INAE Letters

, Volume 3, Issue 1, pp 33–39 | Cite as

Formulation and Extension of Diesel-Based Microemulsion Fuels for Compression Ignition Engines

Original Article


There is a need for sustainable alternative fuels which can address and alleviate both economical and environmental issues. The current work is based on alcohol–diesel–water microemulsion fuels. Microemulsions are thermodynamically stable and isotropic dispersions of oil, water, and an amphiphile. Diesel is used as the oil phase in microemulsion while water in it reduces the combustion temperature which, in turn, reduces the NOx and smoke emissions. In this work, an alcohol has been used, that acts both as a surfactant and a co-surfactant, thereby making the process facile and economical. The microemulsion regions are mapped out in ternary phase diagrams. The initial measurements suggest that the microemulsions have a higher calorific value, and lesser soot and residue formation as compared to that with neat diesel. The microemulsions also have properties such as density, viscosity, flash and fire points, and cloud and pour points close to those of neat diesel. The effect of various ionic and non-ionic surfactants on the extent of microemulsion region has also been studied. Hydrophilic surfactant expands the microemulsion domain while hydrophobic surfactant shrinks the same. Cationic surfactant does not have much influence on the microemulsion domain, while an anionic surfactant surprisingly does not yield any microemulsions by the current methodology.


Diesel replacement Microemulsions Alternative fuels CI engines Surfactant 


The production of fossil fuels is limited, while the demand of their utilization is increasing, resulting in a substantial increase in costs. Moreover, fossil fuels pose environmental threats in the form of global warming and climatic changes. The compression ignition (CI) engines dominate in mass transportation, industrial, and agricultural sectors due to their better power conversion efficiency and better fuel economy. However, they are also main sources of pollution, emitting particulate matter (PM), black smoke, nitrogen oxides (NOx), sulphur oxides (SOx), unburnt hydrocarbons (HC), carbon monoxide (CO), and carbon dioxide (CO2) (Lin and Wang 2003). These pollutants directly affect both health and environment, and are one of the causes of respiratory, and cardiovascular problems, and neuro-degenerative disorders. NOx is responsible for acid rain, and smog and ozone formation at ground level (Varatharajan et al. 2011). Most-researched sustainable alternative fuels include biodiesel, emulsions, gas-to-liquid fuels, and biomass. Standard specifications of diesel fuel oil are given in Table 1 (ASTM: D975-15b 2015).
Table 1

Properties of diesel fuel oil (ASTM: D975-15b 2015)

S. no.


ASTM test method

No. 1 diesel

No. 2 diesel


Kinematic viscosity at 40 °C (mm2/s)

D 445




Sulphur, max (ppm)

D 5453




Cetane number, min

D 613




Flash point, min (°C)

D 93




Ash, max (% mass)

D 482




Ramsbottom carbon residue, max (% mass)

D 524




Lubricity, max (micron)

D 6079




Conductivity, min (pS/m)

D 2624



Biodiesel is produced by thermal cracking, microemulsification, and transesterification. However, the conventional techniques of biodiesel production require high capital costs and long residence times. The first generation biofuels, derived from sugars and vegetable oils, have a negative impact on food security, while the second generation biofuels, derived from ligno-cellulosic biomass such as jatropha, mahua, and jojoba are still in early stages of development. Biodiesel from microalgae poses the challenges of harvesting and extraction. The blending of diesel with oxygenated additives facilitates a cleaner burning by reducing the particulate-matters emissions. Alcohols have high latent heat of vaporization, which causes quenching effect on the walls of combustion chamber, and reduce the peak temperature of combustion, resulting in an incomplete mixing. Thus, CO and hydrocarbon emissions are higher in the combustion of blends (Balamurugan and Nalini 2014). The increase in ignition delay, due to a lowered cetane number, also adds to an increase in the emissions. The brake specific fuel consumption is higher due to lower calorific value of alcohol (Rakopoulos et al. 2010). The NOx emissions, on the other hand, are reduced due to the lower calorific value and higher latent heat of vaporization of the alcohols. The smoke intensity with blends is significantly less due to the oxygenating nature of alcohols (Rakopoulos et al. 2011).

