Study on Volatile Organic Compounds from Diesel Engine Fueled with Palm Oil Biodiesel Blends at Low Idle Speed

applied sciences

Article Study on Volatile Organic Compounds from Diesel Engine Fueled with Palm Oil Biodiesel Blends at Low Idle Speed

Ho Young Kim 1 and Nag Jung Choi 1,*

Division of Mechanical Design Engineering, Jeonbuk National University, 567 Baekje-daero, Deokjin-gu, Jeonju-si 54896, Jeollabuk-do, Korea; [email protected] * Correspondence: [email protected]; Tel.: +82-63-270-4765

Received: 22 May 2020; Accepted: 17 July 2020; Published: 19 July 2020

Abstract: This paper presents the combustion and emissions characteristics including volatile organic compound (VOC) of a common rail direct injection diesel engine fueled with palm oil biodiesel blends contained 0%, 10%, 30%, and 100% (by volume) biodiesel at low idle speed, i.e., 750 rpm. 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. Alkane and aromatic VOCs were mostly emitted. Benzene and tetrahydrofuran accounted for the largest percentage of total detected VOCs in all test conditions. Benzene, toluene, ethylbenzene, xylene (BTEX, toxic aromatic VOCs) were detected for all tests. Among BTEX, benzene has the highest emission ratio, followed by xylene, toluene, and ethylbenzene. Benzene increased for all tests. At low engine load, toluene, ethylbenzene, and xylene decreased with increasing blend ratio. However, these increased at high engine load. When pure palm oil biodiesel was applied at high engine load, benzene decreased.

Keywords: palm oil biodiesel; idle; combustion; emission; VOCs; BTEX

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 [5–8]. 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 [11–13]. Benzene is classified as a class 1 carcinogen by the World Health Organization (WHO) and the International

Appl. Sci. 2020, 10, 4969; doi:10.3390/app10144969 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 4969 2 of 18

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 [20–22]. 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 [26–28]. Many studies [29–31] 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 [10,32–36] 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 [37–40] 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, Appl. Sci. 2020, 10, 4969 3 of 18 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 Table1. 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].

Table 1. Properties of test fuels.

Properties Diesel Palm Oil Biodiesel Test Method 3 Density at 15 ◦C (kg/m ) 836.8 877 ASTM D941 2 Viscosity at 40 ◦C (mm /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 Appl. Sci. 2020, 10, 4969 4 of 18

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 Tables2 and3.

Table 2. Engine speciﬁcation.

Engine Parameters Unit Speciﬁcation Hyundai In-line 4 Cylinder, WGT Engine Type - 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

Table 3. Turbocharger speciﬁcation.

Parameters Unit Speciﬁcation Type - Waste Gate Turbocharger (MHI) Inducer/Exducer dia. mm 33.00/44.01 Blade numbers EA 6 + 6 (Trailing angle 60 degree) Compressor wheel Maximum boost bar 5 Material - Forged aluminum Inducer/Exducer dia. mm 36.5/32.2 Turbine Wheel Blade numbers EA 11 Cooling type - Oil cooled system

2.2.2. Experimental Equipment The experimental equipment diagram is shown in Figure1. 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. Appl. Sci. 2020, 10, 4969 5 of 18 Appl. Sci. 2020, 10, x FOR PEER REVIEW 5 of 18 Appl. Sci. 2020, 10, x FOR PEER REVIEW 5 of 18

