28.6% 26% (4–12 ppm)

–

[138] [139] [134]

" Min. 5 g/kW h;

" Max. 10 g/kW h " Max. 350 g/kW h " Max. 100% " Min. 0.5 g/kW h; " Min. 1 g/kW h; –

[129]

; Max. 15 g/kW h ;

; Max. 9 g/kW h ; 48%

; Max. 25 g/kW h ; 70%

[140]

; Min. 50%; " Max. 400% " Min. 350%vol; ; Max. 1140%vol " Min. 3 g/kW h; ; Max. 18.5 g/kW h

" ; " ;

; " " ; "

" Min. 160 ppm; ; Max. 1700 ppm

Min. 7.5%; Max. 80% Min. 13 ppm; Max. 51 ppm

Min. 20%; Max. 90% Min. 0.234%vol; Max. 0.575% vol Min. 1 g/kW h;

– –

[138] 1

" Min. 0.1 m ; [96] ; 5.5 m1 " Opacity max. 1%; [99]

; Max. 58 g/kW h

; Max. 68%

" Min. 1 ppm;

" Min. 220 ppm;

; Max. 1010 ppm

; Max. 1000 ppm

" Opacity max. 0.2%; ; Max. 35%

Engine performance analysis codes: BSFC: brake specific fuel consumption; BTE: brake thermal efficiency; EGT: exhaust gas temperature; BMEP: brake mean effective pressure; BSEC: brake specific energy consumption. Fuel codes: B: biodiesel; E: ethanol/bioethanol; ULSD: ultra-low sulphur diesel. Engine codes: C: cylinder; DI: direct injection; AC: air-cooled; WC: water-cooled; NA: naturally aspirated; CI: compression-ignition. Emission codes: NOx: nitrogen oxides; UHC: unburned hydrocarbons; CO: carbon monoxide; CO2: carbon dioxide; SO2: sulphur dioxide; HSU: Hartridge smoke unit; PM: particulate matter. ": highest; ;: lowest.

[130]


B0, B20 (canola oil) – blended with ULSD B0, B5, B10 (waste sunflower and waste mixed oils)

References


components such as fuel injectors and pumps [143]. Good lubrication helps reduce friction between the engine components, which in turn, reduces energy consumption and wear [144]. Fuels with different compositions have been developed over the years in order to fulfil fuel specification standards in different countries as well as to fulfil environmental regulation standards by limiting the emission of harmful pollutants such as sulphur and nitrogen in the fuel [145]. Even though hydrogenation process is very effective to reduce sulphur compounds, it also removes polyaromatic, nitrogen, oxygen and unsaturated components, which decreases the lubricity of the fuel [145]. Fig. 2 shows the schematic diagram of a four-ball wear test configuration in accordance with the method given in the ASTM D4172 standard [141,146]. 5.1.1. Wear and friction characteristics of engine components in which biodiesels are used as fuels High compression pressure is required in diesel engines in order to improve fuel atomization, which in turn, controls the combustion process and reduces the emission of pollutants into the atmosphere [147]. In order to fulfil the requirements for high compression pressures and temperatures, engine components should be made from materials that are lightweight with high resistance to wear and corrosion and more importantly, the materials should be compatible with the fuel [148]. Several studies have shown that biodiesels provide a protective layer on metal surfaces [141]. Haseeb et al., [149] discovered that the lubricity of the biodiesel decreases with an increase in temperature and this leads to surface deformation due to wear. In addition, it has been shown that wear decreases with an increase in the percentage volume of biodiesel. The wear of the steel ball decreases by 20% for the B100 biodiesel. In contrast, the wear of the steel ball decreases by only 5% when diesel is used as the test fuel [141]. Dhar et al., [2] investigated the wear and durability of engine components whereby the test fuel was composed of diesel blended with 20% of karanja biodiesel. The results showed that the B20 biodiesel–diesel blend results in minor wear of the valves, pistons, piston rings, liners and small end bearings of the connecting rods. Aparecida et al., [150] compared the friction and wear characteristics of soybean biodiesel and petroleum diesel and the results indicated that the coefficient of friction for petroleum diesel is higher than that for soybean biodiesel and the wear coefficient is

Fig. 2. Schematic diagram of a four-ball wear test configuration: (1) rotating gripper for upper ball, (2) test fuel, (3) cup for gripping three stationary balls, (4) rotating ball, (5) stationary ball [141,146].

