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Fundamental droplet evaporation and engine application studies of an alternate fuel produced from waste transformer oil
Fuel 259 (2020) 116253
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Fundamental droplet evaporation and engine application studies of an alternate fuel produced from waste transformer oil
T
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S. Prasanna Raj Yadava, C.G. Saravananb, , S. Karthicka, K. Senthilnathana, A. Gnanaprakasha a b
Department of Mechanical Engineering, Easwari Engineering College, Chennai, India Department of Mechanical Engineering, Annamalai University, Chidambaram, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Waste transformer oil Evaporation Combustion Emission Diesel engine
With a motivation to contribute in the fundamental and application aspects of alternate fuel research, which has been rarely reported in the same study, this research work has been commemorated. Herein, WTO (waste transformer oil) has been considered as a replacement for diesel as tapping the source of energy from waste products would help reduce the fuel cost. As such, WTO was subjected to fuel refinement process by transesterifying it in two stages using acid and alkali catalysts. Characteristically, the estimated physical and thermal properties of TWTO (trans-esterified waste transformer oil) were recognized to be suitable for its use in a diesel engine. Prior to the application study of testing TWTO – diesel blends in a diesel engine, fundamental study on droplet evaporation was performed through suspended droplet experimental facility to appraise their evaporation attributes. Reportedly, the droplet regression, evaporation constant and droplet lifetime were found to be superior for B50 (50% TWTO and 50% diesel) than B100 (100% TWTO). Finally, from the application study, operation of TWTO – diesel blends in a diesel engine divulged comparable performance and combustion for B25 with that of diesel. Similarly, gaseous emissions such as CO (carbon monoxide), smoke and NOX (oxides of nitrogen) were found to be in closer agreement for B25 with diesel, deducing it as an optimum blend for an unmodified diesel engine.
1. Introduction These days, alternate fuels have gained immense attention in respect of their environmental friendliness [1]. However, the availability of these bio-derived alternate fuels to replenish the decreasing petroleum fuel supply is improbable [2], leaving the efforts to attain energy independence at stake. Therefore, advent of several alternate fuels to satiate the petroleum needs of the nation is imperative and in wake of this, researchers have embarked on to harness energy from every possible source [3]. Most of these devised alternate fuels are capitalized for engine application [4] and during the course of development of these fuels, researchers have concocted plethora of strategies for optimizing the production of them [5]. In recent times, investigation of biodiesel, an ester produced from variety of vegetable oils, in a diesel engine has been reported at aplenty and most of the studies concede to the improvement in engine performance in view of thermal efficiency [6] and fuel consumption. Similarly, the engine out emissions such as HC and CO was reported to be decreased, due to the presence of inherent oxygen within the fuel, at the expense of increased NOX emission [7]. However, most of the
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revelations of researchers and engine manufactures have underscored the limitations of deterioration in engine performance and emissions for higher blends of biodiesel with diesel due to the shortcomings with the fuel properties such as increased viscosity, lower calorific value, higher boiling point and flash point [8,9]. By this trend, the nominal blend suitable for its operation in a diesel engine without any modifications is B20 (20% biodiesel and 80% diesel) [10,11], while higher blends do require revamp of engine design or modification with engine operating parameters. Recent research on alternate fuels harps on capitalizing waste oils as source of fuel as it would not only amount to replace diesel but also enables safe disposal of waste materials. In this regard, as a measure to reap the required sources of energy from agricultural and industrial waste products, research on production of biodiesel from kapok oil and CNSL (cashew shell nut liquid) for diesel engine application have been contended [12,13]. While kapok oil is extracted from waste and discarded kapok seeds, CNSL is being produced from the outer shell of cashew nuts, which are thrown out as waste products [14,15]. It is worthwhile to note that the adoption of WTO (waste transformer oil) in a diesel engine, as a measure to harness energy and realize safe disposal
Corresponding author. E-mail address: [email protected] (C.G. Saravanan).
https://doi.org/10.1016/j.fuel.2019.116253 Received 1 August 2019; Received in revised form 4 September 2019; Accepted 21 September 2019 0016-2361/ © 2019 Published by Elsevier Ltd.
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of it, has also been accomplished recently [16,17]. It is worthwhile to note that the adoption of these waste products in diesel engine not only churns out additional sources of fuel [18,19], but also enables reduction of fuel cost unlike vegetable oil based fuels, given the cost of the vegetable oils are reported to have been 2 times higher than that of conventional petroleum fuels [20]. Most of the research activities on characterization and utilization of alternate fuels in a diesel engine have targeted only on the documentation of engine performance and pollutant emission. In addition, few optimization studies to help accustom higher blends of biodiesel in diesel engine such as modification of engine design or operating parameters have also been brought to light [21–23]. However, fundamental studies on fuel spray, evaporation and flame are imperative to comprehend the underlying physics mechanisms preceding the combustion process. As such, few studies reported the evaporation attributes of biodiesel such as droplet regression, evaporation constant and time through suspended droplet experimental study [24,25]. Also, few researchers have also have given a good account of flame characteristics of certain biofuels, which report the flame speed and stability of biofuels under laminar and turbulent conditions [26,27]. Preceding evaporation and combustion, fuel jet from the diesel injector disperses in to spray and the evaluation of the spray characteristic entails measurement of spray tip penetration, cone angle, particle size and velocity. As such, researchers have also delved into the estimation of the reported macroscopic and microscopic spray characteristics for biofuels at pressures prevailing in a diesel engine [28]. The documentation of these fundamental data on evaporation and flame would provide the option to develop chemical kinetic model and build a comprehensive database, which could be latter availed to model combustion and emission after studying the fuel chemical kinetics. It is indispensable to perform a fundamental study while conducting engine experiments and amalgamate them to attain deeper insights. Therefore, besides performing an engine study, fundamental study on fuel evaporation through suspended droplet experimental study has been performed so as to contribute both in fundamental and application front. Herein, waste transformer oil has been considered and refined through trans-esterification process to make the properties conducive for its use in a diesel engine. Initially, the evaporation characteristics of blends of TWTO (trans-esterified waste transformer oil) with diesel viz B50 and B100 such as droplet regression, evaporation rate and time were studied by suspended droplet experimental study. Subsequently, various blends of TWTO with diesel as shown in Table1 (B25, B50, B75 and B100) were tested in a single cylinder diesel engine and the engine characteristics such combustion, performance and emission were analyzed and reported.
