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Effect of fuel preheating with blended fuels and exhaust gas recirculation on diesel engine operating parameters
Renewable Energy Focus Volume 26, Number 00 September 2018
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ORIGINAL RESEARCH ARTICLE
Effect of fuel preheating with blended fuels and exhaust gas recirculation on diesel engine operating parameters Menelik Walle Mekonen* and Niranjan Sahoo Department of Mechanical Engineering, Indian Institute of Technology Guwahati, Guwahati 781039, India
This study investigates the influence of preheating on fuel properties of biodiesel and its blends on the performance of a diesel engine. Subsequently, the effectiveness of exhaust gas recirculation (EGR) rate for reducing the NOx emission has also been explored. At 102 C, fuel-preheating temperature, the average percentage reductions of kinematic viscosity and density of JOME was 49% and 4.3%, respectively with decrease (17.4%) in brake specific fuel consumption (BSFC) and increase (23%) of brake thermal efficiency (BTHE). The comparative study of all fuels with diesel revealed the fact that the blended fuel (40% biodiesel – 40PBD) with preheating (102 C), is seen to be most efficient when operated without EGR mode. At full load engine testing with same test fuel, BSFC reduces by 19%, BTHE increases by 16%, the exhaust gas temperature (EGT) drops by 6% with slight increase in volumetric efficiency (2%). The carbon monoxide and unburned hydrocarbon emission was reduced by 19.5% and 4.8%, respectively while oxides of nitrogen (NOx) emission was increased by 17.5% compared to diesel fuel. The inclusion of the EGR rate of 30% for 40PBD test fuel, meaningfully decreases the average NOx emission by 68.8% with almost insignificant change in engine BSFC and BTHE. Introduction The depletion of fuel reserves, rising fuel price and fast industrialization, pollution rules to mineral diesel fuels has forced many countries to search alternative source of diesel fuel energy to realize their energy requirements. Among several alternatives, methyl ester (Biodiesel) produced mainly from non-edible vegetable oils through transesterification is considered as a probable fuel sources for diesel fuel in a compression ignition engine [1]. ‘‘Methyl ester biodiesel’’ is a domestic renewable fuel generally made from vegetable, animal fats /oil, tallow and waste cooking oil. The chemical process used to convert these oils to biodiesel is called as, ‘‘transesterification’’ [2]. With respect to chemical terminology, ‘‘biodiesel’’ is defined as the mono alkyl esters of long chain fatty acids. In order to prepare biodiesel, ethanol or methanol, combined with a vegetable oil or animal fat in the presence of a catalyst can react to form ethyl esters or methyl esters (biodiesel) and glycerin. These esters are another class of oxygenates like alcohols and ethers. Thus, the biodiesels qualifies as fuel based
*Corresponding author. Mekonen, M.W. ([email protected])
on their comparative characteristics with diesel so that they can be used in compression-ignition (diesel) engines in its pure form with little or no modifications. In specific, Jatropha methyl ester, Pongamia methyl ester and Rapeseed methyl ester are typical choices for fossil diesel fuel substitutes [3]. In many countries, Jatropha curcas is being considered among the favorable choices and extensive research works have done due to its strong adaptableness to the situation, mainly in terms of draught flexibility, great endurance level and maximum seed yield [4]. Moreover, the methyl ester (biodiesels) have significant advantages such as, cleaner combustion, insignificant contribution to environmental pollution, biodegradability, non-toxic and lower emissions to diesel fuel [5]. Even though the methyl ester biodiesels have high combustion efficiency and Cetane number, they face challenges of higher NOx emission with serious difficulties of direct usages in the diesel engines [6]. Biodiesel has higher kinematic viscosity as well as density and lower heating value compared to petroleum fuel (diesel fuel). The properties of injected fuel have a significant effect on fuel injection characteristics which manipulating engine operating parameters of compression ignition engine [7,8]. Past
1755-0084/ã 2018 Elsevier Ltd. All rights reserved. https://doi.org/10.1016/j.ref.2018.07.003
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Nomenclature
Abbreviation BSFC BTHE CR EGR EGT FPT ID IP JOME PCP Vd Cd d0 QLV
Brake specific fuel consumption (kg/kW h) Brake thermal efficiency (%) Compression ratio Exhaust gas recirculation Exhaust fas temperature ( C) Fuel preheat temperature ( C) Ignition delay ( CA) Injection pressure (bar) Jatropha oil methyl ester Peak cylinder pressure (bar) Engine displacement volume (cc) Coefficent of discharge Orifice diameter (mm) Lower heating value (MJ/kg)
Symbol N, W g r t v 100D 100BD 100PBD 40PBD 60PBD ra , rf , rw DH uSC uSI
Engine speed (rpm), engine load (%) Gravitational accelaration (m/s2) Dynamometer arm length (mm) Time taken for consuming v of fuel (s) Constant volume of fuel consumed (cc) 100% diesel fuel 100% Jatropha oil methyl ester 100% preheated neat JOME 40% preheated JOME and 60% diesel fuel 60% preheated JOME and 40% diesel fuel Density of air, fuel and water (kg/m3) Water column (mm) Angle start of ignition before top dead center Angle start of fuel injection before top dead center
studies on biodiesel reported that, a direct usage of biodiesel in compression engine caused a reduction engine efficiency and increased fuel consumption [9]. Methyl ester biodiesel has lower energy content related to mineral diesel fuel leads lower BTHE of engine, hence to compensate this problem biodiesel require longer injection duration [10]. Higher kinematic viscosity of biodiesel causes larger droplet size, poor fuel atomization, narrower injection spray angle, advanced injection, incomplete combustion and injector clogging [9,11]. The kinematic viscosity of methyl ester (biodiesel) is mostly higher than diesel fuel nearly a factor of two [12,13]. Therefore, important fuel properties of biodiesel need modification in order to make improvement for utilization of biodiesel in diesel engines. Until now, numerous methods were introduced to advance the fuel properties of biodiesel, namely blending biodiesel with mineral diesel fuel, usage of chemical (additives), and heating biodiesel [14,15]. It is reported that direct implementation all of these methods to biodiesel are not fully promising and practical. Till date, blending of biodiesel is a widely used methods for reducing the kinematic viscosity and density of JOME and applied as a fuel in compression ignition engine under the ASTM blended fuel standard [16]. However, the selection of maximum possible
ORIGINAL RESEARCH ARTICLE
percentage of biodiesel ratio depends on the biodiesel feedstock and their fuel properties differ with respect to feedstock. Hence, the determination of right blend ratio is the most important criteria for utilizing biodiesel in a diesel engine. A few earlier studies indicated that, the maximum blend ratio of biodiesel currently limited to blends of 20% or less as commercial fuels for many countries in existing diesel engine operate without engine modification [17,18]. It was reported that, the brake thermal efficiency of fuel blends was slightly higher while brake specific fuel consumption was found increased with a significant reduction of CO and HC emissions and increase of NOx emission as compared to diesel fuel. These blends of biodiesel is proved to be very strong potential fuel for the existing diesel engines. Conversely, at higher percentage of blend difficulties associated to degradation of fuel performance [19]. In contrast, plenty of research studies on additives mixed with biodiesel showed that, increasing biodiesel with additive content severely affects engine performance parameters (increased fuel consumption and significantly decreased engine power output) and increased exhaust emission concentrations [20]. However, some previous experimental studies reported that, modifying fuel properties of methyl ester (biodiesel) through heating method at specific temperature before entering the engine fuel injection systems is economical and feasible. It reality it can be possible for static diesel engines without any modifications by operating with biodiesels and overall improvement in efficiency [21,22]. Preheated JOME (biodiesel) can be also be used as a diesel fuel directly or blend in a compression ignition engine. There are limited research studies for use of preheated biodiesel or biodiesel blend in a diesel engine. Many research studies show the effect of preheated methyl ester oil on fuel properties and on diesel engine parameters [22–24]. The fuel preheating of methyl ester biodiesel at optimal temperature can improve overall engine performance parameters with reductions emissions of CO and HC emission at expense of NOx emission and can be substitute a diesel fuel without any engine modification. Further, the effect of fuel preheating and blend ratios on engine performance and emission evaluation of diesel engine operated with blend mixtures has been investigated [25]. It is indicated that, 80% preheated biodiesel– diesel blend and 20% ethanol can offer reasonably performance improvements with reduction of CO, HC and NOx emissions at varying load conditions as compared diesel. The effect of exhaust gas recirculation (EGR) on performance and emission characteristics of diesel engine fueled with 20% biodiesel was investigated [26]. Among, combinations of different EGR rates, 20% EGR rate of diesel engine using 20% biodiesel fuel was suggested optimal for obtaining better performance and minimal NOx emissions. Saravanan [27] examined the effect of EGR on NOx emission of diesel engine running with different biodiesel–diesel blends of fuel. It was reported that, the NOx and smoke emissions were significantly reduced for blends with reasonable drop in engine performance and slight increase exhaust emission (CO and HC) as compared to diesel fuel. After the exhaustive literature studies, the authors present specifically, the effect of preheating on the fuel properties (viscosity and density) of JOME on performance analysis of a diesel engine. During preliminary experiments, the fuel properties Jatropha oil methyl ester (JOME) has been thoroughly examined with heating 59
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Renewable Energy Focus Volume 26, Number 00 September 2018
Renewable Energy Focus Volume 26, Number 00 September 2018
ORIGINAL RESEARCH ARTICLE
ORIGINAL RESEARCH ARTICLE FIGURE 1
Schematic diagram of experimental setup.
biodiesel from 42 C to 126 C fuel temperatures with increments of 12 C. The fuel properties of JOME improved by preheating that brings down the viscosity and density as per international biodiesel standard agreeable limits. Then for a further improvement the fuel properties is achieved by blends of biodiesel with different percentage fraction. Thus, the main objectives this paper is to investigate the combined effect of fuel preheating and blending JOME on its performance, combustion characteristics and emissions behaviors in a diesel engine. Subsequently, the influences of EGR rates on reductions of NOx emission and its effect on engine performance and emission parameters thoroughly examined. Finally, the results each tested fuels have been compared to diesel fuel baseline parameters.
