Vehicle Powertrain Systems E-Book

Behrooz Mashadi

• Herausgeber: John Wiley & Sons
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 693

Veröffentlichungsjahr: 2011

Beschreibung

Vehicle Powertrain Systems

The powertrain is at the heart of vehicle design; the engine – whether it is a conventional, hybrid or electric design – provides the motive power, which is then managed and controlled through the transmission and final drive components. The overall powertrain system therefore defines the dynamic performance and character of the vehicle.

The design of the powertrain has conventionally been tackled by analyzing each of the subsystems individually and the individual components, for example, engine, transmission and driveline have received considerable attention in textbooks over the past decades. The key theme of this book is to take a systems approach – to look at the integration of the components so that the whole powertrain system meets the demands of overall energy efficiency and good drivability.

Vehicle Powertrain Systems provides a thorough description and analysis of all the powertrain components and then treats them together so that the overall performance of the vehicle can be understood and calculated. The text is well supported by practical problems and worked examples. Extensive use is made of the MATLAB software and many example programmes for vehicle calculations are provided in the text.

Key features:

• Structured approach to explaining the fundamentals of powertrain engineering
• Integration of powertrain components into overall vehicle design
• Emphasis on practical vehicle design issues
• Extensive use of practical problems and worked examples
• Provision of MATLAB programmes for the reader to use in vehicle performance calculations

This comprehensive and integrated analysis of vehicle powertrain engineering provides an invaluable resource for undergraduate and postgraduate automotive engineering students and is a useful reference for practicing engineers in the vehicle industry.

Leseprobe

Contents

Cover

Title Page

Copyright

Dedication

About the Authors

Abbreviations

Preface

Chapter 1: Vehicle Powertrain Concepts

1.1 Powertrain Systems

1.2 Powertrain Components

1.3 Vehicle Performance

1.4 Driver Behaviour

1.5 The Role of Modelling

1.6 Aim of the Book

Further Reading

References

Chapter 2: Power Generation Characteristics of Internal Combustion Engines

2.1 Introduction

2.2 Engine Power Generation Principles

2.3 Engine Modelling

2.4 Multi-cylinder Engines

2.5 Engine Torque Maps

2.6 Magic Torque (MT) Formula for Engine Torque

2.7 Engine Management System

2.8 Net Output Power

2.9 Conclusion

2.10 Review Questions

2.11 Problems

Further Reading

References

Chapter 3: Vehicle Longitudinal Dynamics

3.1 Introduction

3.2 Torque Generators

3.3 Tractive Force

3.4 Resistive Forces

3.5 Vehicle Constant Power Performance (CPP)

3.6 Constant Torque Performance (CTP)

3.7 Fixed Throttle Performance (FTP)

3.8 Throttle Pedal Cycle Performance (PCP)

3.9 Effect of Rotating Masses

3.10 Tyre Slip

3.11 Performance on a Slope

3.12 Vehicle Coast Down

3.13 Driveline Losses

3.14 Conclusion

3.15 Review Questions

3.16 Problems

Further Reading

References

Chapter 4: Transmissions

4.1 Introduction

4.2 The Need for a Gearbox

4.3 Design of Gearbox Ratios

4.4 Gearbox Kinematics and Tooth Numbers

4.5 Manual Transmissions

4.6 Automatic Transmissions

4.7 CVTs

4.8 Conclusion

4.9 Review Questions

4.10 Problems

4.11 Further Reading

References

Chapter 5: Fuel Consumption

5.1 Introduction

5.2 Engine Energy Consumption

5.3 Driving Cycles

5.4 Vehicle Fuel Consumption

5.5 Shifting Effects

5.6 Software

5.7 Automated Gearshifts

5.8 Other Solutions for Fuel Efficiency

5.9 Conclusion

5.10 Review Questions

5.11 Problems

Further Reading

References

Chapter 6: Driveline Dynamics

6.1 Introduction

6.2 Modelling Driveline Dynamics

6.3 Bond Graph Models of Driveline Components

6.4 Driveline Models

6.5 Analysis

6.6 Conclusion

6.7 Review Questions

6.8 Problems

6.9 Further Reading

References

Chapter 7: Hybrid Electric Vehicles

7.1 Introduction

7.2 Types of Hybrid Electric Vehicles

7.3 Power Split Devices

7.4 HEV Component Characteristics

7.5 HEV Performance Analysis

7.6 HEV Component Sizing

7.7 Power Management

7.8 Conclusion

7.9 Review Questions

7.10 Problems

Further Reading

References

Appendix: An Introduction to Bond Graph Modelling

Basic Concept

Standard Elements

Constructing Bond Graphs

Equations of Motion

Index

This edition first published 2012

© 2012 John Wiley & Sons, Ltd

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Library of Congress Cataloging-in-Publication Data

Mashadi, Behrooz.