Table 2 Thermal and physical properties of the blend fuels. Property
Measurement Standards
Diesel
50% HCF (B50)
100% HCF (B100)
Kinematic viscosity at 40 °C (CSt) Fire Point (°C) Flash Point (°C) Boiling point (°C) Density at15°C (g/cc) Calculated cetane index
ASTM D445
2.57
3.3
4.03
ASTM ASTM ASTM ASTM ASTM
75 56 250 0.8072 50
89 68 300 0.8234 51.5
93 85 350 0.8355 52
D92 D92 D240 D1298 D976
greater than 2 mg KOH per gram of oil, subjecting it to alkaline transesterification process would render the yield of hydrocarbon fuel unproductive, as inferred from the reports of previous studies [29,30]. Therefore, it is prudent to pretreat the refined WTO with sulphuric acid in the first stage and then commemorate the alkaline trans-esterification process using alcohol, methanol, and alkali, KOH in the second stage. After the two-stage trans-esterification process, the mixture was allowed to settle so as to separate TWTO and glycerol, followed by draining the glycerol from the bottom. Subsequently, washing process was performed by using a sprinkler, which slowly sprinkled water into the oil container and water-hydrocarbon mixture was agitated gently for 20 min, allowing water to settle out of the hydrocarbon mixture. The pure hydrocarbon fuel, produced by these processes, is then examined for its fuel properties as per ASTM standard method and are reported in Table 2. After the double stage trans-esterification process, the fuel viscosity has been ably reduced with an improvement in cetane number and even all other fuel properties were noted to be in compliance with the required standards, making it appropriate for use in diesel engine applications. 2.2. Suspended droplet experimental setup and procedure Evaporation characteristics of TWTO (trans-esterified waste transformer oil) and its diesel blends have been ascertained by a suspended droplet experimental set up. A schematic of the experimental set up is shown in Fig. 1. The experimental arrangement entails a cylindrical tunnel surrounded by a heating tape, quartz fiber with a bead, air compressor, a digital camera and a light source. Through a syringe,
2. Methodology 2.1. Extraction procedure of hydrocarbon fuel The hydrocarbon fuel used in this study was extracted from waste transformer oil by traditional trans-esterification process, enabling breakdown of higher hydrocarbons to lower ones. Preceding the chemical treatment process, collected WTO was treated and purified, as the prolonged use of it in transformers must have contaminated the oil with impurities. Since the total acidity of refined WTO was perceived to be Table 1 Details of the test fuel composition. Sl. No
Fuel
Fuel blended (% vol)
1. 2. 3. 4. 5.
Diesel 25% HCF 50% HCF 75% HCF 100% HCF
100% diesel fuel 25% hydrocarbon fuel + 75% diesel fuel 50% hydrocarbon fuel + 50% diesel fuel 75% hydrocarbon fuel + 25% diesel fuel 100% hydrocarbon fuel
Fig. 1. Schematic diagram of experimental setup for evaporation study. 2
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certain quantity of fuel is impinged over the quartz fiber bead and care is taken to maintain the initial – diameter of the droplet as maximum as possible. The average velocity of air exiting the vertical tunnel is maintained at 0.15 m/s, while the temperature of the air, heated by the heating tape, has been maintained in the range of 100 °C–150 °C. Significantly, the temperature is recorded at the point of suspension of the droplet every time to avoid discrepancies with the measured values. When hot air impinges over the suspended droplet, the droplet gets evaporated and the time evolution of change in droplet diameter is captured using high definition video camera at the rate of 30 frames per second. Subsequently, the high definition video is being converted into digital images using Image J tool and in the image processing, the extracted images are processed using a MATLAB code. The size of the droplet, which reduces as time passes, is processed by the code and surface regression of the droplet with respect to time is obtained as output. In-order to enhance the accuracy of the measured data, the experimentation is repeated for five times.
operating point, the combustion parameters were also processed and stored in the personal computer. Similarly, the time taken for consumption of 10CC of fuel and other gaseous emissions were also measured and recorded for subsequent analysis. The tests were repeated for three times so as to improve the accuracy of the noted readings and each test was done for 3 h. Finally, the average value of the three readings was taken for the calculation Also, an uncertainty analysis was performed using the method of propagation of errors and the total uncertainty of this experiment is calculated from the uncertainty of various parameters and equipment’s as listed in Table 4 and found to be ± 2.79%. 3. Results and discussion 3.1. Evaporation characteristics for TWTO The time evolution of square of droplet diameter (representing its surface area regression) is designated as the droplet regression rate. Typically, the regression curves at 100 °C and 150 °C, as noted from Fig. 3a and 3b, almost follow the conventional d2-law. While volumetric expansion and unsteady evaporation regime are evident observed at lower air temperature (100 °C), these are figured out to be very small at higher air temperature (150 °C) and the curve mostly manifests a steady evaporation regime. Notably, the initial heat up period is perceived to be prominent for B100 at 100 °C on account of volumetric expansion of the droplet when hot air is impinged over it and therefore, the evaporation of B100 droplet is deemed to be gradual. On the other hand, B50 seldom shows a heat up period even at an air temperature of 100 °C, as faster preferential vaporization of diesel is the cause of this behavior. Evidently, this can be noted by the lower flash and boiling point of B50 when compared to those of B100, as evident from Table 2 and thus fuel properties are deemed to affect the evaporation characteristics of test fuels. With an increase in the air temperature to 150 °C, the evaporation characteristics of both the test fuels are enhanced and the initial heat up period is barely noticed in their regression curves. It is clear that the slope of the regression curves, indicated by trend lines, increases as the air temperature is increased and as a result, the droplet lifetime is seen to decrease. Between B50 and B100, at 150 °C, the differences in vaporization characteristics is reduced, when compared to the air temperature of 100 °C; the slope of the trend line is increased by 1.4 and 2.9 times for B50 and B100, respectively, when the air temperature is increased from 100 °C to 150 °C. Previously, researchers reported lower evaporation rates for the vegetable oil based methyl ester due to its distinct fuel properties [31,32] and similar observations are recorded in the present study with TWTO. From the droplet regression curve, two crucial evaporation characteristics of test fuels, namely evaporation rate constant and droplet lifetime, can be obtained. Evaporation rate constant is the absolute rate of change of surface regression curve with respect to time. This is computed by fitting a linear trend line after eliminating the initial heat up period (if present). While doing this, it is ensured that the value of regression coefficient (R2) is above 0.9 and the evaporation constant has been computed as follows, Evaporation constant (mm2/s) = (d0)2 × Slope of the linear trend line The variation of the evaporation constant for B50 and B100, as a function of air temperature is shown in Fig. 4a. The evaporation rate increases with the increase in air temperature, as well as with the addition of diesel. Similar to evaporation constant, droplet lifetime is also computed from regression curve which is indeed obtained by extrapolating the trend line until the droplet diameter reaches a zero value and noting the corresponding time. This extrapolation is required, as the suspended droplet experiments cannot record zero diameter, as the minimum diameter that can be reached is the diameter of the bead of the quartz suspender. In fact, the experimental data will not be extended beyond 1.3 times the bead diameter, as interferences between
2.3. Engine experimental setup and procedure The compression ignition engine used for the study was Kirloskar TV1, single cylinder, four stroke, constant speed, vertical, water cooled and direct injection diesel engine, and the detailed engine specification are given in Table 3. The engine was coupled with an eddy current dynamometer, as schematically described in Fig. 2, to apply different loading condition from low to full load condition. Significantly, the fuel injection system entails a mechanical fuel pump with 3 hole injector nozzle assembly wherein, the fuel injection pressure and timing are fixed at 220 bar and 23°CA BTDC. The fuel supplied to the engine is measured manually using a burette and stopwatch. Indeed, for this constant speed engine that runs at 1500 rpm, the airflow rate is constant and is measured by an u-tube manometer fitted in the inlet manifold. The engine speed was measured by an in-house designed magnetic pickup sensor, connected to frequency meter. Measurement of combustion chamber pressure was obtained by installing an AVL make water-cooled piezoelectric pressure transducer with the sensitivity 16:11 PC/bar, in the cylinder head portion of the combustion chamber. Characteristically, an AVL 3057 charge amplifier converts the charge yielded by the piezoelectric transducer into proportional electric signals. Finally, a personal computer (PC) interfaced with an AVL 619 Indimeter Hardware and Indwin – software version 2.2 data acquisition system collects combustion parameter data that has been averaged over 100 cycles. From the in-cylinder pressure data, the heat release rate and other associated combustion parameters are evaluated using standard formulations. Gaseous emissions such as NOx (nitrous oxides), CO (Carbon monoxide) and HC (Hydrocarbon) were measured by an AVL 444 di gas analyzer in accordance with NDIR (non-dispersive infrared) principle. AVL 437 smoke meter, based on light extinction principle, measured the smoke intensity of the engine exhaust gases. The engine was started, warmed up at every stage and the cold water as well as lubricating oil temperature was maintained accordingly. At each Table 3 . Test engine specification. Make
Kirlosar-TV 1
General details
Four stroke, compression ignition, constant Speed, vertical, water cooled, direct injection One 87.5 mm 110 mm 17.5:1 1500 rpm 5.2 kW 220 bar 23° bTDC SAE 40
Number of cylinder Bore Stroke Compression ratio Rated speed Rated output Injection pressure Fuel injection timing Lubricating oil
3
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Fig. 2. Schematic diagram of experimental setup for engine study.