Emulsions are kinetically stable mixtures of two immiscible liquids, in which one phase is dispersed into another (Griffin 1949). They are stabilized with the help of surfactants, which have polar and non-polar groups. The hydrophilic lipophilic balance, or HLB number was introduced to classify surfactants, and is given by Eq. (1) (Davies 1957). The emulsions may include two or three phases. The two-phase emulsions are oil-in-water emulsions and water-in-oil emulsions, while the three-phase emulsions are oil-in-water-in-oil emulsions and water-in-oil-in-water emulsions.
$${\text{HLB}} = [({\text{n}}_{\text{H}} \times {\text{H}} - {\text{n}}_{\text{L}} \times {\text{L}}) + 7],$$
where hydrophilic and lipophilic groups have been assigned numbers H and L, and number of these groups per surfactant molecule are \({\text{n}}_{\text{H}}\) and \({\text{n}}_{\text{L}}\) respectively.

The droplets in emulsion quickly vaporize first, because of the lower boiling point of water than that of neat diesel. They expand rapidly and disintegrate the diesel layer to form small droplets of diesel, thereby, increasing the surface area of diesel. This phenomenon, is known as microexplosion, and by increasing the contact surface and mixing between air and the atomized fuel, it increases the burning rate and efficiency (Debnath et al. 2015). The particulate matter is also reduced in water-in-diesel emulsions due to the enhanced atomization and mixing caused by microexplosions (Ithnin et al. 2015). It ultimately results in the reduction of the soot formation and total HCs emission (Khan et al. 2014b). The NOx emissions can also be controlled by means of the decrease in temperature, achieved by the introduction of water. The vaporization of water reduces the temperature as well as dilutes the gas phase. As per literature, the particle size and distribution for mechanically stirred emulsions was larger than homogenized emulsions. Moreover, they have more tendency to coalesce thus resulting in microexplosions, while as homogenized emulsions were more stable, and phase change occured without microexplosion (Khan et al. 2014a). Water can be introduced into the combustion chamber with the inlet air in liquid or vapour form, or by parallel injection or as water in diesel emulsion (WiDE) with or without surfactants (Khan et al. 2014b). The first two methods will increase the cost due to addition of injection system and cause corrosion. WiDE is a convenient option as it does not require any prior or post modification of existing engines. Moreover, it helps in simultaneous reduction of both particulate matter and NOx (Fahd et al. 2013).

The alcohol, being an oxygenate, results in a more complete combustion of fuel, and the stoichiometric air to fuel ratio required is also reduced. Although it is a seemingly promising alternative fuel, injection techniques for alcohols, such as fumigation, require post modification. Blending is a limited application due to poor miscibilities of lower alcohols with diesel, and emulsions are not stable over wide ranges of temperature, and time. Microemulsification is one of the techniques which are stable over infinite length of time, if kept under same thermodynamical conditions. The term ‘microemulsion’ was coined by Schulman in 1959. They are thermodynamically stable, optically isotropic mixtures of oil, water, and a surfactant (Danielsson and Lindman 1981). Microemulsification is a simple technique which does not require high costs. No chemical reactions are involved, and no engine modifications required. Microemulsions could successfully replace 19–49% of diesel in compression-ignition engines without any observed problems (Chandra and Kumar 2007), and also help in reduction of NOx formation and CO emissions (Bhattacharya et al. 2006). Ethanol–diesel microemulsions had shown a slight decrease in brake specific fuel consumption due to a smoother operation, which decreased the ignition delay, and increased the thermal efficiency (Mehta et al. 2012). The presence of ethanol caused a quenching effect which reduced the exhaust gas temperature, and hence, NOx emissions were also reduced (Chandra et al. 2006; Mehta et al. 2012). This, however, increased the total HC content inside the cylinder (Neto et al. 2013). Diesel-based microemulsions with a non-ionic surfactant had also shown similar trends (Neto et al. 2011). Ethanol–biodiesel–diesel microemulsions had the same energy content as that of diesel, an increased lubricity, and a reduced sulphur content, and flash point (Fernando and Hanna 2005). The microemulsions could meet the cloud-point and pour-point standards with the choice of appropriate surfactants and co-surfactants (Do et al. 2011).

