- Email: [email protected]
Optimization of palm oil biodiesel blends and engine operating parameters to improve performance and PM morphology in a common rail direct injection diesel engine
Fuel 260 (2020) 116326
Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Full Length Article
Optimization of palm oil biodiesel blends and engine operating parameters to improve performance and PM morphology in a common rail direct injection diesel engine Jun Cong Gea, Ho Young Kima, Sam Ki Yoona, Nag Jung Choia, a
T
⁎
Division of Mechanical Design Engineering, Chonbuk National University, 567 Baekje-daero, Deokjin-gu, Jeonju-si, Jeollabuk-do 54896, Republic of Korea
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Palm oil biodiesel EGR Pilot injection timing PM morphology Mass fraction burned
In this paper, the characteristics of five palm oil biodiesel blends (B0, B10, B20, B30, and B100) have been tested in a common rail direct injection (CRDI) diesel engine with various EGR rates and pilot injection timings under 25% and 75% engine loads. The following were evaluated: combustion characteristics, including in-cylinder pressure, heat release rate (HRR), and mass fraction burned (MFB); engine performance parameters, including brake specific fuel consumption (BSFC) and coefficient of variation of the indicated mean effective pressure (COVimep); and emission characteristics, including CO, HC, NOx, PM and its morphology. By considering the combustion and emission characteristics, we found that the diesel engine fueled with B30 blend fuel with a 10% EGR rate, or with a pilot injection timing of 24° CA BTDC, can effectively reduce PM emissions and simultaneously keep NOx emissions at a low level. In addition, we found that the variations of the engine load and EGR rate, as well as the biodiesel blend ratio, have direct impacts on the PM particle diameter, shape, and other morphology.
⁎
Corresponding author. E-mail address: [email protected] (N.J. Choi).
https://doi.org/10.1016/j.fuel.2019.116326 Received 3 June 2019; Received in revised form 28 September 2019; Accepted 30 September 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
Fuel 260 (2020) 116326
J.C. Ge, et al.
1. Introduction
technology dilutes the concentration of oxygen in the mixture and reduces the combustion efficiency and combustion temperature, which in turn reduces NOx formation. Gomaa et al. [15] found that using jatropha biodiesel with 5–15% EGR rate can effectively balance the relationship between NOx and PM. Agarwal et al. [16] found that a combination of 20% rice bran oil-based biodiesel and 15% EGR rate not only improved thermal efficiency, but also reduced brake specific energy consumption (BSEC) and regulated pollutants from diesel engines. Split injection is also a promising method for reducing emissions and improving engine performance [17,18]. Li et al. [18] pointed out that the flame temperature peak was reduced effectively, using split injection with a small pilot ration and short dwell time in a single-cylinder diesel engine. Jeon et al. [19] found that pilot injection can improve combustion performance and fuel economy, and that NOx can be effectively controlled at BTDC 40 °CA in a single-cylinder diesel engine. The effects of injection modes on combustion, combustion noise, performance, and exhaust emission characteristics were also investigated and evaluated in other research [20–22]. Overall, engine performance, along with combustion and emission characteristics, are not only affected by the fuel properties, but also the engine parameters. However, most of the current research on injection strategy has been performed based on a single-cylinder engine or CFD simulation due to the variability of engine parameters. Therefore, it is necessary to optimize the relationship between fuel properties and engine parameters to improve engine performance and exhaust emissions. In the present study, the effects of various EGR and pilot injection conditions on combustion, engine performance, and regulated emissions were investigated in a CRDI 4-cylinder diesel engine fueled with various palm oil biodiesel blends. Additionally, the soot particle morphology for each test fuel was comparatively observed by transmission electron microscopy (TEM) under each operating condition.
Energy shortages and environmental pollution caused by fossilfueled vehicles have become a global concern. Especially in recent years, the sharp decline in urban air quality has been reported to be directly related to these vehicles. The main harmful air pollutants emitted by fossil-fueled vehicles are CO, HC, NOx, and PM. Compared with gasoline engines, diesel engines emit less CO and HC, but more NOx and PM. These emissions are factors in causing disease and can lead to a decline in human immunity. There may also be photochemical reactions between these emissions causing secondary pollution to further deteriorate urban air quality [1,2]. At present, although electric vehicles (EVs) are an effective way to reduce environmental pollution, their shortcomings (e.g. high capital and maintenance cost, immature technology, and small scale) are restricting their development. On the other hand, the conventional vehicles combined with exhaust aftertreatment equipment can meet the emission standards, but the disadvantage is that the vehicle cost and maintenance cost increse [3]. Therefore, finding a clean, renewable, high-efficiency alternative energy source is still an important challenge. Oxygenated fuels and biodiesel have become popular in recent decades because their high oxygen content can greatly reduce PM emissions [4,5]. Especially biodiesel, which has become the best alternative fuel for diesel because it is green, renewable, and biodegradable. Most importantly, its basic properties are similar to diesel fuel. Biodiesel can be extracted from a variety of vegetable oils, from algae, and from animal fats via simple transesterification. Biodiesel has a higher cetane number and oxygen content than diesel. This provides improved ignition properties and combustion characteristics, resulting in emission reduction at the source for CO, HC, and PM, but also causes a slight increase in NOx [6]. In addition, CO2 emissions, the main greenhouse gas, can be well controlled with the use of biodiesel [7]. However, a significant drawback is that the viscosity, density, and surface tension are higher in biodiesel than in diesel; this is because biodiesel is a monoalkyl ester composed of saturated fatty acids and unsaturated long chain fatty acids. These shortcomings influence the injected fuel atomization effect, leading to injector cooking and piston ring sticking [8–11]. Wang et al. [12,13] reported that the high viscosity of biodiesel leads to the small spray angle, poor spray atomization and air entrainment, longer injection delay and spray tip penetration. Blending biodiesel with diesel is one of the easiest and most effective ways to reduce the negative effects of biodiesel. Ali et al. [14] indicated that the cloud and pour points, as well as the density, viscosity, and acid value of biodiesel have been greatly improved when adding diesel to biodiesel. On the other hand, combustion and emission characteristics, including engine performance, can be improved by optimizing engine parameters such as exhaust gas recirculation (EGR) and split injection strategies. This is especially beneficial for simultaneously reducing NOx and PM emissions because of their trade-off relationship. EGR
2. Experimental setup 2.1. Fuel property measurements In this work, biodiesel was prepared from crude palm oil (CPO) by transesterification. CPO is mainly composed of triacylglycerols, mono and diacylglycerols, and also contains a small amount of free fatty acids (FFAs), non-oil fatty matter, moisture, and impurities. An FE-SEM image and the EDX spectra of CPO are shown in Fig. 1. It can be observed that the content of O in CPO is as much as 20.31% higher than in palm oil biodiesel (POB), which may be attributed to the presence of partial moisture in CPO, which further increases the content of oxygen. The POB was blended with commercial diesel fuel in volume ratios of 0:10, 1:9, 2:8, 3:7, and 10:0, respectively, and were designated as B0, B10, B20, B30, and B100, respectively (see Fig. S1, Supporting File). The main properties of these fuels are listed in Table 1. The POB has high oxygen content and high cetane number, which are favorable for
Fig. 1. FE-SEM image (left side) and EDX spectra (right side) of crude palm oil. 2
Fuel 260 (2020) 116326
J.C. Ge, et al.
operating conditions are given in the Tables S1 and S2 (Supporting File). Fig. 3a and 3b illustrate fuel volume fractions and combustion characteristics curves of the engine, respectively. As shown in Fig. 3b, the CA10 (crank angle of 10% mass fraction burned) and CA90 (crank angle of 90% mass fraction burned) are defined as the start of combustion (SOC) and the end of combustion (EOC), respectively. Therefore, the time period from the start of injection (SOI) to SOC is called the ignition delay (ID), and the crank angle interval from SOC to EOC is called the combustion duration.
Table 1 Fuel properties. Properties 3
Density (kg/m at 15 °C) Viscosity (mm2/s at 40 °C) Calorific value (MJ/kg) Cetane index Flash point (°C) Pour point (°C) Oxidation stability (h/110 °C) Ester content (%) Oxygen content (%)
B0
B10
B20
B30
B100
836.8 2.719 43.96 55.8 55 −21 25 – 0
841 2.893 43.43 – – – – – –
845 2.991 42.83 – – – – – –
849 3.173 42.31 – – – – – –
877 4.56 39.72 57.3 196.0 12.0 9.24 96.5 11.26
2.3. Exhaust emission analyzers A multi-gas analyzer (Eurotron, GreenLine MK2) and emission analyzer (Nantong Huapeng Electronics, HPC501) were used to measure the amount of CO, HC, and NOx emissions. An opacity smoke meter (QROTECH Co., Ltd., OPA-102) with partial flow sampling method was used to measure the amount of PM. Additionally, to observe the structure and morphology of the PM particles, 400 Mesh TEM grid (FCF400-Cu, Electron Microscopy Sciences, USA) was used to collect PM particles for all tested fuels, at 25% and 75% loads, with a constant engine speed of 1500 rpm. The PM particles loaded on the TEM grid were analyzed by a Transmission Electron Microscope (Hitachi, H7650) with a magnification of 200.00 KX.
