Investigation of Impact of CeO2 nanoparticles blended in Nano-emulsion on efficiency and exhaust emission of diesel engine

Cerium oxide (CeO2) nanoparticle was added to prepared water in diesel Nano-emulsion to study its effect on exhaust emission and performance of diesel engine characteristics between neat diesel, Nano-emulsion and Nano-emulsion+ CeO2. Since water and diesel are immiscible, mixed span and Tween 80 surfactants are used for preparation of stable Nano-emulsion. Central composite design, Response surface method is used for optimization process of preparation of Nano-emulsion at optimum suitable condition designed by Design Expert software. Dispersed Water present of Nano size effect directly on diesel fuel characteristics in compression ignition engine, contributes on reduction of NOx and smoke emission and better thermal efficiency, but accompanied by little increase in HC and CO emission and fuel consumption. CeO2 nanoparticles at three levels (50, 100 and 150 ppm) to optimize Nano-emulsion added and mixed by high energy homogenizer. All diesel, Nano-emulsion and Nano-emulsion+ CeO2 are examined by Engine run at 400 bar and 1500 rpm with different load. Improvement in reduction fuel consumption and thermal efficiency are obtained. Exhaust emission (NOx, HC and CO) and smoke reduced much more are observed with CeO2 nanoparticle.


Introduction
Diesel engine plays a considerable role in different fields of industries and technologies in internationally. It could be in production machine or in transportation etc., with increasing in applications of diesel resources impact on environment which is leading to greenhouse effects.
Emission exhaust gases of diesel engine consider one of the main sources of air pollution, especially in crowded and industrial areas, and it has been a main concern of public for a long time. [1] Solutions are found for environment pollution reduction by emissions of diesel engine that makes it critical concern of investigators [11].
Emulsification technique regards one of the effective methods using for reduction emission of exhaust gases from diesel engine motivated by its cheap cost [2]. The suspended water droplets of emulsified fuel by suitable surfactant don't allow direct contact of water with the inside surfaces of engine [3].. Water in diesel emulsion formulation is confirmed to minimum (NOx, HC, SOx, CO) emissions and (PM) particulate matter without recompensing performance of diesel engines [4].
Water vaporization of diesel emulsion fuel helps to decreasing the local flame adiabatic temperature which is the chemical reaction temperature for formation of NO in gas phase. [5,6].
Volatility difference between water and diesel and the water boiling point is lower than the oil, during compression in piston while the temperature is increasing, droplets of water in emulsion exhibit explosive evaporation and diffusing of surrounding oil layers [7].
Huge number of tiny secondary droplets are produced due to secondary atomization induced by these explosions. Secondary explosion is also known "Micro explosion" [8]. Improvement in combustion efficiency and reduction in pollutant exhaust emissions is due to the Micro explosion that enhance the mixing of inlet air and fuel during combustion [7].
When the droplet size of dispersed phase is between 20-500 nm it is known as Nano emulsion.
Better stability can obtain in Nano emulsion droplet aggregation and gravitational separation due to its smaller droplet size in comparison with Micro and macro emulsion [10].