analyzing these characteristics for TWTO – diesel blends in a diesel engine. Fig. 5a and 5b depicts the heat release rate and cumulative heat release rate, respectively, for TWTO – diesel blends at full load condition. With the increase in proportion of TWTO in the blend, the magnitude of peak heat release rate decreases and so does is the trend with the diffusion combustion phase too. Since TWTO is more viscous than diesel [34], the atomization and evaporation of it is anticipated to be poor, hindering the preparation of homogenous air/fuel mixture. Further due to the cetane number of TWTO the ignition delay period is shortened so as to advance the SOC. With this, the formation of well mixed air/fuel mixture is further minimized and the amount of fuel being burnt in premixed combustion phase is decreased, reducing the magnitude of peak heat release rate for higher TWTO blends, with B100 evincing a decrease of 13.1% than diesel. Inference on the reduction in peak heat releases rate for the fuels having higher viscosity and boiling point, similar to TWTO, complies with the results portrayed in the current study. Normally, vegetable oil based fuels tend to possess lower calorific value [35] than diesel and this is deemed to decrease the peak heat release rate for them. However, unlike biodiesel, TWTO doesn’t suffer much drop in calorific value than that of diesel and therefore, it has nothing to do with the reduction in magnitude of peak heat release rate for TWTO. Also, from the Fig. 5b, the accumulated heat release rate has been observed to decrease with the increase in proportion of TWTO in the blend due to its poor combustion, When comparing the combustion characteristics of different TWTO blends with diesel, B25 is scrutinized to show comparable peak heat release rate with diesel, while all other blends showed reduced peak heat release rate. Notably, almost all the properties of B25 were found to be closer to diesel and this is why, the combustion characteristics were observed to be akin to that of diesel. In the same note, the diffusion combustion phase happens to be in par with diesel for B25 and happens to get less pronounced with the increase in proportion of TWTO in the blend.
Table 4 . Instrument details. Instrument
Measurement
Accuracy
Uncertainty
Burette Load cell Speed Sensor Temperature indicator Exhaust gas analyzer
Fuel Consumption Loading device Speed Exhaust gas measurement CO HC NOX Smoke density Cylinder pressure Crank angle
± 0.2 cc ± 10 N ± 10 rpm ± 2 °C ± 0.02% ± 10 ppm ± 12 ppm ±1 ± 0.3 kg ± 1°
1.5 0.2 1.0 0.30 0.2 0.1 0.2 1 0.3 1
Smoke Pressure transducer Crank angle encoder
the liquid and bead will be higher as the diameter decreases beyond that value. The change in droplet lifetime as a function of air temperature for both B50 and B100 is shown in Fig. 4b. As the evaporation constant increases, the droplet lifetime decreases and it is evident from the current study of TWTO- diesel blends. It is clear that when diesel is added to the fuel, the droplet lifetime decreases due to higher heating rate at higher air temperature and higher volatility of the fuel mixture. Further, as the air temperature is increased, droplet lifetime decreases as droplet evaporates faster at higher temperature and it is interesting to note that the difference in droplet lifetime for B100 at 100 °C and 150 °C is higher, while this is observed to be small for B50.
3.2. Engine characteristics for TWTO The combustion process in a diesel engine is influenced by the physical attributes such as fuel spray and evaporation as well as the chemical kinetics governing the fuel oxidation reactions [33]. As such, the amount of energy being released, in-cylinder pressure as well as temperature is duly dependent on the above factors and it is worth 4
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Fig. 3. Time evolution of droplet squared diameter with respect to time for B50 and B100 at air temperature of a) 100 °C b) 150 °C.
such, combustion is more active for it on the lines of diesel and thus BTE for it is akin to that of diesel. As an implication of drop in engine performance with the increase in proportion of TWTO in the blend, BSFC of the engine is noted to be decreased, as evident from Fig. 6b. Basically, fuel consumption depends on the calorific value of the fuel [39], while other physical and thermal properties would also have an impact on it. In our case, the calorific value of TWTO does not seems to have been affected BSFC, given calorific value of it is closer to diesel, in contrary to vegetable oil based fuel. Therefore, increase in BSFC with the increase in TWTO in the blend could be attributed to the deterioration in combustion process in light of its antagonist fuel properties such as viscosity, flash point and boiling point. Conceptually, in the event of decline in combustion, the fuel to air equivalence ratio increased so as to perpetuate the power being produced by the engine. As such, BSFC for B100 was recognized to be 9.1% and 14.3% higher than that of diesel at low and full load conditions, respectively. However, the fuel consumption for B25 remains to be in agreement with diesel, as blending of TWTO in lower proportion with diesel doesn’t warrants for compromise in fuel
BTE of the engine invariably relies on the degree of combustion [36], which in turn depends on the properties of the fuel being used in a diesel engine. Fig. 6a, depicting the variation of BTE for TWTO – diesel blends, shows an increasing trend with respect to load and a decreasing trend with increase in proportion of TWTO in the blend. However, among all the blends, B25 shows a comparable BTE with diesel, while it was observed to be lower for B100. Comparatively, TWTO is a heavier molecule than diesel, ascertained based on its higher viscosity and boiling point, rendering the dispersion of fuel jet into finer droplet very difficult and thereby affecting the combustion process. Further, as inferred from fundamental study on droplet evaporation, evaporation constant and droplet lifetime are recognized to be lower for blends with higher proportion of TWTO, due to higher boiling and flash point of the fuel. As such, uniform mixing of air/fuel mixture is not appreciable and this is deemed not to dully promote the combustion process for higher blends of TWTO and thereby, reducing BTE. Past reports on declined BTE for certain fuels owing to above mentioned reasons [37,38], complies with the results of current study. Nonetheless, B25 has comparable viscosity, boiling point and flash point with that of diesel and as 5
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Fig. 4. Variation of a) evaporation constant and b) evaporation time for B50 and B100 with respect to air temperature.
