A study on diesel engine performance using biodiesel from waste cooking oil (WCO)

PERPUSTAKAAN UMP

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00001 00320

A STUDY ON DIESEL ENGINE PERFORMANCE USING BIODIESEL

FROM WASTE COOKING OIL (WCO)

NUR ATIQAH BINTI RAMLAN

Thesis submitted in fulfillment of the requirements

for the award of the degree of

Master of Engineering in Automotive

Faculty of Mechanical Engineering

UNIVERSITI MALAYSIA PAHANG

ABSTRACT

The energy consumption, especially in the transport sector, has increased tremendously over decades. The extensive usage has led to an increased demand for petroleum, such as gasoline and diesel fuel, hence causing the depletion of fuel stocks. As concerns mount for long-term energy conservation, it becomes necessary to develop alternative fuels that have properties comparable with diesel fuel. Biodiesel from waste cooking oil (WCO) should be highlighted as a potentially economical alternative fuel. Therefore, the objective of this research is to investigate the engine performance as well as exhaust emission of biodiesel blends produced from WCO by using a single cylinder diesel engine (YANMAR TF 120). There were three types of biodiesel blends used throughout this study, namely the B5 WCO, which is 5% WCO biodiesel with 95% diesel, the B20 WCO with 20% WCO biodiesel with 80% diesel and the B100 WCO, with 100% WCO biodiesel without any diesel added. The results produced by biodiesel blends were compared with that of the diesel fuel. The parameters that were measured during the engine testing included in-cylinder pressure, power, torque, and exhaust emissions. From the in-cylinder pressure, calculations were made to obtain the rate of heat release. Results showed that among all the fuels tested, B5 WCO showed the closest trend to diesel. All fuels gave the highest peak pressure at 1500 rpm and B100 WCO marked the highest point compared to other fuels tested. Moreover, for power and torque performance, all test fuels presented the same trend, with diesel dominated the highest value for both results. In terms of rate of heat release, all biodiesel blends also showed similar trends as diesel. For gas emissions, biodiesel blends, especially the B100 WCO showed some improvement in the reduction of NOR, NO, CO, CO 2 and PM. Finally, it can be concluded that the engine performance and exhaust emissions of all biodiesel blends were compatible and can be operated using diesel engine at certain speeds. The results obviously showed that engine performance using 135 WCO was slightly similar to diesel fuel. Therefore, B5 WCO can be used as a diesel substitute.

ABSTRAK

Penggunaan tenaga termasuklah dari sektor pengangkutan telah digunakan secara meluas sejak berdekad yang lalu. Hal mi telah membawa kepada permintaan tinggi terhadap penggunaan petroleum seperti bahan api gasolin dan diesel, lantas menyebabkan berlakunya kekurangan stok bahan api tersebut dalam jangka masa yang panjang. Menyedari hat ini, pembangunan terhadap bahan api altematif yang mempunyai ciri-ciri yang setara dengan bahan api diesel amatlah diperlukan. Biodiesel yang dihasilkan daripada sisa minyak masak dilihat dapat menjadi salah satu potensi sebagai bahan api altematif yang lebih menjimatkan. Oleh itu, tujuan kajian mi dijalankan adalah untuk mengkaji prestasi enjin dan pembebasan gas oleh campuran biodiesel daripada sisa minyak masak (WCO) menggunakan enjin diesel satu silinder (jenama YANMAR TF120). Sepanjang kajian mi, terdapat tiga jenis campuran biodiesel digunakan iaitu 5% biodiesel WCO dengan tambahan 95% diesel, dinamakan B5 WCO, 20% biodiesel WCO dengan tambahan 80% diesel, dinamakan B20 WCO dan 100% biodiesel WCO tanpa sebarang tambahan diesel, dinamakan B100 WCO. Keputusan kajian yang dihasilkan oleh kesemua campuran biodiesel telah dibandingkan dengan diesel. Antara parameter yang telah diukur semasa ujikaji enjin dijalankan termasuklah tekanan dalam silinder, kuasa yang dihasilkan, daya kilasan dan pembebasan gas. Melalui bacaan tekanan dalam silinder, kadar pembebasan haba dapat dikira. Hasil kajian menunjukkan bahawa B5 WCO menghasilkan keputusan yang lebih menghampiri diesel berbanding bahan api lain. Semua bahan api juga menghasilkan tekanan puncak tertinggi pada kelajuan enjin 1500 rpm dengan B100 WCO menandakan aras paling tinggi berbanding bahan api lain. Selain itu, bagi prestasi kuasa dan daya kilasan, kesemua bahan bakar uji menunjukkan hasil yang sama, dengan diesel mendominasi nilai tertinggi bagi kedua-dua keputusan. Dari segi kadar pembebasan haba, semua campuran biodiesel menunjukkan hasil yang setara dengan diesel. Bagi pembebasan gas, campuran biodiesel terutamanya B 100 WCO menujukkan keputusan yang baik dengan pengurangan NOx, NO, CO, CO 2 dan PM. Di akhir kajian, keputusan menunjukkan bahawa prestasi enjin yang dihasilkan oleh kesemua campuran biodiesel adalah bersesuaian dan boleh digunakan ke atas enjin diesel pada sesetengah kelajuan. Keputusan sangat jelas menunjukkan bahawa prestasi enjin menggunakan B5 WCO hampir sama dengan diesel. Oleh itu, B5 WCO boleh digunakan sebagai pengganti diesel.