Thus, there is a need to study the alcohol-based microemulsion fuels which are stable over wide ranges of temperature, and without the need for an additional surfactant (Abrar and Bhaskarwar 2016). This would reduce the cost of raw materials and of the operation for manufacture of the microemulsion fuel. The formulated microemulsions could then be a sustainable and cleaner fuel.

Materials and Methods


Alcohols (Analytical Reagent) were purchased from Fisher Scientific. Triton X-100 (Scintillation Grade), Span-80 (Laboratory Reagent), Sodium Lauryl Sulfate (SLS: Laboratory Reagent) and Cetyl Trimethyl Ammonium Bromide (CTAB: Laboratory Reagent) were purchased from Fisher Scientific, CDH, Fisher Scientific, and Spectrochem, respectively. High speed diesel was procured from the sales end of Indian Oil Corporation Limited (IOCL), which included additives. Deionized water from Organo Biotech Laboratory was used for the formulation of the microemulsions. The properties of the different components of microemulsions are given in Table 2.
Table 2

The properties of different components used for formulating microemulsions




Triton X-100




Molecular weight (g/mol)







Density (kg/m3) at 25 °C







Viscosity at 20 °C (cP)



240.0 (25 °C)




Calorific value (kJ/kg)







Flash point (°C)



> 113








Grade used

No. 2






AR analytical reagent, LR laboratory reagent, DI de-ionized, SC scintillation grade, N.A. not available


The microemulsion domain was initially determined using an experimental bisection method. Further on, the effect of different surfactants on the microemulsion region was ascertained by the titration method. The calorific values of microemulsions were determined using bomb calorimeter (Make Hindustan Apparatus, Model HAMCO 6DA). API gravities were measured with hydrometer and then converted to specific gravity. The viscosities were measured with Cannon-Fenske routine viscometer (Make Hindustan Apparatus, Model HAMCO 50 A-50) kept in constant viscosity bath (Make Scientech Instruments, Model SE-135) at 40 °C. Flash points were determined with the help of Pensky Martens closed cup tester (Make Scientech Instruments, Model SE-223), while the cloud points were determined with a cloud and pour point apparatus (Make Hindustan Apparatus, Model HAMCO 9B). The sulphur content was determined using wavelength dispersive X-ray florescence (XRF) spectrometer, Ramsbottom carbon residue by Ramsbottom carbon residue apparatus (Make Hindustan Apparatus, Model HAMCO 31) and copper strip corrosion rating by copper strip corrosion test apparatus (Make Scientech Instruments, Model SE-297). The samples were also run in engine (Make Kirloskar, Model TAF-1) attached to alternator (Make Wilson) and a loading panel to check the performance and emission characteristics.

Results and Discussion

Formulation of Alcohol–Diesel–Water (ADW) Microemulsions

The ADW microemulsions were formulated by bisection method. Alcohol and diesel were premixed before adding water. The samples were stirred and observed for transparency. The microemulsions are transparent dispersions, as shown in Fig. 1a. With the addition of excess water with mixing, the mixture becomes turbid, as shown in Fig. 1b, and finally separates into two layers with the oil phase on top and water phase at bottom (Fig. 1c). The ADW microemulsion domain is mapped out in a ternary phase diagram, as shown in Fig. 2.
Fig. 1

Alcohol–diesel–water system: a clear microemulsion. b turbid emulsion, c emulsion separated into two phases when kept undisturbed

Fig. 2

Ternary plot of the microemulsion boundary line in ADW system (component concentrations in % v/v, 32–37 °C)

In the current work, alcohol, which acts both as a surfactant and a co-surfactant, is used to replace some percentage of diesel in the fuel. Thus, the microemulsions help in reducing the total amount of the major fossil fuel being consumed today, thereby helping us cut down on the import of fossil fuels and save the important foreign-exchange component.

