Advanced IC Enginesairwalkbooks.com/images/pdf/pdf_55_1.pdfUNIT II COMPRESSION IGNITION ENGINES9 Diesel Fuel Injection Systems - Stages of combustion – Knocking – Factors affecting

Prof. R. DevarajDr. S. Ramachandran

Dr. A. Anderson

AIR WALK PUBLICATIONS

(Near All India Radio)

80, Karneeshwarar Koil Street

Mylapore, Chennai - 600 004.

Ph.: 2466 1909, 94440 81904

Email: [email protected],

[email protected]

www.airwalkpublications.com

AdvancedIC Engines

As per Revised Syllabus of Leading UniversitiesAs per Revised Syllabus of Leading Universities

ProfessorsSchool of Mechanical Engineering

Sathyabama UniversityChennai - 600 119

First Edition : 12-12-2016

ISBN : 978-93-84893-62-0

http://www.foxitsoftware.com/shopping

ME6016 ADVANCED I.C ENGINES

UNIT I SPARK IGNITION ENGINES 9

Mixture requirements – Fuel injection systems – Monopoint, Multipoint

& Direct injection - Stages of combustion – Normal and Abnormal

combustion – Knock - Factors affecting knock – Combustion chambers.

UNIT II COMPRESSION IGNITION ENGINES 9

Diesel Fuel Injection Systems - Stages of combustion – Knocking –

Factors affecting knock – Direct and Indirect injection systems – Combustion

chambers – Fuel Spray behaviour – Spray structure and spray penetration –

Air motion - Introduction to Turbocharging.

UNIT III POLLUTANT FORMATION AND CONTROL 9

Pollutant – Sources – Formation of Carbon Monoxide, Unburnt

hydrocarbon, Oxides of Nitrogen, Smoke and Particulate matter – Methods

of controlling Emissions – Catalytic converters, Selective Catalytic Reduction

and Particulate Traps – Methods of measurement – Emission norms and

Driving cycles.

UNIT IV ALTERNATIVE FUELS 9

Alcohol, Hydrogen, Compressed Natural Gas, Liquefied Petroleum Gas

and Bio Diesel - Properties, Suitability, Merits and Demerits - Engine

Modifications.

UNIT V RECENT TRENDS 9

Air assisted Combustion, Homogeneous charge compression ignition

engines – Variable Geometry turbochargers – Common Rail Direct Injection

Systems - Hybrid Electric Vehicles – NOx Adsorbers - Onboard Diagnostics.

CONTENTS

Chapter - 1

Spark Ignition Engines

1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1

1.2 Mixture Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2

1.2.1 Mixture requirements at full throttle and constant speeds. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.31.2.2 Mixture requirements at various loads . . . . . . . . . . . 1.41.2.1 Idling range . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.51.2.2 Cruising/Normal range/Medium load . . . . . . . . . . . 1.61.2.3 Power range . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.61.2.4 Effects of operating variables on mixture requirements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.7

1.3 Fuel Injection Systems for S.I. Engines . . . . . . . . . . . . . . . 1.7

1.3.1 Different types of Fuel Systems . . . . . . . . . . . . . . . . . 1.81.3.2 Fuel Supply System in SI Engines. . . . . . . . . . . . . . 1.81.3.3 Carburetor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.91.3.4 Simple Carburetor . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.101.3.5 Various Compensation in Carburetors . . . . . . . . . . 1.111.3.6 Types of Carburetors . . . . . . . . . . . . . . . . . . . . . . . . . 1.14

1.4 Gasoline Injection System . . . . . . . . . . . . . . . . . . . . . . . . . . 1.14

1.4.1 Reasons for adopting gasoline injection system. . . 1.141.4.1 Continuous fuel injection system . . . . . . . . . . . . . . . 1.151.4.2 Timed fuel injection system . . . . . . . . . . . . . . . . . . . 1.15

1.5 Monopoint Fuel Injection System . . . . . . . . . . . . . . . . . . . . 1.16

1.6 Multipoint-injection System . . . . . . . . . . . . . . . . . . . . . . . . . 1.17

1.6.1 D-MPFI system. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.191.6.2 L-MPFI system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.20

1.7 Direct Injection System . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.21

Contents C.1

1.8 Electroniclly Controlled Gasoline Injection System . . . . . 1.22

1.9 Stages of Combustion in SI Engines . . . . . . . . . . . . . . . . . 1.26

1.10 Combustion Phenomenon . . . . . . . . . . . . . . . . . . . . . . . . . . 1.28

1.10.1 Normal Combustion . . . . . . . . . . . . . . . . . . . . . . . . . 1.281.10.1.1 Factors affecting normal combustion in S.I Engines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.30

