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. 2023 May 5;8(19):16545–16560. doi: 10.1021/acsomega.3c00387

Challenges and Opportunities of Low Viscous Biofuel—A Prospective Review

Krishnamoorthy Ramalingam †,*, Suresh Vellaiyan ‡,*, Elumalai Perumal Venkatesan §, Sher Afghan Khan ∥, Zakariya Mahmoud ⊥, Chanduveetil Ahamed Saleel #
PMCID: PMC10193552  PMID: 37214702

Abstract

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Under the roof of solid industrialization and accelerated intensification of multiple ranges of mobilization, a huge rise in precious fuel consumption and pollution was observed. Based on the recent hardships of fossil fuels, experts are undoubtedly eager in carrying out their research in renewable environment-friendly fuels. There have been many reviews of works considering the parameters and standards of biodiesel, which is only from various vegetable and seed oils. But very little review work was carried out on only plant-based biofuel. Plant-based fuel has a lower viscosity and higher volatility properties. The target of this review was to make a bridge to overcome these research gaps. This review extensively studies the biological background, production outcome, properties, and reliability of plant-based biofuel and also deeply investigates the feasibility of usage in a diesel engine. From deep investigation it was identified that most of the low viscous fuel had higher brake thermal efficiency (BTE) (2% to 4%) and NOx emission (5% to 10%) than high viscous biodiesel. The formation of hydrocarbon (HC), CO, and smoke emission was similar to high viscous biodiesel. Overall, the low viscous fuel effectively improves the engine behaviors.

1. Introduction

Direct injection diesel engines are the best of the favorable prime movers in a vast range of applications, which is inclusive of the transport and construction sectors; in addition to that, this kind of engine has also been utilized in agricultural, farm, and power generation types of equipment because of its high thermal efficiency. On the one hand, the demand for these machines escalated the usage of fossil diesel fuel and simultaneously increased the demand for it, which in turn increased tailpipe emissions.1−3 CO2 emission in a prime mover causes the earth’s atmosphere to be overheated and results in global warming, which melts the glaciers of many ice mountains of the world. A considerable focus has to be provided for the reduction of these types of harmful gases and also to move onto an alternative fuel that is biodegradable and also renewable.4 All kinds of edible and nonedible oils may provide the usage of alternative fuels all over the world and also be beneficial in many developed countries. However, the application of many vegetable oils is impeded by some barriers including higher viscosity, lower heat capacity, gum formation, auto-oxidation, and reduction of engine durability. To reduce the viscosity of the raw vegetable oil from edible and nonedible sources, a secondary process named transesterification is required to facilitate the fuel for further operation in a diesel engine.5−7

To reduce this kind of barrier, low viscous biofuel may be extracted from leaves, wood, grass and resins that have identical or merely close properties to diesel fuels.8 Biofuels extracted from the peels of fruits have also been found to be an alternative source of biofuels that is biodegradable. The biofuels extracted from this kind of source have improved properties compared to over other transesterified biofuels/biodiesels and are theorized to have low viscosity, low boiling point, and also better atomization with enhanced evaporation; also, the heating values of these fuels are very near to that of diesel.9,10 In favor of these properties Biofuels mainly relied on eucalyptus leaves, pine resin, lemongrass, tea tree oil, camphor oil, turpentine resin, etc. to yield properties close to those of diesel. The focus of this review is the adoption of current-generation low viscous fuels in the diesel engine. The progress in the production and utilization of low viscous biodiesel in the past decade influenced the writing of this review. This progress was initiated with low viscous biofuels from lemongrass, orange, eucalyptus, lemon peel, pine, algae oil, etc., which possess lower viscosity, and an extensive exploration of physical, chemical and thermal properties and also the impact of fuel composition over the extracted fuels has been revealed. Various blends and compositions along with various experimental strategies to achieve maximum thermal and performance effectiveness are also reviewed.

1.1. High Viscous Second-Generation Biofuels

Second-generation biodiesel is derived from nonedible feedstocks such as Jatropha curcas, Calophyllum inophyllum, Mahua indica, karanja, neem, rubber seed, Thevetia peruviana, Nag Champa and many more. Even though they are derived from nonedible feedstocks, these biodiesels are very high in viscosity and have some potential barriers including gum formation during combustion inside the combustion chamber, lower heat carrying capacity, and auto-oxidation, and they also reduce the durability of the engine.11,12 Moreover, these biodiesels require a secondary process to reduce the viscosity, but during the secondary process, only a very reduced quantity of the fuel is obtained because of the operational and reactive parameters of the process.13 Using alcohol for this transesterification process, the calorific value of the fuel is also reduced, which may be one of the biggest barriers to changing the property of the fuel; this is due to the lower latent heat of vaporization of alcohol.14

1.2. Low Viscous Current-Generation Fuels

Generally, fuel with lower viscosity than that of diesel fuel is said to be low viscous fuel. The characterization of the fuel is clearly distinguished from biodiesel synthesized from vegetable oils. This kind of biofuel is extracted from parts of the plant, not from the seed, but is in no need of a secondary process because of its low viscosity and also higher volatility.15 Some of the commonly identified biofuels are eucalyptus oil, pine oil, citronella oil, lemongrass oil, orange peel oil, algae oil, etc.; the attributes of these kinds of biofuels are elaborated below

1.3. Objective of the Review

An extensive literature survey was performed in the area of biofuels, and the key aspects of the properties of the fuel were categorized. The review is focused on current-generation low viscous biofuels, which is one of the most promising emerging energy resources in the research world. This is one of the major contributing factors to writing this perspective, which describes the properties, performance and emission parameters of a diesel engine with current-generation low viscous biofuels.

