Recent Advances in Fuel Additives and Their Spray Characteristics for Diesel-Based Blends

: The spray characteristics play a crucial role in determining the performance and emissions of compression ignition (CI) engines at the pre-combustion stage. With the advent of many types of alternative fuels and their blends with diesel, it is necessary to investigate the effect of fuel properties and various injection conditions to determine the penetration length, spray cone angle and spray area for a viable fuel with similar or better dispersion characteristics to diesel. Hence, this study reviews and summarizes the spray visualization techniques, along with in depth analysis of macroscopic spray properties of various fuel blends with diesel. It was found that higher injection pressures typically led to higher penetration lengths, better atomization with reduced Sauter mean diameter. Liquid properties such as viscosity, surface tension, and evaporation as well as structural properties play a crucial role in spray formation in fuel blends with various types of alcohols, ethers, biodiesel, aliphatic, aromatic, as well as nanoparticle additives. This review compares these fuel additives and their types to present a comparative study with diesel to determine the ideal conditions with minimal changes to the engine for replacing diesel with a sustainable fuel consisting of better combustion efficiency due to its enhanced spray characteristics.


Introduction
Diesel engines continue to be the largest sector for heavy transport as well as power units due to their versatile, thermally efficient, high-power output, and durable nature.Despite strict emission standards being imposed and a greater push for the electric vehicles, diesel engines are still the primary mode of transport for the larger community [1].However, their main adversary are the harmful emissions such as NOx, CO, unburnt hydrocarbons, and particulate matter [2].Various technologies are being reported to improve engine performance, as well as reduce these emissions as much as possible.These include improvements in combustion such as Homogeneous Charge Compression Ignition (HCCI) as well as Premix and Partial Premixed CCI engines which are still undergoing development [3].
Recently, there has been an increasing interest in the usage of alternative fuels mixed with diesel to tackle issues related to performance, emissions, fuel shortage, affordability, as well as make the development process environmentally friendly.One such fuel is biodiesel which is seen as one of the promising alternative which is compatible with diesel and its engine technology [4].Due to its biofriendly nature of production, many see it as a cleaner, renewable, and sustainable energy source for the coming future.Moreover, biodiesel blends have successfully demonstrated at lab scale for the reduced harmful emissions and compatible nature along with diesel.However, it still lacks affordability and requires modifications to the diesel engine for achieving optimum performance and emissions [5].
In order to achieve the standards as well as compete with the carbon neutral or renewable energy technologies, diesel engines need substantial development due to multiple nature of alternative fuel blends used.These developments can be categorized into pre-combustion and post-combustion processes.A common post combustion technique is the usage of catalytic converters, and exhaust gas recirculation.These converters are often expensive since they utilize platinum or rhodium metals.
The long-term storage is also a point of concern for commercialization of novel diesel blends.For instance, the process of autoxidation in biofuels causes variations in composition and fuel properties.Jain and Sharma [6] concluded that the properties like acid value, peroxide value, and kinematic viscosity and density of biodiesel increased during the storage period.These changes in fuel properties are detrimental to the fuel atomization quality.In order to reduce the oxidation, antioxidants are usually added to fulfill the least threshold of oxidative stability established for biodiesel commercialization [7,8].
Employing alternative fuels with diesel is another solution which may also result in poor lubricity and cause wearing of piston head and engine cylinder.Fazal et al. [9] compared the wear of diesel and biodiesel in direct injection CI engine under static and on field trials.They concluded that slightly lower wear was observed for diesel/biodiesel blends in comparison to the neat diesel due to the higher lubricity of biodiesel.Agarwal et al. [10] carried out an endurance test of 512 h. for a single cylinder engine employing 20% blend of linseed biodiesel and concluded that biodiesel does not show any major effect of wear for critical moving parts of an engine.In contrast, Mujtaba et al. [11] tested the wear characteristics of biodiesel produced from Palm-sesame oil blend (B30) with various fuel additives.They found that the addition of ethanol to B30 fuel decreased its lubricity and increased the wear and friction coefficient as compared to other additives.
In order to improve blend properties, fuel spray technology is the pre-combustion phenomenon where proper understanding and research could lead to a sustainable fuel development based on diesel blends.Fuel spray under high pressure causes droplet formation at high chamber pressures and temperatures leading to a homogenous fuel/air mixture [12].The more the mixing, the better the charge and cleaner the exhaust emissions.Different factors affect fuel injection inside the combustion chamber which can be categorized into fuel properties (viscosity, surface tension, latent heat of vaporization), injector geometry (nozzle shape, hole diameter/geometry), and external factors such as chamber temperature/pressure, injection pressure, rate, and duration.
An optical engine to see the actual fuel spray patterns is possible, however, the chamber pressures required for diesel ignition make it an extremely expensive task [13][14][15][16][17][18]. Figure 1a shows a typical experimental setup for fuel spray characterization [19].The experimental setup consists of fuel injection setup for pressurizing, a control volume vessel (CVV) and an optical path system.Common rail injection supplies high pressure fuel through a pump towards the injector.The injector is controlled by a driver that provides a timed pulse causing injection to occur inside the CVV.Injection duration can be varied by changing the pulse width and frequency from the driver.Nitrogen or air can be used to pressurize the chamber and the strength of the chamber, and its dimensions are designed keeping in mind the amount of pressure it must withstand.
Most researchers used a CVV having 3 to 4 optical windows to closely simulate the actual engine conditions without performing combustion (Figure 1b) [20][21][22].These metallic high strength chambers have optical glass/acrylic windows with high intensity light source and a fast camera to capture the spray patterns.Various imaging methods such as Mie schlieren, particle image velocimetry, phase doppler image analyzer (PDIA), phase doppler anemometry (PDA), phase doppler particle analyser, X-ray radiography or shadowgraph using laser can be used [23,24].In order to illuminate the spray field, various light sources are being used such as LED lights, halogen bulbs, xenon lamps and lasers.These light sources are operated via pulses in sync with the injection system.For capturing the spray images, various high-speed cameras are being used.Moreover, CCD and ICCD cameras have also shown promising results for capturing the macroscopic spray behavior for short time durations [25,26].Measuring macroscopic spray parameters is mostly performed by image processing in Matlab or Image J software.A typical image processing sequence is shown in Figure 1c.The fuel spray characteristics are classified into macroscopic and microscopic properties.The macroscopic properties are penetration length, cone angle, and spray volume which can be directly measured using a fast camera or a CCD camera.However, the microscopic properties such as droplet size, Sauter mean diameter (SMD), its density and distribution require high illumination, precise control and high-resolution imaging at the micro-level.These can be achieved using a PIV technique with laser.Moreover, Laser Induced Fluorescence (LIF) or Phase Doppler Particle Anemometry (PDPA) are also utilized to achieve microscale spray characteristics [31,32].
Many types of diesel blends are currently being investigated for use in CI engines.These are categorized in Figure 2 as solid, liquid, gaseous additives for achieving high performance, better emissions, and sustainable fuels for heavy transport.Several studies can be found in literature related to spray behavior with additives.However, a comparative investigation of different fuel additives and their spray performance was still lacking.Hence, in this paper, we have reviewed the macro and microscopic spray characteristics of biodiesel and diesel fuel with five different categories of fuel additives.Alcohol, ether, nanoparticles, aliphatic and aromatic compounds are investigated, and their fluid and spray characteristics are compared in this review.

