Application of waste cooking oil (WCO) biodiesel in a compression ignition engine

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Contents lists available at ScienceDirect

Fuel journal homepage: www.elsevier.com/locate/fuel 5 6

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Application of waste cooking oil (WCO) biodiesel in a compression ignition engine

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Joonsik Hwang a, Choongsik Bae a,⇑, Tarun Gupta b

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a b

Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology, Republic of Korea Department of Civil Engineering, Indian Institute of Technology Kanpur, India

h i g h l i g h t s
The WCO biodiesel had longer injection delay than diesel.

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The WCO biodiesel showed lower equivalence ratio along the spray than diesel.

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The WCO biodiesel was advantageous in reduction of CO, HC and smoke emissions.

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The WCO biodiesel showed lower level of flame luminosity than diesel.

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a r t i c l e 2 3 1 5 22 23 24 25 26 27 28 29 30 31 32 33 34

i n f o

Article history: Received 10 November 2015 Received in revised form 18 January 2016 Accepted 19 February 2016 Available online xxxx Keywords: Waste cooking oil (WCO) Biodiesel Compression ignition engine Spray Combustion Emission

a b s t r a c t The comprehensive spray and combustion characteristics of waste cooking oil (WCO) biodiesel (B100) and conventional diesel fuels were investigated. The injection rate was measured by Bosch method and spray test was performed in a constant volume chamber under non-evaporating condition. The fuels were injected at injection pressures of 80 and 160 MPa with injection durations of 625 and 410 ls, respectively. From the injection rate experiment, the WCO biodiesel exhibited relatively longer injection delay than diesel due to higher viscosity. The WCO biodiesel also showed longer liquid tip penetration length and narrower spray angle regardless of fuel injection pressure. Nevertheless, the calculated equivalence ratio along the axial direction of spray proved that WCO biodiesel had lean fuel–air ratio compared to diesel due to inherent oxygen content in the fuel molecule. A series of engine experiments was performed to investigate the combustion and emission characteristics in an optically accessible compression ignition engine equipped with a common-rail system. The net indicated mean effective pressures (IMEPnet) of 0.16–0.93 MPa were tested under an engine speed of 1400 r/min. The fuel injection timing was modified from 60 to 0 crank angle degree (CAD) before top dead center (bTDC) by 5 CAD. The WCO biodiesel had a slightly lower peak of in-cylinder pressure and fuel efficiency than diesel due to lower heating value. In terms of emissions, the WCO biodiesel had benefits in reduction of carbon monoxide, unburned hydrocarbon and smoke emissions in conventional diesel operating condition. On the other hand, however, the emission characteristics were deteriorated as the injection timing was advanced and the engine load was increased. The combustion imaging showed that the WCO biodiesel had lower flame luminosity and shorter visible flame duration than diesel. Ó 2016 Published by Elsevier Ltd.

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1. Introduction As a consequence of environmental concerns and the increasing costs of fossil fuel, search for alternative fuels has gained importance. Among many alternative fuels, biodiesel is gaining more ground because it is an environmentally clean and renewable ⇑ Corresponding author at: KAIST, 291, Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea. Tel.: +82 42 350 3063; fax: +82 42 350 5023. E-mail address: [email protected] (C. Bae).

energy source. Biodiesel is comprised of fatty acid alkyl esters derived from transesterification of triglycerides present in vegetable oils or animal fats. A number of scientific research has reported successful operation of compression ignition (CI) engines fueled with biodiesels derived from many different feedstocks [1]. Furthermore, nowadays the biodiesel has been actively commercialized in numerous countries such as Europe, USA, and China [2]. In this situation, the use of recycled oil and grease is attracting attention because it utilizes waste products which can eliminate the needs of disposal problems [3].

http://dx.doi.org/10.1016/j.fuel.2016.02.058 0016-2361/Ó 2016 Published by Elsevier Ltd.

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Nomenclature ;ðxÞ
A F st

x

qa qf

Ca do a h

equivalence ratio at any axial location x [a.u.] stoichiometric air–fuel ratio [a.u.] axial location in the spray [mm] ambient air density [kg/m3] fuel density [kg/m3] area contraction coefficient [a.u.] injector nozzle hole diameter [mm] constant [a.u.] spray angle [deg.]