TABLE OF CONTENTS Page SUPERVISOR'S DECLARATION 11 STUDENT'S DECLARATION 111 ACKNOWLEDGEMENTS iv V ABSTRACT Vi ABSTRAK

TABLE OF CONTENTS vi'

LIST OF TABLES xlii

LIST OF FIGURES xiv

LIST OF SYMBOLS XVII

LIST OF ABBREVIATIONS xviii

CHAPTER 1 INTRODUCTION

1.1 Introduction 1

1.2 Problem Statement 3

1.3 Objectives 4

1.4 Scope 4

CHAPTER 2 LITERATURE REVIEW

2.1 Introduction 5

2.2 Diesel Engines 5

2.2.1 Diesel Engines Operation 5

2.2.2 Diesel Fuel Injection 7

viii,

2.3 Diesel Combustion Analysis 8

2.3.1 Diesel Combustion Process 8

2.3.2 Conceptual Model of Combustion for Direct Injection 11 Diesel Engine

2.4 Engine Indicating Measurements 13

2.4.1 Power and Torque 13

2.4.2 Hydraulic Drive System (Dynamometer) 16

2.4.3 In-cylinder Pressure 17

2.4.4 Rate of Heat Release 18

2.5 Biodiesel Overview 20

2.5.1 TransesterificatiOn Process 20

2.5.1.1 Homogenous Catalyzed TransesterificatiOn 21 2.5.1.2 Heterogeneous Catalyzed TransesterificatiOn 22 2.5.2 Combustion Efficiency and Engine Performance of 23