ignition and combustion. However, the calorific value of B100 is 9.65% lower than that of B0. This is one of the main reasons for the increase in biodiesel consumption in diesel engines when they achieve the same power as diesel engines. 2.2. Experimental engine and apparatus A series of experiments were carried out on a 4-cylinder diesel engine with a common-rail direct injection (CRDI) system. The main engine specifications are displayed in Table 2. The schematic of the experimental setup is shown in Fig. 2. The engine was equipped with a turbocharger and an exhaust gas recirculation (EGR) system without cooler. An eddy-current-type water-cooled EC dynamometer (HWAN WOONG, HE-230) with a rating of 230 kW was used to control engine speed and load. The fuel injection strategies included pilot and main injection timing, and injection pressure was controlled by an electronic control unit (ECU) and their variables were allocated using the WinOLS software. A high-precision digital electronic weighing balance (AND, GP-100 K) was used to measure fuel consumption. A Kistler 6056A piezo-electric type pressure sensor was employed to measure in-cylinder pressure at a resolution of 1 °CA, and the signal from the pressure sensor was enlarged by a Kistler 5011B charge amplifier. An Omron Rotary Encoder E6B2-CWZ3E provided a position feedback signal, and was used to detect the angular position of the crankshaft. All combustion data was acquired and recorded by a NI PCI-6040E data acquisition (DAQ) board, and was further analyzed by the Cass combustion analysis software on a PC computer. To reduce the effect of cycle-to-cycle variations, the in-cylinder pressure and indicated mean effective pressure (IMEP) were sampled and recorded on 200 continuous cycles, and were then used to calculate the heat release rate (HRR) and the coefficient of variations in the IMEP (COVimep), respectively, to analyze the combustion characteristics and engine performance. Before recording the experimental data, the engine was run for about 30 min to reach the set cooling temperature (85 ± 3 °C) under idling conditions (750 rpm). The EGR rate and pilot injection timing, as the main variables, were controlled at 0%, 10%, 20%, and 14 °CA, 24 °CA and 34 °CA BTDC, respectively. The main injection timing was fixed at 4 °CA BTDC. Fig. S2 (in Supporting File) shows the timing chart for various pilot injection strategies. In addition, the engine load was controlled at 25% (35 Nm) and 75% (105 Nm), and the main injection pressure and engine speed were fixed at 60 MPa and 1500 rpm, respectively. Other detailed
3. Results and discussion 3.1. Combustion characteristics The in-cylinder pressures (CPs) for B0, B20, and B100 under 0%, 10%, and 20% EGR rate conditions are shown in Fig. 4a, 4b, and 4c. Those for B10 and B30 are shown in Figs. S3a and S3b (Supporting File). The main and pilot injection timings were fixed at 4 °CA and 24 °CA BTDC, respectively. As shown in these Figures, with increasing EGR rate, the CPs of all tested fuels were gradually reduced under all operating conditions. Fig. S4 (Supporting File) shows the peak value of CPs in the cylinder, which clearly shows the peak drop with increasing EGR. This indicates that EGR effectively reduced the charge temperature in the combustion chamber (due to dilution, as well as thermal and chemical effects), resulting in a reduction in combustion pressure [23]. In addition, the EGR rate had a significantly greater impact on the CP at high engine load (75%) than at low engine load (25%), mainly because of the greater oxygen demand during combustion at high load. Under the high engine load, the excess air ratio (λ) was about 1.9, significantly less than its value (3.6) under the low load. This indicates that the original oxygen content was very low at the high engine load, and the application of EGR further diluted the oxygen concentration, thereby curbing combustion and reducing the CP. Moreover, it was found that the CP of diesel was more affected by EGR than that of biodiesel and its blends. This is mainly due to the presence of oxygen in biodiesel, which can improve combustion characteristics in the combustion chamber compared with diesel fuel when a certain amount of EGR replaces the fresh air in the cylinder. At 75% engine load, the rate of rise in pressure was also reduced as EGR increased, which may be due to the lower peak temperatures. This reduction in peak temperature can be attributed to the dilution of oxygen concentration in the inlet air by applying EGR; EGR increases the specific heat capacity of the working fluid, and reduces the overall specific heat ratio due to the presence of H2O and CO2. The water contained in the exhaust gas also absorbs part of the heat in the combustion chamber [24,25]. Fig. 4d, 4e, and 4f illustrate the effects of various pilot injection timings on CP for B0, B20, and B100. Those for B10 and B30 are shown in Figs. S3c and S3d (Supporting File). Various pilot injection timings were controlled at 14 °CA, 24 °CA, and 34 °CA BTDC, respectively. As shown in these Figures, changing the pilot injection timing and engine load significantly affected the CP curve, including the peak CP and start of combustion, for all tested fuels. The peak CPs (see in Fig. S4) of all
Table 2 Engine specifications. Engine type
4-cylinder 4-stroke direct injection
Fuel injection system Air system EGR system Bore (mm) × Stroke (mm) Compression ratio Max. Power (Kw/rpm) Injector hole diameter (mm)
Bosch common-rail Turbocharger with WGT Water-cooled EGR pumps 83 × 92 17.7:1 82/4000 0.17
3
Fuel 260 (2020) 116326
J.C. Ge, et al.
Fig. 2. Schematic diagram of the experimental apparatus.
Fig. 3. (a) Fuel volume fractions and (b) combustion characteristics curves of engine.
S5 (Supporting File). As illustrated in these Figures, the peak HRR of all tested fuels was gradually reduced, and the beginning of heat release was gradually postponed, with increasing EGR rate under all operating conditions. The effect of EGR was more obvious on changes in the HRR curves at high engine load (75%), compared with low engine load (25%). These trends are consistent with those observed in the cylinder pressure curves. The reasons can be summarized as follows: the action of EGR reduced fuel combustion and flame propagation speed, which led to the decrease in the peak temperature and pressure in the combustion chamber under the dilution, thermal, and chemical effects of EGR [27]; moreover, the large specific heat capacity gases, such as water vapor and CO2, in the EGR gas replaced some of the available oxygen in the combustion chamber, and they not only diluted the available oxygen concentration and prevented rapid/violent combustion, but also absorbed a portion of the released heat. As a result, peak combustion temperature and in-cylinder pressure were reduced. Fig. 6 shows the heat release rate (HRR) for B0, B20, and B100 with various pilot injection timings. Those for B10 and B30 are shown in Fig. S6 (Supporting File). It can be observed that the HRR peaks of all fuels were reduced with the advance of pilot injection timing from 14 °CA to
tested fuels were slightly reduced as pilot injection timing increased from 14 °CA to 24 °CA and 34 °CA BTDC. The decrease in CP peak value was mainly due to the longer ignition delay period, which promoted the formation of lean and even mixtures. In addition, the peak CP depends on the combustion rate in the premixed combustion stage, and an earlier pilot injection can also increase the mix time for the fuel and air. Subsequently, the fuel rich areas are greatly reduced and the rapid combustion behavior is well controlled, eventually resulting in lower peak CP [19,26]. However, more interesting is that the start of combustion (SOC) for B0 at 24 °CA BTDC was obviously faster than at 34 °CA BTDC, and these two igniting times began to approach each other as the concentration of biodiesel in diesel increased. This may be caused by the pressure and temperature in the cylinder being too low to allow the fuel to self-ignite when the fuel is injected too early. In addition, the high cetane number of biodiesel itself can improve combustion characteristics. Another reason may be that premature injection causes some fuel to hang on the cylinder wall and the piston head (wall wetting issues), resulting in failure to burn normally. Fig. 5 shows the heat release rate (HRR) for B0, B20, and B100 with 0%, 10%, and 20% EGR rates. Those for B10 and B30 are shown in Fig. 4
Fuel 260 (2020) 116326
J.C. Ge, et al.
Fig. 4. In-cylinder pressure vs. EGR rate and pilot injection timing for B0, B20, and B100.
accumulated mass fraction burned (MFB) was studied with various EGR rates and pilot injection timings. The corresponding results are shown in Figs. 7 and 8, respectively. Those for B10 and B30 are shown in Figs. S7 and S8, respectively. As shown in Figs. 7 and S7 (Supporting File), the MFB curve of B0 combustion in the middle and late stages gradually moved backward as the EGR rate increased. With the addition of biodiesel to diesel fuel, this effect was significantly weakened, and the MFB curves were nearly equal for biodiesel-blended fuels. Overall, the EGR did not have a significant effect on ignition delay for all test fuels, which may be related to the EGR system without cooler used in this study. On the one hand, the hot EGR increased the inlet charge temperature, leading to almost the same start of combustion [28]. On the
24 °CA BTDC. The reason for this may be that a small amount of fuel was pre-injected into the combustion chamber, mixed well with the air, and the homogeneous mixture reduced the intense combustion of the main injected fuel. In addition, the pilot injection seemed to play a role similar to the internal EGR, which consumed part of the oxygen in advance, and kept the burned exhaust gas in the combustion chamber, thereby reducing the availability of the main injection fuel to oxygen. This result is consistent with those found by Jeon et al. [19]. The peak of HHR was unstable when the pilot injection timing occurred at 34 °CA BTDC, due to premature injection, leading to wall wetting issues and abnormal combustion. To further validate the above observations, the S-shaped
Fig. 5. Heat release rate vs. EGR rate for B0, B20, and B100. 5
Fuel 260 (2020) 116326
J.C. Ge, et al.
Fig. 6. Heat release rate vs. pilot injection timing for B0, B20, and B100.
Fig. 7. Mass fraction burned vs. EGR rate for B0, B20, and B100.
the advance injection of fuel will increase the in-cylinder temperature and improve the combustion environment in the combustion chamber [22]. However, the SOC of BTDC34 was slower than BTDC24, which indicates that the pilot injection timing occurred at 34 °CA BTDC was premature injection, resulting in wall wetting issues. This is consistent with the previous analysis of in-cylinder pressure.
other hand, in general, the ignition delay will be shortened with the increase of the proportion of biodiesel in the blend. This suggests that the oxygen content and high cetane number of biodiesel played a beneficial role in promoting combustion. However, these positive effects of biodiesel (increase oxygen concentration) offset some of the negative effects of EGR (reducing oxygen concentration). Figs. 8 and S8 (Supporting File) illustrate the influence of various pilot injection timings on MFB at 25% and 75% engine loads. The starting combustion point for all fuels was fastest when the pilot injection timing occurred at 24 °CA BTDC, followed by 34 °CA to 14 °CA BTDC in descending order. This indicates that the ignition delay at 24 °CA BTDC was the shortest than that at 34 °CA and 14 °CA BTDC. In a normal situation, the ignition timing is shortened with the pilot injection timing advanced. Because
3.2. Engine performance Fig. 9 illustrates the brake specific fuel consumption (BSFC) of all tested fuels at different EGR rates and pilot injection timings. Overall, the BSFC of B0 was the lowest compared with other biodiesel blends under all operating conditions, primarily because the calorific value of 6
Fuel 260 (2020) 116326
J.C. Ge, et al.
Fig. 8. Mass fraction burned vs. pilot injection timing for B0, B20, and B100.
