energies

Article

Influence of Fuel Injection Pressure on the EmissionsCharacteristics and Engine Performance in a CRDIDiesel Engine Fueled with Palm Biodiesel Blends

Sam Ki Yoon, Jun Cong Ge and Nag Jung Choi *

Division of Mechanical Design Engineering, Chonbuk National University, 567 Baekje-daero, Jeonju-si,Jeollabuk-do 54896, Korea; sky596072@hanmail.net (S.K.Y.); freedefeng@naver.com (J.C.G.)* Correspondence: njchoi@jbnu.ac.kr; Tel.: +82-63-270-4765

Received: 5 September 2019; Accepted: 10 October 2019; Published: 11 October 2019�����������������

Abstract: This experiment investigates the combustion and emissions characteristics of a commonrail direct injection (CRDI) diesel engine using various blends of pure diesel fuel and palm biodiesel.Fuel injection pressures of 45 and 65 MPa were investigated under engine loads of 50 and 100 Nm.The fuels studied herein were pure diesel fuel 100 vol.% with 0 vol.% of palm biodiesel (PBD0), purediesel fuel 80 vol.% blended with 20 vol.% of palm biodiesel (PBD20), and pure diesel fuel 50 vol.%blended with 50 vol.% of palm biodiesel (PBD50). As the fuel injection pressure increased from 45 to65 MPa under all engine loads, the combustion pressure and heat release rate also increased. Theindicated mean effective pressure (IMEP) increased with an increase of the fuel injection pressure. Inaddition, for 50 Nm of the engine load, an increase to the fuel injection pressure resulted in a reductionof the brake specific fuel consumption (BSFC) by an average of 2.43%. In comparison, for an engineload of 100 Nm, an increase in the fuel injection pressure decreased BSFC by an average of 0.8%.Hydrocarbon (HC) and particulate matter (PM) decreased as fuel pressure increased, independent ofthe engine load. Increasing fuel injection pressure for 50 Nm engine load using PBD0, PBD20 andPBD50 decreased carbon monoxide (CO) emissions. When the fuel injection pressure was increasedfrom 45 MPa to 65 MPa, oxides of nitrogen (NOx) emissions were increased for both engine loads.For a given fuel injection pressure, NOx emissions increased slightly as the biodiesel content in thefuel blend increased.

Keywords: combustion pressure; engine performance; fuel injection pressure; palm biodiesel;exhaust emissions

1. Introduction

Modern countries consume a lot of energy due to rapid population growth and complex anddiverse industrial development. Thus, many countries are consuming more and more energy based onfossil fuels. The use of fossil fuels is known to cause problems such as global warming, climate change,pollution of the atmospheric environment and depletion of fossil fuels [1]. Additionally, such usagecan be detrimental to human health. Among the emissions from diesel engines, known as exhaust gas,that harm humans are hydrocarbons (HC), carbon monoxide (CO), particulate matter (PM) and oxidesof nitrogen (NOx) [2]. For this reason, many researchers are working to study the use of biodieselfuels [3–5]. Biodiesel oil is known as an alternative fuel for diesel engines. The beneficial characteristicsof biodiesel fuels are high biodegradability, high cetane number, better inherent lubricity, renewability,sustainability, environmental friendliness, superior flash point and non-toxicity [6]. Further, biodieselhas minimal aromatic hydrocarbon and sulfur content. The biodiesel structure contains about 10–12%oxygen by weight. In addition, biodiesel oils can be used without modification of diesel engine parts,

Energies 2019, 12, 3837; doi:10.3390/en12203837 www.mdpi.com/journal/energies

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such as a low-pressure pump, high-pressure pump, and injector [7]. It can also be used in dieselengines either as biodiesel oil or after blending with conventional diesel. Previous research has foundthat carbon monoxide (CO), carbon dioxide (CO2), particulate matter (PM), hydrocarbons (HC), andsulfur dioxide (SO2) amounts were decreased in the exhaust gas from biodiesel oil compared withconventional diesel. However, nitrogen oxide (NOx) emission was increased [8].

Currently, biodiesel is gaining popularity as an alternative renewable fuel. It is a clean oxygenatedfuel derived through the transesterification of vegetable oils and animal fats. However, biodieselfuel also has some disadvantages, such as low calorific value, low volatility, high viscosity and poorlow temperature properties [9]. The disadvantages of biodiesel in engine operation are decreasedcombustion pressure (CP), heat release rate (HRR), indicated mean effective pressure (IMEP) andincreased brake specific fuel consumption (BSFC) [10–12]. The engine performance and exhaustemission characteristics of a diesel engine are derived differently according to the fuel combustionprocess in the combustion chamber. In general, the parameters influencing the combustion of fuel in acylinder include the fuel spray pattern, air swirl, compression ratio, number and size of injector holes,fuel injection pressure, shape and injector location of the combustion chamber, fuel characteristics,engine load and engine speed [11,13–15]. Fuel injection pressure is one of the most important variablesaffecting the fuel combustion process in the combustion chamber and also affects fuel atomization andformation of the mixture [16–18].

Therefore, the fuel injection pressure is also important in that it specifically affects the engineperformance and exhaust emissions.

Many researchers have studied engine performance and emission characteristics when using blendsof biodiesel and diesel fuel in different proportions in common rail-compressed diesel engines [19–23].

Jindal et al. [24] investigated the effects of compression ratio and injection pressure in directinjection diesel engine, injected Jatropha methyl ester. Their experimental results confirmed that as thecompression ratio and injection pressure were increased, break thermal efficiency (BTE) increased andBSFC decreased. They found that the optimum fuel injection pressure was 25 MPa for a small dieselengine (3.5 kW) used in agriculture and they experimentally confirmed that the compression ratiowas 18.

Gumus et al. [25] reported the effect of fuel injection pressure on the exhaust emissions of a directinjection diesel engine fueled by a biodiesel–diesel fuel blend. Four different fuel injection pressures (18,20, 22 and 24 MPa) and average effective pressures (12.5, 25, 37.5, 50 kPa) were used with four differentengine loads supplied. They experimentally demonstrated a decreased smoke opacity, unburnedhydrocarbon (UHC) and carbon monoxide (CO) emissions.

