Stability of diesohol using biodiesel as additive and its performance and emission characteristics in a compression ignition engine under various compression ratios.

Selvan, V. Arul Mozhi ; Anand, R.B. ; Udayakumar, M. 等

Introduction

The compression ignition engines are widely used in the transport sector, a standby power unit in industries and in agricultural fields due to their long life, reliability and economy. Due to the rise of the energy utilization in the recent years, the petroleum reserves are depleting at a faster rate, which results in the scarcity of diesel supply to meet the current demand. In addition, the stringent governmental regulations on emission control made the urgent need for search for an alternative fuel that is renewable and non-fossil fuel nature or at least partly as fuel extender [1].

Diesohol is a homogeneous blend of an alcohol and diesel. Among the various alcohols, Ethanol is the most preferred fuel because it is renewable and produced from various agricultural feed stocks [2]. To utilize ethanol in the compression ignition engines, several techniques have been adopted such as blending ethanol with diesel, duel fuel mode, spark assisted ignition system, use of ignition improvers etc. [3]. Most of the techniques require engine modification or the use of expensive additives for making compatible with compression ignition engines [4-6]. The fuel blending technique is an ideal choice to use ethanol in diesel engines as they do not require any engine modification. But the major challenge in employing this technique is the phase separation.

Letcher [7] suggested the use of an emulsifier or a co-solvent to prevent the phase separation of diesel-ethanol blends. Several studies [8-12] have been reported using commercial additives developed by Pure Energy Corporation (PEC) of Newyork, AAE Technologies of UK and GE Betz. The additives such as tetrahydrofuran, ethyl acetate [7,13], [O.sub.2] Diesel[TM] [14], isopropanol [15], ethyl ter-butyl ether (ETBE) and ter-amyl ethyl ether (TAEE) [16] are used to prevent phase separation among dieselethanol blends. Caro et al. [17] selected additives which had a glycerol skeleton bearing hetero atoms and amino-ether, hydroxyl, nitrate and nitramine functional groups to study the bahaviour of diesel ethanol blend. It was observed that the engine behavior improved in the presence of additives with reduction of pollutant emission, cycle irregularities and ignition delay. Ajav et al. [2] conducted performance and emission test using ethanol diesel blends and found that no significant power reduction in engine operation and the CO, NOx were lower than that of neat diesel. Agarwal et al. [4] reported that ethanol diesel blends up to 20% can be used in the constant speed engines without any hardware modifications and leads to significant reduction in CO and NOx emission. De-gang Li et al. [18] conducted performance and emission to find the optimum percentage of ethanol that gives simultaneously better performance and lower emissions. The results show that the brake thermal efficiency is increasing with increase in ethanol content in the blended fuel at overall operating conditions and the emissions such as CO, NOx and smoke are reduced and total hydrocarbon emission is increased. Prommes kwancheareon et al. [19] conducted solubility test on diesel-biodiesel-ethanol blend using palm oil methyl ester as additive and reported emission test results of the fuel blend. They found that 5% ethanol, 15% Biodiesel and 20% diesel blend was most suitable for diesohol production due to its lower emissions and acceptable fuel properties. X.Shi et al. [20] used 20% methyl soyate as additive with diesel ethanol blend to prepare a stable fuel blend and the performance and emission test on a multi cylinder variable speed engine shown significant reduction in smoke and particulate emission. Violeta Makareviciene et al. [3] conducted solubility test on multi-component biodiesel fuel system. They found that Rapeseed oil ethyl and methyl esters are soluble in ethanol and diesel without limits and the addition of ethanol increases the inter-solubility of ethanol and fossil diesel. Magin Lapuerta et al. [21] used E10 blend without any additives and conducted performance test on stationary engine test bed. They found improvement in the efficiency of the engine and reduction in particulate matter emission. They suggest using cetane number enhancers and co-solvent additives for the blend stability and better performance and emission reduction.

In this investigation, biodiesel (Jatropha Methyl Ester) produced through transesterification is used as a bridging agent between diesel and ethanol to prevent phase separation. Biodiesel has been used not only as an alternative fuel, but also an additive for diesohol [22, 23]. This homogeneity is due to the fact that the biodiesel can act as an amphiphile and form micelles that have nonpolar tails and polar heads. These molecules are attracted to liquid/liquid interfacial films and to each other. These micelles acted as polar or non-polar solutes, depending on the orientation of the biodiesel molecules. When the diesel fuel was in the continuous phase, the polar head in a biodiesel molecule oriented itself to the ethanol, and the non-polar tail was oriented to the diesel [19, 24]. The present investigation is to throw light on the performance and exhaust emission phenomena of a compression ignition engine using ethanol-diesel-biodiesel blends under various compression ratios.