Figure 1. Schematic diagram of the experimental setups. FigureFigure 1. 1.Schematic Schematic diagramdiagram of of thethe experimental experimental setups. setups. 2.2.3.2.2.3. Sampling Sampling and and Analysis Analysis VOC VOC Emissions Emissions 2.2.3. Sampling and Analysis VOC Emissions TheThe exhaust exhaust gas gas for for analyzing analyzing VOCs VOCs was was sampled sampled in in a atedlar tedlar bag bag (5 (5 Liter, Liter, TD TD-AP05,-AP05, Aluminum Aluminum The exhaust gas for analyzing VOCs was sampled in a tedlar bag (5 Liter, TD-AP05, Aluminum GasGas Sampling Sampling Bag, Bag, LKLABKOREA LKLABKOREA Inc., Inc., Gyeonggi Gyeonggi-do,-do, Korea) Korea) with with a fixed a ﬁxed displacement displacement pump. pump. The 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 sampled exhaust gas was diluted with N2 at a ratio of 19:1. sampled exhaust gas was diluted with N2 at a ratio of 19:1. TheThe VOCs VOCs were were analyzed analyzed by by a a purge purge & & trap trap analyzer analyzer (JDT (JDT-505II-505II/2010/2010 GC/QP2010MS, GC/QP2010MS, Japan Japan The VOCs were analyzed by a purge & trap analyzer (JDT-505II/2010 GC/QP2010MS, Japan AnalyticalAnalytical Industry, Tokyo, Tokyo, Japan) Japan) [10 [10,45],45]. JDT-505II. JDT-505II and and 2010GC 2010GC/ QP2010MS / QP2010MS were were used used for the for purge the Analytical Industry, Tokyo, Japan) [10,45]. JDT-505II and 2010GC / QP2010MS were used for the purgeand trap and samplingtrap sampling and analysisand analysis of VOCs. of VOCs. Figure Figure2 shows 2 shows the analysisthe analysis system system for VOCs,for VOCs, and and the purge and trap sampling and analysis of VOCs. Figure 2 shows the analysis system for VOCs, and theprocedures procedures are are as as follows. follows. The The diluted diluted VOCs VOCs in in aa tedlartedlar bagbag werewere absorbed by a a t tenaxenax Absorber Absorber the procedures are as follows. The diluted VOCs in a tedlar bag were absorbed by a tenax Absorber (Tenax(Tenax-GR:-GR: Japan Japan analytical analytical Industry, Industry, Tokyo, Tokyo, Japan). Japan). The The chromatogram chromatogram and and mass mass spectrometer spectrometer separateseparate(Tenax -theGR: the VOCs VOCs Japan sampled sampled analytical from Industry, the exhaust Tokyo, gasgas Japan). andand showshow The informationinformation chromatogram related related and to to themass the composition composition spectrometer of ofVOCsseparate VOCs of of thethe the exhaustVOCs exhaust sampled gas gas sample sample from and and the the theexhaust resultsresults gas relatedrelat anded show toto the information emission quantity related based basedto the on oncomposition the the peak peak areaareaof VOCs of of each each of VOC the VOC exhaust calculated calculated gas withsample with the the and peak peak the height height results and and relat its itsed duration duration to the emission [46,47] [46,47].. quantity based on the peak area of each VOC calculated with the peak height and its duration [46,47].

FigureFigure 2. 2. GasGas Chromatograph Chromatograph / /MassMass Spectrometer Spectrometer (GC/MS) (GC/MS) for for VOCs VOCs analysis. analysis. Figure 2. Gas Chromatograph / Mass Spectrometer (GC/MS) for VOCs analysis. 2.2.4. Test Procedure 2.2.4. Test Procedure 2.2.4.In Test this Procedure experiment, the rotational speed of the engine was set to the low idle, 750 rpm. The engine 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 loads wereIn this no experiment, engine load the (0 Nm)rotational and 40 speed Nm toof reflectthe engine load wasconditions set to the of lowreal idle,vehicles 750 equippedrpm. The enginewith the auxiliary systems at idle. In real cars, devices such as air conditioners and generators are installed, theloads auxiliary were nosystems engine at load idle. (0 In Nm) real cars,and 40 devices Nm to such reflect as airload conditioners conditions ofand real generators vehicles equippedare installed with, andthe the auxiliary engine systems is loaded at toidle. operate In real even cars, at devices low idle such conditions. as air conditioners The main andinjection generators timing are was installed fixed , and the engine is loaded to operate even at low idle conditions. The main injection timing was fixed Appl. Sci. 2020, 10, 4969 6 of 18