2.5.3

Biodiesel

Exhaust Emission Characteristics of Biodiesel 24

2.5.3.1 Oxides of Nitrogen 25

2.5.3.2 Carbon Monoxide 26

2.5.3.3 Carbon Dioxide 27

2.5.3.4 Unburned Hydrocarbon 28

2.5.3.5 Particulate Matter 29

2.6 WCO and Its Potential 30

2.6.1 Fuel Properties of Biodiesel from WCO 31

2.6.1.1 Density 31

2.6.1.2 Acid Number 31

2.6.1.3 Cetane Number 32

2.6.1.4 Calorific Value 33

2.6.1.5 Viscosity 34

2.6.1.6 Cloud Point and Pour Point 35

2.6.2 Biodiesel Production from WCO 36

2.6.3 Economics of Biodiesel from WCO 37

CHAPTER 3 RESEARCH METHODOLOGY

3.1 Introduction 38

ix

3.2 Engine Test Bed 41

3.2.1 Engine Specification 42

3.2.2 Engine Speed Controller 44

3.2.3 Data Acquisition System 45

3.2.4 Dynamometer System 47

3.2.5 Cooling System 48

3.2.6 Fuel Delivery .Unit 50

3.2.7 Air Intake Measurement System 51

3.2.8 Temperature Measuring System 53

3.2.9 Relative Humidity Data Acquisition 54

3.3 Exhaust Monitoring and Analysis 55

3.3.1 Exhaust Gas Analyzer 55

3.3.2 Particulate Matter Measurement 57

3.5 Test Operating Conditions 58

CHAPTER 4 RESULTS AND DISCUSSIONS

4.1 Introduction 59

4.2 Test Fuels Properties 60

4.2 In-Cylinder Pressure 60

4.2.1. Effect of Engine Speed on In-Cylinder Pressure 61

4.2.2 In-cylinder Peak Pressure 67

4.3 Engine Power and Torque 73

4.4 Rate of Heat Release 77

4.5 Gaseous Emission 81

4.5.1 NO Emission 81

4.5.2 NO Emission 83

4.5.3 CO2 Emission 84

4.5.4 02 Emission 85

4.6 Particulate Matter (PM) Concentration 85

4.6.1 Comparison of PM Concentration at Various Speeds 86

X

CHAPTER 5 CONCLUSION AND RECOMMENDATION

5.1 Conclusion 89

5.2 Recommendations 91

5.2.1 Test with Specific WCO 91

5.2.2 Comparison between Experimental and Simulation 91 Results

5.2.3 Transient-state Engine Testing 92

5.2.4 Exhaust Emission Control Technology 92

REFERENCES 93

APPENDICES

Al Past, Current And Proposed Future For European Emission 101 Standards

BI Some Properties of Biodiesel Produced from Selected WCO 102 B2 Typical Fatty Acid Composition (%) for Different Common Oil 103

Source

Cl K-type Thermocouple Probe Specification 104

C2 Charge Module Specification 105

C3 EL-USB-RT Data Logger Specification 106

LIST OF TABLES

Table No. Title Page

2.1 Operating condition for cetane number testing 34

3.1 Testing methods of fuel properties 39

3.2 Engine specifications 43

3.3 Valve timing 44

3.4 Specifications of Autonics MP5W series digital pulse 45 meter

3.5 Specification of engine dynamometer 48

3.6 Specification of manometer 52

3.7 Specification of EL-USB-RT data logger 54

3.8 Specification for gas analyser 56

3.9 Test matrix for fuel testing 58

4.1 Test fuels properties 60

6.1 Past, current and proposed future for European emission 101 standards

6.2 Some properties of biodiesel produced from selected 102 wCO

6.3 Typical fatty acid composition (%) for different common 103 oil source

6.4 K-type thermocouple probe specification 104

6.5 Charge module specification 105

6.6 EL-USB-RT data logger specification 106

LIST OF FIGURES

xiv

Figure No. Title

2.1 Sequence of cycle in four stroke diesel engine 2.2 Comparison between DI and IDI diesel engine 2.3 Block diagram of diesel combustion

2.4 Phases of combustion in diesel engines

2.5 Temporal sequence of schematic showing how diesel

spray develops from start of injection up to the early part of the mixing-controlled bum