Fig. 9. BSFC vs. EGR rate and pilot injection timing. 7
Fuel 260 (2020) 116326
J.C. Ge, et al.
10%. It shows that the CRDI diesel engine combustion of all tested fuels was very stable without large cycle-to-cycle variations under all operating conditions.
the biodiesel (39.72 MJ/kg) was lower than that of the diesel fuel (43.96 MJ/kg). This led to the biodiesel consuming slightly more fuel to achieve the same output. The BSFC under 75% load was significantly lower than that under 25% load, which can be explained by the environmental conditions in the combustion chamber. The high temperature, high pressure, and high turbulence intensity under 75% load were favorable for combustion, and the fuel could be fully burned so that even consumption of a small amount of fuel could achieve the predetermined energy. On the other hand, as EGR rate increased and pilot injection timing was advanced, the BSFC for most fuels increased to varying degrees. The increase of BSFC caused by the increase of EGR was mainly related to the concentration of oxygen in the combustion chamber. The EGR replaced a portion of the fresh air in the combustion chamber, reducing the air-fuel ratio. In addition, the influence of CO2 and water vapor in the EGR gas reduced the burn rate [16,23]. On the other hand, a slightly higher BSFC, due to the advance of pilot injection timing, was mainly affected by the following factors: increasing heat transfer loss, frictional loss, and negative compression work of air/fuel mixture [26]. Fig. S9 (Supporting File) represents the coefficient of variation of the indicated mean effective pressure (COVimep) for all fuels, according to various EGR rates and pilot injection timings. COVimep is an important parameter for evaluating engine cyclic variability in indicated work per cycle. Many researchers have pointed out that COVimep above 10% will cause vehicle drivability problems [29,30]. Overall, it can be seen that the COVimep of most tested fuels showed a slight increasing trend as the EGR rate increased, and a slight decreasing trend with the advance of pilot injection timing. The COVimep at 75% load was slightly lower than at 25% load. The slightly larger variation under low load was mainly related to combustion instability, lower combustion temperature, and incomplete combustion [31]. Looking at the overall situation, the COVimep of all tested fuels changed between 0.49% and 1.42%, far less than the upper limit of
3.3. Emission characteristics Figs. S10 (Supporting File) and 10 depict the variation of CO, HC, NOx, and PM emissions with various EGR rates. As shown in Fig. S10, the CO and HC emissions of all tested fuels increased with the increase in EGR rate under all operating conditions. In addition, the increasing trend of CO was not obvious at 25% load, but was quite significant at 75% load. Owing to the existence of EGR, the temperature, pressure, and oxygen concentration in the combustion chamber were reduced. The lack of oxygen (or the low concentration of oxygen) was the main reason for the formation of large amounts of CO emissions. In addition, the decrease in temperature was also an indirect reason for the formation of CO emissions [32]. The EGR effect reduced the oxygen concentration, environment temperature, and pressure in the cylinder. The low oxygen concentration was the main reason for the formation of large amounts of CO, while the high HC emissions were mainly related to the incorrect combustion of the heterogeneous mixture formed by the low excess oxygen concentration [16]. On the other hand, CO and HC emissions were gradually reduced as the concentration of biodiesel added to the diesel fuel increased, due primarily to the oxygen-containing characteristics of biodiesel. As shown in Fig. 10, with the increase in EGR rate, NOx emissions were gradually reduced, but PM concentration increased due to their trade-off relationship. Especially at 75% load, the changes of NOx and PM were obvious. NOx and PM emissions were reduced respectively by about approximately 54% and increased by about 8.4 times at 20% EGR compared with at 0% EGR. There are two main reasons for NOx formation; the concentration of available oxygen, and high temperature [33]. On the one hand, the existence of EGR plays a role in diluting the oxygen concentration in the
Fig. 10. NOx and PM emissions vs. EGR rate. 8
Fuel 260 (2020) 116326
J.C. Ge, et al.
Fig. 11. NOx and PM emissions vs. pilot injection timing.
near-complete oxidation of HC is lower than for CO, which resulted in HC presenting a slightly upward trend as the pilot injection timing was advanced. This may have contributed to the fuel having sufficient time to mix with the air when a small amount of fuel was injected into the cylinder in advance, thereby reducing the formation of rich fuel zones, improving combustion efficiency, reducing combustion rate and temperature, and resulting in less NOx and PM generation [37]. To further investigate the physical characteristics of the PM particles, the PM particle morphology for all tested fuels was analyzed using TEM images, with varying EGR rates and pilot injection timings, which are shown in Figs. 12 and S12 (Supporting File), respectively. PM particles emitted from diesel engines were deposited by several approximately spherical particles under the action of thermophoretic and Van der Waals forces. These PM particles are composed of volatile (organic, sulfate, nitrate fraction) and non-volatile (soot and ash) mixtures, most of them being carbonaceous soot particles (CSPs) [38,39]. The CSP components of diesel engine are produced by the heterogeneous combustion process, resulting in the formation of solid particle precursors in both the diffusion and the premixed flame. The formation of PM particles undergoes nucleation, surface growth and coagulation, agglomeration and oxidation [39]. The nucleation model of PM particles is mainly composed of volatile organics and the contribution of sulfur can be neglected when ultra-low sulfur fuel is used [38]. As shown in Figs. 12 and S12, it can be clearly observed that the PM morphology appeared as a chain-like aggregate composed of a number of many primary spherical or ellipsoid particles, which formed many condensed agglomeration structures. This is not only related to Van der Waals forces, but also to the collision and aggregation of PM discharged from engine fuel after combustion. Moreover, with the increase of the proportion of biodiesel in the blend, the oxidation of PM particles was promoted, which leaded to the further shrinkage and aggregation of PM particles. These results are consistent to the previous work by Salamanca et al. [40] and Qu et al. [41]. On the other hand,
combustion chamber, hindering the binding reaction between NO and O. On the other hand, the temperature of NOx formation is approximately 1500 °C, and the higher the temperature, the faster the generation rate. Thus, EGR has little effect on NOx at 25% load because the combustion temperature is not high at low loads, resulting in only a small amount of NOx formation. At 75% load, not only does the presence of EGR reduce the concentration of available oxygen, but the CO2 and water vapor in EGR absorb some of the heat in the combustion chamber, greatly reducing the production rate and total amount of NOx emissions [34]. The reduction of PM with the addition of biodiesel is mainly related to the high oxygen content and high cetane number of biodiesel. The biodiesel increases the available oxygen in the combustion chamber and promotes the combination of carbon atoms and oxygen, thus reducing the formation of unsaturated species such as acetylene and ethylene that lead to soot [35]. Based on the complementary effects of EGR and biodiesel, the use of B30 blend fuel with 10% or 20% EGR rate can effectively reduce PM emissions while NOx remains at a low level. Figs. S11 (Supporting File) and 11 represent the variation of CO, HC, NOx, and PM emissions with various pilot injection timings. As shown in Fig. S11, the effect of pilot injection timing on CO emissions is very obvious at low 25% load. CO emissions of B0 at 14° CA BTDC increase from the lowest concentration of 182 ppm, to the highest of 1326 ppm at 34° CA BTDC. The reason for this may be that when the pilot fuel was injected too early, the temperature and pressure in the cylinder were low, and the conditions for fuel spontaneous combustion were not met. This may have resulted in the pre-injection fuel hanging on the cylinder wall and piston head (wall wetting issues), or fill in piston clearance [36]. This seems to be similar to the characteristics of premixed charge compression ignition (PCCI) engines. At high engine loads such as 75% load, the rapidly rising temperature and pressure in the cylinder forces some of the CO emissions to be oxidized. The reason for the formation of HC is similar to that of CO, but the temperature for 9
Fuel 260 (2020) 116326
J.C. Ge, et al.
Fig. 12. EGR rate vs. TEM images at (a) 25% load and (b) 75% load.
diameter due to the increase of biodiesel mixing ratio may be attributed to the advantages of biodiesel, such as high cetane number and oxygen content, which improve combustion characteristics and promote more PM oxidation. Similar results have been reported by other researchers [42,43].
the diameters of the PM particles for all samples and under all operating conditions were much less than 100 nm, and the diameter values were primarily distributed between 20 nm and 50 nm. In addition, the diameter of the PM particles increased slightly with the increase of EGR rate, decreased with the increase of biodiesel blend ratio, and decreased with the increase in engine load from 25% to 75%. The increased EGR ratio leads to the increase in PM particle diameter for two primary reasons. First, EGR reduces the concentration of available oxygen in the combustion chamber, hinders complete combustion, and produces a large number of PM particles and unburned HC. The collision and aggregation of PM particles, together with the adsorption of unburned HC on the surface of PM, result in a direct increase of PM diameter. Second, some particles in EGR gas are reintroduced into the combustion chamber, which collide and aggregate with newly generated particles. However, the pilot injection timing had no obvious effect on PM particle diameter. With the increase in the biodiesel blend ratio, the micromorphology of the PM particles gradually changed from annular or spherical to cluster and catenoid structure, and more agglomerations and superimpositions were formed. This may be related to the shortcomings of biodiesel, such as high viscosity and density. These shortcomings may have resulted in a poor droplet breakup and atomization effect, increasing the amount of unburned fuel and the volatile organic fraction (VOF), thus further increasing the collision frequency and coagulation rate between PM particles. The decrease of PM particle
4. Conclusions As a reliable alternative fuel, palm oil biodiesel (POB) has great practical application value in diesel engines. To comprehensively investigate the performance, combustion and emission characteristics of POB and its blends in a CRDI diesel engine, a series of experimental conditions (such as EGR and pilot injection timing) were optimized. The following conclusions can be drawn from this study: (1) With an increase in EGR rate from 0% to 10% to 20%, the peak of in-cylinder pressure and heat release rate (HRR) are gradually reduced, especially with reduced NOx emissions. The downside is that CO, HC, and PM emissions are slightly increased, but these shortcomings are significantly improved by adding POB to the diesel fuel. (2) By observing the curves of in-cylinder pressure, HRR, mass fraction burned, and emission characteristics, it is seen that the pilot injection timing occurring at 34° CA BTDC is premature injection, 10
Fuel 260 (2020) 116326
J.C. Ge, et al.