75

within a range of 108–109 mm3/N.m. Habibullah et al., [151] also obtained similar results whereby the steady-state friction coefficient of Calophyllum inophyllum biodiesel is higher than that for biodiesel–diesel blends by 4–16%. In addition, the average wear scar diameter is higher by 41.01% for diesel fuel compared to the Calophyllum inophyllum biodiesel. Ashraful et al., [152] investigated the effect of adding anticorrosion additives into palm olein biodiesel–diesel blend on wear characteristics of metals. The results showed that the addition of anti-corrosion additives reduces wear debris by 17.3% for iron (Fe), whereas the wear of aluminium (Al) and copper (Cu) is reduced by 16.1% and 19.3%, respectively. Mosarof et al., [142] conducted wear test on engine components in which palm oil fuel was used as the test fuel and they concluded that the palm oil fuel has good lubricity and therefore, it increases the anti-wear characteristics of the engine components. However, the palm oil fuel produces a comparable amount of carbon deposits. 5.1.2. Wear and friction characteristics of engine components in which ethanol/bioethanol are used as fuels Bioethanol is one of the oxygen-containing compounds commonly tested for use as a component in fuel blends [96]. According to Yücesu et al., [121] anhydrous bioethanol has low kinematic viscosity, poor lubrication properties, low flash point and low cetane number, and it has been shown that the wear characteristics of engine components will increase when the percentage volume of bioethanol is increased in the fuel blend due to the decrease in kinematic viscosity [122]. Kinematic viscosity is a vital property to ensure good lubrication of fuel systems, particularly systems equipped with rotary fuel pumps [33]. In general, a low kinematic viscosity leads to excessive wear of the fuel injection system [153]. A number of researchers have investigated the compatibility between bioethanol and the materials of engine components, particularly components in fuel injection systems [31]. For instance, Bandeira et al., [154] investigated the friction and wear behaviour of plasma-nitrided and CrN-coated AISI 4140 steel in ethanol fuel and ethanol-oil mixture. The results showed that CrN-coated steel run in ethanol fuel has lower coefficient of friction and wear rate compared to the CrN-coated steel in dry sliding condition. In addition, there is a decrease in the friction and wear characteristics of the CrN-coated AISI 4140 steel immersed in engine oil during sliding tests compared to the CrN-coated AISI 4140 steel immersed in ethanol fuel. Tung et al., [155] investigated the friction and wear characteristics of nitrided stainless steel (NSS) piston rings and chrome plated stainless steel sliding rings immersed in a fuel blend composed of 85% ethanol and 15% unleaded gasoline. A high frequency tribometer for reciprocating sliding motion was used to determine the friction and wear characteristics and the results revealed that the wear is not severe for rings coated with diamond-like carbon. The acidic fuel was found to promote the occurrence of scuffing. Lenauer et al., [156] studied the wear behaviour of the piston ring and the formation of tribofilm on the cylinder liner tribofilm using a model tribometer set-up. The engine oils were artificially aged with ethanol combustion products in order to determine the effect of bioethanol on the piston ring-cylinder liner system and the results showed that the artificially-aged engine oils decrease the steady-state wear rate as well as thickness of the tribofilm. 5.1.3. Wear and friction characteristics of engine components in which biodiesel–bioethanol–petroleum diesel blends are used as fuels Wear is a significant issue in engines, which is partially due to the low sulphur content of petroleum diesel as well as the high oxygen content and low kinematic viscosity of bioethanol [89]. In general, the addition of ethanol into diesel fuel lowers the viscosity and lubricity of the fuel [157,158]. The lubricity of e-diesel and

