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Article Effects of Injection on Combustion and Emission Characteristics under Low Speed Conditions in a Fueled with Palm Oil

Ho Young Kim, Jun Cong Ge and Nag Jung Choi *

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

 Received: 23 July 2019; Accepted: 23 August 2019; Published: 24 August 2019 

Abstract: In this study, the effect of injection pressure on combustion and emission characteristics was evaluated on a direct injection diesel engine fueled with palm oil biodiesel. Recently, many studies have been conducted to utilize biodiesel produced from various sources to prevent environmental pollution and the depletion of petroleum resources. The oxygen content and high cetane number of biodiesel can reduce the production of exhaust pollutants by improving the combustion, but its high viscosity deteriorates the atomization of the injected fuel. Particularly at low engine speed conditions like idle, poor atomization and low airflow in the deteriorates the combustion efficiency. Increasing the pressure is one of the effective methods to improve the atomization of biodiesel without mechanical modification of the current diesel engine. In this study, combustion characteristics and emission levels of pollutants were measured by varying the fuel injection pressure applying palm oil biodiesel. As a result, it was confirmed that increasing the injection pressure to apply palm oil biodiesel at low engine speed can reduce ignition delay and improve combustion efficiency so that nitrogen oxides (NOx) is increased but soot formation is reduced. Carbon monoxide (CO) and hydrocarbon (HC) are slightly reduced but these are increased again when using 100% palm oil biodiesel. The increased NOx due to increased injection pressure can be reduced by applying recirculation (EGR).

Keywords: palm oil biodiesel; rail pressure; idle; combustion; ignition delay; emission

1. Introduction Environmental pollution and depletion of petroleum resources are accelerating as the use of internal combustion engines using fossil increases [1]. Thus, the regulations on exhaust pollutants emitted from internal combustion engines have been strengthened [2,3], and many engineers are exploring methods to improve the efficiency of internal combustion engines. The improvement of engine hardware and precise combustion control reduce exhaust emission while maintaining high performance [4]. Another way to improve efficiency is the improvement of the fuel itself, which is the energy source of the internal combustion engine—that is the use of . Bioethanol and biodiesel are the representative biofuels, and they are considered as alternatives to slow down the depletion of petroleum resources while also reducing environmental pollution [5]. Biodiesel can be produced using various animal fat and vegetable raw materials [6]. However, due to its high viscosity, density, and low volatility, bio-oils from raw materials cannot be used in raw form, but through the transesterification process, biodiesel can be produced for use in commercial engines [6].

Energies 2019, 12, 3264; doi:10.3390/en12173264 www.mdpi.com/journal/energies Energies 2019, 12, 3264 2 of 14

Biodiesel is similar to petroleum diesel, thus it can be applied directly without redesigning the diesel engine or additional modification [5,6]. It is also known that the oxygen contained in the biodiesel increases the combustion efficiency, thereby reducing carbon monoxide (CO), hydrocarbon (HC), and smoke, but nitrogen oxide (NOx) is increased [7,8]. Moreover, biodiesel has a low calorific value and high viscosity and density. The low calorific value dictates more fuel is required to produce the same output, which results in more fuel consumption than petroleum fuel. The high viscosity, density, and surface tension also exacerbate the combustion of the fuel injected into the cylinder because of poor atomization and increased mean droplet size of the injected fuel [9,10]. The study of combustion and pollutant emission characteristics applied palm oil biodiesel and shows that the of engine decreased and particulate matter (PM) emission decreased as the blend ratio of palm oil biodiesel increased [11]. At an idling operation condition of low-speed engine rotation, applying the blended palm oil biodiesel deteriorates the fuel-air mixing through poor atomization. The larger droplet by poor atomization of injected fuel leads to the increasing CO and HC, but the lower combustion temperature makes NOx stay constant [12]. Studies on the unregulated environmental pollutants that are known as VOCs emitted from engines using biodiesel are also underway [13,14]. There are various combustion control methods for optimizing combustion in diesel engines. It is known that the increase of injection pressure is the most effective method for improving fuel atomization. The high fuel injection pressure promotes atomization and is advantageous for the starting of combustion [4,15,16]. When the injection pressure is low, the droplet size becomes larger and the droplet evaporation needs more time so that HC and CO levels are increased. Further, the combustion duration is increased so that the combustion progresses during the expansion resulting in increased heat consumption [17]. Therefore, it is necessary to confirm the influence of fuel injection pressure on combustion efficiency and emission to use biodiesel which has high viscosity and density. In this study, palm oil biodiesel is applied to a common rail diesel internal combustion engine used in commercial automobiles. Additionally, the effects of the fuel injection pressure on the engine performance, combustion phenomena, and generation of exhaust pollutants were observed. Further, the feasibility of exhaust gas recirculation (EGR) applying for reducing NOx increased under higher injection pressure was checked.

2. Methodology

2.1. Test Fuels In this study, palm oil biodiesel was selected. Palm has a high oil content and can produce more biodiesel than other materials, resulting in lower production costs. It is also known as a renewable and acceptable alternative fuel that is clean and nontoxic because it produces less greenhouse gases and other toxic pollutants than petroleum diesel [18]. Here, the name of the test fuel is expressed as PD, which is the abbreviation for palm oil biodiesel. PD0 means 100% petroleum diesel and PD100 means 100% palm oil biodiesel. The properties of petroleum diesel and palm oil biodiesel are shown in Table1. Similar to other studies [ 19,20] of palm oil biodiesel applied in diesel engines, this higher viscosity and lower calorific value than petroleum diesel was observed.

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 Calculated cetane index 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 - Sulfur content (wt.%) 0.11 0.004 ASTM D5453 Hydrogen content (wt.%) 13.06 12.35 ASTM D5453 Carbon content (wt.%) 85.73 79.03 ASTM D5291 Energies 2019, 12, x FOR PEER REVIEW 3 of 14

Hydrogen content (wt.%) 13.06 12.35 ASTM D5453 Carbon content (wt.%) 85.73 79.03 ASTM D5291 Energies 2019, 12, 3264 3 of 14 Additionally, PD20 was used. This is the test fuel blended with 20% palm oil biodiesel and 80% petroleum diesel in volumetric ratio. A blend of 20% biodiesel and 80% petroleum diesel is known as Additionally, PD20 was used. This is the test fuel blended with 20% palm oil biodiesel and 80% the optimal ratio compared with other blends [7,21,22]. Minnesota of the United States announced petroleum diesel in volumetric ratio. A blend of 20% biodiesel and 80% petroleum diesel is known as the plan to increase biodiesel content from 10% to 20% from May 2018 [23]. the optimal ratio compared with other blends [7,21,22]. Minnesota of the United States announced the plan to increase biodiesel content from 10% to 20% from May 2018 [23]. 2.2. Experimental Setup and Measurements 2.2. Experimental Setup and Measurements 2.2.1. Engine Setup 2.2.1.The Engine test Setupengine used in this study is a four-cylinder 2.0-liter turbocharged-intercooled common rail directThe test injection engine diesel used inengine this study applied is a in four-cylinder commercial 2.0-litervehicles. turbocharged-intercooled Detailed specifications are common shown railin Table direct 2. injection A Bosch diesel FIE System engine (fuel applied injection in commercial equipment vehicles. system, Detailedi.e., solenoid specifications , fuel are shownpump, incommon Table2. rail A Bosch and ECU FIE System(engine (fuelcontrol injection unit) were equipment applied. system, i.e., solenoid injectors, fuel , common rail and ECU () were applied. Table 2. Engine specification.

