1. Introduction
To power automotive vehicles and industrialized production petroleum fuelsare used for well over 100 years. Most of the petroleum significant fuels are derived from the fossil energy sources [1]. Globe energy consumption refers to the entire energy used by all of human civilization. Globally utilization increased by 840,000 b/d, with emerging economies accounting for all of the developments. Light distillates are the greatest rising refined product category for a second consecutive year [2]. Now currently, world annual energy utilization is about 12.2 × 109 tons of crude oil. Energy utilization is estimated to raise to 1.75 × 109 tons of oil by 2035 [3], [4]. Southeast Asia’s energy require alone will increase by about 75% by 2030 based on the economic growth trends in China and India [4], [5]. In our global total petroleum fuel is decreasing day by day. Therefore, many of the researchers and scientists are concerned about looking for an alternative biodiesel fuel [6], even though the majority of the renewable power technologies are most environmentally friendlier than conventional energy. If we consider on the alternative fuel, the biodiesel will be advantage to meet the future power developing demand [7]. The situation of environmental issue is that the biodiesel is extra adoptable compared to fossil fuel as it forms lower carbon and smoke which are dependable for worldwide warming [8]. Biodiesel has become an attractive for environmental benefits such as non-toxic and eco-friendly and these are made from the renewable resources. Using the biodiesel in CI engines can reduce the hydrocarbons (HC), smoke and carbon monoxide (CO) emissions. But oxides of nitrogen (NOx) will increase due to the oxygen (O2) content is present in biodiesel causes a NOxformation. As the oxygen levels increase in biodiesel the complete combustionwill take place and the temperature increases and produces the NOx formation. But biodiesel are also having few drawbacks, such as higher viscosity, higher molecular weight, lower volatility and higher pour point compared with the diesel. These drawbacks cause poor atomization and lead to incomplete combustion [9], [10]. Many researchers and scientists investigated the engine operated using alternative biodiesels and its blends. In the study by Gopal et al. [11], the results from the experiment were found that SFC increases with increase in percentages of biodiesel blends because of the lesser heating valueof biodiesel and in emission it was found to be significant reduction in HC, CO and smoke emissions for all Pongamia biodiesel blends which were compared to diesel characteristics. However, Pongamia biodiesel of NOx emission is a bit higher than that of diesel characteristics. Devan et al. [12] conclude from the experiments that the increase in smoke emissions was observed by using neat poon biodiesel by 15% at full load and a very slight improvement of brake thermal efficiency was observed with standard diesel characteristic than with neat poon biodiesel and its diesel blends. Sukumar et al. [13] from this article prove that mahua methyl ester and mahua ethyl ester can be used a substitute for diesel fuel in CI engine. From the literature investigation methyl ester lead ethyl ester and diesel fuel in conditions of performance and emission.
In this work, literature survey was done from highly rated journals and also from technical international conference papers. The review that was done on effects on biodiesel by using additives will be very useful for researches, engineers and industrialists who are involved in biodiesel and diesel enginemanufacturers. A number of fuel additives are used on various biodiesels based on properties are initiated.
2. Biodiesel
Biodiesel is a very essential for vehicular fuel. Biodiesel has worthy properties as a diesel fuel, so it is capable for using in diesel engines. Biodiesel can be derived from a number of various vegetable oils and animal fat feedstock. Biodiesel is defined by ASTM as a fuel comprised of alkyl (methyl, ethyl or propyl) esters of long chain fatty acids derived from different animal fats and vegetable oils [14]. Biodiesel is formed by the process of chemical known as transesterification and also esterification. Since most of the general alcohol is utilized to produce biodiesel are methanol and ethanol and another name for biodiesel is fatty acid methyl esters (FAME). 100% FAME is often designated as B100. Slight concentration, such as B20 is suitably referred to as biodiesel blends.