Fig. 5. (a) Heat release rate and (b) cumulative heat release rate curve for TWTO blends under full load conditions.
properties as against the higher blends. Gaseous emissions such as CO, smoke and NOX were captured for TWTO – diesel blends and the rationale for observed occurrences in their trend are descried herein. The variation of CO emission with the increase in BP, as depicted in Fig. 7, portrays an increasing trend due to the increase in fuel to air equivalence ratio. With the increase in proportion of TWTO in the blend, the CO emission increases as the oxidation of larger droplets of higher TWTO blends in the confines of combustion chamber is affected and this culminates in incomplete combustion. In hindsight, the amount of fuel being injected is higher for higher TWTO blends in order to maintain the required power output and this increases the fuel to air equivalence ratio, decreasing the prevalence of oxygen that is required for the proper oxidation of CO. Further, the in-cylinder temperature for the higher blends of TWTO is believed to be lower on the grounds of reduction in magnitude of peak heat release rate, as detailed above. The increased fuel to air equivalence ratio, coupled by the lowered in-cylinder temperature, impedes CO oxidation and as a result, CO emission was noted to be increased for higher blends of TWTO than lower blends. In respect of better fuel properties for B25, CO emission are evinced to be in the closer vicinity of diesel at low and full load conditions, respectively. The smoke emission for various blend fuels, which is understood to have been formed during diffusion combustion zone, has been exhibited in Fig. 8. As evident from the cumulative heat release rate(Fig. 5b), the accumulated heat release rate is figured out to be lower with the increase in proportion of TWTO in the blend, indicating much less
pronounced diffusion combustion phase. As such, the soot precursors formed during the premixed combustion phase are not duly oxidized in the rather less active diffusion combustion phase. In addition, poor fuel atomization would not pave way for the invasion of oxygen molecules in to the bigger droplets and thereby, inhibiting the soot oxidation process. Besides the disregard in the diffusion combustion phase, the premixed combustion is also noted to be less pronounced for higher blends of TWTO and this reduces the in-cylinder temperature so as to prohibit the oxidation of soot in the fuel rich regions of spray to some extent. In view of all these substantiations for the improper oxidation of soot, smoke emission for B100 is elucidated to be higher than diesel by 9.6%. On the other hand, the smoke emission for B25 is duly noted to be comparable to that of diesel for the justifications as accounted above. With the general token that much pronounced premixed combustion phase would elevate in-cylinder temperature to promote NOX formation [40], it is appropriate to note a reduction in NOX emission for B100, as it exhibits much lower peak heat release rate than all other blends. Evidently, the much lower in-cylinder temperature has forbidden active combustion and therefore, NOX emission is deemed to be reduced by 17.8% than B25, as evinced in Fig. 9. Adding to this, the deprivation in oxygen due to increased fuel to air equivalence ratio further de-promoted NOX formation and thereby, reduced it. Over and all, B25 showed comparable NOX emission with diesel, emerging as an optimum blend for its operation in an unmodified diesel engine (Fig. 10).
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Fig. 8. CO (carbon monoxide) emission for TWTO blends.
Fig. 6. In-cylinder pressure curve for TWTO blends under full load condition.
Fig. 9. Smoke emission for TWTO blends.
Fig. 7. (a) BTE (brake thermal efficiency) (b) BSFC (brake specific fuel consumption) for TWTO blends.
Fig. 10. NOX (nitrogen oxides) emission for TWTO blends.
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4. Conclusion [16]
Considering very few attempts to perform a fundamental study along with an engine experimental work, endeavors to accomplish both have been made in this study. As such, we have considered to zero-in waste transformer oil for the fundamental and engine study. Given waste transformer oil cannot be used as such in a diesel engine, it was purified and refined through conventional trans-esterification process. Categorically, the properties of TWTO were found to be appropriate for diesel engine application and suitable blends of it with diesel are tested in a diesel engine. Prior to that, fundamental study on fuel evaporation was accomplished by conducting suspended droplet experimental study. The fundamental study revealed that the evaporation characteristics such as droplet regression, evaporation rate and time were far better for B50 than that of B100. From the engine experimental study, the combustion characteristics such as peak heat release rate and in-cylinder pressure decreased with increase in proportion of TWTO in the blend, while these were noted to be in par with diesel for B25. On the other hand, BTE for B25 was figured out to be incompliance with diesel and so are the gaseous emissions such as CO, smoke and NOX. Since the operation of higher blends suffered loss in performance and emission, modifications pertaining to engine or fuel are desiderate to attain enhanced engine characteristics in the near future.
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performance and exhaust emissions of a diesel engine fuelled with Ceibapentandra biodiesel blends. Energy Convers Manage 2013;76:828–36. Arpa O, Yumruta R, Argunhan Z. Experimental investigation of the effects of diesellike fuel obtained from waste lubrication oil on engine performance and exhaust emission. Fuel Process Technol 2010;91:1241–9. Murugan S, Ramaswamy M, Nagarajan G. Performance, emission and combustion studiesofa DI diesel engine using Distilled Tyre pyrolysis oil-diesel blends. Fuel Process Technol 2008;89:152–9. Behera P, Murugan S. Combustion, performance and emission parameters of used transformer oil and its diesel blends in a DI diesel engine. Fuel 2013;104:147–54. Behera P, Murugan S, Nagarajan G. Dual fuel operation of used transformer oil with acetylene in a DI diesel engine. Energy Convers Manage 2014;87:840–7. Phan AN, Phan TM. Biodiesel production from waste cooking oils. Fuel 2008;87:3490–6. Sayin C, Gumus M. Impact of compression ratio and injection parameters on the performance and emissions of a DI diesel engine fueled with biodiesel-blended diesel fuel. Appl Therm Eng 2011;31:3182–8. Jaichandar S, Annamalai K. Effects of open combustion chamber geometries on the performance of pongamia biodiesel in a DI diesel engine. Fuel 2012;98:272–9. Mohan B, Yang W, Kiang Chou S. Fuel injection strategies for performance improvement and emissions reduction in compression ignition engines—a review. Renew Sustain Energy Rev 2013;28:664–76. Vallinayagam R, Vedharaj S, Yang W, Raghavan V, Saravanan C, Lee P, et al. Investigation of evaporation and engine characteristics of pine oil biofuel fumigated in the inlet manifold of a diesel engine. Appl Energy 2014;115:514–24. Morin C, Chauveau C, Gökalp I. Droplet vaporisation characteristics of vege table oil derived biofuels at high temperatures. Exp Therm Fluid Sci 2000;21:41–50. Broustail G, Seers P, Halter F, Moréac G, Mounaïm-Rousselle C. Experimental determination of laminar burning velocity