andAppl. the Sci. engine 2020, 10 is, x loaded FOR PEER to REVIEW operate even at low idle conditions. The main injection timing was6ﬁxed of 18 at 2 degrees of crank angle (◦CA) before top dead center (BTDC) and pilot 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 idleat condition.20 °CA BTDC, The injectionand the injection timings pressure and durations was applied by injection at 280 pressuresbar. EGR was measured not operated of the currentat a low for idle condition. The injection timings and durations by injection pressures measured of the current for injection from ECU are shown in Figure3. As the engine load increased, the main injection duration 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 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 of both engine loads were all kept at 0.35 ms◦ (1.5 °CA). This means that only the main injection was increased to meet an engine load of 40 Nm without increasing pilot injection quantity. quantity was increased to meet an engine load of 40 Nm without increasing pilot injection quantity. TheThe combustion combustion pressure pressure and and exhaustexhaust measurementmeasurement were were started started when when the the engine engine speed speed was was stabilized within 750 10 rpm for each experimental condition. Coolant temperature was maintained 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. at 85± ± ◦5 °C. The combustion pressure was calculated as the average of 200 cycles at each engine loads. AfterAfter analyzing analyzing the the combustion combustion pressure pressure andand exhaustexhaust gas, collect collect the the exhaust exhaust gas gas in in the the tedlar tedlar bag bag usingusing a constanta constant capacity capacity pumppump while while the the engine engine is isstable. stable. The The collected collected exhaust exhaust gas was gas diluted was diluted and andanalyzed analyzed in in a ashort short time time without without exposing exposing it it to to direct direct sunlight. sunlight. The The experimental experimental conditions conditions are are summarizedsummarized in in Table Table4. 4.

FigureFigure 3. 3.Injection Injection timings timings and and durationsdurations atat engine loads: loads: (a (a) )00 Nm, Nm, and and (b ()b 40) 40 Nm. Nm.

TableTable 4.4. TestTest conditions.conditions.

TableTable Unit UnitCondition Condition EngineEngine Speed Speed rpm rpm750 ± 10 (idle 750 speed)10 (idle speed) ± EngineEngine Load Load Nm Nm0 & 40 0 & 40 Total injectionTotal injection 0 Nm0 Nm mcc mcc7 7 Quantity Quantity 40 Nm40 Nm mcc mcc13 13 Cooling Water Temperature °C 85 5 Cooling Water Temperature ℃ 85 ± 5 ± Intake Air Temperature °C 25 5 Intake Air Temperature ℃ 25 ± 5 ± Fuel Injection Pressure bar 280 Fuel Injection Pressure bar 280 Injection Timing ◦CA Main BTDC 2/Pilot BTDC 20 Injection Timing °CA Main BTDC 2/Pilot BTDC 20

3. Results and Discussion 3. Results and Discussion 3.1. Engine Performance 3.1. Engine Performance 3.1.1. Combustion Characteristics 3.1.1. Combustion Characteristics Figure4a,b shows the combustion pressure and heat release rate for engine loads and test fuels. Figure 4a,b shows the combustion pressure and heat release rate for engine loads and test fuels. Figure4c,d show the rate of combustion pressure according to the crank angle for engine loads. Figure 4c,d show the rate of combustion pressure according to the crank angle for engine loads. An Ananalysis analysis of of combustion combustion data data is shown is shown in Table in Table 5. In5 .this In study, this study, both botha pilot a and pilot main and injection main injection were wereapplied applied,, and andteststests were wereperformed performed at the lowest at the lowestengine speed. engine Pilot speed. injection Pilot quantities injection were quantities the same were thefor same all test for conditions all test conditions with an injection with an duration injection of duration 0.35 ms as of shown 0.35 ms in Figure as shown 3. For in all Figure engine3. loads For all engineand all loads blend and ratios, all blend the SOC ratios, of thethe SOCpilot ofinjection the pilot for injection all conditions for all was conditions 16 °CA wasBTDC. 16 Under◦CA BTDC. all Underconditions, all conditions, the heat therelease heat rate release, and rate,the rate and of the combustion rate of combustion pressure by pressure pilot injection by pilot were injection different were diffaccordingerent according to engine to load, engine even load, though even the though ignition the delays ignition were delays all the were same. all At the an same. engine At load anengine of 0 loadNm, of 0the Nm, heat the release heat releaserate and rate the andrate theof combustion rate of combustion pressure pressureof the pilot of theinjection pilot injectionappeared appearedalmost the same for the blend ratios, but PD100 showed the lowest values. At an engine load of 40 Nm, the Appl. Sci. 2020, 10, 4969 7 of 18 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 [48–50]. 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 Figure4d 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 Table5, 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. Appl. Sci. 2020, 10, 4969 8 of 18

Appl. Sci. 2020, 10, x FOR PEER REVIEW 8 of 18

FigureFigure 4. 4Combustion. Combustion pressurepressure andand heat release rate rate of of (a (a) )0 0Nm, Nm, and and (b ()b 40) 40 Nm; Nm; combustion combustion pressure pressure riserise rate rate of of (c ()c 0) 0 Nm, Nm, and and ( d(d)) 4040 Nm.Nm.

