Effect of two-stage injection dwell angle on engine combustion and performance characteristics of a common-rail diesel engine fueled with coconut oil methyl esters-diesel fuel blends

Fuel 234 (2018) 227–237

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Full Length Article

Effect of two-stage injection dwell angle on engine combustion and performance characteristics of a common-rail diesel engine fueled with coconut oil methyl esters-diesel fuel blends ⁎

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T

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Y.H. Teoha, , H.H. Masjukib, , H.G. Howb,c, , M.A. Kalamb, K.H. Yub, A. Alabdulkaremd a

School of Mechanical Engineering, Universiti Sains Malaysia, Engineering Campus, 14300 Nibong Tebal, Penang, Malaysia Centre for Energy Sciences, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia c Department of Engineering, School of Engineering, Computing and Built Environment, KDU Penang University College, 32, Jalan Anson, 10400 Georgetown, Penang, Malaysia d Mechanical Engineering Department, College of Engineering, King Saud University, 11421 Riyadh, Saudi Arabia b

A R T I C LE I N FO

A B S T R A C T

Keywords: Diesel engine NOx Injection timing Dwell angle Biodiesel

Diesel engine is widely used as prime mover due to its high thermal efficiency. Usage of renewable biodiesel in diesel engine is also widely studied due to its potential in reducing emission and as a replacement of conventional diesel. Biodiesel performance could be improved by blending it with petroleum diesel besides introducing appropriate injection strategies. In this experiment, the effect of percentage of biodiesel blends and injection strategies such as variations in start of injection (SOI) timing and dwell angle on diesel engine performance were investigated. The test engine used is four-stroke turbocharged direct injection diesel engine. Results show that exhaust emissions, engine performance and combustion characteristics are substantially affected by biodiesel blending ratio and SOI timing but slightly influenced by two-stage injection dwell angle. Biodiesel blends percentage could be raised to improve NOx and smoke emissions. Even though SOI performed at a later timing could reduce NOx emission, smoke emission increased. Dwell angle between two successive injections could be prolonged to lower the effect of the increase in smoke emission. It could also be inferred that by setting a proper SOI timing and dwell angle under two-stage injection scheme when suitable biodiesel blend is used, the engine performance could be optimized.

1. Introduction Diesel engine is ubiquitously used as primary mover by virtue of its high efficiency. It has been used in many areas such as transportation, power generation, irrigation and construction equipment. However, the use of diesel fuel has posed two main problems – fossil fuel depletion and air pollution issues. The demand of liquid fuels is expected to increase in the near future as depletion problem may worsen. Besides, usage of diesel engine has caused emission of air pollutants. Nitrogen oxides (NOx) (nitric oxide and nitrogen dioxide), carbon monoxide (CO) and unburned hydrocarbons (HC) are some of the main components of air pollutants produced when diesel engine is used [1]. The air pollutants may cause various health problems, formation of acid rain, destruction of ozone layer, etc. Steps have to be taken to overcome the problems due to the usage of diesel engine.

Biodiesel fuel is one of the alternatives to solve the diesel depletion problem. It can be derived from plants and animals such as coconut, rapeseed, soybean, animal fats, etc [2]. It is nontoxic, renewable and biodegradable [3]. Emission characteristics of biodiesel fuel are different from conventional diesel. Combustion of biodiesel emits a lower level of CO, smoke and particulate matter concentration due to the higher oxygen content in it [4]. According to Shivakumar et al. [5], NOx emission of waste cooking oil biodiesel blends is higher than conventional diesel. However, the smoke emission of waste cooking oil biodiesel blends is less than conventional diesel. Ganapathy et al. [6] studied about the emission of Jatropha biodiesel. They found that Jatropha biodiesel always exhibits a higher NOx emission compared to conventional diesel. The smoke density observed when Jatropha biodiesel is used is lower than conventional diesel. Brake thermal efficiency (BTE) of Jatropha biodiesel is lower than conventional diesel.

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Corresponding authors at: School of Mechanical Engineering, Universiti Sains Malaysia, Engineering Campus, 14300 Nibong Tebal, Penang, Malaysia (Y.H. Teoh). Centre for Energy Sciences, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia (H.G. How and H.H. Masjuki). E-mail addresses: [email protected] (Y.H. Teoh), [email protected] (H.H. Masjuki), [email protected] (H.G. How), [email protected] (M.A. Kalam), [email protected] (K.H. Yu). https://doi.org/10.1016/j.fuel.2018.07.036 Received 7 April 2018; Received in revised form 4 July 2018; Accepted 7 July 2018 0016-2361/ © 2018 Published by Elsevier Ltd.