Abbreviations aTDC after top dead center BSFC brake specific fuel consumption bTDC before top dead center

75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120

There are several studies which investigated spray, combustion and emission characteristics in a compression ignition engines fueled with biodiesel [4–7]. Lin et al. studied the spray characteristics of WCO biodiesel in a constant volume chamber [8]. In order to compare the macroscopic spray characteristics, liquid tip penetration length and spray angle were measured by high speed imaging. The Sauter mean diameter (SMD) was also measured by a laser particle size analyzer. They found that the liquid tip penetration length of WCO biodiesel spray was longer and spray angle was narrower than those of diesel. The droplet size distribution result showed that the mean SMD was increased with WCO biodiesel. Mohan et al. investigated the spray and air–fuel mixing characteristics of neat WCO biodiesel and its blend with diesel B20 [9]. Spray characteristics such as spray tip penetration, spray angle, spray velocity and spray morphology were studied under high injection and ambient pressure conditions in a constant volume chamber. The result showed that the WCO biodiesel gave longer liquid tip penetration length with narrower spray angle similar to previous researches. The fuel volume and amount of air entrainment within the spray revealed that neat biodiesel exhibited poor air–fuel mixing compared to diesel. However, the spray of WCO biodiesel had lean equivalence ratio than diesel due to the presence of inherent oxygen content in the fuel molecule. Cura et al. examined the microscopic spray characteristics of rapeseed methyl ester (RME) under diesel engine-like operating conditions in a reciprocating rapid compression machine (RCM) [10]. The ultra-fast framing camera was fitted with the long range microscope, resulting in an effective viewing region of 995  746 lm. The microscopic image showed that the RME had longer injection delay than diesel. Based on the images, the liquid core of the RME jet did not breakup directly into ligaments and droplets due to its higher viscosity and surface tension. Many researchers have reported that using WCO biodiesel instead of conventional diesel decreases harmful exhaust emissions with equivalent engine performance [11,12]. Hwang et al. investigated the effect of injection parameters on combustion and emission characteristics in a heavy duty diesel engine fueled with WCO biodiesel [13]. They concluded that WCO biodiesel was advantageous in reducing carbon monoxide (CO), hydrocarbon (HC), and smoke emissions especially with high injection pressure. However, WCO biodiesel showed higher nitrogen oxide (NOx) emissions than diesel under all operating conditions. Can et al. studied the combustion, performance and exhaust emissions of WCO biodiesel in a single cylinder diesel engine under four different engine loads [14]. The maximum in-cylinder pressure and heat release rate with WCO biodiesel were slightly lower than those of

CAD CI CO FLOL HC ISFC LHV NOx PAHs RME SINL SMD WCO

crank angle degree compression ignition carbon monoxide flame lift-off length hydrocarbon indicated specific fuel consumption lower heating value nitrogen oxide polycyclic aromatic hydrocarbons rapeseed methyl ester spatially integrated natural luminosity Sauter mean diameter waste cooking oil

diesel. It was found that the addition of biodiesel resulted in increment on brake specific fuel consumption and reduction on break thermal efficiency. The CO, HC and smoke emissions were decreased but the NOx emissions were increased with WCO biodiesel. The investigation on combustion flame of biodiesel have also been performed. Fang et al. examined the spray and combustion processes in an optically accessible single cylinder compression ignition engine fueled with soybean biodiesel [15]. They confirmed that the ignition and peak of heat release were occurred later with increasing biodiesel content. Under all experimental conditions, biodiesel showed lower soot luminosity than diesel. Menkiel et al. investigated the combustion and soot processes for RME in an optical diesel engine [16]. The flame lift off length (FLOL) based on the appearance of soot were longer for RME. The planar laser induced incandescence (PLII) data confirmed that the relative amount of soot left in the cylinder after the end of visible luminous combustion was less with RME compared to diesel. Cheng et al. studied the impact of soybean biodiesel fueling on NOx emissions in an optically accessible diesel engine [17]. The spatially integrated natural luminosity (SINL) was generally decreased but the NOx emissions were increased with biodiesel. The correlation between SINL and NOx data suggested that the NOx could be increased with biodiesel due to lower radiative heat transfer from soot particles. Despite this kind of effort, comprehensive assessment of WCO biodiesel (B100) application in compression ignition engine have not been fully clarified. In this research, therefore, spray tests including injection rate experiment and macroscopic spray imaging were performed in an injection rate meter and a constant volume chamber, respectively. Furthermore, the combustion and emission characteristics were investigated in an optically accessible single cylinder compression ignition engine. The combustion process was also compared by means of image analysis focusing on the flame structure and luminosity intensity.

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2. Experimental

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2.1. WCO biodiesel production, composition and property

157

The biodiesel was produced from WCO through transesterification process with methanol (CH3OH) catalyzed by sodium methoxide (NaOCH3). A titration was performed to determine proper amount of NaOCH3 needed to neutralize the free fatty acids in WCO. For the trans-esterification, 220 g of CH3OH and 18 g of

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J. Hwang et al. / Fuel xxx (2016) xxx–xxx Table 1 Fatty acid composition of waste cooking oil biodiesel.