Expressions regarding “first generation” and “second generation” of biofuels are in popular usage. Generally first generations of biofuels are produced from regularly obtainable edible feedstocks. A good number of biofuels are used today in our daily life classified as first generation, this contains ethanol produced by fermentation of sugar from corn, sorghum, and sugar cane and the biodiesel is produced by transesterification process. The first generations of edible oil are coconut raw oil, sunflower raw oil, palm raw oil, peanut raw oil. The term of “second generation” can be referred to biofuels produced from nonedible feedstocks. The second generations of non-edible oils are jatropha raw oil, mahua raw oil, Pongamia raw oil, etc. Therefore the main advantages and disadvantages of first generation and advantages of second generation of biofuels are shown below in Table 1.
Table 1. Comparison of 1st and 2nd generations of biofuels.
| 1st Generation of biofuels | 2nd Generation of biofuels | |
|---|---|---|
| Type | Edible oils | Non-edible oils |
| Feedstocks |
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| Products |
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| Name of oils |
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| Problems |
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| Benefits |
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Here the fuel properties define its physical and chemical characteristics. The fuel properties are essential to the design of the engine combustion chambersystem, vehicle fuel systems, fuel storage and fuel dispensing systems. Fuel properties will impact on vehicle performance, emissions reliability, fuel efficiency and durability. The properties of fuel are important in defining the safely hazard posed by a fuel. Since fuels are flammable, fire and explosion hazard are possible. Fuel specification represents an attempt to mold and limit fuel properties to build and make use of vehicle and limit the hazards presented in storing and handling the fuels (see Table 2).
Table 2. European Union (EN 14214) standard requirements for quality biodiesel fuels.
| Property | Units | Lower limit | Upper limit | Test-method |
|---|---|---|---|---|
| Ester content | % (m/m) | 96.5 | – | EN 14103 |
| Density at 15 °C | kg/m3 | 860 | 900 | EN ISO 3675/EN ISO 12185/EN12185 |
| Viscosity at 40 °C | mm2/s | 3.5 | 5.0 | EN ISO 3104/EN 14105 |
| Flash point | °C | >101 | – | EN ISO 2719/EN ISO 3679. |
| Sulfur content | mg/kg | – | 10 | - EN ISO 20846/EN ISO 20884. |
| Cetane number | – | 51.0 | – | EN ISO 5165 |
| Sulfated ash content | % (m/m) | – | 0.02 | ISO 3987 |
| Water content | mg/kg | – | 500 | EN ISO 12937 |
| Total contamination | mg/kg | – | 24 | EN 12662 |
| Copper band corrosion (3 hours at 50 °C) | Rating | Class 1 | Class 1 | EN ISO 2160 |
| Oxidation stability, 110 °C | Hours | 8 | – | EN 14112 |
| Acid value | mg KOH/g | – | 0.5 | EN 14104 |
| Iodine value | – | – | 120 | EN 14111 |
| Linolenic acid methylester | % (m/m) | – | 12 | EN 14103 |
| Polyunsaturated ( > = 4 double bonds) Methylester | % (m/m) | – | 1 | EN 14103 |
| Methanol content | % (m/m) | – | 0.2 | EN 14110 |
| Monoglyceride content | % (m/m) | – | 0.7 | EN 14105 |
| Diglyceride content | % (m/m) | – | 0.2 | EN 14105 |
| Triglyceride content | % (m/m) | – | 0.2 | EN 14105 |
| Free glycerine | % (m/m) | – | 0.02 | EN 14105/EN 14106 |
| Total glycerine | % (m/m) | – | 0.25 | EN 14105 |
| Group I metals (Na + K) | mg/kg | – | 5 | EN 14108/EN 14109/EN 14538 |
| Group II metals (Ca + Mg) | mg/kg | – | 5 | EN 14538 |
| Phosphorus content | mg/kg | – | 4 | EN14107 |
The main important point of biodiesel is high quantity that is the observance to biodiesel fuel standard specifications. This fuel standard requirement might also be followed by the European Union (EN 14214) or American Standard for Testing Materials (ASTM D-6751) for biodiesel fuel quality requirement. For biodiesel fuel quality and purity standard are listed out below.
