Impact of two-stage injection fuel quantity on engine-out responses of a common-rail diesel engine fueled with coconut oil methyl esters-diesel fuel blends

Accepted Manuscript Impact of two-stage injection fuel quantity on engine-out responses of a commonrail diesel engine fueled with coconut oil methyl esters-diesel fuel blends

H.G. How, Y.H. Teoh, H.H. Masjuki, H.-T. Nguyen, M.A. Kalam, H.G. Chuah, A. Alabdulkarem PII:

S0960-1481(19)30277-0

DOI:

10.1016/j.renene.2019.02.112

Reference:

RENE 11244

To appear in:

Renewable Energy

Received Date:

05 February 2018

Accepted Date:

20 February 2019

Please cite this article as: H.G. How, Y.H. Teoh, H.H. Masjuki, H.-T. Nguyen, M.A. Kalam, H.G. Chuah, A. Alabdulkarem, Impact of two-stage injection fuel quantity on engine-out responses of a common-rail diesel engine fueled with coconut oil methyl esters-diesel fuel blends, Renewable Energy (2019), doi: 10.1016/j.renene.2019.02.112

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ACCEPTED MANUSCRIPT 1

Impact of two-stage injection fuel quantity on engine-out responses of a

2

common-rail diesel engine fueled with coconut oil methyl esters-diesel fuel

3

blends

4

H.G. How 1,a, Y.H. Teoh 2,b*, H.H. Masjuki 3,c, H.-T. Nguyen 4,d, M.A. Kalam3,e,

5

H.G. Chuah1,f and A. Alabdulkarem 5

6

1 Department

University College, 32, Jalan Anson, 10400 Georgetown, Penang, Malaysia

7

2 School

8

11 12

of Mechanical Engineering, Universiti Sains Malaysia, Engineering Campus, 14300 Nibong Tebal, Penang, Malaysia

9 10

of Engineering, School of Engineering, Computing and Built Environment, KDU Penang

3 Centre

for Energy Sciences, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia

4 Ho

Chi Minh City University of Food Industry (HUFI), Ho Chi Minh City 700000, Vietnam

5 Mechanical

Engineering Department, College of Engineering, King Saud University, 11421 Riyadh, Saudi

13

Arabia

14

a [email protected], a [email protected], b [email protected], [email protected],

15

d [email protected], e [email protected], f [email protected]

16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34

* Corresponding authors. E-mail addresses: [email protected] or [email protected] (H.G. How), [email protected] (Y.H. Teoh). 1

ACCEPTED MANUSCRIPT 36

Abstract

37 38

Two-stage injection with different biodiesel percentage is investigated where first and second

39

injections were implemented with different SOI timings at various mass ratio under constant speed

40

of 2000 rpm and 60 Nm of torque. The results reveal that maximum BTE of 32.4% and minimum

41

BSFC of 245.5 g/kWh can be achieved simultaneously with injection mass ratio of 50:50 at advanced

42

SOI timing using baseline diesel. A considerably lower level of NOx below 90 ppm is achievable via

43

late SOI timing by using B20 or B50 biodiesel blends with injection mass ratio of 25:75. Specifically,

44

the lowest NOx of 82 ppm can be achieved with smoke emission level still remains below 5% when

45

B50 biodiesel blend and 25:75 injection mass ratio is tested. The highest reduction of 5.3 % of smoke

46

compared to diesel was achieved when B50 was used with 50:50 mass ratio at retarded SOI of 2

47

°ATDC. It was found that simultaneous NOx and smoke reduction compared to that of fossil diesel

48

is feasible with the application of B50 biodiesel blend and execution of retarded SOI timing and

49

injection mass ratio of 25:75. Lastly, two-stage fuel injection is a practical strategy to simultaneously

50

decrease NOx and smoke emissions.

51 52 53 54 55 56 57

Keywords: NOx; Diesel engine; biodiesel; fuel quantity; injection timing; two-stage injection

2

ACCEPTED MANUSCRIPT 59

Highlight

60

►Effect of biodiesel blend, SOI and mass ratio of two-stage injection was investigated

61

►Optimum BTE and BSFC were obtained with early SOI and 75:25 injection mass ratio

62

►Lowest NOx of 82 ppm was achieved with smoke emission level still remains below 5%

63

►Simultaneous NOx and smoke reduction with B50, late SOI and 25:75 injection ratio

3

ACCEPTED MANUSCRIPT 64

Nomenclature and symbol ASTM

American Society for Testing and Materials

HRR

heat release rate

ATDC

after top dead centre

ID

ignition delay

B100

neat biodiesel

IMEP

indicated mean effective pressure

B20

20% COB + 80% diesel fuel

KOME

karanja oil methyl ester

B50

50% COB + 50% diesel fuel

LTC

low temperature combustion

BMEP

brake mean effective pressure

NOx

nitrogen oxides

BSFC

brake specific fuel consumption

PAH

polycyclic aromatic hydrocarbon

BTE

brake thermal efficiency

PHRR

peak heat release rate

CA

crank angle

PM

particulate matter

CO

carbon monoxide

PMGT

peak mean gas temperature

COB

coconut oil biodiesel

PO

proportional-integral

DAQ

data acquisition

ppm

part per million

ECM

electronic control module

PW

pulse-width

ECM

engine control module

PWM

pulse-width-modulation

EGR

exhaust gas recirculation

rpm

revolution per minute

GUI

graphic user interface

SOC

start of combustion

HC

hydrocarbon

SOI

start of injection

TDC

top dead centre

65 66 67 68 69 70 71 72 73 4

ACCEPTED MANUSCRIPT 74

1. Introduction

75

Our world population is increasing at an alarming rate. With the growing population, the

76

demand of human being in every aspect is expected to surge. One of the most important demands is

77

energy. Many types of resource are used to provide energy to sustain human daily lives necessities

78

and most of the energy sources are non-renewable. Diesel fuel is a type of fossil fuel employed in

79

various fields such as transportation, power generation, construction, boiler and others. The limited

80

availability of diesel fuel will be unsustainable in the near future and alternatives have to be found.

81

One of the candidates to replace diesel fuel, especially in the diesel engine operation is biodiesel.

82

However, due to the difference of properties between petroleum diesel and biodiesel, some

83

modifications have to be done to enhance the diesel engine performance [1].

84 85

Biodiesel can be derived from plants and animals, with the presence of suitable catalyst to

86

form fatty acid esters [2]. This makes it a renewable sources compared to diesel which originates

87

from petroleum. Biodiesel possesses some superior properties such as non-toxic, free of sulphur,

88

higher oxygen content, higher lubricity and others [3]. However, neat biodiesel is not an effective

89

fuel to be utilized in diesel engine especially in term of efficiency [4]. The higher density and viscosity

90

of biodiesel will interfere with the engine combustion process. Biodiesel has a higher cetane number

91

in comparison with that of petroleum diesel. Several researchers also reported lower NOx emission

92

with the use of biodiesels [5-8]. For instance, due to its shorter hydrocarbon chain length compared

93

to diesel, combustion at lower adiabatic flame temperature can be attained by coconut-based biodiesel

94

and thus reduce thermal NOx formation [9]. A lot of researches on the combustion characteristics and

95

performance of biodiesel have been done to evaluate the appropriateness of biodiesel to replace diesel

96

in engine.

97 98

To improve fuel combustion in diesel engines, many combustion and injection strategies were

99

proposed in recent decades. For instance, Low Temperature Combustion (LTC), Exhaust Gas

5

ACCEPTED MANUSCRIPT 100

Circulation (EGR), high injection pressure strategy etc. [10-12]. Besides manipulating injection

101

parameters like injection pressure and timing, multiple-injection strategies such as split injection,

102

pilot injection and more have also been suggested and studied due to the availability of high speed

103

injectors and common-rail technology that provides control for precise injections [13, 14]. In split

104

injection, two equal proportions of fuel are injected into engine cylinders at different timings. The

105

total fuel quantity injected is being divided into proportions of dissimilar masses in pilot injection.

106

The injection mass for first injection is relatively smaller than the second, which usually referred as

107

main injection in pilot injection mode [15]. On the other hand, post injection comprises of a light fuel

108

injection after main fuel combustion near TDC.

109 110

In a study by Suh on combustion and exhaust emissions characteristics in a low compression

111

ratio engine, the use of multiple injection strategies (single and double pilot injections prior to main

112

injection) showed remarkable reductions in NOx, soot and HC emissions. Nonetheless, greater CO

113

emission and lower peak HRR were also demonstrated by the strategies compared with conventional

114

single injection mode [16]. Generally, the use of multiple injection strategy in diesel engines were

115

found to have reducing effect to emission of NOx and engine noise [10, 17-21]. Zhang and Boehman

116

revealed the effectiveness of pilot injection, which substantially decreased NOx emission at low load

117

condition [22]. Similarly, Herfatmanesh et al. have revealed the potential of two-stage injection in

118

NOx and soot emissions reductions [23]. However, increment in NOx due to pilot injection strategy

119

was also reported by some researchers [15, 24]. Also, improved soot formation in smoke may also be

120

achieved by applying post injection after main combustion [25].

121 122

In two-stage injection, pilot injection parameters such as pilot duration, pilot mass ratio and

123

pilot timing were reported to have significant influences on combustion of main injection, the effect

124

was reported to be dependent on the combustion phase of pilot injection at which main injection

125

started to ignite [13, 26-29]. Khandal et al. [30] reported that advanced pilot injection will give rise

6

ACCEPTED MANUSCRIPT 126

to reduction in NOx emission quantity due to the shorter ignition delay. However, the increase in

127

brake mean effective pressure (BMEP) will lead to elevation of smoke amount. Nehmer et al. also

128

revealed the dependency of NOx emission on first injection quantity that reduced as less fuel was

129

injected in first injection, while maintaining similar particulate matter emission [31].

