Diesel Emulsified Fuel

ARTICLE pubs.acs.org/EF

Effect of Dimethyl Ether in a Selected Ethanol/Diesel Emulsified Fuel Ratio and Comparing the Performance and Emission of the Same to Diesel Fuel M. P. Ashok* Department of Mechanical Engineering, Faculty of Engineering and Technology, Annamalai University, Annamalai Nagar 608 002, Tamil Nadu, India ABSTRACT: In this research work, in phase I, identification of the best emulsified fuel ratio under the water-in-oil (W/O)-type emulsion method using surfactant 1 has been carried out. On the basis of the best performance and less emission, 50% diesel and 50% ethanol [(50D/50E); 100% proof] has been selected as the best emulsified fuel ratio in comparison to the other emulsified fuel ratios of 90D/10E, 80D/20E, 70D/30E, 60D/40E, and diesel no. 2 fuel. Then, in phase II, oxygen-enriched additive dimethyl ether has been added to the selected best ratio of 50D/50E emulsified fuel. Then, the performance and emission tests for diesel, 50D/50E emulsified fuel ratio, and oxygen-enriched additive-added emulsified fuel have been conducted. Finally, it has been found that the oxygen-enriched additive-added emulsified fuel has given the best performance and less emission when compared to the other two fuels. All of the tests have been carried out in a single-cylinder, water-cooled, four-stroke direct-injection (DI) diesel engine. Also, the entire experiment has been carried out at a constant speed of 1500 revolutions/min. In comparison to diesel and the selected best ratio of the emulsified fuels, the oxygen-enriched additive-added emulsified fuel shows an increase in brake thermal efficiency and a decrease in specific fuel consumption, particulate matter, smoke density, and oxides of nitrogen.

’ INTRODUCTION The increase in prices of petroleum-based fuels, strict government regulations on exhaust emissions, and future depletion of worldwide petroleum reserves encourage studies to search for alternate fuels. Ethanol has been considered as an alternate fuel for diesel.1 Ethanol is a biomass-based renewable fuel, which can be produced from vegetable materials. It has a higher miscibility with diesel fuel when compared to other alternate fuels. Koganti et al. have stated that the use of ethanol in compression ignition (CI) engines has received considerable attention in recent years.2 Also, diesel engines, rigid and simple in structure and known for their fuel economy, remain the major source of inland transportation and industrial power plants. Owing to their dominant advantages of high thermal efficiency, they are found to be the most advantageous fuel combustion engines and are expected to remain so in the foreseeable future. In short, they have proven their role for consumption of diesel in the past as well as in the present. Also, the number of vehicles in industries, transportation, and agriculture will increase in the future. Therefore, use of diesel engines will definitely increase in the future. However, the pollutants emitted from the diesel engines are detrimental to human health and the ecological environment. Hence, diesel engines have been considered as one of the major air pollution sources. The major pollutants from diesel engines are particulate matter (PM), smoke density (SD), oxides of nitrogen (NOx), and other harmful emissions. These pollutants cause damage to the ozone layer, enhance the greenhouse effect, and produce acid rain. The photochemical smog formed from the reaction of NOx with ultraviolet (UV) sunlight might also damage the respiratory system, throat, and eyes. PM taken with polycyclic aromatic r 2011 American Chemical Society

hydrocarbon (PAH) or metallic compounds, if inhaled continuously, might cause carcinogen diseases.3 An emulsification technique is one of the possible approaches to improve fuel economy and reduce emissions of pollutants from diesel engines.4 In this technique, ethanol has a higher miscibility with diesel fuel. Therefore, the use of ethanol in CI engines has received considerable attention in recent years.5 Ethanol addition to diesel fuel results in different physicochemical changes in diesel fuel properties, particularly a reduction in cetane number, viscosity, and heating value.6 Therefore, different techniques involving ethanol diesel fuel operation have been developed to make the diesel engine technology compatible with the properties of ethanol-based fuels. The emulsified fuel ratios of 90D/10E, 80D/20E, 70D/30E, 60D/40E, and 50D/50E have been prepared on the basis of the water-in-oil (W/O)-type emulsion method. Finally, the 50D/50E has been selected as the best emulsified fuel ratio. The reason behind the selection of the above-mentioned ratio is due to its increase in brake thermal efficiency and decrease in specific fuel consumption (SFC), SD, and PM. Even though this emulsified fuel gives better brake thermal efficiency and gives less SFC, SD, and PM based on the performance and emissions, respectively, in comparison to diesel no. 2, there is a significant increase in NOx emission over diesel no. 2. This is due to the lower cetane number of ethanol, which causes high temperatures, resulting in a longer ignition delay. This increases the NOx emission for this fuel. This emission of NOx can be controlled by adding a suitable additive, which must Received: May 20, 2011 Revised: July 15, 2011 Published: July 18, 2011 3799

