1. Introduction
Using internal combustion engines provide convenient transportation. However, air pollution and depletion of the resources caused by internal combustion engines are serious problems. The pollutant emissions from internal combustion engines, such as nitrogen oxides (NOx), carbon monoxide (CO), unburned hydrocarbon (HC), and particulate matter (PM), are subject to strict regulation [1]. Further, global efforts are underway to reduce CO2, CH4, and N2O, the GHGs (greenhouse gasses) that affect global warming and climate change problems [2,3]. The exhaust gas emitted from diesel engine contains higher amount of NOx and particulate matter that causes of severe environmental problems affecting human health [4]. Regulated emissions are not the only pollutants from engines. There are volatile organic compounds (VOCs) that are emitted in small quantities but make photo-chemical smog from a reaction with nitrogen oxide and have an important role in the formation of ozone [5,6,7,8]. VOCs are emitted from various pollution sources, among them, vehicles using internal combustion engines are known to be the major source of VOCs in metropolitan areas with high densities of people [7,9]. Thus, VOCs are known as the precursors of photochemical smog and ozone [6,7]. The important characteristic of VOCs is toxicity, and some VOCs are toxic for human and animals [10]. Representative toxic VOCs are benzene, toluene, ethylbenzene, and xylene in the aromatic family [11,12,13]. Benzene is classified as a class 1 carcinogen by the World Health Organization (WHO) and the International Agency for Research (IARC) [14]. Long exposure to benzene may cause bone marrow damage, aplastic anemia, and leukemia. Ethylbenzene is known as a potential carcinogenic material. Toluene and xylene are harmful and classified as group d, which means non-carcinogenic. Toluene may cause eye, nose, and throat irritation as well as headaches and dizziness by impacting the central nervous system. Long exposure to xylene irritates eyes and may cause blindness. Chen et al. [15] reported that long exposure to toluene, xylene, and ethylbenzene may cause chronic nerve damage. For these reasons, the risk rate for emissions from diesel engines was raised to group 1 from group 2a by IARC and WHO in 2012 June [16]. Many researchers [5,7,9,17,18] analyzed harmful VOCs by collecting air from major cities around the world. Na et al. [17] studied VOCs in Seoul, Korea from 1997 to 1999 and showed that 58% of aromatic VOCs were emitted from vehicles. Also, Kim et al. [9] reported that the major source of VOCs was vehicles, and toluene was emitted the most among aromatic VOCs from vehicles. Tsai et al. [19] studied the VOCs from light-duty diesel vehicles with a chassis dynamometer and showed that aromatic VOCs represented the high portion of pollutants. Wang et al. [7] checked the levels of VOCs using light-duty diesel vehicles under different regulation levels operated on real roads in a big city in China. They reported that benzene is the most common material among the aromatic VOCs. The development of catalysts for reducing aromatic VOCs is ongoing using ceria, CeOy and MnOx, and Manganese [20,21,22].
One of the effective ways to slow the depletion of resources and reduce pollutant emissions is to use biofuel. The representative biofuels are biodiesel and bioethanol [3]. The physical properties of biodiesel are similar to petroleum diesel so that it can be used without mechanical modification of diesel engines [3,23]. Biodiesels contain 10–12% oxygen by weight, which can improve combustion efficiency [24,25]. In contrast, the calorific value is lower than petroleum diesel, which results in more fuel consumption than using a petroleum diesel to produce the same output. The high viscosity, density, and surface tension can deteriorate the combustion because of poor atomization of injected fuel, which means droplet sizes are much larger [26,27,28]. Many studies [29,30,31] show the possibility of reducing pollutant emissions, such as HC, CO, and PM. In addition to the effects on regulated emissions by biodiesels, many researchers [10,32,33,34,35,36] are studying the possibility of reducing VOCs using biodiesels. Ge et al. [10] used a common rail direct injection diesel engine by applying canola biodiesel under different engine loads and showed the possibility of reducing VOCs by biodiesel. Di et al. [32] reported the emission trend of VOCs and pollutants using waste cooking oil blends corresponding to 2%, 4%, 6%, and 8% by mass of oxygen content under the five engine loads at 1800 rpm. In that study, with an increase of biodiesel, benzene increased for each engine load, however toluene and xylene decreased. Peng et al. [33] studied the effects of the 20% soybean oil biodiesel blends on VOC emissions, and reported a reduction in aromatic VOCs such as toluene and xylene. Also, a higher oxygen content in the biodiesel blend may enhance combustion efficiency, tending to lower VOCs, while oxygen blending increased the probability of oxygen VOCs. Correa et al. [34] used a six-cylinder heavy-duty engine for studying aromatic hydrocarbons and reducing VOCs with biodiesel and its blends of 2%, 5%, and 20%. In that study, all BTEX levels reduced and the total reduced level of VOCs was 21.5%. The highest emitted aromatic VOC was toluene, followed by benzene, xylene, and ethylbenzene. Man et al. [35] compared the effect of the level of regulated pollutant emissions and VOCs on the Japanese-13 mode with waste cooking oil blends of 10%, 20%, 30%, and 100%. This research showed that aromatic VOCs were reduced with increasing engine load. Benzene increased with biodiesel blend ratio, even though toluene and xylene were reduced.