TableTable 5.5. CombustionCombustion characteristic under under test test conditions. conditions.

Engine Max CombustionMax Location Exhaust Gas Fuel Engine Location of Exhaust Gas Fuel BTE COVIMEP Load Test PressureCombustion (Pmax) of Pmax Temperature Consumption BTE COVIMEP Load Test Pmax Temperature Consumption FuelFuel Pressure (Pmax) (°CA (Nm) (kPa) (K) (g/h) (%) (%) (Nm) (kPa) ATDC)(◦CA ATDC) (K) (g/h) (%) (%) 0 0PD0 PD0 5089 5089 8 8 390390 455455 -- 2.7 PD10PD10 5062 5062 8 8 390390 464464 -- 2.7 PD30 5085 8 391 502 - 2.7 PD30 5085 8 391 502 - 2.7 PD100 5149 8 390 545 - 3.3 PD100 5149 8 390 545 - 3.3 40 PD0 6125 11 485 1027 26.8 1.0 40 PD0 6125 11 485 1027 26.8 1.0 PD10 6109 11 490 1076 25.2 1.0 PD10PD30 6109 6074 11 11 490493 11061076 24.825.2 1.0 0.9 PD30PD100 6074 6138 11 12 493489 12241106 24.724.8 0.9 1.3 PD100 6138 12 489 1224 24.7 1.3 3.1.2. Combustion Phasing 3.1.2. Combustion phasing The mass fraction burned (MFB) was calculated using the heat release rate as shown in Figure5. 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 The combustion condition in the cylinder can be verified in this manner. The crank position where MFB is 10% from the start of the pilot injection is denoted as CA10, 50% is CA50, and the point where the MFB is 10% from the start of the pilot injection is denoted as CA10, 50% is CA50, and the point the MFB becomes 90% is denoted as CA90. The difference between the fuel injection start point and where the MFB becomes 90% is denoted as CA90. The difference between the fuel injection start point CA10and isCA10 called is called the flame-development the flame-development angle angle (or duration), (or duration), and and the ditheff erencedifference between between CA10 CA10 and and CA90 isCA90 called is the called rapid-burning the rapid-burning angle or angle combustion or combustion duration. duration. CA50 CA50 shows shows where where the MFBthe MFB is 50%, is 50%, which meanswhich that means 50% that of the50% injected of the injected fuel is convertedfuel is converted to energy to energy [57,58 ].[57,58] Table. 6Table shows 6 shows the rapid-burning the rapid- angleburning and angle rapid-burning and rapid angle-burning calculated angle calculated by analyzing by analyzing the MFB the at MFB each at condition. each condition. Appl. Sci. 2020, 10, 4969 9 of 18 Appl. Sci. 2020, 10, x FOR PEER REVIEW 9 of 18

FigureFigure 5. 5.Mass Mass fraction fraction burned burned at at engine engine load load of of (a ()a 0) 0 Nm, Nm, (b ()b 40) 40 Nm. Nm.

TableTable 6. 6.Combustion Combustion phases phases under under test test conditions. conditions.