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Table 1. Properties of DME, Ethanol, and Diesel No. 2 ethanol

diesel

DME

(100% proof)

no. 2

chemical formula

CH3OCH3

CH3CH2OH

C12H26

cetane number

>125

8

50

self-ignition temperature (°C)

125

420

200 420

stoichiometric air/

10.9

9

14.6

specific gravity

33900

0.783

0.894

lower heating value (kJ/kg) density (kg/m3)

1.59 734.7

27000 794

42800 830

fuel ratio (w/w)

have the property of a high cetane number. This leads to a reduction of the temperature, resulting in a smaller ignition delay, thus reducing emission of NOx. Hence, for this present work, dimethyl ether (DME) has been selected as an additive, because of its high cetane number, noncorrosiveness, and low volatility.7 This additive is added on a 7% by volume basis with the selected emulsified fuel ratio of 50D/50E, and the performance, emission, and combustion tests have been carried out. Similarly, diesel no. 2 and emulsified fuel 50D/50E have been tested on the basis of the performance and emission, and all three results are compared. Table 1 indicates the properties of DME, ethanol, and diesel no. 2.

’ PRESENT WORK In phase I, emulsified fuel at different ratios of 50D/50E, 60D/ 40E, 70D/30E, 80D/20E, and 90D/10E have been prepared using the surfactant 1 [Tween-80; hydrophilic lipophilic balance (HLB) value of 15] based on the W/O-type emulsion method. In phase II, the objective of this work is to reduce NOx emission by adding the oxygen-enriched additive DME to the selected emulsified fuel ratio fuel and to study the performance and emission tests. DME is added (7% by volume basis) to the selected emulsified fuel ratio 50D/50E, and the results have been compared to the same emulsified fuel ratio of 50D/50E and diesel no. 2. Hence, the basic aim of the present work is to identify the emulsified fuel, which emits less NOx emission. Therefore, the reduction of NOx emission is performed by adding the required percentage of the additive DME to the emulsified fuel, because of its high cetane number. ’ PROCEDURE FOR THE PREPARATION OF THE EMULSIFIED FUEL For phase I, in the ethanol-in-diesel emulsion fuel preparation method, diesel and ethanol are the dispersion and dispersed medium, respectively. Hence, the dispersed medium is added slowly to the dispersion medium. For example, in the 50D/50E ratio, 49.5% diesel is mixed with the surfactant, Tween-80. Then, 49.5% ethanol is added to the mixture by volume basis slowly. Tween-80 reduces the interfacial tension between the two liquid phases to form a homogenized stable solution.8 Tween-80 is selected based on its HLB of 15. After adding all of the above, the mixture is placed in a special type of mechanical stirrer to obtain an emulsified fuel. Similarly, in phase II, the above-mentioned emulsified fuel is further added to the known quantity of 7% by volume basis of DME. Then, the mixture is placed in a special type of mechanical

Figure 1. Experimental setup.

Table 2. Properties of Test Instruments and Their Values of Accuracy instruments

accuracy

fuel measurement (mL)

+5

temperature measurement (°C)

+3

dynamometer (load applying error) (N m) tachometer (speed) (revolutions/min)

+5 +5

cooling water flow rate (mL)

+10

digas analyzer (%)

+5 in all measurements

stirrer, which has the specifications of three-phase, alternating current (AC) supply, 0 10 000 revolutions/min variable speed, vertical motor with twin blades, helical shape attached with the vertical shaft of the motor, and four numbers of zig-zag-shaped blades, which are fixed in the emulsified fuel containing a drum vessel to obtain a swirl motion for better mixing. After a required time of interval, a good emulsion is formed because of the sharing effects produced by the helical blades of the shaft and fixed blades in the emulsified fuel vessel. The surfactant, 1% by weight, is used, and the stability time is about 1.5 days. All of the tests are carried out at a constant speed of 1500 revolutions/min under variable-load conditions.