Properties of ADW Microemulsions

The properties of various microemulsion fuels formulated were measured as per ASTM International Standards, and compared against those of neat diesel. The calorific values of the fuel samples were measured in a bomb calorimeter, as per ASTM D240. There is a slight reduction in the calorific value of a microemulsion fuel as the percentage of diesel is reduced. The API gravities of the microemulsions were measured as per ASTM D287, and then converted into specific gravities using Eq. (2). The specific gravities were found to be close to that of neat diesel.
$${\text{API}}\,{\text{gravity}} = \frac{141.5}{{{\text{Specific}}\,{\text{gravity}}}} - 131.5.$$

The viscosities of the fuel samples were measured as per ASTM D445. The viscosity of diesel should be in the range of 1.9–4.1 mm2/s as per ASTM D975 in order to have proper atomization and combustion of the fuel. The viscosities of all the samples were in this range, and also close to that of neat diesel. The flash points were measured by Pensky Martin closed cup tester, as per ASTM D93. It was observed that the flash point of microemulsions was slightly lower than that of neat diesel due to the presence of alcohol. Thus, the samples needed to be handled and stored carefully. The cloud and pour points of the samples were measured in cloud and pour point apparatus, respectively, as per ASTM D2500. The standards do not specify the lowest limits of the temperatures for having a satisfactory operation under all ambient conditions. The cloud point values of all the samples were lower than 0 °C. A fuel can work beyond the cloud point, but not after the pour point is attained. The sulfur content was measured using a wavelength dispersive X-ray florescence spectrometer, as per ASTM D2622. The maximum allowable sulfur content was 15 ppm for S15, 500 ppm for S500, and 5000 ppm for S5000 as per ASTM D975. The diesel was procured from IOCL outlet which specifies the sulfur content to be less than 50 ppm. All of the formulated microemulsion samples had sulfur content less than that of neat diesel. The Ramsbottom carbon residue on 10% distillate has been measured as per ASTM D524 by using Ramsbottom carbon-residue apparatus. The maximum permissible mass percentage is 0.35 as per ASTM D975. The carbon residue forms deposits in the combustion chamber under high pressures. All the samples had the residue within the permissible limits. The copper-strip corrosion rating was measured at 50 °C for 3 h as per ASTM D130. All the samples showed rating of no. 1 which indicated that microemulsions were not corrosive to copper, brass, or bronze parts of the engine.

Engine Testing of ADW Microemulsions

The microemulsion fuels satisfied the ASTM standards, and hence were further tested in a diesel engine to check the performance and emissions. A schematic diagram of the test engine along with all necessary accessories is presented in Fig. 3. The engine had been operated at different loads, while keeping speed constant at 1500 rpm.
Fig. 3

Schematic diagram of diesel engine with all necessary accessories

It was observed that the fuel consumption decreased with increase in the engine load. The fuel consumption for microemulsions (samples 2–5) was higher than that of neat diesel (sample 1) at lower loads, as shown in Fig. 4. This is due to the lower calorific values of the microemulsions caused by addition of alcohol and water, and higher fuel–air mixture at lower loads which causes incomplete combustion. However, at higher loads the consumption of some of the microemulsion fuels had decreased below than that of neat diesel indicating considerable savings of diesel, because of more complete combustion due to the presence of oxygenates.
Fig. 4

Variation of brake specific fuel consumption with increase in load for microemulsions with different percentages of diesel