1.10.2 Abnormal Combustion . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.31

1.10.2.1 Pre-ignition. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.311.10.2.2 Knocking (or) Detonation (or) Pinking. . . . . . . . 1.32

1.11 Flame Front Propagation . . . . . . . . . . . . . . . . . . . . . . . . . . 1.33

1.12 Importance of Flame Speed And Effect of Engine Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.34

1.12.1 Factors affecting flame speed . . . . . . . . . . . . . . . . . 1.341.13 The Phenomenon of Knock in SI Engine . . . . . . . . . . . . 1.36

1.13.1 Effects of knocking in SI Engine . . . . . . . . . . . . . 1.371.14 Factors Affecting Knock in SI Engines . . . . . . . . . . . . . . 1.38

1.15 Fuel Requirement And Fuel Rating . . . . . . . . . . . . . . . . . 1.41

1.15.1 Important properties of fuel in SI Engine . . . . . . 1.411.15.2 Important characteristics of SI Engine fuel. . . . . 1.411.15.3 Fuel properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.421.15.4 Octane Number. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.44

1.16 Anti-knock Additives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.45

1.16.1 Anti-knock Agents. . . . . . . . . . . . . . . . . . . . . . . . . . . 1.461.16.2 Effects of Anti knock additives . . . . . . . . . . . . . . . 1.461.16.3 Factors affecting Detonation and Remedies . . . . . 1.47

1.17 Combustion Chamber for SI Engines. . . . . . . . . . . . . . . . 1.47

1.17.1 Types of combustion chambers. . . . . . . . . . . . . . . . 1.48

C.2 Advanced IC Engines

Chapter - 2

Compression Ignition Engines2.1 Diesel Fuel Injection Systems . . . . . . . . . . . . . . . . . . . . . . . . 2.1

2.1.1 Fuel Pump (C.I. Engine) . . . . . . . . . . . . . . . . . . . . . . 2.12.1.2 Fuel Injection System . . . . . . . . . . . . . . . . . . . . . . . . . 2.3

2.2 Electronically Controlled Diesel Injection Systems . . . . . . 2.4

2.2.1 Classification of Diesel Fuel Injection Pumps . . . . . 2.52.2.2 Rotary Distributor Type Fuel Injection System – Electronically Controlled . . . . . . . . . . . . . . . . . . . . . . . 2.52.2.3 Unit Injector System . . . . . . . . . . . . . . . . . . . . . . . . . . 2.72.2.4 Electronic Controlled Common Rail Type Fuel Injection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.9

2.3 Stages of Combustion in CI Engines . . . . . . . . . . . . . . . . . 2.10

2.4 Factors That Affect Delay Period in Diesel Engine . . . . 2.13

2.4.1 Effect of variables on the Delay period . . . . . . . . . 2.142.5 Knocking (or) Diesel Knock . . . . . . . . . . . . . . . . . . . . . . . . . 2.15

2.5.1 Phenomenon of knock in CI engine . . . . . . . . . . . . 2.152.4.2 Comparison of knock on SI and CI Engines . . . . 2.172.4.3 Characteristics Tending to Reduce Detonation . . . 2.19

2.5 Need for Air Motion in Diesel Engine . . . . . . . . . . . . . . . 2.19

2.6 Types of Injection Systems . . . . . . . . . . . . . . . . . . . . . . . . . 2.20

2.6.1 Direct injection system . . . . . . . . . . . . . . . . . . . . . . . 2.212.6.2 Indirect injection system . . . . . . . . . . . . . . . . . . . . . . 2.22

2.7 Combustion Chamber Design for Compression Ignition Engine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.24

2.7.1 Open combustion chamber . . . . . . . . . . . . . . . . . . . . 2.242.7.2 Divided combustion chamber (or) Indirect combustion chamber . . . . . . . . . . . . . . . . . . . . . . . . . 2.262.7.3 Open and Divided combustion chambers . . . . . . . . 2.31

Contents C.3

2.7.4 Characteristics of Common Diesel Combustion Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.32

2.8 Diesel Fuel Requirement : For Compression Ignition Engines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.33

2.8.1 Cetane Number. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.342.8.2 Fuel Rating for CI Engine . . . . . . . . . . . . . . . . . . . . 2.34