2. Biography of Low Viscous Fuels

2.1. Eucalyptus Oil

A plant-based renewable fuel should be an alternative to conventional fuels. One plant-based fuel that has emerged is eucalyptus oil, which is likely to bear the characteristics of ethanol or methanol. Oil extracted from seeds is chained with triglycerides, but eucalyptus oil is extruded from parts of a plant family named Myrtaceae and has been categorized as a light biofuel. The fuel was synthesized from steam-distillation of the leaves of the tree. The oil has been extensively used for medicinal purposes including as a repellent and pesticide for plants. The oil is qualified as a cosolvent owing to its easy miscibility with most of the oil since the chemical structure of eucalyptus resembles a naphthenic base.16 This oil has a very high octane rating, which makes it ideal for us as fuel in engines, but has the drawback of massive cost, which makes it irrelevant to be utilized as fuel. However, due to the depletion of fossil fuels, researchers are reconsidering eucalyptus oil as an alternative replacement for fossil fuel that may be used to run the engines.17 The major constituent of eucalyptus oil is identified to be 1,8-cineole with the chemical structure having 10 carbons, 18 hydrogens, and 1 oxygen (C10H18O). The chemical structure of the oil itself shows that it has a lower heating value than diesel and lower viscosity as it contains the structure of gasoline (C8H18) and additionally has an oxygen atom in it. The properties of this oil not only include low viscosity and lower heating value but also increased fuel evaporation and an atomization combustion process, as compared to diesel fuel.18 The main hindrance to using alcohol as an alternative fuel is its lower calorific value, but the calorific value of the fuel made from eucalyptus oil the fuel is highly comparable to that of diesel, hence making it of interest as an alternative fuel.

2.2. Pine Oil

Pine oil is also one of the low viscous fuels of the current generation. The oil is derived from the oleoresin of the pine tree, which is obtained from bark, wood, and tar. The oil is very stable under all conditions and is easily blendable with all kinds of petroleum products. The oil is pale yellow and has a fresh forest smell and odor; it is easily soluble with alcohol and other mineral oils. The oil is extracted from the oleoresin by a steam-distillation method before it is treated with some acids to synthesize the required pine oil.19,20 The oil in appearance has much lower viscosity, and this property is closely comparable with that of eucalyptus. The chemical structure of pine oil consists of hydrocarbon molecules with oxygen, and it is called an alicyclic hydrocarbon. This type of hydrocarbon mainly consists of cyclic terpene alcohol and alternatively may be called alpha-pinene. The chemical structure of these hydrocarbon molecules is C10H16O, which suggests that they possess inherent oxygen with reduced molecular weight and shorter carbon chain length than diesel or biodiesel. The latent heat of vaporization of this pine oil is lower than much of the alcohol and hence may not contain a cooling effect.21,2 The oil also holds a lower viscosity and boiling point and hence should increase fuel atomization and its vaporization, making possible its use as an alternative fuel.

2.3. Lemongrass Oil

Cymbopogon flexuosus oil, also simply identified as lemongrass oil, is extracted from a plant that belongs to the family Poaceae and is mostly found in Kerala and Tamilnadu in the south Indian region. The oil is lemony and dark yellow and is chemically formulated with 51 carbon, 84 hydrogen, and 5 oxygen molecules bonded together with the hydrocarbon chain. The chemical formulation of the oil consists of citral(C10H16O) at 63% and 12% geranyl acetate and also includes a minimal level of aromatic compounds.23,24 The molecular weight of lemongrass oil is 777.2, which includes 80% carbon, 10% hydrogen, and 10% oxygen. The latent heat of vaporization is a lower chemical formulation of the oil itself, proving that the aromatic oil is low in viscosity and low in boiling point, which leads to good atomization during combustion and makes it a good reactive alternative fuel for conventional engines.25

2.4. Lemon Peel Oil

As the name “lemon peel oil” implies, the oil is extracted from the fresh rinds of lemon peel. The peel is available for a very low cost or free as it is a waste product obtained after peeling a lemon for some other use. The lemon fruit is easily available all over the world, and scientifically the fruit is from the family Rutaceae and has much medicinal use.26 The oil contains many chemicals that frame the oil to hold lower heating values, and these include lemonal, terpinolene, diene, pinene and a few others.27 The chemical properties of lemon peel are identified by chromatography operated at 300 °C for 6 min with a lamp rate of 10 C/min and a split ratio of 10:1. The flash and fire point of lemon peel oil are very close to those of the conventional compressed ignited fuel. The cetane index and viscosity of the lemon peel oil are very near those of the conventional fuel (diesel), which makes it possible to be easily transportable. The closeness of the cetane number and viscosity remains lower than conventional fuel. The absence of fatty acids tends to allow easily sprayable atomization during the combustion process. An additional value that makes lemon peel oil a considerable option for the replacement of conventional fuel is its boiling point temperature, which is 176 °C.28,29 The calorific value is very close to that of diesel fuel, and it is also added with an oxygen atom that escalates the combustion process parameters.30

2.5. Orange Peel Oil

Orange fruit is from the family Rutaceae, and its main important role is in the food and cosmetic industry. The oil is extracted from orange peels, which are waste byproducts, and 27,600 tons of oil are utilized in the production of orange peel oil up to 0.6% by the cold-press process.31 The low demand for orange peel makes it appealing for use in combustion in internal combustion (IC) engines. The extraction of orange peel oil may be done with a cold-press or steam-distillation method. The oil contains 90% of d-limonene and is chemically formulated as C10H16, which implies that it is a hydrocarbon chain. The chemical formulation insinuates the resemblance of the hydrocarbon chain present in conventional fuel (diesel). The low viscosity and higher calorific value of the oil make it compelling to be used as an alternative for conventional fuel.32,33

2.6. Citronella Oil

The citronella oil was extracted from citronella grass, which is largely available in India, Srilanka, Bangladesh, etc.; these types of grasses come under the family Poaceae.34 It is easily nurtured in agricultural land and also in nonagricultural land, roadside, riverside, etc.; it typically grows up to 2 m in height. Citronella oil is extracted by an energy-intensive process; the oil yield is a yellowish-brown color. Most commonly these types of citronella oil are used in the cosmetic, food, beverage and health care industries. These characteristics prove that citronella oil is not hazardous for humans and the environment. The extracted oil has a lower viscosity, high volatility, and higher oxygen content.35,36 These thermo-physical properties support its use to replace diesel in existing diesel engines.