Fig. 12. (continued)
No. 2019R1I1A1A01057727), the Korea government (MSIT) (No. 2019R1F1A1063154), and the Technology Development Program of Ministry of SMEs and Startups (MSS, Korea) (Project No. S2671652).
which causes some fuel to hang on the cylinder wall and the piston head (wall wetting issues), resulting in failure to burn normally. (3) Based on the engine performance and combustion characteristics, the diesel engine fueled with B30 blend fuel at a 10% EGR rate or with a pilot injection timing of 24° CA BTDC can effectively reduce PM emissions and simultaneously keep NOx emissions at low levels. (4) The diameters of the PM particles for all tested fuels were much less than 100 nm under all operating conditions, as shown by TEM image analysis, and the diameters are primarily distributed between 20 nm and 50 nm. In addition, the fuel properties and some engine operating conditions are the main factors affecting PM morphology.
Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.fuel.2019.116326. References [1] Lee YJ, Lim YW, Yang JY, Kim CS, Shin YC, Shin DC. Evaluating the PM damage cost due to urban air pollution and vehicle emissions in Seoul, Korea. J Environ Manage 2011;92(3):603–9. [2] Smit R, Ntziachristos L, Boulter P. Validation of road vehicle and traffic emission models–a review and meta-analysis. Atmos Environ 2010;44(25):2943–53. [3] Mohan B, Yang W, kiang Chou S. Fuel injection strategies for performance improvement and emissions reduction in compression ignition engines—A review. Renew Sustain Energy Rev 2013;28:664–76. [4] Wang X, Cheung C, Di Y, Huang Z. Diesel engine gaseous and particle emissions fueled with diesel-oxygenate blends. Fuel 2012;94:317–23. [5] Ren Y, Huang Z, Miao H, Di Y, Jiang D, Zeng K, et al. Combustion and emissions of a DI diesel engine fuelled with diesel-oxygenate blends. Fuel 2008;87(12):2691–7. [6] Mofijur M, Masjuki HH, Kalam MA, Atabani AE, Shahabuddin M, Palash SM, et al. Effect of biodiesel from various feedstocks on combustion characteristics, engine durability and materials compatibility: a review. Renew Sustain Energy Rev 2013;28:441–55. [7] Wang X, Zheng B, Huang Z, Zhang N, Zhang Y, Hu E. Performance and emissions of
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (Project No. 2016R1D1A1B03931616 and 11
Fuel 260 (2020) 116326
J.C. Ge, et al.
[8]
[9] [10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18] [19]
[20]
[21]
[22]
[23]
[24]
[25]
a turbocharged, high-pressure common rail diesel engine operating on biodiesel/ diesel blends. Proc Inst Mech Eng, Part D: J Autom Eng 2011;225(1):127–39. Gumus M, Kasifoglu S. Performance and emission evaluation of a compression ignition engine using a biodiesel (apricot seed kernel oil methyl ester) and its blends with diesel fuel. Biomass Bioenergy 2010;34(1):134–9. Atadashi I, Aroua M, Aziz AA. High quality biodiesel and its diesel engine application: a review. Renew Sustain Energy Rev 2010;14(7):1999–2008. Atabani AE, Silitonga AS, Badruddin IA, Mahlia T, Masjuki H, Mekhilef S. A comprehensive review on biodiesel as an alternative energy resource and its characteristics. Renew Sustain Energy Rev 2012;16(4):2070–93. Tang C, Guan L, Feng Z, Zhan C, Yang K, Huang Z. Effect of di-n-butyl ether blending with soybean-biodiesel on the near-nozzle spray characteristics. Fuel 2017;191:300–11. Wang X, Huang Z, Kuti OA, Zhang W, Nishida K. An experimental investigation on spray, ignition and combustion characteristics of biodiesels. Proc Combust Inst 2011;33(2):2071–7. Wang X, Huang Z, Kuti OA, Zhang W, Nishida K. Experimental and analytical study on biodiesel and diesel spray characteristics under ultra-high injection pressure. Int J Heat Fluid Flow 2010;31(4):659–66. Ali OM, Mamat R, Abdullah NR, Abdullah AA. Analysis of blended fuel properties and engine performance with palm biodiesel–diesel blended fuel. Renewable Energy 2016;86:59–67. Gomaa M, Alimin A, Kamarudin K. The effect of EGR rates on NOX and smoke emissions of an IDI diesel engine fuelled with Jatropha biodiesel blends. Int J Energy Environ 2011;2(3):477–90. Agarwal D, Sinha S, Agarwal AK. Experimental investigation of control of NOx emissions in biodiesel-fueled compression ignition engine. Renewable Energy 2006;31(14):2356–69. Mobasheri R, Peng Z, Mirsalim SM. Analysis the effect of advanced injection strategies on engine performance and pollutant emissions in a heavy duty DI-diesel engine by CFD modeling. Int J Heat Fluid Flow 2012;33(1):59–69. Zhang X, Liang Y, Yu E, Rao R, Xie J. Review of electric vehicle policies in China: Content summary and effect analysis. Renew Sustain Energy Rev 2017;70:698–714. Jeon J, Park S. Effects of pilot injection strategies on the flame temperature and soot distributions in an optical CI engine fueled with biodiesel and conventional diesel. Appl Energy 2015;160:581–91. d’Ambrosio S, Ferrari A. Potential of double pilot injection strategies optimized with the design of experiments procedure to improve diesel engine emissions and performance. Appl Energy 2015;155:918–32. Agarwal AK, Srivastava DK, Dhar A, Maurya RK, Shukla PC, Singh AP. Effect of fuel injection timing and pressure on combustion, emissions and performance characteristics of a single cylinder diesel engine. Fuel 2013;111:374–83. Ryu K. Effects of pilot injection timing on the combustion and emissions characteristics in a diesel engine using biodiesel–CNG dual fuel. Appl Energy 2013;111:721–30. Qi D, Leick M, Liu Y, Chia-fon FL. Effect of EGR and injection timing on combustion and emission characteristics of split injection strategy DI-diesel engine fueled with biodiesel. Fuel 2011;90(5):1884–91. Fathi M, Saray RK, Checkel MD. The influence of Exhaust Gas Recirculation (EGR) on combustion and emissions of n-heptane/natural gas fueled Homogeneous Charge Compression Ignition (HCCI) engines. Appl Energy 2011;88(12):4719–24. Yoon SH, Suh HK, Lee CS. Effect of spray and EGR rate on the combustion and
[26]
[27]
[28]
[29]
[30]
[31]
[32]
[33] [34] [35] [36]
[37]
[38]
[39] [40]
[41]
[42]
[43]
12
emission characteristics of biodiesel fuel in a compression ignition engine. Energy Fuels 2009;23(3):1486–93. Huang H, Liu Q, Yang R, Zhu T, Zhao R, Wang Y. Investigation on the effects of pilot injection on low temperature combustion in high-speed diesel engine fueled with nbutanol–diesel blends. Energy Convers Manage 2015;106:748–58. Hu E, Huang Z, Liu B, Zheng J, Gu X. Experimental study on combustion characteristics of a spark-ignition engine fueled with natural gas–hydrogen blends combining with EGR. Int J Hydrogen Energy 2009;34(2):1035–44. Tsolakis A, Megaritis A, Wyszynski ML, Theinnoi K. Engine performance and emissions of a diesel engine operating on diesel-RME (rapeseed methyl ester) blends with EGR (exhaust gas recirculation). Energy 2007;32(11):2072–80. Yao M, Chen Z, Zheng Z, Zhang B, Xing Y. Study on the controlling strategies of homogeneous charge compression ignition combustion with fuel of dimethyl ether and methanol. Fuel 2006;85(14–15):2046–56. Jamrozik A, Tutak W, Pyrc M, Sobiepański M. Effect of diesel/biodiesel/ethanol blend on combustion, performance and emissions characteristics on a direct injection diesel engine. Therm Sci 2017;21(1B):591–604. Ghadikolaei MA, Cheung CS, Yung K-F. Study of combustion, performance and emissions of diesel engine fueled with diesel/biodiesel/alcohol blends having the same oxygen concentration. Energy 2018;157:258–69. Deqing M, Junnan Q, Ping S, Yan M, Shuang Z, Yongjun C. Study on the combustion process and emissions of a turbocharged diesel engine with EGR. J Combust 2012;2012. Abd-Alla GH. Using exhaust gas recirculation in internal combustion engines: a review. Energy Convers Manage 2002;43(8):1027–42. Hoekman SK, Robbins C. Review of the effects of biodiesel on NOx emissions. Fuel Process Technol 2012;96:237–49. Tsolakis A. Effects on particle size distribution from the diesel engine operating on RME-biodiesel with EGR. Energy Fuels 2006;20(4):1418–24. Kiplimo R, Tomita E, Kawahara N, Yokobe S. Effects of spray impingement, injection parameters, and EGR on the combustion and emission characteristics of a PCCI diesel engine. Appl Therm Eng 2012;37:165–75. Liu J, Yang F, Wang H, Ouyang M, Hao S. Effects of pilot fuel quantity on the emissions characteristics of a CNG/diesel dual fuel engine with optimized pilot injection timing. Appl Energy 2013;110:201–6. Choi S, Myung C, Park S. Review on characterization of nano-particle emissions and PM morphology from internal combustion engines: Part 2. Int J Automot Technol 2014;15(2):219–27. Li R, Wang Z. Study on status characteristics and oxidation reactivity of biodiesel particulate matter. Fuel 2018;218:218–26. Salamanca M, Mondragón F, Agudelo JR, Benjumea P, Santamaría A. Variations in the chemical composition and morphology of soot induced by the unsaturation degree of biodiesel and a biodiesel blend. Combust Flame 2012;159(3):1100–8. Qu L, Wang Z, Zhang J. Influence of waste cooking oil biodiesel on oxidation reactivity and nanostructure of particulate matter from diesel engine. Fuel 2016;181:389–95. Yang K, Wei L, Cheung C, Tang C, Huang Z. The effect of pentanol addition on the particulate emission characteristics of a biodiesel operated diesel engine. Fuel 2017;209:132–40. Su J, Zhu H, Bohac SV. Particulate matter emission comparison from conventional and premixed low temperature combustion with diesel, biodiesel and biodiesel–ethanol fuels. Fuel 2013;113:221–7.
Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Full Length Article
Optimization of palm oil biodiesel blends and engine operating parameters to improve performance and PM morphology in a common rail direct injection diesel engine Jun Cong Gea, Ho Young Kima, Sam Ki Yoona, Nag Jung Choia, a
T
⁎
Division of Mechanical Design Engineering, Chonbuk National University, 567 Baekje-daero, Deokjin-gu, Jeonju-si, Jeollabuk-do 54896, Republic of Korea
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Palm oil biodiesel EGR Pilot injection timing PM morphology Mass fraction burned
In this paper, the characteristics of five palm oil biodiesel blends (B0, B10, B20, B30, and B100) have been tested in a common rail direct injection (CRDI) diesel engine with various EGR rates and pilot injection timings under 25% and 75% engine loads. The following were evaluated: combustion characteristics, including in-cylinder pressure, heat release rate (HRR), and mass fraction burned (MFB); engine performance parameters, including brake specific fuel consumption (BSFC) and coefficient of variation of the indicated mean effective pressure (COVimep); and emission characteristics, including CO, HC, NOx, PM and its morphology. By considering the combustion and emission characteristics, we found that the diesel engine fueled with B30 blend fuel with a 10% EGR rate, or with a pilot injection timing of 24° CA BTDC, can effectively reduce PM emissions and simultaneously keep NOx emissions at a low level. In addition, we found that the variations of the engine load and EGR rate, as well as the biodiesel blend ratio, have direct impacts on the PM particle diameter, shape, and other morphology.
⁎
Corresponding author. E-mail address: [email protected] (N.J. Choi).
https://doi.org/10.1016/j.fuel.2019.116326 Received 3 June 2019; Received in revised form 28 September 2019; Accepted 30 September 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
Fuel 260 (2020) 116326
J.C. Ge, et al.
1. Introduction
technology dilutes the concentration of oxygen in the mixture and reduces the combustion efficiency and combustion temperature, which in turn reduces NOx formation. Gomaa et al. [15] found that using jatropha biodiesel with 5–15% EGR rate can effectively balance the relationship between NOx and PM. Agarwal et al. [16] found that a combination of 20% rice bran oil-based biodiesel and 15% EGR rate not only improved thermal efficiency, but also reduced brake specific energy consumption (BSEC) and regulated pollutants from diesel engines. Split injection is also a promising method for reducing emissions and improving engine performance [17,18]. Li et al. [18] pointed out that the flame temperature peak was reduced effectively, using split injection with a small pilot ration and short dwell time in a single-cylinder diesel engine. Jeon et al. [19] found that pilot injection can improve combustion performance and fuel economy, and that NOx can be effectively controlled at BTDC 40 °CA in a single-cylinder diesel engine. The effects of injection modes on combustion, combustion noise, performance, and exhaust emission characteristics were also investigated and evaluated in other research [20–22]. Overall, engine performance, along with combustion and emission characteristics, are not only affected by the fuel properties, but also the engine parameters. However, most of the current research on injection strategy has been performed based on a single-cylinder engine or CFD simulation due to the variability of engine parameters. Therefore, it is necessary to optimize the relationship between fuel properties and engine parameters to improve engine performance and exhaust emissions. In the present study, the effects of various EGR and pilot injection conditions on combustion, engine performance, and regulated emissions were investigated in a CRDI 4-cylinder diesel engine fueled with various palm oil biodiesel blends. Additionally, the soot particle morphology for each test fuel was comparatively observed by transmission electron microscopy (TEM) under each operating condition.
Energy shortages and environmental pollution caused by fossilfueled vehicles have become a global concern. Especially in recent years, the sharp decline in urban air quality has been reported to be directly related to these vehicles. The main harmful air pollutants emitted by fossil-fueled vehicles are CO, HC, NOx, and PM. Compared with gasoline engines, diesel engines emit less CO and HC, but more NOx and PM. These emissions are factors in causing disease and can lead to a decline in human immunity. There may also be photochemical reactions between these emissions causing secondary pollution to further deteriorate urban air quality [1,2]. At present, although electric vehicles (EVs) are an effective way to reduce environmental pollution, their shortcomings (e.g. high capital and maintenance cost, immature technology, and small scale) are restricting their development. On the other hand, the conventional vehicles combined with exhaust aftertreatment equipment can meet the emission standards, but the disadvantage is that the vehicle cost and maintenance cost increse [3]. Therefore, finding a clean, renewable, high-efficiency alternative energy source is still an important challenge. Oxygenated fuels and biodiesel have become popular in recent decades because their high oxygen content can greatly reduce PM emissions [4,5]. Especially biodiesel, which has become the best alternative fuel for diesel because it is green, renewable, and biodegradable. Most importantly, its basic properties are similar to diesel fuel. Biodiesel can be extracted from a variety of vegetable oils, from algae, and from animal fats via simple transesterification. Biodiesel has a higher cetane number and oxygen content than diesel. This provides improved ignition properties and combustion characteristics, resulting in emission reduction at the source for CO, HC, and PM, but also causes a slight increase in NOx [6]. In addition, CO2 emissions, the main greenhouse gas, can be well controlled with the use of biodiesel [7]. However, a significant drawback is that the viscosity, density, and surface tension are higher in biodiesel than in diesel; this is because biodiesel is a monoalkyl ester composed of saturated fatty acids and unsaturated long chain fatty acids. These shortcomings influence the injected fuel atomization effect, leading to injector cooking and piston ring sticking [8–11]. Wang et al. [12,13] reported that the high viscosity of biodiesel leads to the small spray angle, poor spray atomization and air entrainment, longer injection delay and spray tip penetration. Blending biodiesel with diesel is one of the easiest and most effective ways to reduce the negative effects of biodiesel. Ali et al. [14] indicated that the cloud and pour points, as well as the density, viscosity, and acid value of biodiesel have been greatly improved when adding diesel to biodiesel. On the other hand, combustion and emission characteristics, including engine performance, can be improved by optimizing engine parameters such as exhaust gas recirculation (EGR) and split injection strategies. This is especially beneficial for simultaneously reducing NOx and PM emissions because of their trade-off relationship. EGR
2. Experimental setup 2.1. Fuel property measurements In this work, biodiesel was prepared from crude palm oil (CPO) by transesterification. CPO is mainly composed of triacylglycerols, mono and diacylglycerols, and also contains a small amount of free fatty acids (FFAs), non-oil fatty matter, moisture, and impurities. An FE-SEM image and the EDX spectra of CPO are shown in Fig. 1. It can be observed that the content of O in CPO is as much as 20.31% higher than in palm oil biodiesel (POB), which may be attributed to the presence of partial moisture in CPO, which further increases the content of oxygen. The POB was blended with commercial diesel fuel in volume ratios of 0:10, 1:9, 2:8, 3:7, and 10:0, respectively, and were designated as B0, B10, B20, B30, and B100, respectively (see Fig. S1, Supporting File). The main properties of these fuels are listed in Table 1. The POB has high oxygen content and high cetane number, which are favorable for
Fig. 1. FE-SEM image (left side) and EDX spectra (right side) of crude palm oil. 2
Fuel 260 (2020) 116326
J.C. Ge, et al.
operating conditions are given in the Tables S1 and S2 (Supporting File). Fig. 3a and 3b illustrate fuel volume fractions and combustion characteristics curves of the engine, respectively. As shown in Fig. 3b, the CA10 (crank angle of 10% mass fraction burned) and CA90 (crank angle of 90% mass fraction burned) are defined as the start of combustion (SOC) and the end of combustion (EOC), respectively. Therefore, the time period from the start of injection (SOI) to SOC is called the ignition delay (ID), and the crank angle interval from SOC to EOC is called the combustion duration.
Table 1 Fuel properties. Properties 3
Density (kg/m at 15 °C) Viscosity (mm2/s at 40 °C) Calorific value (MJ/kg) Cetane index Flash point (°C) Pour point (°C) Oxidation stability (h/110 °C) Ester content (%) Oxygen content (%)
B0
B10
B20
B30
B100
836.8 2.719 43.96 55.8 55 −21 25 – 0
841 2.893 43.43 – – – – – –
845 2.991 42.83 – – – – – –
849 3.173 42.31 – – – – – –
877 4.56 39.72 57.3 196.0 12.0 9.24 96.5 11.26
2.3. Exhaust emission analyzers A multi-gas analyzer (Eurotron, GreenLine MK2) and emission analyzer (Nantong Huapeng Electronics, HPC501) were used to measure the amount of CO, HC, and NOx emissions. An opacity smoke meter (QROTECH Co., Ltd., OPA-102) with partial flow sampling method was used to measure the amount of PM. Additionally, to observe the structure and morphology of the PM particles, 400 Mesh TEM grid (FCF400-Cu, Electron Microscopy Sciences, USA) was used to collect PM particles for all tested fuels, at 25% and 75% loads, with a constant engine speed of 1500 rpm. The PM particles loaded on the TEM grid were analyzed by a Transmission Electron Microscope (Hitachi, H7650) with a magnification of 200.00 KX.