2.9 Fuel Spray Behaviour. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.35

2.9.1 Fuel injection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.362.9.2 Spray structure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.362.9.3 Spray penetration. . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.382.9.4 Droplet size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.40

2.10 Supercharging. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.41

2.11 Introduction To Turbocharging . . . . . . . . . . . . . . . . . . . . . 2.42

2.11.1 Principle of the turbocharger operation . . . . . . . . 2.422.11.2 Advantages of turbochargers . . . . . . . . . . . . . . . . . 2.422.11.3 Waste Gate Turbocharger (WGT). . . . . . . . . . . . . . 2.43

2.12 Comparison Between Petrol Engine And Diesel Engine 2.45

Chapter - 3

Pollutant Formation and Control

3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1

3.1.1 Pollution and pollutants . . . . . . . . . . . . . . . . . . . . . . . 3.13.2 Sources of Pollutants From IC Engine . . . . . . . . . . . . . . . . 3.2

3.2.1 Crankcase emissions . . . . . . . . . . . . . . . . . . . . . . . . . . 3.33.2.2 Evaporative emissions . . . . . . . . . . . . . . . . . . . . . . . . . 3.33.2.3 Exhaust emissions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4

3.3 Carbon Monoxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5

3.3.1 Formation of CO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.53.4 Unburnt Hydrocarbons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6

3.4.1 Formation of HC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6


3.5 Oxides of Nitrogen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7

3.5.1 Formation of NO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.73.6 Smoke . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7

3.6.1 Causes of smoke . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.83.6.2 Formation of smoke . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.83.6.3 Smoke emissions in IC engine . . . . . . . . . . . . . . . . . . 3.9

3.7 Particulate Matter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.10

3.8 Methods of Controlling Emissions . . . . . . . . . . . . . . . . . . . 3.10

3.9 Emission Control By Chemical Methods . . . . . . . . . . . . . . 3.10

3.9.1 Control of Sulphur Dioxide . . . . . . . . . . . . . . . . . . . 3.103.9.2 Control of Nitrogen Oxides. . . . . . . . . . . . . . . . . . . . 3.113.9.3 Control of Carbon Monoxide and Hydrocarbon. . . 3.11

3.10 Emission Control in SI Engines . . . . . . . . . . . . . . . . . . . . 3.12

3.10.1 Engine design modifications. . . . . . . . . . . . . . . . . . 3.123.10.2 Operating parameter modifications . . . . . . . . . . . . 3.133.10.3 Treatment of exhaust products of combustion . . . 3.133.10.4 Fuel modifications . . . . . . . . . . . . . . . . . . . . . . . . . . 3.13

3.11 Control of Oxides of Nitrogen . . . . . . . . . . . . . . . . . . . . . . 3.14

3.11.1 Exhaust Gas Recirculation . . . . . . . . . . . . . . . . . . . 3.143.11.2 Catalyst . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.163.11.3 Water injection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.16

3.12 Emission Control Using Converters . . . . . . . . . . . . . . . . . 3.16

3.12.1 Thermal converter . . . . . . . . . . . . . . . . . . . . . . . . . . 3.173.12.2 Catalytic converter . . . . . . . . . . . . . . . . . . . . . . . . . . 3.173.12.2.3 Engine Emission Control by Three way catalytic converter system . . . . . . . . . . . . . . . . . . 3.203.12.2.4 Oxygen storage. . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.213.12.2.5 Diesel Oxidation Catalyst . . . . . . . . . . . . . . . . . . 3.21

3.13 Effect of Engine Emission on Human Health . . . . . . . . 3.22

3.14 Selective Catalytic Reduction. . . . . . . . . . . . . . . . . . . . . . . 3.24

Contents C.5

3.15 Particulate Traps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.25

3.15.1 Regeneration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.273.15.2 Other methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.283.15.3 Methods to determine soot capacity in particulate traps . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.28

3.16 Methods of Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . 3.29

3.16.1 NDIR Method (Non Dispersive Infra-Red method)3.293.16.2 Flame ionization detector . . . . . . . . . . . . . . . . . . . 3.303.16.3 Chemiluminescence analyzes - NOx detector . . . . 3.32

3.16.4 Smoke measurement . . . . . . . . . . . . . . . . . . . . . . . . 3.323.16.5 Measurement of particulate . . . . . . . . . . . . . . . . . . 3.37

3.17 Emission Norms (EURO and BS). . . . . . . . . . . . . . . . . . . 3.38

3.17.1 Euro Norms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.393.17.2 BS Norms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.39