Vehicle powertrain systems / Behrooz Mashadi, David Crolla.

p. cm.

Includes bibliographical references.

ISBN 978-0-470-66602-9 (cloth) – ISBN 978-1-119-95836-9 (ePDF) – ISBN 978-1-119-95837-6 (oBook) – ISBN 978-1-119-96102-4 (ePub) – ISBN 978-1-119-96103-1 (Mobi)

1. Automobiles–Power trains. 2. Automobiles–Dynamics. I. Crolla, David

A. II. Title.

TL260.M37 2012

629.25'2–dc23                                                                                               2011027302

A catalogue record for this book is available from the British Library.

This book is dedicated to Professor David Crolla who passed away unexpectedly while the book was in production. David led an unusually full and productive life both in work and play, achieving great success and popularity. David was a leading researcher, an inspiring teacher, an excellent supervisor of research postgraduates and a friend to many. David's energy, enthusiasm and irrepressible humour made a lasting impression on me and everyone who knew him. He is sorely missed and his essential contribution to the publication of this book will always be remembered.

About the Authors

Behrooz Mashadi is an Associate Professor in the Department of Automotive Engineering, Iran University of Science and Technology (IUST), Tehran, Iran. He received his BSc and MSc in Mechanical Engineering from Isfahan University of Technology (IUT), Isfahan, Iran, and his PhD degree in Vehicle Dynamics Engineering from the University of Leeds, in 1996 under the supervision of Professor D. A. Crolla. He was then engaged in several R&D projects in the automotive engineering industry and joined the academic staff at IUST in 2002.

He has developed and taught a wide range of courses for undergraduate and postgraduate students in the field of Automotive Engineering. He served as Deputy for Education in the Department of Automotive Engineering and is currently Deputy of the Automotive Research Centre at IUST, which is the leading centre for automotive R&D in Iran.

His current research interests include vehicle powertrain systems, hybrid propulsion systems, vehicle dynamics, vehicle modelling, simulation and control. He has presented and published over 100 papers in journals and conferences. He also serves on the editorial board of several international journals.

David Crolla, FREng, was a Visiting Professor of Automotive Engineering at the Universities of Leeds, Sunderland and Cranfield. After graduating from Loughborough University, he first worked as a research engineer in off-road vehicle design, and then joined the University of Leeds (1979–2001) becoming head of the Mechanical Engineering Department. His research interests included vehicle dynamics, chassis control systems, powertrain systems, suspensions and terramechanics, and he had published and presented over 250 papers in journals and conferences.

His activities included research in low carbon vehicles, industrial short courses in vehicle dynamics and chassis control, and engineering consultancy, for example, the BLOODHOUND SSC 1000mph land speed record attempt.

He was Editor-in-Chief of the world's first Encyclopedia of Automotive Engineering to be published in 2013.

Abbreviations

2WD2-wheel drive4WD4-wheel driveACalternating currentAFRair-fuel ratioAhamp-hourAMTautomated manual transmissionATautomatic transmissionBASbelted alternator starterBDblock diagramBDCbottom dead centreBLDCbrushless DCBMEPbrake mean effective pressureBMSbattery management systemBSFCbrake specific fuel consumptionCAFEcorporate average fuel economyCIcompression ignitionCO2carbon dioxideCOPconformity of productionCPPconstant power performanceCSMcharge sustaining modeCTPconstant torque performanceCVTcontinuously variable transmissionDCdirect currentDCTdual clutch transmissiondeg.degreeDOFdegree of freedomDOHdegree of hybridizationECeddy currentECUengine control unitEFCCEfficient Fuel Consumption CurveEGRexhaust gas recirculationEMelectric motorEMSengine management systemEOPengine operating pointEPAEnvironmental Protection AgencyEREVextended range electric vehicleEUDCextra-urban European driving cycleEVelectric vehicleFBDfree body diagramFCfuel consumptionFCVsfuel cell vehiclesFEADfront engine accessory driveFEMfinite elements methodsFTPFederal Test ProcedureFTPfixed throttle performanceFWDfront wheel driveGDIgasoline direct injectionHChydrocarbonsHCCIhomogeneous charge compression ignitionHEVhybrid electric vehicleICinternal combustionICEinternal combustion engineIMEPindicated mean effective pressureI/Oinput/outputISGintegrated starter-generatorISOInternational Standard OrganizationIVTinfinitely variable transmissionkg/Jkilogram per joulekWhkW-hourllitreLCVlow carbon vehicleLSlow speedMAPmanifold absolute pressureMCmotor controllerMCUmotor control unitMGmotor/generatorMPamega PascalMPDmechanical power distributionMPImulti-point (port) injectionMTMagic Torque (formula)MTmanual transmissionNEDCNew European Driving CycleNOxoxides of nitrogenNRFno-resistive-forceNVHnoise, vibration and harshnessOOLoptimal operating linePCPpedal cycle performancePGSplanetary gear setPHEVplug-in hybrid electric vehiclePIDproportional integral derivativePSDpower split deviceRMSroot mean squarerpmrevs per minuteRWDrear wheel driveSCUsupervisory control unitSFGsingle flow graphSIspark-ignitionSOCstate of chargeSPHseries-parallel hybridTAtype approvalTADtorque amplification deviceTBIthrottle body injectionTCtorque converterTDCtop dead centreTHSToyota Hybrid SystemTPSthrottle position sensorVVTvariable valve timingWhWatt-hourWOTwide-open throttle