Engine Parameters Table 2. EngineUnit specification. Specification EngineEngine Parameters Type Unit - In-line 4 Cylinder, Specification Turbocharged, EGR MaximumEngine TypePower/ kW/Nm - 83.5(@4000rpm)/255(@2000rpm) In-line 4 Cylinder, Turbocharged, EGR MaximumBore Power x Stroke/Torque ㎜ kW ×/ Nm㎜ 83.5(@4000rpm)83 × 92/ 255(@2000rpm) Stroke mm mm 83 92 Displacement× cc× 1991 × Displacement cc 1991 - 17.7: 1 Compression Ratio - 17.7: 1 NumberNumber of of Injector nozzle nozzle holes holes - - 5 5 InjectorInjector type type - - Solenoid Solenoid InjectorInjector hole hole diameter diameter mm㎜ 0.17 0.17

2.2.2.2.2.2. Experimental EquipmentEquipment AsAs shownshown inin thethe experimentalexperimental equipmentequipment diagramdiagram ofof FigureFigure1 1,, the the test test engine engine was was installed installed on on anan eddyeddy currentcurrent typetype dynamometerdynamometer (DY-230kW,(DY-230kW, HwanwoongHwanwoong Mechatronics,Mechatronics, Gyeongsangnam-do,Gyeongsangnam-do, Korea)Korea) equippedequipped withwith aa fuelfuel supplysupply systemsystem andand aa fuelfuel pump.pump. A multi-gas analyzer (HPC501, PantongPantong HuapengHuapeng Electronics,Electronics, China)China) measuredmeasured thethe levelslevels ofof CO,CO, HC,HC, andand NOx.NOx. AA partialpartial flowflow collectingcollecting typetype sootsoot analyzeranalyzer (OPA-102,(OPA-102, Qurotech,Qurotech, Korea)Korea) waswas usedused toto measuremeasure thethe smokesmoke opacity.opacity. TheThe combustioncombustion pressurepressure forfor combustioncombustion analysisanalysis waswas acquiredacquired usingusing aa piezo-electricpiezo-electric type pressure sensor (Kistler, 6056a,6056a, Winterthur,Winterthur, Switzerland)Switzerland) atat thethe positionposition ofof thethe glowglow plugplug andand waswas recordedrecorded byby aa DAQDAQ boardboard (PCI(PCI 6040e,6040e, NationalNational Instrument,Instrument, Austin,Austin, TX,TX, USA).USA).

FigureFigure 1.1. SchematicSchematic diagramdiagram ofof thethe experimentalexperimental setups.setups. Energies 2019, 12, 3264 4 of 14

Energies 2019, 12, x FOR PEER REVIEW 4 of 14 2.2.3. Test Procedure 2.2.3. Test Procedure In this experiment, the rotational speed of the engine was set to idle—750 rpm as the ECU setting. The engineIn this loads experiment, were 40 the Nm rotational to simulate speed theof the condition engine was of set an to external idle—750 load rpm on as thethe ECU actual setting. vehicle. TheThe pilot engine injection loads waswere applied 40 Nm into asimulate similar the fashion condition to that of currentlyan external used load in on diesel the actual automobile vehicle. engines The pilot injection was applied in a similar fashion to that currently used in diesel automobile engines today. The main injection timing was fixed at before top dead center (BTDC) 2 angle (◦CA) and today. The main injection timing was fixed at before top dead center (BTDC) 2 crank angle (°CA) and pilot injection timing was fixed at BTDC 20 CA and the injection pressure was applied at 200 and pilot injection timing was fixed at BTDC 20 ◦°CA and the injection pressure was applied at 200 and 380 , respectively. The injection timings and durations by injection are shown in Figure2. 380 bar, respectively. The injection timings and durations by injection pressures are shown in Figure As injection pressure increased, the pilot duration is reduced from 1.7 CA to 1.3 CA and the main 2. As injection pressure increased, the pilot duration is reduced from 1.7 °CA◦ to 1.3 °CA◦ and the main injectioninjection duration duration is is reduced reduced from from 5.2 5.2◦ °CACA toto 2.7 °CA.◦CA. This This is is because because an an increase increase inin the the fuel fuel injection injection pressurepressure shortens shortens the the period period of of time time required required toto injectinject the same fuel fuel quantity quantity [17]. [17].

Figure 2. a b Figure Injection2. Injection timings timings and and duration duration of pilot of pilot and and main. main. ( ) Injection (a) Injection Pressure Pressure 200 bar 200 and bar ( and) injection (b) pressureinjection 380 pressure bar. 380 bar.

TheThe combustion combustion pressure pressure andand exhaustexhaust measurement were were started started when when the the engine engine speed speed was was stabilized within 750 10 rpm and coolant temperature 85 5 C for each experimental condition. stabilized within 750± ± 10 rpm and coolant temperature 85 ±± 5 ℃◦ for each experimental condition. TheThe combustion combustion pressure pressure is is calculated calculated as as the the average average of of 200 200 cycles. cycles. TheThe EGREGR raterate ofof 10% was applied at injectionat injection pressure pressure 380 bar 380 to evaluatebar to evaluate the feasibility the feasibility of reducing of thereducing increased the NOx.increased The experimentalNOx. The conditionsexperimental are summarized conditions ar ine summarized Table3. in Table 3.

TableTable 3.3. Test conditions.

Table.Table. Unit Unit Condition Condition EngineEngine Speed Speed rpm rpm 750 ± 10 750 (idle10 speed) (idle speed) ± EngineEngine Load Load Nm Nm 40* 40 * Cooling Water Temperature C 85 5 ◦ ± IntakeCooling Air Temperature Water Temperature ℃C 85 ± 5 25 5 ◦ ± FuelIntake Injection Air Pressure Temperature bar℃ 25 ± 2005 & 380 Injection Timing CA Main BTDC 2/Pilot BTDC 20 Fuel Injection Pressure ◦ bar 200 & 380 EGR rate % 0 & 10 Injection Timing °CA Main BTDC 2/Pilot BTDC 20 * To simulate the use of externalEGR load rate devices of the actual % vehicle such as air0 & conditioners 10 and electric devices. * To simulate the use of external load devices of the actual vehicle such as air conditioners and electric devices. 2.2.4. Data Analysis 2.2.4.The Data combustion Analysis pressure and heat release rate were averaged over 200 cycles of the engine runningThe through combustion the combustion pressure and analyzer. heat release The heat rate releasewere averaged rate was over calculated 200 cycles using of thethe followingengine formularunning [12 through,13,24,25 the]: combustion analyzer. The heat release rate was calculated using the following dQ k dV 1 dP formula [12,13,24,25]: = P + V (1) dθ k 1 dθ k 1 dθ 𝑑𝑄 −k 𝑑𝑉 1− 𝑑𝑃 Here, k is the specific heat ratio and= used𝑃 as 1.35.+ Additionally,𝑉 P is the combustion pressure,(1) 𝑑𝜃 k −1 𝑑𝜃 k −1 𝑑𝜃 and θ is the crank angle. The cylinder volume (V, m3) can be calculated as a function of crank Here, k is the specific heat ratio and used as 1.35. Additionally, P is the combustion pressure, and θ is the crank angle. The piston cylinder volume (V, ㎥) can be calculated as a function of crank Energies 2019, 12, 3264 5 of 14 angle from the compression ratio, bore, stroke, and length of connecting the rod vis the sider-crank system. The volume of the cylinder (V, m3) is given by Equation (2) as follows [12,24,25]:

V V  p  V = d + d 1 + R cos θ R2 sin2θ (2) r 1 2 − − − − 3 where Vd is the displacement volume (m ), r is the compression ratio, and R is the stroke-to-bore ratio. The mass fraction burned (MFB) is calculated as a function of crank angle using the following formula [26,27]: R θ dQ d θ dθ θ MFB(θ) = s (3) R θ e dQ dθ θs dθ

Here, θs is the start of energy release and θe is the end of energy release. The brake specific fuel consumption (BSFC) is calculated using the following formula [12,24]:

. m f BSFC = (4) 2πNT . Here, m f is the fuel flow rate, N is the engine speed, and T is the brake torque from the engine dynamometer for an engine load of 40 Nm. The brake thermal efficiency (BTE) is calculated using the following formula [26]:

1 BTE = (5) BSFC LHV × Here, LHV is the lower heating value of blended palm oil biodiesel. The LHV of the blended test fuel, PD20, is calculated [28]. To verify the combustion stability of the diesel engine operation at idle using each test fuel, the coefficient of variation (COV) for the indicated mean effective pressure was used, which is defined as [12,13,29]: q 1 PN  2 N i=1 IMEP(i) X COV = − (6) IMEP 1 PN N i=1 IMEP(i) Here, the numerator is the standard deviation of indicated mean effective pressure (IMEP) over 200 cycles (here, N = 200) and the denominator is the average of IMEP over 200 cycles. IMEP(i) is the indicated mean effective pressure (here, i = 1, 2, 3, ... , 200). The formula for the rate of EGR is as follows [8,24]: m EGR(%) = EGR 100 (7) mi ×

Here, mEGR is the mass of the recycled exhaust gas and mi is the total gas including the recycled exhaust gas.

3. Results

3.1. Engine Performance

3.1.1. Combustion Characteristics The properties of biodiesel in which oxygen content, high cetane number, low calorific value, and high viscosity have a great influence on fuel injection and combustion in the cylinder [19,20]. The viscosity of biodiesel, such as palm oil biodiesel, is higher than that of petroleum diesel and has a high surface tension so that the atomization of injected fuel in the cylinder is not well developed at the same injection pressure [30,31]. Particularly, while idling with a low engine rotational speed, mixing of Energies 2019, 12, 3264 6 of 14 the intake air into the cylinder with the injected fuel becomes poor. Moreover, under idling conditions, the lower fuel injection pressure yields much poorer atomization of the injected fuel. As the blend ratio of palm oil biodiesel increases, the deterioration of the atomization also increases. Figure3a–c show the combustion pressure and heat release rate under injection pressures of 200 and 380 bar applied each test fuel. It can be seen that the combustion pressure and heat release rate are higher under higher injection pressure in all test fuels. As the injection pressure is reduced, the points ofEnergies the maximum 2019, 12, x FOR combustion PEER REVIEW pressure and heat release rate are also retarded and the combustion6 of 14 of the pilot injection is deteriorated, as shown at the graph of heat release rate. As shown in Figure3d, asconditions, the blend the ratio lower of biodiesel fuel injection increases, pressure the combustionyields much ispoorer improved atomization via the of influence the injected of oxygen fuel. As so thatthe theblend maximum ratio of palm combustion oil biodiesel pressure increases, and heat the releasedeterioration rate are of increased. the atomization also increases.

Figure 3. Combustion pressure and heat release rate. (a) PD0, (b) PD20, (c) PD100, and (d) all test fuels Figure 3. Combustion pressure and heat release rate. (a) PD0, (b) PD20, (c) PD100, and (d) all test fuels under 380 bar. under 380 bar. As the combustion pressure increases, the atomization of the fuel injected into the cylinder is Figure 3 (a), (b), and (c) show the combustion pressure and heat release rate under injection improved, and the combustion of the pilot injection is surely started. As the combustion environment pressures of 200 and 380 bar applied each test fuel. It can be seen that the combustion pressure and of the main injection is improved, the values of maximum pressure and heat release rate are increased. heat release rate are higher under higher injection pressure in all test fuels. As the injection pressure Additionally, the maximum pressure and heat release points are advanced. is reduced, the points of the maximum combustion pressure and heat release rate are also retarded Figure4 shows the combustion pressure and heat release rate under the fuel injection pressure and the combustion of the pilot injection is deteriorated, as shown at the graph of heat release rate. of 380 bar with EGR applied. It can be seen that applying EGR reduces the maximum combustion As shown in Figure 3 (d), as the blend ratio of biodiesel increases, the combustion is improved via pressurethe influence and reduces of oxygen the so heat that release the maximum rate of thecombusti pilot injection.on pressure This and leads heat torelease reducing rate are NOx. increased. As the combustion pressure increases, the atomization of the fuel injected into the cylinder is improved, and the combustion of the pilot injection is surely started. As the combustion environment of the main injection is improved, the values of maximum pressure and heat release rate are increased. Additionally, the maximum pressure and heat release points are advanced. Figure 4 shows the combustion pressure and heat release rate under the fuel injection pressure of 380 bar with EGR applied. It can be seen that applying EGR reduces the maximum combustion pressure and reduces the heat release rate of the pilot injection. This leads to reducing NOx. Energies 2019, 12, 3264 7 of 14 Energies 2019, 12, x FOR PEER REVIEW 7 of 14

(c)

FigureFigure 4. 4.Combustion Combustion pressure pressure andand heatheat releaserelease rate wi withth exhaust exhaust gas gas recirculation recirculation (EGR) (EGR) rate rate 10% 10% underunder 380 380 bar. bar. (a ()a PD0,) PD0, (b ()b PD20,) PD20, and and ( c(c)) PD100.PD100.