Engine Pilot PilotMass Mass Fraction Fraction Burned Burned (ATDC) (ATDC) Flame-Flame Rapid-Rapid Test LoadEngine Load TestTiming Development Burning Fuel TimingCA10 CA10CA50 CA50CA90 CA90 -Development -Burning Fuel 1 1 1 1 (Nm)(Nm) (◦CA)(°CA) (◦CA) (°CA)(◦ CA) (°CA)(◦ CA)(°CA) (◦CA) (°CA)(ms )(ms ()◦ CA)(°CA) (ms(ms) ) 00 PD0 PD0 20 −20 9.2 −9.2 5.1 5.1 12.212.2 10.810.8 2.40 2.40 21.421.4 4.764.76 − − PD10 -20 8.9 5.2 12.5 11.1 2.47 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 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 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 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 PD1020 −20 7.0 −7.1 7.5 7.3 14.614.5 13.012.9 2.89 2.87 21.621.6 4.804.80 − − PD100 PD3020 −20 5.0 −7.0 8.3 7.5 15.414.6 15.013.0 3.33 2.89 20.421.6 4.534.80 − − 1 The flame-development PD100 and rapid-burning−20 − duration5.0 can8.3 be converted15.4 to the time15.0 by Time3.33 (ms) = ◦CA20.4/(0.006 *4.53 N). Here1 The N is flame the engine-development speed (rpm). and rapid-burning duration can be converted to the time by Time (ms) = °CA/(0.006 * N). Here N is the engine speed (rpm). The CA10 of all blend ratios at an engine load of 0 Nm were at approximately -9 ◦CA ATDC, and theThe flame-development CA10 of all blend durationsratios at an of engine all blends load varied of 0 Nm from were 2.40 at msapproximately to 2.49 ms. As-9 °CA described ATDC, above,and the the flame pilot injection-development quantity durations was small of comparedall blends withvaried the from total 2.40 injection, ms to and 2.49 the ms. oxygen As described content andabove, poor the atomization pilot injection influence quantity each was other small to o ff comparedset this result with so the that total the injection, ignition delaysand the of oxygen pilot injectionscontent and were poor similar atomization at idle. Atinfluence an engine each load other 40 to Nm, offset the this CA10 result of PD0,so that PD10, the ignition and PD30 delays were of similarpilot injections at 7 CA were ATDC, similar but CA10 at idle. retarded At an engine to 5 loadCA ATDC40 Nm, for the PD100. CA10 Asof PD0, seen inPD10, Figures and4 dPD30 and 5wereb, − ◦ − ◦ thesimilar combustion at −7 °CA phases ATDC, of the but pilot CA10 injection retarded of to PD0, −5 °CA PD10, ATDC and PD30for PD100. dramatically As seen improvedin Figures due 4d and to increased5b, the combustion pressure in phases the cylinder of the withpilot increasedinjection of engine PD0, PD10, loads ofand 40 PD30 Nm, butdramatically the combustion improved phase due wasto increased slower in pressure PD100. Thein the flame-development cylinder with increased durations engine of loads PD0, of PD10, 40 Nm, and but PD30 the atcombustion 40 Nm engine phase loadwas were slower about in PD100. 2.87 ms, The and flame the rapid-burning-development durationsdurations variedof PD0, from PD10, 4.0 and ms toPD30 4.82 at ms. 40 ForNm PD100, engine theload flame-development were about 2.87 ms, duration and the decreased rapid-burning to 3.33 durations ms, and varied the rapid-burning from 4.0 ms to duration 4.82 ms. decreasedFor PD100, sharplythe flame to 4.53-development ms. Also, CA50 duration of PD100 decreased at an engineto 3.33 load ms, ofand 40 Nmthe wasrapid delayed-burning from duration 7.4 ms fordecreased PD30 tosharply 8.3 ms. to This 4.53 may ms. be Also, because CA50 the of combustion PD100 at an conditions engine load were of improved 40 Nm was by the delayed increased from pressure 7.4 ms infor thePD30 cylinder, to 8.3 but ms. the This combustion may be because reaction the did combustion not improve conditio becausens of were the poor improved atomization by the of increased PD100. Afterpressure the main in theinjection, cylinder, the MFB but of the PD100 combustion was delayed reaction as a whole, did not but improve it rose rapidly. because This of is thebecause poor theatomization ignition delay of PD100. of the mainAfter injectionthe main was injection, increased the byMFB the of slower PD100 and was deteriorated delayed as combustiona whole, but of it therose pilotrapidly. injection This due is because to the poor the atomizationignition delay of PD100. of the Qimain et al.injection [49] also was reported increased that theby combustionthe slower and of thedeteriorated main injection combustion was delayed of the duepilot to injection deterioration due to of the the poor pilot atomization injection. Thus,of PD100. when Qi using et al. [49] PD100 also underreported idle conditions, that the combustion the combustion of the characteristics main injection were was more delayed affected due by tothe deterioration poor atomization of the due pilot toinjection. the higher Thus, viscosity when than using by PD100 the eff ectunder of the idle oxygen. 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 Appl. Sci. 2020, 10, 4969 10 of 18

3.2. Emissions Characteristics

3.2.1. Regulated Gaseous Emissions Table7 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 [29–31]. 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 Appl. Sci. 2020, 10, 4969 11 of 18 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.