3.18 Driving Cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.42

3.18.1 Constant Volume Sampler . . . . . . . . . . . . . . . . . . . 3.423.18.2 Indian Driving Cycle . . . . . . . . . . . . . . . . . . . . . . . . 3.44

Chapter 4

Alternative Fuels4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1

4.2 An Outlook on the Properties of Alternative Fuels. . . . . . 4.1

4.3 Alternate Sources of Energy . . . . . . . . . . . . . . . . . . . . . . . . . 4.2

4.4 Natural Gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2

4.4.1 Formation of Natural gas. . . . . . . . . . . . . . . . . . . . . . 4.24.4.2 Components of Natural gas . . . . . . . . . . . . . . . . . . . . 4.44.4.3 Characteristics of Natural gas . . . . . . . . . . . . . . . . . . 4.44.4.4 Production of Natural gas . . . . . . . . . . . . . . . . . . . . . 4.54.4.5 Types of Natural gas. . . . . . . . . . . . . . . . . . . . . . . . . . 4.64.4.6 Compressed Natural Gas . . . . . . . . . . . . . . . . . . . . . . 4.6 4.4.6.1 Compressed Natural Gas for SI Engines . . 4.7


4.4.6.2 Engine modifications for Compressed Natural gas in CI engines . . . . . . . . . . . . . . 4.7 4.4.6.3 Merits of Compressed Natural Gas . . . . . . . 4.8 4.4.6.4 Demerits of Compressed Natural Gas. . . . . 4.84.4.7 Liquefied Natural Gas . . . . . . . . . . . . . . . . . . . . . . . . 4.84.4.8 Applications of Natural gas . . . . . . . . . . . . . . . . . . . . 4.9

4.5 Biodiesel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.10

4.5.1 Production of Biodiesel . . . . . . . . . . . . . . . . . . . . . . . 4.114.5.2 Engine modifications for Biodiesel . . . . . . . . . . . . . 4.134.5.3 Merits of Biodiesel . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.134.5.4 Demerits of Biodiesel . . . . . . . . . . . . . . . . . . . . . . . . . 4.14

4.6 LPG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.15

4.6.1 Production of Liquefied Petroleum Gas . . . . . . . . . 4.164.6.2 Properties of LPG . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.16

4.7 Methanol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.18

4.8 Ethanol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.19

4.8.1 Production of Ethanol . . . . . . . . . . . . . . . . . . . . . . . . 4.204.8.2 Another methods of Ethanol production. . . . . . . . . 4.214.8.2 Merits of ethanol as a fuel . . . . . . . . . . . . . . . . . . . 4.244.8.3 Demerits of ethanol as a fuel . . . . . . . . . . . . . . . . . 4.244.8.4 Alcohol for SI engines . . . . . . . . . . . . . . . . . . . . . . . . 4.244.8.5 Alcohol for CI Engines . . . . . . . . . . . . . . . . . . . . . . . 4.25

4.9 Hydrogen. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.26

4.9.1 Hydrogen properties . . . . . . . . . . . . . . . . . . . . . . . . . . 4.274.9.2 Production of Hydrogen . . . . . . . . . . . . . . . . . . . . . . 4.334.9.3 Hydrogen storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.374.9.4 Engine modifications for hydrogen fuel in SI and CI engines . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.384.9.5 Merits of Hydrogen fuel . . . . . . . . . . . . . . . . . . . . . . 4.404.9.6 Demerits of Hydrogen fuel . . . . . . . . . . . . . . . . . . . . 4.40

Contents C.7

Chapter - 5

Recent Trends5.1 Air Assisted Combustion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1

5.1.1 Air Assisted combustion system . . . . . . . . . . . . . . . . . 5.35.2 Homogeneous Charge Compression Ignition (HCCI) Engines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3

5.3. Variable Geometry Turbocharger (VGT) . . . . . . . . . . . . . . . 5.8

5.4 Common Rail Direct Injection (CRDI) . . . . . . . . . . . . . . . . . 5.9

5.4.1 Components of CRDI System . . . . . . . . . . . . . . . . . . 5.105.4.2 Working of CRDI . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.115.4.3 Benefits of CRDI . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.125.4.4 Advantages of the Common Rail System over the Conventional System . . . . . . . . . . . . . . . . . . . . . . 5.12

5.5 Electric Vehicles (EV) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.12

5.5.1 Types of Electric Vehicles . . . . . . . . . . . . . . . . . . . . . 5.135.5.2 General Configuration of Electric Vehicle . . . . . . . 5.135.5.3 Advantages of Electric vehicle . . . . . . . . . . . . . . . . . 5.15

