The pollutant emissions and performance of a four stroke CI engine operating on Diesel-Water blends has been investigated theoretically and experimentally. In the theoretical study, fuel combustion CFD analysis was carried out using FLUENT software to study the effect of performance and emissions for different blend ratios. Meshing of the combustion chamber was carried out using ICEM CFD by tetrahedral element. Dynamic meshing and layering techniques were used to define the movement of piston inside the cylinder. Experiments were performed in a single cylinder four stroke diesel engine with an eddy current dynamometer and AVL five gas analyzer. Emulsion fuels with varying contents of water in diesel (5%, 10%, 15% and 20%) were prepared and stabilized by sorbitan monooleate surfactant. The experimental results revealed a substantial reduction in harmful emissions without compensating on the engine's performance. Results obtained from both theoretical and experimental studies were compared. The CFD analysis results have been validated against data from experiments and it shows a good agreement between predicted and experimental values.
Keywords: Engine performance, Emulsified fuels, Emission characteristics, CFD analysis
1. Introduction
Compression ignition (CI) engines are the most fuel efficient internal combustion engines with an overall efficiency greater than spark ignition engines. High combustion temperature and lean stoichiometric Air fuel ratios (A/F) are the main reasons for the high efficiency. However, the CI engine advantage is overshadowed by its emission characteristics like particulate matters (PM), Oxides of nitrogen (NOx), Oxides of carbon (CO2 & CO). These diesel fuel emissions, especially NOx emission are dangerous to human health. Hence stringent regulations are required to control the NOx emission from Diesel engine.
Low engine pollution has been achieved by adopting pre-formation and/or post formation emission control techniques like biodiesel, particulate filters, catalyst converters and introduction of water in combustion chamber. Out of these, water in diesel has been found to be the best economical solution that can introduce in CI engine without retrofitting [1].
Computer simulation of a wide range of phenomena in internal combustion engines has been a fertile research area over the past three decades. In internal combustion engine computer modeling is now becomes very significant in combustion research [2]. Most of the simulation software use CFD codes that rely on simple models for combustion reactions and pollutants. In CFD codes, combustion mechanism is single-step where the fuel and oxidizer (oxygen and nitrogen) are directly converted into their final products of CO2 and H2O. Inter mediate and partial combustion products such as CO and H2 are not taken into account [3]. For NO analysis, CFD-FLUENT codes include models of Zeldovich mechanism. The Zeldovich mechanism is extremely sensitive to temperature, which is activated at temperatures over 1800K in mixtures where stoichiometry is close to unity [4].
An emulsion is a blend of two or more liquids which are unblended in nature, one present in continuous phase and others are in dispersed phase. It is achieved with the help of mechanical agitator in the presence of surfactants for emulsion stability [5]. The surfactants (emulsifier) are the surface active agents which is used to weaken the surface tension of the medium (dispersed phase), in which they dissolve (continuous phase). When the water is placed in the Diesel, the surfactants lower the interfacial tension between the Diesel and water phase [6]. Out of different types of surfactants like cationic, anionic, amphoteric and nonionic, nonionic surfactants (Span 80 or Sorbiton monooleate) is the most common surfactants used in water-in-diesel emulsion research. Low Hydrophilic-Lipophilic Balance (HLB), burn with no soot, free of sulphur and nitrogen are the desired characteristics of Span 80 [7].
Emulsion is formed by mechanical agitation. The mechanical agitation can be generated by ultrasonic vibration machine, magnetic stirrer and centrifugal type mixer. Lin and Wang [8] have reported the negative impact of ultrasonic vibration as far as NOx emission and smoke concerned. Combined process of centrifugal mixer and magnetic stirrer are applied for stable emulsion in this experiment.
2. Numerical Simulation Method
The specifications of diesel engine used in the simulation are shown in Table 1
Table.1. Engine Specifications
In this theoretical study, ANSYS FLUENT 13.0 software was used to stimulate the engine process. The combustion chamber boundaries are assumed to be adiabatic. Details of piston rings and crevices are not depicted since it is irrelevant to the simulation. The clearance between the heads and pistons are fixed by tetrahedral cells. The species model of Diesel-Water reaction was used to account for combustion. The turbulence model used was k-epsilon turbulence model. Initial operating pressure and temperature were set to 101325 Pa and 298 K respectively.