2.7. Turpentine Oil

Coniferous trees, like pine trees, naturally produce a resin that is abundant in chemicals, including terpenes, fatty acids, waxes, tannins, and phenolics. The major purposes of resin are to serve as an energy reserve and as a barrier against illnesses, pests, and insects for the tree.37 Turpentine oil, which is made by a steam-distillation process and dissolving it in a volatile liquid, is thought to be one of the most promising diesel fuel substitutes.38 Since the 1970s, turpentine oil has been used commercially as a burning substance in lighting, boilers, furnaces, and other equipment. Before the introduction of fossil fuels, turpentine oil was utilized in engines, but then it was no longer produced because of its unavailability and high manufacturing cost.39

3. Production and Property Analysis

Low viscous fuel (LV fuels) can be extracted from LV fuel feedstocks using a variety of techniques. The methods include mechanical extrusion, solvent extraction, steam-distillation, chemical extraction, and pyrolysis, among others; steam-distillation was deemed to be the most advantageous approach economically.40−42 The technique entails the extraction of oil using heat energy. As shown in the figure, LV oil is extracted from LV feedstocks via steam-distillation (Figure 1).

Figure 1.

Figure 1

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Steam-distillation setup.

The steam-distillation process consists of many chambers. The main chamber is in the bottom section and may be made of glass to withstand the heat energy supplied; the chamber is called the steam-distillation chamber. The water in the chamber is transformed into steam by absorbing the heat through the bottom walls of the combustion chamber. The LV feedstocks are placed over the top section of the distillation chamber, and the feedstocks are heated by the steam from the distillation chamber. The fumes consisting of the feed essence and steam vapor come out of the distillation chamber, and they are routed to the condensation chamber for cooling. The combination of LV oil and water is collected in a collecting tank. The LV oil is in the upper part, and water will be in the lower segment of the condensed liquid due to its variation in density and viscosity. The LV oil is separated from the mixture easily, and any other impurity found is carefully removed. The liquid is mixed with a small quantity of ether, placed in a water bath, and slightly heated to allow any leftover volatile substance to evaporate with ether, leaving behind LV oil. Some of the solid particles are removed by filtering with 40 mm filter paper.43,44

3.1. Inferences from the Review of Literature on the Production of Biodiesel

The inference from the literature reveals that a significant number of researchers extended their research by extracting biodiesel from LV fuels using the steam-distillation method. The yield of LV fuels not only depends on the feedstock but also depends on the quality of the catalyst and response conditions, including temperature and also sometimes pressure.45,46 The literature also reveals that the properties of biodiesel are within the ASTM/EN/IS standards and are also close to the properties of conventional fuel.

3.2. Property Analysis of Current-Generation Low Viscous Biodiesel

The operational feasibility of a diesel engine is determined by the thermal and physical properties of fuel apart from the design and operating parameters. The synthesis, characteristics and utilization of alternative fuels for diesel engines and an in-depth analysis of the properties of the fuel are required for any research. The physicochemical properties of the fuel are determined or experimented with before powering the fuel into an engine, and it should be ensured that the biofuels reach international standards. Many researchers have worked on plant-based biofuel and have framed many of the basic physicochemical properties of the fuel and also its thermal properties. This review is based on the current generation of fuels, that is, LV fuels, utilized in a diesel engine and also summarizes the properties and the effect in performance, emission and combustion characteristics. The chemical composition of the fuel and the influence of functional group parameters are connected to have an intensified effect on the physicochemical properties of the fuel. The influence of the parameters is emphasized and observed, including the effects of functional group, molecular structure, molecular weight and boiling point on all of the properties. A summary of the properties of LV fuels is presented in Table 1.

Table 1. Comparison of Properties of Low Viscous Biofuels with Diesel.

S. No. Fuel Type Kinematic Viscosity @ 40 °C (cSt) Calorific Value (MJ/kg) Cetane Index Density (kg/L) Flashpoint (°C) Boiling Point Auto Ignition Temperature (°C)
1 Diesel4−7 3–4 43.8 50 0.855 76 350 210
2 Eucalyptus oil16−18,43−46,67,68 3–3.25 43.2–44.1 52–54 0. 913–920 93–97 175–178 300–325
3 Pine oil19−22,69−71 1.3–1.5 42.2–42.8 11 0.875 52 150–175 300
4 Lemongrass oil,23−25,72−74 3.6 37 48 0.905 55 210 275
5 Lemon peel oil26−30 1.06 41.5 54 0.853 54 176 280
6 Orange peel oil31−33,75−78 3.5 39 47 0.817 56 177 245
7 Citronella oil34−36,79−82 3–3.6 37.19 52 0.897 69 221 312
8 Turpentine oil37−39,83,84 2.5 44 47 0.920 38 171 290
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3.2.1. Cetane Number

The ignition quality of a fuel is determined by the cetane number. All HC fuels are made of a chain of hydrocarbon atoms. When the chain of hydrocarbon atoms increases, the value of the cetane number of the fuel increases; the cetane number decreases when there are more double bonds, showing that the cetane number is inversely proportional to the degree of saturation of the fuel.47,48 The presence of a higher number of saturated double bonds in pine and turpentine oil, and also shorter carbon chains, results in a lower cetane number.49 Despite the presence of double bonds, lemon peel oil, citronella oil and eucalyptus oil have a higher cetane value because the carbon chains of these oils are alicyclic hydrocarbons in nature.50−53 The presence of 10 carbon atoms in pine and turpentine oil results in a lower cetane value due to the existence of an inherent double bond and their distinct nature, which is illustrated in Table 1. According to the table, pine oil had a minimum value of 11, whereas the biofuel made from lemon peel had a maximum value of 54.

3.2.2. Kinematic Viscosity

Kinematic viscosity is the most important characteristic of a fuel, and it signifies the flow property of the fuel. The viscosity of the fuel indicates the resistance to flow and plays a major role in the spray property of the fuel, which includes the atomization and penetration properties of the fuel.54 LV fuels cause sufficient atomization and thus have better thermal efficiency and soot deposits. The low viscous property of the fuel also gives a better penetration property and thus can have very fine fuel droplets for combustion.55,56 The kinematic viscosity of the fuel is measured and standardized using ASTM standards D445 and up to 1.90–6 mm2/s.57Table 1 depicts the kinematic viscosity of all of the LV fuels. According to the table, the kinematic viscosity of lemongrass oil showed a maximum viscosity value of 3.6 cSt (at 40 °C), whereas the biofuel made from pine oil had a minimum value of 1.5 cSt.