76


5.2. Corrosion

on copper, mild carbon steel, aluminium and stainless steel samples upon exposure to rapeseed biodiesel and the corrosion level is more severe for copper and mild carbon steel samples immersed in rapeseed biodiesel. In contrast, the corrosion level is minor for aluminium and stainless steel – the effect is similar to that for samples immersed in commercial diesel fuel. This is attributed to the formation of a thin oxide layer on the surface of the aluminium and stainless steel, which serves as barrier that prevents these metals from further oxidation, resulting in a minor corrosion effect. The results indicated that chemical corrosion is the primary mechanism of corrosion and the products of corrosion are metal oxides and salts of fatty acids – the latter product is the consequence of the reaction between the metal oxides in the oxide layer of the metals and the fatty acids present in the rapeseed biodiesel. According to Singh et al., [163] biodiesel reduces auto-oxidation and therefore, it is the presence of moisture that increases the corrosion rate. This is due to the formation of a new phase and secondary products by degradation of the metal strip upon exposure to biodiesel–mineral diesel blends. Advanced anode reaction can be used to identify the characteristics of electrochemical metal corrosion and block a number of rusty areas on the metal. This helps prevent corrosion of engine components due to exposure to biodiesel [168].

5.2.1. Corrosion behaviour of metals and their alloys immersed in biodiesels Corrosion is measured using the method outlined in the ASTM D130 standard, which involves comparing the changes in colour of copper strips with respect to time, volume and temperature for the biodiesel or bioethanol [108,163]. According to Fazal et al., [164] corrosion of the copper occurs with an increase in the time in which the copper is immersed in the biodiesel. The immersion time was set at 200, 300, 600, 1200 and 2880 h and the temperature range was 25–27 °C. Haseeb et al., [149] investigated the corrosion rate of commercially pure copper and bronze immersed in B0, B50 and B100 biodiesel–diesel blends within a range of 25–30 °C. The immersion time was 2640 h. They also compared the corrosion rate of the copper and bronze samples immersed in these fuels at a temperature of 60 °C over a period of 840 h. The results showed that the corrosion rate is dependent on the percentage volume of biodiesel in the biodiesel–diesel blend. In general, the copper and bronze samples immersed in the biodiesel–diesel blends have higher corrosion levels compared to those immersed in petroleum diesel, [31,141] which is due to the higher unsaturated fatty acid content of the biodiesels such as oleic and linoleic acids. Both of these unsaturated fatty acids are prone to react with metals, resulting in corrosion [68,146]. It has also been shown that the degradation of fuel is dependent on the type of metal to which the fuel is exposed as well as water absorption, auto-oxidation and microbial attacks during storage of the fuel [165]. Fazal et al., [166] investigated the corrosion behaviour of copper, aluminium and stainless steel test coupons immersed in palm biodiesel at 80 °C. The immersion test was carried out over a period of 600 and 1200 h, whereby the fuels were stirred continuously at an agitation speed of 250 rpm using a magnetic stirrer. The corrosion rate of copper, aluminium and steel was found to be 0.586, 0.202 and 0.015 mils per year (mpy), respectively. Even though corrosion of engine components is a common issue which needs to be addressed, several researchers have noted that there are no specific tolerance limits for corrosion of engine components [163,167]. In general, increasing the percentage volume of biodiesel in biodiesel–petroleum diesel blends will increase the level of corrosion and oxidation stability [31]. Copper alloys are more susceptible to corrosion compared to iron and aluminium alloys [31]. Enzhu et al., [167] observed the occurrence of corrosion