Experimental methodology Test engine setup An experimental test rig is developed to undertake the desired engine experiments with different test fuels in a single cylinder a variable compression ratio (VCR) direct injection diesel engine. The test engine (Kirloskar make) at rated speed of 1500 RPM and 3.5 kW power output. The layout diagram of testbed is shown in Figure 1, while the detailed specification of the test engine is given in Table 1. The setup consists EGR system, biodiesel preheating mechanism, and fuel tanks with a fuel switching control valves, one for normal diesel fuel (100D) and the other for JOME. Fuel 60
filter (‘‘F’’ in Figure 1) is fixed at the biodiesel supply line (before preheater) for removing impurities from high viscous biodiesel and to minimize clogging of fuel supply line. An optical sensor (‘‘F1’’ in Figure 1) measures the fuel consumption and air (‘‘F2’’ in Figure 1). Engine is coupled with an eddy current dynamometer. Engine speed and load are controlled by using manually operated knob. Transducers (‘‘Pz1 and Pz2’’ in Figure 1) are used to measure the cylinder gas pressure and other one to detect the fuel injection pressure of fuel. The test engine is interfaced with exhaust gas analyzer (Make: AVL India, Model: DIGAS 444) for measuring important exhaust emission parameters samples (CO, HC and TABLE 1
Test engine specification. Engine type
Single cylinder, 4 stroke, water cooled, DI diesel engine
Bore stroke, displacement Connecting road length Compression ratio
87.5 mm 110 mm, 661 cc 234 mm 17.5:1, modified to VCR engine CR range: 12–18:1 185 mm 3.5 kW at 1500 rpm 23 BTDC fixed, Injection Variation : 0–25 BTDC 200 bar
Dynamometer arm length Rated power and speed Injection timing Injector opening pressure
Renewable Energy Focus Volume 26, Number 00 September 2018
TABLE 2
The fuel properties of JOME and diesel fuel used in the study. Property
Diesel JOME International standard Fuel specification
Density at 27 C (kg/m3) Kinematic viscosity at 40 C (mm2/s) Flash point ( C) Lower heating value (MJ/kg) Cetane number
846 3.43 68 45.61 47.48
ASTM D6751 EN14214 884 10.37 225 43.04 53.8
– 1.9–6.0 130.0 min – 47 min
860–900 3.5–5.0 120 min – 51 min
have been obtained to achieve high heat transfer coefficient. While designing a helical coil heat exchanger, the geometric effects of curvature ratio and pitch ratio have been considered by following important design steps and procedures [29,30]. The preheating device (Figure 2) consists of a helical bend copper tube aligned perpendicular for better heat exchange process and is placed in a shell maintained at a different heating temperature. A digital temperature measuring device and thermocouples have been used to regulate the temperature.
Exhaust gas recirculation system NOx). The important properties of biodiesel (JOME) and diesel mentioned in Table, were measured (offline) based on ASTM D6751 and EN 14214 standard [28].
Fuel preheating mechanisms In this study, a shell and helical coil heat exchanger developed inhouse, is chosen for fuel preheating (‘‘Hx’’ in Figure 1). Having a compact structure, the designed geometrical parameters (Table 2)
In this experimental investigation, a water-cooled EGR system (Figure 3) is being incorporated in which the exhaust gases passes in the tubes and water is circulated over these tubes. Thereby, the high exhaust gas temperature reduces but it is warmer than the intake air charge. Both intake charge and the exhaust gas mix in the air box (Figure 1) and subsequently enters into the combustion chamber of the engine. The supply of exhaust gas to the engine cylinder is controlled by using gate valves (‘‘gv1 and gv3’’ in Figure 1) and a U-tube manometer measures the flow of exhaust gases. By using
FIGURE 2
Layout diagram (a) biodiesel preheater, and (b) EGR system.
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Renewable Energy Focus Volume 26, Number 00 September 2018
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ORIGINAL RESEARCH ARTICLE FIGURE 3
The effect of fuel preheating temperature on: (a) fuel properties of JOME, (b) on engine performance parameters.
Eq. (1), the EGR rates kept as 10%, 20%, 30% and 40% for the present case [31]. The primary purpose of using EGR rate is to reduce oxides of nitrogen (NOx) emission. It may be emphasized that EGR has no significant effect on engine performance parameters if the components works properly. However, a defective system may affect engine parameters. Where, mEGR is the mass of exhaust gas (kg/h), and mtotal is total mass charge into cylinder (kg/h). mEGR EGRð%Þ ¼ 100 mtotal
ð1Þ
Experimental procedures The experimental fuel used in the present study contain diesel fuel and Jatropha Oil Methyl Ester (JOME). In this study, two major works were carried out. First, the test was conducted with neat JOME to study the effect of preheating inlet temperature on fuel properties of JOME and engine performance analysis with comparison of diesel as baseline data. In this study, tests were conducted with increasing fuel-preheating temperature up to eight different preheating temperatures (in the range of 42–126 C). Simultaneously, important tests were done to examine the outcome of preheating on performance parameters of diesel engine. Here the fuel properties of JOME were measured offline test using a hydrometer and Redwood viscometer Marton apparatus. In the entire study, attention is focused towards diesel engine’s performance, combustion and emission analysis with different test fuels such as pure biodiesel (100BD),
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preheated biodiesel (100PBD) and blend biodiesel with preheating. The blend fuels are prepared and tested in a diesel engine with preheating mode for two different blend ratios by volume i.e. 40% biodiesel – 60% diesel (40PBD) and 60% biodiesel – 40% diesel (60PBD). The individual effect was examined for engine BSFC and BTHE, peak cylinder pressure(PCP), ignition delay (ID) and emissions (CO, HC and NOx) in a compression ignition engine at varying loading conditions without EGR rate and with inclusion of EGR rate (from 0 to 40% at 100% loading condition). Here the neat JOME was preheated at 102 C through engine waste heat exhaust gases. For this purpose, the engine has to be started first with diesel fuel to warm up and brings to engine optimum operating temperature. When the engine reached its steady state, exhaust flow control valve (gv2) was opened fully to supply exhaust gases to preheater at specified temperature. Then, the preheated JOME at elevated temperature goes through insulated separate fuel pipeline to the fuel blend burette. Then, mixing takes place with diesel in different proportion or direct supply to the fuel pump for pumping to fuel injector to pressurized the fuel. During experimentation, first baseline data of diesel engine were executed with a diesel fuel at reference engine running operating conditions of CR 17.5, fuel injector opening pressure 200 bar and 23 BTDC fuel injection timing with a varying engine load. Before proceeding the next experimental work, existing test fuel has to be consumed fully and run for a enough time to expel the previous fuel in the fuel line. Then, considering the proposed engine operating condition, important tests were conducted with other test fuels i.e. 100BD, 100PBD, 40PBD and 60PBD. For every experimental fuels and its running condition (each load or EGR rate) three repeated tests and reading were taken. The glass tube burette (50 ml) was used to fuel consumption for every engine test. The performance (BSFC, BTHE, EGT and volumetric efficiency), combustion (PCP and ID) were calculated by using Eqs. (2)–(8) while the emission (CO, HC and NOx) data were also measured during experiments. The results are presented graphically with necessary comparisons among test fuels. Mass flow rate of fuel (kg/h) [32,33], v 3600 ð2Þ m:f ¼ rf t 1000 Mass flow rate of air, (kg/h) [33], m:a ¼ Cd
pd20 ð2gDHrw ra Þ1=2 4
ð3Þ
Brake power, BP [32], BPðkWÞ ¼
2pNWr 60; 000
ð4Þ
Brake specific fuel consumption, BSFC [33], BSFCðkg=kWhÞ ¼
m:f BP
ð5Þ
Brake thermal efficiency, BTHE [33], BTHEð%Þ ¼
m:f m:f
Q LV
ð6Þ
Volumetric efficiency, hv [32], hv ð%Þ ¼
m:a 2 ra V d N
ð7Þ
Renewable Energy Focus Volume 26, Number 00 September 2018
Ignition delay, ID [32], 0
IDð CAÞ ¼ uSC uSI
ð8Þ
Result and discussion Enhancement of fuel properties of JOME Here, experiments were conducted in the engine with neat JOME under preheated conditions of nine different fuel inlet temperatures (42, 54, 66, 78, 90, 102, 114 and 126 C). All the tests of preheated JOME are undertaken at constant full load (12 kg) and engine speed of 1500 rpm. First engine tested with diesel fuel for warmup and bring waste exhaust gas at a minimum preheating fuel temperature. Then, complete experimentation was conducted as per the engine test matrix (Table 3) using preheated JOME at specific fuel preheating temperature at standard operating condition (CR 17.5, IP 200 bar, IT 23 BTDC and full load). The experimental fuel requires it to meet a set of specifications which are defined in ASTM D 6751 and EN 14214 standards. The properties of diesel fuel (100D) and JOME (100BD) are presented in Table 2. It is seen that density, kinematic viscosity, flashpoint, calorific values and cetane number JOME were, 884 kg/m3, 10.37 mm2/s, 225 C, 43.04 MJ/kg and 53.8, respectively. Except the kinematic viscosity, most of the properties JOME are within the limits of both standards and close to mineral diesel fuel. The lower heating values of JOME is 5.6% lower than diesel. Thus using neat JOME directly in a variable compression ratio (VCR) direct injection(DI) compression ignition engine influence adversely on performance and emission parameters. The fuel properties of JOME need to meet minimum biodiesel standards (ASTM D6751 and EN 14214) to avoid fuel atomization, vaporizations and fuelair mixing process problems in combustion chamber comes with high oil viscosity and density methyl ester oils [34]. In this study, the effect of kinematic viscosity and density of JOME with increasing fuel preheating temperature were investigated and illustrated in Figure 3(a). Noticeably, the kinematic viscosity and density of JOME gradually decrease with increasing fuel preheating inlet
temperatures [22,35]. By comparing the data from Table 2, it is seen that when preheated temperature of JOME is increased to 126 C, the kinematic viscosity and density values closely match with the values as per the international standards highlighted in Table 2. Beyond fuel preheating temperature value of 102 C, there is increase of BSFC and drop in BTHE as seen in Figure 3(b). It creates ‘‘vapor lock’’ in the fuel line and has a negative consequence with respect to lubricity problems due to severe fuel leakage. Due to this fact, there is intermittent fuel supply and engine stops suddenly. For this reason, the maximum fuel preheating temperature of JOME in the present case, is chosen as 102 C for which the minimum BSFC and maximum BTHE diesel engine were obtained (0.31 kg/kW.hr and 27.1%). The BSFC obtained with preheated JOME at 114 C and 126 C were on averages of 0.6% and 1.8% higher than that with BSFC (102 C). The BTHE with preheated JOME at 114 C and 126 C were on averages of 0.6% and 1.6% lower than that with BTHE at 102 C operating condition.
Engine performance study with fuel preheating and EGR In this section, the aim of the investigation is to study the effect of fuel preheating inlet temperature (102 C) on engine performance parameters with a variation of engine load. When the engine is operated in EGR mode, it is very crucial to have adequate flow rate so that it does not have adverse impact on engine performance. While conducting the experiment, it was observed that the effect of EGR seems to have insignificant effect on engine performance at part loads. Hence, the desired choice is to incorporate EGR mechanism at full load by regulating suitable flow rate of exhaust gases with an objective to find the optimum EGR rate.
Brake specific fuel consumption and brake thermal efficiency Figure 4(a) shows BSFC values for different tested fuels (100D, 100BD, 100PBD, 40PBD, 60PBD) at full load condition without considering EGR. It is seen in BSFC all tested fuels gradually
TABLE 3
The engine test matrix (every test was carried out at a constant engine operating conditions of CR 17.5, fuel IP 200 bar and 23 BTDC IT and 1500 rpm engine speed). Mode and operating condition
Test fuels
Fuel preheating temperature
Remarks
Variation fuel preheating (a) At full (12 kg) load without EGR.