Table 2 Properties of waste cooking oil biodiesel and diesel fuel.

Type of fatty acid

Molecular weight

Molecular formula

Carbon chain

% (By mass)

Item

WCO biodiesel

Diesel

Analytic method

Myristic Palmitic Palmitoleic Stearic Oleic Linoleic Linolenic Arachidic Eicosenoic Behenic Unknown components

228 256 254 284 282 280 278 312 310 340

C14H28O2 C16H32O2 C16H30O2 C18H36O2 C18H34O2 C18H32O2 C18H30O2 C20H40O2 C20H38O2 C22H44O2

C14:0 C16:0 C16:1 C18:0 C18:1 C18:2 C18:3 C20:0 C20:1 C22:0

0.24 13.12 0.82 3.23 33.4 39.2 4.33 0.39 0.47 0.2 4.6

Cetane number Lower heating value [MJ/kg] Density (@288 K) [kg/m3] Kinematic viscosity (@313 K) [mm2/s] Flash point [K] Sulfur content [mg/kg] Surface tension (@313 K) [N/m]

51.34 38.85 878 4.4 463 1.00 0.032

50.88 42.98 820 2.2 329 3.93 0.026

ASTM D4737 ASTM D3338 ASTM D1298 ASTM D445 ASTM D93 ASTM D5454 Ref. [20]

Total

100

189

NaOCH3 were added to 1 kg of WCO at 343 K. The mixture was separated into crude glycerin and biodiesel in this process. The separated crude biodiesel was washed with mildly acidic water to remove the neutralized catalysts, water-soluble glycerin and soaps. The composition of fatty acids in the biodiesel was measured by gas chromatography as shown in Table 1. The fatty acids in the WCO biodiesel were identified as approximately 35% monounsaturated, 44% poly-unsaturated, and 17% saturated. Oleic and linoleic acid were the major fatty acids. The major fuel properties were measured according to ASTM standards and compared with each other, as shown in Table 2. The viscosity and surface tension of WCO biodiesel were higher than those of diesel. The density of WCO biodiesel was 6.6% higher and the LHV was 9.6% lower than diesel. The injection quantity of WCO biodiesel needed to be increased to produce the same amount of power. The sulfur content of WCO biodiesel was 1.0 mg/kg, which is evidently lower than that of diesel. WCO biodiesel had a slightly lower cetane number than diesel but the difference was insignificant. Fuel volatility strongly affects engine performance and emission characteristics. The distillation characteristics of WCO biodiesel and diesel were investigated as shown in Fig. 1. It indicates that the WCO biodiesel had a narrow evaporation range from 603 K to 658 K, while diesel had a uniform range between 415 K and 648 K. The distillation temperature influences the vaporization rate and air fuel mixing. The diesel is expected to have better evaporation characteristics compared to that of WCO biodiesel due to lower distillation temperature.

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2.2. Injection rate and macroscopic spray experiment

191

The injection rate was measured according to the Bosch ‘‘longtube” method. It recorded an injection rate by measuring the pressure wave which was produced by an injector when it injected into a length of compressible fluid. Fig. 2(a) shows a schematic of the Bosch type injection rate meter. It was comprised of an injector adaptor, a measuring tube, a needle valve, an accumulator, and a relief valve. The injector adaptor held the injector so that the tip of injector was positioned at the beginning of the measuring tube. The pressure sensor was installed in the injector adaptor to record the pressure waves. An orifice was made by the needle valve between the measuring tube and the accumulator. The area of the orifice determined the portion of reflected pressure wave. A relief valve located at the end of the accumulator adjusted the back pressure on the closed volumes. The concept of measuring the pressure wave to determine injection rate is based on the pressure–velocity equation valid for a single pressure wave in an instationary flow. The pressure inside of the tube can be written as following equation. It was derived from the hydraulic pulse theorem assuming one-dimensional motion [18].

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Fig. 1. Distillation temperature curve for WCO biodiesel and diesel.