Experimental setup

The whole investigation is conducted in two phases; in the first phase, the stability of ethanol-diesel-biodiesel blends varying in the proportions is investigated at the temperatures of 0[degrees]C, 30[degrees]C and 45[degrees]C. A set of twenty six sample blends are prepared using commercially available diesel, ethanol (99.9% purity) and a bio-diesel (Jatropha methyl ester, which is prepared from jatropha oil through transesterification) varying in their volume proportions in individual test tubes. The properties of the fuel blends are shown in the Table 1. Each test tube containing the fuel blend is sealed to prevent leakage and weighed separately using a digital weighing machine to cross check for any weight loss in a later stage as the constituents are volatile in nature. The blends are mechanically agitated uniformly and kept idle for 48 hours in a temperature-controlled environment and the temperature is recorded using a digital thermometer. The samples have been monitored carefully at an interval of fifteen minutes and the blend stability is recorded using a digital camera. The experimental setup and procedure are described in detail by Arul Mozhi Selvan et al. [25].

In the second phase, performance and emission tests are carried out in a test rig consists of a computerised single cylinder four stroke direct injection variable compression ratio engine, eddy current dynamometer, data acquisition system, exhaust gas analyzer and a smoke meter. The schematic diagram of the test rig and engine specification is given in the Fig.1. The engine has a provision for changing the compression ratio over a range of 5 to 20. The eddy current dynamometer is directly coupled with the engine output shaft and the load applied on the engine is measured using the load cell of the dynamometer. The data acquisition system is used to collect and store data related with the engine output, engine speed, air flow rate, fuel mass flow-rate and cooling water flow-rate. Infra red sensor with a burette is used to measure the fuel flow rate. A non-contact type speed sensor, MAP sensor and turbine type flow meter are employed to measure the engine speed, inlet manifold pressure and cooling water flow rate respectively. AVL DIGAS exhaust gas analyzer is used for the measurement of CO, HC and NO emission and AVL Smoke meter is used for the measurement of smoke absorption coefficient ([m.sup.-1]). The estimated uncertainty for the measured and evaluated quantities is shown in Table 2. The experiments are carried out at a constant speed of 1500rpm on various loads under steady state conditions using neat diesel and different stable fuel blends (D85E5B10, D80E10B10, D75E15B10, D70E20B10 and D65E25B10) for the compression ratios of 15, 17 and 19.

[ILLUSTRATION OMITTED]

Results and Discussion

The following sections illustrate the results obtained from the studies on stability of diesel-ethanol-biodiesel blends and its performance and emission characteristics on the CI engine. The stability analysis results are presented in the form of ternary plots and the performance & emission characteristics are presented as the variation of brake specific energy consumption, brake thermal efficiency, quantity of CO, HC, NO and smoke absorption coefficient of exhaust gases with respect the brake mean effective pressure.

Effect of temperature on the stability of fuel blends:

The test tubes containing the blends are observed for the phase separation every fifteen minutes and the conditions of the blends are recorded by taking photographs using a digital camera. The conditions of blends are shown the Fig.2 for the temperatures 0[degrees]C, 30[degrees]C and 45[degrees]C. From the figure, the blends with phase separation and the stable one are clearly seen. In the phase separation studies at 0[degrees]C, the blends are found to exist in three forms: liquid, crystalline with or without phase separation and liquid-crystalline with phase separation. Since the freezing points of both diesel and ethanol are below 0[degrees]C, the mixtures did not freeze into crystalline form. But biodiesel is found to have frozen into the crystalline form, as the freezing point is higher than ethanol and diesel, which leads for the freezing of the blends. From the Fig.2(a), at 0[degrees]C, it is observed that most of the samples are unstable; few samples are in pure crystalline form and few are in liquid-crystalline form that indicates at lower temperatures, the blends become unstable and in turn are unsuitable for the engine operation. The state of samples at 30[degrees]C is shown in Fig.2(b). It is observed that phase separation is prevented by increasing the quantity of biodiesel. The Fig.2 (c) shows that the stability of the blend increases with increase in temperature. For example the unstable blend D60E30B10 at 30[degrees]C is stable at higher temperature at 45[degrees]C. Further, the addition of 10% biodiesel prevents phase separation for the samples up to E25 blend. However, 20% biodiesel is required to keep the samples of E30.

