Potential of Waste Cooking Oil Biodiesel as Renewable Fuel in Combustion Engines: A Review

: As non-renewable conventional fossil fuel sources are depleting day by day, researchers are continually ﬁnding new ways of producing and utilizing alternative, renewable, and reliable fuels. Due to conventional technologies, the environment has been degraded seriously, which profoundly impacts life on earth. To reduce the emissions caused by running the compression ignition engines, waste cooking oil (WCO) biodiesel is one of the best alternative fuels locally available in all parts of the world. Different study results are reviewed with a clear focus on combustion, performance, and emission characteristics, and the impact on engine durability. Moreover, the environmental and economic impacts are also reviewed in this study. When determining the combustion characteristics of WCO biodiesel, the cylinder peak pressure value increases and the heat release rate and ignition delay period decreases. In performance characteristics, brake-speciﬁc fuel consumption increases while brake-speciﬁc energy consumption, brake power, and torque decrease. WCO biodiesel cuts down the emissions value by 85% due to decreased hydrocarbon, SO 2 , CO, and smoke emissions in the exhaust that will effectively save the environment. However, CO 2 and NO x generally increase when compared to diesel. The overall economic impact of production on the utilization of this resource is also elaborated. The results show that the use of WCO biodiesel is technically, economically, environmentally, and tribologically appropriate for any diesel engine.


Introduction
Petroleum, coal, and natural gas cover the significant contribution of energy in the world [1]. However, these supplies are depleting day by day [2], and if countries continue to depend on them without changing their sources, they will quickly run out of fossil fuel reserves [3]. These typical sources are continuous sources of greenhouse gas (GHG) emissions resulting in climate change through global warming [4]. The developed world is seriously considering reducing GHG emissions, and they have already met their targets [5]. However, they estimate that the pace of emissions reduction will slow down after 2020, making it challenging to meet other targets, such as reducing the domestic emissions sively covered the physiochemical properties, combustion, performance, and emission characteristics of WCO, as well as its environmental and economic impacts. This study provides a review of waste cooking oil biodiesel in CI engines from a technical perspective. The study's layout has a comprehensive overview of the physicochemical properties, combustion characteristics, performance characteristics, and emissions characteristics. The environmental impact of the use of conventional fuels and renewables is also summarized. Finally, the economics associated with the use of biodiesel from preparation to end use are reviewed, along with the impacts caused by biodiesel on the durability of engines.

Physicochemical Properties
WCO is not directly used in the compression ignition (CI) engine due to the viscosity and acid number difference. However, other properties also vary from petroleum diesel. The limits defined by ASTM and European standards provide allowable values for its permissible use. Therefore, some essential physicochemical properties are first presented for comparison in Table 1.  [18] It is noted that the biodiesel (ester) made with saturated or long-chain fatty acid gives relatively high cetane numbers and cloud point values and also clogs the nozzle, while the esters of unsaturated fatty acid give a relatively low cetane number but oxidize quickly. In general, the heat of combustion, cetane number, viscosity, and melting points of fatty acid decrease with unsaturation and increase with chain lengths [39]. Compared to diesel, WCO biodiesel has less Sulphur content, aromatic content, flash point, and biodegradability [40]. The significant findings of the physicochemical properties (e.g., cetane number, density, viscosity, calorific value, and liquid length) of waste cooking oil are discussed in Table 2.

Topic Findings References
Viscosity • Due to the high viscosity of the WCO biodiesel, the diesel blends are more viscous than petroleum diesel. Therefore, up to 20% of a biodiesel blend is recommended for CI engines without modification.

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Kinematic viscosity is the key to determining the fuel injection regime measured by the atomization of the fuel. The viscosity above the limit reduces the amount of atomized fuel before the combustion.

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The viscosity of WCO can also be reduced by blending it with n-propanol.