100BD
(42–126 C) with 12 C increment
Baseline test (a) At varying load (0–12 kg) with 2.4 kg increment without EGR. (b) At varying EGR rate (0–40%) with 10% increment at full load (12 kg). Preheating and blending fuel (a) At varying load (0–12 kg) with 2.4 kg increment without EGR. (b) At varying EGR rate (0–40%) with 10% increment at full load (12 kg). Unheated neat biodiesel (a) At varying load (0–12 kg) with 2.4 kg increment without EGR. (b) At varying EGR rate (0–40%) with 10% increment at full load (12 kg).
100D
Without preheating
40PBD 60PBD 100PBD
102 C Preheating Temperature
To study the influence of heating on fuel properties of biodiesel and to recommend the optimum fuel preheating temperature that offers best engine performance parameters (BSFC and BTHE). To examine the consequence of different preheated blends of fuel on BSFC, BTHE, PCP, ID, CO, HC and NOx with respect to diesel and recommend the best fuel. Mainly to study the outcome of EGR on NOx emission for different preheated blends of fuel and recommend the efficient EGR rate with best blend of fuel. Furthermore the effect of EGR on BSFC, BTHE, PCP, ID, CO, HC of diesel engine also examined.
100BD
Without preheating
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Renewable Energy Focus Volume 26, Number 00 September 2018
ORIGINAL RESEARCH ARTICLE FIGURE 4
Variation of brake specific fuel consumption and brake thermal efficiency: (a) BSFC with load, (b) BTHE with load, (c) BSFC with EGR rates, and (d) BTHE with EGR rates.
decreases with increasing engine load in a similar trend as mentioned in references [22,35]. It is mainly due to better combustion characteristic due to improved fuel-air mixing rate at high engine loads. It is clearly observed in Figure 4(a) that BSFC for unheated neat JOME (100BD) has a highest value compared to other tested fuels over a given load ranges. In case of preheated neat JOME (100PBD), the BSFC for preheated neat JOME at varying engine load shows improvement. At full load and with respect to 100BD, the average reduction of BSFC for 40PBD, 60PBD 100PBD was found to be 19.3%, 12.2% and 6.5%, respectively. It clearly indicates that 40PBD offered better BSFC compared to other tested fuels. With respect to pure diesel (100D), the BSFC is seen to be 2.8% higher for 40PBD. By referring to Figure 4(b), the brake thermal efficiency (BTHE), show opposite trends with respect to BSFC. It may be emphasized that the calorific value increases at lower blending ratios as compared to neat biodiesel and drops when the contents of biodiesel in the blended fuel increase. Again, preheating boosts further increase of the calorific value of biodiesel as compared to neat biodiesel. Considering these facts, it is necessary to have a balanced blending ratio with appropriate preheating for optimum engine BTHE. It can be seen from Figure 4(b) that the average increase of BTHE for 40PBD and 60PBD was, 16% and 12.5%, 64
respectively as compared to unheated JOME. Thus, it is understood that, BTHE for JOME was significantly improved with a combined effects of heating and blending with diesel fuel. However, increasing the content of biodiesel in blended fuel drops the BTHE, because of lesser energy content of higher percentage of JOME. Relating to neat diesel (100D), the BTHE of 40PBD was slightly lower (4%). Hence, 40PBDD test fuel offers the most efficient BTHE as compared with other tested fuels. Figure 4(c) illustrate the BSFC variation for different fuels with the influence of exhaust gas recirculation rates (EGR) at full-load condition. BSFC for all tested increases with increasing the percentage of EGR rate [34,36]. This is due to the reduced oxygen availability in cylinder for combustion and occurrence of higher amount of CO2 in the engine cylinder. It is also seen that BSFC for all tested fuels were same for engine operated at lower EGR rate (10%) as compared to ‘‘no EGR’’. However, increasing percentage of EGR rate, amount of exhaust gas recirculated to the engine increases which causes reduction of the availability of oxygen in engine cylinder for combustion. Since the engine is operated at full load, the volume of fuel delivered to engine cylinder is higher. Because of lack of oxygen, the air-fuel ratio is seriously affected and there is incomplete combustion with higher BSFC. Further, it is indicated in Figure 4(b) that BSFC for 100BD is higher for every
Renewable Energy Focus Volume 26, Number 00 September 2018
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EGR rates as compared to other tested fuels. This is because of reduction in in-cylinder gas temperature due to higher oil viscosity leading to incomplete combustion. However, BSFC for preheating and blended JOME (40PBD) is seen to be decreased as compared to other test fuels. Thus, it offers improved fuel properties, increased in-cylinder temperature, better air-fuel mixing process and combustion characteristic. The variations of BTHE for different test fuels with percentage of EGR at full load displayed in Figure 4(d). It is seen that BTHE all test fuels decrease slightly with increasing percentage of EGR rates [31,37]. The increase of the percentage of exhaust gas recirculated to the engine threatens the normal engine combustion process because it decreases in-cylinder gas temperature along with oxygen deficiency. The BTHE for 40PBD test fuel at 30% EGR rate is found to be the optimum as compared to other EGR rates studied in these investigations.
Exhaust gas temperature Another performance indicator of the fuel quality during combustion process is the exhaust gas temperature (EGT). The higher value of EGT implies increased in-cylinder gas temperature and thus ensures efficient combustion characteristics of the fuel. The EGT for different test fuels with a variation of load is illustrated in Figure 5(a). The EGT for all fuels increased with increasing load. It essentially means fuel energy has been efficiently used at higher loads because of improved its oil quality and the presence of oxygen content in biodiesel. It is seen that all the test fuels of JOME have higher EGT as compared diesel. The heating values has fuel less effect on the combustion characteristics of a diesel engine, instead the Cetane number of a fuel play major role for controlling the ignition delay period. The earlier burning fuel takes place in case of biodiesel due to shorter premixed combustion period. Since the biodiesels have higher Cetane numbers, the premixing time reduces and combustion phasing moves earlier towards the compression stroke. In addition, the occurrence of higher oxygen content in JOME facilitates better combustion and causes EGT to be higher. The earlier experimental findings do highlight similar results [38]. In general, the preheating ensures higher EGT but with lower blending ratio, the EGT drops with heating. At full load condition, the overall increase of EGT for 100PBD, 100BD, 60PBD and 40PBD were, 11.6%, 8.2%, 4.5% and 2%, respectively, as compared to diesel even though it has higher calorific value. Figure 5(b) shows the EGT for different test fuels with percentage of EGR rates at full load. The EGT for all tested fuels dropped with increased EGR rate [31,37]. The main reasons of reduction EGT is deficiency of oxygen in combustion chamber due to recirculation of higher amount exhaust gas. It displaces the oxygen contents in combustion chamber resulting drop of in-cylinder gas temperature. At EGR 40%, the overall reduction of EGT is about 25% for all test fuels as compared to EGR 0% (without EGR).
FIGURE 5
Variations exhaust gas temperature (a) EGT with load (no-EGR), (b) EGT with EGR.
engine with increasing load. The effect of preheating JOME helps to improve volumetric efficiency of diesel engine. The heated JOME blend fuels causes improvement in fuel injection characteristics and combustion efficiency leading to increased amount of air intake. Due to this fact, there is a slight increase of volumetric efficiency, but it is still somehow lower than the mineral diesel fuel. At 100% loading condition, the average value of volumetric efficiency is about 80% for all test fuels including diesel. The variations of volumetric efficiency for different fuels with EGR rates at full engine loading condition is shown in Figure 6(b). All the test fuels show marginal drop (2.5%) with increasing the EGR rates which is in line with reported literatures [31]. It is mainly due to the reduction of amount of intake air mass flow into engine cylinder for combustion and supply of exhaust gases to in the engine cylinder through EGR system.
Volumetric efficiency
Effect of fuel preheating and EGR on engine combustion characteristics
Figure 6(a) illustrate the volumetric efficiency of different test fuels with variation of loads. For all fuels, it drops slightly with increase of load [22]. This is because of increased in-cylinder gas temperature at higher engine load leading to higher gas pressure in the cylinder. Hence, it restricts the air quantity at inlet of the engine cylinder resulting slightly drop of the volumetric efficiency of the
The ‘‘peak cylinder pressure (PCP) and ignition delay (ID) are two important parameters that needs to be evaluated for test fuels while looking into engine combustion behavior at different loads along with EGR variation. Both PCP and ID indicate the fuel efficiency during combustion and they are inversely related i.e. higher ID implies lower PCP and vice versa. 65
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in Figure 7(b). It is seen that the ID for all tested fuels decreases with increasing of load. This is as a result of the increase in-cylinder gas temperature with increasing load ensuing a shorter ID for tested fuels. For 100BD and 100PBD, the IDs were shorter compared to other test fuels over the operating loading conditions [39]. In this case, the higher Cetane number of JOME plays a vital role on affecting the chemical delay period for ignition compared to other tested fuels properties. At full load, the IDs for unheated JOME (100BD) and diesel fuel (100D) was 13 C A and 17 C A respectively. The IDs for preheating and blending (40PBD and 60PBD) was increased to 16 C A and 15 C A, respectively due to a decrement of kinematic viscosity and density of JOME. The effect of EGR rate on PCP for different test fuels at full load is shown in Figure 7(c). The PCP for all fuels decreases with higher EGR rate. This because of the shortage of oxygen in combustion chamber because of occurrence of higher CO2 in the exhaust gas. These gases can simple dilute the intake air availability in combustion chamber and affects air-fuel mixing process and combustion characteristics. With respect to the test fuel 40PBD at 30% EGR rate, the drop in PCP is 2.1% as compared diesel (100D) with same EGR rate and 8.1% as compared to the engine operated without EGR. The variation of IDs for test fuels shows increasing trends with higher EGR rate at full engine load (Figure 7(d)). This due to the supply of more exhaust gas to the engine cylinder that decreases the quantity oxygen needed for combustion. Hence, ID increases causing to drop in combustion efficiency. With respect to the test fuel 40PBD at 30% EGR rate, the drop in ID is 5.3% as compared diesel with same EGR rate, and increased by 12.5% as compared to the engine operated without EGR. FIGURE 6
Variation of volumetric efficiency for different fuels: (a) volumetric efficiency with load (no-EGR), (b) volumetric efficiency with EGR rates.