3. Additives
At present for vehicular fuels, combustion of numerous chemical additives is used to improve the quality of biodiesel fuel and diesel fuel to convene up the most wanted performance level. Additives will help out the petroleum to recover its engine combustion, performance and emission environmental standards. The additives selection will be based upon the drawbacks of biodiesel fuel such as density, toxicity, viscosity, economic feasibility, additives solubility, auto ignition temperature, flash point, and cetane number for the fuel blending process. The concentration of fuel additives is not regulated. The additives for diesel engine are discussed and few reasons are listed out below [15].
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Enhanced the nagging properties and immovability of the fuel.
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Shrinking the harmful emission from fuel combustion.
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Developing the combustion and performance properties of the fuel.
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Here to afford engine protection and cleanliness.
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Saving the fuel from optimized engine economy and performance.
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Protecting the petroleum tank, pipeline and other massively expensive corrosion.
Additives are being alienated in terms of their application and drawbacks are listed out.
3.1. Metal based additives
Here to get better quality of fuel, metal based additives are used in various applications. Mostly these additives are used in diesel and biodiesel to diminish the unburn hydrocarbons in the IC engines and to minimize the exhaust gas harmful emissions. The metal based additives consist of catalytic effects. These additives consist of copper(II) chloride (CuCl2), iron(III) chloride (FeCl3), copper(II) oxide (CuO-nano structured), cobalt (II) chloride (CoCl2) and copper(II) sulfate (CuSO4) are added in fuel catalyst for diesel and biodiesel based on the necessity requirement. The availability of catalyst in any chemical private limited and these catalysts must be in a nano powder form [16]. By using these additives the exhaust gas harmful emission is reducing the reason of reducing exhaust that is metals are reacting with water vapor to make hydroxyl and also reacts with carbon atoms, therefore discharging the oxidationtemperature [17]. Regularly, in the diesel engine nano powder metal additives are used as a metal organic compound [18]. Here nano powder metal based plays a vital task as a fuel additive in improving the engine performance characteristics and reducing the harmful exhaust gas harmful emission of internal combustion engine [19].
3.2. Oxygenated additives
The most important additives for diesel and otto engine are oxygenated additives. The fuels that are containing oxygen and blending components contain at least one oxygen atom by the molecules at the side of the hydrogen and carbon atoms. The oxygenated additives are very useful to develop the combustion process and octane rating. Oxygenated additives are blended with diesel fuels and the oxygenated additives must be capable of mixing any ratio without separation of its two phases with various diesel and biodiesel fuels. By blending oxygenated additives in biodiesel and diesel sufficient, sufficient cetane number should be there in oxygenated additives and allowed the blend to increase the cetane number. The oxygen helps to support for burning the fuel without emitting any high amount of inert material such as nitrogen into the air and it causes the harmful material such as NOx emission at some operating load condition in CI diesel engine. The generally used oxygenated additives are alcohols, ether and ester. The few names of alcohols contain butanol, propanol, methanol and ethanol. On the other hand for ether diethyl ether, diisopropyl ether, methyl tert-butyl ether, dimethyl ether, and ethyl tertiary butyl ether are included and for ester acetoacetic esters, dicarboxylic acid esters and dimethyl carbonate esters are included are the efficient groups [20]. By addition of oxygenated additives, the ignition temperature of biodiesel will be minimized and also reduction in smoke emission is observed in the diesel engine [21]. According to the composition of diesel and biodiesel, the oxygenated additives will affect directly the properties such as cetane number, density, viscosity, volatility, flash point and calorific value [22]. To ignite the fuel more efficiently oxygenated additives will support and as well as diminish environment pollution. The engine fuels will burn more completely due to the presence of oxygenated additives.