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The high viscosity of biodiesel decreases the discharge coefficient, mass flow rate, and injection velocity. To compensate for these factors, biodiesel is put at a higher injection temperature of about 60 K than petroleum diesel. • Due to high viscosity, biodiesel's penetration depth in the cylinder increases but reduces the atomization during the injection. [36,[41][42][43][44] Density • CI engines can produce more power with more dense fuel, but the soot emission also increases for high-density fuels. [41] Calorific value

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The calorific value of WCO biodiesel is about 12% lower than diesel due to the oxygen present in its molecule, which also reduces the thermal efficiency of the biodiesel-powered engine as compared to the petroleum diesel-fueled engine. [45] Liquid length

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The liquid length or penetration depth of biodiesel is higher than petroleum diesel due to high viscosity. [43] There are many ways of process waste cooking oil (WCO), but the most appreciated treatment known for its use is transesterification. This treatment makes WCO more compatible with compression ignition engines by modifying the physicochemical properties. The most notable changes are seen in the viscous properties of the oil when it is transesterified. The kinematic viscosity at 40 • C reduces from 32.52 mm 2 /s for WCO to 4.915 mm 2 /s for WCO biodiesel. These modified properties affect the fuel's spray characteristics, transforming the combustion characteristics when burnt in the engine.

Combustion Characteristics
Cylinder pressure is one of the critical factors determining engine performance as it is used to calculate how much work is transferred from burnt gases to the piston. Cylinder pressure is measured using some sophisticated displacement sensors and strain gauges [46,47]. It is measured either in terms of indicated mean effective pressure (IMEP), which is the ratio of work output and the engine swept volume or cylinder peak pressure (CPP). The IMEP leads to the assessment of the engine's mechanical efficiency [48]. The use of biodiesel in the engine increases the cylinder peak pressure [32,33].
Ignition delay (ID) is defined as the period between the start of fuel injection to the onset of combustion, which is one of the fundamental parameters to quantify combustion. The prolonged ID period corresponds to the intensity of the premixed combustion phase's heat release rate, as the amount of air-fuel mixture increases with time [49]. ID limits the operating and combustion range of the CI engine. A prolonged ID period can cause very high in-cylinder temperature and pressure at the end of the compression stroke. At this stage, the charge mixture combusts suddenly, which can sometimes cause knocking [50]. ID is reduced for WCO biodiesel for its high cetane number as it improves the auto-ignition property and causes complete combustion of the fuel [51]. The shorter ID enhances the en- gine's fuel consumption characteristics due to higher oxygen content in WCO biodiesel [52]. The ID period is reduced by increasing the engine load because brake power (BP) increases with the load, increasing the combustion chamber's heat. In this way, the charge gets ignited sooner and is observed using high proportions of WCO biodiesel in the blend [33]. Pressure increase, HRR, and overall pressure can be measured using the ignition delay values [53].
After the ignition delay period is over, the combustion process starts from the heat release rate (HRR), which changes from negative to positive with a crank angle [51]. The effect of higher HRR in the premixed combustion phase for the WCO biodiesel blends is observed in the form of high cylinder pressure [54]. Some studies also claim to reduce the HRR value for biodiesel and the subsequent blends even though the cylinder pressure rises in their case [32,33,55]. For biodiesel and blends, the increase or decrease in exhaust gas temperature (EGT) value with a reference diesel fuel has been studied by various authors. Yesilyurt et al. [18] revealed that EGT value decreases for B20 (20% biodiesel, 80% diesel fuel) compared to petroleum diesel. Yamin et al. [56] reported a decrease for 100% biodiesel (B100), and Muralidharan et al. [32] reported an increase for B40. The EGT value increases with the increase in engine load for a specific fuel, as more fuel is burnt to compensate for the extra required power. EGT value increases the WCO biodiesel amount within a range of 50% to 100% mix in the diesel as the heating value decreases [57,58]. The reason behind this reduction in EGT at higher CR is the lower calorific value and shorter ignition delay, hence the low temperature after compression stroke (peak cylinder temperature) and the performance increase by lower exhaust losses [56].
The significant findings of the combustion characteristics (i.e., cylinder pressure, ignition delay, and heat release rate) of waste cooking oil in a diesel engine are discussed in Table 3. Table 3. Combustion characteristics of waste cooking oil biodiesel blended with diesel fuel in diesel engines.