However, many studies on biofuels are focused on medium and high speed and medium and high load conditions based on an analysis of the above literature. Some studies [37,38,39,40] are performed at high idle, over 1000 rpm, with heavy-duty diesel engines using biodiesel. At present, research on low speed (especially idling) is still lacking. A lot of harmful emissions are emitted from engines under idling conditions due to the poor combustion environment. In particular, the poor atomization by biodiesel has a greater effect on combustion at low engine speed and low injection pressure. The condition of the lowest speed and the lowest injection pressure of the real vehicle engine is low idle. BTEX (Benzene, toluene, ethylbenzene, xylene), the toxic aromatic VOCs, emitted from engines of vehicles can directly affect people in the city at the low idle operation of real vehicles when parking or stopping at traffic lights. In an analysis of driving patterns in the city, the portion of idle operation is 17% [41]. In this condition, the injection quantity is a little higher than the optimum amount of fuel for stability. So, the oxygen content, the high viscosity, and other properties of the biodiesel affect combustion in a complex way. From the results obtained in actual vehicles and engines mentioned above, it was found that more VOCs were emitted at lower speed. The above studies were performed under relatively good conditions above medium speed and not in the low idle state of the actual vehicles.
Therefore, to thoroughly investigate the combustion and emission characteristics of the diesel engine fueled with biodiesel blends under idling conditions, we applied palm oil biodiesel and its blends to a common rail direct injection diesel engine at the lowest speed of 750 rpm. The combustion and exhaust characteristics were analyzed, including the regulated and unregulated pollutant emissions (VOCs and toxic aromatic VOCs-BTEX). 2. Methodology 2.1. Test Fuels
In this study, palm oil biodiesel was selected among many biodiesels. Palm has the highest production rate among raw materials because it has the highest oil content among raw materials for biodiesel production, and the process of converting to biodiesel is the same as using other raw materials [42]. Ong et al. [43] compared several biodiesels and reported that palm oil biodiesel has a high potential for production to meet future demand because of high oil productivity about 13 times better than soybean oil. Moreover, a life cycle analysis (LCA) revealed that palm oil biodiesel could reduce greenhouse gases (GHG) emissions by 62% compared to others (soybean oil 40%, rapeseed oil 45%, and sunflower oil 58%). It is known that the physical properties such as viscosity, cetane number, and heating value of palm oil biodiesel are better than other types of biodiesel. Kalam et al. [44] reported that the physio-chemical properties of palm oil biodiesel met the requirement of diesel engines compared with other biodiesels such as soybean and rapeseed oil.
Here, the name of the test fuels of palm oil biodiesel and its blends is expressed as PD, which is the abbreviation for palm oil biodiesel. PD0 means 100% petroleum diesel and PD100 means 100% palm oil biodiesel. PD10 and PD30 have blended 10% and 30% proportions of palm oil biodiesel by volume with pure petroleum diesel. The properties of petroleum diesel and palm oil biodiesel are shown in Table 1. Normally, it is known that the viscosity of biodiesels is higher than that of diesel and has a high surface tension which can affect the atomization of injected fuel in the cylinder at the same injection pressure [28].
2.2. Experimental Setup and Measurements
2.2.1. Engine Setup
The four-cylinder 2.0 L turbocharged-intercooled common rail direct injection Hyundai motor company diesel engine (Euro-3) applied in commercial vehicles was used for this test. A Bosch fuel injection system (Solenoid injectors, fuel pump, common rail) and ECU (engine control unit) were applied. Turbocharger type is waste gate turbocharger. The detailed specifications of engine and turbocharger are shown in Table 2 and Table 3.
2.2.2. Experimental Equipment
The experimental equipment diagram is shown in Figure 1. The test engine was installed on an eddy current dynamometer (DY-230 kW, Hwanwoong Mechatronics, Gyeongsangnam-do, Korea). The combustion pressure was measured using a piezo-electric type pressure sensor (Kistler, 6056a, Winterthur, Switzerland) located at the position of the glow plug, and the data were recorded and analyzed by a DAQ board (PCI 6040e, National Instrument, Austin, TX, USA). The levels of NOx and CO were measured using a multi-gas analyzer MK2 (Euroton, Italy), and the level of HC was measured by an HPC501 analyzer (Pantong Huapeng Electronics, China). The smoke opacity level was measured using a partial flow collecting type soot analyzer (OPA-102, Qurotech, Korea) based on the level of opacity. The fuel flow was calculated by measuring the fuel weight change over 10 min on a high-precision digital electronic weighing balance (AND, GP-30 K). The exhaust gas temperature was measured after the turbocharger.
2.2.3. Sampling and Analysis VOC Emissions
The exhaust gas for analyzing VOCs was sampled in a tedlar bag (5 Liter, TD-AP05, Aluminum Gas Sampling Bag, LKLABKOREA Inc., Gyeonggi-do, Korea) with a fixed displacement pump. The sampled exhaust gas was diluted with N2 at a ratio of 19:1.
The VOCs were analyzed by a purge & trap analyzer (JDT-505II/2010 GC/QP2010MS, Japan Analytical Industry, Tokyo, Japan) [10,45]. JDT-505II and 2010GC / QP2010MS were used for the purge and trap sampling and analysis of VOCs. Figure 2 shows the analysis system for VOCs, and the procedures are as follows. The diluted VOCs in a tedlar bag were absorbed by a tenax Absorber (Tenax-GR: Japan analytical Industry, Tokyo, Japan). The chromatogram and mass spectrometer separate the VOCs sampled from the exhaust gas and show information related to the composition of VOCs of the exhaust gas sample and the results related to the emission quantity based on the peak area of each VOC calculated with the peak height and its duration [46,47].