Liu et al. [26] studied the effects of diesel injection pressure on the performance and emissions of aheavy duty (HD) common-rail diesel engine fueled with a diesel/methanol dual fuel. The experimentalresults show that at a low injection pressure, the IMEP of the diesel methanol dual fuel (DMDF) modeis lower than that of the pure diesel combustion mode. COVIEMP of the DMDF mode first decreasesand then increases with increasing injection pressure, and it remained under 2.1% for all the tests.

Yesilyurt et al. [27] investigated the effect of fuel injection pressure on the performance andemission characteristics of a diesel engine using waste cooking oil biodiesel–diesel blends. Comparedto diesel fuel, biodiesel fuel showed a decrease in engine torque, brake power, CO, UHC, and smokeopacity, but BSFC, exhaust gas temperature, and NOx and CO2 emissions increased. On the otherhand, the increased injection pressure to 21 MPa increased the engine torque, brake power, and BTE.

Despite these investigations, it is still not completely clear how the fuel injection pressure affectsthe engine performance and the exhaust emissions of a direct injection diesel engine using a blendwith biodiesel oil and standard diesel fuel. Most biodiesel research focuses on the production ofbiodiesel oil, its fuel characteristics and the application to diesel engines. Therefore, the purpose of thisexperiment is to investigate the performance and exhaust emission characteristics of the common-raildiesel engine using a blend of biodiesel oil and standard fuel at a specific engine speed. Additionally,

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to accurately determine the exhaust emission characteristics, the experiment was carried out withvarying fuel injection pressure.

2. Materials and Methods

2.1. Test Fuels and Operating Conditions

2.1.1. Test Fuels

The fuel for the experiment consisted of blends of pure diesel and 0%, 20% and 50% palm oilbased on the volume, referred to as PBD0, PBD20 and PBD50, respectively. The selection of fuels usedin the experimental engine were characterized by determining their viscosities, densities, flow points,distillation temperatures, flaming points, acidities, ester contents, total glass glycerin and calculatedindices of fuel [28–30]. Further, the fuel characteristics of pure diesel and PBD blends were measuredusing ASTM-D6751 and EN-14214 standard test methods. Fuel characteristics of pure diesel and PBDblend fuels are presented in Table 1.

Table 1. Properties of pure diesel and palm biodiesel (PBD).

Properties Diesel Palm Biodiesel Test Method

Density at 15 ◦C (kg/m3) 836.8 877 ASTM D941Viscosity at 40 ◦C (mm2/s) 2.719 4.56 ASTM D445Lower heating value (MJ/kg) 43.96 39.72 ASTM D4809Calculated cetane index 55.8 57.3 ASTM D4737Flash point (◦C) 55 196.0 ASTM D93Pour point (◦C) −21 12.0 ASTM D97Oxidation stability (h/110 ◦C) 25 9.24 EN14112Ester content (%) - 96.5 EN14103Oxygen content (wt%) 0 11.26 -Sulfur content (wt%) 0.11 0.004 ASTM D5453Hydrogen content (wt%) 13.06 12.35 ASTM D5453Carbon content (wt%) 85.73 79.03 ASTM D5291

2.1.2. Operating Conditions

In this experiment, PBD oil with 0.004% sulfur content was blended with pure diesel, and thefuel injection pressure was changed from 45 to 65 MPa using these fuel blends. To investigate thecharacteristics as a function of PBD blending ratio, the engine was sufficiently warmed up to normaloperating temperature before the experiment. The pilot and main injection timings were fixed at beforetop dead center (BTDC) 27 ◦CA and 3.5 ◦CA, respectively, to reduce the effects of engine and exhaustemissions due to changes in injection timing. The engine rpm was set to 1700 rpm and the experimentwas conducted with engine loads of 50 and 100 Nm. In addition, the coolant temperature of the enginewas 80 ± 5 ◦C and the intake air temperature was maintained at 25 ± 3 ◦C. The experimental operatingconditions are summarized in Table 2.

Table 2. Experiment and operating conditions.

Test Parameters Unit Condition

Engine Speed rpm 1700Engine Load Nm 50, 100Cooling Water Temperature ◦C 80 ± 5Intake Air Temperature ◦C 25 ± 3Fuel Injection Pressure MPa 45, 65Injection Timing ◦CA Pilot BTDC27/ Main BTDC 3.5

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The engine speed was fixed at 1700 rpm to isolate dependence of the fuel injection time on fuelinjection pressure. In addition, the engine load was set to 50 Nm or 100 Nm to determine the fuelinjection amount as a function of the engine load. The fuel injection timing was set in a range in whichthe engine operating state was stabilized. Therefore, the pilot injection was fixed at BTDC 27 ◦CA andmain injection at BTDC3.5 ◦CA. Fuel injection pressures were applied at 45 and 65 MPa. The injectiontiming and duration for different injection pressures are shown in Figure 1. When the injection pressurewas increased for an engine load of 50 Nm, the pilot injection duration decreased by 0.06 ms and themain injection duration decreased by 0.17 ms. When the injection pressure was increased and theengine load was 100Nm, the pilot injection duration was confirmed to decrease by 0.02 ms and themain injection duration decreased by 0.19 ms. Increasing the fuel injection pressure at the same loadled to an increase in the fuel injection amount, thereby reducing the fuel injection duration time.

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Table 2. Experiment and operating conditions. 120

Test Parameters Unit Condition Engine Speed rpm 1700 Engine Load Nm 50, 100 Cooling Water Temperature °C 80 ± 5 Intake Air Temperature °C 25 ± 3 Fuel Injection Pressure MPa 45, 65 Injection Timing °CA Pilot BTDC27/ Main BTDC 3.5

The engine speed was fixed at 1700 rpm to isolate dependence of the fuel injection time on fuel 121 injection pressure. In addition, the engine load was set to 50 Nm or 100 Nm to determine the fuel 122 injection amount as a function of the engine load. The fuel injection timing was set in a range in 123 which the engine operating state was stabilized. Therefore, the pilot injection was fixed at BTDC 27 124 °CA and main injection at BTDC3.5 °CA. Fuel injection pressures were applied at 45 and 65 MPa. 125 The injection timing and duration for different injection pressures are shown in Figure 1. When the 126 injection pressure was increased for an engine load of 50 Nm, the pilot injection duration decreased 127 by 0.06 ms and the main injection duration decreased by 0.17 ms. When the injection pressure was 128 increased and the engine load was 100Nm, the pilot injection duration was confirmed to decrease by 129 0.02 ms and the main injection duration decreased by 0.19 ms. Increasing the fuel injection pressure 130 at the same load led to an increase in the fuel injection amount, thereby reducing the fuel injection 131 duration time. 132