130 131

In spite of its many merits when used in diesel engines, biodiesel is known to produce

132

worsened NOx emission, while multiple injection strategy demonstrates potential in engine emission

133

control. Thus, attempts to combine the uses of both in diesel engines have also been initiated. In a

134

study on effects of pilot injection parameters with the uses of soya biodiesel and diesel fuel, Jeon et

135

al. noticed reduction in peak heat release and heat release rate for pilot-injected fuels compared to

136

when they were in single-injection mode when employing constant 2 mg of pilot injection fuel per

137

cycle. Improved energy efficiency was also observed for both biodiesel and diesel, especially when

138

pilot injection timing was timed nearer to TDC. With higher pilot injection quantity, rises in in-

139

cylinder pressure before main combustion were observed for both fuels, which improved their

140

combustion performance than in single-injection mode. However, brake specific energy consumption

141

(BSEC) deteriorated when pilot injection mass increased for both fuels [32].

142 143

Park et al. also investigated multiple-injection modes (split injection and pilot injection) in a

144

single cylinder common-rail biodiesel-fueled diesel engine with injections timed at 30º, 20º and 10º

145

BTDC for first injection and TDC for the following. The authors reported higher Indicated Mean

146

Effective Pressure (IMEP) for both modes as compared to single-injection mode at same injection

147

timing. The IMEP achieved by pilot injection was also found to be significantly higher than split

148

injection. Advantages on HC, CO and soot emissions were also reported for multiple-injection modes,

149

while a contrasting effect is noticed for NOx. The authors also observed reduction in quantity of large

150

particles produced in the engine when using multiple-injection modes than single-injection mode

151

[15].

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ACCEPTED MANUSCRIPT 152

Similarly, Fang et al. demonstrated reduction of NOx emission up to 34% lower for B100

153

biodiesel compared to diesel fuel with retarded main injections. The authors have emphasized the

154

potential of advanced low combustion injection strategies in mitigating the NOx emission problem

155

that mainly found in the use of biodiesel [33]. In addition, a study by Yehliu et al. found that the use

156

of B100 soybean methyl ester biodiesel increased NOx and PM emissions at high load condition with

157

single injection mode. However, NOx emissions using the same fuel showed reduction when split

158

injection was applied [34]. Dhar also experimented on the use of Karanja biodiesel blends with two-

159

stage injection strategy and studied the effects on particulate matter emission. The author reported

160

benefits in reduction of PM emission by KOME20 blend and potential of injection timing retardation

161

in offsetting the NOx-PM trade-off relationship [35].

162 163

In the conventional single injection diesel combustion, the early direct fuel injection is

164

problematic due to the difficulties in fuel vaporization and fuel spray over-penetration. To tackle this

165

issue, the strategy of two-stage diesel fuel injection had been applied by Kook and Bae [36]. In this

166

approach, the fuel was divided and supplied in two injections. The first injection is typically carried

167

out in the compression stroke and followed by the second injection near TDC in the expansion stroke.

168

The mass ratio between pulses in two-stage injections strategy also significantly affects the

169

performance and emission characteristics of diesel engine. Cylinder pressure and mean temperature

170

increase when mass of pilot injection is increased, according to the research of Wei et al. [37].

171

Increase in mass of pilot injection also causes an increase in mass of fuel burnt. Torregrosa et al. [38]

172

found that when pilot injection mass is increased and pilot injection timing is advanced, NOx emission

173

will rise. Mathivanan et al. [39] discovered that peak heat release rate (PHRR) will decrease and

174

retardation of combustion phase will occur if the first injection pulse duration is increased. When the

175

last injection quantity decreases, more fuel will be injected in earlier injections and this produces a

176

more homogeneous mixture and increases PHRR. According to Juneja et al. [40], when fuel is

177

injected close to top dead center (TDC), a longer injection duration will cause incomplete combustion

8

ACCEPTED MANUSCRIPT 178

to happen due to the insufficient mixing time. By carrying out split injection scheme with four pilot

179

injections and one main injection, Su et al. [41] discovered that NOx emission decreases from 400

180

ppm to 300 ppm while HC emission remains almost constant when small amount of pilot injections

181

is used. When large amount of pilot injections is applied, HC emission increases obviously. Smoke

182

emission is found to be decreasing with increasing pilot injection quantity.

183 184

From the summarized literature review as tabulated in Table 1, it is evident that many studies

185

have been done on experimental investigation of multiple injection strategies fueled with fossil diesel

186

fuel. However, the investigation on the combination effect of different biodiesel-diesel blend ratios

187

and two-stage injection timing variation at various mass ratio proportions was rarely found in

188

previous research studies. In fact, most of the studies on advanced multiple-injection fuel combustion

189

have been conducted on single-cylinder research engine, which is not practical representative of the

190

production engine adopted in commercial vehicles. As a consequence, there is a research gap remains

191

in these areas which will be explored in the present paper. In this research study, the effects of

192

biodiesel blends, SOI timing and injection mass ratios on the engine performance, combustion

193

characteristics and exhaust emissions are investigated. Two-stage injection scheme for fuel of

194

different biodiesel percentage is carried out where first and second injections were implemented with

195

different start of injection (SOI) timings at various mass ratio. The tests were performed at constant

196

speed of 2000 rpm and 60 Nm of torque operation with baseline diesel, B20 and B50 fuels. From the

197

results, the effect of each of the injection strategy will be determined to enhance the research in

198

developing biodiesel to overcome the air pollution and fossil fuels depletion issues.

199 200 201 202 203

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ACCEPTED MANUSCRIPT Table 1: Previous findings on multiple injection strategies in diesel engine

204 No

Injection strategy

Engine type

Engine operating condition 800 rpm, fixed fuel injection pressure (27 MPa)

Fuel type

Results

Ref.

1

Multiple injection strategies (i.e. one pilot and two pilot injections prior to main injection)

Single-cylinder Compression Ignition (CI) engine, low compression ratio (15.3:1), Electronically controlled fuel injection

Ultra-low sulfur diesel

Reductions in NOx, soot and HC emissions. Greater CO emission and lower peak HRR.

[16]

2

Pilot and main injection

2.5 L, 4-cylinder, turbocharged, common rail, direct injection, lightduty diesel engine, Bosch electronically controlled common rail injection system 0.499 L, Single-cylinder, high-speed optical engine equipped with a production cylinder head, Firstgeneration common-rail system

1600 rpm, 25% and 75% load, pilot and main injection timing variation, with EGR

BP (BP15), a blend of 20 vol % biodiesel in BP15 (B20), and a blend of 40 vol % biodiesel in BP15 (B40)

Pilot injection have substantially decreased NOx emission at low load condition.

[22]

3

Two-stage injection (30%/70%, 50%/50%, and 70%/30%) with various dwell angles

1500 rpm, 72% of full load, 27.7:1 AFR, injection pressure of 1200 bar

Petroleum diesel fuel

Simultaneous reduction of NOx, soot and unburned hydrocarbon emissions can be achieved with the added benefits of improved engine performance, fuel economy and combustion noise. However, higher soot emissions were produced.

[23]

4

Single (10 mg) and multiple injection strategies (3 mg +7 mg and 5 mg +5 mg)

0.3733 L, Single-cylinder, direct injection diesel engine, Bosch common-rail type

1400 rpm, injection pressure of 60 MPa and 120 MPa

Biodiesel fuel

Increment in IMEP for multiple injection strategy. Reduction in large-sized particles, soot, HC and CO emissions at the expense of higher NOx emission for multiple injection strategy.

[15]

5

Pilot, main and post injection

6-cylinder, turbocharged, heavy duty four-stroke diesel engine, compression ratio of 17.25:1, commonrail injection system

1200 rpm, constant engine torque of 1900 Nm, EGR of 24.8%, RAFR of 1.65

Market diesel fuel

Pilot injection raised the NOx emission. Post injection reduced the NOx emission but it raised the exhaust temperature, soot masses and particle numbers.

[24]

6

Post injection

1.9L, 4-cylinder, turbodiesel engine, compression ratio of 17.5:1, common-rail injection system with solenoid-type Bosch injectors

1800 rpm, lowto-moderate load of 4.8 bar BMEP, lambda of 2.50, rail pressure of 500 bar

Ultra-low sulfur diesel

Close-coupled post injections marked greatest soot reduction and fuel efficiency improvement at the expense of higher NOx emission. Long-dwell post injections indicated great soot reduction, but it showed no improvement in fuel efficiency and emitted more THC.

[25]

7

Single injection

Single-cylinder, 4-stroke, direct injection diesel engine, compression ratio of 17.5:1, common-rail injection system

1500 rpm, variable load condition

Uppage oil methyl ester (UOME)

Gain in BTE and reduction in HC and CO at IT of 10° BTDC and IP of 900 bar.

[30]

8

Single and split injection (10%/90%, 25%/75%, 50%/50% and 75%/25%)

2.44L, Single-cylinder, 4stroke, simulated turbocharging diesel engine, compression ratio of 15.0:1, electronically controlled common-rail injection system

1600 rpm, 80% load, inlet air pressure of 184kPa

Diesel fuel

[31]

9

Pilot and main injection

0.5107 L, naturallyaspirated CI Singlecylinder research engine equipped with Bosch common-rail injection system, compression ratio of 17.1:1

1500 rpm, injection pressure of 50 MPa

Dimethyl ether and ultra-low-sulfur diesel fuels

Split injection reduced peak pressure by more than 45%. If the amount of first fuel injection was reduced, NOx emission can be reduced with slower rise of particulate emissions. Also, split injection utilized air charge better and allows combustion to continue later than a single injection case. Retarded pilot injection increased local temperature slightly for both fuels, led to higher soot formation. Reduction in peak pressure was observed for DME fuel with retarded pilot injection, but the peak HRR was raised. Meanwhile, ULSD fuel showed the opposite trend to the DME fuel.