2.2.4. Test Procedure
In this experiment, the rotational speed of the engine was set to the low idle, 750 rpm. The engine loads were no engine load (0 Nm) and 40 Nm to reflect load conditions of real vehicles equipped with the auxiliary systems at idle. In real cars, devices such as air conditioners and generators are installed, and the engine is loaded to operate even at low idle conditions. The main injection timing was fixed at 2 degrees of crank angle (°CA) before top dead center (BTDC) and pilot injection timing was fixed at 20 °CA BTDC, and the injection pressure was applied at 280 bar. EGR was not operated at a low idle condition. The injection timings and durations by injection pressures measured of the current for injection from ECU are shown in Figure 3. As the engine load increased, the main injection duration was increased from 0.50 ms to 0.75 ms (2.3 °CA to 3.4 °CA). However, the durations of pilot injections of both engine loads were all kept at 0.35 ms (1.5 °CA). This means that only the main injection quantity was increased to meet an engine load of 40 Nm without increasing pilot injection quantity.
The combustion pressure and exhaust measurement were started when the engine speed was stabilized within 750 ± 10 rpm for each experimental condition. Coolant temperature was maintained at 85 ± 5 °C. The combustion pressure was calculated as the average of 200 cycles at each engine loads. After analyzing the combustion pressure and exhaust gas, collect the exhaust gas in the tedlar bag using a constant capacity pump while the engine is stable. The collected exhaust gas was diluted and analyzed in a short time without exposing it to direct sunlight. The experimental conditions are summarized in Table 4.
3. Results and Discussion 3.1. Engine Performance
3.1.1. Combustion Characteristics
Figure 4a,b shows the combustion pressure and heat release rate for engine loads and test fuels. Figure 4c,d show the rate of combustion pressure according to the crank angle for engine loads. An analysis of combustion data is shown in Table 5. In this study, both a pilot and main injection were applied, and tests were performed at the lowest engine speed. Pilot injection quantities were the same for all test conditions with an injection duration of 0.35 ms as shown in Figure 3. For all engine loads and all blend ratios, the SOC of the pilot injection for all conditions was 16 °CA BTDC. Under all conditions, the heat release rate, and the rate of combustion pressure by pilot injection were different according to engine load, even though the ignition delays were all the same. At an engine load of 0 Nm, the heat release rate and the rate of combustion pressure of the pilot injection appeared almost the same for the blend ratios, but PD100 showed the lowest values. At an engine load of 40 Nm, the heat release rate and the rate of combustion pressure were reduced and were the lowest in PD100. This is because the deterioration of the injection atomization had a greater effect on combustion than the oxygen content, slowing the combustion speed of the PD100. A higher heat release rate and rate of combustion pressure of the pilot injection were observed at an engine load of 40 Nm because of the rapid pre-mixed combustion. Specifically, higher pressure and temperature were observed in the cylinder as the engine load increased. The heat release rate and the rate of combustion pressure by the pilot injection on PD100 with engine loads 0 Nm and 40 Nm were similar. This result suggests that the main factor which affects the combustion of pilot injection is the poor atomization by palm oil biodiesel blending under idle conditions.
The combustion of the main injection is strongly affected by the combustion of the pilot injection [48,49,50]. Moreover, the maximum combustion pressure depends on the burned fuel during the pre-mixed phase depending on the properties of biodiesel, such as high viscosity, high cetane number, and low volatility [51,52]. At an engine load of 0 Nm, the maximum combustion pressure of PD100 was the highest at 5149 kPa. Similarly, the highest pressure observed at an engine load of 40 Nm was for PD100 at 6138 kPa. In PD100, the deteriorated combustion by the poor atomization of pilot injection led to an increase in the ignition delay so that the maximum combustion pressure increased. Monirul et al. [52] reported the biodiesel blends showed higher peak combustion pressure than diesel fuel. And Gattamaneni et al. [53] and Wakil et al. [54] also showed that the peak combustion pressure of biodiesel was higher than diesel. This phenomenon was more clearly visible on the graph of the rate of combustion pressure in Figure 4d at an engine load of 40 Nm. The rate of combustion pressure by the pilot injection of PD100 was slower, and the point at which combustion increased in the main injection was retarded, and the rising phase was slower compared to other blends. The points of maximum combustion pressure were 8 °CA ATDC for all blends at an engine load of 0 Nm and 12 °CA ATDC at PD100 only at an engine load of 40 Nm, and the others were 11 °CA ATDC. Exhaust gas temperatures were similar (390 K) at all blends at 0 Nm engine load, but at a 40 Nm engine load, the temperature increased from 485 K to 493 K with increasing blend ratio from PD0 to PD30. The exhaust gas temperature dropped to 489 K when PD100 was applied. The combustion efficiency was reduced by the poor atomization under low idle conditions with increasing injected fuel quantity, however, it increased due to rapid combustion after the ignition delay in the main injection for PD100, while the combustion at pilot injection and the beginning of the main injection deteriorated. As the injected fuel quantity increased, the combustion efficiency was reduced due to the poor atomization in this condition. However, the combustion efficiency of PD100 increased due to rapid combustion after the ignition delay of the main injection, even though the combustion of pilot injection and the beginning of the main injection deteriorated. As shown in Table 5, BTE at an engine load of 40 Nm decreased as the blend ratio increased from PD0 to PD30, but it increased at PD100. The stability of combustion can be observed by the COV of IMEP, where a lower COV means higher stability. If the COV of IMEP exceeds 10%, it means the engine has a problem to operate. but less 5%, it is judged to have a stable combustion state [55,56]. The stability of combustion is better at an engine load of 40 Nm, and it became worse as the blend rate increased.
3.1.2. Combustion Phasing
The mass fraction burned (MFB) was calculated using the heat release rate as shown in Figure 5. The combustion condition in the cylinder can be verified in this manner. The crank position where the MFB is 10% from the start of the pilot injection is denoted as CA10, 50% is CA50, and the point where the MFB becomes 90% is denoted as CA90. The difference between the fuel injection start point and CA10 is called the flame-development angle (or duration), and the difference between CA10 and CA90 is called the rapid-burning angle or combustion duration. CA50 shows where the MFB is 50%, which means that 50% of the injected fuel is converted to energy [57,58]. Table 6 shows the rapid-burning angle and rapid-burning angle calculated by analyzing the MFB at each condition.