[FIGURE 2 OMITTED]

Performance Characteristics: variation of BSEC

The brake specific energy consumption (BSEC) is a more reliable criterion compared to brake specific fuel consumption for comparing the fuels having different calorific value and density. The variation of BSEC with respect to brake mean effective pressure for the stable fuel blends and neat diesel under the compression ratio of 15, 17 and 19 is shown in the Fig.3. From the figure, it is seen that the BSEC decreases with the increase in brake mean effective pressure as expected and the least BSEC is recorded as 13860kJ/kW-hr under the compression ratio of 17 at the bmep of 0.44MPa when the engine is run by neat diesel. Further addition of load increases the brake specific energy consumption. For all the compression ratios, the least BSEC is found while running the engine at the bmep of 0.44MPa; hence the economic load condition is identified. Even though the engine is tested up to 20% more load than the economic load, all the performance and emission parameters are discussed with respect to the economic conditions to identify optimum working conditions. The maximum BSEC is found as 15924.98kJ/kW-hr under the compression ratio of 19 for the E25 blend.

[FIGURE 3 OMITTED]

Performance Characteristics: variation of BTE

The variation of brake thermal efficiency with respect to the brake mean effective pressure under various compression ratios is shown in the Fig.4. From the figure, it is observed that the brake thermal efficiency increases continuously with the increase in the brake mean effective pressure (up to 0.44MPa) for all the cases and then decreases with further loading conditions. Among all the cases, the highest brake thermal efficiency is 26.15% for neat diesel at the brake mean effective pressure of 0.44MPa under the compression ratio of 17, whereas it is 23.74% for E25 blend under the same condition. The least brake thermal efficiency is observed as 22.61% for the E25 blend at the bmep of 0.44MPa under the compression ratio of 15. The lower calorific value of the fuel blend and its higher fuel consumption than neat diesel to produce same power output is the cause for this trend [2]. However the brake thermal efficiency is slightly higher or equal for the diesel-ethanol-biodiesel blends with lower ethanol and higher biodiesel content and found decreases as the percentage of ethanol increases. This reason being that higher cetane index due to the addition of biodiesel and the ethanol content due to the improved quality of fuel spray with blended fuels since the boiling point of ethanol is lower than that of neat diesel, higher reaction activity in the fuel rich zone due to the oxygenate of ethanol and the reduction in heat losses due to the lower flame temperature [17, 18, 26-29].

[FIGURE 4 OMITTED]

Exhaust emission characteristics: variation of CO

The variation of CO emission on volumetric basis with respect to brake mean effective pressure for the various fuel blends and neat diesel are shown in Fig.5. The percentage of CO increases as brake mean effective pressure increases. The variation of CO emission is marginal among the blends and neat diesel up to the brake mean effective pressure of 0.3MPa. Significant variation of CO is observed for the further increase in brake mean effective pressure. However, the percentage of CO emission is lower for all the blends when compared with the neat diesel for all the compression ratios. At lighter loads, the increase in CO level with ethanol blend is a result of incomplete combustion of the blend. Factors involving combustion deterioration such as high latent heat of evaporation may be responsible for the poor oxidation reaction rate of CO and the increase in CO emission. A thick quench layer created by the cooling effect of vaporizing alcohol also play a major role on CO emission at part loads. At the full load, rich combustion invariably produces CO and the emission increase almost linearly with the deviation from the stoichiometry [30]. In addition, presence of ethanol leads to ignition delay and which causes the increase in CO emission. The lowest CO emission of 0.37% is obtained at the brake mean effective pressure of 0.44Mpa for the compression ratio of 15 when the engine is run by the fuel blend E25.

[FIGURE 5 OMITTED]

Exhaust emission characteristics: variation of HC

Fig.6 shows the variation of hydrocarbon emission with brake mean effective pressure under various compression ratios. It is observed that hydrocarbon emission increases as brake mean effective pressure increases for all the blends and neat diesel. The least HC emission is observed as 110ppm for the E5 blend under the economic loading condition (bmep of 0.44MPa) at the compression ratio of 19. The maximum HC emission is observed for the E25 blend as 198ppm at the compression ratio of 17 under the same load. The blends containing higher biodiesel percentage will have lower HC emission due to the higher cetane number than diesel resulting in more complete combustion. The blends containing higher percentage of ethanol produce higher hydrocarbon emission, which indicates that the presence of ethanol might be a factor for the increase in hydrocarbon emissions [19].