5.6 Hybrid Electric Vehicles . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.15

5.6.1 Series hybrid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.155.6.2 Parallel hybrid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.16

5.7 Adsorber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.17

5.8 NOx Adsorber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.18

5.9 On-Board Diagnostics (OBD) . . . . . . . . . . . . . . . . . . . . . . . . 5.20

5.9.1 OBD system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.205.9.2 Basic OBD procedure . . . . . . . . . . . . . . . . . . . . . . . . 5.21


Chapter - 1

Spark Ignition Engines

Mixture requirements - Fuel injection systems - Monopoint, Multipoint

& Direct injection - Stages of combustion - Normal and Abnormal combustion

- knock - Factors affecting knock - Combustion chambers.

1.1 INTRODUCTION

Internal combustion engines are basically classified into

(i) Spark ignition engine

(ii) Compression ignition engine

Spark ignition engines (or) petrol engines work on otto cycle (or)

constant volume heat addition cycle.

In a typical four stroke SI engine, the cycle of operation is completed

in four strokes of the piston or two revolutions of the crankshaft.

The cycle of operation for an ideal four stroke SI engine consists of

the following four strokes.

1. Suction (or) Intake stroke

2. Compression stroke

3. Power (or) expansion stroke

4. Exhaust stroke

In SI engines, during the suction stroke the mixture of air-fuel is

injected into the cylinder. The air-fuel mixture is injected via the carburetor

that controls the quantity and quality of the injected mixture. Then the air-fuel

mixture is ignited with the help of spark from the spark plug. Here the

compression ratio ranges from 6 to 10 depending on the size of the engine

and power required.

SI engines are high speed engines because of the following reasons.

(i) Engine is light in weight.

(ii) Fuel used in SI engines are burnt homogeneously.

Air-fuel ratio (AFR)

It is the ratio of air to fuel in the working charge of an internal

combustion engine or in other combustion mixtures.

It is generally expressed by weight for liquid fuels and by volume for

gaseous fuels.

AFR mair

mfuel

mair mass of air ; mfuel mass of fuel

Fuel-air ratio (FAR)

It is the ratio of the mass of fuel to the mass of air in the fuel-air

mixture.

FAR 1

AFR

mfuel

mair

1.2 MIXTURE REQUIREMENTS

Oxygen is very much necessary to burn the fuel. This oxygen is taken

from atmospheric air. The proper proportion of air and fuel mixture should

be obtained for complete combustion of fuel.

For complete combustion, the Air-Fuel ratio should be approximately15:1 by weight. This is known as chemically correct (or) stoichiometric airfuel ratio.

The normal range of Air-fuel ratio is in between 20:1 to 8:1approximately.

Air-fuel ratio during starting is approximately 10:1 - i.e., very richmixture.

Air-fuel ratio during idling speed (low speed) is approximately 12:1 -i.e., rich mixture.

Air-fuel ratio during normal running condition, is approximately 15:1neither rich nor lean mixture.

Air-fuel ratio for economic running (medium load), is approximately17:1 - economic mixture.

Air-fuel ratio during overtaking, is approximately 12:1 - rich mixture.

1.2 Advanced IC Engines - www.airwalkpublications.com

1.2.1 Mixture requirements at full throttle and constant speeds

The air-fuel ratio at which an engine operates has a considerable

influence on its performance.

Consider a spark ignition engine operating at full throttle and constant

speed with varying Air/fuel ratio. Refer Fig. 1.2.

Excess Fuel Excess air

9 15 19

Fig:1.1.Air Fuel Ratio

TooRich

TooLean

Best Pow er

bsfc

Best Econo my

Pow er ou tput

S toich iom etric M ix ture

8 10 12 14 16 18 20 22

A/F R a tio (kg of air / kg o f F ue l )

bsfc

(kg

/kW

h)

Pow

er o

utpu

t (kW

)

Fig:1 .2

Spark Ignition Engines 1.3

On this condition, the air/fuel ratio has a major impact on both power

output and brake specific fuel consumption.

The A/F mixture corresponding to the maximum power output on the

curve is called best power mixture with an A/F ratio of 12:1.

The mixture corresponding to the minimum point on the brake specific

fuel consumption (bsfc) curve is called the best economy mixture with an

A/F ratio of about 16:1.

NOTE

Best power mixture is much richer than the chemically correct mixture.