Preface

In writing this book, we have aimed it at the needs of both students and practising engineers in the automotive industry. For engineering students, we hope we have provided a sound explanation of the principles behind the design of vehicle powertrain systems. For practising engineers, we have tried to provide a comprehensive introduction to the subject area, which will set the scene for more specialized texts on, for example, engines, transmissions or hybrid electric components.

The book has arisen from our combined teaching experiences at a range of institutions including the Iran University of Science and Technology (IUST), Tehran, and the Universities of Leeds, Sunderland and Cranfield. We have attempted to incorporate two important themes which distinguish our book from other texts:

1. The inclusion of numerous worked examples and the provision of a MATLAB® code for many of the problems.

2. A systems approach to powertrain design – focusing on the integration and interactions of all the components, e.g. engine, transmission final drive, wheels and tyres – in analyzing the overall vehicle performance.

Our experience of teaching engineering students suggests that one of the most useful ways of learning engineering principles is through actually doing problems oneself. Hence, we have tried to provide a wide range of examples together with worked solutions, often with an accompanying MATLAB code. We hope that readers will run these short programmes themselves and modify them to examine other performance issues.

The term ‘systems approach’ is widely used in engineering but is not always clarified in the particular context. Here, we simply mean that in order to understand vehicle performance, it is necessary to analyze all the powertrain components together and examine how they interact, and how the designer tries to integrate them in a coordinated way. Our experience suggests that there are relatively few texts which deal comprehensively with this critical aspect of integration.

At the time of writing, there is considerable pressure on the automotive industry to minimize energy consumption and reduce global emissions. This has led to a huge upsurge in interest in alternative powertrain systems – and the development of a range of electric and hybrid electric vehicles. However, consumers do not appear to be willing to compromise some of the traditional aspects of vehicle performance, e.g. acceleration, speed, etc. in the interests of overall energy consumption. Drivability remains a key commercial issue and there is a demand for vehicles which are ‘fun-to-drive’. Hence, the design challenge continues to involve a compromise between vehicle performance and energy usage. We have tried in this book to provide a comprehensive coverage of both these – often conflicting – aspects of vehicle behaviour.

Vehicle Powertrain Systems is accompanied by a website (www.wiley.com/go/mashadi) housing a solution manual with detailed explanations for the solution methods of more than a hundred exercises in this book. The solutions of the majority of the problems are carried out in MATLAB environment and the program listings are also provided. In addition to the worked examples of the book itself, the website offers invaluable guidance and understanding to students.

Finally, we would like to thank all our colleagues and friends over the years who have contributed in some way or influenced us in writing this text.

Chapter 2

Power Generation Characteristics of Internal Combustion Engines

2.1 Introduction

The engine plays a dominant role in overall vehicle performance and it is essential to learn about its behaviour prior to performing vehicle studies. The internal combustion engine is a complicated system and its thorough analysis requires a multi-disciplinary knowledge of physics, chemistry, thermodynamics, fluid dynamics, mechanics, electrics, electronics and control. Electronics and control are becoming crucial parts of all modern engines and engine control units (ECUs) manage the engine operating parameters to try to achieve a good compromise between drivability, fuel consumption and emissions control.