’ EXPERIMENTAL SETUP The experimental setup of the diesel engine is shown in Figure 1. Specifications of the engine are given in Table 3, and properties of test instruments and their values of accuracy are shown in Table 2. The fuel flow rate is obtained using the buret method, and the airflow rate is obtained on a volumetric basis. NOx emission is obtained using an analyzer working on chemiluminescence principle. The PM from the exhaust is measured with the help of the micro-high-volume sampler. An AVL smoke meter is used to measure the smoke capacity. An AVL DIGAS 444 (DITEST) five-gas analyzer is used to measure the rest of the pollutants and delay period. This five-gas analyzer has been purchased from Digital Electronics, Ltd., Japan. All of the measurements are collected and recorded by a data acquisition system. A buret is used to measure the fuel consumption for a specified time interval. During this interval of time, the fuel consumed by the engine is measured, with the help of a stopwatch. At the pressure of 220 kgf/cm2, the same engine is made to work with both emulsified fuel and diesel fuel. This is because ethanol is an easily combustible fuel. 3800

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Table 3. Specifications of the Diesel Engine type

vertical, water-cooled, four-stroke

number of cylinder

1

bore (mm)

87.5

stroke (mm)

110

compression ratio maximum power (kW)

17.5:1 5.2

speed (revolutions/min)

1500

dynamometer

eddy current

injection timing (deg)

23 before TDC

injection pressure (kgf/cm2)

220

Figure 3. Variation of SFC.

Figure 2. Variation of brake thermal efficiency. All of the tests have been carried out at a constant speed of 1500 revolutions/min under variable-load conditions. Performance and emission tests have been carried out for the emulsified fuel to find the optimum quality of DME, and the results are compared to diesel no. 2. Throughout the experiment, the static injection timing has been maintained at 23° before top dead center (TDC), which is optimal for the base diesel engine.

’ RESULTS AND DISCUSSION Phase I: Identification of the Best Emulsified Fuel Ratio. Figure 2 shows the variation of brake thermal efficiency. All of the emulsified fuel ratios have given better efficiency than the diesel fuel. The difference in the value of the brake thermal efficiency at 5 kW between the emulsified fuel ratio of 50D/50E and diesel fuel is 6.6%. This is due to more quantity of oxygen-enriched air present in ethanol fuel than in diesel fuel (the presence of the volume of air in ethanol and diesel fuel is 4.3 19 and 1.5 8.2, respectively). The possible reason for this increase in efficiency is that ethanol contains oxygen atoms, which are freely available for combustion.9 The oxygen present in ethanol generally improves the brake thermal efficiency when it is mixed with neat diesel. Because of this reason, the brake thermal efficiency increases as the concentration of ethanol is increased. From this, it is understood that the emulsified fuel gives comparatively better efficiency without any modification of the current diesel engine. However, considerable attention has to be paid to the compatibility and corrosiveness of the material.

Figure 4. Variation of SD.

Figure 3 shows the variation of brake power versus SFC. SFC takes lower values for the emulsified fuels than the diesel fuel. This is because of the reduction of energy content because of the addition of ethanol.9 Because the energy content is low for ethanol, when it is mixed with diesel, it makes the emulsified fuel mixture become poor in energy content. Also, the heating value of ethanol is lower when compared to diesel. Because of this reason, the SFC is lower for the emulsified fuel ratio 50D/50E. Because the brake thermal efficiency and SFC are inverse, the two basic parameters are most essential for the good performance of an engine. This could be achieved by the emulsified fuel ratio 50D/50E. Therefore, the performance of the engine will be good if it is run with emulsified fuel. All of the emulsified fuel ratios have taken less values of SD than the diesel fuel. The lowest value is taken by the emulsified fuel ratio 50D/50E, as shown in the Figure 4. The reason is that the addition of ethanol causes a decrease in the smoke level because of the better mixing of the air and fuel and the increase in the OH radical concentration.10 Also, smoke emission of the ethanol-in-diesel fuel emulsion is lower than those obtained with neat diesel fuel because of the soot-free combustion of ethanol under normal diesel-engine-operating conditions. Hence, as the ethanol concentration increases, the SD decreases. Figure 5 shows the variation of PM during this test. As the SD decreases for the emulsified fuel, the same result has been 3801

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Figure 5. Variation of PM.

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Figure 7. Comparison of brake thermal efficiency.