[FIGURE 6 OMITTED]

Exhaust emission characteristics: variation of NO

The NO emissions of the engine using different fuel blends and neat diesel with respect to brake mean effective pressure for the compression ratios 15, 17 and 19 are shown in Fig.7. The NO emission is lower for all the fuels at lower loads and increases as the load increases. The lowest NO emission is observed as 250 ppm for neat diesel under the compression ratio of 19 at the brake mean effective pressure of 0.44 MPa. Also it is observed that NO emission is lesser for few blends (E20, E25) when comparing to the neat diesel under the lower load conditions. The reason attributed being that the addition of ethanol which high specific heat and high latent heat of vaporization causes decreased flame temperature which results reduction in the NO emission [2, 7, 31, 32, 33]. The diesel-ethanol-biodiesel fuel blends produce higher NO emission than neat diesel at higher loading conditions. The highest NO emission is observed as 350 ppm for the E20 blend under the compression ratio of 15 at the brake mean effective pressure of 0.44MPa. The reason for the higher NO emission is the decrease of cetane number with addition of ethanol. A lower cetane number means an increase in ignition delay and more accumulated fuel/air mixture which causes a rapid heat release in the beginning of the combustion resulting in high temperature and high NOx formation [19, 20].

[FIGURE 7 OMITTED]

Exhaust emission characteristics: variation of smoke absorption coefficient

Smoke absorption coefficient (K) is the number, which gives an indication about exhaust emission density. The black smoke emission resulting from combustion of diesel-ethanol-biodiesel blends and neat diesel are plotted against the brake mean effective pressure in the Fig.8. Smoke levels are higher at higher brake mean effective pressure for all the fuel blends and neat diesel due to the fuel rich core at high loads. The highest smoke level is found as 5.6m-1 for diesel at the compression ratio of 15 at the brake mean effective pressure of 0.44MPa, whereas the lowest smoke level is 2.1[m.sup.-1] for E25 blend at the compression ratio of 19. Higher smoke values may be due to unburned and partially reacted hydrocarbons of the fuel. The reduction of smoke is due to the presence of more oxygen and low carbon in the fuel blends due to the higher ethanol content. Also the charge cooling due to ethanol addition increases the ignition delay and thus enhances the mixing of air and fuel which in turn makes better air utilization. The high oxygen content of the blends combined with low C/H ratio contributes for the reduction of smoke [34].

[FIGURE 8 OMITTED]

Conclusion

The stability, performance and emission characteristics of different diesohol blends are investigated to evaluate the potential of using biodiesel as an additive. The conclusions of this investigation are as follows:

The phase separation of ethanol-diesel blends can be prevented using desired quantity of biodiesel (Jatropha methyl ester) as additive. The stability of the blend increases with the increase in the temperature. 10% biodiesel by volume prevents the phase separation at 30[degrees]C for the blends E5, E10, E15, E20 and E25. For further increase in ethanol percentages require more biodiesel i.e. E30 blend need 20% biodiesel.

The lowest specific energy consumption is observed as 13860kJ/kW-hr for neat diesel under the compression ratio of 17 and the highest specific energy consumption is observed as 15924.98kJ/kW-hr for E25 blend at the compression ratio of 19 under the economic loading condition at the brake mean effective pressure of 0.44MPa.

The brake thermal efficiency of the engine fueled with diesohol blends is slightly higher or equal with lower ethanol and higher biodiesel proportion and found decreases with the increases in percentage of ethanol at the compression ratios 15, 17 and 19. Highest brake thermal efficiency is observed as 26.15% for neat diesel under the compression ratio of 17 at the brake mean effective pressure of 0.44MPa, whereas the lowest brake thermal efficiency is observed as 22.61% for the E25 blend under the compression ratio of 15 under the same loading condition.

The least CO emission is observed as 0.37% for the E25 blend at the compression ratio of 15 and the highest CO emission is 0.88% for neat diesel under the compression ratio of 17 at the bmep of 0.44MPa. The least hydrocarbon emission is found as 110ppm for the E5 blend under the compression ratio of 19 and highest as 198ppm for the E25 blend under the compression ratio of 17 at the bmep of 0.44MPa.