133

Figure 1. Curves of combustion characteristic of all tested fuels: (a) fuel injection pressure of 45 MPa 134 and engine load of 50 Nm, (b) fuel injection pressure of 65 MPa and engine load of 50 Nm, (c) fuel 135 injection pressure of 45 MPa and engine load of 100 Nm and (d) fuel injection pressure 65 MPa and 136 engine load 100 Nm. 137

2.2. Test Engine and Experimental Procedure 138

Figure 1. Curves of combustion characteristic of all tested fuels: (a) fuel injection pressure of 45 MPaand engine load of 50 Nm, (b) fuel injection pressure of 65 MPa and engine load of 50 Nm, (c) fuelinjection pressure of 45 MPa and engine load of 100 Nm and (d) fuel injection pressure 65 MPa andengine load 100 Nm.

2.2. Test Engine and Experimental Procedure

2.2.1. Test Engine

A four-cylinder in-line turbocharged common rail direct injection diesel engine was used forthis experiment. It is equipped with a crankshaft position sensor to detect the engine speed and acombustion pressure sensor to detect the combustion pressure of the combustion chamber. Thereis also a fuel rail pressure sensor that measures the rail pressure as the fuel pressure changes. Tocontrol the operation of the engine in real time, the engine control unit—electronic control unit (ECU)is mounted. Details of the engine are shown in Table 3.

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Table 3. Specifications of the test engine.

Engine Parameters Unit Specification

Bore × Stroke mm ×mm 83 × 92Displacement cc 1991Compression ratio - 17.7 : 1Maximum power/torque kW/Nm 83.5 (at 4000 rpm)/255 (at 2000 rpm)Engine type - In-line 4 Cylinder, Turbocharged, EGRNumber of injector nozzle holes - 5Injector type - SolenoidInjector hole diameter mm 0.17Fuel control - ECUManufacture, Model - Hyundai Motor, Santafe

2.2.2. Experimental Equipment Set Up

An eddy current dynamometer (DY-230 kW, Hwanwoong Mechatronics, Gyeongsangnam-do,Korea) was installed to control the engine power. In addition, a multi-gas analyzer (HPC501, NantongHuapeng Electronics, Jiangsu, China) was installed to measure CO, HC, and NOx in the exhaustemissions. A multiple gas analyzer (GreenLine MK2, Eurotron (Korea) Ltd., Seoul, Korea) wasinstalled for accurate and comparative analysis of exhaust emissions. A partial flow collection analyzer(OPA-102, QROTECH Co., Ltd., Gyeonggi-do, Korea) was used to measure particle matter. Combustionpressure for combustion analysis of the combustion chamber was obtained using a piezoelectricpressure sensor (KISTLER Type 6056A, Kistler Korea Co., Ltd., Gyeonggi-do, Korea) at the glow plugposition. For analysis of the combustion pressure, data were obtained over an average of 200 cycles.Data acquisition was performed and recorded using a DAQ board (PCI 6040e, National Instrument,Austin, TX, USA). The released particle matter was collected by a copper grid (FCF400-CU, ElectronMicroscopy Sciences, PA, USA) and transmission electron microscopy (H-7650 TEM, Hitachi Prefecture,Fukuoka, Japan) was used to analyze the shape of the particles. Figure 2 is schematic diagram of theinline experimental equipment.Energies 2019, 12, x FOR PEER REVIEW 6 of 18

162 Figure 2. Schematic of the inline experimental equipment. 163

2.2.3. Data Analysis 164 The heat release rate is a value calculated based on the combustion chamber pressure using the 165

first law of thermodynamics. The heat release rate represents the progress of normal combustion. 166 The following formula was used to calculate heat release rate ( ) [31]: 167 𝑑𝑄𝑑𝜃 = kk − 1 𝑃 𝑑𝑉𝑑𝜃 1k − 1 𝑉 𝑑𝑃𝑑𝜃 (1)

where k is the specific heat ratio, V is the cylinder volume, P is the combustion pressure, and θ is 168 the crank angle. In these experiments, the average value of 200 cycles was calculated to ensure the 169 reliability of the data acquisition. The cylinder volume is a function of the gap volume (Vc), cylinder 170 diameter (D), the length of the connecting rod (b), the radius of the crankshaft (a), and the distance 171 between the piston pin and the crankshaft (L). 172 𝑉 = 𝑉 𝜋𝐷4 (b a − L) (2)

The ratio of fuel consumption to braking power of an engine is defined as the brake specific 173 fuel consumption. Brake specific fuel consumption data are calculated based on the fuel 174 consumption, engine torque and speed values using the following formula [32]: 175 BSFC = 𝑚2𝜋𝑁𝑇 (3)

where 𝒎𝒇 is the fuel flow rate, N is the engine speed, and T is the brake torque. 176 To verify the combustion stability of the engine when using the test fuel in the experimental 177

engine, the coefficient of variation (COV) for the indicated mean effective pressure was used. The 178 coefficient of variation is the standard deviation divided by the arithmetic mean, also known as the 179 relative standard deviation. The larger the coefficient of variation, the greater the relative difference. 180 The coefficient of variation of IMEP (COVIMEP) is given by [31]: 181

Figure 2. Schematic of the inline experimental equipment.

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2.2.3. Data Analysis

The heat release rate is a value calculated based on the combustion chamber pressure using thefirst law of thermodynamics. The heat release rate represents the progress of normal combustion. Thefollowing formula was used to calculate heat release rate ( dQ

dθ ) [31]:

dQdθ

=k

k− 1P

dVdθ

+1

k− 1V

dPdθ

(1)

where k is the specific heat ratio, V is the cylinder volume, P is the combustion pressure, and θ isthe crank angle. In these experiments, the average value of 200 cycles was calculated to ensure thereliability of the data acquisition. The cylinder volume is a function of the gap volume (Vc), cylinderdiameter (D), the length of the connecting rod (b), the radius of the crankshaft (a), and the distancebetween the piston pin and the crankshaft (L).