Alcohol Additives
Alcohol consists of carbon chains with hydroxyl groups.This family includes compounds starting from methanol with a single carbon atom and reaches twenty carbon atoms compound called phytol.Table 1 details the important molecular and fluid-based properties essential for choosing the right additive for diesel engines.Alcohols are abundant and easily processed through anaerobic fermentation of biomass wastes like wood pulp, bagasse, sugarcane etc. [33].Their fluid properties such as viscosity and density are favorable for spray quality, as they are less dense and viscous than the biofuels.Their cetane number, evaporation/ignition characteristics may cause the extended ignition delay (ID) and enhance the fuel air mixture formation.Boiling point of alcohols are favorable for fuel vaporization, and their C/H ratio along with oxygen content, assists in reducing soot emissions.

Methanol
Zhang et al. [40] found that the penetration length (PL) of methanol is slightly longer than diesel at ambient pressure of 0.1 MPa, however, opposite results were observed at 2.2 MPa because of the difference in surface tension and viscosity.Methanol and diesel showed average spray cone angle (SCA) of 26.5° and 21.1° at the Pamb of 2.2 MPa.Anupam et al. [41] studied liquid and vapor penetration of methanol with diesel and 3 different surfactants.With 10 wt.% of methanol in diesel blended with the surfactant, its PL was 25% higher.This occurs since surfactants have a higher boiling point.Using 1dodecanol as surfactant, the PL was similar to diesel, hence, the blend was found compatible without any engine modification.