The brake thermal efficiency of the fuel is inversely proportional to brake specific fuel consumption. Moreover, the brake thermal efficiency initially increases with the increase in engine load, and then decreases after reaching a maxima. The microemulsions have shown comparable or higher efficiencies at higher loads, due to lower fuel consumption of microemulsions, as shown in Fig. 5. The microemulsion fuel with 70% diesel (sample 5) showed increase in the efficiency by 2% and decrease in brake specific fuel consumption by 5%, as compared to those for neat diesel (sample 1). The microemulsions could thus possibly be used as a replacement of diesel.
Fig. 5

Variation in brake thermal efficiency with increase in load for microemulsions with different percentages of diesel

Extension of ADW Microemulsion Region

Experiments were carried out to determine the effect of both ionic and non-ionic surfactants on ADW microemulsion region. Diesel, alcohol, and surfactant were premixed in desired ratios, and then water was gradually added, noting the volume of water added before the ‘end point’ corresponding to change from transparency to turbidity. The quaternary mixture formulated as such, are represented by ternary diagrams, by calculating the percentage of alcohol, diesel and water on surfactant free basis. Non-ionic surfactants, namely, Triton X-100 and Span-80 were used initially. Triton X-100 is a hydrophilic surfactant with a HLB of 13.5. The effect of different concentrations of Triton X-100 on the microemulsion boundary line is shown in Fig. 6.
Fig. 6

Effect of different concentrations of Triton X-100 on ADW microemulsion region (component concentrations in % v/v, 27–34 °C)

At very low concentrations of Triton X-100, the microemulsion region shrinks as there is not enough surfactant present to reduce the surface are per unit volume in order to form the microemulsion. With an increase in the surfactant concentration from 0.0005 to 0.05% v/v, the microemulsion region expands, as expected. The microemulsions should be compatible with the existing engines and infrastructure, or at worse require the least modifications in those. The attention was hence focused on diesel-rich microemulsion region. Span-80 is a hydrophobic surfactant with a HLB of 4.3. It adds to the hydrophobicity of diesel, thereby shrinking the microemulsion region, as shown in Fig. 7. Moreover, with an increase in concentration of the surfactant Span-80, the microemulsion region is further reduced.
Fig. 7

Effect of different concentrations of Span-80 on ADW microemulsion region (component concentrations in % v/v, 26–33 °C)

CTAB is a cationic surfactant with an HLB of 10. It has comparatively equal hydrophilic and lipophilic parts, and thus did not have any observable effect on the microemulsion region. The same was observed even at higher concentrations of CTAB, as shown in Fig. 8. SLS is an anionic surfactant insoluble in diesel. It settled down at the bottom of the flask as soon as stirring was stopped, even at concentrations as low as 0.0005% v/v mixture. No microemulsions could therefore be formulated with SLS.
Fig. 8

Effect of different concentrations of CTAB on ADW microemulsion region (component concentrations in % v/v, 29–37 °C)


Alcohol–diesel–water microemulsions were formulated, using alcohol as a surfactant, in which no other co-surfactant or additive was required. The microemulsions satisfied the ASTM standards and were compatible with the existing engine and infrastructure. It was observed that the microemulsions with 30% diesel replacement showed 2% increase in brake thermal efficiency, and 5% decrease in brake specific fuel consumption as compared to neat diesel, thus making them more economical compared to the other alternatives. The effect of different ionic and non-ionic surfactants on the microemulsion domain was also studied. Hydrophilic non-ionic surfactant, Triton X-100, expanded the microemulsion domain while hydrophobic non-ionic surfactant, Span-80 shrank it. Cationic surfactant, CTAB, did not have much influence on the microemulsion domain. The anionic surfactant, SLS, did not yield any microemulsion at all.



The authors would like to acknowledge DST INSPIRE (IF150907) Fellowship to Ms. Iyman Abrar, and DST SERB (EMR/2016/004152) research grant for the current research work.


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Copyright information

© Indian National Academy of Engineering 2018

Authors and Affiliations

  1. 1.Department of Chemical EngineeringIndian Institute of Technology DelhiNew DelhiIndia

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