P ¼ a  qfuel  u

ð1Þ

where P is the pressure, a is the speed of sound in fluid, qfuel is the density of fuel, and u is the flow velocity. Combined with the continuity equation, the governing equation for the injection rate meter can be derived as following equation [19].

dq A ¼ P dt a  qfuel

ð2Þ

where q is the volume of fuel, t is the time, A is the area of the tube. Therefore, integration of Eq. (2) gives the injected volume. A schematic diagram of the injection rate experiment is shown in Fig. 2 (b). An eight-hole solenoid type injector (Bosch) with a hole diameter of 0.131 mm and an injection angle of 150° was used for whole experiments. The back pressure was maintained at 2 MPa by a relief valve. The tube pressure was measured by a piezoelectric pressure transducer (Kistler, 6052C) coupled with a charge amplifier (Kistler, 5011). The pressure was recorded with the resolution of 150 kHz by data acquisition system. The results of 1000 times injection were averaged to compare fuel flow characteristics according to fuels. The macroscopic spray experiments were performed in a constant volume chamber. A schematic diagram of the optical diagnostic system for non-evaporating macroscopic spray imaging is shown in Fig. 2(c). The spay images were taken by direct photography using Mie-scattering. Optical access to the constant volume chamber was available through three quartz windows which have diameter of 90 mm. The chamber was pressurized up to 2 MPa by nitrogen under room temperature condition. Two discharge lamps with 85 W power (HID Fire, HF-8500) were used to obtain sufficient intensity of the Mie-scattered light from the fuel spray. A high-speed digital video camera (Photron, FASTCAM SA1.1) equipped with a prime lens (Nikkor, 60 mm f/2.8D) was used to capture images of the non-evaporating spray of WCO biodiesel and diesel. A signal from the fuel injection system was used to trigger the high speed camera. The direct imaging was performed at a shutter speed of 20,000 frames per second (fps). The aperture of

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Fig. 2. Experimental setup (a) injection rate meter, (b) schematic diagram of injection rate measuring system, and (c) schematic diagram of the constant volume chamber.

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the lens and the exposure time of the high speed camera were set to 2.8 and 1/62,000 s, respectively. From the spray imaging, five images were averaged to estimate the liquid penetration length

and the spray angle. This procedure was repeated for each nozzle hole. To measure the liquid penetration length and the spray angle, the spray boundary was determined by selecting a threshold value

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Specification

Number of cylinder Injection type Valve per cylinder Intake valve opening [CAD bTDC] Intake valve closing [CAD aBDC] Exhaust valve opening [CAD bBDC] Exhaust valve closing [CAD aTDC] Bore  stroke [mm] Injection system Compression ratio Displacement [cc]

Single Direct injection 4 21 33 56.2 22.2 100  125 Common-rail 17.4:1 980

257

for the intensity. The liquid penetration length was defined as the distance between the nozzle-tip to the farthest axial location of the spray boundary. The spray angle was measured at the middle of the liquid penetration length. The definition of these parameters are also shown in Fig. 2(c).

258

2.3. Engine performance and combustion imaging experiment

259

288

The test engine was a single-cylinder direct-injection diesel engine with a bore of 100 mm, stroke of 125 mm, displacement of 980 cc and compression ratio of 17.4. It was equipped with a high pressure common-rail injection system. The specifications of the engine are presented in Table 3. Fig. 3(a) shows a schematic of the experimental apparatus used for the metal engine test. The injection pressure, injection timing and injection quantity were controlled by a common-rail engine controller (Zenobalti, ZB9013). Engine speed was controlled constantly by a 82 kW DC (Direct current) dynamometer. A rotary encoder (Autonics, E40S) was mounted on the crankshaft. In-cylinder pressure was recorded every 0.2 crank angle degree (CAD) by a piezoelectric pressure transducer (Kistler, 6052C) coupled with a charge amplifier (Kistler, 5011). The in-cylinder pressure of 100 engine cycles were averaged to calculate the heat release rate. The CO, HC and NOx emissions were measured using an exhaust gas analyzer (Horiba, MEXA 1500D). A smoke meter (AVL, 415S) was used to measure the engine-out smoke emissions. Fig. 3(b) shows a cross sectional view of the optical engine system. The engine was modified to allow optical access to the combustion chamber. An elongated piston was equipped to enable the mounting of a 45° mirror beneath the piston quartz window. The surface of the quartz was flat, rather than curved as in the production engine, so that undistorted images of the combustion event could be captured while the optical engine was designed to have a geometric compression ratio of 17.4, which was the same as the metal engine. The identical camera setup with spray test was utilized to capture images of the combustion process. The images of the combustion flame were taken directly without a filter.

