Alexandria Engineering Journal (2016) 55, 2463–2472

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REVIEW

The role of water-in-diesel emulsion and its

additives on diesel engine performance and

emission levels: A retrospective review

* Corresponding author. Fax: +91 044 24357591.

E-mail address: suresh.vellaiyan@gmail.com (S. Vellaiyan).

Peer review under responsibility of Faculty of Engineering, Alexandria University.

http://dx.doi.org/10.1016/j.aej.2016.07.0211110-0168 � 2016 Faculty of Engineering, Alexandria University. Production and hosting by Elsevier B.V.This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Suresh Vellaiyana,*, K.S. Amirthagadeswaran

b

aDepartment of Mechanical Engineering, Vel Tech Dr.RR & Dr.SR Technical University, Avadi, Chennai 600 062, IndiabDepartment of Mechanical Engineering, Government College of Technology, Coimbatore, India

Received 8 March 2016; revised 21 May 2016; accepted 19 July 2016

Available online 4 August 2016

KEYWORDS

Combustion characteristics;

Emission characteristics;

Emulsion fuel stability;

Nano-particles;

Performance parameter;

Water-in-diesel emulsion fuel

Abstract Air pollution caused by the diesel engines has shown much interest in the domain of eco-

friendly diesel fuels since enhanced environmental and human health are of concern. In order to

obtain the efficient energy development with less polluted environment with existing diesel engines,

continuous efforts have gone into research and development of water-in-diesel (W/D) emulsion

fuels, which have the potential to reduce nitric oxides (NOx) and particulate matter (PM) emissions

simultaneously with improved performance level. The current discussion addresses the principle of

W/D emulsion fuel, emulsion fuel stability, physico-chemical properties and effect of W/D emulsion

fuel on combustion, performance and emission characteristics. In addition, role of nano-additives in

W/D emulsion fuel, nano-fuel synthesis and its influence on engine performance and emission char-

acteristics are also discussed. The survey of earlier reports led to the decision that the optimization

of engine parameters, nano-particle size confirmation in nano-fuel and its handling from engine

exhaust are the details to be studied in the domain of W/D emulsion fuel engine.� 2016 Faculty of Engineering, Alexandria University. Production and hosting by Elsevier B.V. This is an

open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2464

2. Principle of water-in-diesel emulsion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24642.1. Role of surfactants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24642.2. Different phases of emulsion fuel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2465

3. Properties of water-in-diesel emulsion fuel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2466

3.1. Emulsion fuel stability. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24663.2. Physico-chemical properties of W/D emulsion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2466

2464 S. Vellaiyan, K.S. Amirthagadeswaran

4. Impact of water-in-diesel emulsion on combustion process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2466

4.1. Fundamentals of micro-explosion phenomena . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24664.2. Factors influencing the micro-explosion behavior. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2467

5. Impact of water-in-diesel emulsion on engine performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2468

5.1. Brake power and torque . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24685.2. Brake specific fuel consumption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24685.3. Brake thermal efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2468

6. Impact of water-in-diesel emulsion on exhaust emission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2468

6.1. Nitrogen oxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24686.2. Soot and particulate matter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24686.3. Carbon monoxide and un-burnt hydrocarbon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2468

7. Role of additives for diesel and emulsion fuel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24697.1. Nanofuels synthesis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2469

7.1.1. One-step method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2469

7.1.2. Two-step method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24697.2. Nanofuel properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24697.3. Nanofuel in combustion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2470

8. Conclusions and future recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2470

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2470

1. Introduction

Diesel engines are dominating in power generation and trans-portation sectors due their better efficiency and durability overgasoline engines. However, diesel engines are noted as major

emission contributor to the environment, which is dangerousto human health. Hence, stringent regulations are to be fol-lowed to control the emission levels from diesel engines.