10

[32]

ACCEPTED MANUSCRIPT 10

Multiple injection strategies (first injection prior to main injection)

Single-cylinder, highspeed, direct-injection diesel engine, Bosch common-rail electronic injection system

Fixed 1.5-cubicmetre first injection fuel, IMEP of 4 bar

European low sulfur diesel and soybean biodiesel fuels

Simultaneous reduction of soot and NOx emission.

[33]

11

Single and split injection (pilot and main injection)

2.5 L, 4-cylinder, turbocharged, direct injection light-duty diesel engine, Bosch common-rail injection system

1850 and 2400 rpm, engine torque of 64 Nm and 110 Nm, constant SOI timing

Ultra-low sulfur diesel (BP15), Fischer-Tropsch (FT) fuel and soybean biodiesel (B100) fuel

Single injection increased NOx emission at high load and increased PM emission for B100, but split injection indicated a contrary result for the NOx emission.

[34]

12

Multiple injection strategies (pilot and main injection)

Single-cylinder diesel engine equipped with common rail direct injection system, compression ratio of 17.5:1

1500 rpm, 5 bar BMEP engine load

Mineral diesel, KOME20 and KOME50

Advanced pilot injections reduced total number concentration of particulates.

[35]

13

Single and two-stage injection (main injection and second injection)

Single-cylinder, direct injection, four-valves, optical diesel engine, compression ratio of 18.9:1, common-rail injection system

800 rpm, no load, injection pressure of 30 and 120 MPa

Diesel fuel

Two-stage injection strategy improved combustion efficiency with higher IMEP.

[36]

14

Single and pilot injection

9.7 L, 6-cylinder, direct injection diesel engine, compression ratio of 17:1, common rail injection system

1340 rpm, 25% of full load

Diesel/methanol dual fuel (DMDF)

Pilot injection enhanced combustion stability and fuel economy at high MSR. Lower regulated emissions (CO, THC, except NOx) and unregulated emissions (except CO2 on M0 & M10 mode and toluene on M50 mode) was indicated for pilot injection.

[37]

15

Pilot injection

1500 rpm, injection pressure of 800 bar, EGR up to 50%

Diesel fuel

Reduction in NOx and soot emission level. Combustion noise can be reduced with higher amount of pilot fuel injection at the expense of decreasing engine BMEP.

[38]

16

Multiple pulse (MP) and single pulse (SP) injection strategies

1.6 L, light-duty fourcylinder Euro IV turbocharged DI diesel engine, compression ratio of 18:1, solenoid controlled common rail injection system 4-cylinder, common rail, direct injection, turbocharged (with VGT) and intercooled engine

1800 rpm, injection pressure of 1200 bar

Diesel fuel

MP fuel injection indicated higher thermal efficiency and NOx emission than SP injection.

[39]

17

Injection rate shaping

2.44 L, Single cylinder, direct injection engine, compression ratio of 16.1:1, common rail injection system

821 rpm, 25% load, EGR Rate of 48.34%

Diesel fuel

Reduction in soot formation and NOx emission with optimization of injection rate-shape.

[40]

18

Multi-pulse injection

6-cylinder, heavy-duty truck engine, compression ratio of 15, FIRCRI common rail injection system

Injection pressure of 80 MPa

Diesel fuel

Reduction in NOx and soot emissions greatly, with load less than IMEP of 0.93MPa.

[41]

205 206

2. Experimental Apparatus and Procedure

207 208

2.1. Apparatus setup

209

The experimental study was carried out with three kinds of fuel samples. The samples made

210

up of B20, B50 of coconut oil biodiesel (COB) blends and baseline diesel. A four-cylinder diesel

211

engine consist of Delphi common-rail fuel injection system and turbocharger system was employed 11

ACCEPTED MANUSCRIPT 212

in this investigation. Engine load and speed were varied by using eddy current engine dynamometer

213

with the rating of 150 kW. A positive displacement gear wheel flow meter (Kobold DOM-A05

214

HR11H00) with measuring range of 0.5- 36 L/hr, which interfaced with a flow rate counter (Kobold

215

ZOD-Z3KS2F300) is employed to measure the fuel consumption of the engine. K-type

216

thermocouples were used to obtain the temperature of engine lubricant oil, surrounding air, engine

217

coolant and exhaust gas emitted. Table 2 shows the test engine information and specifications.

218

Table 2: Test engine information and specifications Engine Type

Diesel, turbocharged direct injection engine, 4-stroke

Fuel injection supply system

Diesel common-rail with rail pressure 140 MPa max.

Combustion chamber type

Bowl-in-piston

Valve per each cylinder

2

Cylinder

4

Connecting rod length

135 mm

Bore x stroke

76.0 mm x 80.5 mm

Compression ratio

18.25 to 1

Displacement

1461 cm3

Maximum torque & power

160 Nm at 2000 rpm & 48 kW at 4000 rpm

Type of engine lubricant

Pennzoil, SAE 15W-40 API heavy duty motor oil

219 220

A commercially available Arduino microcontroller was used as the fuel injection engine control

221

module (ECM) for the engine. Three interrupt service routines in the microcontroller were used to

222

collect the signals from incremental encoder and engine camshaft. Furthermore, programming coding

223

was run by using the C programming language. The codes were uploaded to microcontroller through

224

serial communication with personal computer. By using LabVIEW to create the graphic user interface

225

(GUI) program in order to control and investigate the engine parameters containing engine speed, 12

ACCEPTED MANUSCRIPT 226

start of injection (SOI) timing, number of injections (single, double and triple injection), closed-loop

227

engine speed control mode selection and opening pulse-width (PW). A dedicated engine speed

228

controller was used to regulate the amount of diesel injected in order to keep engine rpm to within

229

±10 rpm from the set point. This engine speed controller consist of a fine-tuned proportional-integral

230

(PI) control loop. By establish this approach, the speed controller could banish a large amount of

231

minor steady-state error and disturbance spanning the whole engine operating range. Besides,

232

programmable peak and hold pulse-width-modulation (PWM) was incorporated in engine controller

233

to vary the current supplied to solenoid injectors for common-rail direct injection to operate

234

efficiently. With these specifically designed control unit, all engine parameters could be flexibly and

235

fully controlled.

236 237

In order to perform the combustion process analysis, in-cylinder pressure is obtained with a

238

Kistler 6058A piezoelectric sensor and its signal was recorded by using high speed data acquisition

239

system. The pressure sensor was fixed in the first cylinder’s head by utilizing the glow plug adapter.

240

The signal from pressure sensor was conditioned by using DAQ-Charge-B charge amplifier. The

241

crankshaft rotation angle was measured by using incremental encoder with the resolution of

242

0.125°CA. Cylinder pressure data for 100 successive engine revolutions were collected and averaged

243

for each test. The concentrations of NOx is measured with AVL DICOM 4000 gas analyzer and the

244

smoke opacity was obtained with AVL DiSmoke 4000 in order to evaluate pollutant emission. The

245

schematic diagram of the experiment setup is shown in Fig. 1. The measurement range and resolution

246

of both of the instruments are provided in Table 3.

247

13

ACCEPTED MANUSCRIPT

248

Fig. 1: Experimental setup arrangement

249 250 251 252 253

Table 3: Measuring components, ranges and resolution of the AVL DICOM 4000 gas analyzer and DiSmoke 4000 smoke analyzer Equipment Gas analyzer

Smoke opacimeter

Measurement principle Non-dispersive infrared Electrochemical Calculation Photodiode detector

Component Carbon monoxide (CO) Nitrogen oxides (NOx) Excess air ratio (λ) Opacity (%)

Measurement range

Resolution

0-10% Vol.

0.01% Vol.

0-5,000 ppm 0-9,999

1 ppm 0.001

0-100%

0.10%

254 255

2.2. Experimental methods and procedures

256

Under normal operating conditions, the engines used in medium-duty diesel powered urban

257

vehicles are typically operated under partial load condition. Therefore, the choices of experimental

258

cases will be the part load operation. In the present engine testing, the engine torque is held constant

259

at 60 Nm and speed with 2000 rpm, as show in Table 4. This moderate engine speed of 2000 rpm was

260

chosen to represent a typical steady mode driving condition for a medium-duty vehicle cruising on

261

highway. The effects of biodiesel blended fuels on engine out-responses, like performance

262

characteristics, combustion and tail-pipe emissions under different first injection SOI timing (-12° 14

ACCEPTED MANUSCRIPT 263

ATDC to 2° ATDC) conditions and two-stage fuel injection were studied. Two-stage injection

264

approach can be applied by splitting the single injection event as in conventional diesel engine into

265

two succeeding injection events for each engine combustion cycle with a fixed dwell timing of 15°CA

266

between the succeeding injections. The benefits of this injection strategy is it able to decrease the

267

combustion flame temperature and allow ample fuel and air mixing so that charge homogeneity can

268

be improved. Fig. 2 describes the timing chart for various kind of two-stage fuel injection strategies

269

investigated in the current work. Clearly, it could be seen that two-stage injection strategy at various

270

mass ratio of first injection to second injection of 25:75, 50:50 and 75:25 was proposed and carried

271

out in this study.