Figure 7(a) demonstrate the variation of peak cylinder pressure for different test fuels with engine load without considering EGR rate. It is seen that the PCP all tested fuels were increased with increasing load [22]. At lower loads up to 40%, the PCP values of biodiesel (100BD and 100PBD) are higher (10.16% and 8.98%) as compared to diesel. All test fuels have identical values (71 bar) of PCP at higher loads. It is because of a shorter ID for biodiesels, thereby injection timing (IT) is advanced because of a higher bulk modulus and density of JOME. Even the unheated JOME (higher density) has lower ID, the oxygenated nature of the fuel improves ignitability and produced maximum cylinder gas temperature even within the existing delay period [36]. The PCP for diesel fuel is less at lower engine load due to a longer ID as compared to biodiesel and occurred in working stroke far from the position of top dead center (TDC). However, at higher load, PCP occurred at near to TDC at same crank angle position with biodiesel. It is realized in Figure 7(a) that, when the load increases, the ID for test fuels (100D, 60PBD and 40PBD) is shortened that lead to start of ignition before TDC and PCP increases more rapidly. The preheating and blending JOME improved the fuel properties of biodiesel resulting marginal increase of ID compared to unheated 100BD. In diesel engine, the time lag between fuel injection and combustion initiation is called ‘‘ignition delay’’ [32]. The ID for different test fuels with load without EGR consideration is shown 66
Effect of fuel preheating and EGR on engine emission In this section, important engine emission elements for example carbon monoxide (CO), hydrocarbon (HC) and oxide of nitrogen (NOx) are discussed for all test fuels at varying engine loads. It is important to note that the significant regulation of NOx emissions can be achieved with controlled EGR rate. Hence, it is imperative to study the effects of preheating and blending for the 40PBD on engine emissions with EGR variation rates and compare it with the base fuel (100D).
Carbon monoxide emission The CO emission in exhaust gas is an indication of incomplete combustion due to deficiency of oxygen available in the combustion process. Figure 8(a) displays the carbon monoxide (CO) for test fuels with load without considering EGR. The CO emissions were higher at low load with gradually drop with increasing load [40]. The increase of in-cylinder gas temperature with increasing load tends to make the fuel-air mixture leaner so that combustion is complete. It is evidently realized that the operations with unheated JOME (100BD) results in lower CO emissions compared with diesel (100D). The presence of CO emission in exhaust gas indicates to the shortage oxygen in combustion process. The high oxygen content and lower carbon content (less Carbon/Hydrogen ratio) in JOME (biodiesel) which acts as combustion promoter inside the cylinder and causes complete combustion. This sufficient amount of inherited oxygen in biodiesel which encourages in oxidation of carbon in the fuel throughout combustion. The
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FIGURE 7
Variations of Peak cylinder pressure and ignition delay for fuels: (a) PCP with load (no-EGR), (b) ID with load (no-EGR), (c) PCP with EGR rates, and (d) ID with EGR rates.
presence of enough oxygen in the combustion chamber plays important role for lesser CO emissions [41]. The preheated and blended fuels (60PBD and 40PBD) have lower CO emissions over diesel. However, the CO values of neat biodiesels with preheating ensures significant reduction with respect to unheated biodiesel (100BD). This is because of reduction of high viscosity and density of biodiesel due to heating process that helps to improve the oxidization in combustion chamber beside the presence of higher oxygen content. At full load condition, the overall percentage decrease of CO emission for 40PBD was 37% as compared to diesel (100D). It is seen in Figure 8(a) that the engine consumes more amount fuel to generate appropriate engine power beyond 80% load. In this instance, only limited oxygen available for combustion in engine cylinder. This makes poor mixture in cylinder and may be the cause of increased CO, as it could not be oxidized to CO2. The similar results were also reported in references [42]. The variation of CO emission for different test fuels with percentage of EGR rate at full engine load (100%) is presented in Figure 8(b). It has been indicated that, CO emission for all tested fuels was increased with increasing percentage of EGR rate [33,39]. This is because of a shortage of air (oxygen) content in the engine cylinder for burning. The exhaust gas dilutes with the intake airfuel mixture and make a heterogeneous mixture. It becomes a
challenging task to burn completely. With 40% EGR rate, the percentage of increased CO emission with 40PBD was 27.1% compared the case without EGR. The results also revealed that, at EGR rate of 40% running condition, the amount of CO emission for tested fuel shows worsening trend. Hence, it is recommended operate the engine with EGR rate less than 30% (maximum averaged increased by 22.4%).
Unburned hydrocarbon emission The variation of unburned hydrocarbon (HC) for different test fuels with load (without EGR) is presented in Figure 9(a). The amount of HC all fuels gradually drop with increasing load [40]. The HC for unheated neat JOME fuel has lower values compared to other tested fuels over the load range. This may be a shorter ignition delay and early combustion attributed because of the higher Cetane number of biodiesel. This results sufficient time for the combustion of fuel and decrease HC emission. The presence of more oxygen content in the biodiesel also contributing to have a better combustion, and this results in lowering HC emissions. The results and trends are supported by available literatures [43]. At full engine load, the average reduction of HC emission for 40PBD was 5% with respect to neat diesel fuel (100D). The effect of preheating JOME help to improve the spray characteristics, 67
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ORIGINAL RESEARCH ARTICLE FIGURE 8
Variation of CO emission for different blends of fuel: (a) CO emission with load (no-EGR), (b) CO emission with EGR rates.
atomization, mixing rate and combustion efficiency of injected fuel in addition to the oxygenated nature of JOME (biodiesel), promoting better combustion in the engine combustion process and producing complete oxidation of HC emissions. In the case of EGR considerations with full load, HC emission for all tested fuels was seen higher at low load and increases slightly with increasing of EGR rates [33,37]. Similar reasoning can be made in this case as it was done for CO emissions. The air-fuel mixture is diluted and form heterogeneous mixture at higher EGR rate, which cannot combust fully. With respect to the case of without EGR consideration, the average HC emission decreased for 40PBD with EGR rate of 30% was about 17.5%, and increased by 2.08% as related to diesel fuel (100D) the engine operated with same EGR rate 30% (in Figure 9(b)).
Oxide of nitrogen emission The main drawback of biodiesel usages in diesel engine is the release of high amount oxides of nitrogen (NOx) emission in exhaust gas, as a result of the attendance of more oxygen in combustion chamber. The oxygenated nature of biodiesel obtained from vegetable oils is the source of the oxygen contents. Figure 10(a) illustrate the NOx emission for various test fuels with loads without consideration of EGR. The results showed increasing 68
FIGURE 9
Variation of HC emission for different blends of fuel: (a) HC emission with load (no-EGR), (b) HC emission with EGR rates.
trends of NOx emissions due to increased in-cylinder gas temperature at higher engine loads [40]. Moreover, the oxygen contents of neat biodiesel is higher than blended as well as diesel fuel. Preheating neat biodiesel also enhances the level of NOx emission compared with unheated fuel over the entire load range. At full engine loading condition, the overall increased of NOx emission with 40PBD was 17.6% higher compared to 100D. The major concern that limits the biodiesel usage in a diesel engine is the NOx formation. Under these circumstances, the effectiveness of EGR is more promising as it can significantly decrease the NOx emissions. The reduction in the engine incylinder gas temperature, is a key to reduce level of NOx emission in the exhaust gas. In this regard, EGR technique helps the recirculation of burned exhaust gas product (mainly carbon dioxide) in the engine cylinder and increased heat capacity of mixture compositions, thereby lowering the quantity of oxygen and reducing the in-cylinder gas temperature [32,33,37]. Figure 10(b) represent the NOx emission for different test fuels with variation of EGR rates at full engine load. At low EGR rates, the decrement of NOx emission for all tested fuels has no significant impact. When EGR rates increased beyond 10%, its effectiveness in terms of NOx reduction seems more justified. At maximum EGR rate of 40% considered in present investigation, the overall percentage of
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ORIGINAL RESEARCH ARTICLE
For the engine’s BP, BSFC, BTHE, volumetric efficiency, CO, HC and NOx emissions, the uncertainty analysis was carried out at standard engine operating conditions (CR 17.5, fuel IP 200 bar, and IT 23 BTDC at 1500 rpm) and for blend of fuel 40PBD. It has been evaluated based on the methodology adopted in the reference [44] and given in Table 4.
Conclusion
FIGURE 10
Variation of NOx emission for different blends of fuel: (a) NOx emission with load (no-EGR), (b) NOx emission with EGR rates.
reduction NOx emission for 100D, 40PBD, 60PBD, 100PBD and 100BD test fuels were noted to be, 89.5%, 86.5%, 84.5%, 75.1% and 76.3%, respectively, as compared to the case with engine running without EGR at full load. The results also reveal that, the engine performance parameters (such as BTHE, BSFC) significantly worsen at this EGR operation. Moreover, the engine operation with EGR should be limited within 20% to 30% EGR rate while using the biodiesel. At EGR rate of 30%, the test fuel 40PBD showed promising drop in NOx emission (68.8%) regarding engine operation without EGR. When compared with diesel at same EGR rate (30%), the NOx emission is seen to be 12.9% higher.
TABLE 4
Conflicts of interest
Experimental uncertainties of test results. Quantity
Value
1500 rpm Speed (N) Full load (W) 12.0 kg Fuel flow rate (m_ f Þ 1.01 kg/h _ a Þ 23.11 kg/h Air flow rateðm CO emission HC emission NOx emission
This research study has two major objectives; firstly, the effects of fuel preheating inlet temperature on the fuel properties of biodiesel, and then to examine on engine performance parameters. Later, the effects blending on preheated biodiesel operated on a diesel engine are investigated with respect to engine performance, combustion and emission studies along with engine load and EGR rate. The inferences from experimental findings have been described as follows: Heating significantly reduces the kinematic viscosity of biodiesel, while the density drops linearly. At 102 C, these values meet property values close to diesel as per international standard. The average reduction of kinematic viscosity for biodiesel was found to be 49% as compared to initial values at 40 C, whereas the density was drops by 4%. The fuel preheating inlet temperature of 102 C is found to be the optimal because the engine performance parameters shows downward trends beyond this temperature. The percentage reduction of BSFC for biodiesel at 102 C was 17% (with increase of BTHE of 23%) compared to the corresponding values at 42 C. With respect to utilization of test fuels in the diesel engine, the biodiesel blended with diesel (40PBD) with fuel preheating at 102 C and EGR rate of 30% offers best alternative for replacement of diesel. BSFC increased by 6.2%, BTHE decreased by 4.7%, EGT increased by 5.7%, volumetric efficiency decreased marginal by 0.9%, PCP decreased by 2.1%, ID decreased by 5.6%, CO increased by 21.04%, HC increased by 2.1%, NOx increased by 12.9% at full load operated conditions. As per the experimental observation, the combined effect of preheating biodiesel and diesel blend is the best solution for utilization of biodiesel oil in a diesel engine. The optimal EGR rate of 30% is an effective solution for a high level of NOx reduction with a minor affects engine performance parameters, combustion and emission characteristics. However, a higher percentage of EGR rate of 40% can be functional at low engine load ranges for a reduction of high amount of toxic oxides of nitrogen (NOx) emissions.