ignition and combustion. However, the calorific value of B100 is 9.65% lower than that of B0. This is one of the main reasons for the increase in biodiesel consumption in diesel engines when they achieve the same power as diesel engines. 2.2. Experimental engine and apparatus A series of experiments were carried out on a 4-cylinder diesel engine with a common-rail direct injection (CRDI) system. The main engine specifications are displayed in Table 2. The schematic of the experimental setup is shown in Fig. 2. The engine was equipped with a turbocharger and an exhaust gas recirculation (EGR) system without cooler. An eddy-current-type water-cooled EC dynamometer (HWAN WOONG, HE-230) with a rating of 230 kW was used to control engine speed and load. The fuel injection strategies included pilot and main injection timing, and injection pressure was controlled by an electronic control unit (ECU) and their variables were allocated using the WinOLS software. A high-precision digital electronic weighing balance (AND, GP-100 K) was used to measure fuel consumption. A Kistler 6056A piezo-electric type pressure sensor was employed to measure in-cylinder pressure at a resolution of 1 °CA, and the signal from the pressure sensor was enlarged by a Kistler 5011B charge amplifier. An Omron Rotary Encoder E6B2-CWZ3E provided a position feedback signal, and was used to detect the angular position of the crankshaft. All combustion data was acquired and recorded by a NI PCI-6040E data acquisition (DAQ) board, and was further analyzed by the Cass combustion analysis software on a PC computer. To reduce the effect of cycle-to-cycle variations, the in-cylinder pressure and indicated mean effective pressure (IMEP) were sampled and recorded on 200 continuous cycles, and were then used to calculate the heat release rate (HRR) and the coefficient of variations in the IMEP (COVimep), respectively, to analyze the combustion characteristics and engine performance. Before recording the experimental data, the engine was run for about 30 min to reach the set cooling temperature (85 ± 3 °C) under idling conditions (750 rpm). The EGR rate and pilot injection timing, as the main variables, were controlled at 0%, 10%, 20%, and 14 °CA, 24 °CA and 34 °CA BTDC, respectively. The main injection timing was fixed at 4 °CA BTDC. Fig. S2 (in Supporting File) shows the timing chart for various pilot injection strategies. In addition, the engine load was controlled at 25% (35 Nm) and 75% (105 Nm), and the main injection pressure and engine speed were fixed at 60 MPa and 1500 rpm, respectively. Other detailed
3. Results and discussion 3.1. Combustion characteristics The in-cylinder pressures (CPs) for B0, B20, and B100 under 0%, 10%, and 20% EGR rate conditions are shown in Fig. 4a, 4b, and 4c. Those for B10 and B30 are shown in Figs. S3a and S3b (Supporting File). The main and pilot injection timings were fixed at 4 °CA and 24 °CA BTDC, respectively. As shown in these Figures, with increasing EGR rate, the CPs of all tested fuels were gradually reduced under all operating conditions. Fig. S4 (Supporting File) shows the peak value of CPs in the cylinder, which clearly shows the peak drop with increasing EGR. This indicates that EGR effectively reduced the charge temperature in the combustion chamber (due to dilution, as well as thermal and chemical effects), resulting in a reduction in combustion pressure [23]. In addition, the EGR rate had a significantly greater impact on the CP at high engine load (75%) than at low engine load (25%), mainly because of the greater oxygen demand during combustion at high load. Under the high engine load, the excess air ratio (λ) was about 1.9, significantly less than its value (3.6) under the low load. This indicates that the original oxygen content was very low at the high engine load, and the application of EGR further diluted the oxygen concentration, thereby curbing combustion and reducing the CP. Moreover, it was found that the CP of diesel was more affected by EGR than that of biodiesel and its blends. This is mainly due to the presence of oxygen in biodiesel, which can improve combustion characteristics in the combustion chamber compared with diesel fuel when a certain amount of EGR replaces the fresh air in the cylinder. At 75% engine load, the rate of rise in pressure was also reduced as EGR increased, which may be due to the lower peak temperatures. This reduction in peak temperature can be attributed to the dilution of oxygen concentration in the inlet air by applying EGR; EGR increases the specific heat capacity of the working fluid, and reduces the overall specific heat ratio due to the presence of H2O and CO2. The water contained in the exhaust gas also absorbs part of the heat in the combustion chamber [24,25]. Fig. 4d, 4e, and 4f illustrate the effects of various pilot injection timings on CP for B0, B20, and B100. Those for B10 and B30 are shown in Figs. S3c and S3d (Supporting File). Various pilot injection timings were controlled at 14 °CA, 24 °CA, and 34 °CA BTDC, respectively. As shown in these Figures, changing the pilot injection timing and engine load significantly affected the CP curve, including the peak CP and start of combustion, for all tested fuels. The peak CPs (see in Fig. S4) of all
Table 2 Engine specifications. Engine type
4-cylinder 4-stroke direct injection
Fuel injection system Air system EGR system Bore (mm) × Stroke (mm) Compression ratio Max. Power (Kw/rpm) Injector hole diameter (mm)
Bosch common-rail Turbocharger with WGT Water-cooled EGR pumps 83 × 92 17.7:1 82/4000 0.17
3
Fuel 260 (2020) 116326
J.C. Ge, et al.
Fig. 2. Schematic diagram of the experimental apparatus.
Fig. 3. (a) Fuel volume fractions and (b) combustion characteristics curves of engine.
S5 (Supporting File). As illustrated in these Figures, the peak HRR of all tested fuels was gradually reduced, and the beginning of heat release was gradually postponed, with increasing EGR rate under all operating conditions. The effect of EGR was more obvious on changes in the HRR curves at high engine load (75%), compared with low engine load (25%). These trends are consistent with those observed in the cylinder pressure curves. The reasons can be summarized as follows: the action of EGR reduced fuel combustion and flame propagation speed, which led to the decrease in the peak temperature and pressure in the combustion chamber under the dilution, thermal, and chemical effects of EGR [27]; moreover, the large specific heat capacity gases, such as water vapor and CO2, in the EGR gas replaced some of the available oxygen in the combustion chamber, and they not only diluted the available oxygen concentration and prevented rapid/violent combustion, but also absorbed a portion of the released heat. As a result, peak combustion temperature and in-cylinder pressure were reduced. Fig. 6 shows the heat release rate (HRR) for B0, B20, and B100 with various pilot injection timings. Those for B10 and B30 are shown in Fig. S6 (Supporting File). It can be observed that the HRR peaks of all fuels were reduced with the advance of pilot injection timing from 14 °CA to
tested fuels were slightly reduced as pilot injection timing increased from 14 °CA to 24 °CA and 34 °CA BTDC. The decrease in CP peak value was mainly due to the longer ignition delay period, which promoted the formation of lean and even mixtures. In addition, the peak CP depends on the combustion rate in the premixed combustion stage, and an earlier pilot injection can also increase the mix time for the fuel and air. Subsequently, the fuel rich areas are greatly reduced and the rapid combustion behavior is well controlled, eventually resulting in lower peak CP [19,26]. However, more interesting is that the start of combustion (SOC) for B0 at 24 °CA BTDC was obviously faster than at 34 °CA BTDC, and these two igniting times began to approach each other as the concentration of biodiesel in diesel increased. This may be caused by the pressure and temperature in the cylinder being too low to allow the fuel to self-ignite when the fuel is injected too early. In addition, the high cetane number of biodiesel itself can improve combustion characteristics. Another reason may be that premature injection causes some fuel to hang on the cylinder wall and the piston head (wall wetting issues), resulting in failure to burn normally. Fig. 5 shows the heat release rate (HRR) for B0, B20, and B100 with 0%, 10%, and 20% EGR rates. Those for B10 and B30 are shown in Fig. 4
Fuel 260 (2020) 116326
J.C. Ge, et al.
Fig. 4. In-cylinder pressure vs. EGR rate and pilot injection timing for B0, B20, and B100.
accumulated mass fraction burned (MFB) was studied with various EGR rates and pilot injection timings. The corresponding results are shown in Figs. 7 and 8, respectively. Those for B10 and B30 are shown in Figs. S7 and S8, respectively. As shown in Figs. 7 and S7 (Supporting File), the MFB curve of B0 combustion in the middle and late stages gradually moved backward as the EGR rate increased. With the addition of biodiesel to diesel fuel, this effect was significantly weakened, and the MFB curves were nearly equal for biodiesel-blended fuels. Overall, the EGR did not have a significant effect on ignition delay for all test fuels, which may be related to the EGR system without cooler used in this study. On the one hand, the hot EGR increased the inlet charge temperature, leading to almost the same start of combustion [28]. On the
24 °CA BTDC. The reason for this may be that a small amount of fuel was pre-injected into the combustion chamber, mixed well with the air, and the homogeneous mixture reduced the intense combustion of the main injected fuel. In addition, the pilot injection seemed to play a role similar to the internal EGR, which consumed part of the oxygen in advance, and kept the burned exhaust gas in the combustion chamber, thereby reducing the availability of the main injection fuel to oxygen. This result is consistent with those found by Jeon et al. [19]. The peak of HHR was unstable when the pilot injection timing occurred at 34 °CA BTDC, due to premature injection, leading to wall wetting issues and abnormal combustion. To further validate the above observations, the S-shaped
Fig. 5. Heat release rate vs. EGR rate for B0, B20, and B100. 5
Fuel 260 (2020) 116326
J.C. Ge, et al.
Fig. 6. Heat release rate vs. pilot injection timing for B0, B20, and B100.
Fig. 7. Mass fraction burned vs. EGR rate for B0, B20, and B100.
the advance injection of fuel will increase the in-cylinder temperature and improve the combustion environment in the combustion chamber [22]. However, the SOC of BTDC34 was slower than BTDC24, which indicates that the pilot injection timing occurred at 34 °CA BTDC was premature injection, resulting in wall wetting issues. This is consistent with the previous analysis of in-cylinder pressure.
other hand, in general, the ignition delay will be shortened with the increase of the proportion of biodiesel in the blend. This suggests that the oxygen content and high cetane number of biodiesel played a beneficial role in promoting combustion. However, these positive effects of biodiesel (increase oxygen concentration) offset some of the negative effects of EGR (reducing oxygen concentration). Figs. 8 and S8 (Supporting File) illustrate the influence of various pilot injection timings on MFB at 25% and 75% engine loads. The starting combustion point for all fuels was fastest when the pilot injection timing occurred at 24 °CA BTDC, followed by 34 °CA to 14 °CA BTDC in descending order. This indicates that the ignition delay at 24 °CA BTDC was the shortest than that at 34 °CA and 14 °CA BTDC. In a normal situation, the ignition timing is shortened with the pilot injection timing advanced. Because
3.2. Engine performance Fig. 9 illustrates the brake specific fuel consumption (BSFC) of all tested fuels at different EGR rates and pilot injection timings. Overall, the BSFC of B0 was the lowest compared with other biodiesel blends under all operating conditions, primarily because the calorific value of 6
Fuel 260 (2020) 116326
J.C. Ge, et al.
Fig. 8. Mass fraction burned vs. pilot injection timing for B0, B20, and B100.
Fig. 9. BSFC vs. EGR rate and pilot injection timing. 7
Fuel 260 (2020) 116326
J.C. Ge, et al.