The CA10 of all blend ratios at an engine load of 0 Nm were at approximately -9 °CA ATDC, and the flame-development durations of all blends varied from 2.40 ms to 2.49 ms. As described above, the pilot injection quantity was small compared with the total injection, and the oxygen content and poor atomization influence each other to offset this result so that the ignition delays of pilot injections were similar at idle. At an engine load 40 Nm, the CA10 of PD0, PD10, and PD30 were similar at −7 °CA ATDC, but CA10 retarded to −5 °CA ATDC for PD100. As seen in Figure 4d and Figure 5b, the combustion phases of the pilot injection of PD0, PD10, and PD30 dramatically improved due to increased pressure in the cylinder with increased engine loads of 40 Nm, but the combustion phase was slower in PD100. The flame-development durations of PD0, PD10, and PD30 at 40 Nm engine load were about 2.87 ms, and the rapid-burning durations varied from 4.0 ms to 4.82 ms. For PD100, the flame-development duration decreased to 3.33 ms, and the rapid-burning duration decreased sharply to 4.53 ms. Also, CA50 of PD100 at an engine load of 40 Nm was delayed from 7.4 ms for PD30 to 8.3 ms. This may be because the combustion conditions were improved by the increased pressure in the cylinder, but the combustion reaction did not improve because of the poor atomization of PD100. After the main injection, the MFB of PD100 was delayed as a whole, but it rose rapidly. This is because the ignition delay of the main injection was increased by the slower and deteriorated combustion of the pilot injection due to the poor atomization of PD100. Qi et al. [49] also reported that the combustion of the main injection was delayed due to deterioration of the pilot injection. Thus, when using PD100 under idle conditions, the combustion characteristics were more affected by the poor atomization due to the higher viscosity than by the effect of the oxygen.
3.2. Emissions Characteristics
3.2.1. Regulated Gaseous Emissions
Table 7 summarizes the emission characteristics of NOx, PM, HC, and CO, which are regulated gaseous emissions. NOx emissions of palm oil biodiesel blends under all engine load conditions were lower than those of pure diesel fuel. Under pure diesel fuel condition of 0 Nm engine load, NOx produced 258 ppm, PD10 decreased by about 1.2% to 255 ppm, PD30 decreased by about 5.0% to 245 ppm, and PD100 decreased by 2.7% to 251 ppm. And under 40 Nm engine load, NOx of pure diesel fuel was 835 ppm, PD10 decreased by about 1.2% to 825 ppm, PD30 decreased by about 5.9% to 786 ppm, and PD100 decreased by 2.8% to 812 ppm. In particular, with PD100, the slow combustion of pilot injection and the ignition delay of the main injection increased because of poor atomization caused by the high viscosity of palm oil biodiesel so that the premixed combustion increased, resulting in increased generation of NOx. Opinions and results on the generation of NOx in diesel engines with Biodiesel are divided [29,59]. Mirhashemi et al. [59] reviewed the NOx emissions of diesel engines fueled with various biodiesels. In that study, it was said that the NOx generation of biodiesel blends was complex and not conclusive and there were many factors that can influence NOx emissions such as fuel cetane number, density, volatility, degree of unsaturation, the chemically bound oxygen content, equivalent ratio or aromatic fuel composition. Mangus et al. [60] used four biodiesels (palm, jatropha, soybean, beef tallow) and its blends (0%, 5%, 10%, 20%, 50%, and 100% by volume) to compare the emission characteristics of NOx after application to diesel engines. In this study, NOx decreased with increasing the biodiesel blend rate. The reasons were explained as follows: (i) Reduced atomization and less premixed burn because of the high viscosity and a reduced volatility, (ii) lower cylinder temperatures for the higher blend percentage, (iii) an increase in unsaturated hydrocarbons, reducing energy release rate during oxidation through strong bonds, and (iv) prompt NOx reduction as oxygen present in fuel oxidizes radical combustion. Puhan et al. [61] tested with mahua oil ethyl ester and reported a 12% reduction compared to pure diesel fuel. It also reported that the generation of NOx is sensitive to oxygen content, adiabatic frame property, and spray characters. Banapurmath et al. [62] also applied various biodiesel fuels to a single-cylinder 4-stroke diesel engine, and reported the emission of NOx reduced when biodiesel applied.
Using biodiesel or its blends is generally known to decrease PM, HC, and CO due to the effects of the oxygen content of biodiesel [29,30,31]. Smoke opacity decreased from 3.1% (PD0) to 1.2% (PD100) at an engine load 0 Nm and from 4.8% (PD0) to 1.8% (PD100) at 40 Nm. HC also decreased by about 26% from 54 ppm (PD0) to 40 ppm (PD100) at an engine load of 0 Nm, and by about 60% from 62 ppm (PD0) to 25 ppm (PD100). Most of the studies on the application of biodiesels found reductions in smoke opacity and HC. The emission of CO depended on the engine loads. At an engine load 0 Nm, CO was reduced in the fuels with a low blend ratio but increased with the pure palm oil biodiesel. The CO emission of PD0 was 493 ppm, PD10 was 475 ppm, and PD30 was 439 ppm, and PD100 was 492 ppm. Conversely, at 40 Nm, CO of the fuels with a low blending ratio tended to increase but reduced with the pure palm oil biodiesel. The CO emission of PD0 was 241 ppm, PD10 was 270 ppm, PD30 was 315 ppm, and PD100 was 245 ppm. The following analyses can be made on the causes affecting the properties of CO emissions by the emission results and combustion characteristics. At a low engine load of 0 Nm, the increasing blend ratio of in biodiesel can make close to complete combustion by the added oxygen from biodiesel. Thus, the CO emissions of PD10 and PD30 tend to decrease compared to PD0. However, in the case of applying pure biodiesel, the oxygen content increases, but the deterioration of the atomization of injected fuel due to the high viscosity of the palm oil biodiesel increases the tendency to incomplete combustion, thus increasing CO emissions. When the engine load condition is 40 Nm, the amount of fuel injection doubles. However, the amount of air intake is the same as the engine load of 0 Nm. The increase in the biodiesel blending ratio increases the amount of oxygen content, but the higher viscosity of biodiesel is believed to have a greater effect. Thus, the CO emissions of PD10 and PD30 increase. According to the previous combustion analysis, the application of pure biodiesel slowed the combustion of the pilot injection, but the heat release rate increased rapidly during the combustion of the main injection so that CO is reduced. Banapurmatha et al. [62] reported higher CO emissions with biodiesels compared to diesel at high load conditions. An et al. [63] reported that CO emissions increase as increasing biodiesel blend ratio and decrease as an increasing engine load under the same fuel conditions.