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Water in diesel Nano emulsion preparation including three basic composition: continuous phase diesel, dispersed phase water and mixture of emulsifiers. Selection of suitable surfactant contribute have great impact to produce Nano emulsion fuel keeping fine droplet of water in diesel for much longer time. Hydrophilic-lipophilic balance of the produced emulsion should be chosen 4-6 and according to specific equation [16].
More water fine droplets and more surface area of Nano emulsion in comparison with microemulsion due to its smaller droplets size [12], participating more effectively in fuel/air mixing for complete combustion.
Brake specific fuel consumption of emulsion fuel is higher than net diesel because of the lower heating value of emulsion fuel due to presence of quantity of water [17,18].
For obtaining emulsion with higher stability and less exhaust emission as a fuel, its component percentage should be at optimum content. Performance and starting ignition problems of fuel take place if too much water percent present in emulsion fuel [19]. Addition of water to certain limit had positive effect, but continuing in increasing water percent decrease emulsion stability [20]. The reason behind of the increasing emission of larger amount of CO concentration and engine efficiency problems is too much loss in latent heat in combustion of fuel to completely burn [21,22]. Nano emulsion has taken attention a lot of investigators of use as fuel emulsion as a potential source of energy in compression combustion engine due to its supplying minimum emission and contribute to convert CO to CO 2 than the regular fuels [1,17,18,23]. Several of investigations proved that addition nanofluids can enhance the combustion efficiency and reduce emissions. [24,25,26].
Nano Metal oxides such as copper, cerium and iron are regarded fuel additives. Metal nanofluids can react with water molecule for enhance the oxidation of soot or can react directly with carbon atoms in soot for minimizing the oxidation temperature [27]. Providing more oxygen for combustion process due to Ability of CeO 2 nanoparticles act like oxygen buffers source, oxidation of hydrocarbons in fuel concurrently, therefore lowering the nitrogen oxides, especially at stoichiometric condition [25].
High oxygen storage capacity and mobility inside its lattice and ease changing between Ce3+ and Ce4+ states. All these properties and multitude cerium in earth, high effectively and low cost makes it better alternative to other metal oxides nanoparticles [28,29]. The major aim of

No.30-(3) 2021 Journal of Petroleum Research & Studies (JPRS)
E72 this study is finding out optimum condition of preparing Nano-emulsion and the impact of CeO 2 nanoparticle on the efficiency and exhaust gases emissions of diesel engine.

Materials
Preparation of Nano-emulsions in experiments were done by distillated water at

Preparation and characterization of Nano emulsion
Six parameters choosen to study their effects on the prepared Nano emulsion and interaction between the variables to the stability of optimized sample. Response surface method with 54 runs are built by Design Expert software version 10 for preparation of different Nano emulsions with varying the variables in each run. Water content is varying between (0-25 wt%) with pH range from (7)(8)(9)(10)(11)(12)(13)(14) at basic circumference. After preparation of mixed surfactants, the accepted value of hydrophilic-lipophilic balance for water in oil emulsion is chosen to be (4.5-6.5) at (1-5 wt%) content of emulsifiers. High energy method is established by high speed rotator-stator Heidolph DIAX 900 homogenizer for rupture water phase to nanometer size. Surfactants are mixed for certain HLB 6 preparation and mixed with the diesel that carrying CeO 2 nanoparticles, this step takes 10 minutes with 5000 rpm. last step is based according to instruction of optimum condition: water percent is 12 wt% of the system with pH equal to 10.5.

Droplet size distribution Analysis:
Hydrodynamic diameter and size distribution for water droplets by the technique of dynamic light scattering using (Brookhaven 90 Plus) instrument. Each test of sample takes 3 minutes at 25 o C.

Viscosity measurement
For measuring viscosity of emulsified fuel Brookfield viscometer was used. The RV7 spindle is used. The capacity of the sample is 100 ml and temperatures are varying from 25 o C to 100 0 C to study the effect of the temperature on emulsifier's fuel viscosity.

Calorific Value Measurement
PARR 1266 instrument, isopribol bomb calorimeter was used to determine the gross heat combustion of an emulsified diesel fuel test. The heat obtained fromk emulsified fuel is compared with the heat obtained from the combustion in same quantity of Benzoic acid sample that its calorific value is known.
The gross calorific value of emulsified fuel is in equation (1):

Characterization of Nano emulsion
The difference in calorific value between pure diesel and emulsified fuel 12% water content is shown in Table (2). Replacement in Nano emulsion fuel by water instead of diesel participates to reduce formation of NOx by minimizing of reaction temperature, thus exhaust emission and soot formation is also reduced. The improvement in calorific value of emulsions could be