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2.4. Experimental conditions

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The experimental conditions are listed in Table 4. The tested fuels were WCO biodiesel (B100) and commercial diesel. For the injection rate and macroscopic spray experiments, identical injection duration was applied for WCO biodiesel and diesel. The injection duration was set to 625 ls and 410 ls for injection pressure of 80 MPa and 160 MPa respectively. The corresponding injection quantity for diesel was 20 mg. The backpressure and ambient air pressure were maintained at 2 MPa under room temperature. The net indicated mean effective pressures (IMEPnet) of 0.16– 0.93 MPa were tested under an engine speed of 1400 r/min. The

253 254 255 256

260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287

291 292 293 294 295 296 297 298 299

5

fuel and coolant temperature were controlled at 313 K and 353 K respectively. Exhaust gas recirculation was not introduced during the investigation. Injection pressures of 80 MPa of the commonrail injection system was adopted. The injection timing was varied from to 0 crank angle degree (CAD) before top dead center (bTDC) to 60 CAD bTDC to examine the effect of the injection timing on the combustion characteristics. The injection quantity for the combustion imaging experiments was set to 44.3 mg/stroke for WCO biodiesel and 40 mg/stroke for diesel in order to compensate for the lower heating value of WCO biodiesel.

300

3. Results and discussion

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3.1. Injection rate and macroscopic spray characteristics

311

The injection rates according to fuels for injection pressures of 80 MPa and 160 MPa are presented in Fig. 4(a). The injection duration was set to identical for each injection pressure case and the injection quantity was 20 mg for diesel. It can be seen that the peak of injection rate became higher with injection pressure of 160 MPa due to increased fuel momentum. In terms of fuel, the WCO biodiesel showed higher peak of injection rate than diesel. The injection quantity of WCO biodiesel was also larger due to higher fuel density despite the identical injection duration. At the needle opening transient period, WCO biodiesel had the retarded start of injection as shown in Fig. 4(b). From the research of Moon et al. slower increase in needle speed and exit velocity of biodiesel were also verified by ultrafast X-ray phase-contrast imaging technique [20]. The retarded start of injection is attributed to the effects of fuel properties on the frictional loss and needle lift motion in the nozzle [21,22]. In spite of high lubricity, which should reduce friction in the nozzle, the viscosity and surface tension appeared to play a more dominant role on frictional losses in the nozzle for the WCO biodiesel. The higher fuel viscosity also made the fuel flow harder to be discharged from the control chamber in the injector that reduced the pressure decrease rate in the control chamber, finally decreasing the speed of needle motion. According to fuel injection pressure, the difference in start of injection was decreased at high injection pressure. Fig. 5 shows the macroscopic spray of WCO biodiesel and diesel at injection pressure of 80 MPa and 160 MPa under ambient pressure of 0.1 MPa and temperature of 300 K. The time after the start of injection (SOI) was defined as the first appearance of a liquid spray at the nozzle tip. The spatial resolution of the images was 0.27 mm/pixel. It is noted that the spray development was facilitated by higher injection pressure of 160 MPa case showing longer liquid penetration length than 80 MPa condition. The observed overall spray structure was similar for both fuels. To compare the spray characteristics quantitatively, liquid tip penetration length and spray angle were measured. The measured liquid penetration tip length and spray angle are presented in Fig. 6. As shown in the figures, the WCO biodiesel had relatively longer liquid tip penetration length and narrower spray angle than diesel regardless of injection pressure. This result was attributed to higher density, viscosity and surface tension of WCO biodiesel compared to diesel. It has been reported that higher fuel density induces a lower ambient air entrainment [23,24]. Lower air entrainment attenuates the fuel atomization and evaporation process. On the other hand, higher viscosity and surface tension of WCO biodiesel also resisted the formation of surface waves, ligaments and droplets [25]. The level of turbulence in spray boundary was lower with WCO biodiesel than diesel due to more stable spray structure [26]. As a result, the diffusion of spray along the radial direction was deteriorated for WCO biodiesel case and finally resulted in longer liquid tip penetration length and nar-

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Fig. 3. Experimental setup (a) schematic diagram of metal engine test. (b) Schematic diagram of the optical engine system for direct imaging of flame; a comparison of cylinder bowl with metal engine and optical quartz is presented in the left upper part.

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rower spray angle. In the previous research of Fang et al., the stronger fuel impingement was detected with biodiesel fuel especially under advanced injection timing condition in an optical diesel engine [15].

The equivalence ratio along the injected fuel spray is important because it affects combustion and emission characteristics in compression ignition engines. It is one of the dominant parameters in governing the soot formation. Therefore it has to be well under-

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J. Hwang et al. / Fuel xxx (2016) xxx–xxx Table 4 Experimental conditions. Condition

Injection rate

Macroscopic spray

Engine test

Cycle number Back pressure [MPa] Injection duration [ls] Injection pressure [MPa] Fuel temperature [K] Engine speed [rpm] Injection timing [CAD bTDC] Injection quantity [mg/stroke]

1000 2 625 (80 MPa), 410 (160 MPa) 80, 160 313 – – –

5 2 625 (80 MPa), 410 (160 MPa) 80, 160 313 – – –

Coolant temperature [K]