Low engine pollution has been promoted by adopting pre-

formation and/or post formation emission control techniques

such as the introduction of biodiesel, particulate filters,

catalytic converters and W/D emulsion fuel. Out of these,

W/D emulsion has been found to be the most economical

solution that can be introduced in diesel engine without engine

modification. Efficient combustion and better fuel economy

are the added advantages of W/D emulsion fuel [1]. A

decrease in temperature of the combustion products leads to

a drop in NOx formation in diesel engines due to an endother-

mic reaction of water particles in W/D emulsion fuel during

combustion. The presence of water also reduces the rate of

formation of soot particles and enhances their burnout

characteristics by increasing the concentration of oxidation

species [2].

The use of W/D emulsion fuel in existing diesel engines isthe active field of research for the past two decades. The incor-

poration of nano-particles in W/D emulsion fuel enhanced thecombustion and emission characteristics. However, the com-plexity of analysing the combustion behaviour associated with

micro-explosion, physical path of emulsion and soot formationleads to inconsistent reports in terms of SFC, BTE, CO andHC emissions. In addition, engine operating variables and

ambient conditions also affect atomization and general com-bustion process and restrict drawing a general conclusion withW/D emulsion fuel. As a consequence, further research isneeded to examine the combustion characteristics of W/D

emulsion fuel under fluctuating conditions. This studyaddresses the basics and current status of W/D emulsion fuel,and enlightens the future intervention in the domain of W/D

emulsion fuel.

2. Principle of water-in-diesel emulsion

Introduction of water in the diesel engine combustion zonewas initially proposed by Prof. B. Hopkinson to improve thethermal efficiency, reduce the exhaust emissions and make bet-

ter inter cooling of the gas engines [3]. There are generallythree major approaches followed to introduce water in dieselengine: (i) direct injection into the combustion chamberthrough a normal injector [4], (ii) fumigating the water into

the engine intake air [5] and (iii) in the form of emulsion [6].The experimental investigation of all the above methodsconfirmed the advantages of water in diesel fuel and indicated

the simultaneous reduction of NOx and PM emissions. Out ofthe above methods, diesel-water emulsion fuels appear to bethe most appropriate since the desired emission characteristics

can be obtained without engine retrofitting [7]. Besides, themicro-explosion of water particles during the combustionprocess helps to improve atomization and mixing of air-fuel

mixture [8].An emulsion consists of two incomplete immiscible liquids,

with one of the liquid dispersed as small spherical droplets inthe other [9]. The emulsion is generated by means of a high-

energy emulsification method in the presence of surface-active agents (or) surfactants. These high-energy methodsinclude mechanical agitation, high-pressure homogenizers,

ultrasonic and supersonic vibrations [10]. As far as the smokeand NOx emissions are concerned, ultrasonic and supersonicvibrations have negative impact compared to emulsions

prepared by mechanical homogenization [11].

2.1. Role of surfactants

In general, emulsion fuels are thermodynamically unstable

systems. Over a period, emulsion fuels are slowly separatedinto two immiscible phases. The most common processes ofan emulsion fuel destabilization are droplet-droplet coales-

cence, flocculation and creaming [12]. To form a kineticallystable emulsion fuel, the surfactants are used to lower the sur-face tension between water and diesel molecules.

Figure 1 Schematic representation of W/O and O/W emulsions [31].

Figure 2 Schematic representation of O/W/O and W/O/W emulsions [31].

The role of water-in-diesel emulsion and its additives 2465

The surfactants contain a polar (or) hydrophilic head and a

non-polar (or) hydrophilic tail that is incorporated to weaken

the surface tension of the medium in which it is dissolved. In

water-diesel emulsion, the polar group orients toward the

water and the nonpolar group orients toward the diesel and

lowers the interfacial tension between diesel and water phases

[13]. The key factors to form a stable emulsion with nominal

droplets are appropriate hydrophilic-lipophilic balance

(HLB) value and molecular structure of the surfactant. Low

HLB has a tendency to make stable water-in-oil emulsion

whereas high HLB has a tendency to make stable oil-in-

water emulsion [14].