V = VC +πD2

4(b + a− L) (2)

The ratio of fuel consumption to braking power of an engine is defined as the brake specific fuelconsumption. Brake specific fuel consumption data are calculated based on the fuel consumption,engine torque and speed values using the following formula [32]:

BSFC =

.m f

2πNT(3)

where.

m f is the fuel flow rate, N is the engine speed, and T is the brake torque.To verify the combustion stability of the engine when using the test fuel in the experimental

engine, the coefficient of variation (COV) for the indicated mean effective pressure was used. Thecoefficient of variation is the standard deviation divided by the arithmetic mean, also known as therelative standard deviation. The larger the coefficient of variation, the greater the relative difference.The coefficient of variation of IMEP (COVIMEP) is given by [31]:

COVIMEP =

√1N

∑Ni=1

{IMEP(i) −X

}2

1N

∑Ni=1 IMEP(i)

(4)

where IMEP(i) is the indicated mean effective pressure for each cycle. Here, the numerator is thestandard deviation of indicated mean effective pressure (IMEP) over 200 cycles (N = 200) and thedenominator is the average of these values. The brake thermal efficiency is the value of the brakepower divided by the thermal energy of the feed fuel [32]:

BTE =Ne

B×HL(5)

where Ne is the brake power output, B is the fuel consumption per unit time, and HL is the low calorificvalue of the fuel.

3. Results and Discussion

3.1. Combustion Characteristics

3.1.1. Combustion Pressure and Heat Release Rate

Figure 3 shows the combustion pressure and heat release rate when the fuel injection pressureis increased in the biodiesel blended oil in the common rail compressed diesel engine. As seen inthe figure, when the engine load was held constant at 50 Nm and the fuel injection pressure was

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increased from 45 to 65 MPa, the combustion pressure increased by 9.6, 9.6, and 10.4% for PBD0,PBD20, and PBD50, respectively. Additionally, the heat release rate under the same conditions wasconfirmed to increase by 12.1, 16.1, 11.2% in PBD0, PBD20, and PBD50, respectively. Keeping theengine load constant at 100 Nm and increasing the fuel injection pressure from 45 MPa to 65 MParesulted in an increase in combustion pressure by 12.1, 13.5 and 13% for PBD0, PBD20 and PBD50,respectively. In addition, the heat release rate increased by 14.1, 15.4, 9.2% under the same conditions.The reason is the high fuel injection pressure, which improves spraying and mixing. Therefore, fuelevaporation is activated in the boundary layer where compressed air meets injected fuel, resulting inbetter combustion. Thereby combustion pressure and heat release rate increased with increasing fuelinjection pressure [33,34].

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204 Figure 3. Combustion pressure and heat release rate in the combustion chamber for (a) PBD0 for an 205 engine load of 50 Nm, (b) PBD0 for an engine load of 100 Nm, (c) PBD20 for an engine load of 50Nm, 206 (d) PBD20 for an engine load of 100 Nm, (e) PBD50 for an engine load of 50 Nm and (f) PBD50 for 207 an engine load of 100 Nm. 208

Table 4 shows location of the maximum combustion pressure and the maximum heat release 209 rate when the fuel injection pressure is increased. When the engine load was held at 50 Nm and the 210 fuel injection pressure was increased from 45 to 65 MPa, the average time needed to achieve the 211 maximum combustion pressure and maximum heat release rate was approximately 0.098ms faster 212 for all fuels. When the engine load was held constant at 100 Nm and the fuel injection pressure was 213 increased from 45 to 65 MPa, the average time to reach maximum combustion pressure and 214 maximum heat release rate was 0.196 and 0.264 ms faster, respectively for all fuels. 215 216

Figure 3. Combustion pressure and heat release rate in the combustion chamber for (a) PBD0 for anengine load of 50 Nm, (b) PBD0 for an engine load of 100 Nm, (c) PBD20 for an engine load of 50Nm,(d) PBD20 for an engine load of 100 Nm, (e) PBD50 for an engine load of 50 Nm and (f) PBD50 for anengine load of 100 Nm.

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Table 4 shows location of the maximum combustion pressure and the maximum heat release ratewhen the fuel injection pressure is increased. When the engine load was held at 50 Nm and the fuelinjection pressure was increased from 45 to 65 MPa, the average time needed to achieve the maximumcombustion pressure and maximum heat release rate was approximately 0.098ms faster for all fuels.When the engine load was held constant at 100 Nm and the fuel injection pressure was increased from45 to 65 MPa, the average time to reach maximum combustion pressure and maximum heat releaserate was 0.196 and 0.264 ms faster, respectively for all fuels.

Table 4. Crankshaft angle (ATDC) of the maximum combustion pressure and heat release rate.

FIP 1 PBD 0 2

50 NmPBD 0

100 NmPBD 20 3

50 NmPBD 20100Nm

PBD 50 4

50 NmPBD 50100 Nm

(MPa) CP 5

(◦CA)HRR6

(◦CA)CP

(◦CA)HRR(◦CA)

CP(◦CA)

HRR(◦CA)

CP(◦CA)

HRR(◦CA)

CP(◦CA)

HRR(◦CA)

CP(◦CA)

HRR(◦CA)

45 12 14 15 17 13 14 16 17 12 15 16 1865 11 13 13 14 12 14 14 15 11 13 14 15

1FIP: Fuel Injection Pressure. 2 PBD 0: 0% Palm Biodiesel + 100% Diesel. 3 PBD 20: 20% Palm Biodiesel + 80%Diesel. 4 PBD 50: 50% Palm Biodiesel + 50% Diesel. 5 CP: Combustion Pressure. 6 HRR: Heat Release Rate.

3.1.2. Combustion Peak Pressure

To investigate the change in maximum combustion pressure over each cycle, the experiment wascarried out over 200 cycles. The fuel injection pressure was then increased from 45 MPa to 65 MPa.The experimental results showed that the average change in maximum combustion pressure for eachcycle of the PBD0, PBD20, and PBD50 fuels increased by 10.32% at a 50 Nm engine load.