AccuracyCalculated parametersUncertainty 50 rpm BP 0.1 kg BSFC 0.01 kg/hBTHE 0.01 kg/hhv
0–10% vol. 0.01% 0–2000 ppm1 ppm 0–5000 ppm1 ppm
NOx CO HC
3.44% 3.58% 3.58% 0.043% 0.02% 0.1% 0.05%
The authors declare that there is no conflict of interest regarding publication of this paper. References [1] W. Parawira, Sci. Res. Essays 5 (14) (2010) 1796–1808. [2] S.K. Dash, P. Lingfa, AIP Conference Proceedings (Vol. 1859, No. 1, p. 020100), AIP Publishing, 2017, July. [3] L. Lin, Z. Cunshan, S. Vittayapadung, S. Xiangqian, D. Mingdong, Appl. Energy 88 (4) (2011) 1020–1031. [4] Z. Wang, M.M. Calderon, Y. Lu, Biomass Bioenergy 35 (7) (2011) 2893–2902.
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Uncertainty analysis
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ORIGINAL RESEARCH ARTICLE
Effect of fuel preheating with blended fuels and exhaust gas recirculation on diesel engine operating parameters Menelik Walle Mekonen* and Niranjan Sahoo Department of Mechanical Engineering, Indian Institute of Technology Guwahati, Guwahati 781039, India
This study investigates the influence of preheating on fuel properties of biodiesel and its blends on the performance of a diesel engine. Subsequently, the effectiveness of exhaust gas recirculation (EGR) rate for reducing the NOx emission has also been explored. At 102 C, fuel-preheating temperature, the average percentage reductions of kinematic viscosity and density of JOME was 49% and 4.3%, respectively with decrease (17.4%) in brake specific fuel consumption (BSFC) and increase (23%) of brake thermal efficiency (BTHE). The comparative study of all fuels with diesel revealed the fact that the blended fuel (40% biodiesel – 40PBD) with preheating (102 C), is seen to be most efficient when operated without EGR mode. At full load engine testing with same test fuel, BSFC reduces by 19%, BTHE increases by 16%, the exhaust gas temperature (EGT) drops by 6% with slight increase in volumetric efficiency (2%). The carbon monoxide and unburned hydrocarbon emission was reduced by 19.5% and 4.8%, respectively while oxides of nitrogen (NOx) emission was increased by 17.5% compared to diesel fuel. The inclusion of the EGR rate of 30% for 40PBD test fuel, meaningfully decreases the average NOx emission by 68.8% with almost insignificant change in engine BSFC and BTHE. Introduction The depletion of fuel reserves, rising fuel price and fast industrialization, pollution rules to mineral diesel fuels has forced many countries to search alternative source of diesel fuel energy to realize their energy requirements. Among several alternatives, methyl ester (Biodiesel) produced mainly from non-edible vegetable oils through transesterification is considered as a probable fuel sources for diesel fuel in a compression ignition engine [1]. ‘‘Methyl ester biodiesel’’ is a domestic renewable fuel generally made from vegetable, animal fats /oil, tallow and waste cooking oil. The chemical process used to convert these oils to biodiesel is called as, ‘‘transesterification’’ [2]. With respect to chemical terminology, ‘‘biodiesel’’ is defined as the mono alkyl esters of long chain fatty acids. In order to prepare biodiesel, ethanol or methanol, combined with a vegetable oil or animal fat in the presence of a catalyst can react to form ethyl esters or methyl esters (biodiesel) and glycerin. These esters are another class of oxygenates like alcohols and ethers. Thus, the biodiesels qualifies as fuel based
*Corresponding author. Mekonen, M.W. ([email protected])
on their comparative characteristics with diesel so that they can be used in compression-ignition (diesel) engines in its pure form with little or no modifications. In specific, Jatropha methyl ester, Pongamia methyl ester and Rapeseed methyl ester are typical choices for fossil diesel fuel substitutes [3]. In many countries, Jatropha curcas is being considered among the favorable choices and extensive research works have done due to its strong adaptableness to the situation, mainly in terms of draught flexibility, great endurance level and maximum seed yield [4]. Moreover, the methyl ester (biodiesels) have significant advantages such as, cleaner combustion, insignificant contribution to environmental pollution, biodegradability, non-toxic and lower emissions to diesel fuel [5]. Even though the methyl ester biodiesels have high combustion efficiency and Cetane number, they face challenges of higher NOx emission with serious difficulties of direct usages in the diesel engines [6]. Biodiesel has higher kinematic viscosity as well as density and lower heating value compared to petroleum fuel (diesel fuel). The properties of injected fuel have a significant effect on fuel injection characteristics which manipulating engine operating parameters of compression ignition engine [7,8]. Past
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Nomenclature
Abbreviation BSFC BTHE CR EGR EGT FPT ID IP JOME PCP Vd Cd d0 QLV
Brake specific fuel consumption (kg/kW h) Brake thermal efficiency (%) Compression ratio Exhaust gas recirculation Exhaust fas temperature ( C) Fuel preheat temperature ( C) Ignition delay ( CA) Injection pressure (bar) Jatropha oil methyl ester Peak cylinder pressure (bar) Engine displacement volume (cc) Coefficent of discharge Orifice diameter (mm) Lower heating value (MJ/kg)
Symbol N, W g r t v 100D 100BD 100PBD 40PBD 60PBD ra , rf , rw DH uSC uSI
Engine speed (rpm), engine load (%) Gravitational accelaration (m/s2) Dynamometer arm length (mm) Time taken for consuming v of fuel (s) Constant volume of fuel consumed (cc) 100% diesel fuel 100% Jatropha oil methyl ester 100% preheated neat JOME 40% preheated JOME and 60% diesel fuel 60% preheated JOME and 40% diesel fuel Density of air, fuel and water (kg/m3) Water column (mm) Angle start of ignition before top dead center Angle start of fuel injection before top dead center
studies on biodiesel reported that, a direct usage of biodiesel in compression engine caused a reduction engine efficiency and increased fuel consumption [9]. Methyl ester biodiesel has lower energy content related to mineral diesel fuel leads lower BTHE of engine, hence to compensate this problem biodiesel require longer injection duration [10]. Higher kinematic viscosity of biodiesel causes larger droplet size, poor fuel atomization, narrower injection spray angle, advanced injection, incomplete combustion and injector clogging [9,11]. The kinematic viscosity of methyl ester (biodiesel) is mostly higher than diesel fuel nearly a factor of two [12,13]. Therefore, important fuel properties of biodiesel need modification in order to make improvement for utilization of biodiesel in diesel engines. Until now, numerous methods were introduced to advance the fuel properties of biodiesel, namely blending biodiesel with mineral diesel fuel, usage of chemical (additives), and heating biodiesel [14,15]. It is reported that direct implementation all of these methods to biodiesel are not fully promising and practical. Till date, blending of biodiesel is a widely used methods for reducing the kinematic viscosity and density of JOME and applied as a fuel in compression ignition engine under the ASTM blended fuel standard [16]. However, the selection of maximum possible
ORIGINAL RESEARCH ARTICLE
percentage of biodiesel ratio depends on the biodiesel feedstock and their fuel properties differ with respect to feedstock. Hence, the determination of right blend ratio is the most important criteria for utilizing biodiesel in a diesel engine. A few earlier studies indicated that, the maximum blend ratio of biodiesel currently limited to blends of 20% or less as commercial fuels for many countries in existing diesel engine operate without engine modification [17,18]. It was reported that, the brake thermal efficiency of fuel blends was slightly higher while brake specific fuel consumption was found increased with a significant reduction of CO and HC emissions and increase of NOx emission as compared to diesel fuel. These blends of biodiesel is proved to be very strong potential fuel for the existing diesel engines. Conversely, at higher percentage of blend difficulties associated to degradation of fuel performance [19]. In contrast, plenty of research studies on additives mixed with biodiesel showed that, increasing biodiesel with additive content severely affects engine performance parameters (increased fuel consumption and significantly decreased engine power output) and increased exhaust emission concentrations [20]. However, some previous experimental studies reported that, modifying fuel properties of methyl ester (biodiesel) through heating method at specific temperature before entering the engine fuel injection systems is economical and feasible. It reality it can be possible for static diesel engines without any modifications by operating with biodiesels and overall improvement in efficiency [21,22]. Preheated JOME (biodiesel) can be also be used as a diesel fuel directly or blend in a compression ignition engine. There are limited research studies for use of preheated biodiesel or biodiesel blend in a diesel engine. Many research studies show the effect of preheated methyl ester oil on fuel properties and on diesel engine parameters [22–24]. The fuel preheating of methyl ester biodiesel at optimal temperature can improve overall engine performance parameters with reductions emissions of CO and HC emission at expense of NOx emission and can be substitute a diesel fuel without any engine modification. Further, the effect of fuel preheating and blend ratios on engine performance and emission evaluation of diesel engine operated with blend mixtures has been investigated [25]. It is indicated that, 80% preheated biodiesel– diesel blend and 20% ethanol can offer reasonably performance improvements with reduction of CO, HC and NOx emissions at varying load conditions as compared diesel. The effect of exhaust gas recirculation (EGR) on performance and emission characteristics of diesel engine fueled with 20% biodiesel was investigated [26]. Among, combinations of different EGR rates, 20% EGR rate of diesel engine using 20% biodiesel fuel was suggested optimal for obtaining better performance and minimal NOx emissions. Saravanan [27] examined the effect of EGR on NOx emission of diesel engine running with different biodiesel–diesel blends of fuel. It was reported that, the NOx and smoke emissions were significantly reduced for blends with reasonable drop in engine performance and slight increase exhaust emission (CO and HC) as compared to diesel fuel. After the exhaustive literature studies, the authors present specifically, the effect of preheating on the fuel properties (viscosity and density) of JOME on performance analysis of a diesel engine. During preliminary experiments, the fuel properties Jatropha oil methyl ester (JOME) has been thoroughly examined with heating 59
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Renewable Energy Focus Volume 26, Number 00 September 2018
Renewable Energy Focus Volume 26, Number 00 September 2018
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ORIGINAL RESEARCH ARTICLE FIGURE 1
Schematic diagram of experimental setup.
biodiesel from 42 C to 126 C fuel temperatures with increments of 12 C. The fuel properties of JOME improved by preheating that brings down the viscosity and density as per international biodiesel standard agreeable limits. Then for a further improvement the fuel properties is achieved by blends of biodiesel with different percentage fraction. Thus, the main objectives this paper is to investigate the combined effect of fuel preheating and blending JOME on its performance, combustion characteristics and emissions behaviors in a diesel engine. Subsequently, the influences of EGR rates on reductions of NOx emission and its effect on engine performance and emission parameters thoroughly examined. Finally, the results each tested fuels have been compared to diesel fuel baseline parameters.