The quantity of surfactant in the emulsion varies from

0.5% to 2% of the total volume. As the percentage increasesfurther, the stability of the emulsion will decrease due to rapidcoalescence. The common surfactants used in the water-in-diesel emulsion are sorbitan monooleate, sorbitan monooleate

and polyethylene glycol sorbitan monooleate mixture, poly-ethylene glycol sorbitan monooleate and sorbitol sesquioleatemixture, sorbitan monolaurate, gemini, polyoxyethylene non-

ylphenyl ether, solgen 40 and noigen TDS-30, and detergent/liquid soap. Based on the polar group the surfactants are clas-sified as cationic, anionic, amphoteric and non-ionic. Out of

these, non-ionic surfactants have the desired characteristics

of sting with no shoot and free of sulphur and nitrogen [15].

Enhanced lubrication, inhibition of corrosion and protectionagainst freezing are the added advantages of non-ionic surfac-tants in diesel-water emulsion [16].

2.2. Different phases of emulsion fuel

Diesel-water emulsions are classified based on the relative spa-tial distribution of diesel and aqueous phases. In general, there

are two types of emulsification techniques, namely, two-phaseand three-phase emulsion. The formation of three-phase emul-sion is in need of two-stages of emulsification process with dif-

ferent surfactant and a result of increases in process cost. Inaddition, the stability period of three-phase and variation inphysico-chemical properties are not clear in the previous liter-

atures. A further study on the physico-chemical propertiesvariation, combustion and emission behaviors is recommendedwith three-phase emulsion.

The two-phase emulsion comprises one continuous phaseand one dispersed phase while the three-phase comprises onecontinuous phase and two are dispersed phases [17]. Thereare generally two distinct forms of two-phase emulsion:

(i) water-in-oil (W/O) and (ii) oil-in-water (O/W). An emulsionthat consists of water droplets dispersed in an oil phase is

2466 S. Vellaiyan, K.S. Amirthagadeswaran

known as W/O emulsion, and the emulsion that consists of oildroplets dispersed in an aqueous phase is known as O/W emul-sion. The schematic representation of W/O and O/W emul-

sions is shown in Fig. 1.Large fragmentation, less change in viscosity and micro-

explosion phenomena of the water droplet are appreciated

characteristics of water-in-diesel emulsion over diesel-in-water. In addition, in O/W emulsion, more quantity ofwater comes into direct contact with the combustion system,

which will result in rough engine operation and incompletecombustion [18,19].

Multiple emulsions comprise three phases such that theinner and outer phases are separated by a dispersed phase.

The schematic representation of oil-in-water-in-oil (O/W/O)and water-in-oil-in-water (W/O/W) emulsions is shown inFig. 2. O/W/O emulsions are recommended for fuel prepara-

tion and W/O/W emulsions are recommended for pharmaceu-tical applications [20,21].

Three-phase emulsions are prepared by the following meth-

ods: (i) phase inversion, (ii) mechanical agitation and (iii) two-stage emulsion. Out of these, two-stage emulsification tech-nique has been widely used for O/W/O emulsion preparation

and reported by many studies [22,23]. In two-stage emulsiontechnique, first two-phase O/W emulsion is prepared using ahydrophilic surfactant and then lipophilic surfactant is usedto form three-phase emulsion.

On a study of combustion and emission characteristics ofthree-phase emulsion in marine engine at a compression ratioof 17, Lin and Chen [11] have reported that the three-phase

emulsion has low BSFC, CO and NOx and higher exhausttemperature over two-phase emulsion and base diesel (BD)at all loading conditions. The same authors also concluded

that the complexity of analysing the micro-explosion behaviorof O/W/O emulsion and its high process cost are the undesir-able characteristics of three-phase emulsion over two-phase

emulsion. In addition, the water droplets are suspended inthe fuel in diesel-water emulsion and water does not come intodirect contact with engine surface. Hence, two-phase emul-sions are noted to be more suitable for further studies.

3. Properties of water-in-diesel emulsion fuel

The stability period of emulsion fuel and its physico-chemical

properties are the crucial factors in order to ensure that thediesel engine can run with emulsion fuel without any retrofit-ting. If the emulsion is destabilized during the engine running

condition, it will damage the combustion system and its func-tioning will fail. Besides, the variation in the physico-chemicalproperties of emulsion fuel should be within limits so as to les-

sen modifications in the injection and combustion systems.