Ethanol
Zhan et al. [42] found that the PL of diesel/biodiesel/ethanol (D64B16E20) increased with injection pressure from 80 mm (Pinj = 50 MPa) to 100 mm (Pinj = 100 MPa).A slight decrease of its spray cone angle (SCA) was observed with increase of injection pressure.The longest PL was observed for biodiesel/diesel blend.The PL of diesel/butanol/ethanol were shorter because of higher SCA at any given injection condition.The spray projected area (SPA) of diesel/butanol/ethanol blend was greater than diesel/butanol blend.Hence, lower kinematic viscosity of ethanol favored fuel atomization.Lin Bao et al. [43] studied the macroscopic spray behavior of diesel-ethanol DE20 fuel.In comparison to diesel (D100), the PL, SCA and SPA of DE20, increased by −3%.Nilaphai et al. [29] showed that ethanol/butanol/diesel blend had greater PL.He concluded that the addition of ethanol favored the PL and enhanced spray quality.Geng et al. [44] found that the addition of ethanol to biodiesel enhanced the SCA and reduced PL.Its SMD decreased with the increase of ethanol in biodiesel blends.Yu et al. [45] concluded that ethanol volume fraction in fuel blends and the fuel injection pressure defined the fuel spray behavior.Fuel was injected with greater force at elevated injection pressure resulting in higher velocities and more shock waves associated with the jet.Hence, longer PL was observed at higher injection pressure and with lesser ethanol proportions in the fuel.

Propanol
Lee et al. [46] found that the PL decreased slightly while the SCA was increased with the addition of isopropanol-butanol-ethanol (IBE) to diesel at various concentrations.This effect was deteriorated at elevated temperature for evaporation.The ambient temperature had a negligible effect on SCA on the IBE fuel blend.Almost all the fuels at higher environmental temperature showed similar ignition delay.

Butanol
Huang et al. [47] reported that diesel/n-butanol blend showed increased PL due to high volatility and quick evaporation.The SCA of biodiesel decreased while PL was improved by blending butanol-acetone mixture with diesel owing to the lower viscosity and good volatility of BA mixture [48].Mo et al. [49] found that the addition of n-butanol to biodiesel reduced the PL and the spray volume.This is because of lower viscosity and smaller surface tension of butanol compared to biodiesel.It also caused reduction of drop sizes especially in the core region of spray for butanol blended fuels.He concluded that microscopic spray quality was improved more as compared to macroscopic properties using butanol as an additive.Algayyim et al. [50] revealed the impact of nozzle hole diameter using butanol/diesel blends.Two injectors of Delphi and Bosh were used having hole diameters of 0.198 mm and 0.18 mm respectively.Their experimental results showed that greater hole diameter enhanced the PL.Butanol blend had slightly longer PL than diesel for each hole diameter and the SCA was increased slightly with the increase of PL and nozzle diameter.
Li et al. [51] investigated spray properties of n-butanol and n-pentanol with diesel (Figure 3).They revealed that for same blending ratio and injection conditions, butanol showed greater PL, larger SCA, SPA area and lower peak tip velocity as compared to the n-pentanol blends.The comparison of spray morphology showed that n-butanol-diesel blends have largest SPA and thinnest spray boundaries, followed by n-pentanol blend and then diesel.Zhang et al. [52] compared the spray behavior of n-butanol/diesel blends with hydrogenated vegetable oil.The vapor phase penetration was almost similar for all fuel blends; however, the liquid penetration was affected by the physical fuel properties like boiling point.HVO yielded marginally longer liquid penetration than diesel.

Pentanol
Pan et al. [53] showed that the addition of pentanol reduced the spray permeability of diesel blend and enhanced the fuel SCA.The PL of pentanol and diesel in equal proportions (P50) was 24 mm, while the maximum and minimum SCAs of P50 are 25.06° and 17.13° which are greater than diesel having 23.24° and 15.83°.The smallest SPA was 62.21 mm 2 for the P50 fuel.Li et al. [19] also studied spray characteristics of biodiesel with pentanol (Figures 4-6).The results for four different fuel blends BDP10, BDP20, BDP30 and BDP40 having (10 v/v%, 20 v/v%, 30 v/v% and 40 v/v% pentanol) were compared with diesel.PL of biodiesel/pentanol blends decreased and the SCA was increased with the increase in concentration of pentanol.BDP40 fuel having (60% biodiesel and 40% pentanol) behaved like diesel.Comparison of spray tip velocity revealed that addition of pentanol reduced the axial momentum of the fuel jet.Ma et al. [54] concluded that adding pentanol affects the PL and SCA significantly at elevated ambient temperature.However, with the increase of ambient pressure, the PL was not much affected, with SCA decreasing for the pentanol blends.Hence, employing pentanol additives was more suited at higher ambient pressures and temperatures.