–

–

– – – 80 313 1400 60–0 (5 CAD sweep) 10, 20, 30, 40, 50, 60 (for diesel) 11, 22.1, 33.4, 44.3, 55.4, 66.2 (for WCO biodiesel) 353

x is the characteristic length scale. The x can be calculated by following equation.

sffiffiffiffiffiffi x ¼

ð4Þ

where qf is the fuel density, qa is the ambient air density, Ca is the area contraction coefficient, d0 is the injector nozzle hole diameter, a is a constant, and h is the spray angle. The constants Ca and a are assumed as 0.8 and 0.75 respectively. The calculated equivalence ratio at the leading edge of the spray is shown in Fig. 7. The equivalence ratio result shows a decreasing trend according to spray tip penetration. This implies the air–fuel mixture becomes lean indicating oxidizer rich environment in downstream of the spray. It can also be seen that the equivalence with 160 MPa injection pressure was lower than that of 80 MPa injection pressure due to enhanced fuel atomization and vaporization. In terms of fuel, the WCO biodiesel had lower level of equivalence ratio along the spray even though it showed deteriorated spray characteristics. The reason is that WCO biodiesel have oxygen inherent in its chemical structure which has lower stoichiometric air–fuel ratio compared to diesel. This may result in lower soot emissions as reported by many researchers [28–30].

370 371 372 373 374 375

376

378

379 380

where ;ðxÞ is the equivalence ratio at any axial location x, F st is the stoichiometric air–fuel ratio, x is the axial location in the spray, and

388 389 390 391 392 393 394 395 396 397 398 399 400 401 402

404

where


A

387

Fig. 8 shows in-cylinder pressure trace and heat release rate with respect to crank angle for the WCO biodiesel and diesel at injection pressure of 80 MPa and injection timing of 5 CAD bTDC. The heat release rate was calculated by single zone First Law model. This model is normally employed in preference to the potentially more accurate multi-dimensional thermodynamic models since it is less complex, numerically more efficient and normally yield similar results. The First Law equation utilized in this study can be written as follow [31].

stood for different fuels. The equivalence ratio along the axial direction of the spray can be calculated based on the correlation suggested by Naber et al. [27] by assuming that the fuel spray behaves similar to that of gaseous turbulent jets. The equivalence ratio at any axial location in the spray was calculated by following equation.

ð3Þ

386

403

dQ n dQ ch dQ ht c dV 1 dp þ ¼  ¼ p V c  1 dt c  1 dt dt dt dt

2  F st ;ðxÞ ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2ffi 1 þ 16  xx  1

385

3.2. Combustion and emission characteristics

Fig. 4. Injection rate (a) entire injection event, (b) at the initial stage according to fuels for injection pressures of 80 MPa and 160 MPa.


A

382

383

pffiffiffiffiffi

qf C a  d0
  qa a  tan 2h

381

405 406 407 408 409 410 411 412

413

ð5Þ

dQ n is the net heat release rate, dQdtch is the gross heat release dt dQ ht is the heat-transfer rate to the walls, is the ratio of specidt

rate, c fic heats, p is the in-cylinder pressure, V is the cylinder volume, and t is the time. In the calculation, the heat transfer to wall was ignored for simplicity so the heat release rate determined was referred to as ‘net’ heat release rate. From previous research, it was revealed that the net heat release values were typically 15% lower than those obtained on a gross heat release basis [32]. After the ignition delay, premixed fuel–air mixture burns rapidly, followed by diffusion combustion, where the burn rate is controlled by air–fuel mixing. From the heat release result, a slightly longer ignition delay was observed for WCO biodiesel. Although the cetane number of WCO biodiesel was a little higher

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Fig. 5. Macroscopic spray images at injection pressure of 80 MPa and 160 MPa.

Fig. 7. Calculated equivalence ratio ð£Þ along axial direction of spray according to fuels and injection pressures.

Fig. 6. Comparison of (a) liquid tip penetration length and (b) spray angle at different injection pressure for WCO biodiesel and diesel fuel, respectively, according to time after start of injection.

than that of diesel, the lower volatility led to poor atomization and evaporation, which resulted in a later start of combustion. The peak in-cylinder pressures were obtained as 6.56 MPa (at 12.6 CAD aTDC) and 6.58 MPa (at 12 CAD aTDC) for WCO biodiesel and diesel, respectively. Meanwhile, the WCO biodiesel showed higher peak of heat release rate in premixed combustion phase because the peak of heat release rate is related to the amount of prepared fuel within the ignition delay period. Another possible reason is that even though the physical properties of WCO biodiesel were less favorable regarding mixing and evaporation, the presence of fuel bound oxygen enhanced the combustion. Fig. 9 displays the indicated specific fuel consumption (ISFC) according to various injection timing at injection pressure of 80 MPa. At the injection timing between 15 and 0 CAD bTDC, the combustion process occurs near TDC so negative work could be minimized and resulted in lower ISFC [33]. The ISFC reached the minimum for the both fuels at injection pressure of 10 CAD bTDC.