2.6 Schematic diagram of operating principle of a dynamometer

2.7 Transesterification processes of triglycerides

2.8 General transesterification equations of triglycerides 2.9 Schematic representation of PM formed during

combustion of atomized fuel droplets

2.10 Composition of PM emitted from diesel engine 2.11 Schematic diagram of the biodiesel production from

WCO

3.1 Methodology flow chart

3.2 Schematic diagram of engine test bed

3.3 Single cylinder YANMAR TF12O diesel engine

3.4 Sectional view of the combustion chamber of the diesel, engine Page 6 7 9 10 12 14 20 20 29 30 36 38 41 42 43

xv

3.5 Autonics MP5W series digital pulse meter 44

3.6 Data acquisition system by TFX Engineering 46

3.7 In-cylinder pressure sensor with the close view 47

3.8 Dynamometer system 48

(a) Gear pump (b) Pressure gauge and control valve

3.9 Schematic diagram of engine cooling system 49

3.10 Dynamometer cooling system 50

3.11 Fuel delivery unit 51

3.12 Air intake measurement system 52

3.13 Temperature measurement device 54

(a) Temperature indicator (b) In-cylinder temperature sensor

3.14 Exhaust channel 55

3.15 Exhaust gas analyzer 56

3.16 Particulate matter apparatus 57

4.1 Diesel at various engine speeds 62

4.2 B100 at various engine speeds 63

4.3 B5, B20 and B100 at 1200 RPM engine speed 65

4.4 B5, B20 and B100 at 2400 RPM engine speed 66

4.5 In-cylinder peak pressure for diesel 68

4.6 In-cylinder peak pressure for B100 70

4.7 In-cylinder peak pressure for biodiesel blends 72

4.8 Comparison power for diesel and B100 74

4.9 Comparison torque for diesel and B100 75

xv'

4.11 Comparison torque for biodiesel blends 77

4.12 ROHR at1200RPM 79

4.13 ROHR at2400R.PM 80

4.14 Comparison of NO content at various engine speeds 82

4.15 Comparison of NO content at various engine speeds 83

4.16 Comparison of CO2 content at various engine speeds 84

4.17 Comparison of 02 content at various engine speeds 85

4.18 Comparison of PM concentration at various engine 87

speeds

LIST OF SYMBOLS

A Cylinder area,

L Stroke length, m

rhf Mass flow per unit time, g/hr

N Angular engine speed, rpm

P Density, kg/M3

Pb Brake power, kW

Pf Friction power, kW

P1 Indicated power, kW

p In-cylinder pressure, bar

T Brake torque, Nm

t Time, S

Vd Displaced volume, dm3

LIST OF ABBREVIATIONS

AS! _{After Start of Injection}

ASTM _{American Society of Testing Materials}

ATDC After Top Dead Center

BDC Bottom Dead Center

BMEP _{Brake Mean Effective Pressure}

BSFC _{Brake-Specific Fuel Consumption}

BTDC Bottom Dead Center

CO Carbon Monoxide

CO₂ Carbon Dioxide

DAQ Data Acquisition

DI Direct Injection

DPF _{Diesel Particulate Filter}

ECU Engine Control Unit

EGR _{Exhaust Gas Recirculation}

EOC _{End of Combustion}

EPA _{U.S Environmental Protection Agency}

EU European Union

FFA Free Fatty Acid

HC _Hydrocarbon

IDI Indirect Injection

IMEP Indicated Mean Effective Pressure

NO Nitrogen Oxide

NO Nitrogen Dioxide

NOx Oxides of Nitrogen

Oxygen

PAHs Polycyclic Aromatic Hydrocarbons

PM Particulate Matter

ROHR Rate of Heat Release

rpm Radius per Minute

SO Sulfur Dioxide

2

SOI Start of Injection

SOC Start of Combustion

TDC Top Dead Center

WCO Waste Cooking Oil

CHAPTER 1

INTRODUCTION

1.1 INTRODUCTION

Nowadays, the world energy usage has increased significantly due to the effect of industrialization. This includes the usage of petroleum products, natural gas, electricity as well as coal and coke. The Malaysian Energy Commission has reported that in 2009, Malaysia was one of the leading energy exporters with 13.7% of its export earnings derived from crude oil, liquefied natural gas (LNG) and petroleum products. At the same time, its energy consumption comprised approximately 42.6% from the coal and coke sector, 36.5% from the transportation sector and the balance of 13.8% was from the residential and commercial sector (Energy Commission, Malaysia). Focusing on a transportation sector, it is among the largest consumers of petroleum products, predominantly gasoline and diesel.