Octanol
Tian et al. [55] used four unique blends of octanol with diesel to investigate their spray properties.OC30BD70 fuel having 30% octanol showed the longest SPL, SPA; while the OC20BD80 fuel with 20% octanol had the largest spray cone angle.Hiroyasu and Arai model for predicting the fuel spray was also modified by including density, viscosity and the correction factor.The modified model was in good agreement for macroscopic properties for high injection pressures.

Acetone-Butanol-Ethanol (ABE) Mixture
The ABE mixture is a new potential additive as compared to other alcohols for fuels in IC engines [56].Few authors have experimentally examined the impact of ABE on spray characteristics [57][58][59].Algayyim et al. [58] studied the effect of ABE additive on macroscopic spray characteristics of cottonseed biodiesel and diesel fuel at injection pressure of 30 MPa. Results revealed that addition of ABE reduced the SPL of diesel and biodiesel because of viscosity and surface tension.Thus, it enhanced the rate of vaporization and hence, combustion efficiency.Wu et al. [60] investigated the effect of ABE and butanol on spray characteristics for various temperatures in a CVV.The captured spray images revealed that SPL of n-butanol and ABE are significantly less than diesel for all testing conditions.An increase in ambient temperature causes viscosity and surface tension to decrease and vapor pressure to increase.These changes significantly accelerate the atomization and evaporation of the liquid spray.Higher temperature led to reduced penetration length.The cause for the lower ambient temperature was the differences in physical properties of the fuels amplified under low temperature relative to those at high temperature.These modifications significantly speed up the atomization and evaporation of the fuel jet.The SPL was reduced as the temperature rose.The difference in physical properties was significant at low temperature relative to those at high temperature.It is a difficult task to study the blends with mixtures like ABE rather than a blend of individual species such as ethanol, butanol, or acetone.This is because ABE has three different alcohols, each having its unique chemical properties.Hence, more research is required to fill the gaps and to identify optimal blend ratios and operating conditions.
Table 2 summarizes the fuel spray properties of various types of alcohols used in spray analysis.

Ether Additives
Ether additives are partially renewable compounds that are obtained by reacting isobutene and iso-amylenes with ethanol [62].Ether additives catch fire readily.They have high cetane number due to which they are used as an additive to enhance fuel properties.They are less dense and viscous than biofuels, and their addition enhances injection quality.Ether additives not only improve engine performance but also reduce the emissions of harmful gases [63].
They mainly consist of ethyl ter-butyl ether (ETBE), Ter-amyl ethyl ether (TAEE), 2ethoxy ethyl ether (EXEE), 2-methoxy ethyl ether (MXEE), dimethyl ether (DME), diethyl ether (DEE), and di-n-butyl ether (DNBE).Physical and chemical properties of ethers are shown in Table 3.It is seen that Polyoxymethylene dimethyl ether (PODE) has received more attention in recent years because of its ability to enhance the fuel spray quality.PODE and DME cannot prolong the ignition delay (ID).Longer air-fuel mixing time is detrimental to the mixture.The boiling points of PODE and DME favor fuel vaporization, and their molecular structure assists low soot combustion.9 show the results of various ether-based blends.Dimethyl ether has 35wt.% of oxygen, high cetane number and no carbon-carbon bonds, which makes it an effective alternative fuel for diesel engines [67].Investigation of fuel spray properties revealed that the addition of DME decreased PL, and increased SCA [68,69].This is because the compressibility of dimethyl ether/diesel blend is enhanced as DME is denser and volatile than diesel.Atomization quality of DME20 is better than diesel [70,71].The effect of enhancing ambient pressure is more noteworthy for DME as compared to diesel.For both fuels, the PL decreased and the SCA increased with increase of ambient pressure [72,73].DME had less soot emissions because of better spray quality than diesel [36].Zhan et al. [42] investigated the fuel spray behavior of diethyl ether (DEE) with biodiesel.The PL decreased while SPA increased for DEE/biodiesel blend as compared to the diesel/biodiesel blend.Particle droplet image analysis (PDIA) revealed that microscopic spray properties are significantly affected by injection and ambient pressure.The number fraction of smaller droplets was increased with addition of DEE to diesel/biodiesel blend in addition to fixed injection and ambient pressure.Moreover, SMD of the tested fuels decreased with the addition of DEE and indicated enhancement of the atomization.