272 273

According to Table 4, test cases examined in this research are designed by manipulating types

274

of fuel, first injection SOI timings and first injection to second injection mass ratios. Different fuel

275

types contain different biodiesel composition and this will greatly affect the exhaust emission. On the

276

other hand, change in SOI timings cause the start of combustion to happen at different crank position,

277

thus resulting in longer or shorter ignition delay. Also, variation in fuel injection mass ratio of first

278

injection to second injection affect the combustion characteristics and engine performance. Thus, the

279

study of simultaneous effects of these three parameters has to be done to optimize the engine

280

performance. In this study, two parameters at three levels and one parameter at 8 levels, which has

281

resulted in a total of 72 combinations were tested. First, combination of baseline diesel and mass ratio

282

of 25:75 is fixed in studying the effect of variation in SOI timings. Then, mass ratio is changed and

283

the steps are repeated. When all mass ratios and SOI timings have been tested using baseline diesel,

284

the entire procedures are carried out using B20 and B50 biodiesel.

285 286

A commercial diesel fuel was used as baseline fuel for comparison purposes in each test

287

scheme. The engine has no starting difficulty and it functioned adequately over the whole test when

288

biodiesel blends were used to operate the engine at room temperature. The tests were carried out when 15

ACCEPTED MANUSCRIPT 289

the steady-state conditions were reached. Exhaust gas warmed sufficiently is necessary to ensure all

290

tests are performed under thermally stable condition. Under this condition, the exhaust gas has

291

reached a sufficiently high final temperature and with minimum fluctuation. Besides, the temperature

292

range for coolant and lubricant temperatures are both controlled to within 85 to 90 °C. Every test case

293

was repeated for two times to obtain average value in order to improve the accuracy in the study.

294

Repeatability was as high as 95% for every case tested.

295 296 297 298

Fig. 2. Timing chart for various test cases with three different injection mass ratios at SOI= 6ºATDC.

299

Table 4: Engine operating condition and test cases Parameter

Value

Engine angular velocity

2000 rpm

Torque

60 Nm

Fuel injection strategy

Two-stage

Dwell angles (°CA)

15

Types of fuel

Baseline diesel, B20, and B50

First injection SOI timings (°ATDC)

-12, -10, -8, -6, -4, -2, 0, 2

Mass ratio (first: second)

25:75, 50:50 and 75:25

300 301 302

16

ACCEPTED MANUSCRIPT 303

2.3. Biodiesel production and property test

304

Generally, there are many methods to convert biodiesel from vegetable oil such as pyrolysis,

305

dilution, microemulsion, and transesterification. However, the most attractive and economic one is

306

still by transesterification process. This reaction has been extensively used to reduce the viscosity of

307

crude vegetable oil and conversion of the triglycerides into ester and glycerol. A catalyst is typically

308

employed to enhance the reaction rate and yield. In the present study, the acid value of crude coconut

309

oil is measured to be 6.0 mg KOH/g. Due to the high content of FFA of the crude coconut oil, two

310

step acid-base catalyst processes are employed to convert this crude oil to biodiesel. The esterified

311

crude oil was then transferred into a preheated reactor at a temperature of 60 °C. The oil was reacted

312

with 25% (v/v oil) methanol and 1% by weight of alkali catalyst (KOH). The reaction mixture was

313

maintained at 60 °C for 2 hours with stirring at the constant speed of 800 rpm. After the completion

314

of the reaction, the produced methyl esters were poured into a separation funnel for 24 hours to

315

separate the glycerol from the biodiesel. The lower layer, which consists of impurities and glycerin,

316

was drawn off. Then, the methyl ester was washed with warm distilled water and evaporated with a

317

rotary evaporator at 65 °C for 30 minutes to remove residual methanol and water. Lastly, the methyl

318

ester was dried using Na2SO4 and filtered using qualitative filter paper to collect the final product.

319

Subsequently, the fuel properties of methyl ester produced was investigated thoroughly. The

320

comparison of fuel properties with biodiesel standards was conducted after transesterification process

321

was done. The information of the important physicochemical properties possessed by the COB

322

compared with ASTM standard is shown in Table 5. The table also shows the important properties of

323

fossil diesel fuel. The physiochemical properties of biodiesel produced were benchmarked against the

324

ASTM D6751 which has been used as biodiesel standard. The physicochemical properties of the COB

325

meet the ASTM standards for the biodiesel. Particularly, the kinematic viscosity of the transesterified

326

coconut oil was improved a lot. Nevertheless, it was slightly higher than that of conventional diesel.

327

In additions, COB had a greater flash point than conventional diesel and thus appropriate to be utilized

328

as transportation fuel. One of the demerit of COB is it lower calorific value when compared to 17

ACCEPTED MANUSCRIPT 329

petroleum diesel fuel. Another influence which affect engine performance and combustion

330

characteristics is the distillation temperature of fuel. Usually, the parameter for examination of fuel

331

quality is the distillation temperature. In this study, the distillation temperature analyzer has been

332

employed to obtain the entire ranges of distillation temperatures of the fuel sample Tx, in which “x”

333

means distillation temperatures corresponding to x vol% of the distilled and condensed liquid fuel. It

334

is found that the distillation temperatures of T50 of diesel fuel and COB are 298.5°C and 284°C,

335

respectively. The information about the measurement devices technical specification is tabulated in

336

Table 6. In this study, two type of fuel blends with different percentage of methyl ester blends, which

337

were B20 (80% petroleum diesel, 20% biodiesel) and B50 (50% petroleum diesel, 50% biodiesel)

338

were formulated and tested. Table 7 shows the vital physicochemical properties of the biodiesel

339

blends and conventional diesel. The mixing of conventional diesel with biodiesel could significantly

340

improve the resultant biodiesel blend properties. Specifically, the increase of the ratio of conventional

341

diesel in the blends decreased the kinematic viscosity. Furthermore, the flash points of both of the

342

B20 and B50 biodiesel blends were comparably larger than that of pure diesel. Therefore, they are

343

suitable to be used as transportation fuel. Yet, biodiesel blends have lesser calorific value when

344

compared to conventional diesel. Table 5: Physicochemical properties of neat COB and baseline fossil diesel.

345

Biodiesel Limit Parameters

Units

Standards

COB

Diesel (ASTM D6751)

Kinematic viscosity mm2s-1

ASTM D445

4.02

1.9-6.0

2.99

Density @ 40°C

kg/m3

ASTM D1298

856.0

-

825.6

Flash point

°C

ASTM D93

145.5

130 min

71.5

Cloud point

°C

ASTM D2500

4

Not stated

3

Pour point

°C

ASTM D97

3

Not stated

0

CFPP

°C

ASTM D6371

7

Not stated

5

Calorific value

MJ/kg

ASTM D240

39.92

-

45.21

Acid value

mg KOH/g

ASTM D664

0.05

0.5 max

-

Oxidation stability

h

EN ISO 14112

15.8

3 min

>100.0

@ 40 °C

18

ACCEPTED MANUSCRIPT Carbon

%wt

73.2

-

86.1

Hydrogen

%wt

12.5

-

13.8

Nitrogen

%wt

Oxygen

%wt

Calculation

< 0.1

-

< 0.1

14.3

-

0.1

Water content

ppm

EN ISO 12937

210

<500 ppm

120

-

ASTM D 130

1a

1

1a

ASTM D 5291

Copper Strip Corrosion (3 h @ 50˚C) Distillation: Initial boiling point

92

165.5

Recovery of 5%

240

220

Recovery of 10%

249

Recovery of 20%

260

Distillation

262.5

269

temperature, 90%

276.5

Recovery of 30% Recovery of 40% recovery Recovery of 50%

346 347 348 349 350 351 352 353

˚C

D86

276 284

240

288.5

recovered

298.5

(T90) = 360 °C max

Recovery of 60%

294

309

Recovery of 70%

312

320

Recovery of 80%

321

333

Recovery of 90%

324

351

Final boiling point

324

374

Table 6: Summary of fuel physicochemical properties measurement devices technical specifications Parameter

Equipment

Measuring range

Kinematic viscosity

Anton Paar SVM 3000 viscometer

0.2 to 10 000 mm2/s

Density

Anton Paar SVM 3000 viscometer

0.65 to 2 g/cm3

Flash point

Ambient to 400°C

Cloud point

Normalab-fully automatic pensky martens flash point tester model NPM 440 Normalab Cloud point and Pour tester NTE 450

Pour point

Normalab Cloud point and Pour tester NTE 450

-75 to 51°C

CFPP

-80 to +20°C

Calorific value

Normalab fully automated cold filter plugging point model NTL 450 IKA C 2000 calorimeter

Acid value

Mettler-Toledo G20S Compact Titrator

± 2000 mV

Oxidation stability

Metrohm- 873 Biodiesel Rancimat

50 to 220°C

Carbon, Hydrogen, Nitrogen, Oxygen Water content

CE-440 Elemental Analyzer, Exeter Analytical

100 ppm to 100%

Metrohm- KF 831 coulometer

10 µg to 200 mg

Copper Strip Corrosion

Stanhope-Seta- 11300-0 copper corrosion bath

40 to 100°C

Distillation

Anton Paar ADU 5

0 to 450 °C 19

-75 to 49°C

40,000 J

ACCEPTED MANUSCRIPT 354 355 356 357

Table 7: The key fuel physicochemical properties of neat COB, petroleum diesel, B20 and B50. Properties

Units

Diesel

COB

B20

B50

Test method

Calorific Value

MJ/kg

45.21

39.92

44.60

42.79

ASTM D240

Density @ 40°C

kg/m3

825.6

856.0

831.3

840.1

ASTM D1298

Flash Point

°C

71.5

145.5

74

80.5

ASTM D93

Kinematic viscosity at 40°C

mm2s-1

2.985

4.02

3.204

3.491

ASTM D445

Oxidation Stability

hr

>100.0

15.8

89.48

51.44

EN ISO14112

358 359

2.4. Uncertainty analysis

360

Uncertainty in measurements is always inevitable in any experiments, therefore there is a need

361

for uncertainty analysis to verify the experimental results obtained in this study. Errors may arise

362

from various aspects including inherent instrument repeatability, fluctuating environmental

363

conditions, human blunders and more. Information including instrument measurement range,

364

calculated accuracy, percentage uncertainties and measurement techniques employed in this study is

365

listed in Table 8. Percentage uncertainties of brake specific fuel consumption (BSFC) and brake

366

thermal efficiency (BTE) were computed from the percentage uncertainties of instruments employed

367

for each parameter measurement. The overall percentage uncertainty was estimated as ±2.9% by the

368

principle of propagation of errors. The uncertainty analysis was performed using the method

369

described by [42, 43]. The overall experimental uncertainty was computed by the following formula:

370 371

Experimental uncertainty = √ [ (Fuel Flow Rate uncertainty)2 + (BSFC uncertainty)2 + (BTE

372

uncertainty)2 + (NOx uncertainty)2 + (Smoke uncertainty)2 + (Pressure sensor uncertainty)2 + (Crank

373

angle encoder uncertainty)2] = √ [(0.5)2 + (1.5)2 + (1.7)2 + (1.3)2 + (1)2 + (0.5)2 + (0.03)2] = ±2.9%

374 375

Table 8: Summary of measurement range, accuracy and percentage uncertainties.