3.2.2. Unregulated Gaseous Emissions–VOCs
The results of VOCs analyzed with GC/MS for each test fuel are shown as the VOCs detected for each test condition in Table 8 and Table 9. At an engine load of 0 Nm, 10 types were detected for PD0, 11 types for PD10, 10 types for PD30, and 14 types for PD100. At 40 Nm, 10 types were observed for PD0, 10 types for PD30, and 12 types for PD100. Higher blend ratios of palm oil biodiesel result in more complex combustion reactions within the cylinder.
Most of the VOCs emitted are alkanes and aromatics. Alkanes include nonane, octane, decane, tetradecane, and undecane. One alkene was detected, 1-butene. Ethyl alcohol and are common VOCs that were detected in all conditions. The emission compositions of tetrahydrofuran and benzene were highest in all conditions, accounting for about 60% or more. Tetrahydrofuran is an ether (R-O-R′), a highly volatile, colorless liquid with four carbon atoms and one oxygen atom with a pentagon ring structure [64]. Tetrahydrofuran is known as a toxic substance and can cause symptoms of nausea, headache, and central nervous suppression when inhaled. It can also irritate the skin and affect white blood cell reduction and livers and kidneys with chronic effects. It is also a highly probable cause of cancer [64]. The emission of tetrahydrofuran increased due to the increase in biodiesel content, but research on this is lacking. Benzene, toluene, ethylbenzene, and xylene (BTEX), all known toxic aromatic VOCs, were detected in all test conditions.
3.2.3. Toxic Aromatic VOCs, BTEX
BTEX was detected under all test conditions in this study. Xylene has three isomers, i.e., meta-xylene, para-xylene, and ortho-xylene, depending on the location of methylene (CH3) in the benzene ring. Xylene was analyzed by combining the emission ratio of all these xylene isomers in this study. Figure 6 and Table 10 summarize the emission ratio of BTEX based on engine load and test fuel conditions. Total BTEX accounts for approximately 50% or more of the total VOCs emitted under each test condition. The largest proportion of each test fuel discharged was benzene, followed by xylene, and toluene. The smallest percentage emitted was ethylbenzene. In the studies of Di et al. [65] and Cheung et al. [66] (conducted using waste cooking oil biodiesel) and Man et al. [35] (conducted under Japanese-13 test mode), benzene was also the largest emission followed by xylene and toluene. Unlike the above studies, Correa et al. [34] used a heavy-duty diesel engine to show that toluene was emitted most, followed by benzene, xylene, and ethylbenzene. The above results indicate that the emissions of BTEX vary depending on the fuel used, the engine used, and the test conditions. In this study, benzene tends to increase with an increasing rate of the blend of palm oil biodiesel. The emission ratio of benzene increased from 26.7% for PD0 to 36.5% for PD100 at an engine load of 0 Nm. The benzene emission ratio also increased from 38% for PD0 to 43% for PD30 at 40 Nm. Turrio-Baldassarri et al. [67] showed that biodiesel produced higher benzene emissions than when pure diesel fuel was used. They reported benzene emissions of 6.8 g/kWh for B20, which is a 62% increase compared with 4.2 g/kWh for diesel. A study by Man et al. [35] with waste cooking oil biodiesel also showed that benzene increased with increasing blend ratio of biodiesel under any engine load. Many studies were indicated the major factor of increasing benzene was the low exhaust temperature. Takada et al. [68] pointed out that benzene emissions increase under low exhaust temperature conditions. Di et al. [65] and Man et al. [35] pointed out that the addition of biodiesel at lower engine loads lowered exhaust temperatures, thereby increasing benzene emissions by slowing benzene’s oxidation while increasing the oxygen content due to the biodiesel. In other words, the exhaust temperature and oxygen content of biodiesel have a combined effect on benzene emission. However, as shown in Table 4, the emission ratio of benzene increased in this study, even though the exhaust gas temperature was the same (390 K) for all blends at an engine load 0 Nm. It also increased when the temperatures increased from 485 K to 493 K using PD0 to PD30 at an engine load of 40 Nm. The exhaust gas temperature was reduced to 489 K at PD100 with an engine load of 40 Nm, but the benzene emission ratio was reduced. This can be inferred from the fact that the increased oxygen content or the high exhaust gas temperature is not the main factor for reducing benzene under idle conditions with low injection pressure and low engine running speed.