3.1. Emulsion fuel stability

The stability of the emulsion fuel depends on emulsificationtechnique, process duration, concentration of water, stirrerspeed and surfactant concentration. Chen and Tao [24]reported that an increase in process duration, stirrer speed

and diesel to water ratio, increases the emulsion stability.On the other hand, an increase in process temperature reducesthe emulsion stability. Watanabe et al. [25] reported that

the selection of appropriate surfactant, suitable agitation

frequency, and process duration has equal importance to formstable emulsion. Ghannam and selim [26] also indicated thatincrease in surfactant concentration, stirrer speed and process

time is required for higher water concentration in diesel.Lin and Wang [27] measured the emulsion stability based

on layer separation in graduated scale test tube under static

conditions. Basha and Anand [28] also followed the same pro-cedure to measure the emulsion stability in their studies. Notmuch work has been recorded to measure the stability of emul-

sion. Hence, it is necessary to develop accurate stability mea-surement system since the emulsion stability has directinfluence on fuel ignition system.

3.2. Physico-chemical properties of W/D emulsion

Shorter ignition delay, high cetane number, suitable volatilityin the operating range of temperatures, limited smoke and

odor, free from corrosion and wear are the required character-istics of fuels used in CI engine. These characteristics mainlydepend on physico-chemical properties such as density, viscos-

ity, elasticity and compressibility. Siegmund et al. [29] andPark et al. [30] reported that increase in water concentrationraises the density and viscosity of the emulsion fuel due to high

density of water over BD [29]. Increase in bulk modulus ofelasticity and compressibility with increase in water concentra-tion has been recorded by Armas et al. [30]. Khan et al. [31]pointed out that the flash point is increased and the heating

value is reduced with increase in water concentration. The vari-ation in physico-chemical properties suggests further studies toobtain optimal water concentration with diesel.

4. Impact of water-in-diesel emulsion on combustion process

Micro-explosion phenomena of water-in-diesel emulsion and

its influencing parameters have a huge impact on CI enginecombustion process. The detailed study on the basics ofmicro-explosion and its influencing factors are discussed as

follows.

4.1. Fundamentals of micro-explosion phenomena

The micro-explosion phenomena in W/D emulsion fuel werefirst reported by Ivanov and Nevedov [32]. They reported thatthe suspended water particles in emulsion fuel reached theirsuperheated stage faster than the diesel and created vapor

expansion breakup (spontaneous explosion) during the com-bustion, forming very fine particles. More surface area of thefine particles with intake air leads to an improvement in the

air and fuel mixing and increases the combustion efficiency.The secondary atomization of fuel droplet and enhancedair-fuel mixing is illustrated in Fig. 3. Sheng et al. [3] also

supported the micro-explosion behavior of emulsion fuel by theirobservation in flame characteristics and flame angle study.

Several studies have been carried out to research the micro-

explosion behavior of emulsion fuels. Mura et al. [34] andTanaka et al. [35] adopted a hot plate method to observe theoccurrence of micro-explosion using a high-speed camera.Abu-Zaid [4] used different hot plates (stainless steel and alu-

minum) to estimate the evaporation time of droplets. Watan-abe et al. [33] used the fine wire technique (ceramic fibrewire) to study the breakup characteristics of the secondary

Figure 3 Schematic diagram of secondary atomization and micro-explosion [31].

The role of water-in-diesel emulsion and its additives 2467

atomization. Mie scatter imaging system with high-speed cam-era was developed by Mizutani et al. [36] to observe the micro-

explosion occurrence in the spray flame. On the other hand, Fuet al. [37] developed numerical models of micro-explosion andvalidated the data against the experimental values.

4.2. Factors influencing the micro-explosion behavior

The percentage of water concentration in the emulsion, size of

dispersed water particle, droplet size, ambient pressure andtemperature are the key factors that directly affect the strengthof micro-explosion. Jeong and Lee [38] observed that the

increase in water concentration in emulsion increases the inten-sity of micro-explosion and results in prolonged duration ofexplosion. They also reported that the strength of micro-explosion is enhanced with increase in water concentration.