Fig.1. volume mesh of the combustion chamber
Fig.1. shows the volume mesh of the combustion chamber at the end of exhaust stroke. The computational simulation of diesel fuel and emulsified diesel fuel was carried out.
Intake valve and exhaust valve lift profiles are shown in Table.2.
Table.2. Intake valve and exhaust valve lift profiles
3. Experimental details
The experimental setup consisting of a single cylinder four stroke Diesel engine (brake power: 5 HP/3.7 kW, bore: 80 mm, stroke: 110 mm) coupled to an eddy current dynamometer and AVL five gas analyzer. The engine was running at constant speed (1500 RPM) throughout the experiment. The load applied on the engine was measured by the local cell connected to the eddy current dynamometer. The schematic diagram of the experimental setup is shown in Fig.2.
Fig.2. Schematic diagram of Experimental Setup
The capacity of AVL five gas analyzer are listed in Table.3
Table.3. Capacity of AVL five gas analyzer
The water-in-diesel emulsion consisting of Diesel fuel and distilled water were prepared in a combined setup of mechanical stirrer and centrifugal mixer at a speed about 1500 rpm. To stabilize the emulsion, a 1-2% by volume Sorbiton monooleate was used. Water was added in the ratio of 5%, 10%, 15% and 20% with diesel by volume and emulsified. The properties of emulsified fuels are given below in Table.4.
Table.4. Properties of emulsified fuels.
The experiments were conducted in different load conditions like no load condition, 25%, 50%, 75% of full load and full load condition at constant speed.
4. RESULTS AND DISCUSSIONS
CFD analysis has been carried out to analyze the effect of water substitution on Diesel combustion. Fig.3 shows the formation of NOx in Diesel engine combustion chamber during the exhaust stroke where the fuel is pure Diesel. The NOx formation starts-off at the location of the initial combustion onset and in the premixed combustion zones. Then it continuously gets formed in the slightly fuel lean and high temperature region.
Fig.3. CFD analysis of NOx for pure Diesel
Fig.4. CFD analysis of NOx for 5% water emulsified Diesel
Fig.5. CFD analysis of NOx for 10% water emulsified Diesel
Fig.4-7 illustrates contours and graphs representing change in mole fraction of NOx with increase percentage of water. The main reason being, as water percentage increase in the Diesel, mole fraction of H2O increases. This leads to reduction of net combustion temperature [9]. As the combustion temperature falls down the tendency of NOx formation reduces, which finally results in reduction in NOx with increase in percentage water substitution. 20% emulsified fuel shows 48% NOx reduction than in pure Diesel.
Fig.6. CFD analysis of NOx for 15% water emulsified Diesel
Fig.7. CFD analysis of NOx for 20% water emulsified Diesel
Experimental analysis has also been carried out to analyze the effect of water substitution on Diesel combustion. Fig.8 shows the variation of the NOx emissions with engine brake power for the different emulsions. The formation of NOx in diesel engine is due to high temperature in combustion chamber and excess oxygen availability during the combustion. NOx emission increases while the brake power is increased due to high exhaust gas temperature as has been seen already. In case of emulsion fuel, the magnitude of NOx emission is drastically reduced than that of pure diesel due to micro explosion and high latent heat of vaporization (2257 kJ/kg) of water [10]. 20% emulsified diesel shows 45% NOx reduction than in pure diesel at full load condition.
Fig.8. Brake Power versus NOx
The variation of total fuel consumption (TFC) with engine brake power for the different emulsion is shown in Fig.9 and Fig.10. Two caucuses are adopted for TFC. In the first convention, the total fuel is calculated by considering the diesel fuel plus water as the total fuel. By considering the total fuel as strictly the amount of diesel the second convention is adopted. The TFC increases in both conventions as engine brake power increases. . Fig.9. shows that the percentage of water in the emulsion increases, the TFC increases. This is because; a larger amount of diesel is displaced by an equal amount of water [11]. Fig.10 clearly indicated that the TFC consumed in second caucus is less than first caucus. This means that less diesel fuel is actually encountered within each volume of the emulsion. Tsukahar [9] has also reported that the water-in-diesel emulsion fuel reduced the TFC.