Best economy mixture is slightly leaner than the chemically correct

mixture.

1.2.2 Mixture requirements at various loads

In practical cases, the air-fuel mixture requirements in an automobile

engine vary considerably from the ideal condition discussed above.

For effective operation of the SI engine, the carburetor has to provide

Air/fuel mixtures which follow the shape of the curve PQRS incase of single

cylinder and P Q R S incase of multi-cylinder engine as shown in Fig. 1.3

The carburetor should be suitably designed in order to meet the various

engine requirements.

Thro ttle O pen ing (% )50 1000

Best Economy

S

S

P

P

Best Pow er

20

10

5

C hem ica lly C orrect M ix tu re

A/F

Ra

tio (

kg o

f ai

r / k

g o

f

Fu

el)

Lea

n R

ich

Q R

Q R

Fig: 1.3 Anticipated Carburetor Performance to fu lfill Engine Requirements

M ulti C ylinde rS ing le C ylinder


There are three different ranges of throttle operations as shown in

Fig.1.3. They are

1. Idling (requires rich mixture)

2. Cruising (requires lean mixture)

3. High power (requires rich mixture)

In each of the above mentioned cases, the Air/fuel mixture

requirements may subject to vary.

1.2.1 Idling range (Requires rich mixture)

Running of engine under no-load condition is called idling.

During idling range, the throttle is nearly closed and the suction

pressure is very low i.e pressure in the intake manifold is below the

atmospheric pressure due to restriction in the air flow.

When the intake valve opens, the pressure difference between the

combustion chamber and intake manifold leads to backward flow of exhaust

gases into the intake manifold.

As the piston moves down on the intake stroke, these exhaust gases

are drawn back into the combustion chamber and mixes with the fresh charge

entering into the combustion chamber.

As a result, the final air-fuel mixture in the combustion chamber gets

diluted which leads to poor combustion and loss of power.

Therefore it is necessary to provide more fuel particles by richening

the air-fuel mixture.

The richening of mixture increases the probability of contact between

fuel and air particles and thus improves combustion process.

In short we can say that the A/F ratio for idling and low loads should

be rich for smooth operation.

FuelAir

0.08 or AirFuel

12.5

Refer Fig. 1.3. The curve PQ in the graph represents the idling range

of SI engine.


1.2.2 Cruising/Normal range/Medium load (requires lean mixture)

As the throttle is gradually opened from point P to Q (refer Fig. 1.3),

the pressure difference between the intake manifold and the combustion

chamber becomes smaller and exhaust gas dilution of the fresh charge

diminishes.

Mixture requirements then proceed further along line PQ to a leanerA/F ratio required for the cruising operation.

The line QR (refer Fig. 1.3) represents the cruising range of SI engine.

This is the region where engine runs most of the time therefore it isdesirable that the running should be most economical in this condition.

So a lean mixture can be supplied, as engine has low fuel consumptionat medium load.

Lean mixture at medium load is necessary for good fuel economy.

1.2.3 Power range (requires rich mixture)

From Fig. 1.3, the curve RS represents the maximum power range

zone.

When maximum power is required, the engine must be supplied with

rich mixture as the economy is not a consideration.

As the engine enters in the power range, the spark must be retarded

otherwise knocking would occur.

The rich mixture at the time of maximum power range is required for

following reasons.

1. To provide maximum power and

2. To prevent overheating of exhaust valve and the area near it.

The mixture requirements for maximum power is a rich mixture of

A/F about 14:1 or F/A = 0.07.

In multicylinder engines, the A/F ratios are slightly richer mixture in

order to overcome the mal distribution of air-fuel mixture in different

cylinders. Maximum power/acceleration is required at the time of

(i) Overtaking a vehicle

(ii) Climbing up a hill


For starting under extremely cold conditions, a very rich mixture will

be required.

1.2.4 Effects of operating variables on mixture requirements

Some of the effects of operating variables such as inlet and exhaust

pressures. Spark timing and friction are described below.

1. Inlet and exhaust pressure

(i) Decrease in inlet pressure due to throttling or operating at higher

altitudes leads to reduction in flame speed and increase in fuel/air ratio

for best economy.

(ii) Increase in exhaust pressure results in reduced flame speeds and increase

in fuel/air ratio for best economy.

2. Spark timing

Any variation from the optimum spark timing will lead to the increase

of best economy Fuel/Air ratio, since it will increase the time losses.

3. Friction

By keeping the indicated mean effective pressure (I.M.E.P) constant,the increase in friction mean effective pressure (F.M.E.P) will result in theincrease of fuel/air ratio for best economy.