However, too much increase in water concentration in emul-sion led to premature failure of injection system as reportedby Sheng et al. [3]. Fu et al. [37] reported that the optimum

level of water concentration in emulsion resulted in betterexplosion and shorter burnout time.

Marrone et al. [39] reported that increase in water particle

size of emulsion increases the ability of emulsified water to dis-perse in the oil droplet. Fu et al. [37] observed the low strengthof micro-explosion and weak expansion, with relatively smallsize of emulsified water particle. Concurrently, the larger size

of emulsified particles also weakens the strength of

micro-explosion due to much amount of water accumulatedduring the combustion. Fu et al. [37] and Mura et al. [40]

reported that the dispersed water particles of size between4.5 and 4.7 lm exhibit crucial explosion.

The small size of emulsion droplet at the entry of combus-

tion process weakens the strength of micro-explosion due to

the low residual mass of water. The increase in droplet size

enhances the intensity of micro-explosion and results in earlier

occurrence of explosion. Fu et al. [37] concluded that the min-

imum droplet size should be twice the size of the dispersed

water droplet or more.

Sheng et al. [3] observed that the increase in ambient pres-

sure weakens the strength of micro-explosion due to the

shorter penetration of the droplet at higher gas density. They

also reported that vigorous explosion occurs at an ambient

temperature of 773 K. At higher temperatures a weaker explo-

sion is observed by the same authors due to partial attainment

of super heated state of water droplets. However, Tanaka et al.

[35] and Fu et al. [37] stated that the increase in ambient pres-

sure and temperature increases the intensity of micro-

explosion.

Similar to micro-explosion phenomena, ignition delay isanother crucial factor in emulsion that can influence the com-bustion process. The water concentration in emulsion, fuel

injection pressure and ambient temperature predominantlyaffect the ignition delay. Ghojel and Honnery [41] and Armaset al. [30] reported the increase in ignition delay with emulsion

2468 S. Vellaiyan, K.S. Amirthagadeswaran

fuel due to lower flame temperature. Subramanian [42] alsorecorded similar trends in ignition delay. Ghojel and Tran[15] observed that the ambient temperature significantly

affected the ignition delay. Hence, it is important to controlthe changes in ignition delay period with W/D emulsion fuelin order to keep smooth engine operations with desired com-

bustion characteristics.

5. Impact of water-in-diesel emulsion on engine performance

The performance characteristics play a vital role on fuel modifi-cation techniques. The improvement in performance level leadsto economic feasibility to replace the existing diesel fuel. Several

researches have been conducted to study the impact of emulsionfuel with various concentration of water (5–40%) on engineperformance in terms of brake power (BP), torque, brake speci-

fic fuel consumption (BSFC) and thermal efficiency under differ-ent conditions. The observations are summarized as follows.

5.1. Brake power and torque

Abu-Zaid [4] and Alahmer et al. [43] reported the increase intorque and engine power with increase in water concentrationof emulsions. They also reported that additional force acts on

top of the piston due to the pressure exerted by the steam andresults in more engine power and torque. Concurrently, manyresearchers concluded that there is a slight drop in power and

torque with emulsion fuel due to their lower heating value[44,45]. They also reported that the longer ignition delay andthe maximum pressure rise with emulsion fuel increase thecompression work and reduce the network output of the cycle.

However, the variations in engine power and torque are rela-tively low and can be tolerated compared to significant emis-sion reduction.

5.2. Brake specific fuel consumption

Abu-Zaid [4] and Tsukahara et al. [46] report a drop in BSFC

with increase in water concentration due to the following rea-sons: (i) intensity of micro-explosion behavior, (ii) improvedair-entraining in the spray, (iii) higher premixed combustion,

(iv) lower combustion temperature and (v) more product ofcombustion gas due to presence of water in the emulsion.Experimental results of Suresh and Amirthagadeswaran [1]also supported the improvement of BSFC in emulsion fuel.