for butanol and ethanol iso-octane blends. Fuel 2011;90:1–6. Chong CT, Hochgreb S. Measurements of laminar flame speeds of liquid fuels: JetA1, diesel, palm methyl esters and blends using particle imaging velocimetry (PIV). Proc Combust Inst 2011;33:979–86. Kim HJ, Suh HK, Park SH, Lee CS. An experimental and numerical investigation of atomization characteristics of biodiesel, dimethyl ether, and biodiesel-ethanol blended fuel. Energy Fuels 2008;22:2091–8. Sivakumar P, Sindhanaiselvan S, Gandhi NN, Devi SS, Renganathan S. Optimization and kinetic studies on biodiesel production from underutilized CeibaPentandra oil. Fuel 2013;103:693–8. Ramadhas AS, Jayaraj S, Muraleedharan C. Biodiesel production from high FFA rubber seedoil. Fuel 2005;84:335–40. Manjunath M, Prakash P, Raghavan V, Mehta PS. Composition effects on thermophysical properties and evaporation of suspended droplets of biodiesel fuels. SAE Technical Paper 2014. Morin C, Chauveau C, Dagaut P, Goekalp I, Cathonnet M. Vaporization and oxidation ofliquid fuel droplets at high temperature and high pressure: application to n-alkanes and vegetableoil methyl esters. Combust Sci Technol 2004;176:499–529. Ganesan V. Internal combustion engines. McGraw Hill Education (India) Pvt Ltd; 2012. Gumus M, Kasifoglu S. Performance and emission evaluation of a compression ignition engine using a biodiesel (apricot seed kernel oil methyl ester) and its blends with diesel fuel. Biomass Bioenergy 2010;34:134–9. Arbab M, Masjuki H, Varman M, Kalam M, Imtenan S, Sajjad H. Fuel properties, engine performance and emission characteristic of common biodiesels as a renewable and sustainable source of fuel. Renew Sustain Energy Rev 2013;22:133–47. Vallinayagam R, Vedharaj S, Yang W, Lee P, Chua K, Chou S. Combustion performance and emission characteristics study of pine oil in a diesel engine. Energy 2013;57:344–51. Ramadhas A, Muraleedharan C, Jayaraj S. Performance and emission evaluation of a diesel engine fueled with methyl esters of rubber seed oil. Renew Energy 2005;30:1789–800. Chauhan BS, Kumar N, Cho HM. A study on the performance and emission of a diesel engine fueled with Jatropha biodiesel oil and its blends. Energy. 2012;37:616–22. Vallinayagam R, Vedharaj S, Yang W, Saravanan C, Lee P, Chua K, et al. Impact of ignitionpromoting additives on the characteristics of a diesel engine powered by pine oil–diesel blend. Fuel 2014;117:278–85. Vedharaj S, Vallinayagam R, Yang W, Chou S, Lee P. Effect of adding 1, 4-Dioxane with kapok biodiesel on the characteristics of a diesel engine. Appl Energy 2014;136:1166–73.
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Fuel journal homepage: www.elsevier.com/locate/fuel
Fundamental droplet evaporation and engine application studies of an alternate fuel produced from waste transformer oil
T
⁎
S. Prasanna Raj Yadava, C.G. Saravananb, , S. Karthicka, K. Senthilnathana, A. Gnanaprakasha a b
Department of Mechanical Engineering, Easwari Engineering College, Chennai, India Department of Mechanical Engineering, Annamalai University, Chidambaram, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Waste transformer oil Evaporation Combustion Emission Diesel engine
With a motivation to contribute in the fundamental and application aspects of alternate fuel research, which has been rarely reported in the same study, this research work has been commemorated. Herein, WTO (waste transformer oil) has been considered as a replacement for diesel as tapping the source of energy from waste products would help reduce the fuel cost. As such, WTO was subjected to fuel refinement process by transesterifying it in two stages using acid and alkali catalysts. Characteristically, the estimated physical and thermal properties of TWTO (trans-esterified waste transformer oil) were recognized to be suitable for its use in a diesel engine. Prior to the application study of testing TWTO – diesel blends in a diesel engine, fundamental study on droplet evaporation was performed through suspended droplet experimental facility to appraise their evaporation attributes. Reportedly, the droplet regression, evaporation constant and droplet lifetime were found to be superior for B50 (50% TWTO and 50% diesel) than B100 (100% TWTO). Finally, from the application study, operation of TWTO – diesel blends in a diesel engine divulged comparable performance and combustion for B25 with that of diesel. Similarly, gaseous emissions such as CO (carbon monoxide), smoke and NOX (oxides of nitrogen) were found to be in closer agreement for B25 with diesel, deducing it as an optimum blend for an unmodified diesel engine.
1. Introduction These days, alternate fuels have gained immense attention in respect of their environmental friendliness [1]. However, the availability of these bio-derived alternate fuels to replenish the decreasing petroleum fuel supply is improbable [2], leaving the efforts to attain energy independence at stake. Therefore, advent of several alternate fuels to satiate the petroleum needs of the nation is imperative and in wake of this, researchers have embarked on to harness energy from every possible source [3]. Most of these devised alternate fuels are capitalized for engine application [4] and during the course of development of these fuels, researchers have concocted plethora of strategies for optimizing the production of them [5]. In recent times, investigation of biodiesel, an ester produced from variety of vegetable oils, in a diesel engine has been reported at aplenty and most of the studies concede to the improvement in engine performance in view of thermal efficiency [6] and fuel consumption. Similarly, the engine out emissions such as HC and CO was reported to be decreased, due to the presence of inherent oxygen within the fuel, at the expense of increased NOX emission [7]. However, most of the
⁎
revelations of researchers and engine manufactures have underscored the limitations of deterioration in engine performance and emissions for higher blends of biodiesel with diesel due to the shortcomings with the fuel properties such as increased viscosity, lower calorific value, higher boiling point and flash point [8,9]. By this trend, the nominal blend suitable for its operation in a diesel engine without any modifications is B20 (20% biodiesel and 80% diesel) [10,11], while higher blends do require revamp of engine design or modification with engine operating parameters. Recent research on alternate fuels harps on capitalizing waste oils as source of fuel as it would not only amount to replace diesel but also enables safe disposal of waste materials. In this regard, as a measure to reap the required sources of energy from agricultural and industrial waste products, research on production of biodiesel from kapok oil and CNSL (cashew shell nut liquid) for diesel engine application have been contended [12,13]. While kapok oil is extracted from waste and discarded kapok seeds, CNSL is being produced from the outer shell of cashew nuts, which are thrown out as waste products [14,15]. It is worthwhile to note that the adoption of WTO (waste transformer oil) in a diesel engine, as a measure to harness energy and realize safe disposal
Corresponding author. E-mail address: [email protected] (C.G. Saravanan).
https://doi.org/10.1016/j.fuel.2019.116253 Received 1 August 2019; Received in revised form 4 September 2019; Accepted 21 September 2019 0016-2361/ © 2019 Published by Elsevier Ltd.