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obtaining a much cleaner exhaust emissions and engine performance. In this study, effect of different start of injection timing and dwell angle between injections in two-stage injection scheme on the performance and emission characteristics will be investigated and discussed. Start of injection timing is one of the factors which greatly affect diesel engine performance and emission characteristics. Shivakumar et al. [5] found that NOx emitted was reduced when waste cooking oil biodiesel blends were used when SOI is retarded while advanced SOI caused reduction in amount of smoke emitted. The same results were obtained by Sayin et al. [22] using C18.08H34.86O2 biodiesel and Ganapathy et al. [6] using Jatropha biodiesel. According to Jeon and Park [23], when pilot injection was applied, the retardation of SOI of pilot injection caused higher emission of NOx compared to single injection scheme. Adam et al. [24] used gasoline and soy methyl ester biodiesel in investigating engine performance and emission characteristics. They found that with retarded injection timing, gas temperature and HRR increased, leading to rise in NOx emission amount. Besides SOI, dwell angle is also an important factor. According to Cung et al. [25], when the dwell angle was reduced, the penetration rate was increased. However, a dwell angle which is too short will cause the soot to increase. With a longer dwell angle, the peak heat release rate (PHRR) and total heat released were increased. Rohani and Bae [26] found that longer dwell angle of about 30°CA produced a more premixed charge. In split injection with five times of injection, Mathivanan et al. [27] discovered that smoke level was the lowest when the dwell angle between the fourth injection and the last injection was the longest. Liu and Song [28] found that cylindrical pressure decreased with increasing dwell angle under constant fuel quantity in double injection scheme. When the post injection was retarded and the dwell angle was increased, NOx emission could be reduced. A shorter dwell angle causes limitation in oxidation of CO and hydrocarbon (HC) due to insufficient mixing time. Using Karanja biodiesel as fuel, Dhar and Agarwal [29] found that NOx emission decreased with retarding start of main injection timing for every start of pilot timing. However, at fixed start of main injection, NOx emission was almost constant with the change in start of pilot injection. Some studies about impacts of biodiesel blends, start of injection (SOI) timing and multiple injection dwell angle on efficiency and exhaust emission of diesel engine have been carried out as stated above. However, the study on the combination effect of these three factors is less. In fact, most of them have been carried out on single-cylinder research engine, which is not a practical representative of the production multi-cylinder engine adopted in commercial vehicles. This might be due to single-cylinder engine that essentially differs from multi-cylinder engine in term of rotational dynamics, gas intake dynamics, heat transfer dynamics, dynamic coupling between cylinders, and in other aspects [30]. For instance, the difference in gas intake dynamics arises especially in the case of turbocharged engines, which can significantly alter the in-cylinder intake air charge motion and momentum, thus impacting on in-cylinder emission production due to the differences in heat transfer. Consequently, there is a research gap existing in these disciplines. Hence, the topic will be explored in the present paper. In this study, effects of start of injection timing and dwell angle on diesel engine performance and characteristics emission will be investigated by using biodiesel blended fuels in multi-cylinder turbocharged diesel engine. Combination of different parameters will be carried out to understand the combination which can lead to optimum performance and emission characteristics of diesel engine.

Raheman and Ghadge [7] investigated the performance of diesel engine when Mahua biodiesel was used. They showed that the brake specific fuel consumption (BSFC) increased while the brake thermal efficiency (BTE) decreased when the percentage of biodiesel blend increased. The NOx and smoke emission trends are the same as what were observed when waste cooking oil biodiesel blend and Jatropha biodiesel were used. In order to improve the performance and emission characteristics of diesel engine when biodiesel is used to replace conventional diesel, a more advanced injection strategies have to be carried out. Multipleinjection strategy has been proposed as one of the methods to reduce the air pollutants emitted by diesel engine. Generally, two kinds of multiple-injection schemes which are pilot injection and split injection are applied in diesel engine. Pilot and split injections are distinguished by the injection quantity of the first injection. Usually, in pilot injection, a relatively small quantity of first injection fuel is introduced before the second main injection. While in split injection, the injection of fuel is divided into two or more equal portions, where each portion is injected consecutively but non-continuously. One of the purposes is to ensure that before main combustion occurs, a highly premixed fuel can be formed. The benefits of multiple-injections, which includes pilot and split injections, have been studied by many researchers before [8–14]. Jafarmadar and Nemati [15] applied the split injection scheme in an indirect injection diesel engine to study the exergetic performance using three-dimensional CFD code. The first injection pulse was altered from 75% to 90% and the duration of the dwelling angle was maintained at 20° crank angle. As the results showed, with longer first injection pulse, the heat loss, work, and fuel burn exergies as well as the exergy efficiency increased. Besides, Choi and Reitz [16] noted the NOx suppression effects with both late and split injection strategies when using biodiesel in a single-cylinder heavy-duty diesel engine. In the case of double injection strategies, the first injection pulse and dwell time were fixed at 50% and 0.97 ms for low load and at 61% and 1.18 ms for high load conditions. The authors found that these injection schemes reduced the burning rate during premixed combustion phase, thereby lowered in-cylinder combustion temperatures. Another study [17] used neat biodiesel from soybean oil for the study of the effect of injection timing and exhaust gas recirculation (EGR) rate on the combustion and emissions. The results showed that biodiesel exhibited lower NOx with split injection strategy at retarded SOI timing. Also, the BSFC was slightly increased with retarded SOI. The testing was carried out with small fuel quantity of the first injection compared to the second injection (main injection) on the Ford Lion V6 DI diesel engine. This injection technique was rather common but no in-depth studies were conducted on the effect of same fuel injection quantities for the first and second injection. Kim et al. [18] experimentally assessed split injection on engine performance, exhaust emissions and soot particulates on single-cylinder common-rail injection diesel engine fueled with neat biodiesel derived from soybean. The results indicated that the split injection reduced NOx emissions remarkably without a significant increase in soot emissions. The benefits of split injection for NOx reduction were further affirmed by Stringer and co-worker [19] using an optical access research engine. Additionally, post injection applied during split injection serves to reduce the amount of smoke, particulate matter and unburned hydrocarbon [20]. The parameters in split injection can be changed and improved to produce a better effect on the performance and emissions characteristics of diesel engine. These parameters include injection timing, dwell angle between injections, mass injected, mass ratio of injections, number of injections, injection pressure and others. Recently, the authors of this paper [21] examined the effects of combining biodiesel blended fuels and split injection (i.e. single, double and triple injection) in a four-cylinder common-rail diesel engine at various SOI and reported simultaneous decrement of NOx and smoke emissions with a drop in performance. Besides, the outcomes also suggested that the dwell angle between consecutive injections for double injection scheme could be further optimized for

2. Experimental apparatus and procedure 2.1. Apparatus setup Table 1 shows the specifications of the engine used in this experiment. The experimental setup was the same as that described in [31]. Four-stroke diesel engine with turbocharger was used instead of single228

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injection parameters and engine parameters. By employing a closedloop feedback of engine speed via incremental encoder, the period of injection could be determined for ECM to send appropriate signal to injector. In this way, the angular velocity of engine could be kept constant. Disturbance and steady state error which occurs during the experiment could be alleviated by implementing proportional-integral control loop. To avoid injector from being heated up, which would subsequently affect the fuel temperature and engine performance, peak and hold method was applied to trigger the solenoid type fuel injectors. Aside from combustion characteristics, engine performance and exhaust emission were also investigated in this experiment. In order to quantify the efficiency of engine, the quantity of fuel used had to be determined. Kobold fuel flow meter was used to measure the quantity of fuel consumed during the combustion. On the other hand, exhaust emission could be studied by using Bosch BEA 350 gas analyzer, AVL DICOM 4000 gas analyzer and smoke opacity meter. K-type thermocouples was employed to measure the temperature of surrounding air, lubricant oil, exhaust gas and engine coolant.