20

ACCEPTED MANUSCRIPT Measurement

Measurement range

Accuracy

Measurement techniques

% Uncertainty

Load

±600 Nm

±0.1 Nm

Strain gauge type load cell

±0.25

Speed

0-10,000 rpm

±1 rpm

Magnetic pick up type

±0.1

Time

-

±0.1s

-

±0.2 ±0.5

Fuel flow measurement

0.5-36 L/hr

±0.04 L/hr

Positive displacement gear wheel flow meter

NOx

0-5,000 ppm

±1ppm

Electrochemical

±1.3

Smoke

0-100%

±0.1%

Photodiode detector

±1

Pressure sensor

0-25,000 kPa

±10 kPa

Piezoelectric crystal type

±0.5

Crank angle encoder

0-12,000 rpm

±0.125°

Incremental optical encoder

±0.03

BSFC

-

±5 g/kWh

-

±1.5

BTE

-

±0.5 %

-

±1.7

Computed

21

ACCEPTED MANUSCRIPT 377

3. Results and Discussion

378

3.1. Performance characteristics

379

Fig. 3 represents the changes in brake specific fuel consumption (BSFC) and brake thermal

380

efficiency (BTE) with various kinds of fuel, SOI timings and injection mass ratios. Generally, it was

381

observed that blending of biodiesel into conventional diesel tend to reduce the BTE of the engine.

382

This happens for all SOI timings and injection mass ratios tested. Interestingly, for the maximum

383

BTE across all SOI timings, baseline diesel achieved the highest BTE of 29.8%, 32.4% and 32.0% at

384

-12 °ATDC for the injection mass ratio 25:75, 50:50 and 75:25 respectively, whereas the lowest was

385

recorded with 27.9% for B20, 30.6% for B50 and B20 at the respective injection mass ratio with the

386

same SOI timings. The phenomenon is similar to the observation reported by Chhabra et al. [44],

387

where an increase in biodiesel blends causes a slight decrease in BTE. The cause of this outcome may

388

be due to the lower calorific value of biodiesel. Biodiesel which contains higher oxygen content will

389

exhibit a decrement in calorific value. Besides, the retardation of SOI timing causes decrease in BTE

390

when different percentage of biodiesel blends are used under various injection mass ratio conditions.

391

When injection mass ratio is set as 25:75, the peak pressure occurs almost exactly at TDC (refer to

392

Fig. 5). The pressure level is more symmetrical about TDC compared to other injection mass ratio

393

strategies, where the relatively high in-cylinder combustion pressure during the compression stroke

394

will cause inefficiency in producing useful work. When SOI timing is retarded, peak pressure will

395

increase. The rise of pressure will cause the pressure level to become more symmetrical about TDC,

396

causing the BTE to reduce. Besides, the pressure curve also reveals that decrement and retardation of

397

peak pressure in expansion stroke will happen when SOI timing is retarded. Peak pressure which

398

happens later will have its magnitude reduced due to cylinder expansion. As a result the useful work

399

done to the piston of cylinder will be lower. Consequently, BTE will decrease.

400 401

According to Fig. 3, when the engine is operated at first SOI of -6°ATDC by using baseline

402

diesel as fuel, it is observed that BTE increases with increasing mass ratio of first injection. First 22

ACCEPTED MANUSCRIPT 403

injection combustion phase is nearer to TDC compared to second injection combustion phase. The

404

increase in mass of first injection will cause the peak pressure of first injection combustion phase to

405

rise more sharply compared to the increase in peak pressure of second injection combustion phase

406

when the mass of second injection is increased [15, 37, 45]. This phenomenon can be observed with

407

the peak pressure formed due to combustion of first injection diesel increases with increasing mass

408

ratio of first injection (refer to Fig. 5). The combustion phasing of second injection occurs at retarded

409

timing towards the expansion stroke. Thus, the temperature, heat release rate (HRR) and gas pressure

410

are generally lower, causing the work done produced to be less. Hence, an increase in mass of later

411

injection will cause the BTE to decrease [39]. Besides, the decreasing trend gradient of BTE with

412

retarding SOI timing becomes less steep when the mass ratio of first injection is increased. This

413

indicates that the sensitivity of BTE towards the change in SOI timing decreases with rise in mass

414

ratio of first injection. When mass ratio of injection is 25:75, the deterioration in BTE is due to the

415

increase in peak pressure in compression stroke. At mass ratio of 50:50 and 75:25, the reduction in

416

BTE is due to the peak pressure which occurs further away from TDC. It can be seen that the effect

417

of increasing peak pressure occurs at compression stroke is more significant than the effect of

418

retardation of peak pressure, hence the sensitivity of BTE decreases with increasing first injection

419

mass ratio.

420

According to Fig. 3, the increase in percentage of biodiesel blend leads to the increase in

421

BSFC across all SOI timing and injection mass ratio. For instance, B50 recorded the highest value of

422

minimum BSFC across all SOI timings, with the values of 299.9 g/kWh, 245.5 g/kWh and 271.7

423

g/kWh respectively at the injection mass ratio of 25:75, 50:50 and 75:25, compared to the baseline

424

diesel, with the values of 270 g/kWh, 274.7 g/kWh and 248 g/kWh accordingly. The same result was

425

discovered by Bhusnoor et al. [46] and Ozsezen et al. [47]. The observation can be attributed to the

426

decreasing BTE when biodiesel is utilized. With a lower BTE, more fuel has to be injected to maintain

427

the engine speed at 2000 rpm under a load of 60 Nm, resulting in a higher BSFC. In addition, results

428

shows that the BSFC increases with SOI carried out later, which is compatible with observation of

23

ACCEPTED MANUSCRIPT Weall et al. [48] and Zhu et al. [49]. The changes are again related to the decrease in BTE with

430

retardation of SOI timings as explained before. Besides, it can be observed that for the respective

431

SOI, the BSFC drops when the mass of first injection increases. Also, it can be observed that the

432

sensitivity of BSFC towards the retardation of SOI timing reduces with the increasing amount of first

433

injection.

35 33 31 29 27 25 23 21 19 17 15 13 11 9 7 5

Injection mass ratio 25:75

Injection mass ratio 50:50

Injection mass ratio 75:25

650 600 550 500

Diesel (BTE) B20 (BTE) B50 (BTE) Diesel (BSFC) B20 (BSFC) B50 (BSFC)

450 400 350 300 250 200

SOI for first injection pulse, °ATDC -12 -10 -8 -6 -4 -2 0

2 -12 -10 -8 -6 -4 -2 0

2 -12 -10 -8 -6 -4 -2 0

BSFC (g/kWh)

BTE (%)

429

2

434

SOI for first injection pulse, °ATDC

435

Fig. 3: BSFC and BTE for different fuels, injection mass ratios and start of injection timings

436 437

3.2. Combustion characteristics

438

With the use of piezoelectric pressure sensor, the variation of in-cylinder pressure during

439

combustion event can be precisely measured and recorded for 100 successive cycles. The average

440

value of pressure is processed for each crank angle. Meanwhile, HRR can be determined from

441

pressure data. Fig. 4 shows the combustion pressure curve, HRR curve and profile of injector current

442

of test engine using baseline diesel at SOI timing of -6ºATDC with varying injection mass ratios.

443

Based on injection current profile, it can be seen that every test case investigated involves two

444

consecutive injection pulses. Also, it can be seen that the first and second SOI timing remain constant

445

when injection strategies of different mass ratios are compared. Due to the extended combustion 24

ACCEPTED MANUSCRIPT 446

period, heat losses when the approach of 25:75 injection mass ratio is applied is more significant,

447

hence greater quantity of fuel need to be injected to compensate the energy loss. This causes in longer

448

main injection opening timing to enable sufficient quantity of fuel to be injected into the engine.