Topic Findings References
Cylinder pressure • At full load conditions, biodiesel blends give higher cylinder pressure values compared to petroleum diesel.

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The indicated MEP for blend B40 is low at high CR and high at low CR compared with the standard diesel. At CR 21, its value is 5.58 bar for B40 and 5.77 bar for diesel. [32,33] Ignition delay

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The higher proportion of biodiesel in the blends lowers the ignition delays.

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The ignition delay for biodiesel is shortened when compared to the ignition delay for petroleum biodiesel. [33,44] Heat release rate • It also shows similar trends like ignition delay. A more excellent ratio of biodiesel decreases the value of HRR. [33] The use of biodiesel generally delivers a rise in peak cylinder pressure compared to petroleum diesel. Another combustion property, like ignition delay, is reduced, as claimed by various authors. The heat release rate is declined generally for biodiesel and its blends compared with petroleum diesel. The EGT value depends on CR, engine load, blends proportion, heating value, and ignition delay. Therefore, different studies have published either an increase or decrease in the value of the reference fuel. The combustion properties are directly linked to the performance characteristics, such as engine torque, BP, BTE, BSFC, BSEC, and EGT.

Performance Characteristics
Due to the high viscosity of the WCO biodiesel, the blends also get more viscous than the pure diesel, which affects the fuel's atomization during injection and disturbs the spray characteristics. In this way, the evaporation and the burning period during expansion are prolonged, reducing the engine torque. However, this torque value increases the fuel injection pressure, which improves the fuel's spray characteristics [18,44,59]. Spray characteristics play an important role in engine performance and exhaust emissions. According to Som et al. [43], some of the fuels may require slight design modifications to the engine, like piston bowl design, due to differences in spray and injection characteristics. Sometimes improvement of the injection or ambient conditions like density and temperature can solve the problem. The nozzle shape is also a factor that can improve the spray characteristics, as a non-circular orifice enhances the air entrance [60]. Similarly, Yu et al. [61] recommend a triangular orifice to serve this purpose. Wang et al. [62] and Agarwal et al. [63] suggest that fuel injection pressure is the best way for solving this issue as it improves the equivalence ratio and spray tip penetration and shrinks the spray cone angle and area [64]. All these improvements can enhance the engine torque output for WCO biodiesel blended fuels.
Brake power is reduced by using the biodiesel blend as compared to petroleum diesel. This is due to the small heating value of WCO biodiesel [44]. The brake power is, however, improved by increasing the fuel injection pressure [18]. BSFC is defined as the amount of fuel consumed to produce a unit output of power, which is a measure of the engine's economic performance. Using B100, the BSFC of a diesel engine is relatively higher than using B0 fuel. The higher density and viscosity and the lower calorific value of the B100 with increasing brake mean effective pressure (BMEP) [51]. The BSFC value decreases with increasing engine load because heat loss is reduced [65]. Abed et al. [66] reveal that WCO biodiesel blends show a higher value of BSFC than pure diesel for the same power output.
Thermal efficiency is the ratio of power output and the energy produced by the injected fuel. This energy comes by taking the product of lower heating value and the mass flow rate of injected fuel [28], also called fuel conversion efficiency [51]. The BTE decreases for the biodiesel blends compared to the pure diesel, which is due to the poor atomization and combustion of the viscous and dense blended fuel [66,67]. Brake-specific energy consumption is another valuable factor for observing different heating value fuels in a CI engine. It is the product of the heating value and BSFC of the fuel [51]. For biodiesel blend B80, brake-specific energy consumption (BSEC) value decreases [55]. The significant findings of the performance characteristics (i.e., brake power, brake specific fuel consumption, brake thermal efficiency, mechanical efficiency, exhaust gas temperature, and engine torque) of waste cooking oil in a diesel engine are discussed in Table 4. Table 4. Performance characteristics of waste cooking oil biodiesel blended with diesel fuel in diesel engines.