Figure 8. Comparison of SFC. Figure 6. Variation of oxides of nitrogen.

obtained for the PM. Because the SD and PM are directly proportional, they have given a higher PM for a higher SD and vice versa. PM is reduced because of the fact that ethanol can be combusted essentially soot-free under typical combustion conditions.11 Here, PM emission is higher for the diesel fuel than for all of the emulsified fuel ratios. The ratio 50D/50E has taken the lowest range of emission. Other ratios are lying between the diesel fuel and 50D/50E emulsified fuel ratio. The difference in value of the PM emission between the 50D/50E ratio and the diesel fuel at 5 kW brake power is 0.277 g/h. It is clear that the PM emission decreases as the ethanol concentration increases. All of the emulsified fuels emit a higher range of NOx than diesel fuel. Masahiro et al. have stated that generally alcohol/ diesel fuel emulsion causes a higher NOx emission because of the cetane-depressing properties of alcohol. Ethanol diesel fuel emulsion causes a high NOx emission because of the low cetane number of ethanol. A low cetane number leads the fuel to increase the ignition delay and greater rates of pressure rise, resulting in higher peak cylinder pressures and high peak combustion temperatures. This high peak temperature increases NOx emission.12 From the experiment, it is observed that, as the ethanol content increases, emission of NOx also increases (Figure 6). Phase II: Selection of the Best Emulsified Fuel with the Selected Emulsified Fuel Ratio. From Figure 7, the brake

thermal efficiency is almost equal for all of the fuels at lower loads. This is due to the increase in ignition delay and the adverse effect of ethanol on the combustion process when the engine temperature is low. At higher load conditions, because of the higher cetane number of the additive, brake thermal efficiency slightly increases for DME-added emulsified fuel than the remaining two fuels.2 At higher load conditions, the longer ignition delay leads to a rapid increase in the premixed heat release rate that affects brake thermal efficiency favorably. The ignition delay period is slightly decreased for the emulsified fuel and diesel no. 2. On the basis of this reason, there is a smaller decrease in brake thermal efficiency for the emulsified fuel than for diesel no. 2 fuel and the DME-added emulsified fuel. The difference in value between diesel no. 2 and DME-added emulsified fuel is 9.1%, at maximum load conditions. Figure 8 shows that DME-added emulsified fuel attains less SFC. The next consecutive roles are taken by the emulsified fuel 50D/50E and diesel no. 2, respectively. This is based on the energy content of the fuel. Normally, ethanol has less energy content than diesel fuel. On the basis of this, the emulsified fuel shows a lower value of SFC than diesel fuel. Also, DME has the property of a lower energy content value than ethanol.9 Hence, a lower specific fuel value is attained by DME-added emulsified fuel than the emulsified fuel and diesel no. 2, as shown in Figure 2. The difference in value obtained between 3802

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Figure 9. Comparison of SD.

Figure 11. Comparison of oxides of nitrogen.

Figure 10. Comparison of PM.

Figure 12. Comparison of ignition delay.

diesel no. 2 and DME-added emulsified fuel at the maximum load is 0.178 kg/kWh. The SD level is less for emulsified fuel than for diesel fuel no. 2, because of better mixing of the air and fuel and increase in the OH radical concentration.13 The peak value attained by diesel no. 2 and the emulsified fuel at maximum load conditions are 22.8 Hartridge smoke units (HSU) and 14.9 HSU, respectively. SD is further decreased with DME addition because of the presence of oxygen in the additive. The difference in value between the emulsified fuel and DME-added emulsified fuel is 5.1 HSU (Figure 9). Because SD and PM are directly proportional with each other, the same result is obtained. PM is reduced for the emulsified fuel because of the fact that ethanol can be combusted essentially soot-free under typical combustion conditions.11 It is much more reduced for DME-added emulsified fuel because of the presence of oxygen. On the basis of that, diesel no. 2 has taken the peak value, next to the role of emulsified fuel, and finally, DME-added emulsified fuel has taken the least role of PM (Figure 10). The low cetane depressing properties cause an increase in ignition delay and greater rates of pressure rise, resulting in high peak cylinder pressure and high peak combustion temperatures. The peak temperature always increases NOx formation.10 Because of this fact, the emulsified fuel containing ethanol, which