3.2.3. Density

The estimation of the fuel quantity flowing into the injector is measured by the density of the fuel. The density of the fuel depends on many factors; some of the important factors are the feedstock used for extraction and the method used for LV fuel conversion. The density of the fuel plays a very important role in the design of the nozzle and may also affect the operational feasibility of the engine.58,59 The property directly influences the atomization of the fuel, which may in turn affect the thermal efficiency of the engine.60 The property is identified as the most critical barrier for LV fuels. ASTM D1298 and EN ISO 3675/12185 have the density guidelines for LV fuels. From Table 1, it can be understood that the range for third-generation LV fuels is from 860 to 900 kg/m. The maximum value of 920 kg/m is for turpentine oil, and the lowest value of 817 kg/m is for orange peel oil.

3.2.4. Flashpoint

The flashpoint (FP) property is measured by fuel vapor getting flashed when exposed to an ignition source. The property is very useful in judging the volatile fuel vapor getting fired at the combustion chamber.60,61 The FP of diesel fuel is 52–96 °C, whereas many biodiesels possess an FP of 150 °C. This also signifies the safety property of the fuel. The poorly volatile vegetable oil biodiesel has a very high FP when compared to LV fuels.62,63 ASTM D93 and EN ISO 3679 have flashpoint guidelines. According to Table 1, the lowest FP value of the biodiesel, 38 °C, was obtained for the turpentine, and the highest FP value, for eucalyptus oil, was 95 °C.

3.2.5. Calorific Value

The amount of heat energy expelled from the burning or combustion of a unit value of fuel is defined as the calorific value (CV) of the fuel.64 So the CV of the fuel is a very important and desirable factor for an IC engine.64 The CV of the LV fuel is less than the CV of conventional diesel fuel. The CV of fuel is calculated or referred to as per the terms of EN 14213 as 35 MJ/kg. From Table 1, the calorific value of turpentine oil showed a maximum heating value of 44 MJ/kg, whereas the biofuel made from lemongrass had a lower heating value of 37 MJ/kg.

3.2.6. Boiling Point

The volatility of a substance can be measured using the boiling point of the substance. The boiling point of a substance is the temperature at which the vapor pressure of the substance equals the atmospheric pressure. The value of the boiling point is indirectly proportional to the volatility of the substance and vice versa.65,66 The BP may also explain the bonding of molecules inside a chemical or biodiesel. From Table 1, the boiling point of citronella oil showed a maximum of 221 °C, whereas the biofuel made from pine oil had a lower BP of 150 °C.

4. Low Viscous Fuel in Diesel Engine

4.1. Eucalyptus Oil

Recently, plant-based fuels have gained more attention as alternate energy sources for diesel engines because of their low viscosity and higher calorific value properties.67 Many of the researchers chose eucalyptus oil for diesel engines due to its large availability and minimal oil production cost. Interestingly, eucalyptus oil recovers engine tailpipe emissions compared with diesel fuel in conventional CI engines. On performance output, large gaps were found. To overcome this step many of the researchers have chosen a fuel blending technique, and many proved that diesel-blended eucalyptus oil exhibits improved performance output (BTE and brake specific fuel consumption (BSFC)) compared to eucalyptus oil. Notably, E20 (20% eucalyptus + 80% diesel) had improved BTE and BSFC; at higher eucalyptus mixing proportions, the results were on the opposite trend.16,18 Improving the diesel replacement ratio without losing BTE and BSFC was a tedious job. There were several possible methods available, namely, water addition, biofuel addition and additive addition.68 The author proposed that biofuel addition was a feasible method for improving the diesel fuel replacement ratio and also implemented the same in a diesel engine. The research was carried out in a diesel engine with mixed methyl ester of paradise oil blended with eucalyptus biodiesel. The blend PE50 (25% methyl ester of paradise oil + 25% eucalyptus oil) displays a 2.5% improvement in BTE compared to B20. There was a 50% reduction in smoke, 35% reduction in HC and 37% reduction in CO for the PE50 blend. Except for NOx emission, these results may be the reason for abundant oxygen availability. Author P. K. Devan reported that the diesel engine was smoothly operated with 100% elimination of diesel fuel.44 The present work concentrates on the study of the conventional diesel engine employed with B100 fuel (mixed methyl ester of paradise oil with eucalyptus oil by 50:50). The B100 blend had improved performance and emission parameters. On the other hand, the major concern was raised on NOx emission, which is the main drawback associated with plant-based fuel.69 There were some possible ways available to mitigate the NOx formation such as alcohol addition, water addition (emulsion fuel), gas addition (dual fuel mode) and nano-additive addition (nanofluid). The researcher proposed alcohol mixing in the eucalyptus oil method for effective NOx mitigation, implemented in a normal diesel engine. In the present study, butanol was selected and mixed with eucalyptus biodiesel by 5% (E20Bu5), 10% (E20Bu10), and 15% (E20Bu15). The NOx was diminished by 23%, 21%, and 25% for E20Bu5, (E20Bu10), and (E20Bu15), respectively. The mitigation of NOx was done by the cooling effect of butanol.46 Dual fuel mode was introduced in ordinary diesel engines for evaluating the effectiveness of eucalyptus oil. In the present work, natural gas (NG) was introduced at the inlet manifold, and eucalyptus biodiesel acted as the pilot fuel. The NG-biodiesel had improved performance and emission output. In comparison with E20Bu15, NG-biodiesel had higher NOx emissions.68 In the last decade, nanotechnology was unavoidable in all fields. The researcher proposed that nano-addition (nanofluid) was the best method for mitigating NOx in biodiesel and implemented the same in common rail direct injection (CRDi) engines. In this experiment, aluminum oxide nanopowder was an addition with eucalyptus oil as a fuel. It had a significant improvement in both performance and emission output response.43 A summary of studies conducted with eucalyptus oil and its comparison with diesel fuel is shown in Table 2. The inference of the above study is shown in Figure 2.

Table 2. Summary of Studies Conducted with Eucalyptus Oil and Its Comparison with Diesel Fuel.