Topic Findings References
Brake power • At higher compression ratios (CR), the BP value decreases for higher blend proportions as the energy is converted from chemical to mechanical. At CR 21, BP for diesel and B40 is 2.12 kW and 2.07 kW, respectively. • Maximum BP is observed for the minor biodiesel proportion of B5 to be 7.9 kW and 5.5 kW for B100 fuel. • Engine power is reduced by 6, 8, and 10 kW for B20, B70, and B100 blends. [18,32,44] Brake-specific fuel consumption

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The specific fuel consumption of the B40 blend is lower than that of all other blends at the compression ratios of 20 and 21. Its value for blend B40 at the compression ratio of 21 is 0.259 kg/kWh, whereas for diesel it is 0.314 kg/kWh, which can be due to viscosity, density, or the heating value of fuels. • At maximum BP, brake-specific fuel consumption (BSFC) value for B100 is 0. 35   The research abridgement shows that engine torque, BP, and BSEC decreases for using biodiesel and blends, and the BTE value either decreases or increases according to operating conditions such as injection pressure and spray nozzle geometry. BSFC is higher for most cases. However, a few studies claim a decrease.

Emission Characteristics
The amount of unburnt HC in the exhaust depends on the maxing of air and fuel within the engine cylinder [51]. The longer ignition delay can also cause high HC emission as the fuel is accumulated in the combustion chamber. The amount of HC emissions decreases for the higher proportions of the WCO biodiesel blends at all engine loads due to the higher oxygen content and higher cetane number [66]. The lower HC emission ensures that the combustion is perfect with the fuel's good atomization [71,72]. Redfern et al. [73] performed a 60,000 km durability test on a EURO II and a EURO IV diesel engine using B10 and B8 blends. They found that total polycyclic in the EURO II engine aromatic hydrocarbons emissions was less when biodiesel was used. The EURO IV engine did not show a significant change in PAH and PCDD/F (polychlorinated dibenzo-p-dioxins and dibenzofurans) emissions.
The amount of CO in the engine emissions is directly related to the fuel's physicochemical properties like peak temperature within the engine cylinder, air to fuel ratio, time available for the complete combustion, and the oxygen availability at high engine speed [74]. However, the higher viscosity of the WCO blends generally increases CO emission due to lower atomization in the unmodified engines [75]. At lower loads, the CO emission is even less than the diesel, but it increases at the higher loads. The decrease is due to more oxygen and less carbon in the biodiesel molecule than diesel, which helps fuel burn completely [66]. CO 2 is reported as the least harmful greenhouse gas as its life cycle can easily be regulated by growing energy crops globally. The CO 2 emission depends mainly on compression ratio and exhaust gas temperature. At lower CR, the emission content is high due to proper combustion [32]. The CO 2 amount increases for higher biodiesel proportions in diesel. Its trend rises for the engine running at higher loads due to more fuel burning at higher loads and more oxygen available in the biodiesel molecule [66]. Xue et al. [76] report about CO 2 emission that its increasing trend is for biodiesel and diesel with increasing engine load, and a similar trend has been supported by many others in the research.
Diesel emission contains harmful gases like NO x , the acid rain source when accumulated in the environment [50]. It is produced due to very high temperature in the premixed combustion phase, available oxygen amount, and reaction time. The NO x amount increases with increasing engine load no matter which fuel is being used. This is because more fuel is burnt and the rise of peak cylinder temperature is the cause of thermal/Zeldovich NO x synthesis. The peak cylinder temperature is directly related to the adiabatic flame temperature, which controls the NO x emission rate. High adiabatic flame temperature causes higher peak cylinder temperature and higher percentages of NO x . The biodiesel increases the cylinder temperature compared to diesel and more oxygen is contained in the biodiesel molecule, therefore the No x emissions increase. In this way, biodiesel blends increase the NO x amount [66]. A similar concept is given by Alessandro et al. and Valente [77,78]. Reduction in the emission of oxides of nitrogen is one of the prime focuses of engine researchers. Generally, NO x emission increases with an increase in CR.
The smoke amount in the engine exhaust emission is due to the incomplete burning of fuel, and engines with lower smoke emission are signs of good combustion of fuel [79,80]. This occurs due to the poor atomization of the fuel. The smoke emission increases with the increase in output power due to more fuel burning inside the engine, applied to all the fuels. Particularly for diesel fuel, this increase is due to the branched and ring structure; however, the emission level decreases for the biodiesel blends due to oxygen in the biodiesel molecular structure [66,76]. Yang et al. [81] tested for durability (80,000 km) two brand new identical diesel engines fueled by B0 and B20. At 0 km, the B20 engine showed lower HC, PM, and CO emissions than the B0 engine. After 20,000 km and above, the B0 emissions were less than B20.
The significant findings of the emission characteristics (i.e., HC, CO, CO 2 , NO x , and smoke emission) of waste cooking oil in a diesel engine are discussed in Table 5.  The effect of transformed physical and chemical properties is also observed on the fuel's emission characteristics. In this regard, biodiesel and blends show a reduction in unburnt HC emissions, CO, SO 2 , and smoke. CO 2 shows an increase because the biodiesel molecule has higher oxygen content as compared to the diesel molecule. The value of NO x emissions mainly depends on EGT, therefore the contrasting results have been published by different authors. To sum up the emission characteristics, it can be said that WCO biodiesel emissions are reduced and positively impact the environment by CO 2 equivalent emissions reduction. Of course, global warming will be controlled by creating legislation to make the use of biodiesel mandatory worldwide. It will also provide a way through finding an alternative, sustainable energy source.