Diesel engines are generally preferred compared to gasoline engines due to their undoubted benefits of fuel economy and torque output. In addition, carbon monoxide, carbon dioxide and hydrocarbon produced in diesel engines are much lower compared to gasoline engines (Brijesh and Sreedhara, 2013). At the beginning, diesel engines were known to be noisy, large and slow features. Nowadays, diesel engines hold a commanding position, not only for light and heavy-duty vehicles, but also for other applications such as marine, construction, military as well as agricultural areas. This can be proven by the increased sales of diesel engine cars. For example, in 2006, Europe had a 51% growth of diesel-powered cars compared to gasoline-powered cars (Adam et al., 2010).

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However, the extensive usage of diesel engines has led to an increase in demand for diesel fuel, thus causing the decrease of petroleum fuel stocks. Due to concerns of long-term energy sustainability, it has become paramount to develop alternative fuels from natural sources that have properties comparable to petroleum-based fuels. Furthermore, the ordinary diesel fuels are available only in certain regions of the world and the source has reached its maximum production (Obed and Mamat, 2013). In view of this, considerable attention has been given to the production of biodiesel as a diesel substitute.

In addition, biodiesel is a more attractive substitute because it is a non-polluting fuel and produced from renewable biomass sources. The fuel properties of biodiesel are also comparable to diesel fuel since it promotes lubricating properties besides producing low exhaust emissions. In Malaysia for example, the government had already mandate the use of B5 nationwide starting on July 1st, 2014. Nevertheless, the biodiesel feedstock from vegetable oil is quite limited since the price is higher. Waste cooking oil becomes an effective way of reducing biodiesel production since it reduces feedstock cost by 70-95% (Chhetri et al., 2008), while at the same time it complies with international standard ASTM D6751 (Kumaran et at., 2011). The production of biodiesel from WCO basically does not vary much from standard biodiesel production as it also undergoes the transesterification process with alkaline, acidic and enzyme as catalyst (Enweremadu and Mbarawa, 2009).

Many researchers have proved that biodiesel from WCO can improve exhaust emissions by reducing smoke, hydrocarbon, carbon monoxide and sulphur oxide (Lin et al., 2011; Muralidharan and Vasudevan, 2011 and Abu-Jrai et al., 2011). However, the high viscosity of WCO, similar to other biodiesel, is a major problem because high viscosity could cause numerous engine problems such as poor atomization, carbon deposition and inaccurate operation of fuel injectors (Adam et al., 2011 and Mamat et al., 2012). Due to these problems, most researchers have used a small percentage (5%) of WCO to be blended and tested for engine compatibility.

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1.2 PROBLEM STATEMENT

With the rapid increase in prices and the uncertainties abound concerning fuel availability nowadays, the revival of diesel engine related research has been gaining momentum. At this point, attention has been given towards the production of biodiesel as an alternative fuel. This is because the characteristics of biodiesel are similar to conventional diesel and it is much oxygenated, which leads to a more complete combustion. Moreover, the feedstock for biodiesel such as by-products of plants and animals as well as edible and non-edible oils are renewable. However, the use of edible oils such as palm, corn, soybean and sunflower for biodiesel has become an environmental issue since it causes unnecessary clearing of forests for plantation. The European Union has criticised biodiesel production using edible oil at the expense of the millions of people facing hunger and starvation around the world (Gimbun et al., 2013).

In developed countries, there are large productions of municipal and industrial wastes. These wastes comprise of activated sludge, food and organic waste (Hosseini Koupaie et al., 2014) as well as cooking oil waste. The management of these waste products pose a significant challenge because of the problems in disposing them. The uncontrolled disposal of these wastes will cause problems such as contamination of water and land resources. Moreover, there are irresponsible parties who choose to dump these wastes into rivers and landfills, thus causing environmental pollution. In the case of waste cooking oil, which can also be used for soap production, the major part of it is still discharged into the environment.