3.1.2.3.1.2. Ignition Ignition Delay Delay and and Combustion Combustion DurationDuration MassMass Fraction Fraction Burned Burned (MFB) (MFB) isis usedused toto evaluateevaluate ignition delay delay and and combustion combustion duration duration by by dividingdividing the the amount amount of of accumulated accumulated fuelfuel oror accumulatedaccumulated heat heat during during the the combustion combustion period period by by the the totaltotal fuel fuel or or total total heat heat release release [17 [17,24,32].,24,32]. FigureFigure5 5 shows shows thethe MFB at each each blend blend ratio ratio as as well well as as 200 200 bar bar andand 380 380 bar bar injection injection pressure. pressure. As As shown shown in in (a–c), (a), (b), a lowerand (c), fuel a lower injection fuel pressure injection yieldspressure a slower yields risea ofslower the MFB rise profile of the MFB according profile to according crank angle. to crank There angle. is There no clear is no symptom clear symptom for the for combustion the combustion of the pilotof the injection pilot injection at low injection at low injection pressure. pressure. When EGRWhen is EGR applied is applied as shown as shown as (d), as the (d), initial the initial rise of rise MFB becomesof MFB slightly becomes slower. slightly slower. TheThe combustion combustion state state occurring occurring in in the the cylinder cylinder can can be be distinguished distinguished byby thethe MFB.MFB. FromFrom thethe start of fuelof injection,fuel injection, the crank the crank position position where where the MFB the MFB becomes becomes 10% 10% is noted is noted as CA10, as CA10, and theand pointthe point where where the MFB becomes 90% is noted as CA90 [24,25,32,33]. The difference between the fuel injection the MFB becomes 90% is noted as CA90 [24,25,32,33]. The difference between the fuel injection start start point and CA10 is called the flame-development angle or ignition delay, and the difference point and CA10 is called the flame-development angle or ignition delay, and the difference between between CA10 and CA90 is called the rapid-burning angle or combustion duration. CA50 denotes CA10 and CA90 is called the rapid-burning angle or combustion duration. CA50 denotes where the where the MFB is 50%, which means that 50% of the injected fuel is converted to energy [32,33]. Table MFB is 50%, which means that 50% of the injected fuel is converted to energy [32,33]. Table4 shows the 4 shows the ignition delay and combustion delay calculated by analyzing the MFB at each ignitionexperimental delay and condition. combustion Figure delay 6 is a calculatedgraph compar by analyzinging ignition the delay MFB and at combustion each experimental duration condition. under Figureeach6 condition. is a graph comparing ignition delay and combustion duration under each condition. As shown in Figure6a,b, the ignition delay becomes shorter and the combustion duration becomes longer as the fuel injection pressure increases. However, ignition delay due to EGR application at a high injection pressure and the variation of combustion duration are small. The CA50 in Figure6c is advanced with increasing fuel injection pressure and does not change with increasing biodiesel blend ratio and EGR. Energies 2019, 12, x FOR PEER REVIEW 8 of 14

Energies 2019, 12, 3264 8 of 14 Energies 2019, 12, x FOR PEER REVIEW 8 of 14

Figure 5. Mass Fraction Burned. (a) PD0, (b) PD20, (c) PD100, and (d) PD100 under 380 bar with EGR rate 10%.

Table 4. Ignition delay and combustion duration under test conditions.

Fuel Start of Mass Fraction Burned FigureFigure 5. 5. MassMass Fraction FractionEGR Burned. Burned. (a (a) )PD0, PD0, (b (b) )PD20, PD20, ( (cc)) PD100, PD100, and and ( (dd)) PD100 PD100Ignition under under 380 380 bar barCombustion with with EGR EGR Testrate 10%.Injection Pilot rate 10%. Rate CA10* CA50* CA90* Delay Duration Fuels Pressure Injection* Table 4. Ignition delay and combustion duration under test conditions. (bar) Table(%) 4. Ignition (°CA) delay and combustion(°CA) (°CA) duration (°CA) under test(°CA) conditions. (ms**) (°CA) (ms**) Fuel200 Injection - EGR Start –20 of Pilot –3 Mass Fraction9.2 Burned17.0 17 3.7 Combustion20 4.4 Test Ignition Delay FuelPressure Rate StartInjection of * Mass Fraction Burned Duration PD0Fuels 380 EGR- –20 –8.2CA10 *6.4 CA50 *13.8 CA90 * 11.8Ignition 2.6 Combustion22 4.9 Test Injection Pilot 380(bar) Rate10 (%) –20 (◦CA) CA10 –7.9 (◦CA)* CA506.8 (◦ CA)* CA9014.5 (◦CA)* 12.1 (◦DelayCA) (ms 2.7 **) (22.4◦DurationCA) (ms5.0 **) Fuels Pressure Injection* 200200 - - –20 20 –3.3 39 9.216.6 17.0 16.7 17 3.7 3.7 19.9 20 4.44.4 − − PD0 (bar)380 (%) - (°CA)20 (°CA)8.2 (°CA) 6.4 (°CA) 13.8 (°CA) 11.8 (ms 2.6**) (°CA) 22 (ms 4.9 **) PD20 380 - –20− –8.9− 6.4 13.8 11.1 2.5 22.7 5.0 380 10 20 7.9 6.8 14.5 12.1 2.7 22.4 5.0 200380 10- –20 –20− –8.3 –3 − 9.26.6 17.013.9 11.717 3.72.6 22.220 4.44.9 200 - 20 3.3 9 16.6 16.7 3.7 19.9 4.4 PD0 380200 -- –20 –20− –8.2–1.4− 6.49.2 13.816.5 11.818.6 2.64.1 17.922 4.94.0 PD20 380 - 20 8.9 6.4 13.8 11.1 2.5 22.7 5.0 − − PD100 380380380 10- 10 –20 –20 20 –7.9–8.5 8.36.86.3 6.614.513.2 13.9 12.111.5 11.7 2.72.5 2.6 22.421.7 22.2 4.95.04.8 200 - –20− –3.3− 9 16.6 16.7 3.7 19.9 4.4 380200 10 - –20 20 –7.2 1.46.6 9.213.5 16.5 12.8 18.6 2.8 4.1 20.7 17.9 4.04.6 − − PD20PD100* 380380 - - –20 20 –8.9 8.56.4 6.313.8 13.2 11.1 11.5 2.5 2.5 22.7 21.7 4.85.0 In the columns of Start of Pilot Injection,− CA10,− CA50, and CA90 in Mass Fraction Burned, a negative 380 10 20 7.2 6.6 13.5 12.8 2.8 20.7 4.6 number 380means BTDC10 and a positive –20− number –8.3− means6.6 ATDC. 13.9 ** The ignition11.7 delay2.6 and 22.2combustion 4.9 * In the columns of Start of Pilot Injection, CA10, CA50, and CA90 in Mass Fraction Burned, a negative number duration200 can be converted- to time –20 by time –1.4(ms) = °CA/(0.0069.2 ×16.5 N) × 1000.18.6 Here, N4.1 is the engine17.9 speed4.0 PD100means BTDC380 and a positive- number –20 means ATDC.–8.5 ** The ignition6.3 delay13.2 and combustion11.5 duration2.5 can21.7 be converted 4.8 to(rpm) time [24,25]. by time (ms) = ◦CA/(0.006 N) 1000. Here, N is the engine speed (rpm) [24,25]. 380 10 × –20 × –7.2 6.6 13.5 12.8 2.8 20.7 4.6 * In the columns of Start of Pilot Injection, CA10, CA50, and CA90 in Mass Fraction Burned, a negative number means BTDC and a positive number means ATDC. ** The ignition delay and combustion duration can be converted to time by time (ms) = °CA/(0.006 × N) × 1000. Here, N is the engine speed (rpm) [24,25].