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Fig. 11. The engine out HC emissions ratio of WCO biodiesel to diesel.

Fig. 8. Comparison of in-cylinder pressure and heat release rate at injection timing of 5 CAD bTDC with injection pressure of 80 MPa for WCO biodiesel and diesel.

Fig. 12. The engine out smoke emissions ratio of WCO biodiesel to diesel.

Fig. 9. Comparison of indicated specific fuel consumption according to injection timing at injection pressure of 80 MPa.

Fig. 13. The engine out NOx emissions ratio of WCO biodiesel to diesel.

Fig. 10. The engine out CO emissions ratio of WCO biodiesel to diesel.

The advancement or retardation from this optimum value of injection timing caused the deterioration of ISFC. The ISFC for WCO biodiesel was approximately 10% higher than that of diesel due to its lower heating value. The effects of the injection timing and engine load on the CO emissions are shown in Fig. 10. The result was plotted by the ratio of emission level between two fuels. The color map on the right

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Fig. 14. Sequential flame images of WCO biodiesel and diesel combustion at injection pressure of 80 MPa and injection timing of 5 CAD bTDC.

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side of the graph indicates the ratio of the emission level. Within conventional diesel operating condition, the CO emissions of the WCO biodiesel was lower than those of diesel. The reason is that the oxygen content of the biodiesel resulted in more complete combustion [34]. However, in high engine load and advanced injection timing conditions, the emission level increased abruptly more than that of diesel. The possible reason is that the incomplete combustion was occurred by cylinder wall and piston impingement due to poor atomization and vaporization characteristics of WCO biodiesel. The stronger impingement for WCO biodiesel could be attributed to a few factors. At first, WCO biodiesel had higher boiling point with a low volatility causing longer liquid tip penetration length as seen in spray experiment. Secondly, the injection quantity for WCO biodiesel was slightly higher than diesel for the identical IMEPnet due to a lower heating value. The stronger impingements of soybean biodiesel on cylinder wall and piston were confirmed by Fang et al. through optical diesel engine [15]. Based on this situation, the CO emissions of WCO biodiesel could be deteriorated especially in early injection case because the fuel was injected in significantly lower in-cylinder pressure and temperature compared to conventional injection timing condition. Fig. 11 compares the HC emissions between WCO biodiesel and diesel according to injection timings and engine load. Typically, HC emissions are a serious problem at low load condition for diesel engines. At low load condition, the lean fuel and air mixture may survive to escape into exhaust because of poor fuel distribution even though less fuel is impinged on surface [35]. In this situation, WCO biodiesel showed lower HC emissions than diesel. This is because of better combustion of WCO biodiesel inside the combustion chamber due to the availability of oxygen atom in the fuel molecule. However, in high load condition, the emission level suddenly increased similar to the CO emissions trend. As explained above, the larger fuel rich region could be formed by strong impingement with WCO biodiesel. Thus, hydrocarbons were not consumed due to incomplete mixing or to quenching of the oxidation process [31]. Fig. 12 displays the smoke emissions of WCO biodiesel and diesel at various fuel injection timing and engine load. As shown in the figure, the WCO biodiesel was advantageous for the reduction of smoke emissions under conventional diesel operating conditions. This is because the oxygen content in the WCO biodiesel assisted smoke oxidation during the diffusion combustion phase [36]. Venkanna et al. stated that aromatics are known as contributor of soot formation, while the inherent oxygen molecules in the biodiesel help to promote stable and complete combustion by delivering oxygen to the pyrolysis zone [37]. The oxygen content can reduce locally over rich regions and limit primary smoke formation. On the other hand, the emission level was increased in

early injection timing conditions due to stronger impingement of WCO biodiesel in the combustion chamber. Fig. 13 illustrates NOx emissions of WCO biodiesel and diesel under various injection timing and engine load. By comparing with Fig. 12, the trade-off between NOx and soot can be seen. From the result, the WCO biodiesel showed higher NOx emission level than diesel in conventional diesel operating condition. The oxygen content of biodiesel is an important factor of the high NOx formation because they provide a higher local peak temperature and a corresponding excess of air. It was confirmed that the radiative cooling by soot particles reduces flame temperature by 25–50 K, corresponding to an estimated NOx reduction of 12–25% [38]. Therefore, the combustion of WCO biodiesel could be maintained at higher incylinder temperature than diesel and finally resulted in high NOx emissions. Meanwhile, the level of NOx emissions were significantly decreased in early injection timing conditions. The reason is that the in-cylinder temperature was decreased due to incomplete combustion.