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of it, has also been accomplished recently [16,17]. It is worthwhile to note that the adoption of these waste products in diesel engine not only churns out additional sources of fuel [18,19], but also enables reduction of fuel cost unlike vegetable oil based fuels, given the cost of the vegetable oils are reported to have been 2 times higher than that of conventional petroleum fuels [20]. Most of the research activities on characterization and utilization of alternate fuels in a diesel engine have targeted only on the documentation of engine performance and pollutant emission. In addition, few optimization studies to help accustom higher blends of biodiesel in diesel engine such as modification of engine design or operating parameters have also been brought to light [21–23]. However, fundamental studies on fuel spray, evaporation and flame are imperative to comprehend the underlying physics mechanisms preceding the combustion process. As such, few studies reported the evaporation attributes of biodiesel such as droplet regression, evaporation constant and time through suspended droplet experimental study [24,25]. Also, few researchers have also have given a good account of flame characteristics of certain biofuels, which report the flame speed and stability of biofuels under laminar and turbulent conditions [26,27]. Preceding evaporation and combustion, fuel jet from the diesel injector disperses in to spray and the evaluation of the spray characteristic entails measurement of spray tip penetration, cone angle, particle size and velocity. As such, researchers have also delved into the estimation of the reported macroscopic and microscopic spray characteristics for biofuels at pressures prevailing in a diesel engine [28]. The documentation of these fundamental data on evaporation and flame would provide the option to develop chemical kinetic model and build a comprehensive database, which could be latter availed to model combustion and emission after studying the fuel chemical kinetics. It is indispensable to perform a fundamental study while conducting engine experiments and amalgamate them to attain deeper insights. Therefore, besides performing an engine study, fundamental study on fuel evaporation through suspended droplet experimental study has been performed so as to contribute both in fundamental and application front. Herein, waste transformer oil has been considered and refined through trans-esterification process to make the properties conducive for its use in a diesel engine. Initially, the evaporation characteristics of blends of TWTO (trans-esterified waste transformer oil) with diesel viz B50 and B100 such as droplet regression, evaporation rate and time were studied by suspended droplet experimental study. Subsequently, various blends of TWTO with diesel as shown in Table1 (B25, B50, B75 and B100) were tested in a single cylinder diesel engine and the engine characteristics such combustion, performance and emission were analyzed and reported.
Table 2 Thermal and physical properties of the blend fuels. Property
Measurement Standards
Diesel
50% HCF (B50)
100% HCF (B100)
Kinematic viscosity at 40 °C (CSt) Fire Point (°C) Flash Point (°C) Boiling point (°C) Density at15°C (g/cc) Calculated cetane index
ASTM D445
2.57
3.3
4.03
ASTM ASTM ASTM ASTM ASTM
75 56 250 0.8072 50
89 68 300 0.8234 51.5
93 85 350 0.8355 52
D92 D92 D240 D1298 D976
greater than 2 mg KOH per gram of oil, subjecting it to alkaline transesterification process would render the yield of hydrocarbon fuel unproductive, as inferred from the reports of previous studies [29,30]. Therefore, it is prudent to pretreat the refined WTO with sulphuric acid in the first stage and then commemorate the alkaline trans-esterification process using alcohol, methanol, and alkali, KOH in the second stage. After the two-stage trans-esterification process, the mixture was allowed to settle so as to separate TWTO and glycerol, followed by draining the glycerol from the bottom. Subsequently, washing process was performed by using a sprinkler, which slowly sprinkled water into the oil container and water-hydrocarbon mixture was agitated gently for 20 min, allowing water to settle out of the hydrocarbon mixture. The pure hydrocarbon fuel, produced by these processes, is then examined for its fuel properties as per ASTM standard method and are reported in Table 2. After the double stage trans-esterification process, the fuel viscosity has been ably reduced with an improvement in cetane number and even all other fuel properties were noted to be in compliance with the required standards, making it appropriate for use in diesel engine applications. 2.2. Suspended droplet experimental setup and procedure Evaporation characteristics of TWTO (trans-esterified waste transformer oil) and its diesel blends have been ascertained by a suspended droplet experimental set up. A schematic of the experimental set up is shown in Fig. 1. The experimental arrangement entails a cylindrical tunnel surrounded by a heating tape, quartz fiber with a bead, air compressor, a digital camera and a light source. Through a syringe,
2. Methodology 2.1. Extraction procedure of hydrocarbon fuel The hydrocarbon fuel used in this study was extracted from waste transformer oil by traditional trans-esterification process, enabling breakdown of higher hydrocarbons to lower ones. Preceding the chemical treatment process, collected WTO was treated and purified, as the prolonged use of it in transformers must have contaminated the oil with impurities. Since the total acidity of refined WTO was perceived to be Table 1 Details of the test fuel composition. Sl. No
Fuel
Fuel blended (% vol)
1. 2. 3. 4. 5.
Diesel 25% HCF 50% HCF 75% HCF 100% HCF
100% diesel fuel 25% hydrocarbon fuel + 75% diesel fuel 50% hydrocarbon fuel + 50% diesel fuel 75% hydrocarbon fuel + 25% diesel fuel 100% hydrocarbon fuel
Fig. 1. Schematic diagram of experimental setup for evaporation study. 2
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certain quantity of fuel is impinged over the quartz fiber bead and care is taken to maintain the initial – diameter of the droplet as maximum as possible. The average velocity of air exiting the vertical tunnel is maintained at 0.15 m/s, while the temperature of the air, heated by the heating tape, has been maintained in the range of 100 °C–150 °C. Significantly, the temperature is recorded at the point of suspension of the droplet every time to avoid discrepancies with the measured values. When hot air impinges over the suspended droplet, the droplet gets evaporated and the time evolution of change in droplet diameter is captured using high definition video camera at the rate of 30 frames per second. Subsequently, the high definition video is being converted into digital images using Image J tool and in the image processing, the extracted images are processed using a MATLAB code. The size of the droplet, which reduces as time passes, is processed by the code and surface regression of the droplet with respect to time is obtained as output. In-order to enhance the accuracy of the measured data, the experimentation is repeated for five times.