10%. It shows that the CRDI diesel engine combustion of all tested fuels was very stable without large cycle-to-cycle variations under all operating conditions.
the biodiesel (39.72 MJ/kg) was lower than that of the diesel fuel (43.96 MJ/kg). This led to the biodiesel consuming slightly more fuel to achieve the same output. The BSFC under 75% load was significantly lower than that under 25% load, which can be explained by the environmental conditions in the combustion chamber. The high temperature, high pressure, and high turbulence intensity under 75% load were favorable for combustion, and the fuel could be fully burned so that even consumption of a small amount of fuel could achieve the predetermined energy. On the other hand, as EGR rate increased and pilot injection timing was advanced, the BSFC for most fuels increased to varying degrees. The increase of BSFC caused by the increase of EGR was mainly related to the concentration of oxygen in the combustion chamber. The EGR replaced a portion of the fresh air in the combustion chamber, reducing the air-fuel ratio. In addition, the influence of CO2 and water vapor in the EGR gas reduced the burn rate [16,23]. On the other hand, a slightly higher BSFC, due to the advance of pilot injection timing, was mainly affected by the following factors: increasing heat transfer loss, frictional loss, and negative compression work of air/fuel mixture [26]. Fig. S9 (Supporting File) represents the coefficient of variation of the indicated mean effective pressure (COVimep) for all fuels, according to various EGR rates and pilot injection timings. COVimep is an important parameter for evaluating engine cyclic variability in indicated work per cycle. Many researchers have pointed out that COVimep above 10% will cause vehicle drivability problems [29,30]. Overall, it can be seen that the COVimep of most tested fuels showed a slight increasing trend as the EGR rate increased, and a slight decreasing trend with the advance of pilot injection timing. The COVimep at 75% load was slightly lower than at 25% load. The slightly larger variation under low load was mainly related to combustion instability, lower combustion temperature, and incomplete combustion [31]. Looking at the overall situation, the COVimep of all tested fuels changed between 0.49% and 1.42%, far less than the upper limit of
3.3. Emission characteristics Figs. S10 (Supporting File) and 10 depict the variation of CO, HC, NOx, and PM emissions with various EGR rates. As shown in Fig. S10, the CO and HC emissions of all tested fuels increased with the increase in EGR rate under all operating conditions. In addition, the increasing trend of CO was not obvious at 25% load, but was quite significant at 75% load. Owing to the existence of EGR, the temperature, pressure, and oxygen concentration in the combustion chamber were reduced. The lack of oxygen (or the low concentration of oxygen) was the main reason for the formation of large amounts of CO emissions. In addition, the decrease in temperature was also an indirect reason for the formation of CO emissions [32]. The EGR effect reduced the oxygen concentration, environment temperature, and pressure in the cylinder. The low oxygen concentration was the main reason for the formation of large amounts of CO, while the high HC emissions were mainly related to the incorrect combustion of the heterogeneous mixture formed by the low excess oxygen concentration [16]. On the other hand, CO and HC emissions were gradually reduced as the concentration of biodiesel added to the diesel fuel increased, due primarily to the oxygen-containing characteristics of biodiesel. As shown in Fig. 10, with the increase in EGR rate, NOx emissions were gradually reduced, but PM concentration increased due to their trade-off relationship. Especially at 75% load, the changes of NOx and PM were obvious. NOx and PM emissions were reduced respectively by about approximately 54% and increased by about 8.4 times at 20% EGR compared with at 0% EGR. There are two main reasons for NOx formation; the concentration of available oxygen, and high temperature [33]. On the one hand, the existence of EGR plays a role in diluting the oxygen concentration in the
Fig. 10. NOx and PM emissions vs. EGR rate. 8
Fuel 260 (2020) 116326
J.C. Ge, et al.
Fig. 11. NOx and PM emissions vs. pilot injection timing.
near-complete oxidation of HC is lower than for CO, which resulted in HC presenting a slightly upward trend as the pilot injection timing was advanced. This may have contributed to the fuel having sufficient time to mix with the air when a small amount of fuel was injected into the cylinder in advance, thereby reducing the formation of rich fuel zones, improving combustion efficiency, reducing combustion rate and temperature, and resulting in less NOx and PM generation [37]. To further investigate the physical characteristics of the PM particles, the PM particle morphology for all tested fuels was analyzed using TEM images, with varying EGR rates and pilot injection timings, which are shown in Figs. 12 and S12 (Supporting File), respectively. PM particles emitted from diesel engines were deposited by several approximately spherical particles under the action of thermophoretic and Van der Waals forces. These PM particles are composed of volatile (organic, sulfate, nitrate fraction) and non-volatile (soot and ash) mixtures, most of them being carbonaceous soot particles (CSPs) [38,39]. The CSP components of diesel engine are produced by the heterogeneous combustion process, resulting in the formation of solid particle precursors in both the diffusion and the premixed flame. The formation of PM particles undergoes nucleation, surface growth and coagulation, agglomeration and oxidation [39]. The nucleation model of PM particles is mainly composed of volatile organics and the contribution of sulfur can be neglected when ultra-low sulfur fuel is used [38]. As shown in Figs. 12 and S12, it can be clearly observed that the PM morphology appeared as a chain-like aggregate composed of a number of many primary spherical or ellipsoid particles, which formed many condensed agglomeration structures. This is not only related to Van der Waals forces, but also to the collision and aggregation of PM discharged from engine fuel after combustion. Moreover, with the increase of the proportion of biodiesel in the blend, the oxidation of PM particles was promoted, which leaded to the further shrinkage and aggregation of PM particles. These results are consistent to the previous work by Salamanca et al. [40] and Qu et al. [41]. On the other hand,
combustion chamber, hindering the binding reaction between NO and O. On the other hand, the temperature of NOx formation is approximately 1500 °C, and the higher the temperature, the faster the generation rate. Thus, EGR has little effect on NOx at 25% load because the combustion temperature is not high at low loads, resulting in only a small amount of NOx formation. At 75% load, not only does the presence of EGR reduce the concentration of available oxygen, but the CO2 and water vapor in EGR absorb some of the heat in the combustion chamber, greatly reducing the production rate and total amount of NOx emissions [34]. The reduction of PM with the addition of biodiesel is mainly related to the high oxygen content and high cetane number of biodiesel. The biodiesel increases the available oxygen in the combustion chamber and promotes the combination of carbon atoms and oxygen, thus reducing the formation of unsaturated species such as acetylene and ethylene that lead to soot [35]. Based on the complementary effects of EGR and biodiesel, the use of B30 blend fuel with 10% or 20% EGR rate can effectively reduce PM emissions while NOx remains at a low level. Figs. S11 (Supporting File) and 11 represent the variation of CO, HC, NOx, and PM emissions with various pilot injection timings. As shown in Fig. S11, the effect of pilot injection timing on CO emissions is very obvious at low 25% load. CO emissions of B0 at 14° CA BTDC increase from the lowest concentration of 182 ppm, to the highest of 1326 ppm at 34° CA BTDC. The reason for this may be that when the pilot fuel was injected too early, the temperature and pressure in the cylinder were low, and the conditions for fuel spontaneous combustion were not met. This may have resulted in the pre-injection fuel hanging on the cylinder wall and piston head (wall wetting issues), or fill in piston clearance [36]. This seems to be similar to the characteristics of premixed charge compression ignition (PCCI) engines. At high engine loads such as 75% load, the rapidly rising temperature and pressure in the cylinder forces some of the CO emissions to be oxidized. The reason for the formation of HC is similar to that of CO, but the temperature for 9
Fuel 260 (2020) 116326
J.C. Ge, et al.
Fig. 12. EGR rate vs. TEM images at (a) 25% load and (b) 75% load.
diameter due to the increase of biodiesel mixing ratio may be attributed to the advantages of biodiesel, such as high cetane number and oxygen content, which improve combustion characteristics and promote more PM oxidation. Similar results have been reported by other researchers [42,43].
the diameters of the PM particles for all samples and under all operating conditions were much less than 100 nm, and the diameter values were primarily distributed between 20 nm and 50 nm. In addition, the diameter of the PM particles increased slightly with the increase of EGR rate, decreased with the increase of biodiesel blend ratio, and decreased with the increase in engine load from 25% to 75%. The increased EGR ratio leads to the increase in PM particle diameter for two primary reasons. First, EGR reduces the concentration of available oxygen in the combustion chamber, hinders complete combustion, and produces a large number of PM particles and unburned HC. The collision and aggregation of PM particles, together with the adsorption of unburned HC on the surface of PM, result in a direct increase of PM diameter. Second, some particles in EGR gas are reintroduced into the combustion chamber, which collide and aggregate with newly generated particles. However, the pilot injection timing had no obvious effect on PM particle diameter. With the increase in the biodiesel blend ratio, the micromorphology of the PM particles gradually changed from annular or spherical to cluster and catenoid structure, and more agglomerations and superimpositions were formed. This may be related to the shortcomings of biodiesel, such as high viscosity and density. These shortcomings may have resulted in a poor droplet breakup and atomization effect, increasing the amount of unburned fuel and the volatile organic fraction (VOF), thus further increasing the collision frequency and coagulation rate between PM particles. The decrease of PM particle
4. Conclusions As a reliable alternative fuel, palm oil biodiesel (POB) has great practical application value in diesel engines. To comprehensively investigate the performance, combustion and emission characteristics of POB and its blends in a CRDI diesel engine, a series of experimental conditions (such as EGR and pilot injection timing) were optimized. The following conclusions can be drawn from this study: (1) With an increase in EGR rate from 0% to 10% to 20%, the peak of in-cylinder pressure and heat release rate (HRR) are gradually reduced, especially with reduced NOx emissions. The downside is that CO, HC, and PM emissions are slightly increased, but these shortcomings are significantly improved by adding POB to the diesel fuel. (2) By observing the curves of in-cylinder pressure, HRR, mass fraction burned, and emission characteristics, it is seen that the pilot injection timing occurring at 34° CA BTDC is premature injection, 10
Fuel 260 (2020) 116326
J.C. Ge, et al.