In this study, benzene tended to increase with an increased blend rate of palm oil biodiesel regardless of engine load. However, the emission trends of toluene, ethylbenzene, and xylene varied depending on engine load. Dealkylation of aromatics in the fuel-rich area increased benzene. Correa et al. [34] reported that the main causes of BTEX emissions from an engine are pyrosynthesis occurring during combustion in the cylinder and structural modification, while Zervas et al. [69] reported benzene was generated by unburned fuel under different combustion conditions in a fuel-rich state. Liu et al. [70] also reported an increase in benzene due to the poor atomization of biodiesel and a decrease in toluene and xylene due to the low aromatic content of biodiesel. At an engine load 40 Nm, Benzene except PD100 increased with an increasing blend ratio of palm oil biodiesel. Further, Toluene, Ethylbenzene, and xylene of blended fuels were lower than those of pure diesel and increased with an increasing blend ratio of palm oil biodiesel. At this condition, the fuel consumption was more than doubled, as shown in Figure 3. This means that the amount of injected fuel will be more than double. Under idle conditions, the amount of air intake was the same at all engine loads, so under an engine load of 40 Nm, more fuel-rich areas occurred in the combustion chamber due to the increased fuel injection of high viscosity palm oil biodiesel. Thus, more benzene is produced than at an engine load of 0 Nm. The generation of toluene, ethylbenzene, and xylene increased under these conditions. The emission rate of benzene was drastically reduced for PD100 with an engine load of 40 Nm. This was likely the result of rapid oxidation of benzene due to the rapid combustion reaction that occurs during the combustion of the main injection. In other words, the high combustion temperature due to the rapid burning with increased oxygen content of palm oil biodiesel after the late ignition delay of the main injection reduced benzene. Sagese et al. [71] studied the pyrolysis and oxidation of benzene under various combustion conditions. They showed that temperature greatly affected the pyrolysis of benzene. Phenyl radicals produced by benzene at high temperature broke the aromatic rings and made C2 and C4 species.
As shown in the above studies, applying biodiesel and its blends has a great effect on the generation of aromatic VOCs. The results of the emission trends of VOCs are all different. In particular, researchers have found different results of the emission characteristics of benzene. This likely affects the generation of BTEX as a result of the fuel, but it depends on the conditions of the experiment, the type, and the condition of the engine. 4. Conclusions Various palm oil biodiesel blends (PD0, PD10, PD30, PD100) in a common rail direct injection diesel engine were used under low idle speed conditions (750 rpm) applying pilot injection. The combustion characteristics were analyzed and the regulated and unregulated gaseous emissions (VOCs and toxic aromatic VOCs-BTEX) were studied. Our conclusions are as follows:
i. The nitrogen oxide (NOx) emissions of biodiesel blends were lower than that of pure diesel and NOx tended to decrease as the blending ratio increased. Soot opacity and hydrocarbon (HC) were reduced with an increasing blend ratio. Carbon monoxide (CO) varied with the engine load conditions. Under low load, CO emissions tended to decrease with increasing blending ratio and increased under high load.
ii. The VOCs emitted from engine are mostly alkane and aromatic, and benzene and tetrahydrofuran have the highest emission ratios.
iii. BTEX (i.e., toxic aromatic VOCs) were detected under all test conditions, and benzene has the highest emission ratio, followed by xylene, toluene, and ethylbenzene.
iv. Benzene increased regardless of engine load under all blends except at engine load 40 Nm with PD100. At low engine load, benzene was increased while toluene and xylene were reduced. At high engine load, the levels of toluene, ethylbenzene, and xylene from test fuels blended with palm oil biodiesel were lower than those of diesel. And these increased as the increasing blending ratio. However, benzene from pure palm oil biodiesel under high engine loads were sharply reduced.
When biodiesel was applied under low idle speed conditions, the oxygen content of biodiesel, which is an advantage, and high density and low LHV, which are disadvantages, affect the combustion and exhaust pollutants in a complex way. In particular, it was confirmed that different types of VOC emissions were detected. The emission of BTEX, toxic VOCs, under all fuel blending ratios has been confirmed. Benzene, which can cause cancer, took the highest portion of emitted VOCs and increased with the blending ratio of biodiesel. Toluene, Ethylbenzene, and Xylene were relatively low compared to benzene emissions and their emission tendencies varied depending on engine load conditions. Therefore, further research to reduce the levels of toxic VOCs, especially BTEX, from a diesel engine fueled with biodiesel should be carried out.
Enlarge this image.Figure 3. Injection timings and durations at engine loads: (a) 0 Nm, and (b) 40 Nm.
Enlarge this image.Figure 4. Combustion pressure and heat release rate of (a) 0 Nm, and (b) 40 Nm; combustion pressure rise rate of (c) 0 Nm, and (d) 40 Nm.