Experimental methodology Test engine setup An experimental test rig is developed to undertake the desired engine experiments with different test fuels in a single cylinder a variable compression ratio (VCR) direct injection diesel engine. The test engine (Kirloskar make) at rated speed of 1500 RPM and 3.5 kW power output. The layout diagram of testbed is shown in Figure 1, while the detailed specification of the test engine is given in Table 1. The setup consists EGR system, biodiesel preheating mechanism, and fuel tanks with a fuel switching control valves, one for normal diesel fuel (100D) and the other for JOME. Fuel 60
filter (‘‘F’’ in Figure 1) is fixed at the biodiesel supply line (before preheater) for removing impurities from high viscous biodiesel and to minimize clogging of fuel supply line. An optical sensor (‘‘F1’’ in Figure 1) measures the fuel consumption and air (‘‘F2’’ in Figure 1). Engine is coupled with an eddy current dynamometer. Engine speed and load are controlled by using manually operated knob. Transducers (‘‘Pz1 and Pz2’’ in Figure 1) are used to measure the cylinder gas pressure and other one to detect the fuel injection pressure of fuel. The test engine is interfaced with exhaust gas analyzer (Make: AVL India, Model: DIGAS 444) for measuring important exhaust emission parameters samples (CO, HC and TABLE 1
Test engine specification. Engine type
Single cylinder, 4 stroke, water cooled, DI diesel engine
Bore stroke, displacement Connecting road length Compression ratio
87.5 mm 110 mm, 661 cc 234 mm 17.5:1, modified to VCR engine CR range: 12–18:1 185 mm 3.5 kW at 1500 rpm 23 BTDC fixed, Injection Variation : 0–25 BTDC 200 bar
Dynamometer arm length Rated power and speed Injection timing Injector opening pressure
Renewable Energy Focus Volume 26, Number 00 September 2018
TABLE 2
The fuel properties of JOME and diesel fuel used in the study. Property
Diesel JOME International standard Fuel specification
Density at 27 C (kg/m3) Kinematic viscosity at 40 C (mm2/s) Flash point ( C) Lower heating value (MJ/kg) Cetane number
846 3.43 68 45.61 47.48
ASTM D6751 EN14214 884 10.37 225 43.04 53.8
– 1.9–6.0 130.0 min – 47 min
860–900 3.5–5.0 120 min – 51 min
have been obtained to achieve high heat transfer coefficient. While designing a helical coil heat exchanger, the geometric effects of curvature ratio and pitch ratio have been considered by following important design steps and procedures [29,30]. The preheating device (Figure 2) consists of a helical bend copper tube aligned perpendicular for better heat exchange process and is placed in a shell maintained at a different heating temperature. A digital temperature measuring device and thermocouples have been used to regulate the temperature.
Exhaust gas recirculation system NOx). The important properties of biodiesel (JOME) and diesel mentioned in Table, were measured (offline) based on ASTM D6751 and EN 14214 standard [28].
Fuel preheating mechanisms In this study, a shell and helical coil heat exchanger developed inhouse, is chosen for fuel preheating (‘‘Hx’’ in Figure 1). Having a compact structure, the designed geometrical parameters (Table 2)
In this experimental investigation, a water-cooled EGR system (Figure 3) is being incorporated in which the exhaust gases passes in the tubes and water is circulated over these tubes. Thereby, the high exhaust gas temperature reduces but it is warmer than the intake air charge. Both intake charge and the exhaust gas mix in the air box (Figure 1) and subsequently enters into the combustion chamber of the engine. The supply of exhaust gas to the engine cylinder is controlled by using gate valves (‘‘gv1 and gv3’’ in Figure 1) and a U-tube manometer measures the flow of exhaust gases. By using
FIGURE 2
Layout diagram (a) biodiesel preheater, and (b) EGR system.
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The effect of fuel preheating temperature on: (a) fuel properties of JOME, (b) on engine performance parameters.
Eq. (1), the EGR rates kept as 10%, 20%, 30% and 40% for the present case [31]. The primary purpose of using EGR rate is to reduce oxides of nitrogen (NOx) emission. It may be emphasized that EGR has no significant effect on engine performance parameters if the components works properly. However, a defective system may affect engine parameters. Where, mEGR is the mass of exhaust gas (kg/h), and mtotal is total mass charge into cylinder (kg/h). mEGR EGRð%Þ ¼ 100 mtotal
ð1Þ
Experimental procedures The experimental fuel used in the present study contain diesel fuel and Jatropha Oil Methyl Ester (JOME). In this study, two major works were carried out. First, the test was conducted with neat JOME to study the effect of preheating inlet temperature on fuel properties of JOME and engine performance analysis with comparison of diesel as baseline data. In this study, tests were conducted with increasing fuel-preheating temperature up to eight different preheating temperatures (in the range of 42–126 C). Simultaneously, important tests were done to examine the outcome of preheating on performance parameters of diesel engine. Here the fuel properties of JOME were measured offline test using a hydrometer and Redwood viscometer Marton apparatus. In the entire study, attention is focused towards diesel engine’s performance, combustion and emission analysis with different test fuels such as pure biodiesel (100BD),
62
preheated biodiesel (100PBD) and blend biodiesel with preheating. The blend fuels are prepared and tested in a diesel engine with preheating mode for two different blend ratios by volume i.e. 40% biodiesel – 60% diesel (40PBD) and 60% biodiesel – 40% diesel (60PBD). The individual effect was examined for engine BSFC and BTHE, peak cylinder pressure(PCP), ignition delay (ID) and emissions (CO, HC and NOx) in a compression ignition engine at varying loading conditions without EGR rate and with inclusion of EGR rate (from 0 to 40% at 100% loading condition). Here the neat JOME was preheated at 102 C through engine waste heat exhaust gases. For this purpose, the engine has to be started first with diesel fuel to warm up and brings to engine optimum operating temperature. When the engine reached its steady state, exhaust flow control valve (gv2) was opened fully to supply exhaust gases to preheater at specified temperature. Then, the preheated JOME at elevated temperature goes through insulated separate fuel pipeline to the fuel blend burette. Then, mixing takes place with diesel in different proportion or direct supply to the fuel pump for pumping to fuel injector to pressurized the fuel. During experimentation, first baseline data of diesel engine were executed with a diesel fuel at reference engine running operating conditions of CR 17.5, fuel injector opening pressure 200 bar and 23 BTDC fuel injection timing with a varying engine load. Before proceeding the next experimental work, existing test fuel has to be consumed fully and run for a enough time to expel the previous fuel in the fuel line. Then, considering the proposed engine operating condition, important tests were conducted with other test fuels i.e. 100BD, 100PBD, 40PBD and 60PBD. For every experimental fuels and its running condition (each load or EGR rate) three repeated tests and reading were taken. The glass tube burette (50 ml) was used to fuel consumption for every engine test. The performance (BSFC, BTHE, EGT and volumetric efficiency), combustion (PCP and ID) were calculated by using Eqs. (2)–(8) while the emission (CO, HC and NOx) data were also measured during experiments. The results are presented graphically with necessary comparisons among test fuels. Mass flow rate of fuel (kg/h) [32,33], v 3600 ð2Þ m:f ¼ rf t 1000 Mass flow rate of air, (kg/h) [33], m:a ¼ Cd
pd20 ð2gDHrw ra Þ1=2 4
ð3Þ
Brake power, BP [32], BPðkWÞ ¼
2pNWr 60; 000
ð4Þ
Brake specific fuel consumption, BSFC [33], BSFCðkg=kWhÞ ¼
m:f BP
ð5Þ
Brake thermal efficiency, BTHE [33], BTHEð%Þ ¼
m:f m:f
Q LV
ð6Þ
Volumetric efficiency, hv [32], hv ð%Þ ¼
m:a 2 ra V d N
ð7Þ
Renewable Energy Focus Volume 26, Number 00 September 2018
Ignition delay, ID [32], 0
IDð CAÞ ¼ uSC uSI
ð8Þ
Result and discussion Enhancement of fuel properties of JOME Here, experiments were conducted in the engine with neat JOME under preheated conditions of nine different fuel inlet temperatures (42, 54, 66, 78, 90, 102, 114 and 126 C). All the tests of preheated JOME are undertaken at constant full load (12 kg) and engine speed of 1500 rpm. First engine tested with diesel fuel for warmup and bring waste exhaust gas at a minimum preheating fuel temperature. Then, complete experimentation was conducted as per the engine test matrix (Table 3) using preheated JOME at specific fuel preheating temperature at standard operating condition (CR 17.5, IP 200 bar, IT 23 BTDC and full load). The experimental fuel requires it to meet a set of specifications which are defined in ASTM D 6751 and EN 14214 standards. The properties of diesel fuel (100D) and JOME (100BD) are presented in Table 2. It is seen that density, kinematic viscosity, flashpoint, calorific values and cetane number JOME were, 884 kg/m3, 10.37 mm2/s, 225 C, 43.04 MJ/kg and 53.8, respectively. Except the kinematic viscosity, most of the properties JOME are within the limits of both standards and close to mineral diesel fuel. The lower heating values of JOME is 5.6% lower than diesel. Thus using neat JOME directly in a variable compression ratio (VCR) direct injection(DI) compression ignition engine influence adversely on performance and emission parameters. The fuel properties of JOME need to meet minimum biodiesel standards (ASTM D6751 and EN 14214) to avoid fuel atomization, vaporizations and fuelair mixing process problems in combustion chamber comes with high oil viscosity and density methyl ester oils [34]. In this study, the effect of kinematic viscosity and density of JOME with increasing fuel preheating temperature were investigated and illustrated in Figure 3(a). Noticeably, the kinematic viscosity and density of JOME gradually decrease with increasing fuel preheating inlet
temperatures [22,35]. By comparing the data from Table 2, it is seen that when preheated temperature of JOME is increased to 126 C, the kinematic viscosity and density values closely match with the values as per the international standards highlighted in Table 2. Beyond fuel preheating temperature value of 102 C, there is increase of BSFC and drop in BTHE as seen in Figure 3(b). It creates ‘‘vapor lock’’ in the fuel line and has a negative consequence with respect to lubricity problems due to severe fuel leakage. Due to this fact, there is intermittent fuel supply and engine stops suddenly. For this reason, the maximum fuel preheating temperature of JOME in the present case, is chosen as 102 C for which the minimum BSFC and maximum BTHE diesel engine were obtained (0.31 kg/kW.hr and 27.1%). The BSFC obtained with preheated JOME at 114 C and 126 C were on averages of 0.6% and 1.8% higher than that with BSFC (102 C). The BTHE with preheated JOME at 114 C and 126 C were on averages of 0.6% and 1.6% lower than that with BTHE at 102 C operating condition.
Engine performance study with fuel preheating and EGR In this section, the aim of the investigation is to study the effect of fuel preheating inlet temperature (102 C) on engine performance parameters with a variation of engine load. When the engine is operated in EGR mode, it is very crucial to have adequate flow rate so that it does not have adverse impact on engine performance. While conducting the experiment, it was observed that the effect of EGR seems to have insignificant effect on engine performance at part loads. Hence, the desired choice is to incorporate EGR mechanism at full load by regulating suitable flow rate of exhaust gases with an objective to find the optimum EGR rate.