Traditionally, in the literature on internal combustion engine design, the material discussed included: working fluids, thermodynamics, gas dynamics, combustion processes and chamber design, heat transfer, engine efficiency, friction, emissions and pollution. Also, the dynamics of engine moving parts and loads acting on the engine bearings and components are traditionally discussed in books on mechanism design or the dynamics of machinery. On the other hand, in areas related to the vehicle powertrain designs, the engine properties are needed as inputs to the system. Such vital information suitable for powertrain analysis cannot be found in the aforementioned books. Students have always seemed to have difficulties relating the engine design materials to powertrain design requirements. Moreover, it has been found that the engine performance characteristics described by full throttle engine maps usually given in the engine design books are misleading and confuse students, due to the fact that they try to explain the vehicle motion without sufficient information.

In this chapter, a review of internal combustion engine behaviour over a full range of operations is provided. This includes torque generation principles and characteristics as well as engine modelling for both petrol and diesel engines. This chapter is not intended to explain those materials generally covered by books written on the topics of internal combustion engines; instead, the torque generation principles of engines that are required in powertrain analysis will be the focus of this chapter.

2.2 Engine Power Generation Principles

In vehicle powertrain studies, the power generation properties of engines are of vital importance as the torque produced by the engine drives the vehicle in different and diverse driving situations. Internal combustion engines convert chemical energy contained in the fuel into mechanical power that is usually made available at a rotating output shaft. The fuel includes chemical energy that is converted to thermal energy by means of combustion or oxidation with air inside the engine. The pressure of the gases within the engine builds up because of the combustion process that is generating heat. The high pressure gas then expands and pushes the surfaces inside the engine. This expansion force moves the mechanical linkages of the engine and eventually rotates a crankshaft. The output shaft of an internal combustion engine is usually coupled to a gear box, as in the case of transport vehicles.

Most internal combustion engines are of the reciprocating type, having pistons that move back and forth inside cylinders fixed to the engine blocks. Reciprocating engines range from single cylinder engines up to several cylinders arranged in many different geometric configurations. Internal combustion engines can be classified in different ways but the classifying method according to ignition type is most common. Two major ignition types are spark-ignition (SI) and compression-ignition (CI) types. Details of the combustion processes in SI and CI engines depend entirely on the characteristics of the fuel used in each type. Since the combustion process is quite different between SI and CI engines, the types and quantities of the various exhaust emission materials that are formed vary as a result.

2.2.1 Engine Operating Modes

The slider-crank mechanism is a basic linkage to convert the reciprocating motion of the piston into the rotating motion of a crankshaft in reciprocating engines. The piston acts as the slider and moves inside the cylinder and with the provision of the valves and manifolds, an engine with the ability to compress and expand gases results. Figure 2.1 shows the schematic of a typical slider-crank mechanism used in a single cylinder engine. At zero crank angle θ, the piston is at the position known as top dead centre (TDC), because the piston speed reaches zero at this point. Rotation of the crank arm through 180° displaces the piston from TDC to the other bottom extreme, again with zero piston speed, called bottom dead centre (BDC). The total distance that the piston travels during this 180° rotation of the crank is called one stroke that is twice the radius of the crank. Returning from BDC to TDC will take another 180° rotation of the crank and the piston behaviour is reverse of that between zero and 180°.

Figure 2.1 Schematic of engine slider-crank mechanism

Reciprocating engines, both spark ignition and compression ignition, need four basic phases, namely intake (or induction), compression, combustion (or power) and exhaust to complete a combustion cycle.

2.2.1.1 Four-Stroke Engines

Some engines are designed to have four distinctive strokes for the piston in a complete working cycle and are called four stroke engines. In a four-stroke engine, the piston has to go through four strokes in order to complete the cyclic thermodynamic processes. The crankshaft must perform two full turns in order that the piston completes four strokes. Figure 2.2 illustrates the basic parts of a four-stroke engine including the cylinder, the piston, the cylinder head, ports and valves.

Figure 2.2 A classic four-stroke engine components

Starting from TDC at the beginning of intake stroke, the inlet valve opens and the outlet valve closes. With the piston motion towards the BDC, fresh air (or mixture) flows into the cylinder. At BDC, the first stroke is complete and the inlet valve closes and the piston moves towards the TDC, compressing the gases inside the cylinder. At the TDC, the compression stroke ends and while both valves are closed, the power stroke starts with combustion and the resulting gases expand, pushing the piston down to the BDC at which the fourth and last stroke starts by opening the outlet valve to let the pressurized combustion products leave the cylinder. The motion of the piston to TDC helps the exhaust process by pushing the gases out. Table 2.1 summarizes these four strokes.