operating point, the combustion parameters were also processed and stored in the personal computer. Similarly, the time taken for consumption of 10CC of fuel and other gaseous emissions were also measured and recorded for subsequent analysis. The tests were repeated for three times so as to improve the accuracy of the noted readings and each test was done for 3 h. Finally, the average value of the three readings was taken for the calculation Also, an uncertainty analysis was performed using the method of propagation of errors and the total uncertainty of this experiment is calculated from the uncertainty of various parameters and equipment’s as listed in Table 4 and found to be ± 2.79%. 3. Results and discussion 3.1. Evaporation characteristics for TWTO The time evolution of square of droplet diameter (representing its surface area regression) is designated as the droplet regression rate. Typically, the regression curves at 100 °C and 150 °C, as noted from Fig. 3a and 3b, almost follow the conventional d2-law. While volumetric expansion and unsteady evaporation regime are evident observed at lower air temperature (100 °C), these are figured out to be very small at higher air temperature (150 °C) and the curve mostly manifests a steady evaporation regime. Notably, the initial heat up period is perceived to be prominent for B100 at 100 °C on account of volumetric expansion of the droplet when hot air is impinged over it and therefore, the evaporation of B100 droplet is deemed to be gradual. On the other hand, B50 seldom shows a heat up period even at an air temperature of 100 °C, as faster preferential vaporization of diesel is the cause of this behavior. Evidently, this can be noted by the lower flash and boiling point of B50 when compared to those of B100, as evident from Table 2 and thus fuel properties are deemed to affect the evaporation characteristics of test fuels. With an increase in the air temperature to 150 °C, the evaporation characteristics of both the test fuels are enhanced and the initial heat up period is barely noticed in their regression curves. It is clear that the slope of the regression curves, indicated by trend lines, increases as the air temperature is increased and as a result, the droplet lifetime is seen to decrease. Between B50 and B100, at 150 °C, the differences in vaporization characteristics is reduced, when compared to the air temperature of 100 °C; the slope of the trend line is increased by 1.4 and 2.9 times for B50 and B100, respectively, when the air temperature is increased from 100 °C to 150 °C. Previously, researchers reported lower evaporation rates for the vegetable oil based methyl ester due to its distinct fuel properties [31,32] and similar observations are recorded in the present study with TWTO. From the droplet regression curve, two crucial evaporation characteristics of test fuels, namely evaporation rate constant and droplet lifetime, can be obtained. Evaporation rate constant is the absolute rate of change of surface regression curve with respect to time. This is computed by fitting a linear trend line after eliminating the initial heat up period (if present). While doing this, it is ensured that the value of regression coefficient (R2) is above 0.9 and the evaporation constant has been computed as follows, Evaporation constant (mm2/s) = (d0)2 × Slope of the linear trend line The variation of the evaporation constant for B50 and B100, as a function of air temperature is shown in Fig. 4a. The evaporation rate increases with the increase in air temperature, as well as with the addition of diesel. Similar to evaporation constant, droplet lifetime is also computed from regression curve which is indeed obtained by extrapolating the trend line until the droplet diameter reaches a zero value and noting the corresponding time. This extrapolation is required, as the suspended droplet experiments cannot record zero diameter, as the minimum diameter that can be reached is the diameter of the bead of the quartz suspender. In fact, the experimental data will not be extended beyond 1.3 times the bead diameter, as interferences between
2.3. Engine experimental setup and procedure The compression ignition engine used for the study was Kirloskar TV1, single cylinder, four stroke, constant speed, vertical, water cooled and direct injection diesel engine, and the detailed engine specification are given in Table 3. The engine was coupled with an eddy current dynamometer, as schematically described in Fig. 2, to apply different loading condition from low to full load condition. Significantly, the fuel injection system entails a mechanical fuel pump with 3 hole injector nozzle assembly wherein, the fuel injection pressure and timing are fixed at 220 bar and 23°CA BTDC. The fuel supplied to the engine is measured manually using a burette and stopwatch. Indeed, for this constant speed engine that runs at 1500 rpm, the airflow rate is constant and is measured by an u-tube manometer fitted in the inlet manifold. The engine speed was measured by an in-house designed magnetic pickup sensor, connected to frequency meter. Measurement of combustion chamber pressure was obtained by installing an AVL make water-cooled piezoelectric pressure transducer with the sensitivity 16:11 PC/bar, in the cylinder head portion of the combustion chamber. Characteristically, an AVL 3057 charge amplifier converts the charge yielded by the piezoelectric transducer into proportional electric signals. Finally, a personal computer (PC) interfaced with an AVL 619 Indimeter Hardware and Indwin – software version 2.2 data acquisition system collects combustion parameter data that has been averaged over 100 cycles. From the in-cylinder pressure data, the heat release rate and other associated combustion parameters are evaluated using standard formulations. Gaseous emissions such as NOx (nitrous oxides), CO (Carbon monoxide) and HC (Hydrocarbon) were measured by an AVL 444 di gas analyzer in accordance with NDIR (non-dispersive infrared) principle. AVL 437 smoke meter, based on light extinction principle, measured the smoke intensity of the engine exhaust gases. The engine was started, warmed up at every stage and the cold water as well as lubricating oil temperature was maintained accordingly. At each Table 3 . Test engine specification. Make
Kirlosar-TV 1
General details
Four stroke, compression ignition, constant Speed, vertical, water cooled, direct injection One 87.5 mm 110 mm 17.5:1 1500 rpm 5.2 kW 220 bar 23° bTDC SAE 40
Number of cylinder Bore Stroke Compression ratio Rated speed Rated output Injection pressure Fuel injection timing Lubricating oil
3
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Fig. 2. Schematic diagram of experimental setup for engine study.
analyzing these characteristics for TWTO – diesel blends in a diesel engine. Fig. 5a and 5b depicts the heat release rate and cumulative heat release rate, respectively, for TWTO – diesel blends at full load condition. With the increase in proportion of TWTO in the blend, the magnitude of peak heat release rate decreases and so does is the trend with the diffusion combustion phase too. Since TWTO is more viscous than diesel [34], the atomization and evaporation of it is anticipated to be poor, hindering the preparation of homogenous air/fuel mixture. Further due to the cetane number of TWTO the ignition delay period is shortened so as to advance the SOC. With this, the formation of well mixed air/fuel mixture is further minimized and the amount of fuel being burnt in premixed combustion phase is decreased, reducing the magnitude of peak heat release rate for higher TWTO blends, with B100 evincing a decrease of 13.1% than diesel. Inference on the reduction in peak heat releases rate for the fuels having higher viscosity and boiling point, similar to TWTO, complies with the results portrayed in the current study. Normally, vegetable oil based fuels tend to possess lower calorific value [35] than diesel and this is deemed to decrease the peak heat release rate for them. However, unlike biodiesel, TWTO doesn’t suffer much drop in calorific value than that of diesel and therefore, it has nothing to do with the reduction in magnitude of peak heat release rate for TWTO. Also, from the Fig. 5b, the accumulated heat release rate has been observed to decrease with the increase in proportion of TWTO in the blend due to its poor combustion, When comparing the combustion characteristics of different TWTO blends with diesel, B25 is scrutinized to show comparable peak heat release rate with diesel, while all other blends showed reduced peak heat release rate. Notably, almost all the properties of B25 were found to be closer to diesel and this is why, the combustion characteristics were observed to be akin to that of diesel. In the same note, the diffusion combustion phase happens to be in par with diesel for B25 and happens to get less pronounced with the increase in proportion of TWTO in the blend.
Table 4 . Instrument details. Instrument
Measurement
Accuracy
Uncertainty
Burette Load cell Speed Sensor Temperature indicator Exhaust gas analyzer
Fuel Consumption Loading device Speed Exhaust gas measurement CO HC NOX Smoke density Cylinder pressure Crank angle
± 0.2 cc ± 10 N ± 10 rpm ± 2 °C ± 0.02% ± 10 ppm ± 12 ppm ±1 ± 0.3 kg ± 1°
1.5 0.2 1.0 0.30 0.2 0.1 0.2 1 0.3 1
Smoke Pressure transducer Crank angle encoder
the liquid and bead will be higher as the diameter decreases beyond that value. The change in droplet lifetime as a function of air temperature for both B50 and B100 is shown in Fig. 4b. As the evaporation constant increases, the droplet lifetime decreases and it is evident from the current study of TWTO- diesel blends. It is clear that when diesel is added to the fuel, the droplet lifetime decreases due to higher heating rate at higher air temperature and higher volatility of the fuel mixture. Further, as the air temperature is increased, droplet lifetime decreases as droplet evaporates faster at higher temperature and it is interesting to note that the difference in droplet lifetime for B100 at 100 °C and 150 °C is higher, while this is observed to be small for B50.