Dibutyl Ether (DBE)
Guan et al. [75] investigated the spray behavior of dibutyl ether with soybean biodiesel.Their results showed that PL decreased while SCA and SPA increased with an increase of blending ratio of DBE up to 30%.The spray properties were comparable to that of diesel.PDIA was employed for determining the microscopic spray characteristics like SMD and statistical size distributions.Drop density distribution was maximum near the central axis of the spray and decreased sharply towards the edge for all tested fuels.Increasing the blending ratio of DBE caused a reduction in SMD, indicating improved spray quality.Liu et al. [76] revealed the spray behavior of 5 different fuels.His results showed that increased amount of DBE in fuel caused reduction of PL while the SCA and SPA were enhanced.D64B16DBE20 fuel with 20% by volume of DBE had almost similar spray behavior with diesel.Addition of DBE also improved air entrainment characteristics of diesel/biodiesel blends.Moreover, it had similar fuel-air mixing as diesel.Fu et al. [79] investigated the spray characteristics of biodiesel with di-n-butyl ether (DBE15, DBE30).The PL decreased while the SCA and spray width increased with the addition of DBE.Light intensity level for biodiesel was the narrowest, while the biodiesel-DBE blends showed similar light intensity level as of diesel fuel.DBE30 showed the highest gas entrainment mass and lowest equivalence ratio, like diesel.In summary, it is concluded that poor spray quality of biodiesels can be improved by adding DBE.

PODE
PODE showed promising results for enhancing fuel spray quality and combustion efficiency because of its low viscosities, pour points, low distillation temperatures, high oxygen contents, high CNs and absence of C-C bond [80].The PL for PODE was slightly longer than diesel under non evaporating conditions due to higher density of PODE [81].Liu et al. [77] revealed spray behavior of blending PODE with biodiesel in various ratios (10%, 20% and 30% by volume) and the results were compared with diesel.Results showed that addition of PODE enhanced PL initially, however, with increase from 20% in blending proportion, the PL started decreasing.The SCA and SPA increased with the increase of PODE blending ratio for all testing conditions.Chen et al. [74] studied spray of two fuel blends having 20% (P20) and 50% (P50) PODE concentrations respectively.Spray was analyzed by employing Mie scattering and Schlieren imaging techniques.Results showed that increment of injection pressure (IP) enhanced PL, SCA while SMD decreased for all the fuels.Increasing ambient temperature increased vapor phase area and cone angle but decreased SMD for both P20 and P50 fuel.Largest SMD was found for P50 followed by P20 and diesel.They concluded that P20 and P50 had better spray quality especially at low temperatures.Liu et al. [30] confirmed that PODE showed longer PL and larger SCA under non-evaporating conditions.PODE also reduced the equivalence ratio in both axial and radial directions for gasoline and diesel due to its high oxygen content.It had larger SMD as compared to gasoline.He also observed that PODE showed shorter PL compared to diesel under evaporating conditions.PODE had a high evaporating rate owing to the higher volatility.A smaller SCA was achieved by the addition of PODE in diesel as compared to neat diesel.PODE improved atomization quality because of high volatility.In summary, the addition of PODE to diesel was found detrimental to PL while the average SCA and SPA increased slightly.Table 4 summarizes the results of spray characteristics of ether-based additives.