Environmental Impacts
The global CO 2 equivalent emissions were recorded to be 35.65 billion tons in 2017, with a 2.7% increase in 2018 [82]. This amount of emissions is enough to escalate the global warming that affects melting icebergs and glaciers, weather extremes, shifting habitats, and sea-level rise [83]. It also affects marine life by increasing oxygen-consuming rates in fishes, altered emigrational patterns, and foraging in the polar seas [84]. The trees are being affected by extinction in localized species due to climate [85], and infectious diseases, especially mosquito-borne diseases like dengue, malaria, and viral encephalitis, are also influenced by the environment [86]. Figure 1 shows the emissions caused by significant sectors globally, and diesel is common in all industries [87].
GHG emissions are directly related to the world's energy requirement for both industrial and domestic purposes [88], and it is also the primary source of emissions. Out of all the energy resources that contribute to global energy demand, the crude oil portion is the highest of all, i.e., 31% [89]. The contribution of other primary energy resources globally, including crude oil, is shown in Figure 2 [89].
In the developing world, diesel fuel has an important place in the industrial economy in countries directly concerned with energy production and consumption. The end products of crude oil include fuel gas, LPG, kerosene, gasoline, diesel, fuel oil, and naphtha. The percentages of all these products distilled from a unit mass of crude oil are shown in Figure 3 [90]. It shows that diesel is about 20% of all the end products obtained from a refinery [90]. Therefore, taking 20% of the 31% energy demand which is met by crude oil, diesel's contribution to the global energy mix comes out to be 6.2%. In this way, diesel produces 3.2 billion tons of life cycle CO 2 emissions out of 35.65 billion tons of global CO 2 eq. emissions.
The CO 2 equivalent of diesel is 87 g/MJ, and that of WCO biodiesel is 13 g/MJ. This shows that WCO biodiesel causes 85% fewer emissions than diesel [91]. Utilizing WCO as biodiesel, the pollution is controlled through wastewater reduction by 79%, hazardous waste reduction by 96%, particulate matter reduction by 47%, and HC emissions by 67%. Moreover, 3.5 renewable units of energy are extracted for the expenditure of 1 unit of energy from fossil fuel for biodiesel production [92]. This confirms that the use of biodiesel is environmentally friendly, but the production is also evidence of a clean atmosphere with energy security. GHG emissions are directly related to the world's energy requirement for bo dustrial and domestic purposes [88], and it is also the primary source of emissions. all the energy resources that contribute to global energy demand, the crude oil por the highest of all, i.e., 31% [89]. The contribution of other primary energy resources ally, including crude oil, is shown in Figure 2 [89].   Figure 1. CO 2 equivalent emissions by sector [87]. GHG emissions are directly related to the world's energy requirement for both industrial and domestic purposes [88], and it is also the primary source of emissions. Out of all the energy resources that contribute to global energy demand, the crude oil portion is the highest of all, i.e., 31% [89]. The contribution of other primary energy resources globally, including crude oil, is shown in Figure 2 [89].   Figure 2. Major contributors to global energy demand [89].
Energies 2021, 14,2565 Figure 3 [90]. It shows that diesel is about 20% of all the end products obtained from a refinery [90]. Therefore, taking 20% of the 31% energy demand which is met by crude oil, diesel's contribution to the global energy mix comes out to be 6.2%. In this way, diesel produces 3.2 billion tons of life cycle CO2 emissions out of 35.65 billion tons of global CO2 eq. emissions. The CO2 equivalent of diesel is 87 g/MJ, and that of WCO biodiesel is 13 g/MJ. This shows that WCO biodiesel causes 85% fewer emissions than diesel [91]. Utilizing WCO as biodiesel, the pollution is controlled through wastewater reduction by 79%, hazardous waste reduction by 96%, particulate matter reduction by 47%, and HC emissions by 67%. Moreover, 3.5 renewable units of energy are extracted for the expenditure of 1 unit of energy from fossil fuel for biodiesel production [92]. This confirms that the use of biodiesel is environmentally friendly, but the production is also evidence of a clean atmosphere with energy security.