Table 2.1 The four strokes of a reciprocating engine.

Note that the valve opening/closing crank angles given in Table 2.1 are only theoretical values and will be different in practice. For example, when the next intake process starts, it is better to leave the outlet valve open for a while in order that the burned gases leaving the combustion chamber continue their flow due to their momentum (also the fresh air can push them out). This will provide more room for fresh air and increase the combustion efficiency. Similarly when the piston is starting to move towards the TDC at the beginning of compression stroke, it is better to leave the inlet valve open for a while, so that the incoming air continues to flow into the cylinder due to its momentum.

2.2.1.2 Two-Stroke Engines

A two-stroke engine performs the four basic phases of a combustion cycle only in two piston strokes. In the two-stroke engine, the inlet and exhaust valves are eliminated and the ports for the entrance and exit of the gases are built on the cylinder walls and crankcase instead. The piston covers and uncovers the ports when it moves back and forth inside the cylinder (see Figure 2.3).

Figure 2.3 Schematic of a classic two-stroke engine

Let us start the cycle with the combustion stroke. The mixture in the combustion chamber is ignited in the same way as in the four-stroke engine at the top of the stroke. The piston moves downwards and uncovers the outlet port, allowing the pressurized burned gases to flow out of the cylinder. The downward movement of the piston at the same time compresses the gases in the crankcase. Further down, the piston uncovers the transfer port and the compressed gases in the crankcase flow through the channel into the combustion chamber and push the combustion products out through the outlet port. So, in a single stroke of the piston both combustion and exhaust cycles are accomplished. The upward movement of the piston compresses the gases in the combustion chamber and simultaneously depressurizes the crankcase to allow the pressure of the atmosphere to fill the crankcase with fresh air. Further up, the compression stroke will end and a new cycle will start by the combustion process. Again in a single upward piston stroke, both induction and compression cycles are accomplished.

It appears that two-stroke engines are more advantageous since they perform the power cycle faster than four-stroke engines and do not need the valves and valve trains either. But, in practice, two-stroke engines are not as efficient as four-stroke cycle engines, especially at high speeds. Two-stroke engines are generally used in small SI engines for motorcycles and in large CI engines for locomotives and marine applications that work in lower speeds. At large CI engine sizes, the two-stroke cycle is competitive with the four-stroke cycle, because in the CI cycle, only air is lost in the cylinder (see Section 2.2.2).

In the rest of this chapter it will be assumed that the engine works only on the four-stroke basis.

2.2.2 Engine Combustion Review

It is common to refer to engines as either petrol (gasoline in the USA) or diesel, according to the nature of the ignition and combustion, however, the terms ‘spark ignition’ (SI) and ‘compression ignition’ (CI) are also used. In SI engines the air and fuel are usually premixed before the initiation of combustion by sparking. In CI engines the fuel burns as it is injected into hot compressed air and produces a combustible mixture.

In order to have ideal combustion, the amount of fuel must be related exactly to the amount of intake air. In fact, according to the burning chemistry, for a specific amount of air molecules, there must be specific number of fuel molecules for perfect burning of the fuel. This fuel/air ratio is called the stoichiometric ratio and the objective in engine combustion is to produce fuel/air ratios as close to the stoichiometric as possible. More details will be discussed in the following sections.

2.2.2.1 SI Engine Combustion

In SI engines, the fuel is mixed with the air in the intake system prior to entry to the cylinder. In the past, carburettors were used for the homogenized mixing of the air and fuel. The basis of a carburettor operation was a pressure drop when air passed through a venturi and an appropriate amount of fuel (at higher pressure) surged into the air flow at the venturi throat from the float chamber. The throttle opening controlled the air flow inside the venturi and as a result the amount of fuel entering the engine was adjusted accordingly. This type of fuel metering was very sensitive to atmospheric changes and could not maintain accurate fuel to air ratios and resulted in poor engine performance and high levels of pollution.

In newer generations of engines, fuel injection systems that replace the carburettors inject the fuel in more accurate amounts. Injection systems are electronically controlled systems – the air flow rate must be measured and the desired amount of fuel per cylinder that is required for a proper combustion must be calculated and injected accordingly.