1.3 FUEL INJECTION SYSTEMS FOR S.I. ENGINESTo run S.I. engine, the petrol from the fuel tank must reach cylinder.

The petrol vaporize easily at atmospheric condition, therefore the enginesuction is sufficient to vaporize petrol. In petrol engine, the petrol from thefuel tank reaches the cylinder through fuel pump, filter and carburetor. Thus,the fuel feed system of a petrol engine consists of the following components.

1. Fuel tank, 2. Fuel pump, 3. Fuel filter 4. Carburetor, 5. Intake

manifold, 6. Fuel tubes for necessary connections, 7. Gauge to indicate the

driver about the fuel level in the fuel tank.

The fuel system is used for the following reasons.

To store fuel in the fuel tank

To supply fuel in the required amount and at proper condition

To indicate the driver about the fuel level in the fuel tank.


1.3.1 Different types of Fuel Systems

The fuel from the fuel tank is supplied to the engine cylinder by the

following systems: (a) Gravity system, (b) Pressure system, (c) Vacuum

system, (d) Pump system, (e) Fuel injection system

In gravity system, the fuel tank is placed above the carburetor. The

fuel flows from the tank to the carburetor due to the gravitational force. Thus

the system does not have fuel pump. This system is cheap and simple one.

The fuel tank is directly connected to the carburetor. Motor cycles and

scooters use this system.

In pressure system, the pressure is created inside the tank by means

of a pump, and the fuel flows to the carburetor. In this system the tank can

be placed above or below the carburetor.

In a vacuum system, the engine suction is used for sucking the petrol

from the main tank to the auxiliary fuel tank and then it flows by gravity

to the carburetor.

In pump system, a fuel feed pump is used to feed the petrol from the

fuel tank to the carburetor. In this system the fuel tank can be placed at any

suitable position in the vehicle.

In fuel injection system, a fuel injection pump is used in place of

carburetor. The fuel is atomized by means of a nozzle and then delivered

into an air stream.

1.3.2 FUEL SUPPLY SYSTEM IN SI ENGINES

A schematic diagram of fuel supply system is shown in Fig.1.4. Here,

the storage tank is located below the carburetor the fuel pump sucks the

petrol from tank and pumps it to carburetor through fuel filter. Filter is used

to prevent the dust and other materials going along with petrol.

StorageTank

FuelPump

Fuel filter Carburettor

Fig 1.4 A Schem atic diagram of fuel supply system.

Engine


Fuel Pump (for S.I. Engine)

Refer Fig. 1.5. This type of pump is used in petrol engine when the

cam shaft rotates, it pushes the lever in upward direction. This upward

movement pulls the diaphragm downward. It creates a vacuum in the pump

chamber and the petrol comes to pump chamber from the glass bowl. Strainer

is used to prevent the impurities of the fuel coming along with fuel. On the

return stroke, the spring pushes the diaphragm in the upward direction and

the petrol is forced to carburetor.

1.3.3 Carburetor

Carburetor is a device which is used for atomizing and vaporizing the

fuel (petrol) and mixing it with the air in varying proportions, to suit the

changing operating conditions of the engine.

Atomization is the breaking up the liquid fuel (petrol) into very small

particles so that it is properly mixed with the air. But vaporization is the

change of state of the fuel from liquid to vapour. Carburetor performs both

the process i.e., atomization of the fuel and vaporization of the fuel.

Outle t valve

Diaphragm

Pum p Chamber

Spring

Hinged poin t

Cam

Strainer

G lass bowl

Fig.1.5 Fuel pump for SI Engine


1.3.4 SIMPLE CARBURETOR

A simple carburetor refer Fig. 1.6 consists of following 1. float and

float chamber, 2. venturi and throttle valves and 3. choke valve.

1. Float and Float chamber

The petrol is supplied to the float chamber from the fuel tank through

the filter and fuel pump. When the petrol in float chamber reaches a particular

level, the needle valve blocks the inlet passage and thus cuts off the petrol

supply. On the fall of the petrol level in the float chamber, the float descends

down and inlet passage opens. The petrol is supplied to the chamber again.

Thus a constant fuel (petrol) level is maintained in the float chamber. The

float chamber supplies the petrol to the main discharge jet placed in venturi

tube. The level of fuel in the float chamber is kept slightly below the top

of the jet to prevent the leakage when not operating.