Spray and Atomization of Biodiesel and Its Blends
The physical and chemical properties of a fuel have a significant effect on spray quality [85].In comparison to diesel, biodiesel shows longer PL, higher tip velocity and smaller SCA.Biodiesels have higher density and kinematic viscosity that cause dense fuel spray at the core region of the fuel jet (Table 5).This reduces the area occupied by biofuels compared to diesel.Higher injection pressure can be employed to mitigate the effect of poor atomization of biodiesel [19,86,87].The difference in spray behavior of biofuels is due to the varying cavitation structure in the injector nozzle hole.
For Karanja and pine biodiesel, it was observed that STP, SCA and SPA all were increased, however, the atomization quality was comparable to diesel [47,88].Figure 10 shows the spray profiles of Karanja biodiesel for various time durations.Another study showed shorter PL for Kerosene and hydrogenated catalytic biodiesel (HCB) compared to diesel, especially at lower injection pressure [89].Increase of PL at ultrahigh injection pressure was less significant and the SCA was almost insensitive to injection pressure [42,49].Liu et al. [90] revealed that higher viscosity, air entrainment and lower fuel injection speeds for biodiesel results in producing larger drop sizes, especially along the radial direction.In summary, biodiesel show larger SMD because of greater surface tension and viscosity [87].They have poor atomization due to the highest viscosity and surface tension, the lowest gas entrainment mass and poor gas-fuel mixing [79,91].
Ulu et al. [92] compared the fuel spray properties of canola, corn, cottonseed, and sunflower biodiesel.All biodiesels on average showed 3-20% longer PL, 5-30% narrower SCA, and 5-18% less SPA than diesel under ambient pressures of 5 and 10 bar.The difference between the spray properties of biodiesel and diesel was diminished at 15 bar ambient.Analytical and empirical estimation revealed that biodiesels had around 21-35% greater SMD and approximately 7% lower air entrainment.Sathiyamoorthi et al. [93] investigated the fuel spray behavior of four unique blends of Palmorsa biodiesel with diesel fuel (PMO25, PMO50, PMO75, PMO100) under injection pressures of 200 bars, 225 bars, and 250 bars in a constant volume chamber (CVC).Their results revealed that longest and shortest PL were for neat biodiesel and neat diesel respectively.SMD for tested blends reduced with the elevation of FIP.The spray volume and spray area were reduced by 0.4 cm 3 and 5cm 2 respectively at 250 bars injection pressure.Suraj et al. [94] compared the spray behavior of fresh Karanja biodiesel (FKBD) and aged Karanja biodiesel (AKBD).Mass flow rate of both biodiesels reduced at lower injection pressures than diesel.Similar spray results were observed for both biodiesel and diesel in terms of PL and SCA, however, the SPA for diesel was higher than biodiesel.They concluded that long-term storage of biodiesel may cause variations in physiochemical properties, but no significant variations were observed for macroscopic spray characteristics.Bohl et al. [95] compared spray properties of hydrogenated vegetable oil, palm, soy and used cooking oil biodiesel.He observed that the longest PL is for the denser and viscous fuel that was Soybean biodiesel and HVO showed max SCA and spray area.He also modified Hiroyasi and Arai [96] PL model for prediction of biodiesel sprays.Figure 11 shows the results of PL at 90 MPa, 100 MPa and 120 MPa injection pressure for biodiesels derived from various sources.It was observed that higher FIP caused longer PL and higher ambient pressure decreased PL.The PL at 120 MPa FIP for Pi40 and Pi50 fuel was much shorter than BDFp and BDFc at 100 MPa injection pressures.This is because the effect of ambient pressure, which was 5 MPa for Pi40 and Pi50, and 1.5 MPa for BDFp and BDFc, was dominant as compared to the FIP at the later stages of spray.Moreover, it also depends on the operating conditions as well such as the type of injector, fuel injection system, and most importantly the physiochemical properties of the fuel blends used.Figures 12 and 13 show the comparison of SCA and SMD for various biodiesel fuels.
In summary, the poor atomization quality of biodiesel can be improved using various blends.Hoang [97] mixed biodiesels with evaporative fuels, like alcohols, gasoline, ethers and esters and found it to be quite effective and practical to improve spray quality and atomization characteristics of biodiesel blends.Table 6 summarizes spray characteristics of biodiesel and its blends with diesel fuel.

Ester Fuels
Ester compounds are obtained from esterification reaction between carboxylic acid and alcohols.They are oxygen rich compounds that are helpful in reducing emissions.Most commonly known ester compounds that are used as an additive are 2-ethoxy ethyl acetate (EEA), 2-methoxy ethyl acetate (MEA), ethylene glycol mono acetate (EGM), methyl oleate and ethyl oleate.Physical and chemical properties of various esters can be seen in Table 7.The macromolecular ester fuels are favorable for improving liquid spray, while the esters with small molecular structure are unfavorable, since their capacity to improve spray quality is related to length of its carbon chain.Ester also reduces soot emissions.Sukjit et al. [105] employed methyl ester as a tertiary additive in alcohol/diesel blends.They found that ester additives improved stability and lubricity of fuel blends thus enhancing combustion efficiency.Han et al., 2017 [106] found that high viscosity and surface tension of methyl oleate and ethyl oleate produced longer PL and lower SPA with larger droplets in the axial direction of spray.For the same operating conditions, the diffusion and atomization tendency of methyl laurate are more desirable than methyl oleate and ethyl oleate.The micro explosion of an emulsion droplet caused larger SCA and smaller SMD at elevated temperatures especially for emulsified waste cooking oil methyl esters (EME) [107].