          Engine Output Response
  
Year Research Group Base Fuel Fuel Modification Engine Modification Performance Emission Inferences
2011 Anandavelu et al.16 Diesel Eucalyptus oil blend with diesel - BTE-↓ HC↓ NOx was major concern
BSFC-↑ CO↓
NOx↑
2011 Tarabet et al.18 Diesel Eucalyptus oil blend with diesel - BTE-↓ HC↓ NOx was major concern
BSFC-↑ CO↓
NOx↑
2009 P. K. Devan et al.44 Eucalyptus oil Methyl ester of paradise oil blend with eucalyptus oil - BTE-↓ HC↓ NOx was major concern
BSFC-↑ CO↓
NOx↑
2016 Srinivas Komanna et al.67 Palm kernel methyl ester Methyl ester of palm kernel oil blend with eucalyptus oil VCR BTE-↓ HC↓ NOx was major concern
BSFC-↑ CO↓
NOx↑
2020 Rickwinder Singh et al.46 Diesel Eucalyptus oil blend with diesel - BTE-↓ HC↓ Significant drop in BTE and BSFC
Addition of n-butanol with eucalyptus biodiesel BSFC-↑ CO↓
NOx↓
2014 Tarabet et al.68 Diesel Eucalyptus oil blend with diesel - BTE-↓ HC↓ Significant drop in BTE and BSFC
Addition of natural gas with eucalyptus biodiesel BSFC-↑ CO↓
NOx↓
2019 N. Senthur et al.43 Diesel Eucalyptus oil blend with diesel CRDI engine BTE-↑ HC↓ Significant improvement in both performance and emission output response
Addition of aluminum oxide nanoparticle with eucalyptus biodiesel BSFC-↓ CO↓
NOx↓
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Figure 2.

Figure 2

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Inference of the eucalyptus oil study.

4.2. Pine Oil

The author evaluated the pine oil–diesel blend in a diesel engine; the result proved that the performance deed was poorer for pine oil, but emission formation was more positive except for the oxide of nitrogen when compared with diesel fuel.22 Undoubtedly, alternate fuel researchers work toward NOx elimination from biodiesel/biofuel. There were many routes found such as water emulsion, nanoemulsion, nano-additive, catalytic converter, selective catalytic reduction (SCR), exhaust gas recirculation (EGR), etc. for reducing NOx emission from biodiesel/biofuel.70 The researcher proposed that both SCR and catalytic converter assembly are more suitable techniques for effective NOx reduction, and they were implemented in single-cylinder engines. From the work, it was revealed that NOx emission was lowered by 30–35%. This may be the reason for the thermo-hydrolysis reaction of SCR + CC assembly.21 Most of the researchers proved that fuel injection pressure played a major role in engine fuel consumption by atomization action. The researcher proposed the CRDi system as a more suitable technique for pine oil fuel and implemented it in a single-cylinder engine.19 From the work, it was revealed that BTE and fuel consumption of the engine were promoted at higher fuel injection pressure. This may be the reason for improved fuel atomization and air fuel entrainment action in the combustion chamber.71 On the other hand, NOx emission was promoted. The researcher proposed that a water-emulsified nano-addition (nanofluid) was the best method for simultaneously mitigating NOx and improving the performance deeds. In this experiment rice husk nanopowder was an addition with water-emulsified pine oil. It had significant improvement in both performance and emission output response.19 A summary of the studies conducted with pine oil and its comparison with diesel fuel is shown in Table 3. The inference of the above study is shown in Figure 3.

Table 3. Summary of Studies Conducted with Pine Oil and Its Comparison.

          Engine Output Response
  
Year Research Group Base Fuel Fuel Modification Engine Modification Performance Emission Inferences
2013 Vallinayagam et al.22 Diesel Pine oil blend with diesel - BTE-↓ HC↓ NOx was major concern
BSFC-↑ CO↓
NOx↑
2014 Vallinayagam et al.69 Diesel Pine oil blend with diesel Ignition assistance BTE-↓ HC↓ NOx was major concern
BSFC-↑ CO↓
NOx↑
2014 Vallinayagam et al.21 Diesel Pine oil blend with diesel SCR + catalytic converter BTE-↓ HC↓ Slight drop in BTE and BSFC
BSFC-↑ CO↓
NOx↓
2019 Saravanan et al.20 Diesel Pine oil blend with diesel Varying injection pressure (IP 200–250 bar) BTE-↓ HC↓ Slight improvement in performance output response but NOx was increased
BSFC-↓ CO↓
NOx↑
2020 Venkatesan et al.71 Diesel Pine oil blend with diesel 10% of EGR BTE-↓ HC↓ Significant improvement in emission output response
BSFC-↑ CO↓
NOx↓
2020 Panithasan et al.19 Diesel Nanomixed pine oil with water emulsion CRDI BTE-↑ HC↓ Significant improvement in both performance and emission output response
BSFC-↓ CO↓
NOx↓
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Figure 3.

Figure 3

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Inference of the pine oil study.

4.3. Lemongrass Oil

The investigator explored whether lemongrass oil can smoothly run the diesel engine without any major modification, and they also compared the performance and emissions results with diesel fuel. Interestingly, lemongrass oil emits lower tailpipe emissions (HC, CO and CO2) except NOx emission. This may be a reason for the effective oxidation process in the combustion phase. The author explored the diesel engine behavior with lemongrass oil and its blends with diesel. It was revealed from the research that the LGO20 (20% lemongrass oil + 80% diesel) blend shows higher BTE than other LGO blends (LGO40, LGO60). This may be a cause of the lower calorific value of lemongrass oil. Notably, CO and HC emission was enormously reduced for all of the lemongrass blends compared to diesel. This was possibly the reason for the abundant O2 availability in the lemongrass oil blends. It was found that lemongrass oil exhibits peak heat release compared ot diesel fuel due to better evaporation of the low viscous lemongrass oil.25 In terms of emission reduction in diesel engines, NOx emission is a major concern in LGO blends; many routes such as water emulsion, nanoemulsion, nano-additive, SCR, EGR, etc. were studied to mitigate this.72−74 Annamalai et al.23 projected that water emulsion is a more suitable technique and executed it in monocylinder diesel fuel engines. In their work, the emission and performance output response of LGO5W (5% water emulsion + LGO) were investigated in a conventional engine. The NOx emission was effectively reduced by 20% due to the low heating value (LHV) of water accumulated in the LGO emulsion; it absorbed the peak combustion chamber temperature but slightly increased the HC and CO emissions. Alagumalai et al.24 reported that lemongrass oil fueled improved NOx emissions in the premixed charge compression ignition (PCCI) engine compared to the conventional diesel engine, but there is no improvement in performance concerns.