Currently there are two different fuel injection systems, namely, throttle body injection (TBI) and multi-port injection (MPI). TBI systems are something like a carburettor which contains one or more injectors. When fuel is injected, it will be mixed with the air and the mixture will move in the inlet manifold exactly like in the case of carburettor. In MPI systems, instead of having a throttle body for all cylinders, air is moved directly to the inlet port of each cylinder without mixing. The fuel is injected just at the entrance to each cylinder and is mixed with the air. In MPI systems, therefore, the number of injectors is equal to the number of cylinders. MPI systems are more efficient than TBI systems; first, because the fuel is more precisely metered for each cylinder in MPI systems and, second, the fuel is completely moved into the cylinder, whereas in TBI some part of the fuel in contact with the surface of intake manifold will stick and remain.

The newer generation of injection systems for SI engines includes the gasoline direct injection (GDI) systems that use the injection concept in CI engines (see Section 2.2.2.2) in which the fuel is injected into the combustion chamber inside the cylinder. These systems allow the achievement of both the fuel efficiency of a diesel engine and the high output of a conventional petrol engine.

Regardless of the injection type, the SI engine cycle can be described as follows. During the intake process the inlet valve is open and the air and fuel mixture is inducted in the cylinder. After the inlet valve closes, the cylinder contents are compressed by the piston movement upwards. Before the piston gets to the TDC, a high voltage electric discharge across the spark plug starts the combustion process. Burning the fuel during the combustion process increases the temperature in the cylinder to a very high peak value. This, in turn, raises the pressure in the cylinder to a very high peak value. This pressure forces the piston down and a torque about the crank axis is developed. The expansion stroke causes the pressure and the temperature to drop in the cylinder. For a given mass of fuel and air inside the cylinder, an optimum spark timing produces the maximum torque.

Before the end of the expansion stroke, the exhaust valve starts to open and the burned gases find a way through the valve into the exhaust port and into the manifold. Pressure in the cylinder is still high relative to the exhaust manifold and this pressure differential causes much of the hot products to be blown out of the cylinder before the piston starts its upward motion. The piston motion during the exhaust stroke transfers the remaining combustion products into the exhaust manifold. The timing for the exhaust valve opening is important since an early opening will reduce the work on the piston (less output torque) and a late opening will need external work delivered to the piston during the exhaust phase (see Section 2.2.3.1).

The intake valve opens before TDC and the exhaust valve closes some time after in order to help the combustion products trapped in the clearance volume when the piston reaches TDC to leave and to replace them with a fresh mixture. This period when both the inlet and exhaust valves are open is called valve overlap. The combustion process of SI engines is divided into four phases, namely ignition, flame development, flame propagation and flame termination [1]. Flame development is sometimes taken as part of the first phase and a total of three phases is considered [2]. The flame development interval is between the spark discharge and the time when a fraction of the fuel-air mixture has burned. This fraction is defined differently such as 1, 5 or 10%. During this period, although ignition occurs and the combustion process starts, very little pressure rise and useful work is achieved.

In the interval between the end of the flame development stage and the end of the flame propagation process, usually the bulk of the fuel and air mass is burned and an energy release of about 90% is achieved. During this period, the pressure in the cylinder is greatly increased and thus the useful work of an engine cycle is the result of the flame propagation period. The remaining 5–10% of the fuel-air mass burning takes place in the flame termination phase. During this time, the pressure quickly decreases and combustion stops. The combined duration of the flame development and propagation phases is typically between 30° and 90° of the crank angle.

2.2.2.2 CI Engine Combustion

The operation of a typical four-stroke CI engine during the intake stroke is the same as for the intake stroke in an SI engine in terms of valve openings. The only difference is that air alone is inducted into the cylinder in this stroke. The compression ratio is higher for CI engines and during the compression stroke, air is compressed to higher pressures and temperatures than SI engines. The fuel is injected directly into the cylinder in the combustion stroke where it mixes with the very hot air, causing the fuel to evaporate and self-ignite and combustion to start. The power stroke continues as combustion ends and the piston travels towards BDC. The exhaust stroke is also the same as for SI engines.

In a CI engine at a given engine speed, the air flow is unchanged and the output power is controlled by only adjusting the amount of fuel injected. The nature of the fuel-air mixture in compression ignition engines is essentially different from SI engines. In SI engines, a homogeneous mixture is available and during the combustion process a flame moves through the mixture. In CI engines, however, the liquid fuel that is injected at high velocities through small nozzles in the injector tip, atomizes into small drops and penetrates into the hot compressed air inside combustion chamber. As a result, the nature of combustion is an unsteady process occurring simultaneously at many spots in a very non-homogeneous fuel-air mixture.