Table 1 Key information and specifications of test engine. Type of engine Injection system Cylinder number Valve number per cylinder Bore diameter Stroke length Length of connecting rod Compression ratio Displacement Maximum torque Maximum power output

Four-cycle, turbocharged, direct injection diesel engine High-pressure (over 140 MPa) common-rail 4 2 76.0 mm 80.5 mm 135 mm 18.25:1 1461 cm3 160 Nm at 2000 rpm 48 kW at 4000 rpm

cylinder diesel engine due to its more common usage in real world. By using Delphi common–rail fuel injection system, it was possible to control the injection parameter electronically to manipulate the combustion condition. High pressure direct fuel injection was able to be performed. Other devices were also connected to test engine or interconnected with each other to implement corresponding function according to Fig. 1. Eddy current dynamometer with absorption power of 150 kW was connected to the engine shaft to provide constant load and engine angular velocity. The dynamometer was controlled by a dynamometer controller while hot-wire air mass sensor was used to measure the mass flow rate of air which entered the engine. Pressure developed in cylinder during combustion was monitored by using Kistler 6058A piezoelectric sensor. The pressure sensor was mounted on engine by using glow plug adapter while charge amplifier, DAQ-Charge-B was utilized to amplify the signal from pressure sensor. Based on the pressure data obtained, the corresponding HRR and temperature values could be calculated using law of thermodynamics. Incremental encoder was also used so that at every interval of 0.125°CA, the in-cylinder gas pressure reading could be taken. The engine was started and run steadily for a certain amount of time, before the taking of pressure readings began. The pressure data of 100 consecutive cycles was taken and the average pressure of each data point was obtained. The signals produced by the encoder and pressure sensor were feed into a high-speed data acquisition system. Other lower speed data acquisition system and engine control module (ECM) were inhouse built by utilizing Arduino microcontrollers. Three I/O pins on Arduino were connected to incremental encoder and camshaft to act as receiver. Arduino was linked to a personal computer, where a graphic user interface (GUI) developed using LabVIEW was employed to set the

2.2. Experimental methods and procedures The engine operating conditions were fixed as shown in Table 2 throughout this experiment. Load which was lower than the maximum value was applied by dynamometer to the engine as vehicles normally work under part load condition. It could be seen that two-stage injection strategy was carried out for all test cases. The mass ratio of first injection to second injection was 50:50 as depicted in Fig. 2, where the quantity of fuel injected during first injection was same as that of second injection. Two-stage injection strategy was investigated due to its advantage in improving the exhaust emissions. By dividing fuel to be introduced into cylinder via two injections, the maximum temperature achieved can be reduced owing to the longer combustion period and better fuel air mixture can be formed. First injection combustion can create a suitable environment with higher temperature for the combustion of second injection to occur effectively. Late injection combustion serves to promote complete reaction between fuel and air. According to Table 3, test cases examined in this research are designed by manipulating biodiesel blending ratios, SOI timings and dwell angles. Different fuel types contain different biodiesel composition and this will greatly affect the exhaust emission. On the other hand, change in SOI timings cause the start of combustion to happen at different position with corresponding condition and the ignition delay to be lengthened or shortened. Dwell angles affect the combustion

Fig. 1. Apparatus setup schematic diagram. 229

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Table 2 Engine operating condition.

Table 3 Test cases.

Fixed parameter

Value

Variable parameter

Level

Engine angular velocity Torque Fuel injection strategy Mass ratio (first: second)

2000 rpm 60 Nm Two-stage 50:50

Types of fuel First injection SOI timings (°ATDC) Dwell angles (°CA)

Baseline diesel, B20, and B50 −12, −10, −8, −6, −4, −2, 0, 2 12, 15, 18

shown in Table 5. It was observed that calorific value decreased with increasing percentage of biodiesel blends. Low calorific value fuel was not preferable since more fuel will be consumed to produce same amount of energy. Neat COB also had a higher density compared to conventional diesel. When the biodiesel percentage rises, the flash point was observed to be elevated and favorable for the usage in engine. Besides, blends with higher percentage of biodiesel were more likely to be subjected to oxidation.

2.4. Statistical and equipment uncertainty analysis Fig. 2. Timing chart at three different dwell angle and with first SOI of -6°ATDC.

Research measurements always subject to uncertainty, which must be considered when analytical results are used as part of a basis for comparing experimental numbers. All experimental measured result reported should be accompanied by an explicit uncertainty estimate. For the present study, the summary of the equipment used and the corresponding measurement range and accuracy of the instruments is tabulated in Table 6. Uncertainty evaluation is crucial to verify the accuracy of the experiments. Hence, the percentage uncertainties of brake specific fuel consumption (BSFC) and brake thermal efficiency (BTE) were determined based on the percentage uncertainties of the instruments used in the experiments. The uncertainty analysis was performed using the method described by [21,34]. The overall experimental uncertainty was determined using the following equation:

characteristics and engine performance in about the same way as SOI timing where the position of start of combustion is change. Hence, the study of simultaneous effects of these three parameters had to be done to optimize the engine performance. A test case consisted of a combination of three levels, where each level originated from corresponding variable parameter was carried out. All possible combinations were investigated in this experiment. First, combination of baseline diesel and dwell angle of 12°CA was fixed in studying the effect of variation in SOI timings. Then, dwell angle was changed and the steps were re-

Overall experimental uncertainty=Square root of [(uncertainty of Fuel Flow Rate)2+ (uncertainty of BSFC)2 + (uncertainty of BTE)2 + (uncertainty of NOx )2 + (uncertainty of Smoke)2 + (uncertainty of pressure sensor)2 + (uncertainty of crank angle encoder)2] = Square root of [(0.5)2 + (1.5)2 + (1.7)2 + (1.3)2 + (1)2 + (0.5)2

peated. When all dwell angles and SOI timings have been tested using baseline diesel, the entire procedures were repeated using B20 and B50 biodiesel respectively.