In addition, it was confirmed that the average change in maximum combustion pressure of eachcycle increased by 12.7% in each experimental fuel at an engine load of 100 Nm. Figure 4 showsthe maximum combustion pressure as a function of the engine load. Biodiesel’s high viscosity andconcentrations can lead to poor spray and volatility, resulting in poor combustion. However, the highoxygen content of biodiesel can improve combustion [11]. Many other researchers have reported thathigh fuel injection pressure can improve fuel atomization, even for high viscosity biodiesel [35].

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Table 4. Crankshaft angle (ATDC) of the maximum combustion pressure and heat release rate. 217

FIP 1

(MPa)

PBD 0 2

50Nm

PBD 0

100Nm

PBD 20 3

50Nm

PBD 20

100Nm

PBD 50 4

50Nm

PBD 50

100Nm

CP 5

(°CA)

HRR6

(°CA)

CP

(°CA)

HRR

(°CA)

CP

(°CA)

HRR

(°CA)

CP

(°CA)

HRR

(°CA)

CP

(°CA)

HRR

(°CA)

CP

(°CA)

HRR

(°CA)

45 12 14 15 17 13 14 16 17 12 15 16 18

65 11 13 13 14 12 14 14 15 11 13 14 15

1 FIP: Fuel Injection Pressure. 2 PBD 0: 0% Palm Biodiesel + 100% Diesel. 3 PBD 20: 20% Palm Biodiesel + 80% 218 Diesel. 4 PBD 50: 50% Palm Biodiesel + 50% Diesel. 5 CP: Combustion Pressure. 6 HRR: Heat Release Rate. 219

3.1.2. Combustion Peak Pressure 220 To investigate the change in maximum combustion pressure over each cycle, the experiment 221

was carried out over 200 cycles. The fuel injection pressure was then increased from 45 MPa to 65 222 MPa. The experimental results showed that the average change in maximum combustion pressure 223 for each cycle of the PBD0, PBD20, and PBD50 fuels increased by 10.32% at a 50 Nm engine load. 224

225 Figure 4. Combustion peak pressure at engine loads of (a) 50 Nm and (b) 100 Nm. 226

In addition, it was confirmed that the average change in maximum combustion pressure of 227 each cycle increased by 12.7% in each experimental fuel at an engine load of 100 Nm. Figure 4 228 shows the maximum combustion pressure as a function of the engine load. Biodiesel’s high 229 viscosity and concentrations can lead to poor spray and volatility, resulting in poor combustion. 230 However, the high oxygen content of biodiesel can improve combustion [11]. Many other 231 researchers have reported that high fuel injection pressure can improve fuel atomization, even for 232 high viscosity biodiesel [35]. 233

3.2. Engine Performance 234

3.2.1. COVIMEP and IMEP Analysis 235

Figure 4. Combustion peak pressure at engine loads of (a) 50 Nm and (b) 100 Nm.

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3.2. Engine Performance

3.2.1. COVIMEP and IMEP Analysis

An experiment was conducted to find out the change of the indicated mean effective pressurein the experimental engine. Figure 5a,b shows the change in IMEP, when fuel injection pressure wasincreased from 45 to 65 MPa, IMEP slightly increased by 0.95%, 8.5% and 3.8% for PBD0, PBD20 andPBD50, respectively. In addition, under the same conditions for 100 Nm of engine load, IMEP slightlyincreased by 2.2%, 6.2% and 2.8% when using PBD0, PBD20 and PBD50, respectively.

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An experiment was conducted to find out the change of the indicated mean effective pressure 236 in the experimental engine. Figure 5a,b shows the change in IMEP, when fuel injection pressure was 237 increased from 45 to 65 MPa, IMEP slightly increased by 0.95%, 8.5% and 3.8% for PBD0, PBD20 238 and PBD50, respectively. In addition, under the same conditions for 100 Nm of engine load, IMEP 239 slightly increased by 2.2%, 6.2% and 2.8% when using PBD0, PBD20 and PBD50, respectively. 240

In this experiment, we also determined the COVIMEP values. As seen in Figure 5c,d, the COVIMEP 241 values for the 50 and 100 Nm engine loads are less than 1.6% and 0.9%, respectively, when the fuel 242 injection pressure is changed. This shows that the combustion state of the engine works well within 243 200 cycles. The increase in fuel injection pressure is evidence that the difference in combustion 244 pressure between each cylinder in the four-cylinder cylinder is not large and is relatively uniform. 245 In addition, the COVIMEP value with respect to the biodiesel oil blend rate is lower than that of pure 246 diesel. This is likely because palm oil itself has higher oxygen content than pure diesel oil [21]. 247

248 Figure 5. IMEP for engine loads of (a) 50 Nm and (b) 100 Nm. COVIMEP for engine loads of (c) 50 Nm 249 and (d) 100 Nm. 250

3.2.2. BSFC Analysis 251 As shown in Figure 6, the BSFC with respect to the engine load exhibited an average value that 252

was 16.3% lower for the engine load of 100 Nm than for the engine load of 50 Nm. For an engine 253 load of 50 Nm, BSFC was reduced by an average of 2.43% when the fuel injection pressure was 254 increased from 45 MPa to 65 MPa. In addition, the BSFC for the 100 Nm engine load showed an 255 average reduction of 0.8% when the fuel injection pressure was increased from 45 MPa to 65 MPa. 256 However, comparing the BSFC of pure diesel and biodiesel blends shows that BSFC values are 257 increased in the biodiesel blends. The average value of BSFC increased with increasing biodiesel 258 blended at 50 Nm of engine load. BSFC increased 4.07% and 1.91% at 45 and 65 MPa fuel injection 259 pressures, respectively. In addition, the average value of BSFC increased with increasing biodiesel 260 blended at 100 Nm engine load. BSFC increased 5.83% and 3.7% at 45 and 65 MPa fuel injection 261

Figure 5. IMEP for engine loads of (a) 50 Nm and (b) 100 Nm. COVIMEP for engine loads of (c) 50 Nmand (d) 100 Nm.

In this experiment, we also determined the COVIMEP values. As seen in Figure 5c,d, the COVIMEP

values for the 50 and 100 Nm engine loads are less than 1.6% and 0.9%, respectively, when the fuelinjection pressure is changed. This shows that the combustion state of the engine works well within200 cycles. The increase in fuel injection pressure is evidence that the difference in combustion pressurebetween each cylinder in the four-cylinder cylinder is not large and is relatively uniform. In addition,the COVIMEP value with respect to the biodiesel oil blend rate is lower than that of pure diesel. This islikely because palm oil itself has higher oxygen content than pure diesel oil [21].