Brake specific fuel consumption and brake thermal efficiency Figure 4(a) shows BSFC values for different tested fuels (100D, 100BD, 100PBD, 40PBD, 60PBD) at full load condition without considering EGR. It is seen in BSFC all tested fuels gradually
TABLE 3
The engine test matrix (every test was carried out at a constant engine operating conditions of CR 17.5, fuel IP 200 bar and 23 BTDC IT and 1500 rpm engine speed). Mode and operating condition
Test fuels
Fuel preheating temperature
Remarks
Variation fuel preheating (a) At full (12 kg) load without EGR.
100BD
(42–126 C) with 12 C increment
Baseline test (a) At varying load (0–12 kg) with 2.4 kg increment without EGR. (b) At varying EGR rate (0–40%) with 10% increment at full load (12 kg). Preheating and blending fuel (a) At varying load (0–12 kg) with 2.4 kg increment without EGR. (b) At varying EGR rate (0–40%) with 10% increment at full load (12 kg). Unheated neat biodiesel (a) At varying load (0–12 kg) with 2.4 kg increment without EGR. (b) At varying EGR rate (0–40%) with 10% increment at full load (12 kg).
100D
Without preheating
40PBD 60PBD 100PBD
102 C Preheating Temperature
To study the influence of heating on fuel properties of biodiesel and to recommend the optimum fuel preheating temperature that offers best engine performance parameters (BSFC and BTHE). To examine the consequence of different preheated blends of fuel on BSFC, BTHE, PCP, ID, CO, HC and NOx with respect to diesel and recommend the best fuel. Mainly to study the outcome of EGR on NOx emission for different preheated blends of fuel and recommend the efficient EGR rate with best blend of fuel. Furthermore the effect of EGR on BSFC, BTHE, PCP, ID, CO, HC of diesel engine also examined.
100BD
Without preheating
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Variation of brake specific fuel consumption and brake thermal efficiency: (a) BSFC with load, (b) BTHE with load, (c) BSFC with EGR rates, and (d) BTHE with EGR rates.
decreases with increasing engine load in a similar trend as mentioned in references [22,35]. It is mainly due to better combustion characteristic due to improved fuel-air mixing rate at high engine loads. It is clearly observed in Figure 4(a) that BSFC for unheated neat JOME (100BD) has a highest value compared to other tested fuels over a given load ranges. In case of preheated neat JOME (100PBD), the BSFC for preheated neat JOME at varying engine load shows improvement. At full load and with respect to 100BD, the average reduction of BSFC for 40PBD, 60PBD 100PBD was found to be 19.3%, 12.2% and 6.5%, respectively. It clearly indicates that 40PBD offered better BSFC compared to other tested fuels. With respect to pure diesel (100D), the BSFC is seen to be 2.8% higher for 40PBD. By referring to Figure 4(b), the brake thermal efficiency (BTHE), show opposite trends with respect to BSFC. It may be emphasized that the calorific value increases at lower blending ratios as compared to neat biodiesel and drops when the contents of biodiesel in the blended fuel increase. Again, preheating boosts further increase of the calorific value of biodiesel as compared to neat biodiesel. Considering these facts, it is necessary to have a balanced blending ratio with appropriate preheating for optimum engine BTHE. It can be seen from Figure 4(b) that the average increase of BTHE for 40PBD and 60PBD was, 16% and 12.5%, 64
respectively as compared to unheated JOME. Thus, it is understood that, BTHE for JOME was significantly improved with a combined effects of heating and blending with diesel fuel. However, increasing the content of biodiesel in blended fuel drops the BTHE, because of lesser energy content of higher percentage of JOME. Relating to neat diesel (100D), the BTHE of 40PBD was slightly lower (4%). Hence, 40PBDD test fuel offers the most efficient BTHE as compared with other tested fuels. Figure 4(c) illustrate the BSFC variation for different fuels with the influence of exhaust gas recirculation rates (EGR) at full-load condition. BSFC for all tested increases with increasing the percentage of EGR rate [34,36]. This is due to the reduced oxygen availability in cylinder for combustion and occurrence of higher amount of CO2 in the engine cylinder. It is also seen that BSFC for all tested fuels were same for engine operated at lower EGR rate (10%) as compared to ‘‘no EGR’’. However, increasing percentage of EGR rate, amount of exhaust gas recirculated to the engine increases which causes reduction of the availability of oxygen in engine cylinder for combustion. Since the engine is operated at full load, the volume of fuel delivered to engine cylinder is higher. Because of lack of oxygen, the air-fuel ratio is seriously affected and there is incomplete combustion with higher BSFC. Further, it is indicated in Figure 4(b) that BSFC for 100BD is higher for every
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EGR rates as compared to other tested fuels. This is because of reduction in in-cylinder gas temperature due to higher oil viscosity leading to incomplete combustion. However, BSFC for preheating and blended JOME (40PBD) is seen to be decreased as compared to other test fuels. Thus, it offers improved fuel properties, increased in-cylinder temperature, better air-fuel mixing process and combustion characteristic. The variations of BTHE for different test fuels with percentage of EGR at full load displayed in Figure 4(d). It is seen that BTHE all test fuels decrease slightly with increasing percentage of EGR rates [31,37]. The increase of the percentage of exhaust gas recirculated to the engine threatens the normal engine combustion process because it decreases in-cylinder gas temperature along with oxygen deficiency. The BTHE for 40PBD test fuel at 30% EGR rate is found to be the optimum as compared to other EGR rates studied in these investigations.
Exhaust gas temperature Another performance indicator of the fuel quality during combustion process is the exhaust gas temperature (EGT). The higher value of EGT implies increased in-cylinder gas temperature and thus ensures efficient combustion characteristics of the fuel. The EGT for different test fuels with a variation of load is illustrated in Figure 5(a). The EGT for all fuels increased with increasing load. It essentially means fuel energy has been efficiently used at higher loads because of improved its oil quality and the presence of oxygen content in biodiesel. It is seen that all the test fuels of JOME have higher EGT as compared diesel. The heating values has fuel less effect on the combustion characteristics of a diesel engine, instead the Cetane number of a fuel play major role for controlling the ignition delay period. The earlier burning fuel takes place in case of biodiesel due to shorter premixed combustion period. Since the biodiesels have higher Cetane numbers, the premixing time reduces and combustion phasing moves earlier towards the compression stroke. In addition, the occurrence of higher oxygen content in JOME facilitates better combustion and causes EGT to be higher. The earlier experimental findings do highlight similar results [38]. In general, the preheating ensures higher EGT but with lower blending ratio, the EGT drops with heating. At full load condition, the overall increase of EGT for 100PBD, 100BD, 60PBD and 40PBD were, 11.6%, 8.2%, 4.5% and 2%, respectively, as compared to diesel even though it has higher calorific value. Figure 5(b) shows the EGT for different test fuels with percentage of EGR rates at full load. The EGT for all tested fuels dropped with increased EGR rate [31,37]. The main reasons of reduction EGT is deficiency of oxygen in combustion chamber due to recirculation of higher amount exhaust gas. It displaces the oxygen contents in combustion chamber resulting drop of in-cylinder gas temperature. At EGR 40%, the overall reduction of EGT is about 25% for all test fuels as compared to EGR 0% (without EGR).
FIGURE 5
Variations exhaust gas temperature (a) EGT with load (no-EGR), (b) EGT with EGR.
engine with increasing load. The effect of preheating JOME helps to improve volumetric efficiency of diesel engine. The heated JOME blend fuels causes improvement in fuel injection characteristics and combustion efficiency leading to increased amount of air intake. Due to this fact, there is a slight increase of volumetric efficiency, but it is still somehow lower than the mineral diesel fuel. At 100% loading condition, the average value of volumetric efficiency is about 80% for all test fuels including diesel. The variations of volumetric efficiency for different fuels with EGR rates at full engine loading condition is shown in Figure 6(b). All the test fuels show marginal drop (2.5%) with increasing the EGR rates which is in line with reported literatures [31]. It is mainly due to the reduction of amount of intake air mass flow into engine cylinder for combustion and supply of exhaust gases to in the engine cylinder through EGR system.
Volumetric efficiency
Effect of fuel preheating and EGR on engine combustion characteristics
Figure 6(a) illustrate the volumetric efficiency of different test fuels with variation of loads. For all fuels, it drops slightly with increase of load [22]. This is because of increased in-cylinder gas temperature at higher engine load leading to higher gas pressure in the cylinder. Hence, it restricts the air quantity at inlet of the engine cylinder resulting slightly drop of the volumetric efficiency of the
The ‘‘peak cylinder pressure (PCP) and ignition delay (ID) are two important parameters that needs to be evaluated for test fuels while looking into engine combustion behavior at different loads along with EGR variation. Both PCP and ID indicate the fuel efficiency during combustion and they are inversely related i.e. higher ID implies lower PCP and vice versa. 65
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in Figure 7(b). It is seen that the ID for all tested fuels decreases with increasing of load. This is as a result of the increase in-cylinder gas temperature with increasing load ensuing a shorter ID for tested fuels. For 100BD and 100PBD, the IDs were shorter compared to other test fuels over the operating loading conditions [39]. In this case, the higher Cetane number of JOME plays a vital role on affecting the chemical delay period for ignition compared to other tested fuels properties. At full load, the IDs for unheated JOME (100BD) and diesel fuel (100D) was 13 C A and 17 C A respectively. The IDs for preheating and blending (40PBD and 60PBD) was increased to 16 C A and 15 C A, respectively due to a decrement of kinematic viscosity and density of JOME. The effect of EGR rate on PCP for different test fuels at full load is shown in Figure 7(c). The PCP for all fuels decreases with higher EGR rate. This because of the shortage of oxygen in combustion chamber because of occurrence of higher CO2 in the exhaust gas. These gases can simple dilute the intake air availability in combustion chamber and affects air-fuel mixing process and combustion characteristics. With respect to the test fuel 40PBD at 30% EGR rate, the drop in PCP is 2.1% as compared diesel (100D) with same EGR rate and 8.1% as compared to the engine operated without EGR. The variation of IDs for test fuels shows increasing trends with higher EGR rate at full engine load (Figure 7(d)). This due to the supply of more exhaust gas to the engine cylinder that decreases the quantity oxygen needed for combustion. Hence, ID increases causing to drop in combustion efficiency. With respect to the test fuel 40PBD at 30% EGR rate, the drop in ID is 5.3% as compared diesel with same EGR rate, and increased by 12.5% as compared to the engine operated without EGR. FIGURE 6
Variation of volumetric efficiency for different fuels: (a) volumetric efficiency with load (no-EGR), (b) volumetric efficiency with EGR rates.