has a low cetane number (8), emits higher NOx formation than diesel fuel no. 2, which has a high cetane number (50). However, additive DME-added emulsified fuel having a higher cetane number (>125) than those two fuels emits much lower NOx formation. This is due to the reduction in ignition delay. This reduced ignition delay lowers the mass of the fuel accumulated before combustion and lowers the initial combustion rates, hence decreasing the peak temperature and, thus, reducing the NOx formation.7,13 On the basis of the above reference, the emulsified fuel emits more NOx than diesel fuel no. 2 and then the DMEadded emulsified fuel emits lower NOx than diesel no. 2, because of its higher cetane number (Figure 11). Ignition delay is highest at all loads for the emulsified fuel as compared to diesel no. 2 and the DME-added emulsified fuel. The lowest ignition delay value is obtained by the DME-added emulsified fuel at all loads. Ignition delay values for diesel no. 2 are obtained in between these two curves, as shown in Figure 8. Normally, a low cetane number increases the delay period.14 Here, the emulsified fuel 50D/50E consists of ethanol, which has a low cetane number. Hence, the emulsified fuel curve has taken the higher value of the delay period. The next role has been taken by diesel no. 2 fuel, which has a higher cetane number than the emulsified fuel ratio. At last, 7% DME-added emulsified fuel has a comparatively higher cetane number than the two fuels. Hence, it 3803

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Figure 13. Comparison of peak pressure.

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Figure 15. Comparison of heat release rate.

oxygen-enriched additive-added emulsified fuel releases less heat. Hence, the self-ignition temperature becomes reduced.

Figure 14. Comparison of maximum rate of pressure rise.

has taken the least value of the delay period. With the introduction of DME, the dynamic injection timing is retarded. This leads the fuel to being injected closer to TDC, where the air temperature is high. This phenomenon and the high cetane number of DME lead to lower ignition delay (Figure 12).7 The peak pressure is higher for the emulsified fuel, as shown in Figure 13. It is reduced at high output with the introduction of DME because of the reduction in ignition delay and retarded injection.15 However, it is increased at lower outputs. Diesel no. 2 fuel gives a lower peak pressure value than the two fuels. The difference between the DME-added emulsified fuel and the diesel no. 2 at higher output is 12 bar. As in the previous category, similar trends are seen in the case of the maximum rate of pressure rise, as shown in Figure 14. It is also reduced at high outputs with the introduction of DME because of the reduction in ignition delay and retarded injection.16 The heat release rate is slightly less for the oxygenated additive DME-added emulsified fuel for the best blend ratio of 50D/50E. This is illustrated in Figure 15. In the case of oxygen-enriched additive DME-added emulsified fuel, the cetane number is high. Mixing of additive and diesel leads to a higher cetane number. The higher cetane number of the fuel reduces the delay period. This results in low peak combustion pressure and temperature, which further reduces the heat release.17 Hence, the

’ CONCLUSION For phase I, the 50D/50E ratio of the emulsified fuel has been selected as the best ratio compared to the other ratios based on its increase in brake thermal efficiency, decrease in SFC, SD, and PM, and increase in NOx. For phase II, the performance and emission characteristics of the oxygen-enriched additive DME-added emulsified fuel is compared to the emulsified fuel ratio 50D/50E and diesel no. 2. The following results have been obtained: (1) A higher brake thermal efficiency is achieved by DME-added emulsified fuel. Brake thermal efficiency increases from 35.6% (emulsified fuel) to 38.1% (DME-added emulsified fuel). (2) A lower SFC is achieved by DME-added emulsified fuel. There is a difference in SFC of 0.05 kg/kWh between DME-added emulsified fuel and the emulsified fuel. (3) The best decrease in the SD value is obtained for DME-added emulsified fuel compared to the other two fuels. The value for DME-added emulsified fuel and the emulsified fuels are 9.8 and 14.9 HSU, respectively. (4) PM emission is low at lower outputs and equal to the emulsified fuel values at higher outputs for DME-added emulsified fuel. (5) The NOx value is drastically reduced because of the additive DME. It is usually higher for the emulsified fuel using the normal surfactant. It is reduced from 3.68 to 2.856 g/kWh for 50D/50E emulsified fuel and DME-added emulsified fuel, respectively. (6) Ignition delay is decreased for DME-added emulsified fuel compared to the other two fuels. At lower output, the difference in value is 4.9° crank angle (CA). (7) The peak pressure and the maximum rate of pressure rise decrease because of the reduction in ignition delay. (8) The heat release rate is higher for the emulsified fuel than the diesel and additive DME-added emulsified fuels. The maximum value obtained by the emulsified fuel is 106.591 kJ m 3 deg 1, and the minimum value attained by the additive DME-added emulsified fuel is 53.81 kJ m 3 deg 1. On the whole, the selected ratio of the emulsified fuel 50D/ 50E shows an increase in brake thermal efficiency and decrease in SFC, PM, and SD. However, there is a rise in NOx. When DME is added as an additive to the emulsified fuel (50D/50E), NOx and ignition delay are significantly decreased. Also, the performance of the fuel becomes increased and emission characteristics become decreased. 3804

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’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

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