Figure 6. Mass Fraction Burned. (a) Ignition delay, (b) combustion duration, and (c) CA50. Figure 6. Mass Fraction Burned. (a) Ignition delay, (b) combustion duration, and (c) CA50. 3.1.3. Stability of Combustion As shown in Figure7, the COVs for IMEP under each experimental condition are all below 2.0%, which means the combustion is very stable [12,34]. Figure7a,b are the measurements of PD20 and PD100 for IMEP during 200 cycles, respectively. The COV shown as (c) increases slightly as the fuel Figure 6. Mass Fraction Burned. (a) Ignition delay, (b) combustion duration, and (c) CA50. Energies 2019, 12, x FOR PEER REVIEW 9 of 14

As shown in Figure 6a and 6b, the ignition delay becomes shorter and the combustion duration becomes longer as the fuel injection pressure increases. However, ignition delay due to EGR application at a high injection pressure and the variation of combustion duration are small. The CA50 in Figure 6 (c) is advanced with increasing fuel injection pressure and does not change with increasing biodiesel blend ratio and EGR.

3.1.3. Stability of Combustion As shown in Figure 7, the COVs for IMEP under each experimental condition are all below 2.0%, Energies 2019, 12, 3264 9 of 14 which means the combustion is very stable [12,34]. Figure 7a and 7b are the measurements of PD20 and PD100 for IMEP during 200 cycles, respectively. The COV shown as (c) increases slightly as the fuelinjection injection pressure pressure increases, increases, but thisbut this is considered is considered to be to caused be caused by the by improvedthe improved combustion combustion in the in thecylinder cylinder due todue the to improved the improved atomization atomization of the injectedof the injected palm oil palm biodiesel. oil biodiesel. In addition, In addition, when biodiesel when biodieselis applied, is combustionapplied, combustion due to increased due to increase oxygend content oxygen is content improved is impr andoved COV and is lowered. COV is lowered.

Figure 7. The variation of indicated mean effective pressure (IMEP) and coefficient of variation (COV). Figure 7. The variation of indicated (IMEP) and coefficient of variation (COV). (a) IMEP of PD20, (b) IMEP of PD100, and (c) COV of IMEP. (a) IMEP of PD20, (b) IMEP of PD100, and (c) COV of IMEP. 3.2. Emission Characteristics 3.2. Emission Characteristics 3.2.1. Pollutant Emissions 3.2.1. Pollutant Emissions Table5 shows the levels of pollutant emissions which are NOx, Soot, HC, and CO on each test condition.Table 5 shows Figure the8 levelsshows of the pollutant result of emissions NOx and which Soot. are As theNOx, fuel Soot, injection HC, and pressure CO on increases, each test condition.the amount Figure of NOx 8 generatedshows the increases result of because NOx and the Soot. atomization As the offuel the injection injected fuelpressure is improved increases, and the amountcombustion of NOx efficiency generated is increased. increases The because same the result atom isization obtained of eventhe injected when the fuel blend is improved ratio of palmand the oil combustionbiodiesel increases. efficiency Furthermore, is increased. the The amount same result of soot is decreases obtained aseven the when fuel injection the blend pressure ratio of increases. palm oil biodiesel increases. Furthermore, the amount of soot decreases as the fuel injection pressure increases. Table 5. Pollutant emissions. Table 5. Pollutant emissions. Fuel Injection Pollutant Emissions EGR Rate Test Fuels FuelPressure NOxPollutant Soot Emissions HC CO Test Injection EGR rate (bar) (%) (ppm)NOx (%)Soot (ppm) HC (%) CO Fuels Pressure 200 - 491 7.8 21 0.061 (bar) (%) (ppm) (%) (ppm) (%) PD0 380 - 922 3.4 20 0.060 200 380- 10491 718 4.17.8 2121 0.0600.061 PD0 380 200- -922 567 2.73.4 2020 0.0540.060 PD20 380 38010 - 1011718 1.44.1 1921 0.0500.060 200 380- 10567 824 2.72.7 2120 0.0500.054 PD20 380 200- -1011 582 2.11.4 2119 0.0530.050 PD100 380 38010 - 1095824 1.22.7 2121 0.0530.050 Energies 2019, 12, x FOR PEER REVIEW 10 of 14 200 380- 10582 876 1.92.1 2121 0.0530.053 PD100 380 - 1095 1.2 21 0.053 380 10 876 1.9 21 0.053

Figure 8. NOx and PM. (a) NOx and (b) smoke opacity by PM. Figure 8. NOx and PM. (a) NOx and (b) smoke opacity by PM.

Increased pressure improves atomization, reduces ignition delay and improves mixing with air in the cylinder, improving combustion conditions. This leads to an increase in NOx and at the same time a decrease in soot. By applying EGR at a high fuel injection pressure, the level of NOx generation can be reduced by lower combustion temperature while maintaining a slight increase in soot formation. As shown in Figure 9, the emissions of CO and HC decrease as the fuel injection pressure increases. As the blend ratio of palm oil biodiesel increases, the CO and HC emissions are initially reduced, but increased again above a certain level. In the case of PD100, the CO and HC levels are similar under all injection pressure conditions.

Figure 9. CO (a) and HC (b).

3.2.2. Thermal Efficiency and Fuel Consumption As the fuel injection pressure increases, the brake thermal efficiency increases and the fuel consumption decreases. As shown in Figure 10 (a), the BSFC of the higher injection pressure is lower, while (b) shows the Brake Thermal Efficiency according to fuel injection pressure and shows a much higher efficiency at the higher fuel injection pressure. The exhaust gas temperature rises as the Brake Thermal Efficiency increases, as noted in (c). As the fuel injection pressure is increased, the atomization of the injected fuel is improved, the combustion efficiency is increased, and the fuel consumption rate is decreased. At low fuel injection pressures, an increase in blend ratio of palm oil biodiesel increases the BSFC. Therefore, when palm oil biodiesel is mixed and used, it is advantageous to improve by increasing the fuel injection pressure. Moreover, the increase in BSFC due to the application of EGR is less than that observed with low fuel injection pressure. It can be seen that the increased NOx upon rising fuel injection pressure can be reduced by EGR implementation to an acceptable level without deterioration of fuel efficiency. Energies 2019, 12, x FOR PEER REVIEW 10 of 14