No.30-(3) 2021 Journal of Petroleum Research & Studies (JPRS)
E75 observed with the addition of nanoparticles [30,31]. It is known higher molecular weight of petroleum fractions have higher flash point and flash point is an indicator for fuel volatility.
Water present in Nano emulsified fuel increasing its flash point, depicted in Table (1), Nano emulsion molecule had a higher molecular weight comparing with net diesel. Viscosity of the prepared Nano emulsion had higher value than net diesel. High viscosity results in poor atomization in engine cylinders and combustion of fuel. Density of net diesel are less than the prepared Nano emulsion while pour point is changed from +9 to +7. Problems could take place of high pour point fuels during transporting in pipes. Table (2) illustrated the change in fuel properties as a result of water addition and Nano emulsion formation. Density (g/cm 3 ) 0.87 0.882

Exhaust Emission Gas Concentration
After optimization process and analyzation of factors the present result gained: increasing water content effect on increasing of droplet size, while mixing speed are contribute to certain limit in minimizing of water droplet dispersion size. Emulsifier with 6 HLB saved highly stability of the system at 5 wt% content. Water for the Nano emulsion at base side with pH 10.5.  By increasing the load on the diesel engine, CO emission by Nano emulsion is minimized, controversy with the starting point. In Figure (2a), shows the curve of WiDNE are reducing the carbon monoxide more efficiently than the net diesel.
Hydrocarbon emission is the result of the incomplete combustion of the fuel. Increasing the load reducing the HC emission.net diesel at low load are burn better than the emulsified diesel but at higher loads water phase contribute effectively in complete burning and the reducing HC emission 18.5% at higher load.CO and HC emission are producing from the same reason and highly depending on the viscosity of the fuel. Water in diesel Nano emulsion containing 150 ppm CeO 2 is less emission of HC and CO comparing to another fuel blends as shown in Figure   (2b). [32,33].
The source of the emission of the nitrogen oxide is form the outside 78% from the environment air mixture. When the temperature inside the cylinder is increased, monoatomic N are produced from the rupture of the N 2 . At high temperature 2500 to 3000 K considerable amount of the monoxide nitrogen are produced, which more NOx produced. Water content in the When there is a lack of oxygen in engine cylinder, CeO 2 are reduced to Ce 2 O 3 to provide necessary oxygen for converting carbon monoxide to carbon dioxide [33].it will re-oxidized to  Smoke produced by increasing the load. Effect of Nano emulsion was obvious in minimizing 40.6% of the smoke opacity by the effect of Nano emulsion and the CeO 2 [17,18]. The increasing the amount of the nanoparticles has a limited effect and from economical side the 50 ppm are best for the smoke reduction.

Engine performance
BTE is the measure of how efficiently the engine uses from the fuel.  4.23% than net diesel, this is due to higher viscosity of the emulsion additional to its lower calorific value against net diesel. This lose in heat can restore by the Cerium dioxide due to the improvement in conductivity of ion oxygen, which enable the fuel to operate at reduced temperature.150 ppm of nanoparticle gave 15.5% the highest fuel consumption.

Conclusion
Preparation of Nano emulsion was achieved successfully due to recording high coefficient of determination R 2 by the RSM at optimization method 98.34%. Suitable contents with the desirability 1.00 for compositions in preparation of emulsified diesel assist to produce a new fuel with a least emissions and higher efficiency.
Presence of water 12% in Nano emulsion contribute effectively in reduction of Smoke opacity to 17.18%, 5.35% NOx emission and accompanied with higher HC and CO 4.1% and 4.34% respectively.
Use of 150 ppm of CeO 2 dispersed in Nano emulsion assist in enhance the properties of prepared fuel and compensation from the loss in of fuel consumption by providing higher thermal efficiency. 40.6% NOx, 19.7% CO and 12.85% HC emissions are reduced at higher load. Thermal efficiency and BSFC of new blend fuel are improved 17.23 and 13.4% respectively.