3.2.2. BSFC Analysis

As shown in Figure 6, the BSFC with respect to the engine load exhibited an average value thatwas 16.3% lower for the engine load of 100 Nm than for the engine load of 50 Nm. For an engine load of50 Nm, BSFC was reduced by an average of 2.43% when the fuel injection pressure was increased from45 MPa to 65 MPa. In addition, the BSFC for the 100 Nm engine load showed an average reduction of

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0.8% when the fuel injection pressure was increased from 45 MPa to 65 MPa. However, comparingthe BSFC of pure diesel and biodiesel blends shows that BSFC values are increased in the biodieselblends. The average value of BSFC increased with increasing biodiesel blended at 50 Nm of engineload. BSFC increased 4.07% and 1.91% at 45 and 65 MPa fuel injection pressures, respectively. Inaddition, the average value of BSFC increased with increasing biodiesel blended at 100 Nm engineload. BSFC increased 5.83% and 3.7% at 45 and 65 MPa fuel injection pressure, respectively. This isbecause the density and kinematic viscosity of biodiesel oil (877 kg/m3, 4.56 mm2/s) is higher thanthat of pure diesel (836.8 kg/m3, 2.719 mm2/s) and the lower calorific value is lower than that of purediesel [25,36,37].

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pressure, respectively. This is because the density and kinematic viscosity of biodiesel oil (877 262 kg/m3, 4.56 mm2/s) is higher than that of pure diesel (836.8 kg/m3, 2.719 mm2/s) and the lower 263 calorific value is lower than that of pure diesel [25,36,37]. 264

265 Figure 6. BSFC for engine loads of (a) 50 Nm (b) 100 Nm. 266

3.3. Emission Characteristics 267

3.3.1. HC and PM 268 Figure 7a,b shows that the HC emissions decreased by 29.6%, 19% and 12.5% for PBD0, PBD20, 269

and PBD50, respectively, when the fuel injection pressure was increased from 45 to 65 MPa for a 50 270 Nm engine load. In addition, when the fuel injection pressure was increased from 45 MPa to 65 271 MPa for a 100 Nm engine load, HC emissions decreased by 24%, 15% and 18.2% using PBD0, 272 PBD20 and PBD50 fuels, respectively. As the proportion of biodiesel oil and fuel injection pressure 273 increased, HC emissions decreased because the high amount of oxygen in the biodiesel itself 274 reduces the amount of unburned HC in the exhaust [25,38]. 275

In general, the biodiesel with high oxygen content can effectively deliver oxygen to the 276 pyrolysis zone of combustion spray to reduce smoke emission. In addition, the oxygen in biodiesel 277 ensures post-flame oxidation and increases flame speed during air-fuel interaction resulting in 278 complete hydrocarbon oxidation [39,40]. Figure 7c,d shows the PM emissions when the fuel 279 pressure was increased from 45 to 65 MPa using PBD0, PBD20, and PBD50 for engine loads of 50 280 Nm and 100 Nm. For an engine load of 50 Nm, the PM emissions decreased by 59.5%, 51.6%, and 281 52.1% using PBD0, PBD20, and PBD50, respectively, under these conditions. Additionally, when the 282 fuel pressure was increased from 45 MPa to 65 MPa for an engine load of 100 Nm, the PM emission 283 decreased by 41.7%, 43.5%, 52.7% using PBD0, PBD20, and PBD50, respectively. Overall, PM 284 emissions of all tested fuels decreased with the increase of fuel injection pressure from 45 to 65 MPa. 285 This is because biodiesel itself contains high oxygen content, which can improve the combustion 286 environment of fuel even under partial oxygen deficiency conditions [25,41]. 287

Figure 6. BSFC for engine loads of (a) 50 Nm (b) 100 Nm.

3.3. Emission Characteristics

3.3.1. HC and PM

Figure 7a,b shows that the HC emissions decreased by 29.6%, 19% and 12.5% for PBD0, PBD20,and PBD50, respectively, when the fuel injection pressure was increased from 45 to 65 MPa for a 50 Nmengine load. In addition, when the fuel injection pressure was increased from 45 MPa to 65 MPafor a 100 Nm engine load, HC emissions decreased by 24%, 15% and 18.2% using PBD0, PBD20 andPBD50 fuels, respectively. As the proportion of biodiesel oil and fuel injection pressure increased, HCemissions decreased because the high amount of oxygen in the biodiesel itself reduces the amount ofunburned HC in the exhaust [25,38].

In general, the biodiesel with high oxygen content can effectively deliver oxygen to the pyrolysiszone of combustion spray to reduce smoke emission. In addition, the oxygen in biodiesel ensurespost-flame oxidation and increases flame speed during air-fuel interaction resulting in completehydrocarbon oxidation [39,40]. Figure 7c,d shows the PM emissions when the fuel pressure wasincreased from 45 to 65 MPa using PBD0, PBD20, and PBD50 for engine loads of 50 Nm and 100 Nm.For an engine load of 50 Nm, the PM emissions decreased by 59.5%, 51.6%, and 52.1% using PBD0,PBD20, and PBD50, respectively, under these conditions. Additionally, when the fuel pressure wasincreased from 45 MPa to 65 MPa for an engine load of 100 Nm, the PM emission decreased by 41.7%,43.5%, 52.7% using PBD0, PBD20, and PBD50, respectively. Overall, PM emissions of all tested fuelsdecreased with the increase of fuel injection pressure from 45 to 65 MPa. This is because biodiesel itselfcontains high oxygen content, which can improve the combustion environment of fuel even underpartial oxygen deficiency conditions [25,41].