| Properties | Diesel | Palm Oil Biodiesel | Test Method |
|---|---|---|---|
| Density at 15 °C (kg/m3) | 836.8 | 877 | ASTM D941 |
| Viscosity at 40 °C (mm2/s) | 2.719 | 4.56 | ASTM D445 |
| Lower heating value (MJ/kg) | 43.96 | 39.72 | ASTM D4809 |
| Cetane number | 55.8 | 57.3 | ASTM D4737 |
| Flash point (°C) | 55 | 196.0 | ASTM D93 |
| Pour point (°C) | −21 | 12.0 | ASTM D97 |
| Oxygen content (wt.%) | 0 | 11.26 | - |
| Hydrogen content (wt.%) | 13.06 | 12.35 | ASTM D5453 |
| Carbon content (wt.%) | 85.73 | 79.03 | ASTM D5291 |
| Engine Parameters | Unit | Specification |
|---|---|---|
| Engine Type | - | Hyundai In-line 4 Cylinder, WGT Turbocharged, EGR (Euro-3) |
| Maximum Power/Torque | kW/Nm | 84.6 (@4000 rpm)/260(@2000 rpm) |
| Bore x Stroke | mm × mm | 83 × 92 |
| Displacement | cc | 1991 |
| Compression Ratio | - | 17.7: 1 |
| Number of Injector nozzle holes | - | 5 |
| Injector type | - | Solenoid |
| Injector hole diameter | mm | 0.17 |
| Parameters | Unit | Specification | |
|---|---|---|---|
| Type | - | Waste Gate Turbocharger (MHI) | |
| Compressor wheel | Inducer/Exducer dia. | mm | 33.00/44.01 |
| Blade numbers | EA | 6 + 6 (Trailing angle 60 degree) | |
| Maximum boost | bar | 5 | |
| Material | - | Forged aluminum | |
| Turbine Wheel | Inducer/Exducer dia. | mm | 36.5/32.2 |
| Blade numbers | EA | 11 | |
| Cooling type | - | Oil cooled system | |
| Table | Unit | Condition | |
|---|---|---|---|
| Engine Speed | rpm | 750 ± 10 (idle speed) | |
| Engine Load | Nm | 0 & 40 | |
| Total injection Quantity | 0 Nm | mcc | 7 |
| 40 Nm | mcc | 13 | |
| Cooling Water Temperature | ℃ | 85 ± 5 | |
| Intake Air Temperature | ℃ | 25 ± 5 | |
| Fuel Injection Pressure | bar | 280 | |
| Injection Timing | °CA | Main BTDC 2/Pilot BTDC 20 | |
| Engine Load | Test Fuel | Max Combustion Pressure (Pmax) | Location of Pmax | Exhaust Gas Temperature | Fuel Consumption | BTE | COVIMEP |
|---|---|---|---|---|---|---|---|
| (Nm) | (kPa) | (°CA ATDC) | (K) | (g/h) | (%) | (%) | |
| 0 | PD0 | 5089 | 8 | 390 | 455 | - | 2.7 |
| PD10 | 5062 | 8 | 390 | 464 | - | 2.7 | |
| PD30 | 5085 | 8 | 391 | 502 | - | 2.7 | |
| PD100 | 5149 | 8 | 390 | 545 | - | 3.3 | |
| 40 | PD0 | 6125 | 11 | 485 | 1027 | 26.8 | 1.0 |
| PD10 | 6109 | 11 | 490 | 1076 | 25.2 | 1.0 | |
| PD30 | 6074 | 11 | 493 | 1106 | 24.8 | 0.9 | |
| PD100 | 6138 | 12 | 489 | 1224 | 24.7 | 1.3 |
| Engine Load | Test Fuel | Pilot Timing | Mass Fraction Burned (ATDC) | Flame- Development | Rapid- Burning | ||||
|---|---|---|---|---|---|---|---|---|---|
| CA10 | CA50 | CA90 | |||||||
| (Nm) | (°CA) | (°CA) | (°CA) | (°CA) | (°CA) | (ms 1) | (°CA) | (ms 1) | |
| 0 | PD0 | −20 | −9.2 | 5.1 | 12.2 | 10.8 | 2.40 | 21.4 | 4.76 |
| PD10 | -20 | −8.9 | 5.2 | 12.5 | 11.1 | 2.47 | 21.4 | 4.76 | |
| PD30 | −20 | −8.8 | 5.4 | 12.8 | 11.2 | 2.49 | 21.6 | 4.80 | |
| PD100 | −20 | −9.1 | 5.1 | 12.2 | 11.0 | 2.44 | 21.2 | 4.72 | |
| 40 | PD0 | −20 | −7.2 | 7.4 | 14.5 | 12.8 | 2.84 | 21.7 | 4.82 |
| PD10 | −20 | −7.1 | 7.3 | 14.5 | 12.9 | 2.87 | 21.6 | 4.80 | |
| PD30 | −20 | −7.0 | 7.5 | 14.6 | 13.0 | 2.89 | 21.6 | 4.80 | |
| PD100 | −20 | −5.0 | 8.3 | 15.4 | 15.0 | 3.33 | 20.4 | 4.53 | |
1 The flame-development and rapid-burning duration can be converted to the time by Time (ms) = °CA/(0.006 * N). Here N is the engine speed (rpm).