Economic Impact
Biodiesel is produced by any fatty acid source, such as animal fats, vegetable oil, almonds, fish, etc. Out of all the fatty acid sources, the lowest cost fatty acid source is waste cooking oil [39]. The cost of production of WCO biodiesel is distributed into the feedstock, maintenance, chemicals, energy, labor, and depreciation. Each entity's cost is given in Figure 4, which shows that the feedstock is the most significant cost [93]. The primary feedstock is collected through the wastewater bodies and food industry. The people of the world can be made aware of health issues caused by reusing cooking oil and disposing of it into sinks and garbage by targeted awareness programs. They should be educated to instead sell it to the biodiesel production facilities. In this way, an organized structure can be formulated for the collection of WCO resources.

Economic Impact
Biodiesel is produced by any fatty acid source, such as animal fats, vegetable oil, almonds, fish, etc. Out of all the fatty acid sources, the lowest cost fatty acid source is waste cooking oil [39]. The cost of production of WCO biodiesel is distributed into the feedstock, maintenance, chemicals, energy, labor, and depreciation. Each entity's cost is given in Figure 4, which shows that the feedstock is the most significant cost [93]. The primary feedstock is collected through the wastewater bodies and food industry. The people of the world can be made aware of health issues caused by reusing cooking oil and disposing of it into sinks and garbage by targeted awareness programs. They should be educated to instead sell it to the biodiesel production facilities. In this way, an organized structure can be formulated for the collection of WCO resources.  When the collection and purchasing of the WCO are made less and less expensive, the total production cost will be lessened significantly. The amount of this resource in the world is enough to help meet the environmental cleanliness targets quickly. The availability of WCO is tabulated in Figure 5, which shows the amount of feedstock available in different parts of the world. According to the national biodiesel board, waste cooking oil When the collection and purchasing of the WCO are made less and less expensive, the total production cost will be lessened significantly. The amount of this resource in the world is enough to help meet the environmental cleanliness targets quickly. The availability of WCO is tabulated in Figure 5, which shows the amount of feedstock available in different parts of the world. According to the national biodiesel board, waste cooking oil will become the second-largest feedstock for biodiesel production [94]. When the collection and purchasing of the WCO are made less and less expensive, the total production cost will be lessened significantly. The amount of this resource in the world is enough to help meet the environmental cleanliness targets quickly. The availability of WCO is tabulated in Figure 5, which shows the amount of feedstock available in different parts of the world. According to the national biodiesel board, waste cooking oil will become the second-largest feedstock for biodiesel production [94].  When this much feedstock is available worldwide, an excellent strategic structure can be built to organize the cycle of WCO collection, purchasing, biodiesel production, and supply to end users while prioritizing economic energy and a clean environment. However, the risk factors of production of WCO biodiesel must be taken into account, and proper risk management tools must be applied [101]. The WCO biodiesel does not require any engine modification [102,103]. Owing to its good lubricating properties, it does not require additional lubricants, such as diesel. It also uses local feedstock and is produced locally, so it does not require drilling, refining, and transport like petroleum diesel [40]. All these factors will save the cost and make its use economic.