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288 Figure 7. HC emissions in the exhaust gas for engine loads of (a) 50 Nm and (b) 100 Nm. PM in the 289 exhaust gas for engine loads of (c) 50 Nm and (d) 100 Nm. 290

3.3.2. Particulate Matter Characteristics 291 As shown in Figure 8, transmission electron microscopy (TEM) images were taken to 292

investigate the dependence of PM emission characteristics on the engine load and fuel injection 293 pressure. Figure 9 shows a graph of the mean particle diameter, measured by analyzing the 294 diameter of the PM as well as the shape of the PM using TEM micrographs. When the engine load 295 was 50 Nm and the fuel injection pressure was increased from 45 MPa to 65 MPa, the diameter of 296 PM decreased by 4.7%, 2.5% and 0.4% on average in PBD0, PBD20 and PBD50, respectively. In 297 addition, when the engine load was 100 Nm and the fuel injection pressure was increased from 45 298 MPa to 65 MPa, the diameter of PM decreased by 11.7%, 6%, and 2% on average in PBD0, PBD20, 299 and PBD50, respectively. As a result, it was confirmed that the exhaust particle size of PM was 300 reduced by increasing the fuel injection pressure [11,42]. 301

Figure 7. HC emissions in the exhaust gas for engine loads of (a) 50 Nm and (b) 100 Nm. PM in theexhaust gas for engine loads of (c) 50 Nm and (d) 100 Nm.

3.3.2. Particulate Matter Characteristics

As shown in Figure 8, transmission electron microscopy (TEM) images were taken to investigatethe dependence of PM emission characteristics on the engine load and fuel injection pressure. Figure 9shows a graph of the mean particle diameter, measured by analyzing the diameter of the PM as well asthe shape of the PM using TEM micrographs. When the engine load was 50 Nm and the fuel injectionpressure was increased from 45 MPa to 65 MPa, the diameter of PM decreased by 4.7%, 2.5% and 0.4%on average in PBD0, PBD20 and PBD50, respectively. In addition, when the engine load was 100 Nmand the fuel injection pressure was increased from 45 MPa to 65 MPa, the diameter of PM decreased by11.7%, 6%, and 2% on average in PBD0, PBD20, and PBD50, respectively. As a result, it was confirmedthat the exhaust particle size of PM was reduced by increasing the fuel injection pressure [11,42].

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302 Figure 8. TEM images of the PM emissions for different engine loads and fuel injection pressures 303 changes. 304

305 Figure 9. Average particle size of PM emissions for different engine loads of (a) 50 Nm (b) 100 Nm. 306

3.3.3. CO and NOX 307 CO emission from PBD0, PBD20, and PBD50 fuels used at engine loads of 50 and 100 Nm are 308

shown in Figure 10a,b for when the fuel injection pressure is increased from 45 MPa to 65 MPa. For 309 an engine load of 50 Nm, this fuel injection pressure increase resulted in CO emission decreases of 310 6.1%, 9.7% and 7.4% when using PBD0, PBD20 and PBD50, respectively. In addition, CO emissions 311 from PBD0, PBD20 and PBD50 decreased by 6.3%, 9.5% and 8.9% with this fuel injection pressure 312 increase for an engine load of 100 Nm. From the above results, as the biodiesel blended rate and 313 fuel injection pressure were increased, the CO emissions were decreased compared to pure diesel. 314 This is because biodiesel itself contains about 11% oxygen, which can promote fuel towards 315 complete combustion. Increasing the fuel injection pressure resulted in a good mixture of air and 316

Figure 8. TEM images of the PM emissions for different engine loads and fuel injection pressures changes.

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302 Figure 8. TEM images of the PM emissions for different engine loads and fuel injection pressures 303 changes. 304

305 Figure 9. Average particle size of PM emissions for different engine loads of (a) 50 Nm (b) 100 Nm. 306

3.3.3. CO and NOX 307 CO emission from PBD0, PBD20, and PBD50 fuels used at engine loads of 50 and 100 Nm are 308

shown in Figure 10a,b for when the fuel injection pressure is increased from 45 MPa to 65 MPa. For 309 an engine load of 50 Nm, this fuel injection pressure increase resulted in CO emission decreases of 310 6.1%, 9.7% and 7.4% when using PBD0, PBD20 and PBD50, respectively. In addition, CO emissions 311 from PBD0, PBD20 and PBD50 decreased by 6.3%, 9.5% and 8.9% with this fuel injection pressure 312 increase for an engine load of 100 Nm. From the above results, as the biodiesel blended rate and 313 fuel injection pressure were increased, the CO emissions were decreased compared to pure diesel. 314 This is because biodiesel itself contains about 11% oxygen, which can promote fuel towards 315 complete combustion. Increasing the fuel injection pressure resulted in a good mixture of air and 316

Figure 9. Average particle size of PM emissions for different engine loads of (a) 50 Nm (b) 100 Nm.

3.3.3. CO and NOX

CO emission from PBD0, PBD20, and PBD50 fuels used at engine loads of 50 and 100 Nm areshown in Figure 10a,b for when the fuel injection pressure is increased from 45 MPa to 65 MPa. Foran engine load of 50 Nm, this fuel injection pressure increase resulted in CO emission decreases of6.1%, 9.7% and 7.4% when using PBD0, PBD20 and PBD50, respectively. In addition, CO emissionsfrom PBD0, PBD20 and PBD50 decreased by 6.3%, 9.5% and 8.9% with this fuel injection pressureincrease for an engine load of 100 Nm. From the above results, as the biodiesel blended rate andfuel injection pressure were increased, the CO emissions were decreased compared to pure diesel.This is because biodiesel itself contains about 11% oxygen, which can promote fuel towards completecombustion. Increasing the fuel injection pressure resulted in a good mixture of air and fuel, creating

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good combustion environment, resulting in reduced CO emissions. Other researchers also reportedsimilar results [25,43].

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fuel, creating good combustion environment, resulting in reduced CO emissions. Other researchers 317 also reported similar results [25,43]. 318

NOx emission also changed for engine loads, fuel injection pressures, and fuel blends. The 319 formation of NOx emissions is mainly related to the high temperature conditions in the combustion 320 chamber. In general, the oxygen content in biodiesel increases the highest temperature in 321 combustion chamber compared with that in diesel fuel with no oxygen content. The NOx emissions 322 emitted from diesel engine is predominantly composed of NO, and with lesser amounts of NO2 [44]. 323 When the temperature in the combustion chamber exceeds 1700 K, a series of chemical reactions 324 occur between N2 and O2 through Zeldovich mechanism to form a lot of NOx emissions [45]. The 325 basic equation for the formation of NOx is listed in chemical reactions 6–8. High cetane number of 326 biodiesel can shorten ignition delay, and high oxygen content can ensure better combustion 327 resulting in higher cylinder temperature [46]. Therefore, the higher the oxygen content in fuel, the 328 more the formation of NOx emissions. 329