Aliphatic Compound Additives
Aliphatic compounds have non-aromatic rings and are used as a corrosion inhibitor.Mostly aliphatic compounds are flammable in nature that allows them to be used as fuel additives.These additives increase the ignition delay; however, they may also decrease ignition quality.Aliphatic compound that are used as additives mainly consist of n-heptane, n-octane, iso-octane, etc. [109].A mixture of toluene and n-heptane has been proposed as an effective surrogate for the alkanes and aromatic blends of diesel [110,111].Figures 14 and 15 show the PL and SCA of various aliphatic and aromatic compounds as described in the next section.

Aromatic Compound Additives
Aromatic compounds are organic compounds having a planar unsaturated ring of atoms that are stabilized through bonds forming a ring type structure.Aromatic compounds that are used as additives consist of toluene, dimethyl furan (DMF) etc. [109].Researchers later found out that 2,5-dimethylfuran (DMF) showed better results as biofuel additive for diesel.DMF did not get much recognition from researchers initially because of a difficult preparation process.Tian et al. [113] studied the spray characteristics of DMF with gasoline, with images captured using shadowgraph and microscopic properties from Phase Doppler Particle Analyzer (PDPA).Their experiments showed that DMF had similar spray properties as gasoline.Drop sizes of DMF were smaller than ethanol and their size decreased with enhancement of FIP.Mean spray velocity of DMF fuel was higher than ethanol and similar to gasoline.They concluded that DMF is more suitable than ethanol to be used in SI engine.Zhang et al. [108] investigated macroscopic and microscopic spray characteristics of DMF20 under three FIP of 90 MPa, 120 MPa and 150 MPa and ambient pressure of 5 MPa.Their results showed that DMF20 had longer PL, higher SPA because of higher density, lower latent heat and viscosity.The SCA also increased slightly and drop breakup had an adverse effect owing to the properties of DMF20.They concluded that properties of DMF are favorable for improving spray atomization quality under enhanced FIP.Table 8 summarized the fuel spray characteristics of esters, aliphatic and aromatic compound blends.

Nanoparticles
Nanoparticles have become a recent and an emerging area of research in the past few years in many fields including fuel blends.They come in a variety of sizes ranging from 1 to 100 nm, as well as shapes such as spherical, flat, cylindrical, conical, and tubular [114].Nanoparticles can either be amorphous or crystalline in nature, and may be packed closely, loosely or made up of single or multi crystal solids [115,116].Nanoparticles are classified into carbon, organic, inorganic, and composite for their origin and basic elements forming their structure [117].The dramatic influence of nanoparticles on the ignition and combustion behavior of base fuel has prompted researchers to investigate their potential as an additive.Metal and metal oxide nanoparticles, which fall under the category of inorganic nanoparticles, have far more attractive properties to offer in the CI engine research.Nanoparticles addition increases fuel reactivity, evaporation rates, and also improve calorific values, thermal conductivity, and viscosity of the base fuel [118,119].
Nanoparticles, unlike alcohols, are used as an additive in CI engines because of their ability to improve the fuel thermo-physical properties [120].They have a higher surface to volume ratio due to their nano structure, which is an attractive feature to the blends for use in CI engines as it provides a larger surface area for the chemical process and combustion reaction [121].
Figures 16 and 17 show the PL and SCA of various nano-additive based fuels.Bao et al. [43] studied the macroscopic spray behavior of diesel-ethanol DE20 fuel with dispersed cerium nanoparticles using shadowgraph technique.In comparison to diesel, the PL of DE20, DE20Ce25, DE20Ce50, DE20Ce100 increased by −3%, −1.8%, −0.3% and 9.5%, while the spray cone angle increased by −3%, 4%, 2.8% and 12.5% as well as spray area increased by −3%, −1.3%, 0.6% and 31% respectively.Nano additives boost instabilities of fuel and enhance the breakup phenomena and reduce breakup time.The increase of viscosity and surface tension was balanced out by the disturbance.DE20Ce50 showed similar macroscopic properties as compared to diesel, making it a suitable candidate for use in CI engines.
Zhang et al. [122] investigated the effect of cerium oxide (CeO2) and multi-wall carbon nanotube (CNT) additives in diesel and gas-to-liquid (GTL) fuel.The macroscopic spray characteristics were studied in CVV.His work found that nano additives had negligible effect on SCA for both diesel and GTL fuel.Both CeO2 and CNT reduced PL for diesel at lower FIP, however, GTL fuel had a negligible effect on PL because of its lesser density.The PL was increased for GTL fuel blended with CNT at post injection period.This is because of smaller and lighter molecules of GTL fuel which get absorbed in CNT and reduce evaporation rate.The SCA was insensitive to injection pressure, however it reduced with decrease of ambient pressure and increase of ambient temperature.Table 9 summarizes the fuel spray characteristics of nano-based additives.