3.2. Engine characteristics for TWTO The combustion process in a diesel engine is influenced by the physical attributes such as fuel spray and evaporation as well as the chemical kinetics governing the fuel oxidation reactions [33]. As such, the amount of energy being released, in-cylinder pressure as well as temperature is duly dependent on the above factors and it is worth 4
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Fig. 3. Time evolution of droplet squared diameter with respect to time for B50 and B100 at air temperature of a) 100 °C b) 150 °C.
such, combustion is more active for it on the lines of diesel and thus BTE for it is akin to that of diesel. As an implication of drop in engine performance with the increase in proportion of TWTO in the blend, BSFC of the engine is noted to be decreased, as evident from Fig. 6b. Basically, fuel consumption depends on the calorific value of the fuel [39], while other physical and thermal properties would also have an impact on it. In our case, the calorific value of TWTO does not seems to have been affected BSFC, given calorific value of it is closer to diesel, in contrary to vegetable oil based fuel. Therefore, increase in BSFC with the increase in TWTO in the blend could be attributed to the deterioration in combustion process in light of its antagonist fuel properties such as viscosity, flash point and boiling point. Conceptually, in the event of decline in combustion, the fuel to air equivalence ratio increased so as to perpetuate the power being produced by the engine. As such, BSFC for B100 was recognized to be 9.1% and 14.3% higher than that of diesel at low and full load conditions, respectively. However, the fuel consumption for B25 remains to be in agreement with diesel, as blending of TWTO in lower proportion with diesel doesn’t warrants for compromise in fuel
BTE of the engine invariably relies on the degree of combustion [36], which in turn depends on the properties of the fuel being used in a diesel engine. Fig. 6a, depicting the variation of BTE for TWTO – diesel blends, shows an increasing trend with respect to load and a decreasing trend with increase in proportion of TWTO in the blend. However, among all the blends, B25 shows a comparable BTE with diesel, while it was observed to be lower for B100. Comparatively, TWTO is a heavier molecule than diesel, ascertained based on its higher viscosity and boiling point, rendering the dispersion of fuel jet into finer droplet very difficult and thereby affecting the combustion process. Further, as inferred from fundamental study on droplet evaporation, evaporation constant and droplet lifetime are recognized to be lower for blends with higher proportion of TWTO, due to higher boiling and flash point of the fuel. As such, uniform mixing of air/fuel mixture is not appreciable and this is deemed not to dully promote the combustion process for higher blends of TWTO and thereby, reducing BTE. Past reports on declined BTE for certain fuels owing to above mentioned reasons [37,38], complies with the results of current study. Nonetheless, B25 has comparable viscosity, boiling point and flash point with that of diesel and as 5
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Fig. 4. Variation of a) evaporation constant and b) evaporation time for B50 and B100 with respect to air temperature.
Fig. 5. (a) Heat release rate and (b) cumulative heat release rate curve for TWTO blends under full load conditions.
properties as against the higher blends. Gaseous emissions such as CO, smoke and NOX were captured for TWTO – diesel blends and the rationale for observed occurrences in their trend are descried herein. The variation of CO emission with the increase in BP, as depicted in Fig. 7, portrays an increasing trend due to the increase in fuel to air equivalence ratio. With the increase in proportion of TWTO in the blend, the CO emission increases as the oxidation of larger droplets of higher TWTO blends in the confines of combustion chamber is affected and this culminates in incomplete combustion. In hindsight, the amount of fuel being injected is higher for higher TWTO blends in order to maintain the required power output and this increases the fuel to air equivalence ratio, decreasing the prevalence of oxygen that is required for the proper oxidation of CO. Further, the in-cylinder temperature for the higher blends of TWTO is believed to be lower on the grounds of reduction in magnitude of peak heat release rate, as detailed above. The increased fuel to air equivalence ratio, coupled by the lowered in-cylinder temperature, impedes CO oxidation and as a result, CO emission was noted to be increased for higher blends of TWTO than lower blends. In respect of better fuel properties for B25, CO emission are evinced to be in the closer vicinity of diesel at low and full load conditions, respectively. The smoke emission for various blend fuels, which is understood to have been formed during diffusion combustion zone, has been exhibited in Fig. 8. As evident from the cumulative heat release rate(Fig. 5b), the accumulated heat release rate is figured out to be lower with the increase in proportion of TWTO in the blend, indicating much less
pronounced diffusion combustion phase. As such, the soot precursors formed during the premixed combustion phase are not duly oxidized in the rather less active diffusion combustion phase. In addition, poor fuel atomization would not pave way for the invasion of oxygen molecules in to the bigger droplets and thereby, inhibiting the soot oxidation process. Besides the disregard in the diffusion combustion phase, the premixed combustion is also noted to be less pronounced for higher blends of TWTO and this reduces the in-cylinder temperature so as to prohibit the oxidation of soot in the fuel rich regions of spray to some extent. In view of all these substantiations for the improper oxidation of soot, smoke emission for B100 is elucidated to be higher than diesel by 9.6%. On the other hand, the smoke emission for B25 is duly noted to be comparable to that of diesel for the justifications as accounted above. With the general token that much pronounced premixed combustion phase would elevate in-cylinder temperature to promote NOX formation [40], it is appropriate to note a reduction in NOX emission for B100, as it exhibits much lower peak heat release rate than all other blends. Evidently, the much lower in-cylinder temperature has forbidden active combustion and therefore, NOX emission is deemed to be reduced by 17.8% than B25, as evinced in Fig. 9. Adding to this, the deprivation in oxygen due to increased fuel to air equivalence ratio further de-promoted NOX formation and thereby, reduced it. Over and all, B25 showed comparable NOX emission with diesel, emerging as an optimum blend for its operation in an unmodified diesel engine (Fig. 10).
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Fig. 8. CO (carbon monoxide) emission for TWTO blends.
Fig. 6. In-cylinder pressure curve for TWTO blends under full load condition.
Fig. 9. Smoke emission for TWTO blends.
Fig. 7. (a) BTE (brake thermal efficiency) (b) BSFC (brake specific fuel consumption) for TWTO blends.
Fig. 10. NOX (nitrogen oxides) emission for TWTO blends.
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4. Conclusion [16]
Considering very few attempts to perform a fundamental study along with an engine experimental work, endeavors to accomplish both have been made in this study. As such, we have considered to zero-in waste transformer oil for the fundamental and engine study. Given waste transformer oil cannot be used as such in a diesel engine, it was purified and refined through conventional trans-esterification process. Categorically, the properties of TWTO were found to be appropriate for diesel engine application and suitable blends of it with diesel are tested in a diesel engine. Prior to that, fundamental study on fuel evaporation was accomplished by conducting suspended droplet experimental study. The fundamental study revealed that the evaporation characteristics such as droplet regression, evaporation rate and time were far better for B50 than that of B100. From the engine experimental study, the combustion characteristics such as peak heat release rate and in-cylinder pressure decreased with increase in proportion of TWTO in the blend, while these were noted to be in par with diesel for B25. On the other hand, BTE for B25 was figured out to be incompliance with diesel and so are the gaseous emissions such as CO, smoke and NOX. Since the operation of higher blends suffered loss in performance and emission, modifications pertaining to engine or fuel are desiderate to attain enhanced engine characteristics in the near future.
[17]
[18] [19] [20] [21]
[22] [23]
[24]
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