+ (0.03)2] = ± 2.9%

2.3. Biodiesel property test

3. Results and discussion

In this experiment, the biodiesel used was produced in laboratory rather than purchased in market in which the biodiesel does not have standard composition. Transesterification was carried out to convert triglycerides in coconut oil into methyl ester so that viscosity of coconut oil could be reduced to make it a suitable fuel to be used in diesel engine. After the process was implemented, various tests were conducted to obtain neat coconut oil biodiesel (COB) properties. The neat COB produced had to fulfill ASTM standard. The properties of neat COB, ASTM D6751 standard and baseline diesel properties were listed in Table 4. It can be seen that all the properties of neat COB satisfied the standard employed in this experiment. Distillation characteristics can be discovered from Table 4. Furthermore, the physicochemical properties of the baseline diesel fuels were also measured and compared with those reported in literature. It appeared that most of the physicochemical properties of baseline diesels were in good agreement with the results from literature. To optimize the properties of biodiesel, neat COB had to be blended with petroleum diesel. In this experiment, B20 blend (80:20 petroleum diesel-biodiesel ratio) and B50 blend (50:50 petroleum diesel-biodiesel ratio) were suggested and prepared. The properties of petroleum diesel, B20 blend, B50 blend and neat COB were obtained by carrying out ASTM and EN ISO test methods, as

3.1. Performance characteristics The data obtained had been plotted in graph form. The performance and emission characteristics variations of diesel engine with the percentage of biodiesel blend, start of injection timing (SOI) and dwell angle change are analyzed based on the graphs plotted. In this section, the effects of the stated engine parameters on brake thermal efficiency (BTE) and brake specific fuel consumption (BSFC) will be examined. Fig. 3 shows the BTE and BSFC of diesel engine when the parameters as shown were varied. It can be seen that BTE of biodiesel is always lower than diesel engine at any SOI timing and dwell angle. By focussing on test case with SOI timing of -6°ATDC and dwell angle of 12°CA, comparison can be made where the BTE values were 30.6%, 30.5% and 30.4% for petroleum diesel, B20 and B50 blends respectively. Moreover, the results suggest that SOI timing is one of the key factors which affect BTE. When SOI timing is advanced, the BTE of all type of fuel improves. This is discovered for different dwell angles. This is because when the SOI is advanced, the ignition delay of the first injection will increase, causing more homogeneous mixture to form. This leads to a more complete combustion. Besides, according to Fig. 6, the peak heat release rate (PHRR) occurred nearest to top dead center (TDC) at the 230

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Table 4 Chemical and physical properties of baseline diesel and neat COB. Chemical/Physical Properties

Units

Kinematic viscosity at temperature 40 °C Flash point Density at 40 °C Pour point Cloud point Acid value Cold filter plugging point Calorific/Heating value Oxidation stability Carbon content Hydrogen content Nitrogen content Oxygen content Water content Copper Strip Corrosion (3 h @ 50 °C) Distillation: Initial boiling point 5% recovery 10% recovery 20% recovery 30% recovery 40% recovery 50% recovery 60% recovery 70% recovery 80% recovery 90% recovery Final boiling point

2

mm /s °C kg/m3 °C °C mg KOH/g °C MJ/kg H %wt %wt %wt %wt %wt – °C

Standards

COB

Biodiesel Limit (ASTM D6751)

Diesel

Diesel [32,33]

ASTM D445 ASTM D93 ASTM D1298 ASTM D97 ASTM D2500 ASTM D664 ASTM D6371 ASTM D240 EN ISO 14112 ASTM D 5291

4.02 145.5 856.0 3 4 0.05 7 39.92 15.8 73.2 12.5 < 0.1 14.3 0.021 1a

1.9–6.0 130 min – Not specified Not specified 0.5 max Not specified – 3 min – – – – < 500 ppm 1 Distillation temperature, 90% recovered (T90) = 360 °C max

2.99 71.5 825.6 0 3 – 5 45.21 > 100.0 86.1 13.8 < 0.1 0.1 0.012 1a

2.91 71.5 839.0 1.0 2.0 0.17 −8.0 45.825 23.70 88.5 13.5 – 0.0 0.0038 1a

165.5 220 240 262.5 276.5 288.5 298.5 309 320 333 351 374

182.7 212.4 225.5 246.1 261.0 274.6 286.9 299.4 312.7 327.8 346.1 366.0

Calculation EN ISO 12937 ASTM D 130 D86

192 240 249 260 269 276 284 294 312 321 324 324

be less. Another possible explanation is that at retarded crank angle, cylinder expansion cooling effect may take place. The volume of cylinder will increase quickly where the increment rate of pressure will be lower. Effective pressure which is lower can be achieved and this produces a lower power output [36]. On the other hand, by zeroing in on test case with SOI of -6°ATDC and dwell angle of 12°CA, it can be seen that increasing percentage of biodiesel blend reduced the BTE level. This phenomenon is observed in test case with different SOI timings and dwell angles and the trend is also reported by other researchers [5,7,37]. One of the reasons may be that the properties of biodiesel was different from baseline diesel. Diesel engine used was not specifically tailored to be operated with biodiesel. Furthermore, oxygen content of biodiesel blends was higher than that of pure diesel. This might affect biodiesel blends in such a way that the fuel’s calorific value will be lower compared to that of pure diesel as indicated in Table 5. Lower part of Fig. 3 depicts the variation of BSFC with changing engine parameters. When B20 or B50 biodiesel blend was used to run diesel engine, the BSFC value obtained was invariably larger than that of petroleum diesel across every SOI timing and dwell angle. This phenomenon is aligned with Atul Dhar et al. [29], H. Raheman et al. [7], Cenk Sayin et al. [22] and T. Ganapathy [6]. When higher BSFC is observed, more fuel is necessary to maintain the same power output.