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3.3. Flame image analysis

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The direct combustion images of WCO biodiesel and diesel are shown in Fig. 14. The color1 map on the right of the graph indicates the intensity of the natural flame luminosity, where ‘‘0” is shown by black implying the weakest luminosity and ‘255’ is denoted by white indicating the strongest luminosity. The natural flame luminosity is generally consisted of the incandescence from the hot soot particles and the chemiluminescence from the excited gaseous species that are produced during the combustion process [39]. Among these, soot incandescence was dominant in WCO biodiesel and diesel combustion. The diffusion flame was detected at the downstream of the spray. At 6.38 CAD aTDC, distinguishable diffusion jet structure can be seen entire combustion chamber. After 13.1 CAD aTDC, the flame luminosity decreased while the remaining diffusion flames were carried downstream to the cylinder wall. From the combustion images, it was verified that the WCO biodiesel had a bit longer ignition delay than diesel with the flame luminosity detected at a later crank angle. The soot incandescence was also detected further away from the nozzle tip, closer to the cylinder wall. Longer lift off length results in relatively more entrainment of air upstream of lift-off, which causes the mixture distribution in WCO sprays to be relatively leaner, compared to diesel and this also leads to the reduction of the soot formation [14]. The peak of flame luminosity was calculated to be 168 and 173 for WCO biodiesel and diesel, respectively. This

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1 For interpretation of color in Fig. 14, the reader is referred to the web version of this article.

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result was ascribed to lower smoke emissions because the oxygen content in WCO biodiesel interrupted the production of soot precursors during the combustion process [40]. The flame area and visible flame luminosity duration for WCO biodiesel were also smaller than that for diesel. From previous research, it was verified that the soot particles from the WCO biodiesel had lower carbon weight fraction than diesel by elemental analysis [41]. Meanwhile, the WCO biodiesel soot particles contained larger amount of volatile organic fraction (VOF) than those of diesel. Therefore, the soot particles from the WCO biodiesel were oxidized at a relatively lower in-cylinder temperature and resulted in lower level of flame area and visible flame luminosity duration.

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4. Conclusion

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In this study, comprehensive spray and combustion characteristics of WCO biodiesel and diesel were investigated. The injection rate was measured by using Bosch ‘‘long tube” method and the macroscopic spray characteristics were studied under nonevaporating condition in a constant volume chamber. The engine combustion and emission tests were performed in an optically accessible compression ignition engine equipped with a common-rail system. Emissions including CO, HC, smoke and NOx were measured at injection pressure of 80 MPa and various injection timings. The direct flame imaging was also conducted in order to investigate the different flame characteristics of WCO biodiesel and diesel combustion. The major findings of this study are summarized as follows:

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1. Injection rate result exhibited that the WCO biodiesel had a longer injection delay and higher peak of injection rate due to its higher fuel viscosity and density. The macroscopic spray showed that the WCO biodiesel had longer liquid tip penetration length and narrower spray angle showing poor air–fuel mixing compared to diesel. However, the calculated equivalence ratio of WCO biodiesel was lower than diesel. This was due to the inherent oxygen content present in WCO biodiesel fuel molecule. 2. WCO biodiesel had a slightly lower peak of in-cylinder pressure than diesel combustion. The WCO biodiesel showed the higher peak of premixed burn due to a bit longer ignition delay than diesel. Even though the physical properties of WCO biodiesel were less favorable regarding mixing and evaporation as shown in macroscopic spray characteristics, the presence of oxygen content in the fuel molecule enhanced the combustion. 3. WCO biodiesel had the benefits in CO, HC and PM reduction at low load and conventional diesel operating conditions. On the other hand, the nitrogen oxide emissions were increased with WCO biodiesel. In the high engine load and early injection timing conditions, the emission characteristics of WCO biodiesel was deteriorated than those of diesel. 4. Based on the combustion flame images, the WCO biodiesel showed lower level of flame intensity and shorter visible flame duration than diesel. This implies that the production of soot precursors was suppressed and the oxidation of soot particles was accelerated by inherent oxygen content in WCO biodiesel. The flame area was also smaller with WCO biodiesel because the diffusion burn phase was relatively less dominant than diesel combustion.

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Acknowledgements

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Financial support from National Research Foundation of Korea under Korea-India project (2014K1A3A1A19067560) is gratefully

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acknowledged. India side of this research was supported by the Department of Science and Technology of India (INT/KOREA/P-23 dated 06-07-2015).

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