Figure 8. NOx and PM. (a) NOx and (b) smoke opacity by PM. Energies 2019, 12, 3264 10 of 14 Increased pressure improves atomization, reduces ignition delay and improves mixing with air in theIncreased cylinder, pressure improving improves combustion atomization, conditions. reduces This ignition leads to delay an increase and improves in NOx mixing and at with the airsame in thetime cylinder, a decrease improving in soot. By combustion applying conditions.EGR at a high This fuel leads injection to an increasepressure, in the NOx level and of atNOx the generation same time acan decrease be reduced in soot. by By lower applying combustion EGR at a hightemperature fuel injection while pressure, maintaining the level a slight of NOx increase generation in soot can beformation. reduced by lower combustion temperature while maintaining a slight increase in soot formation. AsAs shownshown in in Figure Figure9, the9, emissionsthe emissions of CO of and CO HC an decreased HC decrease as the fuel as injectionthe fuel pressureinjection increases.pressure Asincreases. the blend As ratio the blend of palm ratio oil biodieselof palm oil increases, biodiesel the increases, CO and HCthe emissionsCO and HC are emissions initially reduced, are initially but increasedreduced, but again increased above a certainagain above level. a In certain the case level. of PD100, In the thecase CO of andPD100, HC levelsthe CO are and similar HC levels under are all injectionsimilar under pressure all injection conditions. pressure conditions.

Figure 9. CO (a) and HC (b). Figure 9. CO (a) and HC (b). 3.2.2. Thermal Efficiency and Fuel Consumption 3.2.2. Thermal Efficiency and Fuel Consumption As the fuel injection pressure increases, the brake thermal efficiency increases and the fuel consumptionAs the fuel decreases. injection As pressure shown inincreases, Figure 10 thea, the brake BSFC thermal of the higherefficiency injection increases pressure and isthe lower, fuel whileconsumption (b) shows decreases. the Brake As Thermal shown in Effi Figureciency 10 according (a), the BSFC to fuel of the injection higher pressure injection and pressure shows is a lower, much higherwhile (b) effi showsciency the at the Brake higher Thermal fuel injection Efficiency pressure. according The to exhaust fuel injection gas temperature pressure and rises shows as the a Brakemuch Energies 2019, 12, x FOR PEER REVIEW 11 of 14 Thermalhigher efficiency Efficiency at increases,the higher as fuel noted injection in (c). pressure. The exhaust gas temperature rises as the Brake Thermal Efficiency increases, as noted in (c). As the fuel injection pressure is increased, the atomization of the injected fuel is improved, the combustion efficiency is increased, and the fuel consumption rate is decreased. At low fuel injection pressures, an increase in blend ratio of palm oil biodiesel increases the BSFC. Therefore, when palm oil biodiesel is mixed and used, it is advantageous to improve fuel efficiency by increasing the fuel injection pressure. Moreover, the increase in BSFC due to the application of EGR is less than that observed with low fuel injection pressure. It can be seen that the increased NOx upon rising fuel injection pressure can be reduced by EGR implementation to an acceptable level without deterioration of fuel efficiency.

Figure 10. Thermal Efficiency and fuel consumption. (a) brake specific fuel consumption (BSFC), (Figureb) brake 10. thermal Thermal effi Efficiencyciency (BTE), and andfuel (cconsumption.) exhaust gas ( temperature.a) brake specific fuel consumption (BSFC), (b) brake thermal efficiency (BTE), and (c) exhaust gas temperature. As the fuel injection pressure is increased, the atomization of the injected fuel is improved, the4. Discussion combustion efficiency is increased, and the fuel consumption rate is decreased. At low fuel injection pressures, an increase in blend ratio of palm oil biodiesel increases the BSFC. Therefore, when palm As a result of this test, it was observed that the combustion pressure and heat release rate in the oil biodiesel is mixed and used, it is advantageous to improve fuel efficiency by increasing the fuel cylinder becomes higher as the fuel injection pressure increases. The increase in fuel injection injection pressure. Moreover, the increase in BSFC due to the application of EGR is less than that pressure promotes the atomization of the injected fuel. Thus, the points of max combustion pressure observed with low fuel injection pressure. It can be seen that the increased NOx upon rising fuel and heat release were advanced. On the contrary, when the injection pressure was low, the size of injection pressure can be reduced by EGR implementation to an acceptable level without deterioration the fuel droplet becomes larger, and the time required for the droplet to evaporate became much of fuel efficiency. longer. Kannan et al. [16] and Agarwal et al. [17,35] investigated the effect of injection pressure on combustion characteristics and these studies showed that the combustion pressure and heat release rate on the condition of higher injection pressure tend to be high. Under low-speed conditions such as idling, the atomization of injected fuel is considered to have a significant effect on combustion. At lower fuel injection pressures in this study, the heat release by the combustion of pilot injection was not clear. This means the combustion of pilot injected fuel did not occur well. It is due to the larger droplet size by lower injection pressure and longer ignition delay. The relationship on ignition delay about injection pressure can be confirmed by analysis of the Mass Fraction Burned (MFB). At lower injection pressures, ignition delay (which is the difference from the start of injection to CA10) increased, but the combustion duration (From CA10 to CA90) decreased. The overall combustion duration (ignition delay + combustion duration) of a lower injection pressure was longer than the one of higher injection pressure so that the combustion is delayed to the expansion phase. This phenomenon reduces combustion efficiency. Additionally, the point of CA50 was advanced. Other researchers [16,17,36] also found similar results about ignition delay and combustion duration by injection. And the study by Kuti et al. [37] investigated the ignition delay of biodiesel and diesel as the injection pressure increased from 100 to 300 MPa under simulated quiescent conditions in a constant-volume vessel. They found that the ignition delay decreased with the increasing of injection pressure. It could be attributed to the enhanced mixing achieved at increasing injection pressure. Along with the oxygen content of biodiesel, the improvement of atomization by high injection pressures leads to increased NOx emissions and reduced CO, HC, and soot. Additionally, as increasing in injection pressure, BSFC was decreased as BTE increased, which shows the improvement of combustion. These results were similar to those by other researchers [4,17,36]. However, exhaust gas temperature was increased on lower injection pressure because of the poor combustion of pilot injection and long combustion duration by lower injection pressure. The increased NOx by the high injection pressure can be reduced by applying EGR to minimize soot increasing and fuel consumption without reduced engine power.