Fig.9. Brake Power versus Total Fuel Consumption (By considering Diesel+water as total fuel)
Fig.10.Brake Power versus Total Fuel Consumption (By considering Diesel as total fuel)
Fig.11 shows the variation of the gases exhaust temperature with engine brake power for the different emulsion. It clearly indicates that the exhaust gas temperature decreases when the percentage of water is increases. The latent heat of water will cool the charges due to the evaporation of water. Jamil [10] has also reported that the water in diesel lowers the cylinder average temperature in diesel engine.
Fig.11. Brake Power versus Exhaust Gas Temperature
The variations of brake thermal efficiency with engine brake power for the different emulsion have been shown in Fig.12. At low load condition, all the samples exhibit similar brake thermal efficiencies and show slight variations in higher loads. Even though the calorific value of the emulsion fuels is less than that of pure diesel, engine's performance is compensated. This is because of micro explosion, volatility difference between water and fuels, enhanced air fuel mixing [12].
Fig.12. Brake Power versus Brake Thermal Efficiency
At idle and light load conditions, the fuel concentration in the spray core is small. In this case increases in local temperature of the combustion region are small and are associated with slower oxidation reaction rates. These concentration rates are further reduced due to very low concentration of fuel molecules as they diffuse in the air around this region. The ratio of unburned hydrocarbons formed in this region to total fuel injected is high. When engine load increases there is sufficient O2 in the mixture so that with increased temperature oxidation reaction rates are enhanced and HC emission reduced [14].
Fig.13. Brake Power versus HC Emission
Fig.13 shows the variation of HC emissions with engine brake power for the different emulsions. HC emissions of emulsified fuel are found to increase when the concentration of water increase. The magnitude of unburnt HC emissions for the emulsion fuels is enhanced due to the higher accumulation of fuel in the premixed combustion phase [13]. 20% emulsified diesel shows 28.5% HC augmented than in pure diesel at no load condition.
Fig.14. Brake Power versus CO Emission
Fig.14 illustrates the variation of CO for the tested fuels. It is observed from the figure that there is an enhancement in CO emission for the emulsion fuel. This is due to the adverse effect of water addition which has lead to accumulation of more fuel on the account of prolonged ignition delay
The smoke opacity at different load for various emulsion fuels has been shown in Fig.15. Water emulsified fuels show marginally reduction in smoke opacity compared to pure Diesel. Since the water get vaporized by absorbing the heat energy during combustion and increase the ignition delay period. This improves the mixing process which leads to faster combustion reaction and reduce the smoke opacity [14]. 20% emulsified diesel shows 42% smoke opacity reduction than pure diesel
Fig.15. Brake Power versus Smoke Opacity
5. ConclusionS
The performance and emissions characteristics of neat Diesel and Diesel-water blends are investigated to evaluate the emission reduction potential on the single cylinder 4S Diesel engine. CFD analysis also has been carried out to analyze the variation of NOx on emulsified fuels. The conclusion of this investigation is as follows:
CFD analysis reveals that 20% water-in-Diesel reduces 48% of NOx emission. Whereas the experimental investigation shows that 46% of NOx reduction on full load condition. This exhibit good agreement between predicted and experimental values.
The TFC calculated by considering the total fuel as strictly the amount of Diesel fuel that is burned in combustion chamber decreases as the water percentage increases.
The slight variations in brake thermal efficiency can be tolerated compared to the expected reduction in exhaust emissions of NOx when water is added to Diesel fuel.
The exhaust gas temperature and smoke emissions are drastically reduced for the emulsion fuels compared to the pure Diesel. The magnitude of exhaust gas temperature and smoke emission observed is 80°C and 17.8% for pure diesel, whereas it is 72°C and 10.2% for 20% emulsion fuel at the full load, respectively.
The unburnt hydrocarbons and CO emissions of emulsified fuels are slightly higher than pure Diesel due to moisture content, poor atomization and incomplete combustion of the emulsion.