Materials U S A U K C a n a d a E U I n d i a C h i n a J a p a n T u r k e y G e r m a n y T a i w a n
On the economic side, this fuel is inexpensive due to low production cost and widespread availability of raw material, which has been wasted for years and is available equally in all parts of the world. Moreover, there is no need for massive investments in extraction and logistics such as in crude oil. It is a privilege for investors and economists to earn and provide an alternative product to the masses. The engine's operational and maintenance costs are also reduced due to its lubricating properties that lengthen the engine life. A summary of the literature is given in Table 6.

Engine Durability
The use of biodiesel and waste tire pyrolysis oil increases the engine life because of its higher lubricating properties [105,106]. WCO biodiesel decreases the wear and tear of the engine which lessens the maintenance requirement [40,107]. Tribological studies show that biodiesel's friction coefficient obtained from cottonseed oil showed a 28% smaller value than petroleum diesel, and the wear scar diameter of the same biodiesel was 47.6% smaller than petroleum diesel [108]. Bietresato et al. [109] performed an 800 h durability test on a 118 kW tractor fueled by B100 and reported that the engine had none of the problems that affect engine life when the lubricant was replaced every 100 h. According to Fazal et al. [110], engines running on biodiesel blended fuels mostly show fuel pump failure problems, coking in fuel injectors, sticky moving parts, and filter plugging. However, most of the published data shows low carbon deposition and low wear by using biodiesel blends, but a few authors also claim higher carbon deposits. Different methods are used to find the tribological performance of the circular and distorted circular bores of internal combustion engines [111].

Conclusions
This study covers a detailed review of the WCO biodiesel use in the CI engine with different blending proportions with petroleum diesel. The physicochemical properties comparison of diesel, biodiesel, and WCO showed whether the values come within the allowable limits by ASTM and European standards or not. This comparison was followed by the combustion, performance, and emission characteristics elucidation of biodiesel blends and reference fuel, i.e., petroleum diesel. The significant findings of the review are as follows: • WCO is a potential source and widely available in the world for producing biodiesel through transesterification.

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The need for transesterification of WCO is due to its viscosity and acid number for direct use in engines because high viscosity disturbs the spray characteristics of fuel and a high acid number causes corrosion of the engine parts.

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In the combustion characteristics comparison of biodiesel with reference diesel, CPP increased, ID period shortened, HRR decreased, and EGT had erratic behavior.
• Similarly, the performance characteristics showed that BP, BSEC, and engine torque decrease for biodiesel. Meanwhile, BSFC increases and BTE indicates an inconsistent trend. • Lastly, the emissions comparison revealed that HC, SO 2 , CO, and smoke decreases, and CO 2 increases in the exhaust aggregate. However, NO x emissions vary inconsistently. • Biodiesel use is economically viable due to expected availability, low processing cost, and no modification required in the CI engines' design or structure. • Engine life is longer for biodiesel-fueled engines because the lubricity of biodiesel is higher than petroleum diesel. • Biodiesel reduces diesel emission values by 85%, which has a 6.2% share in the global energy mix with an emissions share of 3.2 billion tons of CO 2 eq. emissions.
Further work in this field has explored the inclusion of nanoparticles in biodiesel blends due to their positive effects on their physicochemical properties and emission characteristics. Better characterization is also good for enhancing combustion, performance, and emission characteristics. There is also room for improving biodiesel's oxidative stability and the blend's stability, especially with nanoparticle additions. Alternative fuel-related policies should be developed to commercialize the WCO-diesel blended fuel.

Conflicts of Interest:
The authors declare no conflict of interest.