5.2.2. Corrosion behaviour of metals and their alloys immersed in ethanol/bioethanol It has been reported that engine components can become susceptible to corrosion upon exposure to bioethanol [169]. Since bioethanol is a type of alcohol, it absorbs water, which accelerates the corrosion of metals. In addition, bioethanol may contain other impurities such as organic acids resulting from bacterial contamination during the fermentation process which also accelerates the corrosion of metals. In addition, the structural composition of bioethanol–gasoline blends can change during the liquid phase and this change is dependent on the fluid temperature, atmospheric pressure, the amount of polar components in the gasoline as well as aromatic substances [170]. According to Shahir et al., [14] the severity of corrosion is influenced by the quality of the bioethanol. Corrosion due to methanol can be divided into the following categories: (1) general corrosion which is caused by ionic dirt (chloride and acetic acid), (2) dry corrosion which is caused by the polarity of the ethanol molecules, and (3) wet corrosion which is caused by azeotropic water, which oxidizes a variety of metals. Corrosion is undesirable in engine components because it can damage the metal surface [171]. Even though anti-corrosive compounds can be used to reduce the effects of corrosion resulting from the use of bioethanol–gasoline blends, these compounds may alter the exhaust emission profiles of the blends, such that they introduce new emission products to the environment [82]. Boniati et al., [172] and Cao et al., [173] discovered that the bioethanol results in holes, pores and cracks on the surface of AISI 4140 steel due to the decrease in oxidation stability of the fuel. Kruger et al., [174] observed the occurrence of corrosion on the surface of the aluminium alloys immersed in bioethanol–gasoline blends, which is attributed to the high boiling point of the bioethanol, which severely corrodes the metal surface. Likewise, Park et al., [175] also observed the occurrence of corrosion on the surface of aluminium alloys immersed in bioethanol–gasoline blends at 100 °C. In situ two-electrode electrochemical impedance spectroscopy (EIS) was used to assess the corrosion and the results showed that the polarization of the aluminium alloys increases during the initial period of immersion due to the formation of boehemite film (c-AIOOH). However, the polarization of the aluminium alloys decreases thereafter due to the initiation of the reaction of aluminium alkoxide.

e-b-diesel is governed by the tribological properties of the fuel and evaporation of ethanol, and there is no linear relationship between these properties [159]. It is found that the addition of up to 15% bioethanol in a fuel blend is relatively safe such that it does not result in significant wear on the sensitive parts of the fuel pump [160]. Quantitative analysis of the metals present in a lubricant gives an indication of engine component wear whereas qualitative analysis provides information on the origin of these metals [161]. A few researchers have reported the tribological characteristics of biodie sel–bioethanol–petroleum diesel blends [145,162]. Armas et al., [132] investigated the effect of biodiesel–bioethanol–petroleum diesel blends on the wear behaviour of engine components in a light duty diesel engine. The results indicated that these blends do not result in significant wear of the pistons and cylinders. The wear value is 0.55%, which is well within the range of error. Moreover, the results of Lapuerta et al., [159] prove that blending ethanol with biodiesel–diesel blends preserves the engine against frictional wear even better than before when the addition of biodiesel into commercial diesel fuel was still a foreign concept to many.


5.3. Lubricity Lubricity is an important fuel property since it indicates the ability of the fuel to reduce friction and wear of the engine components [150]. However, the lubricity of the fuel itself can be degraded due to contamination from soot, water, metal particles and acidic by-products of the fuel [176]. In addition, lubricants inhibit combustion gases and carbon acids during engine operations [177]. There is evidence that some lubricants help reduce oxidation because of the lower unsaturated fatty acid content [176]. However, a couple of studies have also reported problems in the engine components due to the increase in fluid and engine oil polymerization, which in turn, decreases the lubricity of the lubricant [176]. 5.3.1. Lubricity of biodiesel–petroleum diesel blends One of the promising applications of biodiesels is its use as a lubrication additive [178] which is due to its inherent outstanding lubrication properties [179]. It is known that biodiesels are solvents than petroleum diesel and therefore, they are capable of dissolving layers of foreign materials deposited onto steel surfaces. This is particularly important especially in components such as injection pumps due to clogging of the filters [180]. In general, biodiesels with higher FAME content and monoglycerides have better lubrication properties [181]. However, the presence of free fatty acids and diglycerides will reduce the lubrication properties of biodiesels [181,182]. Dhar et al., [183] found that the density, carbon residue and ash content of a biodiesel–diesel blend (20% karanja biodiesel and 80% petroleum diesel) increases in a 200-h endurance test. The endurance test was also carried out to determine if there is any visible indication of metal friction such as iron, aluminium, copper, chromium and magnesium, and the results showed that there is a significant decrease in the lubricity of the biodiesel–diesel blend compared to mineral diesel after 100 h of testing. 5.3.2. Lubricity of bioethanol–gasoline blends Ethanol is completely miscible with water at all proportions whereas gasoline is immiscible with water [184]. For this reason, bioethanol–gasoline blends may contain water, accelerates the corrosion of mechanical components in engines, particularly components made from brass, copper or aluminium [185]. Moreover, ethanol reacts with rubber, which clogs the fuel pipes [186]. For this reason, it is advisable to use fluorocarbon rubber compared to conventional rubber [184]. It shall be noted that an alcohol content of more than 10% in bioethanol–gasoline blends increases wear in SI engines [187]. Indeed, Bielaczyc et al., [188] found that highest lubricity was attained for the E10 bioethanol–gasoline blend, with a value of 823 lm. In addition, the E5, E10, E25 and E50 bioethanol–gasoline blends help reduce engine exhaust emissions. Dutcher et al., [189] added that the addition of ethanol into the fuel reduces the mass concentration of particulate matter in the exhaust. In addition, the compounds associated with gasoline (e.g. sulphur-containing species) is reduced due to the dilution of ethanol added into the fuel relative to those associated with lubricating oil (e.g. calcium, zinc, phosphate) in a single particle spectrum. 5.3.3. Lubricity of biodiesel–bioethanol–petroleum diesel blends Since fuels play a significant role in lubricating intricate engine components such as injection pumps and fuel injectors, changes in the lubricity of the fuels are also a common problem in diesel engines [130]. In general, the lubricity of biodiesel–bioethanol–petro leum diesel blends is lower though the effect is not as marked as pure bioethanol (E100) [190,191]. The presence of bioethanol in biodiesel–bioethanol–petroleum diesel blends increases the