The researcher proposed that nano-addition (nanofluid) was the best method for simultaneously mitigating NOx, HC and CO emissions and implemented it in an EGR fitted direct injection (DI) engine. In this experiment, cerium oxide nanopowder was an addition with lemongrass oil as a fuel. It had significant improvement in both performance and emission output response. A summary of studies conducted with lemongrass oil and its comparison with diesel fuel is shown in Table 4. The inference of the above study is shown in Figure 4.

Table 4. Summary of Studies Conducted with Lemongrass Oil and Its Comparison with Diesel Fuel.

          Engine Output Response
  
Year Research Group Base Fuel Fuel Modification Engine Modification Performance Emission Inferences
2020 Kolla Kotaiah et al.25 Diesel Lemongrass oil blend with diesel - BTE-↓ HC↓ NOx was major concern
BSFC-↑ CO↓
NOx↑
2015 Avinash Alagumalai et al.24 Diesel Lemongrass oil (LGO) PCCI-DI BTE-↓ HC↓ Major drop in BTE and BSFC
BSFC-↑ CO↓
NOx↓
2016 Annamalai et al.23 Lemongrass oil (LGO) Water emulsion with LGO - BTE-↓ HC↓ Significant drop in BTE and BSFC
BSFC-↑ CO↓
NOx↓
2016 Sathiyamoorthi et al.73 Lemongrass oil (LGO) Antioxidant additive with LGO - BTE-↑ HC↓ Slight improvement in both performance and emission output response
BSFC-↓ CO↓
NOx↓
2020 Saravana Kumar et al.72 Diesel 20% LGO + diesel 10% to 30% of EGR BTE-↓ HC↓ Significant improvement in emission output response
BSFC-↑ CO↓
NOx↓
2017 Sathiyamoorthi et al.74 Diesel Addition of cerium oxide nanoparticle with LGO emulsion EGR BTE-↑ HC↓ Significant improvement in both performance and emission output response
BSFC-↓ CO↓
NOx↓
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Figure 4.

Figure 4

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Inference of the lemongrass oil study.

4.4. Lemon Peel Oil

The researchers revealed that lemon peel oil can smoothly run the diesel engine, and also they compared the results with diesel fuel. Interestingly, lemon peel oil emits minimal tailpipe emissions except for NOx emissions. The HC and CO emissions were noted as less when supplying the lemon peel oil in a diesel engine. This may be a reason for the effective oxidation process in the combustion phase.28 The author explored the diesel engine behavior with lemon peel oil and its blends with diesel. It was revealed from the research that the LPO20 (20% lemon peel oil + 80% diesel) blend shows higher BTE than diesel (D100). However, a further increase of the proportion of lemon peel oil resulted in lower BTE and increased brake-specific fuel consumption. These results may be a cause of the lower calorific value of lemon peel oil. The CO and HC emissions were enormously reduced for all of the lemon peel oil blends compared to diesel. This was possibly the reason for the abundant O2 availability in the lemon peel oil blends.30 It was proved that 100% lemon peel oil can successfully run a diesel engine without any considerable modification; the BTE was nearer to diesel fuel. The NOx emission is the only negative concern with lemon peel oil. It was noted that lemon peel oil exhibits peak heat release compared to diesel fuel due to better evaporation of the low viscous lemon peel oil. Following emission reduction in diesel engines, NOx emission was a major concern for LPO and its blends. There were many routes studied for NOx emission elimination from biodiesel/biofuel such as water emulsion, nanoemulsion, nano-additive, SCR, EGR, etc.29 Vellaiyan et al. projected that water emulsion is a more suitable technique and executed it in a monocylinder diesel fuel engine. In their work, for LPO10W (10% water emulsion + LPO) the emission and performance output response of conventional engines were investigated. The result proved that even for the addition of a water emulsion in LPO there was no drop in the performance output response. In terms of emissions, the NOx emission was effectively reduced by 30% and the HC and CO emissions were also mitigated. However, the brake thermal efficiency and fuel consumption of the engine were not improved by LPO and its blends compared with diesel fuel.29 BTE and BSFC also play a prominent role in selecting alternate fuels. The engine modification technique significantly improved the engine’s performance and emission output response. Most of the researchers proved that fuel injection pressure played a major role in engine fuel consumption by atomization action.27,29 The researcher proposed that the CRDi system was a more suitable technique for LPO fuel and implemented it in an EGR attached single-cylinder engine. From the work, it was revealed that BTE and fuel consumption of the engine were promoted at higher fuel injection pressure. This may be the reason for the improved fuel atomization and air fuel entrainment action in the combustion chamber. NOx emission was promoted by 10% EGR. This may be a reason for the diminished peak cylinder temperature with the help of cooled exhaust gas circulation.26 A summary of studies conducted with lemon peel oil and its comparison with diesel fuel is shown in Table 5. The inference of the above study is shown in Figure 5.

Table 5. Summary of Studies Conducted with Lemon Peel Oil and Its Comparison with Diesel Fuel.

          Engine Output Response
  
Year Research Group Base Fuel Fuel Modification Engine Modification Performance Emission Inferences
2017 Nanthagopal et al.28 Diesel LPO blend with diesel - BTE-↓ HC↓ NOx was major concern
BSFC-↑ CO↓
NOx↑
2017 Ashok et al.30 Lemon peel oil (LPO) - - BTE-↓ HC↓ NOx was major concern
BSFC-↑ CO↓
NOx↑
2020 Suresh Vellaiyan et al.29 Lemon peel oil 10% Water emulsion with LPO - BTE-↓ HC↓ Significant drop in BTE and BSFC
BSFC-↑ CO↓
NOx↓
2018 Naresh Kumar et al.27 Diesel 20% LPO blend with diesel 10 to 30% EGR BTE-↓ HC↓ Significant drop in BTE and BSFC
BSFC-↑ CO↓
NOx↓
2018 Ashok et al.26 Diesel 20% LPO blend with diesel CRDI-Engine IP 400 bar to 600 bar, 10% EGR BTE-↑ HC↓ Significant improvement in both performance and emission output response
BSFC-↓ CO↓
NOx↓
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Figure 5.

Figure 5

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Inference of the lemon peel oil study.