Table 5 Notable chemical and physical properties of baseline diesel, COB, B20 and B50. Physical/Chemical Properties

Units

Diesel

COB

B20

B50

Test method

Calorific/Heating Value Density at 40 °C Flash Point Kinematic viscosity at 40 °C Oxidation Stability

MJ/kg

45.21

39.92

44.60

42.79

ASTM D240

kg/m3 °C mm2/s

825.6 71.5 2.985

856.0 145.5 4.02

831.3 74 3.204

840.1 80.5 3.491

ASTM D1298 ASTM D93 ASTM D445

h

> 100.0

15.8

89.48

51.44

EN ISO14112

most advanced SOI. This will produce higher effective pressure to perform greater useful work [35]. Nevertheless, for the corresponding SOI timing, slight fall of BTE level occurred with a value of 0.1% and 1.6% for the case of dwell angles of 15°CA and 18°CA, respectively, in comparison with that of baseline diesel at 12°CA dwell angle. Peak pressure might decrease when dwell angle is increased where the useful work done on the piston drops [20,28]. Besides, heat loss might increase with longer combustion process for dwell angles of 15°CA and 18°CA. The energy which is converted into useful mechanical work may

Table 6 Summary of measurement range, accuracy and percentage uncertainties. Measurement

Measurement range

Accuracy

Measurement techniques

% Uncertainty

Load Speed Time Fuel flow measurement NOx Smoke Pressure sensor Crank angle encoder

± 600 Nm 0–10,000 rpm – 0.5–36 L/h 0–5000 ppm 0–100% 0–25,000 kPa 0–12,000 rpm

± 0.1 Nm ± 1 rpm ± 0.1 s ± 0.04 L/h ± 1 ppm ± 0.1% ± 10 kPa ± 0.125°

Strain gauge type load cell Magnetic pick up type – Positive displacement gear wheel flow meter Electrochemical Photodiode detector Piezoelectric crystal type Incremental optical encoder

± 0.25 ± 0.1 ± 0.2 ± 0.5 ± 1.3 ±1 ± 0.5 ± 0.03

Computed BSFC BTE

– –

± 5 g/kWh ± 0.5%

– –

± 1.5 ± 1.7

231

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Fig. 3. BTE and BSFC data for different biodiesel blends, SOI timings and dwell angles.

reduce the NOx released across every SOI timing and dwell angle. Besides, the B20 and B50 biodiesel blends have a lower calorific value and greater cetane number in comparison with that of baseline diesel. As a result, lower heat release rate (HRR) and peak mean gas temperature (PMGT) may develop during premix combustion stage. This finding can be explained by observing the peak mean gas temperature (PMGT) data tabulated in Fig. 6. Moreover, a considerably low level of NOx near 110 ppm was achievable via late SOI timing for fuel operations conducted using B20 or B50 biodiesel blends with dwell angle 18°CA. The increase in dwell angle caused slight decrease in NOx emission for all fuels and SOI cases. The observation is compatible with the research results of B. Yang et al. [41], Wenyi Liu et al. [28], Atul Dhar et al. [29], S. Kevin Chen [39] and Donghui Qi et al. [17]. With retarded injection, the combustion phase occurs late and may be located in expansion stroke, away from TDC. This phenomenon results in decreased temperature of combustion and less NOx formation. Besides, the concentration of oxygen is lower at retarded crank angle. Less oxygen atoms were available to react with nitrogen atoms to form NOx molecules. In addition, according to M. Pontoppidan et al. [42], the rise of temperature with respect to single injection containing both first and second injection, occurs at a higher level, if the first injection is kept close to the second injection. However, if the second injection is performed further from first injection, the total duration for the whole process to occur will increase. The injected mass fraction during each injection will be reduced, leading to a leaner mixture burnt at a lower temperature. As a result, NOx emission amount will decrease. According to G.M. Bianchi et al. [43], when dwell angle between two injections is reduced, the mass of fuel burnt at high temperature will increase, causing the NOx emission amount to increase. Smoke formation happens due to ineffective combustion of hydrocarbon and partial reaction of carbon content in diesel. The smoke amount released for test cases with different fuels at various SOI timings and dwell angles is displayed in Fig. 4. Overall, it can be observed that amount of smoke emitted was lower when B20 or B50 biodiesel blend is employed at different SOI timings. Ji Zhang et al. [44] and Shivakumar et al. [5] discovered that the amount of soot which leads to smoke of biodiesel is less than baseline diesel. According to Joonho Jeon et al. [23], even though soot particles are formed at high production rate when biodiesel is burnt, the soot is eliminated by the active oxidation process. Lower smoke level of combustion of biodiesel is attributed to more oxygen, less carbon content and non-existing or low quantity of aromatic compound in biodiesel [45,46]. Moreover, when SOI is retarded, the smoke emission amount generally increased for all fuels across all dwell angle. This