3.3. Cetane number improver additives
Delay period is the one of the most important role plays in diesel engine, in which the delay period varies with chemical and physical properties of the diesel or biodiesel fuels. To differ the delay period cetane number improvers are the main important parameter. To minimize the ignition delay during the combustion cetane number improvers help to get better working in diesel engine ignition. During the combustion process, shorter ignition delay period leads to more complete combustion of the fuel there in the internal combustion engine, reduces the noise of the engine and also reduces the unwanted harmful pollutants in the environment.
In worldwide, most of the diesel fuel specification is been set by the high cetane number targets. Here for the regular refiner, the refinery economic, higher quality of diesel fuel and its flexibility by using the cetane number improvers are improved [23]. Many of the researches had been tried to raise the cetane number of the diesel fuel by different additives such as nitries, peroxides nitrates, aldehydes and tetra-azoles. Alkyl nitrates such as hexyl-nitrates, mixed octyl-nitrates and amyl-nitrate are commercially used and obtained with very good results. Alkyl nitrates have been proved the most important vital in commercially used cetane improvers. The 2-Ethylhexyl nitrate is the cetane improved which has been used very commercially for a number of years, but today it is major cetane number improving additives [24]. In 1940s, the first recognized effective cetane number improver is Di-tertiary butyl peroxide (DTBP). For reducing NOx emissions di-tertiary butyl peroxide has a very good potential than the nitrate. The additive has very good stable conditions such as thermally and oxidatively in diesel and biodiesel fuels at some typical condition temperatures [25]. Due to the insufficient cetane number the diesel and biodiesel lead to higher noise produced in engine, increasing the fuel consumption, poor stating problem in cold weather conditions and producing higher exhaust harmful gas emissions [26].
3.4. Antioxidant additives
The major purpose of using antioxidant to biodiesel fuels was to slow down the development of slush deposits and darkening of color. Petroleum products are very quick moving diesel fuels, but they are very rarely, are stored in for 1–3 years and are subjected to corrosion [27]. Few names of antioxidant additives are butylated hydroxyl anisole (BHA), butylated hydroxyl toluene (BHT), pyrogallol (PL), diphenylamine (DPA), tert-butylhydroxyquinone (TBHQ) and propyl-gallate (PG). From the survey TBHQ, PL and PG are mainly successful among all the antioxidants. These additives are mainly used in biodiesel mixing to get a better stabilizing potential [28], [29]. As the amount of free radicals and peroxide species increases, the oxidative chain reaction raises at a produced exponential rate. By decomposing the peroxides, the antioxidant works by disturbing the chain propagating steps.
4. Effects on diesel engine performance, combustion and emissions by using additives
From the above discussion all different types of additives show that the utilization of additive on biodiesel and diesel fuels is having very good hopeful results found from the articles by researchers, but the selection of additive should be valid based upon the properties of diesel and biodiesel in diesel engine. Many researchers and scientist across the india have utilized the different types of additives such as antioxidant additives, oxygenated additives, metal based additives and cetane number improver additives used in biodiesel blends to improve the performance, emission and combustion of diesel engine. From this article, the literature surveys show the impact of using different types of additives with biodiesel fuels on engine performance, combustion and emission.