4.5. Orange Peel Oil

Globally, India is the third largest harvester of orange fruit. In India approximately 70 million metric tons of orange fruit was yielded every year, among which 15 million metric tons of orange peel was gained from industry and residential areas.75,76 There is not much demand for orange peel oil because it is nonedible. Based on the above-mentioned facts, many of the researchers prefer orange peel oil for operating diesel engines as an alternate fuel. The author evaluated the orange peel oil–diesel blend in a diesel engine, and the result proved that the performance deed was poorer for orange peel oil but the emission formation was improved except for the oxide of nitrogen when compared with diesel fuel.76 To diminish the NOx emission, the researcher proposed alcohol/water emulsion mixing in the orange peel oil method for effective NOx mitigation. Amar Deep et al.31 analyzed the ethanol mixed with orange peel oil in a diesel engine. Interestingly, the output of NOx was minimized. With an increase of the ethanol proportion in the blend, NOx emission was lowered. This may be the reason for the reduction of the peak cylinder temperature during combustion of the alcohol mixed fuel, but HC and CO formation was on the opposite trend. The smoke emission was slightly lower for the ethanol mixed blend at full load condition. In the half loaded state, the formation of smoke was enormously reduced. Sivasubramanian et al.77 studied the orange peel oil–water emulsion (5% water + 95% orange peel oil and 10% water + 90% orange peel oil) in diesel engines, and engine brake thermal efficiency was reduced for both test fuels compared to diesel. Fuel consumption of the orange peel oil–water emulsion was increased for test fuels compared to diesel. This output may be the reason for the lower energy content of the test fuels. BTE and BSFC also play a prominent role in selecting alternate fuels. The engine modification technique significantly improved the engine’s performance and emission output response. Most of the researchers proved that the fuel injection pressure played a major role in engine fuel consumption by atomization action.77,78 The researcher proposed the CRDi system as a more suitable technique for orange peel oil fuel and implemented it in single-cylinder engines. From the work, it was revealed that the BTE and fuel consumption of the engine were promoted at higher fuel injection pressure. This may be a reason for the improved fuel atomization and air fuel entrainment action in the combustion chamber. On the other hand, NOx emission was promoted.32 The researcher proposed that nano-addition (nanofluid) was the best method for simultaneously mitigating NOx and improving the performance deeds. In this experiment, titanium oxide nanopowder was an addition with orange peel oil. It had significant improvement in both performance and emission output response.75 A summary of studies conducted with orange peel oil and its comparison with diesel fuel is shown in Table 6. The inference of the above study is shown in Figure 6.

Table 6. Summary of Studies Conducted with Orange Peel Oil and Its Comparison with Diesel Fuel.

          Engine Output Response
  
Year Research Group Base Fuel Fuel Modification Engine Modification Performance Emission Inferences
2021 Asokan et al.76 Diesel Orange peel oil blend with diesel - BTE-↓ HC↓ NOx was major concern
BSFC-↑ CO↓
NOx↑
2018 Sivasubramanian et al.77 Orange peel biodiesel Addition of water with orange biodiesel - BTE-↓ HC↓ Significant drop in BTE and BSFC
BSFC-↑ CO↓
NOx↓
2018 Siva et al.78 Orange peel biodiesel Addition of water with orange biodiesel - BTE-↓ HC↓ Significant drop in BTE and BSFC
BSFC-↑ CO↓
NOx↓
2020 Asok et al.32 Diesel Orange peel oil blend with diesel CRDI BTE-↑ HC↓ NOx was major concern
BSFC-↑ CO↓
NOx↑
2019 Asok et al.33 Orange peel oil Orange peel oil CRDI BTE-↑ HC↓ NOx was major concern
BSFC-↑ CO↓
NOx↑
2018 Amar Deep et al.31 Orange peel oil Addition of ethanol with orange peel oil - BTE-↓ HC↓ Significant drop in BTE and BSFC
BSFC-↑ CO↓
NOx↓
2020 Kumar et al.75 Diesel Addition of titanium oxide nanoparticle with orange peel oil biodiesel   BTE-↑ HC↓ Significant improvement in both performance and emission output response
BSFC-↑ CO↓
NOx↓
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Figure 6.

Figure 6

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Inference of the orange peel oil study.

4.6. Citronella Oil

The author explored the diesel engine behavior with citronella oil and its blends with diesel. It was revealed from the research79 that the CNO20 (20% citronella oil + 80% diesel) blend shows higher BTE than other CNO blends (CNO40, CNO60). It may be a cause of the lower calorific value of citronella oil. Notably, CO and HC emissions were enormously reduced for all of the citronella blends compared to diesel. This was possibly the reason for the abundant O2 availability in the citronella oil blends. It was found that citronella oil exhibited peak heat release compared to diesel fuel due to the better evaporation of the low viscous citronella oil. In terms of emission reduction in diesel engines, NOx emission was a major concern in CNO blends,35 with many researchers working towards NOx elimination from biodiesel/biofuel.79−80 There were many routes found such as water emulsion, nanoemulsion, nano-additive, SCR, EGR, etc.81−82 The researcher proposed that nano-addition (nanofluid) was the best method for simultaneously mitigating NOx, HC and CO emissions and implemented it in the DI engine. In this experiment cobalt chromite nanopowder was an addition with citronella oil as a fuel. It significantly improved the emission output response. The performance deeds are also an important phenomena in terms of fuel economy. In this regard, the author tried citronella oil fuel in a homogeneous charge compression ignition (HCCI) engine, which improved the NOx emission compared to a conventional diesel engine, but there is a slight improvement in performance concerns. The researcher proposed that water-emulsified nano-addition (nanofluid) was the best method for simultaneously mitigating NOx and improving the performance deeds. In this experiment cobalt chromite nanopowder was an addition with water-emulsified citronella oil. It significantly improved both performance and emission output response.81,82 A summary of studies conducted with citronella oil and its comparison with diesel fuel is shown in Table 7. The inference of the above study is shown in Figure 7.

Table 7. Summary of Studies Conducted with Citronella Oil and Its Comparison.