Fig. 12. (continued)
No. 2019R1I1A1A01057727), the Korea government (MSIT) (No. 2019R1F1A1063154), and the Technology Development Program of Ministry of SMEs and Startups (MSS, Korea) (Project No. S2671652).
which causes some fuel to hang on the cylinder wall and the piston head (wall wetting issues), resulting in failure to burn normally. (3) Based on the engine performance and combustion characteristics, the diesel engine fueled with B30 blend fuel at a 10% EGR rate or with a pilot injection timing of 24° CA BTDC can effectively reduce PM emissions and simultaneously keep NOx emissions at low levels. (4) The diameters of the PM particles for all tested fuels were much less than 100 nm under all operating conditions, as shown by TEM image analysis, and the diameters are primarily distributed between 20 nm and 50 nm. In addition, the fuel properties and some engine operating conditions are the main factors affecting PM morphology.
Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.fuel.2019.116326. References [1] Lee YJ, Lim YW, Yang JY, Kim CS, Shin YC, Shin DC. Evaluating the PM damage cost due to urban air pollution and vehicle emissions in Seoul, Korea. J Environ Manage 2011;92(3):603–9. [2] Smit R, Ntziachristos L, Boulter P. Validation of road vehicle and traffic emission models–a review and meta-analysis. Atmos Environ 2010;44(25):2943–53. [3] Mohan B, Yang W, kiang Chou S. Fuel injection strategies for performance improvement and emissions reduction in compression ignition engines—A review. Renew Sustain Energy Rev 2013;28:664–76. [4] Wang X, Cheung C, Di Y, Huang Z. Diesel engine gaseous and particle emissions fueled with diesel-oxygenate blends. Fuel 2012;94:317–23. [5] Ren Y, Huang Z, Miao H, Di Y, Jiang D, Zeng K, et al. Combustion and emissions of a DI diesel engine fuelled with diesel-oxygenate blends. Fuel 2008;87(12):2691–7. [6] Mofijur M, Masjuki HH, Kalam MA, Atabani AE, Shahabuddin M, Palash SM, et al. Effect of biodiesel from various feedstocks on combustion characteristics, engine durability and materials compatibility: a review. Renew Sustain Energy Rev 2013;28:441–55. [7] Wang X, Zheng B, Huang Z, Zhang N, Zhang Y, Hu E. Performance and emissions of
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (Project No. 2016R1D1A1B03931616 and 11
Fuel 260 (2020) 116326
J.C. Ge, et al.
[8]
[9] [10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18] [19]
[20]
[21]
[22]
[23]
[24]
[25]
a turbocharged, high-pressure common rail diesel engine operating on biodiesel/ diesel blends. Proc Inst Mech Eng, Part D: J Autom Eng 2011;225(1):127–39. Gumus M, Kasifoglu S. Performance and emission evaluation of a compression ignition engine using a biodiesel (apricot seed kernel oil methyl ester) and its blends with diesel fuel. Biomass Bioenergy 2010;34(1):134–9. Atadashi I, Aroua M, Aziz AA. High quality biodiesel and its diesel engine application: a review. Renew Sustain Energy Rev 2010;14(7):1999–2008. Atabani AE, Silitonga AS, Badruddin IA, Mahlia T, Masjuki H, Mekhilef S. A comprehensive review on biodiesel as an alternative energy resource and its characteristics. Renew Sustain Energy Rev 2012;16(4):2070–93. Tang C, Guan L, Feng Z, Zhan C, Yang K, Huang Z. Effect of di-n-butyl ether blending with soybean-biodiesel on the near-nozzle spray characteristics. Fuel 2017;191:300–11. Wang X, Huang Z, Kuti OA, Zhang W, Nishida K. An experimental investigation on spray, ignition and combustion characteristics of biodiesels. Proc Combust Inst 2011;33(2):2071–7. Wang X, Huang Z, Kuti OA, Zhang W, Nishida K. Experimental and analytical study on biodiesel and diesel spray characteristics under ultra-high injection pressure. Int J Heat Fluid Flow 2010;31(4):659–66. Ali OM, Mamat R, Abdullah NR, Abdullah AA. Analysis of blended fuel properties and engine performance with palm biodiesel–diesel blended fuel. Renewable Energy 2016;86:59–67. Gomaa M, Alimin A, Kamarudin K. The effect of EGR rates on NOX and smoke emissions of an IDI diesel engine fuelled with Jatropha biodiesel blends. Int J Energy Environ 2011;2(3):477–90. Agarwal D, Sinha S, Agarwal AK. Experimental investigation of control of NOx emissions in biodiesel-fueled compression ignition engine. Renewable Energy 2006;31(14):2356–69. Mobasheri R, Peng Z, Mirsalim SM. Analysis the effect of advanced injection strategies on engine performance and pollutant emissions in a heavy duty DI-diesel engine by CFD modeling. Int J Heat Fluid Flow 2012;33(1):59–69. Zhang X, Liang Y, Yu E, Rao R, Xie J. Review of electric vehicle policies in China: Content summary and effect analysis. Renew Sustain Energy Rev 2017;70:698–714. Jeon J, Park S. Effects of pilot injection strategies on the flame temperature and soot distributions in an optical CI engine fueled with biodiesel and conventional diesel. Appl Energy 2015;160:581–91. d’Ambrosio S, Ferrari A. Potential of double pilot injection strategies optimized with the design of experiments procedure to improve diesel engine emissions and performance. Appl Energy 2015;155:918–32. Agarwal AK, Srivastava DK, Dhar A, Maurya RK, Shukla PC, Singh AP. Effect of fuel injection timing and pressure on combustion, emissions and performance characteristics of a single cylinder diesel engine. Fuel 2013;111:374–83. Ryu K. Effects of pilot injection timing on the combustion and emissions characteristics in a diesel engine using biodiesel–CNG dual fuel. Appl Energy 2013;111:721–30. Qi D, Leick M, Liu Y, Chia-fon FL. Effect of EGR and injection timing on combustion and emission characteristics of split injection strategy DI-diesel engine fueled with biodiesel. Fuel 2011;90(5):1884–91. Fathi M, Saray RK, Checkel MD. The influence of Exhaust Gas Recirculation (EGR) on combustion and emissions of n-heptane/natural gas fueled Homogeneous Charge Compression Ignition (HCCI) engines. Appl Energy 2011;88(12):4719–24. Yoon SH, Suh HK, Lee CS. Effect of spray and EGR rate on the combustion and
[26]
[27]
[28]
[29]
[30]
[31]
[32]
[33] [34] [35] [36]
[37]
[38]
[39] [40]
[41]
[42]
[43]
12
emission characteristics of biodiesel fuel in a compression ignition engine. Energy Fuels 2009;23(3):1486–93. Huang H, Liu Q, Yang R, Zhu T, Zhao R, Wang Y. Investigation on the effects of pilot injection on low temperature combustion in high-speed diesel engine fueled with nbutanol–diesel blends. Energy Convers Manage 2015;106:748–58. Hu E, Huang Z, Liu B, Zheng J, Gu X. Experimental study on combustion characteristics of a spark-ignition engine fueled with natural gas–hydrogen blends combining with EGR. Int J Hydrogen Energy 2009;34(2):1035–44. Tsolakis A, Megaritis A, Wyszynski ML, Theinnoi K. Engine performance and emissions of a diesel engine operating on diesel-RME (rapeseed methyl ester) blends with EGR (exhaust gas recirculation). Energy 2007;32(11):2072–80. Yao M, Chen Z, Zheng Z, Zhang B, Xing Y. Study on the controlling strategies of homogeneous charge compression ignition combustion with fuel of dimethyl ether and methanol. Fuel 2006;85(14–15):2046–56. Jamrozik A, Tutak W, Pyrc M, Sobiepański M. Effect of diesel/biodiesel/ethanol blend on combustion, performance and emissions characteristics on a direct injection diesel engine. Therm Sci 2017;21(1B):591–604. Ghadikolaei MA, Cheung CS, Yung K-F. Study of combustion, performance and emissions of diesel engine fueled with diesel/biodiesel/alcohol blends having the same oxygen concentration. Energy 2018;157:258–69. Deqing M, Junnan Q, Ping S, Yan M, Shuang Z, Yongjun C. Study on the combustion process and emissions of a turbocharged diesel engine with EGR. J Combust 2012;2012. Abd-Alla GH. Using exhaust gas recirculation in internal combustion engines: a review. Energy Convers Manage 2002;43(8):1027–42. Hoekman SK, Robbins C. Review of the effects of biodiesel on NOx emissions. Fuel Process Technol 2012;96:237–49. Tsolakis A. Effects on particle size distribution from the diesel engine operating on RME-biodiesel with EGR. Energy Fuels 2006;20(4):1418–24. Kiplimo R, Tomita E, Kawahara N, Yokobe S. Effects of spray impingement, injection parameters, and EGR on the combustion and emission characteristics of a PCCI diesel engine. Appl Therm Eng 2012;37:165–75. Liu J, Yang F, Wang H, Ouyang M, Hao S. Effects of pilot fuel quantity on the emissions characteristics of a CNG/diesel dual fuel engine with optimized pilot injection timing. Appl Energy 2013;110:201–6. Choi S, Myung C, Park S. Review on characterization of nano-particle emissions and PM morphology from internal combustion engines: Part 2. Int J Automot Technol 2014;15(2):219–27. Li R, Wang Z. Study on status characteristics and oxidation reactivity of biodiesel particulate matter. Fuel 2018;218:218–26. Salamanca M, Mondragón F, Agudelo JR, Benjumea P, Santamaría A. Variations in the chemical composition and morphology of soot induced by the unsaturation degree of biodiesel and a biodiesel blend. Combust Flame 2012;159(3):1100–8. Qu L, Wang Z, Zhang J. Influence of waste cooking oil biodiesel on oxidation reactivity and nanostructure of particulate matter from diesel engine. Fuel 2016;181:389–95. Yang K, Wei L, Cheung C, Tang C, Huang Z. The effect of pentanol addition on the particulate emission characteristics of a biodiesel operated diesel engine. Fuel 2017;209:132–40. Su J, Zhu H, Bohac SV. Particulate matter emission comparison from conventional and premixed low temperature combustion with diesel, biodiesel and biodiesel–ethanol fuels. Fuel 2013;113:221–7.