| Engine Load | Test Fuel | NOx | Smoke Opacity | HC | CO |
|---|---|---|---|---|---|
| (Nm) | (ppm) | (%) | (ppm) | (ppm) | |
| 0 | PD0 | 258 | 3.1 | 54 | 493 |
| PD10 | 255 | 2.4 | 54 | 475 | |
| PD30 | 247 | 1.5 | 43 | 439 | |
| PD100 | 251 | 1.2 | 40 | 492 | |
| 40 | PD0 | 835 | 4.8 | 62 | 241 |
| PD10 | 825 | 4.4 | 61 | 270 | |
| PD30 | 786 | 3.0 | 36 | 315 | |
| PD100 | 812 | 1.8 | 25 | 245 |
| PD0 | PD10 | PD30 | PD100 | ||||
|---|---|---|---|---|---|---|---|
| Name | Formula | Name | Formula | Name | Formula | Name | Formula |
| Ethyl alcohol | C2H6O | 1-Butene | C4H8 | Ethyl alcohol | C2H6O | 1-Butene | C4H8 |
| Tetrahydrofuran | C4H8O | Ethyl alcohol | C2H6O | Tetrahydrofuran | C4H8O | Ethyl alcohol | C2H6O |
| Benzene | C6H6 | Tetrahydrofuran | C4H8O | Benzene | C6H6 | Tetrahydrofuran | C4H8O |
| Toluene | C7H8 | Benzene | C6H6 | Toluene | C7H8 | Benzene | C6H6 |
| Octane | C8H10 | Toluene | C7H8 | Octane | C8H18 | Toluene | C7H8 |
| Ethylbenzene | C8H10 | Ethylbenzene | C8H10 | Ethylbenzene | C8H10 | Octane | C8H18 |
| m,p-Xylene | C9H20 | m,p-Xylene | C8H10 | m,p-Xylene | C8H10 | Ethylbenzene | C8H10 |
| o-Xylene | C8H10 | Nonane | C9H20 | o-Xylene | C8H10 | m,p-Xylene | C8H10 |
| Decane | C10H22 | o-Xylene | C8H10 | Decane | C10H22 | Nonane | C9H20 |
| Tetradecane | C14H30 | Decane | C10H22 | Tetradecane | C14H30 | o-Xylene | C8H10 |
| Tetradecane | C14H30 | Decane | C10H22 | ||||
| Tetradecane | C14H30 | ||||||
| Undecane | C11H24 | ||||||
| Tetradecane | C14H30 | ||||||
| PD0 | PD10 | PD30 | PD100 | ||||
|---|---|---|---|---|---|---|---|
| Name | Formula | Name | Formula | Name | Formula | Name | Formula |
| Ethyl alcohol | C2H6O | Ethyl alcohol | C2H6O | Ethyl alcohol | C2H6O | Trans-2-Butene | C4H8 |
| Tetrahydrofuran | C4H8O | Tetrahydrofuran | C4H8O | Tetrahydrofuran | C4H8O | Ethyl alcohol | C2H6O |
| Benzene | C6H6 | Benzene | C6H6 | Benzene | C6H6 | Tetrahydrofuran | C4H8O |
| Toluene | C7H8 | Toluene | C7H8 | Toluene | C7H8 | Benzene | C6H6 |
| Ethylbenzene | C8H10 | Ethylbenzene | C8H10 | Octane | C8H18 | Toluene | C7H8 |
| m,p-Xylene | C8H10 | m,p-Xylene | C8H10 | Ethylbenzene | C8H10 | Octane | C8H18 |
| Nonane | C9H20 | Nonane | C9H20 | m,p-Xylene | C8H10 | Ethylbenzene | C8H10 |
| o-Xylene | C8H10 | o-Xylene | C8H10 | o-Xylene | C8H10 | m,p-Xylene | C8H10 |
| Decane | C10H22 | Decane | C10H22 | Decane | C10H22 | o-Xylene | C8H10 |
| Tetradecane | C14H30 | Tetradecane | C14H30 | Tetradecane | C14H30 | Decane | C10H22 |
| 2-Ethyl-1-hexanol | C8H18O | ||||||
| Tetradecane | C14H30 | ||||||
| Engine Load | Test Fuel | Benzene | Toluene | Ethylbenzene | Xylene (m, p, o) |
|---|---|---|---|---|---|
| (Nm) | (%) | (%) | (%) | (%) | |
| 0 | PD0 | 26.7 | 8.9 | 3.7 | 16.7 |
| PD10 | 27.1 | 8.1 | 2.6 | 9.3 | |
| PD30 | 29.9 | 7.1 | 2.1 | 11.1 | |
| PD100 | 36.5 | 6.3 | 1.5 | 5.9 | |
| 40 | PD0 | 38.0 | 8.1 | 3.2 | 10.0 |
| PD10 | 38.5 | 6.0 | 1.9 | 7.2 | |
| PD30 | 43.0 | 6.2 | 2.0 | 8.6 | |
| PD100 | 28.8 | 7.1 | 2.2 | 10.6 |
Author Contributions
H.Y.K. suggested this research, performed the experiments, analyzed all experimental data, and wrote this paper. N.J.C. performed the data analysis and contributed to the discussion, and supervised the work and the manuscript. All authors participated in the evaluation of the data, and reading and approving the final manuscript. All authors have read and agreed to the published version of the manuscript.
Funding
This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the government of Korea (MSIT) (No. 2019R1F1A1063154).
Acknowledgments
The authors also thank the teachers in the Center for University-Wide Research Facilities (CURF) at Jeonbuk National University for their help in collecting some experimental data.
Conflicts of Interest
The authors declare no conflict of interest.
Abbreviations
The following abbreviations are used in this manuscript.
| VOCs | Volatile Organic Compounds |
| PD | Palm Oil Biodiesel |
| °CA | Degree of Crank Angle |
| NOx | Nitrogen oxides |
| PM | Particulate Matter |
| CO | Carbon Monoxide |
| HC | Hydrocarbon |
| PD0 | 0% Palm Oil Biodiesel + 100% Diesel, Pure petroleum diesel |
| PD10 | 10% Palm Oil Biodiesel + 80% Diesel |
| PD30 | 30% Palm Oil Biodiesel + 70% Diesel |
| PD100 | 100% Palm Oil Biodiesel + 0% Diesel, Pure palm oil biodiesel |
| COVIMEP | Coefficient of Variation of Indicated Mean Effective Pressure |
| MFB | Mass Fraction Burned |
| BSFC | Brake Specific Fuel Consumption |
| BTE | Brake Thermal Efficiency |
| LHV | Lower Heating Value |
| CA10 | The crank angle of 10% Mass Fraction Burned |
| CA50 | The crank angle of 50% Mass Fraction Burned |
| CA90 | The crank angle of 90% Mass Fraction Burned |
| ATDC | After Top Dead Center |
| BTDC | Before Top Dead Center |
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Ho Young Kim and Nag Jung Choi*
Division of Mechanical Design Engineering, Jeonbuk National University, 567 Baekje-daero, Deokjin-gu, Jeonju-si 54896, Jeollabuk-do, Korea
*Author to whom correspondence should be addressed.
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