5. Conclusions The effect of injection pressure of combustion and emission characteristics under low speed conditions on a diesel engine fueled with palm oil biodiesel were investigated. The levels of combustion pressure and heat release were increased with the increase injection pressure. At low injection pressure, the combustion of pilot injection did not occur well. Additionally, Energies 2019, 12, 3264 11 of 14

4. Discussion As a result of this test, it was observed that the combustion pressure and heat release rate in the cylinder becomes higher as the fuel injection pressure increases. The increase in fuel injection pressure promotes the atomization of the injected fuel. Thus, the points of max combustion pressure and heat release were advanced. On the contrary, when the injection pressure was low, the size of the fuel droplet becomes larger, and the time required for the droplet to evaporate became much longer. Kannan et al. [16] and Agarwal et al. [17,35] investigated the effect of injection pressure on combustion characteristics and these studies showed that the combustion pressure and heat release rate on the condition of higher injection pressure tend to be high. Under low-speed conditions such as idling, the atomization of injected fuel is considered to have a significant effect on combustion. At lower fuel injection pressures in this study, the heat release by the combustion of pilot injection was not clear. This means the combustion of pilot injected fuel did not occur well. It is due to the larger droplet size by lower injection pressure and longer ignition delay. The relationship on ignition delay about injection pressure can be confirmed by analysis of the Mass Fraction Burned (MFB). At lower injection pressures, ignition delay (which is the difference from the start of injection to CA10) increased, but the combustion duration (From CA10 to CA90) decreased. The overall combustion duration (ignition delay + combustion duration) of a lower injection pressure was longer than the one of higher injection pressure so that the combustion is delayed to the expansion phase. This phenomenon reduces combustion efficiency. Additionally, the point of CA50 was advanced. Other researchers [16,17,36] also found similar results about ignition delay and combustion duration by injection. And the study by Kuti et al. [37] investigated the ignition delay of biodiesel and diesel as the injection pressure increased from 100 to 300 MPa under simulated quiescent conditions in a constant-volume vessel. They found that the ignition delay decreased with the increasing of injection pressure. It could be attributed to the enhanced mixing achieved at increasing injection pressure. Along with the oxygen content of biodiesel, the improvement of atomization by high injection pressures leads to increased NOx emissions and reduced CO, HC, and soot. Additionally, as increasing in injection pressure, BSFC was decreased as BTE increased, which shows the improvement of combustion. These results were similar to those by other researchers [4,17,36]. However, exhaust gas temperature was increased on lower injection pressure because of the poor combustion of pilot injection and long combustion duration by lower injection pressure. The increased NOx by the high injection pressure can be reduced by applying EGR to minimize soot increasing and fuel consumption without reduced engine power.

5. Conclusions The effect of injection pressure of combustion and emission characteristics under low speed conditions on a diesel engine fueled with palm oil biodiesel were investigated. The levels of combustion pressure and heat release were increased with the increase injection pressure. At low injection pressure, the combustion of pilot injection did not occur well. Additionally, the points of max combustion pressure and heat release were advanced, as reduced ignition delay increased injection pressure. As increasing injection pressure, ignition delay was reduced, but combustion duration was increased. The overall combustion duration under lower injection pressure was longer because of longer ignition delay. The points of CA50 were advanced as increasing injection pressure. Higher injection pressure reduced the exhaust emission such as CO, HC, and soot but increased the NOx emission due to shorter ignition delay by improved atomization and oxygen content in palm oil biodiesel. Additionally, BSFC is decreased by improved BTE due to improved atomization because of higher injection pressure. Applying EGR can reduce increased NOx by high injection pressure while minimizing soot emissions, fuel consumption, and power reduction. To use palm oil biodiesel that has a higher viscosity than petroleum diesel in the commercial engines, the control of injection pressure is the most effective and easiest method to improve combustion Energies 2019, 12, 3264 12 of 14 efficiency. Further, another method is to the control of injection timing. Many studies [16,17] have been conducted to determine the optimum combustion state by controlling fuel injection pressure and injection timing simultaneously in a diesel engine fueled with biodiesel. However, in these studies, the speed conditions are higher than idle. In the future, the study on the combustion improvement of the combustion of palm oil biodiesel having high viscosity by injection timing under low-speed condition will be needed.

Author Contributions: H.Y.K. suggested this research, performed the experiments, analyzed all experimental data, and wrote this paper. J.C.G. performed the experiments and contributed to combustion analysis. 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. Funding: This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (No. 2016R1D1A1B03931616 and No. 2019R1I1A1A01057727), and the Korea government (MSIT) (No. 2019R1F1A1063154). Conflicts of Interest: The authors declare no conflict of interest.

Abbreviations The following abbreviations are used in this manuscript.

PD Palm Oil Biodiesel CRDI Common-Rail Direct Injection ◦CA Crank Angle NOx Nitric Oxide PM Particulate Matter CO Carbon Monoxide HC Hydrocarbon PD0 0% Palm Oil Biodiesel + 100% Diesel PD20 20% Palm Oil Biodiesel + 80% Diesel PD100 100% Palm Oil Biodiesel + 0% Diesel COV Coefficient of Variation IMEP Indicated Mean Effective Pressure dQ/dθ Heat Release Rate k Specific heat ratio P Combustion pressure θ Crank angle V Cylinder volume Vd Displacement volume r Compression ratio R Stroke-to-Bore ratio . mf Fuel flow rate N Engine Speed T Engine Torque 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 BTDC Before Top Dead Center ATDC After Top Dead Center Energies 2019, 12, 3264 13 of 14