Therefore, researchers have found that waste cooking oil has a good potential to be biodiesel feedstock. It is a decent approach since it is easy to obtain and can reduce the uncontrolled waste disposal problem. At the same time, the use of waste cooking oil can also decrease the cost of biodiesel production since the cost of waste cooking oil is 2 to 3 times cheaper than edible oils (Phan and Phan, 2008). Furthermore, the waste cooking oil used as biodiesel feedstock can significantly reduce the amount of farmland needed for biodiesel producing crops.

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1.3 OBJECTIVES

i. To investigate the engine performance of biodiesel blends from waste cooking oil using a single cylinder diesel engine.

ii. To investigate the exhaust emission characteristics produced by biodiesel blends from waste cooking oil.

1.4 SCOPE

The research presented in this thesis deals with the:

i. Literature review on the application of diesel and biodiesel in engine testing. ii. Determine the chemical properties of testing fuel, which includes diesel,

biodiesel blends from waste cooking oil that are B5 WCO, B20 WCO and B100 WCO.

iii. Conducting engine testing at limited engine speeds of 1200 rpm, 1500 rpm, 1800 rpm, 2100 rpm and 2400 rpm with 20Nm constant loads.

iv. Estimating the engine performance characteristics and exhaust emission characteristics fuelled with four different testing fuels.

This thesis is organized as follows. After the introduction, the experimental rig and conditions are described, followed by the experimental results and discussions. Finally, the major findings of the study are presented and conclusions drawn.

CHAPTER 2

LITERATURE REVIEW

2.1 INTRODUCTION

The intention of this chapter is to present a review of previous research efforts related to biodiesel fuels and internal combustion engines. The organized review of other applied research studies is provided so that research can be customized appropriately. This review starts with the diesel engine operation and combustion analysis, overview of the biodiesel production, followed by the potential of waste cooking oil and then the brief explanation on exhaust emission from diesel engines. Finally, at the end of the chapter, is the engine testing analysis.

2.2 DIESEL ENGINES

At present, diesel engines play an important role in the economy and in energy efficiency. Diesel engines are widely used in passenger cars, heavy-duty vehicles, locomotives, agricultural and in industrial applications. However, diesel engines face significant challenges with respect to not only fuel economy and performance but also the type and amount of pollutants emitted.

2.2.1 Diesel Engines Operation

Theoretically, diesel engines are commonly known as compression ignition engines and do not differ much from gasoline engines, which are both designed to convert chemical energy from fuel into a series of combustions. In a gasoline engine, fuel is pre-mixed with air in the carburettor, followed by compression by the pistons and

ignition from spark plugs. Compared to gasoline engines, the air in the diesel engine is

compressed before the fuel is injected. The pressure in the cylinder increases and so

does the temperature. Therefore, the air heats-up when the cylinder is compressed,

which then ignites the fuel. Figure 2.1 shows the four-stroke engine cylinder cycle that

includes intake stroke, compression stroke, power stroke and lastly, exhaust stroke.

intake vavia

fuel Injector

exhaust valve

intake

_compression

power

exhaust

Figure 2.1: Sequence of cycle in four stroke diesel engine

Source: Encyclopedia Britannica (2007)

The stroke of the piston in the intake stroke begins at top-dead centre (TDC).

Then, the intake valve opens up, letting in air and moving the piston to the bottom-dead

cylinder (BDC), thus increasing the volume of the cylinder. At the compression stroke,

both intake and exhaust valves are closed. The piston moves to the top of the cylinder

while compressing the air into the cylinder head. During the compression stroke, the

compression ratio can be much higher as more power is generated. Conversely, in

gasoline engines, the higher compression ratio of air-fuel that exceeds the limit will

cause knocking. Next, during the power stroke, as the piston reaches the top, fuel is

injected and ignited under high pressure, which then forces the piston down. Lastly,

during the exhaust stroke, the piston moves back to the TDC while the exhaust valve is

open and pushes out the exhaust created during combustion (Robert, 2005).

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2.2.2 Diesel Fuel Injection

The injector on diesel engines is a complex component and has been the subject of a great deal of experimentation. Diesel engines can be categorized into two different types of injection systems called direct injection (DI) system and indirect injection (IDI) system. Figure 2.2 illustrates the comparison between DI and IDI systems in diesel engines.