N2 +O ↔ NO+N (6)

N +O2 ↔ NO+O (7)

N +OH ↔ NO+H (8)

As shown in Figure 10c,d, NOx emissions increased by 30.8%, 31.3% and 35.6% for an engine 330 load of 50 Nm using PBD0, PBD20, and PBD50, respectively, as the fuel injection pressure increased. 331 In addition, NOx emissions from PBD0, PBD20, and PBD50 increased by 35.8%, 35.9% and 34.9%, 332 respectively, for a 100 Nm engine load. On the other hand, the NOx emissions of PBD20 and PBD50 333 were respectively increased by about 0.7% and 2.8% on average compared to PBD0, when the fuel 334 injection pressures were respectively controlled at 45 and 65 MPa at an engine load of 50 Nm; for 335 100 Nm engine load, the NOx emissions were respectively increased by about 1.7% and 1.5% 336 according to 45 and 65 MPa. The NOx emissions increased as the biodiesel blended rate and fuel 337 injection pressure increased, due to the presence of the high chemically bound oxygen content in 338 biodiesel compared to pure diesel, which resulted in increasing of combustion temperature and 339 pressure in the combustion chamber. Other researchers reported that the oxygen content in 340 biodiesel fuel plays an important role for formation of NOx emissions [25,47]. 341

342 Figure 10. Exhaust gas emissions of CO from engine loads of (a) 50 Nm and (b) 100 Nm and NOx 343 from engine loads of (c) 50 Nm and (d) 100 Nm. 344 Figure 10. Exhaust gas emissions of CO from engine loads of (a) 50 Nm and (b) 100 Nm and NOx fromengine loads of (c) 50 Nm and (d) 100 Nm.

NOx emission also changed for engine loads, fuel injection pressures, and fuel blends. Theformation of NOx emissions is mainly related to the high temperature conditions in the combustionchamber. In general, the oxygen content in biodiesel increases the highest temperature in combustionchamber compared with that in diesel fuel with no oxygen content. The NOx emissions emitted fromdiesel engine is predominantly composed of NO, and with lesser amounts of NO2 [44]. When thetemperature in the combustion chamber exceeds 1700 K, a series of chemical reactions occur betweenN2 and O2 through Zeldovich mechanism to form a lot of NOx emissions [45]. The basic equationfor the formation of NOx is listed in chemical reactions 6–8. High cetane number of biodiesel canshorten ignition delay, and high oxygen content can ensure better combustion resulting in highercylinder temperature [46]. Therefore, the higher the oxygen content in fuel, the more the formation ofNOx emissions.

N2 + O↔ NO + N (6)

N + O2↔ NO + O (7)

N + OH↔ NO + H (8)

As shown in Figure 10c,d, NOx emissions increased by 30.8%, 31.3% and 35.6% for an engineload of 50 Nm using PBD0, PBD20, and PBD50, respectively, as the fuel injection pressure increased.In addition, NOx emissions from PBD0, PBD20, and PBD50 increased by 35.8%, 35.9% and 34.9%,respectively, for a 100 Nm engine load. On the other hand, the NOx emissions of PBD20 and PBD50were respectively increased by about 0.7% and 2.8% on average compared to PBD0, when the fuelinjection pressures were respectively controlled at 45 and 65 MPa at an engine load of 50 Nm; for100 Nm engine load, the NOx emissions were respectively increased by about 1.7% and 1.5% accordingto 45 and 65 MPa. The NOx emissions increased as the biodiesel blended rate and fuel injection

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pressure increased, due to the presence of the high chemically bound oxygen content in biodieselcompared to pure diesel, which resulted in increasing of combustion temperature and pressure in thecombustion chamber. Other researchers reported that the oxygen content in biodiesel fuel plays animportant role for formation of NOx emissions [25,47].

4. Conclusions

This study was conducted to investigate combustion and exhaust emission characteristics at 50and 100 Nm engine loads using PBD0, PBD20, and PBD50 fuels in a common rail diesel engine as theinjection pressure is increased from 45 to 65 MPa.

The combustion pressure and heat release rate increased with increasing fuel injection pressurefor engine loads of 50 and 100 Nm. IMEP also increased when the fuel injection pressure increased for50 and 100 Nm engine loads. COVIMEP values are less than 1.6% and 0.9% for the engine loads of 50and 100 Nm, respectively, with increasing fuel injection pressures. When the fuel injection pressurewas increased for an engine load of 50 Nm, the BSFC was reduced on average by 2.43%. Additionally,when the fuel injection pressure was increased for 100 Nm of engine load, the BSFC decreased by0.8% on average. HC emissions were reduced for all fuels tested with increasing fuel pressure for 50and 100 Nm engine loads. As the biodiesel oil content increased, the emission of HC decreased. PMdecreased when the fuel pressure was increased from 45 to 65 MPa for both 50 and 100 Nm engineloads. CO emissions decreased when the fuel injection pressure was increased for an engine load of50 Nm when using PBD, PBD20 and PBD50. CO emissions were also reduced with increasing fuelinjection pressure for a 100 Nm engine load in PBD0, PBD20 and PBD50. As fuel injection pressure andbiodiesel content were increased, CO emissions were decreased compared to pure diesel. When thefuel injection pressure was increased from 45 MPa to 65 MPa, NOx emissions were increased for 50and 100 Nm engine loads. For a given fuel injection pressure, the amount of NOx emissions increasedslightly as the biodiesel content in the fuel blend increased.

Author Contributions: S.K.Y. performed the experiments, analyzed all experimental data, and wrote this paper.J.C.G. performed the experiments and contributed to engine performance analysis. N.J.C. designed this experimentand contributed to the data analysis supervised the experiment and the paper. All authors participated in theevaluation of the data, reading and approving the final manuscript.

Funding: This research was supported by the Basic Science Research Program through the National ResearchFoundation of Korea (NRF) funded by the Ministry of Education (Project No. 2016R1D1A1B03931616 andNo. 2019R1I1A1A01057727), the Korea government (MSIT) (No. 2019R1F1A1063154), and the TechnologyDevelopment Program of Ministry of SMEs and Startups (MSS, Korea) (Project No. S2671652).

Conflicts of Interest: The authors declare no conflict of interest.

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