On the other hand, the increase in BSFC with increase inwater concentration has been recorded by many studies.Ghojel et al. [7] observed significant increase (22–26%) in

BSFC with 13% of water concentration in diesel. Experimen-tal records of Armas [30], Alahmer et al. [43] and Barnes et al.[47] supported the report of Ghojel. They concluded that the

increase in BSFC is due to increase in the amount of water.

5.3. Brake thermal efficiency

Most of the studies reported that the BTE is increased withincrease in water concentration. Basha and Anand [28] noted3.5% increase in BTE with 20% water in emulsion fuel.Alahmer et al. [43], and Wang and Chen [48] also reported

improvement in BTE. They concluded that the improvement

in BTE is due to longer ignition delay and micro-explosionphenomena of emulsion fuel. Further, they indicated that theincrease in ignition delay leads to accumulating more amount

of fuel, resulting in a higher heat release rate, higher fuel burn-ing in the pre-mixed stage and better BTE. Increase in flamepropagation speed and flame-lift-off length with emulsion fuel,

promotes better mixing of air-fuel mixture and results in com-plete combustion and improved BTE.

6. Impact of water-in-diesel emulsion on exhaust emission

Introduction of water in diesel engine combustion chamber inthe form of emulsion fuel has direct influence on the pollution

formation and the observations are detailed as follows.

6.1. Nitrogen oxide

High latent heat absorption of water particle during the com-bustion will lower the local high temperature resulting in thereduction of NOx. All the studies reported that the formationof NOx is significantly reduced with W/D emulsion fuel. Sur-

esh and Amirthagadeswaran [49] recorded 35% reduction inNOx in their studies, whereas Attia and Kulchitskiy [50] andPark et al. [51] recorded 25% and 20% reduction in NOx

respectively in their studies. Farfaletti et al. [52] reported thatthe heat sink effect of emulsion fuel reduced the combustiontemperature and resulted in low level of NOx formation.

Increase in hydroxyl (OH) concentration due to the presenceof water in emulsion fuel leads to NOx reduction. Numericalstudy of Samec et al. [53] reported a reduction of 20% NOx

emission with 10% water content in the emulsion. The reduc-

tion of NOx when using W/D emulsion fuel is due to lowerpeak flame temperature during the combustion.

6.2. Soot and particulate matter

Lesser formation of soot and PM indicates more efficient com-bustion. Majority of the studies reported that soot and PM are

reduced with increase in water concentration. Ochoterena et al.[54] reported reduction in soot emission by 81% with W/Demulsion fuel. Fu et al. [37] and Nazha and Crookes [55]

observed the reduction in soot and PM due to the better mix-ing and enhanced atomization caused by micro-explosionbehavior of emulsion fuel. Based on the production behaviorof light hydrocarbons (LHCs) and polycyclic aromatic hydro-

carbons (PAHs), PM reduction tendency in emulsion fuel isrecorded. Noge et al. [56] reported that the sulphur contentin the emulsion fuel has a major influence on the formation

of PM emission. Lower flame temperature, rapid evaporationof water, decrease of pyrolysis reaction and enhanced oxida-tion are the other causes to form low level of soot and PM

in emulsion fuel.

6.3. Carbon monoxide and un-burnt hydrocarbon

Incomplete combustion, lack of homogeneity and slow burn-ing of soot are the main causes to form CO and un-burntHC in diesel engine. Many of the studies reported the increasein CO and HC emission with W/D emulsion fuel [42,57] due to

the lower combustion temperature that is not sufficient in

The role of water-in-diesel emulsion and its additives 2469

emulsion fuel combustion to convert the CO into CO2. Kocand Abdullah [58] observed that the increase in CO and HCin emulsion fuel is due to the high amount of OH contribution

resulting in better oxidation of carbon to CO. Increases in igni-tion delay and low flame temperature of emulsion fuel alsocontribute to form higher level of CO and HC emission in die-

sel engine. However, Attia and Kulchitskiy [50] observed thatthe reduction of these emissions due to micro-explosion phe-nomena can lead to complete combustion.