449

Besides, it can be observed that difference in injection mass ratio affects the combustion

450

characteristics significantly. The pressure peak increases with increasing in mass ratio of first

451

injection, from 74.64 bar to 77.26 bar, and ended with 78.24 bar. After the peak pressure occurs, the

452

combustion pressure decreasing rate (i.e. slope of the pressure curve) is more rapidly with larger mass

453

ratio of first injection. The starts of combustion (SOC) of all test cases conducted happened at the

454

difference crank angle. One of the factors which influence SOC timing is the quantity of fuel injected

455

during first injection. If a large amount of fuel is introduced, a longer air fuel mixing time will be

456

required and this will cause a retarded SOC timing. From the results, it can be seen that the occurring

457

crank angle position for the SOC of first injected fuel, when 50:50 and 75:25 injection mass ratios

458

are conducted, is shifted later by 1.375° CA and 1.5° CA respectively, in comparison with injection

459

mass ratio of 25:75, that occurred at -2.125 °CA ATDC. Two noteworthy HRR peaks can be noticed

460

for all injection mass ratios. First peaks of HRR of different test cases are formed at slightly different

461

crank angle (within the range of 2°CA) due to the unequal quantity of fuel injected at the same SOI

462

timing. However, the second peaks of HRR were shifted earlier by 4.75° CA and 12.75° CA for 50:50

463

and 75:25 injection mass ratio operation respectively, and gets lowered in comparison with the case

464

of 25:75 injection mass ratio. This clearly shows that the decrement in the quantity of second injected

465

fuel will cause an advance in second peaks of HRR timing. The main cause for the second HRR peak

466

timing to occur early is the shorter ignition delay (i.e. ID_2) and subsequently leads to the earlier

467

HRR rise. The parameter of ignition delay in a diesel engine is defined as the time interval between

468

the SOI and the SOC. As can be seen, comparing ID_2 of test cases with different injection mass

469

ratio, it is observed that the greater the fraction of second injected fuel, the longer the ID_2. This is

470

attributable to the extension of air fuel mixing time and fuel vaporization of the fuel. Besides, the pre-

471

injection of a small quantity of fuel as in 25:75 injection mass ratio operation permits stable main

25

ACCEPTED MANUSCRIPT 472

combustion as well as allows for extensive combustion phasing retard, which effectively lower the

473

NOx emissions.

130

SOI _1TDC

120

Pressure (bar)

90

60

Mass ratio 50:50 Mass ratio 75:25

Pressure

50 40

c

c

30

b

HRR

b a

20 10

a

Crank angle (ºCA)

0 -20 -15 -10 475 476 477 478 479 480 481 482 483

Injection Current Mass ratio 25:75

2

1

80 70

2

1

100

135 125 115 105 95 85 75 65 55 45 35 25 15 5 -5

EOI 2

EOI 12

1

110

SOI_2

-5

0

5

10 15 20 25 Crank angle (ºCA)

30

35

40

45

Heat release rate (J/ºCA)

474

50

Note: a & a’ represent ignition delay for first and second injected fuel, respectively for 25:75 injection mass ratio b & b’ represent ignition delay for first and second injected fuel, respectively for 50:50 injection mass ratio c & c’ represent ignition delay for first and second injected fuel, respectively for 75:25 injection mass ratio

Fig 4: Combustion pressure, heat release rate and injector current profiles for baseline diesel with various injection mass ratios at SOI of -6°ATDC

484

In the following section, the characteristics of combustion pressure and HRR curves when

485

different injection mass ratios and SOI timings are tested using baseline diesel will be highlighted

486

and discussed. According to Fig. 5, with injection mass ratio of 25:75, it can be seen that the peak

487

pressure of all SOI timing occurs near TDC. For instance, the highest peak pressure of 78.35 bar was

488

obtained at 1.125 °CA for the SOI timing of 2 °ATDC while the SOI timing of -12 °ATDC recorded

489

the lowest peak pressure of 71.70 bar at 1.5 °CA. In general, the peak pressure seems to rise slightly

490

from SOI timing -12ºATDC to 2ºATDC. The possible reason which leads to this condition is the late

491

second injection timing, which causes the in-cylinder gas temperature to remain higher than that of

492

earlier injection timing. After the TDC point, it can be observed that pressure level drops and remains 26

ACCEPTED MANUSCRIPT 493

at a nearly constant value before further decreases to a lower level. In fact, the crank angle position

494

for the plateau pressure phase is shifted earlier toward the TDC point and the magnitude become

495

somewhat higher with advance SOI timing. The plateau pressure phase happens by the reason of the

496

second injection diesel combustion which increases the temperature in cylinder and prevents the drop

497

in pressure. On account of the piston location near TDC, higher magnitude of pressure can be

498

observed with advanced SOI timing. Regarding HRR curve, it is noticed that the combustion of

499

second injected fuel produces a higher HRR peak compared to that of the first stage combustion. The

500

phenomenon occurs by virtue of the larger fuel mass ratio of second injection. Besides, it is interesting

501

to discover that the first combustion HRR peak of 16.47 J/°CA and 17.27 J/°CA produced at 9.625

502

°CA and 11.625 °CA respectively, when SOI timing is equal to 0ºATDC and 2ºATDC accordingly,

503

is much higher than those of advanced SOI timing cases. Earlier first injections are conducted near to

504

the TDC at compression stroke, where the temperature and pressure elevates sharply with retarding

505

crank angle. The high cylinder temperature will cause the ignition delay to decrease and results in

506

lower HRR. When late injection is performed at SOI timing of 0ºATDC and 2ºATDC, cylinder

507

temperature starts to reduce. The ignition delay will be longer where air fuel mixing can be mixed

508

more completely. The combustion of refined mixture will bring about higher HRR. Focusing on

509

second combustion HRR, it can be noticed that PHRR rises with retarded SOI timing from 28.50

510

J/°CA for -12ºATDC to 36.40 J/°CA for -2ºATDC before it start to decreases to 31.09 J/°CA and

511

28.59 J/°CA at SOI timing 0ºATDC and 2ºATDC respectively. The increasing pattern occurs owing

512

to the elevation of fuel consumption. When SOI timing is set as either 0ºATDC or 2ºATDC, first

513

combustion can be carried out effectively, resulting in a higher cylinder temperature and shorter

514

ignition delay of second combustion. Poor air fuel mixing transpires where the second combustion

515

PHRR will be lower. Besides, due to cylinder volume expansion, the cylinder temperature will

516

decrease sharply at retarded crank angle, further reduce the HRR. When injection mass ratio is set as

517

50:50 and 75:25, it can be seen that first injection diesel combustion generates peak pressure and

518

PHRR. This is because of the position of piston which is nearer to TDC when first injection diesel

27

ACCEPTED MANUSCRIPT 519

combustion happens. Another interesting phenomenon is that with retarding SOI timing, the peak

520

pressure and PHRR will shift and occur at a later timing. With a retarded SOI timing, the pressure

521

peak which is formed due to the combustion of first injection diesel has its magnitude reduced [48-

522

50]. Expansion of cylinder volume and shorter ignition delay may be accountable for this

523

phenomenon. SOI timing which is too advanced on the other hand causes a high peak pressure as the

524

enhanced air fuel mixing enables great amount of fuel to be combusted at the same time [46].

525

Observing HRR curve, PHRR increases slightly with retarding SOI timing. A considerable rise in

526

PHRR can be noticed when SOI timing is set as either 0ºATDC or 2ºATDC. This can be explained

527

by applying the same reasons as provided in the discussion when injection mass ratio is set as 25:75.

528

Unlike the case with injection mass ratio of 50:50, the HRR curve does not reveal two visible peaks

529

when injection mass ratio of 75:25 is employed. In fact, the HRR curve of first combustion increases

530

to a maximum value smoothly and decreases afterwards. The trend when injection mass ratio of 75:25

531

is applied can be ascribed to the first injection mass ratio which is too large. Besides, the direct

532

comparison of 50:50 and 75:25 injection mass ratio with the same SOI reveals that the two

533

combustion events resulting from these strategies differ only in the second half (as shown in Fig. 4).

534

This combustion phase is most responsible for overall smoke emissions. The fuel combustion of the

535

small fraction of second injected fuel during cool piston expansion stroke can be utilized to achieve

536

a better oxidation of the fuel-air mixture. This positive effect of the soot oxidation during the post

537

stage of the combustion process can be noticed with the lower smoke emission level compared to the

538

case with injection mass ratio of 50:50, as is reflected in Fig. 8.

28

-20 -15 -10 -5

539

0

5

100 90 80 70 60 50 40 30 20 10 0

Pressure (bar)

100 90 80 70 60 50 40 30 20 10 0

0

Heat release rate (J/°CA)

10 15 20 25 30 35 40 45 50 b) Mass ratio 50:50

-20 -15 -10 -5

540

130 115 100 85 70 55 40 25 10 -5

a) Mass ratio

5

-12°ATDC -10°ATDC -8°ATDC -6°ATDC -4°ATDC -2°ATDC 0°ATDC 2°ATDC

10 15 20 25 30 35 40 45 50

Pressure (bar)

c) Mass ratio 75:25

-20 -15 -10 -5 541 542 543 544

130 115 100 85 70 55 40 25 10 -5

Heat release rate (J/°CA)

TDC

100 90 80 70 60 50 40 30 20 10 0

Crank Angle (°ATDC) 0 5 10 15 20 25 30 35 40 45 50 Crank Angle (°ATDC)

130 115 100 85 70 55 40 25 10 -5

Heat release rate (J/°CA)

Pressure (bar)

ACCEPTED MANUSCRIPT

Fig. 5: Heat release rate and combustion pressure curves for different injection mass ratios at different SOI timings and with baseline diesel.

545 546

Fig. 6 shows the comparison of pressure and HRR results when different types of fuel are

547

used. The comparison is implemented for all cases of injection mass ratio at constant SOI of 29

ACCEPTED MANUSCRIPT 548

6ºATDC. Generally, it can be seen that the peak pressure developed is the highest when baseline

549

diesel is used across all injection mass ratio, with the recorded value of 74.64 bar, 77.26 bar and 78.24

550

bar for the injection mass ratio of 25:75, 50:50 and 75:25 respectively. The trend is in agreement with

551

the study of Bhusnoor et al. [46]. This is because baseline diesel has a higher calorific value compared

552

to biodiesel. With the same fuel amount, the heat energy released to act on the piston is greater for

553

baseline diesel in comparison with that of biodiesel. Another possible reason is that biodiesel has poor

554

volatility and high viscosity, causing ineffective atomization to happen during preparation of mixture.