Figure 7(a) demonstrate the variation of peak cylinder pressure for different test fuels with engine load without considering EGR rate. It is seen that the PCP all tested fuels were increased with increasing load [22]. At lower loads up to 40%, the PCP values of biodiesel (100BD and 100PBD) are higher (10.16% and 8.98%) as compared to diesel. All test fuels have identical values (71 bar) of PCP at higher loads. It is because of a shorter ID for biodiesels, thereby injection timing (IT) is advanced because of a higher bulk modulus and density of JOME. Even the unheated JOME (higher density) has lower ID, the oxygenated nature of the fuel improves ignitability and produced maximum cylinder gas temperature even within the existing delay period [36]. The PCP for diesel fuel is less at lower engine load due to a longer ID as compared to biodiesel and occurred in working stroke far from the position of top dead center (TDC). However, at higher load, PCP occurred at near to TDC at same crank angle position with biodiesel. It is realized in Figure 7(a) that, when the load increases, the ID for test fuels (100D, 60PBD and 40PBD) is shortened that lead to start of ignition before TDC and PCP increases more rapidly. The preheating and blending JOME improved the fuel properties of biodiesel resulting marginal increase of ID compared to unheated 100BD. In diesel engine, the time lag between fuel injection and combustion initiation is called ‘‘ignition delay’’ [32]. The ID for different test fuels with load without EGR consideration is shown 66
Effect of fuel preheating and EGR on engine emission In this section, important engine emission elements for example carbon monoxide (CO), hydrocarbon (HC) and oxide of nitrogen (NOx) are discussed for all test fuels at varying engine loads. It is important to note that the significant regulation of NOx emissions can be achieved with controlled EGR rate. Hence, it is imperative to study the effects of preheating and blending for the 40PBD on engine emissions with EGR variation rates and compare it with the base fuel (100D).
Carbon monoxide emission The CO emission in exhaust gas is an indication of incomplete combustion due to deficiency of oxygen available in the combustion process. Figure 8(a) displays the carbon monoxide (CO) for test fuels with load without considering EGR. The CO emissions were higher at low load with gradually drop with increasing load [40]. The increase of in-cylinder gas temperature with increasing load tends to make the fuel-air mixture leaner so that combustion is complete. It is evidently realized that the operations with unheated JOME (100BD) results in lower CO emissions compared with diesel (100D). The presence of CO emission in exhaust gas indicates to the shortage oxygen in combustion process. The high oxygen content and lower carbon content (less Carbon/Hydrogen ratio) in JOME (biodiesel) which acts as combustion promoter inside the cylinder and causes complete combustion. This sufficient amount of inherited oxygen in biodiesel which encourages in oxidation of carbon in the fuel throughout combustion. The
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FIGURE 7
Variations of Peak cylinder pressure and ignition delay for fuels: (a) PCP with load (no-EGR), (b) ID with load (no-EGR), (c) PCP with EGR rates, and (d) ID with EGR rates.
presence of enough oxygen in the combustion chamber plays important role for lesser CO emissions [41]. The preheated and blended fuels (60PBD and 40PBD) have lower CO emissions over diesel. However, the CO values of neat biodiesels with preheating ensures significant reduction with respect to unheated biodiesel (100BD). This is because of reduction of high viscosity and density of biodiesel due to heating process that helps to improve the oxidization in combustion chamber beside the presence of higher oxygen content. At full load condition, the overall percentage decrease of CO emission for 40PBD was 37% as compared to diesel (100D). It is seen in Figure 8(a) that the engine consumes more amount fuel to generate appropriate engine power beyond 80% load. In this instance, only limited oxygen available for combustion in engine cylinder. This makes poor mixture in cylinder and may be the cause of increased CO, as it could not be oxidized to CO2. The similar results were also reported in references [42]. The variation of CO emission for different test fuels with percentage of EGR rate at full engine load (100%) is presented in Figure 8(b). It has been indicated that, CO emission for all tested fuels was increased with increasing percentage of EGR rate [33,39]. This is because of a shortage of air (oxygen) content in the engine cylinder for burning. The exhaust gas dilutes with the intake airfuel mixture and make a heterogeneous mixture. It becomes a
challenging task to burn completely. With 40% EGR rate, the percentage of increased CO emission with 40PBD was 27.1% compared the case without EGR. The results also revealed that, at EGR rate of 40% running condition, the amount of CO emission for tested fuel shows worsening trend. Hence, it is recommended operate the engine with EGR rate less than 30% (maximum averaged increased by 22.4%).
Unburned hydrocarbon emission The variation of unburned hydrocarbon (HC) for different test fuels with load (without EGR) is presented in Figure 9(a). The amount of HC all fuels gradually drop with increasing load [40]. The HC for unheated neat JOME fuel has lower values compared to other tested fuels over the load range. This may be a shorter ignition delay and early combustion attributed because of the higher Cetane number of biodiesel. This results sufficient time for the combustion of fuel and decrease HC emission. The presence of more oxygen content in the biodiesel also contributing to have a better combustion, and this results in lowering HC emissions. The results and trends are supported by available literatures [43]. At full engine load, the average reduction of HC emission for 40PBD was 5% with respect to neat diesel fuel (100D). The effect of preheating JOME help to improve the spray characteristics, 67
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Variation of CO emission for different blends of fuel: (a) CO emission with load (no-EGR), (b) CO emission with EGR rates.
atomization, mixing rate and combustion efficiency of injected fuel in addition to the oxygenated nature of JOME (biodiesel), promoting better combustion in the engine combustion process and producing complete oxidation of HC emissions. In the case of EGR considerations with full load, HC emission for all tested fuels was seen higher at low load and increases slightly with increasing of EGR rates [33,37]. Similar reasoning can be made in this case as it was done for CO emissions. The air-fuel mixture is diluted and form heterogeneous mixture at higher EGR rate, which cannot combust fully. With respect to the case of without EGR consideration, the average HC emission decreased for 40PBD with EGR rate of 30% was about 17.5%, and increased by 2.08% as related to diesel fuel (100D) the engine operated with same EGR rate 30% (in Figure 9(b)).
Oxide of nitrogen emission The main drawback of biodiesel usages in diesel engine is the release of high amount oxides of nitrogen (NOx) emission in exhaust gas, as a result of the attendance of more oxygen in combustion chamber. The oxygenated nature of biodiesel obtained from vegetable oils is the source of the oxygen contents. Figure 10(a) illustrate the NOx emission for various test fuels with loads without consideration of EGR. The results showed increasing 68
FIGURE 9
Variation of HC emission for different blends of fuel: (a) HC emission with load (no-EGR), (b) HC emission with EGR rates.
trends of NOx emissions due to increased in-cylinder gas temperature at higher engine loads [40]. Moreover, the oxygen contents of neat biodiesel is higher than blended as well as diesel fuel. Preheating neat biodiesel also enhances the level of NOx emission compared with unheated fuel over the entire load range. At full engine loading condition, the overall increased of NOx emission with 40PBD was 17.6% higher compared to 100D. The major concern that limits the biodiesel usage in a diesel engine is the NOx formation. Under these circumstances, the effectiveness of EGR is more promising as it can significantly decrease the NOx emissions. The reduction in the engine incylinder gas temperature, is a key to reduce level of NOx emission in the exhaust gas. In this regard, EGR technique helps the recirculation of burned exhaust gas product (mainly carbon dioxide) in the engine cylinder and increased heat capacity of mixture compositions, thereby lowering the quantity of oxygen and reducing the in-cylinder gas temperature [32,33,37]. Figure 10(b) represent the NOx emission for different test fuels with variation of EGR rates at full engine load. At low EGR rates, the decrement of NOx emission for all tested fuels has no significant impact. When EGR rates increased beyond 10%, its effectiveness in terms of NOx reduction seems more justified. At maximum EGR rate of 40% considered in present investigation, the overall percentage of
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For the engine’s BP, BSFC, BTHE, volumetric efficiency, CO, HC and NOx emissions, the uncertainty analysis was carried out at standard engine operating conditions (CR 17.5, fuel IP 200 bar, and IT 23 BTDC at 1500 rpm) and for blend of fuel 40PBD. It has been evaluated based on the methodology adopted in the reference [44] and given in Table 4.
Conclusion
FIGURE 10
Variation of NOx emission for different blends of fuel: (a) NOx emission with load (no-EGR), (b) NOx emission with EGR rates.
reduction NOx emission for 100D, 40PBD, 60PBD, 100PBD and 100BD test fuels were noted to be, 89.5%, 86.5%, 84.5%, 75.1% and 76.3%, respectively, as compared to the case with engine running without EGR at full load. The results also reveal that, the engine performance parameters (such as BTHE, BSFC) significantly worsen at this EGR operation. Moreover, the engine operation with EGR should be limited within 20% to 30% EGR rate while using the biodiesel. At EGR rate of 30%, the test fuel 40PBD showed promising drop in NOx emission (68.8%) regarding engine operation without EGR. When compared with diesel at same EGR rate (30%), the NOx emission is seen to be 12.9% higher.
TABLE 4
Conflicts of interest
Experimental uncertainties of test results. Quantity
Value
1500 rpm Speed (N) Full load (W) 12.0 kg Fuel flow rate (m_ f Þ 1.01 kg/h _ a Þ 23.11 kg/h Air flow rateðm CO emission HC emission NOx emission
This research study has two major objectives; firstly, the effects of fuel preheating inlet temperature on the fuel properties of biodiesel, and then to examine on engine performance parameters. Later, the effects blending on preheated biodiesel operated on a diesel engine are investigated with respect to engine performance, combustion and emission studies along with engine load and EGR rate. The inferences from experimental findings have been described as follows: Heating significantly reduces the kinematic viscosity of biodiesel, while the density drops linearly. At 102 C, these values meet property values close to diesel as per international standard. The average reduction of kinematic viscosity for biodiesel was found to be 49% as compared to initial values at 40 C, whereas the density was drops by 4%. The fuel preheating inlet temperature of 102 C is found to be the optimal because the engine performance parameters shows downward trends beyond this temperature. The percentage reduction of BSFC for biodiesel at 102 C was 17% (with increase of BTHE of 23%) compared to the corresponding values at 42 C. With respect to utilization of test fuels in the diesel engine, the biodiesel blended with diesel (40PBD) with fuel preheating at 102 C and EGR rate of 30% offers best alternative for replacement of diesel. BSFC increased by 6.2%, BTHE decreased by 4.7%, EGT increased by 5.7%, volumetric efficiency decreased marginal by 0.9%, PCP decreased by 2.1%, ID decreased by 5.6%, CO increased by 21.04%, HC increased by 2.1%, NOx increased by 12.9% at full load operated conditions. As per the experimental observation, the combined effect of preheating biodiesel and diesel blend is the best solution for utilization of biodiesel oil in a diesel engine. The optimal EGR rate of 30% is an effective solution for a high level of NOx reduction with a minor affects engine performance parameters, combustion and emission characteristics. However, a higher percentage of EGR rate of 40% can be functional at low engine load ranges for a reduction of high amount of toxic oxides of nitrogen (NOx) emissions.
AccuracyCalculated parametersUncertainty 50 rpm BP 0.1 kg BSFC 0.01 kg/hBTHE 0.01 kg/hhv
0–10% vol. 0.01% 0–2000 ppm1 ppm 0–5000 ppm1 ppm
NOx CO HC
3.44% 3.58% 3.58% 0.043% 0.02% 0.1% 0.05%
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ORIGINAL RESEARCH ARTICLE
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