This is not surprising in view of the lower calorific value of B20 and B50 biodiesel blends in comparison with that of baseline diesel, which is approximately 1.4% and 5.4%, respectively lower. In addition, changes in injection timing may influence the BSFC profoundly. Advancing SOI timing from crank angle of 2°ATDC to -12°ATDC occasioned the reduction in BSFC when different fuels were tested. This is in accordance with results obtained by M. Badami et al. [38]. According to S. Kevin Chen [39], by performing the first injection earlier, it is possible to reduce BSFC. The occurrence may be explained by the fact that when SOI is carried out earlier, there is a constant refinement in the effectiveness and quality of combustion. With the same magnitude of brake power output, the decreasing BSFC implies that less fuel was supplied to enable a more effective combustion process to occur. Besides, for most of the SOI timings, the BSFC elevated with the larger dwell angles for every type of fuel. These outcomes are congruous with other researchers’ observations [17,38–40]. At retarded SOI timing of second injection due to large dwell angle, turbulence effect may be low. The less enhanced mixing of air and fuel might result in incomplete combustion where energy produced might be lower. Besides, at retarded crank angle, expansion cooling effect may take place and result in lower rate of combustion. Another reasonable explanation is that the longer combustion period might have increased the amount of heat loss when dwell angle is increased. As a consequence, BSFC had to be greater to generate more energy. 3.2. Emissions characteristics Impacts of percentage of biodiesel blends, SOI timing and dwell angle on NOx and smoke emissions are analyzed in the section below. The NOx amount released when different test fuels were utilized at varying SOI timings and dwell angles is delineated in Fig. 4. The graph shows that advancing SOI timing gave rise in NOx level for every test fuel and dwell angle. Research by Atul Dhar et al. [29] also showed that NOx emission amount increased with advancing injection timing. The increasing pattern in NOx indicates that with SOI implemented earlier, the mixture of air and fuel can be ignited and burnt in advance, therefore causing early formation of pressure peak near TDC. This brings a greater combustion temperature and aids in thermal or Zeldovich NOx formation. Moreover, according to Wenyi Liu et al. [28], when late second injection was carried out due to retarded SOI, the cylinder volume expansion and heat transfer reduced the temperature, causing NOx emission amount to decrease. The outcome also pointed out that both of the B20 and B50 biodiesel blends showed a tendency to 232

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Fig. 4. NOx and smoke emission data for different biodiesel blends, SOI timings and dwell angles.

result is compatible with findings of Shivakumar et al. [5], T. Ganapathy et al. [6], S. Saravanan et al. [47] and Takeshi Hashizume et al. [48]. Advanced SOI timing may increase in-cylinder gas temperatures. This enhances the chemical reaction between fuel and oxygen, subsequently giving rise to less smoke emitted. A second possible reason is the fact that sufficient time may be available for the evaporation and mixing of fuel with air to happen, resulting in more effective mixing and combustion. In addition, when SOI of first injection was retarded, the injection was performed nearer to TDC where the piston wall was closer to the injector. The fuel injected may be impinged on the piston wall easily, causing the quenching of fuel by wall to occur [42]. This reduces the temperature of fuel and cause incomplete combustion. When SOI is more than -2°ATDC onwards, the smoke emission amount exhibited a decreasing trend for most of the SOI and fuels when dwell angle was increased. The observation is in accordance with Cheolwoong Park et al. [20], Takeshi Hashizume et al. [48] and Khanh Cung et al. [25] research. When the dwell angle between two injections is small, the ignition delay for second injection will be short. The diesel injected during second injection may not have enough time to mix with the air. Besides, the diesel might be injected directly at the flame produced via first injection. The combustion during first injection may be lack of oxygen because the diesel from second injection entrains the burnt gas of first injection. This causes low heat release rate and temperature, resulting in large amount of smoke released. When the injection strategy employing B50 biodiesel blends and 18°CA dwell angle was conducted, smoke emission amount decreased while showing a reduction in NOx at the same time. The results indicate that with retardation of SOI, a low NOx emission of 110 ppm was achieved with smoke emission level still remained below 6%. Hence, simultaneous NOx and smoke amount decrement compared to that of petroleum diesel is viable with the application of B50 biodiesel blend and implementation of retarded SOI timing and larger dwell angle of 18°CA in the test engine.

Fig. 5. Combustion pressure, HRR and injector current data for test case employing baseline diesel with different dwell angles at -6°ATDC SOI.

two similar injection pulses. Due to longer combustion period, heat loss when longer dwell angle was applied was more and greater amount of fuel was injected to compensate the energy loss. This resulted in longer injection current and injector activation timing. It can be observed that difference in dwell angles affects the combustion characteristics. The pressure peak increased with increasing dwell angles. After the peak pressure occurs, the rate of decrease in pressure was higher with longer dwell angle, where the pressure when dwell angle was set as 12°CA was the highest after 15°ATDC. The starts of combustion (SOC) of all test cases conducted happened at the same crank angle. One of the factors which influences SOC timing is the quantity of fuel injected. If a large amount of fuel is introduced, a longer air-fuel mixing time might be required and this will cause a retarded SOC timing. Since the amount of fuel injected were similar for every test case implemented, the SOC occurred at the same crank angle. Two noteworthy HRR peaks can be noticed for every dwell angle. First peaks of HRR of different test cases were formed at almost the same crank angle due to the equal amount of fuel injected at the same SOI timing. However, the second peaks of HRR were located at different crank angles when different dwell angles were applied. The shorter the dwell angles, the more advanced the second peak of HRR was. Ignition delay_1 is longer than ignition delay_2 when the dwell angle was set as 12°CA and 15°CA. This is due to the high cylinder temperature when second injection is performed. The injection of fuel towards main combustion zone which formed during the first

3.3. Combustion characteristics Using pressure sensor, pressure developed in cylinder can be obtained and recorded for 100 successive cycles. The average value of pressure was calculated for each crank angle. Subsequently HRR was evaluated from pressure value. Fig. 5 depicts the combustion pressure curve, HRR curve and profile of injector current of test engine run using petroleum diesel at -6°ATDC SOI timing with varying dwell angles. It is clearly shown by the Fig. 5 that every test case investigated involved 233

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Fig. 6. Peak mean gas temperature (PMGT) and peak heat release rate (PHRR) for different fuels, SOI timings and dwell angles.