555

On the other hand, there are a few differences observed for HRR curve obtained under different

556

injection mass ratio schemes. It is found that under injection mass ratio of 25:75, the first combustion

557

HRR curves are almost the same magnitude for different fuel types. When 50:50 injection mass ratio

558

is tested, HRR curve for first injected of baseline diesel fuel exhibits the highest peak of 25.09 J/°CA.

559

This is followed by B20 and B50 biodiesel in sequence, with peak HRR of 21.68 J/°CA and 20.90

560

J/°CA accordingly. The observation is compatible with Mizushima et al. [51] results where pilot

561

injection diesel combustion HRR is higher for ultra-low sulfur diesel. Besides, for HRR curve of

562

75:25 injection mass ratio scheme, the first appeared of peak HRR of 23.64 J/°CA at around 4.75

563

°CA reduces to 21.12 J/°CA and 19.82 J/°CA with increasing percentage of biodiesel blend. After

564

the first HRR local maximum, HRR curve rises to a higher HRR peak and it increases with increasing

565

percentage of biodiesel blend. Also, the occurrence of peak values for baseline diesel, B20 and B50

566

biodiesel are located almost at the same crank angle of 9.25 °CA. With a greater quantity of injected

567

fuel, more time is needed for air to mix well with fuel, causing the lower HRR of baseline diesel as

568

compared to that of biodiesel blend fuels. Unlike the B20 and B50 biodiesel blends, the oxygen

569

content in the fuel will ensure a more complete combustion, consequently increasing PHRR as

570

compared to that of baseline diesel. Observing the start of combustion timing, it can be seen that the

571

higher the percentage of biodiesel blends, the more advanced the timing for combustion to begin.

572

Shelke et al. [52], Szybist et al. [53] and Bittle et al. [54] also reported the same results when

30

ACCEPTED MANUSCRIPT 573

comparing pure diesel with biodiesel blends. The phenomenon is due to the higher cetane number

574

and shorter ignition delay of biodiesel as compared to that of baseline diesel.

31

95 85 75 65 55 45 35 25 15 5 -5

Pressure (bar)

a) Mass ratio 25:75

-5

0

5

100 90 80 70 60 50 40 30 20 10 0

10

15

20

25

30

b) Mass ratio 50:50

-5

0

100 90 80 70 60 50 40 30 20 10 0

5

10

15

20

40 95 85 75 65 55 45 35 25 15 5 -5

Diesel B20 B50

Pressure (bar)

-10

576

35

25

30

35

40 95 85 75 65 55 45 35 25 15 5 -5

Pressure (bar)

c) Mass ratio 75:25

-10 577 578 579 580

-5

0

Crank Angle (°ATDC) 5 10 15 20 25 Crank Angle (°ATDC)

30

35

Heat release rate (J/°CA)

-10

575

Heat release rate (J/°CA)

TDC

100 90 80 70 60 50 40 30 20 10 0

Heat release rate (J/°CA)

ACCEPTED MANUSCRIPT

40

Fig. 6: Heat release rate and combustion pressure curves for different injection mass ratios using different types of fuel at -6°ATDC SOI.

581

Fig. 7 shows the variation of peak HRR (PHRR) and peak mean gas temperature (PMGT)

582

with different types of fuel and injection strategies. Generally, it can be noticed that for most of the

583

SOI timings and injection mass ratios tested, PMGT of baseline diesel is higher than that of B20 and 32

ACCEPTED MANUSCRIPT 584

B50. Taking the SOI timing of 2 °ATDC, the baseline diesel marked the PMGT of 2070K compared

585

to 2032K for the B50 at the injection mass ratio of 25:75. Interestingly, the implementation of the

586

stated injection mass ratio and SOI timing resulted the highest PMGT among all conditions tested.

587

This phenomenon is aligned with Mizushima et al. [51]. This can be associated with the lower heating

588

value of B20 and B50 fuels compared to that of baseline diesel. In addition, temperature developed

589

through adiabatic flame is lower due to the disparity of C, H and O ratio in the fuel. The lower

590

hydrogen-carbon (HC) ratio in biodiesel may cause the combustion temperature to be lower. These

591

factors will result in lower PMGT of B20 and B50 in comparison with the baseline diesel operation.

592

Besides, different injection mass ratio strategies exhibit different trends of PMGT with the variation

593

of SOI timings. When injection mass ratio of 25:75 is applied, a retarding SOI timing causes an

594

increase in PMGT. This can be associated with the combustion phase of second injected fuel occurs

595

at a more retarded crank angle which is further away from TDC in the expansion stroke when injection

596

is performed at a later timing. The pressure produced will be lower due to cylinder volume expansion

597

process. The useful work done on the piston will be lower. To compensate the decrease in work done,

598

BSFC is increased where higher amount of fuel is injected (refer to Fig. 3). As a result, the combustion

599

of greater quantity of fuel will lead to a higher cylinder temperature. Besides, with injection mass

600

ratio of 50:50 is applied, it is observed that initially PMGT dropped with retarding SOI timing until

601

a minimum PMGT value is reached near SOI timing of -6ºATDC. Then, with SOI performed later,

602

PMGT will increase. The initial decrement in PMGT can be attributable to the lower peak pressure

603

developed nearer to TDC when SOI timing is retarded. Due to the low peak pressure, PMGT achieved

604

will be lower. With retarding SOI timing from -12ºATDC to -6ºATDC, the peak pressure will occur

605

at late crank angle and decrease on account of the cylinder expansion, causing a decrease in

606

temperature. The elevation of PMGT with retarding SOI timing when SOI is perform late at the range

607

of -6ºATDC to 2ºATDC can be explained by observing BSFC trend and HRR curve. With late SOI

608

timing, combustion will occur at crank angle when the volume of cylinder is large and rate of

609

expansion is rapid. In order to develop pressure which is ample to produce enough effective work

33

ACCEPTED MANUSCRIPT 610

done to maintain equal power output, temperature achieved in cylinder has to be high. Combustion

611

of larger amount of fuel is evident by observing the increasing effect of BSFC with retarding SOI

612

timing. This will lead to a higher HRR peak, which subsequently result in higher PMGT. On the other

613

hand, for the cases with injection mass ratio of 75:25, the PMGT values attained for most of the test

614

cases are higher than their counterparts when injection mass ratio is 50:50. The incremental effect is

615

due to the higher peak combustion pressure, which results in a higher PMGT. Another noteworthy

616

combustion parameter is the PHRR. The significant influence of injection mass ratio on the

617

combustion characteristics is also manifested in the trend of PHRR with change in SOI timings based

618

on Fig. 7. Injection mass ratio of 25:75 is applied with different types of fuel and various SOI timing.

619

Initially, the PHRR increases with retarding SOI timing, from 28.41 J/°CA at -12 °ATDC to 38.00

620

J/°CA at -2 °ATDC for the B20, which was the most significant rise among all types of fuel. This is

621

attributable to the increasing fuel consumption when SOI is carried out late. More fuel will be

622

combusted where the heat release rate will rise. However, after the SOI of -2ºATDC, PHRR begins

623

to decrease to 32.85 J/°CA and 30.97 J/°CA at 0 °ATDC and 2 °ATDC accordingly. The decrement

624

trend can be associated with the considerably late second combustion process of overly late second

625

injected fuel (refer to Fig. 5). When injection mass ratio of 50:50 is employed, PHRR will be located

626

at the first peak of HRR curve. According to Fig. 7, PHRR seems to rise steadily with SOI is perform

627

at the range of -12ºATDC to 0ºATDC and increases exponentially afterward for all types of fuel. For

628

instance, the PHRR rises from 25.50 J/°CA to 32.24 J/°CA with the increase of the SOI timing for

629

the baseline diesel. This is because retardation in SOI timing results in injection of larger quantity of

630

first injected fuel as aforementioned. The combustion will yield higher PHRR.

34

ACCEPTED MANUSCRIPT Injection mass ratio 25:75

Injection mass ratio 50:50

Injection mass ratio 75:25 60

Diesel (PMGT) B20 (PMGT) B50 (PMGT) Diesel (PHRR) B20 (PHRR) B50 (PHRR)

2050 2000 1950

55 50

1900

45

1850

40

1800 35

1750 1700

30

1650

25

PHRR (J/°CA)

PMGT (K)

2100

1600 20

1550 SOI for first injection pulse, °ATDC

1500

-12 -10 -8 -6 -4 -2 0

635

2 -12 -10 -8 -6 -4 -2 0

15 2

SOI for first injection pulse, °ATDC

631 632 633 634

2 -12 -10 -8 -6 -4 -2 0

Fig. 7: Peak heat release rate (PHRR) and peak mean gas temperature (PMGT) for different fuels, injection mass ratios and SOI timings 3.3. Emissions characteristics

636

Effects of biodiesel blend ratios, first SOI timing and fuel injection mass ratio on NOx and

637

smoke emissions are examined in the section below. The NOx amount emitted when different test

638

fuels are used at varying SOI timings and fuel injection mass ratios is delineated in Fig. 8. The graph

639

indicates that retarding SOI timing will reduce in NOx level for all test fuels and fuel injection mass

640

ratio. By applying a late SOI timing, the research done by Weall et al. [48] also yielded a lower NOx

641

emission level. The same observation was also reported by Han et al. [55] and Gomes et al. [50]. The

642

decreasing pattern in NOx indicates that with SOI implemented later, the air-fuel mixture will ignite

643

and burn later, therefore causing later formation of pressure peak near TDC. This lowers the

644

combustion temperature and avoids forming excessive NOx via thermal or Zeldovich mechanism.