Conclusions
In conclusion, we highlighted the fact that spray characterization is crucial in determining the performance and emissions of various fuel blends before using it in CI engine.From this study, we found that higher viscosity and surface tension resulted in reduced injection velocity and created a larger droplet size leading to poor atomization.Alcohol helped in removing this problem due to its lower kinematic viscosity and surface tension.Butanol and pentanol led to improvements in diesel due to their higher mixing capability and higher cetane number.Moreover, despite the addition of nanoparticles causing an increase in viscosity and surface tension, their PL, SCA and tip velocity increased while the breakup time reduced due to disturbance caused by the nano additives.This led to earlier atomization and improvement in spray characteristics.
Moreover, higher density, viscosity and surface tension of biofuels caused longer injection delay and lower jet velocities.This resulted in larger drop sizes and deteriorated atomization quality.Biodiesel without additives showed poor atomization as compared to the ternary blends.Volumetric ratio of 5-40% for alcohols and ethers in biodiesel and diesel blends can be used.The volumetric percentage of biodiesel may vary from 5-80% with typical value of 20%, however, only a few authors used 100% pure biodiesel in diesel engine.Specifically, the spray behavior of DE20Ce50 was similar to diesel.Addition of cerium oxide and alumina nanoparticles increased the surface/volume ratio and caused improvement in chain reactions during combustion.Carbon nanotubes (CNT) enhanced the stability of the blends because of superior heat transfer capabilities and thermal conductivity.PODE alleviated the gas entrainment levels because of wider SCA and greater SPA improving spray atomization.The PL decreased when the PODE concentration was greater than 20%.Alcohol mixtures such as BA and ABE had shorter PL like diesel as these mixtures had higher volatility which favored vaporization.
For future work, we recommend combining both spray analysis with the performance of CI engine especially for ternary, quaternary, and 4th generation biofuel blends with nano-additives and temperature considerations for improvement in fuel atomization and eventually engine performance and emissions characteristics.Moreover, financial considerations related to blending must also be considered to make biodiesel production and utilization a commercially viable and sustainable solution for society.
Author Contributions: Writing-original draft, investigation, M.U.H.; Writing-original draft, methodology, supervision, A.T.J.; formal analysis, review and editing, S.A.; formal analysis, methodology, T.A.C.; formal analysis, M.Q.A.; review and editing, supervision, funding acquisition, N.A.All authors have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.

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

Figure 1 .
Figure 1.(a) Schematic of a spray analysis setup.Reprinted with permission from Ref. [19]; (b) Control volume vessels used for fuel spray analysis with optical windows for visualization.Reprinted with permission from Ref. [27-29]; (c) Sequence for image processing and their results.Reprinted with permission from Ref. [30].

Figure 2 .
Figure 2. List of various additives used with diesel to improve spray performance and combustion properties.

Figure 3 .
Figure 3. Processed spray profiles of long-chain alcohol-diesel fuels in a constant volume chamber for various time duration.Reprinted with permission from Ref. [51].

Figure 10 .
Figure 10.Processed spray profiles of Karanja biodiesel for various time duration.Reprinted with permission from Ref. [98].

Figure 14 .
Figure 14.PL for various esters, aliphatic and aromatic additives with diesel at various injection & ambient pressure conditions [99,106,108,112].

Figure 15 .
Figure 15.Spray cone angle for esters, aliphatic and aromatic additives with diesel at different injection & ambient pressures [99,106,108].

Table 2 .
Summary of spray properties with alcohol additives.

Table 4 .
Summary of spray properties with ether additives.

Table 6 .
Summary of spray properties of biodiesel and its blends with diesel fuel.

Table 8 .
Fuel spray characteristics of esters, aliphatic and aromatic compound blends.

Table 9 .
Fuel spray characteristics of nano-based additives.