injection causes the fuel to burn immediately. Comparing ignition delay_2 of test cases with different dwell angle, it is observed that the greater the dwell angle, the longer the ignition delay_2. This is attributable to the lower cylinder temperature achieved after passing a longer dwell angle. Larger heat loss and expansion cooling effect may have taken place where low temperature results in longer time required to heat up the fuel. Influences of percentage of biodiesel blend, SOI timing and dwell angle on peak mean gas temperature (PMGT) and peak heat release rate (PHRR) are examined in section below. Fig. 6 shows the changes in PMGT with various SOI timings and dwell angles of test engine fuelled with baseline diesel, B20 and B50 biodiesel blends. There was no consistent trend observed with increasing percentage of biodiesel blends. However, the variation in SOI timing did give rise to considerably constant pattern. For every type of fuel and injection dwell angle tested, it was discovered that initially PMGT decreased with retarding SOI timing until a minimum PMGT value was reached around SOI timing of -6°ATDC. Then, with SOI performed later, PMGT rose. The initial decrement in PMGT value can be attributed to the lower peak pressure developed nearer to TDC when SOI timing was retarded based on Fig. 7. Due to the low peak pressure, PMGT achieved will be lower. With retarding SOI timing from -12°ATDC to -6°ATDC, the peak pressure may occur at late crank angle and decrease on account of the cylinder expansion, causing a decrease in temperature. The elevation of PMGT with retarding SOI timing when SOI is perform late at the range of -6°ATDC to 2°ATDC can be explained by observing BSFC trend from Fig. 3 and HRR curve from Fig. 7. With late SOI timing, combustion occurs at crank angle when the volume of cylinder is large and rate of expansion is rapid. In order to develop ample pressure to produce enough effective work done to maintain equal power output, temperature achieved in cylinder has to be high. Combustion of larger amount of fuel is evident by observing Fig. 3 where BSFC increased with retarding SOI timing. This may lead to a higher HRR peak as delineated in Fig. 7, which subsequently resulted in higher PMGT. On the other hand, it can be seen that when dwell angle is fixed as 18°CA, the PMGT values attained for most of the test cases associated were lower than their counterparts when dwell angle was 12°CA or 15°CA. This may indicate that longer dwell angle will cause a decrease in PMGT developed. The same observation was reported by Takeshi Hashizume et al. [48] too. When dwell angle is increased, combustion will continue for a longer duration. The fuel burnt per unit time will reduce, where the rate of rising of temperature will be lower. Expansion cooling effect also further promotes the decrement in cylinder temperature. As a result,

Fig. 7. In-cylinder gas pressure and HRR curves for dwell angle of a) 12°CA, b) 15°CA, and c) 18°CA under different SOI timings using baseline diesel.

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PMGT achieved will be lower. Peak HRR (PHRR) is also one of the noteworthy variables associated with combustion characteristics. From Fig. 6, it is delineated that B20 and B50 biodiesel blends exhibit PHRR which was constantly less than that of pure diesel when different SOI timings and dwell angles were tested. The trend was observed by Seung HyunYoon et al. [49], Ji Zhang et al. [44] and T. Ganapathy et al. [6] in their studies. The trend occurs primarily owing to the greater cetane number of biodiesel in comparison with that of petroleum diesel, thus leading to shorter ignition delay and smaller PHRR, as described in section before. When SOI is retarded, PHRR during the premixed combustion phase may increase when different biodiesel blends are used at any dwell angle. This observation is compatible with Cheolwoong Park et al.’s [20] and S. Kevin Chen’s [39] findings where heat release increases when pilot injection is performed nearer to TDC. The combustion phase may occur at a more retarded crank angle which is further away from TDC when injection is performed at a later timing. The pressure produced will then be lower due to cylinder volume expansion process. The useful work done on the piston will be lower. To compensate the decrease in work done, BSFC increases as higher amount of fuel is injected according to Fig. 3. As a result, the combustion will continue for a longer time where HRR can rise to a higher value before the fuel is burnt out. Also, it can be seen that PHRR increased slightly when dwell angle is increased from dwell angle of 12°CA to 15°CA. This trend happened for most of the SOI applied. At a dwell angle equals to 12°CA, the interval between two injections may be too short [20]. The second injection might be introduced to the flame produced by the combustion of first injection. The fuel injected during second injection will entrain the burnt gas, reducing the amount of oxygen required for combustion. As a result, PHRR achieved when dwell angle was fixed as 12°CA was lower. When dwell angle was increased from 15°CA to 18°CA, PHRR decreased slightly at most of the SOI tested. Combustion duration may increase with increasing dwell angle. Amount of fuel burnt per unit time might decrease, causing PHRR to drop. Dwell angle is one of the parameters which can be adjusted to improve the NOx and smoke emission amount. Fig. 7 displays the data of combustion pressure and HRR for dwell angles of 12°CA, 15°CA and 18°CA at different SOI timings when baseline diesel is utilized. The influences of dwell angle on in-cylinder gas pressure and HRR are analyzed here by directly comparing outcomes of corresponding SOI timings under dwell angle subgroups. The comparison enables one to point out the differences in the combustion events that are responsible for results observed in NOx and smoke emissions. The HRR data collected signified that the two combustion events began to deviate as the first fuel injection pulse of double injection had been stopped after a premixed combustion of about the similar intensity, which can be evidently observed in Fig. 5. Combustion process which occurred after the premixed combustion phase during the duration between SOI_1 and SOI_2 featured a high initial in-cylinder gas temperature by virtue of the burning of a huge amount of fuel injected during the first injection. For this reason, the second injection fuel was combusted in the form of diffusion flame. Only a very short ignition delay was available for the mixing process of fuel and hot gases to transpire. The successive fuel injection led to another lower HRR peak and rise in flame temperature, which happened for approximately 20° CA in combustion cycle. The occurrence enabled lower quantity of NOx to be produced. Thus, further optimization of multiple injection scheme in the context of NOx and smoke emissions amount was achieved by employing a larger dwell angle. From Fig. 7, it is observed that peak pressure increased with advancing SOI when baseline diesel was used. This observation was obtained for different dwell angles. Atul Dhar et al. [29], T. Ganapathy et al. [6], Joonho Jeon et al. [23] and Hongyuan Wei et al. [50] discovered the same trend in their research. According to heat release rate curve, when SOI was advanced, the start of combustion happened nearer to TDC. This produces a higher pressure at TDC and creates a

Fig. 8. In-cylinder gas pressure and HRR curves for dwell angle of a) 12°CA, b) 15°CA, and c) 18°CA using different fuels at SOI of -6°ATDC.