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oxygen content of the fuel [190]. This is desirable since oxygenation can significantly reduce particulate matter emissions and other harmful exhaust emissions such as CO, sulphur oxides SOx and nitrogen NOx [14]. According to Fernando et al., [90] the presence of biodiesel in bioethanol–petroleum diesel blends stabilizes the bioethanol in the blends. Bioethanol–biodiesel–petroleum diesel blends have higher energy content, lower cetane number and lower lubricity compared to biodiesel–petroleum diesel blends. In addition, these blends tend to stabilize the heat of combustion in diesel engines [95]. Armas et al., [132] discovered that the biodiesel–bioetha nol–petroleum diesel blend with the following composition (7.7% bioethanol, 37.69% biodiesel and 69.61% petroleum diesel) affects the durability of the injection pumps and injector nozzles to a similar degree as that for diesel fuel.

6. Life cycle greenhouse gas emissions of biofuels The primary aim of environmental impact assessment is to examine the depletion of resources as well as the emissions of all processes related to the production and usage of fuels [192]. Life-cycle assessment (LCA) is a method used to quantify the potential environmental risks and benefits of biofuel systems [193,194]. Environmental impact assessment of biofuels is typically focused on evaluating the direct and indirect impact of land use, the decrease in carbon stocks, degradation in air quality, as well as water depletion, water pollution and biodiversity losses [195]. A number of researchers have conducted detailed studies on LCA and one of the key works in this area is the work of Rajaeifar et al., [196] who assessed the life cycle energy and CO2 emissions of biodiesel production from soybeans. The results showed that the net energy gain and fossil energy ratio is 8373 MJ/ha and 1.97 MJ/ha, respectively, indicating that soybeans are suitable for use as biodiesel feedstocks. The greenhouse gases produced from biodiesel production is 311.96 kg CO2-eq/ha whereas the total amount of greenhouse gases is 1,710.3 kg CO2-eq/ha. In addition, it is found that the soybean biodiesel significantly reduces fossil fuel-based energy consumption, the use of petroleum as well as greenhouse gas emissions by more than 52%, 88% and 57%, respectively, compared to petroleum-derived fuels [197]. The use of palm oil for biodiesel production has raised concerns regarding pollution, greenhouse gas emissions and land conversions [198]. Hence, one study has been carried out regarding the LCA of palm biodiesel in Malaysia [199]. There is great potential for the production of palm biodiesel in this country since oil palm is widely cultivated in this region and therefore, palm oil is available in abundance. It was found that the use of palm biodiesel gives an energy yield ratio of 3.53, which is positive net energy, indicating the sustainability of palm biodiesel. The energy yield ratio is defined as the ratio of the output energy to the input energy. According to Reijnders et al., [200] the CO2 emissions due to the use of fossil fuels and waste anaerobic conversion of palm oil mills is still appropriate in South Asia, whereby the amount of emissions ranges from 2.8 to 19.7 kg CO2-eq/kg of palm oil. LCA has also been carried out on biodiesel production in which sunflowers are used as the raw materials. According to Spinelli et al., [201] local-scale production of biodiesels from sunflower seeds can reduce fossil fuel consumption in the long term since sunflower biodiesels are good alternative fuels. Iriarte et al., [202] conducted LCA and the results indicated that land use and nitrogen fertilizers are factors which have a significant contribution on greenhouse gas emissions. However, the use of nitrogen fertilizers more than 330 kg N/ha has no effect on greenhouse gas savings.