          Engine Output Response
  
Year Research Group Base Fuel Fuel Modification Engine Modification Performance Emission Inferences
2020 Krishnamoorthy et al.79 Diesel Citronella oil blend with diesel - BTE-↓ HC↓ NOx was major concern but lower performance deeds
BSFC-↑ CO↓
NOx↑
2020 Erdiwansyah et al.35 Diesel Citronella oil Blend with diesel - BTE-↓ HC↓ NOx was major concern but lower performance deeds
BSFC-↑ CO↓
NOx↑
2021 Krishnamoorthy et al.81 Citronella oil Nano-additive mixed with citronella oil - BTE-↓ HC↓ NOx was improved but lower performance deeds
BSFC-↑ CO↓
NOx↑
2022 Senthur et al.82 Citronella oil Nano-additive mixed with citronella oil HCCI BTE-↑ HC↓ Slight improvement in performance output response but NOx was increased
BSFC-↓ CO↓
NOx↑
2020 Asaithambi et al.36 Citronella oil Nano-additive mixed with citronella oil R-EGR BTE-↓ HC↓ Significant improvement in emission output response
BSFC-↑ CO↓
NOx↑
2020 Krishnamoorthy et al.34 Diesel Nano-mixed citronella oil with water emulsion Varying IT and IP BTE-↑ HC↓ Significant improvement in both performance and emission output response
BSFC-↓ CO↓
NOx↓
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Figure 7.

Figure 7

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Inference of the citronella oil study.

4.7. Turpentine Oil

The author studied the behavior of diesel engines when using turpentine oil and its mixtures with diesel. According to the study, TPO20 (80% diesel + 20% turpentine oil) had a greater BTE than the other TPO blends (TPO40, TPO60). It might be the reason turpentine oil has a reduced calorific value. Notably, all of the turpentine blends had significantly lower CO and HC emissions than diesel. This may have been the cause of the abundance of O2 present in the turpentine oil mixtures. Due to the low viscosity of turpentine oil’s improved evaporation, it was discovered that it releases heat at a higher peak than diesel fuel.84 In terms of emission reduction in diesel engines, NOx emission was a major concern in TPO blends. Numerous approaches, including water emulsion, nanoemulsion, nano-additive, SCR, EGR, HCCI, and variable compression ratio (VCR), were discovered.83,84 In this regard, the author experimented by fueling HCCI engines with turpentine oil, which reduced NOx emissions compared to regular diesel engines, but there is no improvement in the performance deeds.83 The researcher has investigated the conventional diesel engine fueled with Jatropha mixed turpentine oil; the result showed that NOx emission was further reduced compared with the turpentine oil fueled engine.37 In the VCR engine, there was a slight improvement in both performance and emission results.38 The researcher proposed that mixing cetane improver with EGR was the best method for simultaneously mitigating NOx and improving the performance deeds. In this experiment, SC5D was an addition to turpentine oil. It significantly improved both performance and emission output response.39 The table presents an overview of experiments using turpentine oil and compares it to diesel fuel in Table 8. The study’s inference is shown in Figure 8.

Table 8. Summary of Studies Conducted with Turpentine Oil and Its Comparison with Diesel Fuel.

          Engine Output Response
  
Year Research Group Base Fuel Fuel Modification Engine Modification Performance Emission Inferences
2010 Prem Anand et al.84 Diesel Turpentine oil blend with diesel - BTE-↓ HC↓ NOx was major concern
BSFC-↑ CO↓
NOx↑
2014 Kannan et al.83 Diesel Turpentine oil (TPO) blend with diesel HCCI BTE-↓ HC↓ Slight improvement in emission output response
BSFC-↑ CO↓
NOx↓
2017 Pankaj Dubey et al.37 Diesel Blending Jatropha oil with turpentine oil - BTE-↓ HC↓ Major drop in BTE and BSFC
BSFC-↑ CO↓
NOx↓
2018 Pankaj Dubey et al.38 Diesel Blending Jatropha oil with turpentine oil VCR BTE-↑ HC↓ Slight improvement in both performance and emission output response
BSFC-↓ CO↓
NOx↓
2019 Jeevanantham et al.39 Diesel 20% TPO + diesel + cetane improver 10% to 30% of EGR BTE-↑ HC↓ Significant improvement in both performance and emission output response
BSFC-↓ CO↓
NOx↓
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Figure 8.

Figure 8

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Inference of the turpentine oil study.

5. Conclusion

In this review work, several biofuels, namely, lemongrass oil, citronella oil, eucalyptus oil, pine oil, turpentine oil, lemon peel oil and orange peel oil, were classified as low viscous fuels, and a detailed review on the operation of a diesel engine with and without modification has been made. An immense effort was undertaken to provide detailed comparisons of diesel and low viscous fuels on their performance, combustion and emission parameters. A large amount of experimental data for low viscous fuels were reviewed, and their inferences are framed for an easy understanding to the readers. Low viscous biofuel is nontoxic, biodegradable, renewable, largely available and eco-friendly. It is easy to store, handle and transport. Low viscous fuels are derived from plant stems, leaves, peels and resins. On a diesel engine, low viscous biofuel emits lower HC, CO and PM than standard diesel. The following points were drawn from the comprehensive review carried out on low viscous biofuels. In comparison with diesel, low viscous fuels and their blends produce slightly lower HRR and BTE and higher BSFC and BSEC due to the lower heating content. Low viscous fuels and their blends produce lower tailpipe emissions such as CO, UBHC and PM and higher CO2 and NOx due to the higher cetane index and rich O2 content that support complete combustion. However, low viscous fuel blends of up to 20% can be surveyed as a substitute fuel for diesel

5.1. Challenges and Opportunities

Yet, some issues are associated with low viscous fuel such as higher lower thermal efficiency and higher fuel economy, which are mainly because of the undesirable properties of low heating value and volatility. The NOx emission was higher for all low viscous fuels because of the abundant O2 content. Even though hopeful work has been employed on examining the emission, performance and combustion behavior of diesel engines fueled with various low viscous fuels, namely, citronella, lemon peel oil, orange peel oil, etc., it is observed from the literature review that a constrained amount of work has been done to examine the multicylinder engine, after the treatment device attached engine.

Nowadays, low viscous fuels are not economically viable due to various factors. Government-supported policies and enhanced technological development are needed to promote biofuel research. Thus, more experimental long-term issues on biofuels such as carbon deposits, choking, and contamination of engines have to be addressed to enhance the engine behavior with improved engine design as per the biofuel properties.

Acknowledgments

The authors extend their appreciation to the Deanship of Scientific Research at King Khalid University for funding this work through large group Research Project under grant number RGP2/45/44

The authors declare no competing financial interest.

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