Table 7. Regulated gaseous emissions under test conditions.

Engine Load Test NOx Smoke Opacity HC CO (Nm)Fuel (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

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 Tables8 and9. 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-R0), 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.

Table 8. List of VOCs observed under an engine load of 0 Nm.

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 Appl. Sci. 2020, 10, 4969 12 of 18

Table 9. List of VOCs observed under an engine load of 40 Nm.

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

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. Figure6 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 Table4, 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. Appl. Sci. 2020, 10, 4969 13 of 18 Appl. Sci. 2020, 10, x FOR PEER REVIEW 13 of 18

FigureFigure 6. 6. ToxicToxic aromatic aromatic VOCs, VOCs, BTEX: BTEX: engine load of ( a) 0 0 Nm, and and ( b) 40 Nm.

TableTable 10. BTEXBTEX results results under under different diﬀerent test conditions.

Engine XyleneXylene Engine Load TestTest BenzeneBenzene Toluene Toluene Ethylbenzene Ethylbenzene Load (m,(m, p, o) p, o) FuelFuel (Nm)(Nm) (%) (%)(%) (%)(%) (%) (%) (%) 00 PD0 PD0 26.7 26.78.9 8.93.7 3.7 16.7 16.7 PD10PD10 27.1 27.18.1 8.12.6 2.6 9.3 9.3 PD30PD30 29.9 29.97.1 7.12.1 2.1 11.1 11.1 PD100 36.5 6.3 1.5 5.9 PD100 36.5 6.3 1.5 5.9 4040 PD0 PD0 38.0 38.08.1 8.13.2 3.2 10.0 10.0 PD10 38.5 6.0 1.9 7.2 PD10PD30 38.5 43.06.0 6.21.9 2.0 7.2 8.6 PD30PD100 43.0 28.86.2 7.12.0 2.2 8.6 10.6 PD100 28.8 7.1 2.2 10.6 In this study, benzene tended to increase with an increased blend rate of palm oil biodiesel 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 regardless of engine load. However, the emission trends of toluene, ethylbenzene, and xylene varied varied depending on engine load. Dealkylation of aromatics in the fuel-rich area increased benzene. depending on engine load. Dealkylation of aromatics in the fuel-rich area increased benzene. Correa Correa et al. [34] reported that the main causes of BTEX emissions from an engine are pyrosynthesis 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] 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 reported benzene was generated by unburned fuel under different combustion conditions in a fuel- state. Liu et al. [70] also reported an increase in benzene due to the poor atomization of biodiesel 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 and a decrease in toluene and xylene due to the low aromatic content of biodiesel. At an engine load load 40 Nm, Benzene except PD100 increased with an increasing blend ratio of palm oil biodiesel. 40 Nm, Benzene except PD100 increased with an increasing blend ratio of palm oil biodiesel. Further, Further, Toluene, Ethylbenzene, and xylene of blended fuels were lower than those of pure diesel and 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 increased with an increasing blend ratio of palm oil biodiesel. At this condition, the fuel consumption was more than doubled, as shown in Figure3. This means that the amount of injected fuel will be 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, 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 so under an engine load of 40 Nm, more fuel-rich areas occurred in the combustion chamber due to to the increased fuel injection of high viscosity palm oil biodiesel. Thus, more benzene is produced the increased fuel injection of high viscosity palm oil biodiesel. Thus, more benzene is produced than than at an engine load of 0 Nm. The generation of toluene, ethylbenzene, and xylene increased under at an engine load of 0 Nm. The generation of toluene, ethylbenzene, and xylene increased under these these conditions. The emission rate of benzene was drastically reduced for PD100 with an engine conditions. The emission rate of benzene was drastically reduced for PD100 with an engine load of load of 40 Nm. This was likely the result of rapid oxidation of benzene due to the rapid combustion 40 Nm. This was likely the result of rapid oxidation of benzene due to the rapid combustion reaction reaction that occurs during the combustion of the main injection. In other words, the high combustion 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 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 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 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 affected the pyrolysis of benzene. Phenyl radicals produced by benzene at high temperature broke aromatic rings and made C2 and C4 species. 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 Appl. Sci. 2020, 10, 4969 14 of 18

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.

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. Appl. Sci. 2020, 10, 4969 15 of 18

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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