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Several researchers have also conducted LCA of bioethanol production. In one study, LCA of lignocellulosic bioethanol production was carried out by comparing its environmental impact against the environmental impact of conventional fuels and the results revealed that greenhouse gas emissions are influenced by the proportion of bioethanol in bioethanol–gasoline blends. Increasing the proportion of bioethanol in bioethanol–gasoline blends reduces greenhouse gas emissions by 10% to over 40% [203]. The life cycle greenhouse gas emissions for an ethanol blended fuel (85%vol denatured ethanol) ranges from 38.5 to 37.2 g CO2-eq/MJ of the fuel produced. In addition, the use of ethanol reduces fossil fuelbased energy consumption by 47% [204]. The amount of greenhouse gas emissions for bioethanol produced from cassava and molasses is within a range of 27–91 and 28–100 g CO2-eq/MJ of ethanol, respectively.[205] In general, the production of biofuels requires lower energy consumption than fossil fuels and the burning of biofuels produce lower CO2 emissions compared to fossil fuels [206].

7. Conclusions Previous studies have shown that blending the three types of fuels (biodiesel, bioethanol and petroleum diesel) is one of the ways to improve the physicochemical properties of the fuel and this in turn, can help reduce our dependency on fossil fuels in the future. Biodiesel–bioethanol–petroleum diesel blends improve auto-ignition delay and combustion pressure, reduce CO and NOx emissions, as well as reduce the opacity of the exhaust emissions. The biodiesel in these blends acts as an amphiphile (surface active agent) which will stabilize the bioethanol in the blends. A low bioethanol content (up to 10% by volume) in biodiesel–bioetha nol–petroleum diesel blends is generally desirable since it fulfils the lubrication specifications of diesel. Blending petroleum diesel with biodiesel and bioethanol is attractive since it substitutes 5– 15% of diesel fuel with biofuels, which helps reduce our dependency on fossil fuels and reduce greenhouse gas emissions. The composition of the biodiesel–bioethanol–petroleum diesel blends influences their physicochemical properties and it is imperative to choose a suitable proportion of each fuel in order to attain fuel properties which fulfil the specifications given in the ASTM D675 standard. In recent years, there are issues concerning the corrosion of components in CI and SI since the components are in direct contact with the biofuel. The addition of bioethanol into biodiesel–petroleum diesel blends can alter the physicochemical properties of the fuel and it has been shown that increasing the percentage volume of bioethanol in the blend decreases the cetane number, kinematic viscosity, lubricity and corrosion resistance of the fuel. Furthermore, the phase behaviour diagram of biodiesel–bioetha nol–petroleum diesel blends may change with an increase in the volume of light fuel fraction. However, further testing is need to be carried out over extended periods in order to gain a better understanding on corrosion, particularly with regards to the compatibility of the engine component materials with bioethanol–bio diesel–petroleum diesel blends.

Acknowledgement The authors are greatly indebted to the Ministry of Education Malaysia and University of Malaya, Kuala Lumpur, Malaysia, for funding this work under the High Impact Research Grant (Project title: Development of alternative and renewable energy carrier, Grant no.: UM.C/HIR/MOE/ENG/60 (D0000060-16001)), Universiti Malaya Research Grant Scheme (Grant No.: RP022A-13AET), SATU

Joint Research Scheme (Grant No.: RU021B-2015) and Postgraduate Research Grant (PPP): PG013-2015A.

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