4.1. Performance
4.1.1. Brake specific fuel consumption
As the quantity of the biodiesel increases, the brake specific fuel consumption(BSFC) increases [30]. At some operating conditions such as load, injection pressure, timing, nozzle size diameter and speed will also affect the fuel consumption of diesel engine fueled with diesel and biodiesel [31]. Some of the scientists and researchers investigated specific fuel consumption by using additives. Jatropha oil (B100) biodiesel was blended with an additive magnalium (Al-Mg) and was tested in a variable load constant speed diesel engine and from the result it was found that BSFC decreases with 3% addition of magnalium (Al-Mg) [32]. BSFC was slightly reduced with a Tall oil (B60) with an addition of cobalt additive [33]. Tall oil (B60) with additives was investigated at a constant load variable speed, and the result of BSFC decreases with 10% addition of magnesium (Mn) and 5% addition of nickel (Ni) [34]. For canola oil(B20) at a variable speed, the BSFC 4.09% was decreased by the addition of BHA, 4.12% decreased with the accumulation of EHN, 4.22% reduced by adding BHT and 10.19% decreased with an addition of TBHQ [35]. For palm oil (B20) at a variable speed, BSFC 0.44% was decreased with addition of BHA and 0.68% was decreased with addition of BHT [36]. It is confirmed that bimetallic cerium and platinum diesel fuel borne catalyst shows the fuel economy improvements for heavy duty and light duty engines [37]. With the effect of metallic based additives, the mean of specific fuel consumption was reduced to 2.01% for D-16 Mg and 1.02% for D-8 Mg and it was also reduced for D-16Mn and D-8Mn [38]. By using nonyl phenoxy acetic acid additive in edible palm biodiesel, lower BSFC was achieved compared to diesel characteristics [39]. The results show the addition of DEE additive in plastic oil biodiesel, and brake specific energy consumption was shown lower [40]. Increasing the dimethyl carbonate additive percentage in mahua methyl ester, the BSFC was reduced with respective to load. This was due to the high volatility, low viscosity and oxygen content present [41].
4.1.2. Brake thermal efficiency
Some of the researches have concluded that the brake thermal efficiency for the biodiesel fuels was lesser than the diesel fuel characteristics [43]. The brake thermal efficiency decreases as the shorter ignition delay of biodiesel and heat loss of diesel engine. The BTE decreased with biodiesel blends because of low calorific value, higher viscosity, high volatility and poor spray properties [44]. Some of the researchers have studied the impact of additives on BTE. Jatropha oil (B100) biodiesel was blended with an additive magnalium (Al-Mg) and was tested in a variable load constant speed diesel engine and from the result it was found that BTE increased with 1% addition of magnalium (Al-Mg) [32]. Jatropha (B15) with addition of cerium oxide nano particles 1.7% BTE was found to be increased [45]. For palm oil (B20) at variable speed, BTE 0.92% was increased with addition of BHA and 0.37% was increased with addition of BHT [36]. Higher brake thermal efficiency was found by using nonyl phenoxy acetic acid additive in biodiesel [39]. The BTE was increased steadily with increasing the DEE additive blend ratio with plastic oil biodiesel [40]. By addition of dimethyl carbonate additive percentage in mahua methyl ester biodiesel, the BTE was increased with respective to load. This was due to the high volatility, lower viscosity; lower density and oxygen content present [41]. The BTE was raised due to the addition of 15% of DEE in mahua methyl ester biodiesel [42].
4.2. Emissions
4.2.1. HC emission
In internal combustion engine the hydrocarbon emission is produced by unburned fuels. A number of researchers have tried to reduce HC emission. HC emission reduced with higher oxygen quantity and higher cetane number. The quantity of the oxygen present in the biodiesel leads to complete combustion and the higher cetane number reduces the ignition delay, as shorter the ignition delay decreases the unburned hydrocarbon [46]. Some of the scientist studied the HC emission by using additives. The use of Al-Mg additive with jatropha (B100) HC 76% was reduced. Cobalt oxide with Jatropha (B100) HC 52% was decreased [32]. Mahua pure biodiesel was used with 5% diethyl ether was investigated and HC was found to decrease [47]. For the accumulation of BHA and BHT in biodiesel fuel, the HC emission was reduced to 23.81% [48]. The comparison of palm biodiesel blend (B20) and palm biodiesel with additive (B20X) illuminates that the fuel B20X turns out to be improved combustion than B20 fuel; thus, the HC was reduced [39].