The least NO emission is observed as 250ppm for the neat diesel under the compression ratio of 19 and the highest NO emission is observed as 350ppm for the E20 blend under the compression ratio of 15 at the bmep of 0.44MPa.

The smoke absorption coefficient is found to decrease remarkably for all the ethanol blends. The lowest smoke absorption coefficient is observed as 2.1[m.sup.-1] K for the E25 blend at the compression ratio of 19 and the highest as 5.6 [m.sup.-1] for diesel at the compression ratio of 15 at the bmep of 0.44 MPa.

Nomenclature

CR     Compression ratio
BP     Brake power
BSEC   Brake specific energy consumption
bmep   Brake mean effective pressure
CO     Carbon monoxide
HC     Hydrocarbon
NO     Nitrogen oxide
PM     Particulate matter
JME    Jatropha methyl ester
E5     85% Diesel+ 5% Ethanol+10%Biodiesel  (D85E5B10)
E10    80% Diesel+10% Ethanol+10%Biodiesel (D80E10B10)
E15    75% Diesel+15% Ethanol+10%Biodiesel (D75E15B10)
E20    70% Diesel+20% Ethanol+10%Biodiesel (D70E20B10)
E25    65% Diesel+25% Ethanol+10%Biodiesel (D65E25B10)

Acknowledgment

The authors are grateful to Dr. M. Chidambaram, Director, National Institute of Technology, Tiruchirappalli, for granting permission to establish I.C. Engines Research Laboratory in the Mechanical Engineering Department with modern computerized experimental facilities to the international standards. Also special thanks to Mr. Palanisamy and Mr. Durairaj for their help rendered during experimentation.

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V. Arul Mozhi Selvan, R.B. Anand and M. Udayakumar Department of Mechanical Engineering, National Institute of Technology, Tiruchirappalli-620 015, Tamilnadu, India. Email:arulmozhi@nitt.edu

Table 1: Properties of diesel-ethanol-biodiesel fuel blends

Properties                      Diesel      Ethanol      JME     E5

Kinematic Viscosity @               2 (a)   1.1314 (d)    5.98    2.91
40[degrees]C,cSt
Density @ 15[degrees]C, gm/cc    0.83 (a)     0.79 (d)   0.893   0.839
Flash Point, [degrees]C            50 (a)     13.5 (c)      88    17.5
Fire Point, [degrees]C             56 (a)        -         106      20
Pour Point, [degrees]C              6 (b)   -117.3 (b)      -7      -5
Copper strip corrosion              -            -           1       1
Cetane Number                      46 (c)        6 (c)    55.4    47.7
Net calorific value, MJ/kg      42.30        25.18 (d)   38.71   40.70

Properties                      E10     E15     E20     E25

Kinematic Viscosity @            2.67    2.51    2.35     2.2
40[degrees]C,cSt
Density @ 15[degrees]C, gm/cc   0.832   0.829   0.827   0.823
Flash Point, [degrees]C            15      13      11       9
Fire Point, [degrees]C             18      17      14      13
Pour Point, [degrees]C             -7     -10     -14     -17
Copper strip corrosion              1       1       1       1
Cetane Number                   46.85    46.1   45.25   44.2
Net calorific value, MJ/kg      40.50   40.30   40.10   39.90

(a) Ref. [5], (b) [19], (c) [20], (d) [21]

Table 2: Estimated uncertainty for the measured and evaluated
quantities

Quantity                   Estimated uncertainty

CO                         [+ or -] 0.01%
HC                         [+ or -] 1ppm for < 2000ppm
                           [+ or -] 10ppm for > 2000ppm
NO                         [+ or -] 1ppm Volume
Smoke Absorption           [+ or -] 0.01[m.sup.1]
Temperature                [+ or -] 3[degrees]C
Brake thermal efficiency   [+ or -] 3.5% of the calculated value

Figure 1: Schematic diagram of the experimental setup

Specification of the test engine:

Item              Specification   Item       Specification

Brake Power       3 kW            Speed      1500 rpm
No. of Cylinder   1               Compress   5:1 to 20:1(Variable)
                                  ratio
Bore              80 mm           Stroke     110 mm
Ignition          Compression     Engine     Water
                  ignition        cooling