References

1. Campbell, C.J.; Laherrère, J.H. The end of cheap oil. Sci. Am. 1998, 278, 78–83. [CrossRef] 2. EU: and Light . Available online: https://www.dieselnet.com/standards/eu/ld.php (accessed on 16 July 2019). 3. Johnson, T.; Joshi, A. Review of Vehicle Engine Efficiency and Emissions. SAE Int. J. Engines 2018, 11, 1307–1330. [CrossRef] 4. Gumus, M.; Sayin, C.; Canakci, M. The impact of fuel injection pressure on the exhaust emissions of a direct injection diesel engine fueled with biodiesel– blends. Fuel 2012, 95, 486–494. [CrossRef] 5. Mahmudul, H.; Hagos, F.Y.; Mamat, R.; Adam, A.A.; Ishak, W.; Alenezi, R. Production, characterization and performance of biodiesel as an alternative fuel in diesel engines—A review. Renew. Sustain. Energy Rev. 2017, 72, 497–509. [CrossRef] 6. Bajpai, D.; Tyagi, V.; Tripathy, D. Biodiesel: Source, Production, Composition, Properties and Its Benefits. J. Oleo Sci. 2006, 55, 487–502. [CrossRef] 7. Ge, J.; Yoon, S.; Kim, M.; Choi, N. Application of canola oil biodiesel/diesel blends in a common rail diesel engine. Appl. Sci. 2017, 7, 34. [CrossRef] 8. Ge, J.C.; Yoon, S.K.; Choi, N.J. Using Canola Oil Biodiesel as an Alternative Fuel in Diesel Engines: A Review. Appl. Sci. 2017, 7, 881. [CrossRef] 9. Seo, Y.T.; Suh, H.K.; Lee, C.S. A study on the injection characteristics of biodiesel fuels injected through common-rail injection system. Trans. Korean Soc. Automot. Eng. 2007, 15, 97–104. 10. Park, S.H.; Kim, H.J.; Suh, H.K.; Lee, C.S. A study on the fuel injection and atomization characteristics of soybean oil methyl ester (SME). Int. J. Heat Fluid Flow 2009, 30, 108–116. [CrossRef] 11. Yusop, A.F.; Mamat, R.; Yusaf, T.; Najafi, G.; Yasin, M.H.M.; Khathri, A.M. Analysis of Particulate Matter (PM) Emissions in Diesel Engines Using Palm Oil Biodiesel Blended with Diesel Fuel. Energies 2018, 11, 1039. [CrossRef] 12. Kim, H.Y.; Ge, J.C.; Choi, N.J. Application of Palm Oil Biodiesel Blends under Idle Operating Conditions in a Common-Rail Direct-Injection Diesel Engine. Appl. Sci. 2018, 8, 2665. [CrossRef] 13. Ge, J.C.; Kim, H.Y.; Yoon, S.K.; Choi, N.J. Reducing volatile organic compound emissions from diesel engines using canola oil biodiesel fuel and blends. Fuel 2018, 218, 266–274. [CrossRef] 14. Karavalakis, G.; Stournas, S.; Bakeas, E. Light vehicle regulated and unregulated emissions from different . Sci. Total Environ. 2009, 407, 3338–3346. [CrossRef] 15. Puhan, S.; Jegan, R.; Balasubbramanian, K.; Nagarajan, G. Effect of injection pressure on performance, emission and combustion characteristics of high linolenic linseed oil methyl ester in a DI diesel engine. Renew. Energy 2009, 34, 1227–1233. [CrossRef] 16. Kannan, G.; Anand, R. Effect of injection pressure and injection timing on DI diesel engine fuelled with biodiesel from waste cooking oil. Biomass Bioenergy 2012, 46, 343–352. [CrossRef] 17. Agarwal, A.K.; Dhar, A.; Gupta, J.G.; Kim, W.I.; Choi, K.; Lee, C.S.; Park, S. Effect of fuel injection pressure and injection timing of Karanja biodiesel blends on fuel spray, engine performance, emissions and combustion characteristics. Energy Convers. Manag. 2015, 91, 302–314. [CrossRef] 18. Zahan, K.A.; Kano, M. Biodiesel Production from Palm Oil, Its By-Products, and Mill Effluent: A Review. Energies 2018, 11, 2132. [CrossRef] 19. El-Araby, R.; Amin, A.; El Morsi, A.; El-Ibiari, N.; El-Diwani, G. Study on the characteristics of palm oil–biodiesel–diesel fuel blend. Egypt. J. Pet. 2018, 27, 187–194. [CrossRef] 20. Lim, Y.K.; Jeon, C.H.; Kim, S.; Yim, E.S.; Song, H.O.; Shin, S.C.; Kim, D.K. Determination of fuel properties for blended biodiesel from various vegetable oils. Korean Chem. Eng. Res. 2009, 47, 237–242. 21. Steiner, S.; Czerwinski, J.; Comte, P.; Popovicheva, O.; Kireeva, E.; Müller, L.; Heeb, N.; Mayer, A.; Fink, A.; Rothen-Rutishauser, B. Comparison of the toxicity of diesel exhaust produced by bio- and fossil diesel combustion in human lung cells in vitro. Atmos. Environ. 2013, 81, 380–388. [CrossRef] 22. Yoon, S.K.; Kim, M.S.; Kim, H.J.; Choi, N.J. Effects of Canola Oil Biodiesel Fuel Blends on Combustion, Performance, and Emissions Reduction in a Common Rail Diesel Engine. Energies 2014, 7, 8132–8149. [CrossRef] 23. Minnesota Statutes, 239.77 Biodiesel Content Madate. 2018. Available online: https://www.revisor.mn.gov/ statutes/cite/239.77 (accessed on 12 August 2019). 24. Heywood, J.B. Internal Combustion Engine Fundamentals; McGraw-Hill Education: New York, NY, USA, 1988. Energies 2019, 12, 3264 14 of 14

25. Alagumalai, A. Combustion characteristics of lemongrass (Cymbopogon flexuosus) oil in a partial premixed charge compression ignition engine. Alex. Eng. J. 2015, 54, 405–413. [CrossRef] 26. Ryu, K. Effects of pilot injection timing on the combustion and emissions characteristics in a diesel engine using biodiesel–CNG dual fuel. Appl. Energy 2013, 111, 721–730. [CrossRef] 27. Huang, Z. Combustion behaviors of a compression-ignition engine fuelled with diesel/methanol blends under various fuel delivery advance angles. Bioresour. Technol. 2004, 95, 331–341. [CrossRef] 28. Amin, A.; Gadallah, A.; El Morsi, A.; El-Ibiari, N.; El-Diwani, G. Experimental and empirical study of diesel and castor biodiesel blending effect, on kinematic viscosity, density and calorific value. Egypt. J. Pet. 2016, 25, 509–514. [CrossRef] 29. Huang, Z.H.; Liu, L.X.; Jiang, D.M.; Ren, Y.; Liu, B.; Zeng, K.; Wang, Q.; Huang, Z. Study on cycle-by-cycle variations of combustion in a natural-gas direct-injection engine. Proc. Inst. Mech. Eng. Part D J. Automob. Eng. 2008, 222, 1657–1667. [CrossRef] 30. Lee, C.S.; Park, S.W.; Kwon, S.I. An Experimental Study on the Atomization and Combustion Characteristics of Biodiesel-Blended Fuels. Energy Fuels 2005, 19, 2201–2208. [CrossRef] 31. Tziourtzioumis, D.N.; Stamatelos, A.M. Experimental Investigation of the Effect of Biodiesel Blends on a DI Diesel Engine’s Injection and Combustion. Energies 2017, 10, 970. [CrossRef] 32. Cooney, C.P.; Yeliana; Worm, J.J.; Naber, J.D. Combustion Characterization in an Internal Combustion Engine with Ethanol Blended Fuels Varying Compression Ratios and . Energy Fuels 2009, − 23, 2319–2324. [CrossRef] 33. Li, B.; Li, Y.; Liu, H.; Liu, F.; Wang, Z.; Wang, J. Combustion and emission characteristics of diesel engine fueled with biodiesel/PODE blends. Appl. Energy 2017, 206, 425–431. [CrossRef] 34. Han, S.B. Investigation of cyclic variations of IMEP under idling operation in spark ignition engines. KSME Int. J. 2001, 15, 81–87. [CrossRef] 35. Agarwal, A.K.; Dhar, A.; Gupta, J.G.; Kim, W.I.; Lee, C.S.; Park, S. Effect of fuel injection pressure and injection timing on spray characteristics and particulate size–number distribution in a biodiesel fuelled common rail direct injection diesel engine. Appl. Energy 2014, 130, 212–221. [CrossRef] 36. Aalam, C.S.; Saravanan, C.; Anand, B.P. Impact of high fuel injection pressure on the characteristics of CRDI diesel engine powered by mahua methyl ester blend. Appl. Therm. Eng. 2016, 106, 702–711. [CrossRef] 37. Kuti, O.A.; Zhu, J.; Nishida, K.; Wang, X.; Huang, Z. Characterization of spray and combustion processes of biodiesel fuel injected by diesel engine common rail system. Fuel 2013, 104, 838–846. [CrossRef]

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