4.2.2. CO emission
CO emission is a toxic gas that has colorless and odorless. High quantity of oxygen present and higher cetane number will reduce the CO emission [49]. Few authors have investigated biodiesel with additives to reduce CO emission. CO emission reduced 66% with Jatropha oil (B100) blended with Al-Mg additive and 33% reduced with cobalt oxide [32]. Mahua oil (B100) with DEE 15% CO emission 67% was reduced [47]. For rice bran (B25) with 10% EHN additive the CO emission was found to decrease [50]. The effects of additives BHA and BHT with palm oil (B20) were investigated and found that the BHA 21.21% and BHT 23.81% were reduced [36]. The metal based additives are most helpful in CO emissions, with Mg and Mn metal based additives helped in reduced CO harmful emissions to 13.43% and 16.35% [38]. It is found that among all the fuels, fuel B20X with additive produces the lowest level of CO emissions [39]. The DEE additive with plastic oil of CO emission concentration was found to drastically reduce the high oxygen content and volatility present in DEE which supports complete combustion [40]. The DMC additive with mahua oil biodiesel has a capability of reducing the CO emission because of good spray atomizationand good air-fuel ratio [41].
4.2.3. Nox emission
One of the most dangerous emissions in diesel engine is NOx. Few additives such as cetane improver, oxygenated additive and metal based additives were blended in biodiesel to decrease the NOx. The jatropha biodiesel fuel with metal based cerium oxide additive on diesel engine was observed that the additive has reduced the NOx by 23.5% against diesel fuel characteristics [45]. The oxygenated additive diethyl ether with mahua biodiesel blends decreased the NOx at low load conditions [47]. The NOx for Palm oil (B20) with BHA and BHT additives was found to be reduced [36]. The cottonseed biodiesel with 0.01% L-ascorbic acid was investigated and results were seen that the NOx 5.65% decreased [51]. Drastically the NOx was reduced 29% at higher loads by using DEE with plastic oil biodiesel [40].
4.2.4. CO2 emission
The oxygen quantity present in biodiesel fuel and lesser quantity of carbon causes the higher carbon dioxide. The pure cottonseed biodiesel with 0.01% L-ascorbic acid decreased 1% of CO2 [51]. Fish biodiesel with 2% DEE was investigated and the CO2 results were found to be decreased [52]. The diminution of CO2 emission compared to the standard diesel fuel characteristics was resulted by using the waste cooking biodiesel with ethanol [53]. The emissions of CO2 are reduced slightly with metal additives at different speed conditions [38].
4.3. Combustion
The start of injection and the start of combustion of time elapsed between these two are defined as ignition delay. The ID period starts at the start of injection was the common agreement by the authors. A number of criteria have been utilized to define the end of SOC or ID, including pressure rise and heat release rise [54]. Increasing the percentage of cetane number improver such as 2-EHN added to the fuel is not linear to decrease in ID. Increasing in quantity of additives in fuel, the rate of ID slowly gradually decreases. An exhaustive investigation of the release heat rate showed the effects of the additive are not only on accelerating the commencement reactions at the early on-stage of auto-ignition process, leading to the higher temperature combustion [55]. The maximum cylinder gas pressure and its maximum gradient reduce by 6.5 % and 9.3 % when running the fully loaded engine on fuel Jet A-1 at low 1400 rpm speed. Adding 0.12 % of 2-ethylhexyl nitrate does not affect the cylinder pressure and pressure gradient at maximum torque; however, the maximum pressure gradient reduces by 14.3 % at rated 2200 rpm mode [56]. However, for jatropha (B20) and with addition of additives, the highest in-cylinder pressure occurred after the top dead center within the range of 8 to 10.5 crank angles. As the speed increases, in-cylinder pressure also increased consequently. For jatropha (B20) at 2000 rpm it is shown the higher maximum in-cylinder pressure than the diesel fuel. For jatropha (B20) blends, higher and slightly maximum pressure can be attributed due to the higher cetane number compared to the diesel fuel. In the case of 10 percent additive blends it is more prominent than 5 percent additive blends. By using the additives the heat release rate during the remixed combustion process was reduced. However, the heat release rate was improved for the adapted blends compared to the Jatropha (B20) [57].