The reports revealed that there is an inconsistency in thedomain of W/D emulsion fuel in terms of specific fuel con-sumption (SFC), brake power (BP), hydrocarbon (HC) andcarbon monoxide (CO) emissions due to the complexity in

combustion analysis. However, in terms of NOx and PM emis-sion levels, all the studies agreed on the improvement in theirlevels. Hence, it is necessary to develop accurate numerical

model to analyse the combustion and emission behaviour ofdiesel engine running with W/D emulsion fuel

7. Role of additives for diesel and emulsion fuel

In order to improve the lubricity, stability and combustion effi-ciency, additives are incorporated with diesel, biodiesel and

alternate fuels. These additives can be classified into pre-flame additives, flame additives, and post-flame additivesbased on their application. Pre-flame additives are used to

overcome the drawbacks such as prior burning and higherpour point in biodiesel and alternate fuels. Flame additivesare designed to improve the combustion efficiency, whereas,post-flame additives are used to reduce carbon deposits in

the engine, smoke, and emissions [59].Metal-based nano-additives have received much attention

in eco-friendly fuel and W/D emulsion fuel. Metal-based addi-

tives effectively reduce the emission levels in diesel engines. Themetal particles either react with water and enhance soot oxida-tion, or directly react with carbon atoms in the soot and lower

the oxidation temperature [60].

7.1. Nanofuels synthesis

Several novel methods are adopted by the researchers to formnanofluids. Out of these, one-step method and two-stepmethod are widely used in nanofluids preparation.

7.1.1. One-step method

To reduce the particle agglomeration in nanofluids, Eastmanet al. [61] developed a one-step physical vapor condensation

method to prepare copper incorporated ethylene glycolnanofluids. This process consists of simultaneously makingand dispersing the nanoparticles in the base fluid. The one-step process uniformly disperses the nanoparticles and

Table 1 Properties of oxides and their nanofluids.

K (W/m K) q (kg/m3) Crystalline

MgO 48.4 2.9 Cubic

TiO2 8.4 4.1 Anatase

ZnO 13.0 5.6 Wurtzite

Al2O3 36.0 3.6 CSiO2 10.4 2.6 Noncrystalline

increases the stability. Bonnemann et al. [62] and Singh et al.[63] also adopted one-step method to prepare mineral-oil basednanofluids and ethanol-based nanofluids, which recorded max-

imum stability.

7.1.2. Two-step method

Two-step method is widely used for nanofuels preparation.

The required nanoparticles, nanofibres, or nanotubes are firstsynthesized as dry powders. This dry powder is dispersed intoa base fluid in the second processing step with the help of agi-

tation. Due to high surface area and surface activity, nanopar-ticles have the tendency to aggregate. To enhance the stabilityof nanoparticles in base fluid, surfactants are used with

nanofluids. Chen et al. [64] reported that the surfactant addi-tion with nanofluids is an effective way to enhance the dis-persibility of nanoparticles.

7.2. Nanofuel properties

Nanoscale particles have maximum stability in nanofuels thanmicroscale particles due to their small diameter and specific

surface area and can be suspended for weeks under certainconditions. In nanofuels, nanoparticles are dispersed at lowconcentrations in terms of ppm. Nanofuels show higher ther-

mal conductivity than the fuel without nanoparticles. Choiet al. [65] recorded 10% conductivity enhancement with lowCNT particle concentrations, whereas, 150% enhancement is

noted with 1% in total volume. An enhancement in thermalconductivity of nanofuels has also been shown to enhancemass diffusivity, radioactive heat transfer and ion transfer

[66]. The other physico-chemical properties of fuel are notgreatly affected with nanoparticles inclusion, since concentra-tion and diameter of particles in nanofuels are too small.The other important characteristic of nano-size materials is

their improved specific surface area, which leads to increasein reactivity [67]. In addition, higher magnetic behavior, higherplasticity, lower sintering temperature and higher densities are

added advantages of nano-size particles over macro-sized ones.The excess energy of surface atoms is the predominant charac-teristic of nanoparticles [68]. Table 1 shows the properties of

oxides and their nanofluids [69].Deluca et al. [70] observed the enhancement in surface-area-

to-volume ratio with nano-fuels which results in more amountof fuel reacting with oxidizer. Kao et al. [71] reported that the

additives in the form of metal oxide in the W/D emulsion, actas catalysts to activate the molecular bonding of the mixture.This can improve the chemical reaction and result in better

performance and reduced emission levels in W/D emulsionfuel. Luisa and Duncan [72] stated that the reduced size ofthe catalyst increased the active surface area, which resulted

in improved reaction efficiency.