Direct Injection (DI) System Indirect Injection (ID!) System

Figure 2.2: Comparison between DI and IDI diesel engine

Source: Romain (2013)

In the DI system, fuel is injected as small droplets and reacts with the hot air inside the cylinder during the compression stroke. The combustion chamber consists of a bowl formed at the top of the piston and the fuel is injected into this volume. Normally, the injector tip has four to eight holes to form multiple spray cones (David and Elsayed, 2011). In this system, the momentum and energy of the injected fuel jets are adequate to achieve sufficient fuel distribution and rate of mixture with the air. A detailed explanation on combustion from DI system will be explained in the combustion subsection.

8

On the other hand, the WI system fuel is injected into the pre-chamber, which is located above the piston crown of the engine block and is connected to the main combustion chamber by a nozzle. The pre-chamber promotes rapid mixing of air and fuel and shortens the ignition delay period. The WI system contains of a glow plug, which functions by heating the combustion chambers and raising the air temperature when the engine is cold so that the engine can be started. The DI system is usually used in the smallest of engine sizes and the strong charge motion required during fuel injection is generated at the compression stroke (Heywood, 1988). However, this system can cause remarkable efficiency losses or thermal losses.

2.3 DIESEL COMBUSTION ANALYSIS

2.3.1 Diesel Combustion Process

Inside the diesel engine, fuel is injected into a highly compressed gas volume and ignition occurs due to the collision between molecules of air mixture and vaporized fuel. Since thermo chemical reactions during ignition do not occur immediately, some residence time called ignition delay is required as fuel mixes with air during this period. For that reason, the initial phase of the combustion process is premixed and then continues with fuel being burnt in mixed and controlled diffusion flames. As mentioned by Hiroyasu and Arai (1990), the main issue that controls the diesel combustion is the mixture formation that is controlled by the spray characteristics, features of the injection system and also the nature of air swirl and turbulence in the cylinder. This can be explained from the block diagram of diesel combustion, as shown in Figure 2.3.

Air

• EGR

• Characteristics of super charge

I

Design inlet port

I

Fuel • Characteristics of fuel

I

Injection system

I

9 mbustion chamber

I

4- Spray Characteristics • Drop distribution • Penetration • Spray angle Injection characteristics • Injection rate • Timing • Injection duration Characteristics of air motion 4- Vaporizing Characteristics Fuel-air mixture

4- _{Characteristics of Ignition Delay}

I

Ignition

I

4- _{Flame Propagation}

1- _{Partially pre-mixed combustion} 4- _{Partially diffusive combustion}

Combustion

I 4- Diffusion of Heat losses ________ combustion products

Heat release rate PM NO^ Exhaust HC emission

Figure 2.3: Block diagram of diesel combustion

4e

C

10

In general, there are four main phases in the combustion process of diesel engines that have to be taken into account. The four phases are ignition delay period, premixed combustion phase, mixing-controlled combustion phase and late combustion phase as illustrated in Figure 2.4. The ignition delay period is an important parameter as it affects the engine's performance and exhaust emissions, especially the formation of NOR. During this period, the phase is counted from the start of injection (a - b) where the fuel is injected directly into the cylinder towards the compression stroke. The fuel then atomizes into small droplets and penetrates into the combustion chamber. Fuel vaporizes and mixes in the midst of air with very high pressure and temperature. The rate of heat released is controlled in this phase by the speed of the chemical reactions and the amount of air-fuel mixture formed (Günter et al., 2011).

a. b

Crank Angle, deg.

Figure 2.4: Phases of combustion in diesel engines

Source: Heywood (1988)

The premixed combustion phase (b - c) is the initial phase of air-fuel mixture becoming unstable and causes the rapid rise of the in-cylinder pressure. A high rate of rising pressure indicates a sudden application of load to the engine structure, which results in fatigue damage to the parts and also produces knocking (Musculus et al.,