645

Another possible explanation is that with retarded SOI, the effects of higher cylinder volume

646

expansion and higher time for heat transfer will reduce the combustion temperature, causing NOx

647

emission amount to decrease. Besides, there is hardly any difference in NOx emissions between B20 35

ACCEPTED MANUSCRIPT 648

and B50 biodiesel blends compared to baseline diesel, for all the combinations tested. Specifically,

649

when injection mass ratio is fixed as 25:75, the increase in percentage of biodiesel blends does not

650

significantly promote decrement in NOx emission. The B50 recorded the lowest NOx emission of

651

82.0 ppm while baseline diesel marked emission of 90.7 ppm, both at the SOI timing of 2 °ATDC. In

652

fact, at injection mass ratio equals 50:50 and 75:25, NOx emission of biodiesel blended fuels is slightly

653

lower than that of baseline diesel across all SOI timings. For instance, the lowest NOx emission was

654

indicated by the B50 with value of 114.6 ppm and 150.0 ppm at the injection mass ratio 50:50 and

655

75:25 respectively, compared to that of baseline diesel with value of 117.8 ppm and 172.4 ppm

656

accordingly, at the SOI timing of 2 °ATDC. This may be due to the higher cetane number and lower

657

calorific value of B20 and B50 fuel blends compared to that of baseline diesel. The combined effects

658

of higher cetane number and lower calorific value caused a reduction in combustion temperature and

659

HRR during premix combustion stage (refer to Fig. 5), thus resulting in lower emission of NOx.

660

Moreover, a considerably lower level of NOx below 90 ppm is achievable via late SOI timing for fuel

661

operations conducted using B20 or B50 biodiesel blends with injection mass ratio of 25:75. Besides,

662

when SOI is fixed at -12°ATDC, it can be seen that NOx rises sharply with increases first injection

663

mass ratio. This phenomenon can be observed for all types of fuels and is compatible with the research

664

done by Nehmer et al. [31], which showed that higher fuel quantity of first injection could cause

665

increment in NOx emission. In addition, Wei et al. [37] and Yang et al. [56] also discovered that NOx

666

emission amount tend to elevate with greater quantity of pilot injection fuel. This occurrence may be

667

explained by the fact that combustion of first injection fuel occurs near TDC where pressure and

668

temperature is considerably high. Hence, the NOx emission will elevate on this account. Moreover,

669

the increment in first injection mass ratio will also cause more fuel to be combusted earlier in the

670

cylinder and at high temperature environment, thus resulting in longer residence time and generate

671

greater amount of NOx. Another interesting trend is the strong correlations between NOx emission

672

and SOI timing variation can be observed with the rising in first injection mass ratio. This suggests

673

that first injection fuel combustion is responsible for the main source of NOx formation. With a large 36

ACCEPTED MANUSCRIPT 674

amount of fuel burnt during first injection, advanced SOI timing will exaggerate the effect of NOx

675

formation. Meanwhile, retarded SOI timing will drastically inhibit NOx production. On the other

676

hand, with reduced first injection fuel fraction, the NOx emission released during first combustion

677

will be reduced and the effect of SOI timing retardation toward NOx variation is less pronounced.

678

This shows that two-stage injection with relatively small fraction of first injection fuel will enable

679

stable combustion event and is an effective approach in reducing NOx emissions. Smoke formation

680

can be related to incomplete combustion of hydrocarbon and partial reaction of carbon content in fuel.

681

The smoke results for all test fuels under various SOI timings and injection mass ratios is displayed

682

in Fig. 8. Overall, it can be observed that amount of smoke emitted is lower when B20 or B50

683

biodiesel blend is employed across all SOI timings and injection mass ratios. For all the injection

684

mass ratios, the smoke emissions for the B50 recorded the lowest percentage of 3.4% at -12 °ATDC,

685

4.8% at -10 °ATDC and 1.3% at -12 °ATDC, when compared to baseline diesel, at the respective

686

injection mass ratio ranging from 25:75 to 75:20. There was a maximum of 5.3% reduction of smoke

687

produced as compared to baseline diesel when equal portions of injection mass ratio was used with

688

first injection at 2 °ATDC and with B50 blend. This is in accordance with the research observation

689

of Bhusnoor et al. [46], Weall et al. [48] and Mizushima et al. [51]. According to Kawano et al. [57],

690

the PM and soot emission of biodiesel blends will be lower than that of pure diesel, thus producing a

691

lower smoke intensity. By utilizing fuel with higher oxygen content, formation of polycyclic aromatic

692

hydrocarbon (PAH) can be contained. Particle inception and coagulation will happen at a slower pace

693

[58]. Incomplete combustion in the local fuel-rich regions will be less likely to occur compared to

694

that of petroleum diesel. As a result, the smoke emission will be diminished when biodiesel is used.

695

The results also indicate that with advanced SOI timings, the smoke emissions were generally

696

decreased for all injection mass ratio. This is due to when SOI timing is advanced, combustion gas

697

temperature is higher, in which fuel oxidation will be improved. Another possible reason is the ample

698

time for the fuel to vaporize and form mixture with air, thus permitting thorough mixing and complete

699

combustion. Moreover, it can be seen that lowest smoke is emitted when injection mass ratio is 75:25.

37

ACCEPTED MANUSCRIPT 700

This is ascribed to the effective oxidation reaction which occurs during the subsequent second

701

injection fuel combustion after the main, thus able to maintain the smoke level well below 5% for all

702

fuel types. The results exhibit some similarities with Mobasheri et al. [59] observation, where the

703

smoke emission decreases with the increasing first main injection mass percentage from 65% to 80%.

704

It is also noteworthy that when B50 biodiesel blend and 25:75 injection mass ratio are tested, the

705

smoke emission amount can be reduced while ensuring a reduction in NOx simultaneously. The

706

results show that retardation of SOI can achieve lower NOx emission of 82 ppm while the smoke

707

emission level remains below 5%. Hence, simultaneous NOx and smoke amount reduction compared

708

to that of fossil diesel is feasible with the application of B50 biodiesel blend and execution of retarded

709

SOI timing and injection mass ratio of 25:75.

710 711

Injection mass ratio 50:50

Injection mass ratio 75:25

NOx (ppm)

600 500 400 300 200 100

Diesel (NOx) B20 (NOx) B50 (NOx) Diesel (Smoke) B20 (Smoke) B50 (Smoke)

0 -100 -200 -300 -400 -500 712 713 714 715 716 717 718 719

SOI for first injection pulse, °ATDC -12-10 -8 -6 -4 -2 0 2 -12-10 -8 -6 -4 -2 0 2 -12-10 -8 -6 -4 -2 0 2 SOI for first injection pulse, °ATDC

28 26 24 22 20 18 16 14 12 10 8 6 4 2 0

Smoke (%)

Injection mass ratio 25:75

Fig. 8: NOx and smoke emissions for different fuels, injection mass ratios and first injection SOI timings.

38

ACCEPTED MANUSCRIPT 720

Conclusion

721

In a two-stage injection operated engine, the engine parameters such as types of fuel, SOI timing and

722

injection mass ratio have been investigated in this paper. The tests have been performed at constant

723

speed of 2000 rpm and 60 Nm of torque operation with baseline diesel, B20 and B50 fuels. Different

724

combinations of the parameters have been implemented to understand their impacts on the

725

characteristics of combustion. Via this study, the optimum conditions for better engine performance,

726

combustion characteristics and emissions (i.e. NOx and smoke) have been inferred. Below are the

727

inferences made from the analysis of data.

728 729

1. Maximum BTE of 32.4% and minimum BSFC of 245.5 g/kWh can be achieved

730

simultaneously with injection mass ratio of 50:50 at advanced SOI timing using baseline

731

diesel.

732 733

2. With higher mass ratio of first injection at SOI of -6 °ATDC, the first peak HRR decreases while the second peak HRR increases due to the variation in ignition delay.

734

3. The HRR curve for the injection mass ratio of 75:25 is different as compared to that for the

735

injection mass ratio of 25:75 and 50:50, where it does not indicate two PHRR. This is because

736

of the large mass ratio of first injection which resulted higher heat release rate. However, this

737

injection strategy has lower smoke emission due to better oxidation process.

738

4. NOx was slightly improved by using biodiesel-diesel blends for all combinations of first

739

injection SOI timing and mass ratio. NOx emission for all fuels generally improved with later

740

SOI for first injection pulse. The minimum 82.0 ppm NOx emission was achieved by using

741

B50 in the engine with injection mass ratio of 25:75 at SOI of 2 °ATDC.

742

5. Smoke emissions was greatly improved with the use of biodiesel-diesel blends. The higher

743

the concentration of biodiesel in the fuel, the lower the smoke produced. The highest reduction

744

of 5.3 % of smoke compared to diesel was achieved when B50 was used with 50:50 mass ratio

745

at retarded SOI of 2 °ATDC. 39

ACCEPTED MANUSCRIPT 746

6. Simultaneous NOx and smoke reduction compared to that of baseline diesel was feasible with

747

the application of B50 biodiesel blend and execution of retarded SOI timing and injection

748

mass ratio of 25:75.

749

7. Two-stage fuel injection with different mass ratio is a practical strategy to simultaneously

750

decrease NOx and smoke emissions when the SOI timing is fine-tuned and is an ideal

751

alternative to operate with biodiesel fuel.

752

Acknowledgments

753

The authors would like to acknowledge the Ministry of Higher Education (MOHE) of Malaysia,

754

Universiti Malaya, KDU Penang University College Internal Research Grant and Universiti Sains

755

Malaysia (BRIDGING research grant scheme- 304/PMEKANIK/6316488) for financial support.

756

757

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ACCEPTED MANUSCRIPT Highlight

►Effect of biodiesel blend, SOI and mass ratio of two-stage injection was investigated ►Optimum BTE and BSFC were obtained with early SOI and 75:25 injection mass ratio ►Lowest NOx of 82 ppm was achieved with smoke emission level still remains below 5% ►Simultaneous NOx and smoke reduction with B50, late SOI and 25:75 injection ratio