t (Cp) with 5.0 vol.% Enhancement in K of nanofluids (%)

17.4 40.6

31.2 27.2

129.2 26.8

28.2 28.2

31.5 25.3

2470 S. Vellaiyan, K.S. Amirthagadeswaran

7.3. Nanofuel in combustion

Tyagi et al. [73] recorded the enhancement in mass transferproperties, ignition temperature, and ignition delay periodwith nano-fuels using hot-plate ignition probability test. The

high reactivity of nano-fuels due to high specific area and highpotential of nano-size metallic powder to store energy canreduce the ignition delay and the soot emission in dieselengines [74].

Basha and Anand [75,76] conducted the experiments withdifferent nanoparticles (CNT, Alumina) mixed emulsion fueland observed significant improvement in combustion, perfor-

mance and emission characteristics. Selvan et al. [77] andSajith et al. [78] conducted experiments with Cerium oxidenanoparticles as additive in biodiesel and noted marginal

improvement in engine efficiency and notable reduction inignition delay, heat release rate, and emission levels. Marquisand Chibante [79] also carried out the experiments with

CNT nanoparticles and recorded enhancement in surface-area-to-volume ratio. The overall observations indicateimprovement in combustion performance with nanoparticlesincorporated diesel and W/D emulsion fuels.

However, no work has been focused in terms of insolubilityand uniform dispersion of nanoparticles in the emulsion fuel.In addition, the effects of nanoparticle distribution to atmo-

sphere through engine exhaust have to be studied, which areessential to improve environmental and human health.

8. Conclusions and future recommendations

The basic principle behind the W/D emulsion fuel, and itseffects on engine combustion, performance and emission char-

acteristics are critically reviewed and its recent advances alsopresented. From the discussions, the following conclusioncan be derived:

� Increase in water concentration of emulsion fuel, increasesthe thermal efficiency of the engine due to the micro-explosion phenomena. Significant improvement in BSFC,

BP and torque is observed with W/D emulsion fuel inmajority of the studies. However, much studies have notbeen conducted with W/D emulsion fuel. As the fuels are

intended to be practically usable by replacing the existingfuels, improvement in stability period assumes importance.

� NOx emission is reduced by 45% with W/D emulsion fuels,

whereas, 80–90% reduction is recorded in PM emission. Interms of HC and CO emissions, inconsistent improvementsare reported due to the complexity of analysing the combus-tion behaviour associated with micro-explosion, physical

path of emulsion, and soot formation. This indicates theneed for possible closer study of the process.

� Incorporation of nano-particles with W/D emulsion fuel

provides better combustion, performance and emissioncharacteristics with shorter ignition delay period and com-plete combustion.

Based on the above conclusions, the following suggestions

are derived for future intervention in the domain of W/Demulsion fuel:

� W/D emulsification process parameters (surfactant concen-

tration, water concentration, stirrer speed) may be opti-mized to obtain better physico-chemical properties withmaximum stability.

� A systematic approach is to be developed to measure theaccurate variation in emulsion stability.

� Numerical models have to be developed to justify theenhancement in engine performance and emission charac-

teristics with W/D emulsion fuel.� The size of nanoparticles incorporated and their dispersioncharacteristics over emulsion surface are to be confirmed

through suitable measurements. To detain the nanoparticlesof engine exhaust entering into atmosphere, an exhaust gasafter-treatment system may be integrated.

� In order to obtain efficient energy development with lesspollution to the environment, engine operating parameterssuch as, fuel injection angle, injection timing and water con-centration in emulsion fuel may be optimized using modern

optimization tools.

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