larger useful work done. Retarding SOI may cause the combustion to occur at a later timing, where cylinder volume expansion happens. The peak pressure achieved will be lower and the useful work done will be reduced. Besides, when SOI is advanced, a longer ignition delay will be available before the pressure and temperature is high enough to initiate combustion [4]. The air-fuel mixing time will be longer to enable a more complete combustion to occur and thus leads to a higher temperature and pressure. Therefore, advancing SOI can cause the peak pressure and PHRR to occur at an earlier crank angle [50]. Referring to Fig. 8, when dwell angle was set as 12°CA and SOI was fixed as -6°ATDC, the start of combustion was advanced when the percentage of biodiesel blends increased. On dwell angle equal to 15°CA and 18°CA, the same trend was observed. The result is compatible with Ji Zhang et al. [44], T. Ganapathy et al. [6] and Seung Hyun Yoon et al. [49] research finding. This is because biodiesel has a higher cetane number, bulk modulus, sound velocity and viscosity [46,51–53]. Its ignition delay is shorter, causing it to burn earlier. However, PHRR decreases with increasing percentage of biodiesel blends [49]. The ignition delay of biodiesel is shorter, resulting in less air-fuel mixing time. The combustion of the biodiesel will be less intense compared to baseline diesel, thus producing a lower PHRR. Besides, the calorific value of biodiesel is also less than baseline diesel [44]. With same 235

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explains the lower peak pressure achieved when shorter dwell angle was set. In addition, it is found that PHRR of second injection combustion was higher when dwell angle of 12°CA was applied compared to dwell angle of 15°CA and 18°CA. Smaller dwell angle may shorten the combustion duration, causing the same amount of fuel to be burnt at a shorter time interval, hence increasing HRR of second peak. When dwell angle increases, the second injection will be performed at a retarded timing further away from TDC. Expansion of cylinder volume will cause reduction in pressure and gas temperature, therefore lowering the rate of combustion of second injection diesel. Besides, a longer dwell angle may increase the heat loss during the entire period of combustion process [29]. Moreover, the direct comparison of various dwell angles under SOI timing of -6°ATDC also showed slight difference in the peak pressure and second PHRR. Combustion phasing is one of the factors which responsible for diesel engine smoke emissions. The diesel combustion with longer dwell angle can be performed to enable a longer ignition delay for better air-fuel mixing. This enables a sufficient oxidation of air-fuel mixture to happen. The favorable impact of soot oxidation in the last combustion phase could be noticed when smoke emission becomes lower, as what is displayed in Fig. 4 when longer dwell angle of 18°CA was applied compared to dwell angle of 15°CA and 12°CA. 4. Conclusion The engine operating parameters such as biodiesel blend ratios, SOI timing and dwell angle have been investigated in this paper. Different combinations of the parameters have been implemented to understand their impacts on the characteristics of combustion. Via this study, the optimum conditions where the pollutant emissions highlighted are lessened simultaneously have been inferred. Conclusions on the optimized injection strategies are made as follow: 1. Biodiesel blend ratio and SOI timing affect exhaust emissions, engine performance and combustion characteristics significantly while dwell angle causes a slight variation in the outcome studied. 2. Increase in percentage of biodiesel blends causes reduction in NOx and smoke emission amount. 3. Retarded SOI timing improves NOx emission but this occurs in the expense of increasing smoke emission. 4. By retarding SOI timing in conjunction with long dwell angle, it is possible to reduce NOx emission without causing much increase in smoke emission.

Fig. 9. In-cylinder gas pressure and HRR curves using a) baseline diesel, b) B20, and c) B50 with different dwell angles at SOI of -6°ATDC.

amount of fuel burnt, the heat released by biodiesel will be lesser. When PHRR is lower, the heat released will be lower. Thus, the peak pressure developed by burning biodiesel at any dwell angle was smaller, as shown in Fig. 8 [49]. It is observed that the start of combustion of second injection was slightly advanced for baseline diesel compared to biodiesel even though the SOI of second injection was fixed at 6°ATDC when dwell angle was 12°CA. This can be due to the higher PHRR achieved during the first injection by using baseline diesel. The gas temperature after the combustion of the first injection of baseline diesel is higher. This condition reduces the ignition delay of second injection, leading to an advanced start of combustion of baseline diesel. Referring to Fig. 9, when baseline diesel was used at SOI equals -6°ATDC, the heat release rate when dwell angle was 12°CA dropped at a higher rate after the first PHRR was reached compared to dwell angles of 15°CA and 18°CA. This happened to B20 and B50 biodiesel as well. When a shorter dwell angle is used, the second injection will be introduced to the flame produced by the combustion of first injection. The gas temperature and the oxygen amount available for combustion decreases due to the second injection diesel vaporization. As a result, the second injection which is close to the combustion of first injection will interrupt with the combustion and causes the rapid drop in heat release rate [38]. The earlier reduction in heat release rate will also cause the peak pressure to decrease due to less energy produced near TDC. This

Acknowledgments The authors would like to acknowledge the Ministry of Higher Education (MOHE) of Malaysia, Universiti Malaya, KDU Penang University College Internal Research Grant and Universiti Sains Malaysia (BRIDGING research grant scheme-304/PMEKANIK/ 6316119) for financial support towards this research project. References [1] Heywood J. Internal combustion engine fundamentals. McGraw-Hill Education; 1988. [2] Demirbas A. Importance of biodiesel as transportation fuel. Energy Policy 2007;35(9):4661–70. [3] Ma F, Hanna MA. Biodiesel production: a review1Journal Series #12109, Agricultural Research Division, Institute of Agriculture and Natural Resources, University of Nebraska-Lincoln. 1. Bioresour Technol 1999;70(1):1–15. [4] Mohamed Shameer P, et al. Effects of fuel injection parameters on emission characteristics of diesel engines operating on various biodiesel: a review. Renew Sustain Energy Rev 2017;67:1267–81. [5] Shivakumar, Srinivasa Pai P, Shrinivasa Rao BR. Artificial Neural Network based prediction of performance and emission characteristics of a variable compression ratio CI engine using WCO as a biodiesel at different injection timings. Appl Energy 2011;88(7):2344–54. [6] Ganapathy T, Gakkhar RP, Murugesan K. Influence of injection timing on

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