ChokeValve

To Engine

M ixtu re

Air

Float cham ber

Float

Vent

Fuel In le t

Need le valve

xVenturi

FuelJet

ThrottleValve

Fig.1.6 Sim ple Carburetor

2 2


2. Venturi and Throttle valve

The carburetor consists of a narrower passage at its centre, called

venturi. One end of the carburetor is connected with the intake manifold of

the engine. During the suction stroke, vacuum is created inside the cylinder.

Due to vacuum, the air is sucked to the carburetor. The velocity of the air

increases as it passes through the venturi where the area of cross section is

minimum. Due to increased velocity of air at the venturi, the pressure at the

venturi decreases. Therefore a low pressure zone is created in the venturi. So

the jet (nozzle) located at the venturi is in the zone of low pressure. The

fuel comes out from jet (nozzle) in the form of fine spray. This fuel spray

is mixed with air and the mixture is supplied to the intake manifold of the

engine. The throttle valve is placed between the jet (nozzle) and the intake

manifold of the engine. The quantity of the mixture is controlled by means

of throttle valve.

3. Choke Valve

While starting in cold weather the engine needs extra rich mixture. So

a choke valve is introduced in the air passage before the venturi. When the

choke valve is closed it creates high vacuum near the fuel jet and small

quantity of air is allowed, to get rich mixture. The fuel flow increases as the

vacuum near the jet increases.

1.3.5 Various Compensation in Carburetors

A simple carburetor cannot supply different air-fuel ratio according to

the speeds and loads of the engine.

Supply of correct airfuel ratio to meet the existing condition is known

as the compensation in carburetor. The various compensations in carburetor

are given below.

1. Auxiliary (or) extra air valve compensation

2. Restricted air bleed compensation

3. Compensating jet compensation

4. Economiser needle in metering jet.


1. Auxiliary (or) Extra air valve compensation

An extra air valve is provided to the carburetor to supply extra air to

mixture, when the throttle valve is opened more and more. So the air-fuel

ratio (mixture strength) is maintained constant.

2. Restricted air-bleed compensation

Air

Part o f floatcham ber

To intake m an ifo ld

Thro ttle

O uterenc lose r

Jet Tub e

R estric te d a irb leed ope ning

Fig:1.8 Restric ted air-bleed com pensation

Air

Extra a irvalve

Air

Part of floatcham ber

To in take m an ifold

Thro ttle

Fig: 1.7 Auxiliary (or) Extra air valve com pensation


Here, a jet tube having openings at its periphery is provided in the

carburetor. Refer Fig. 1.8. A restricted air bleed opening is connecting the

main air passage to the outer enclosure of the jet tube.

During starting and slow speed, more quantity of fuel flows into

venturi to give rich mixture.

During high speed, the throttle valve opens more and the vacuum in

the venturi become more. So more fuel is drawn and sprayed by nozzle. But

at this stage, the air bubbles start bleeding through the jet-tube and make the

mixture lean.

3. Compensating Jet Compensation

In this system, main jet and compensating jet are provided in the

carburetor. Refer Fig. 1.9. The main jet is connected to float chamber directly.

The compensating jet is connected to float chamber through tube C whose

top end is open to atmosphere. For normal throttle valve openings, both the

jets supply fuel to venturi. But when the throttle opens more and more, the

fuel supply from main-jet increases and the fuel supply from compensating

Air

Part o f floatcha m ber

To intake m an ifo ld

Thro ttle

c

O pen to a tm osphere

AM ain je t

B C om pensatin g jet

Fig: 1.9 Com pensating jet Compensation


jet decreases due to falling level of fuel in tube C. Because of atmospheric

pressure acting in this tube C, the richness of mixture decreases.

4. Economiser needle in metering jet

The flow of fuel is controlled by changing the area of the metering

nozzle supplying fuel from float chamber to the main jet. The area is changed

by means of a needle operated by linkage with accelerator pedal.

1.3.6 TYPES OF CARBURETORS

There are four important types of carburetor

1. Zenith Carburetor

2. Solex Carburetor

3. Amal Carburetor

4. Carter Carburetor

1.4 GASOLINE INJECTION SYSTEM

In a multicylinder engine with a carburetor, it is difficult to obtain

uniform mixture in each cylinder. The various cylinders receive the gasoline

mixture in varying quantities and richness. This problem is called

mal-distribution and the above mentioned problem can be solved by using

gasoline injection system.

By adapting gasoline injection, each cylinder can get the same richness

of the air-gasoline mixture and the mal-distribution can be avoided to a great

extent.

1.4.1 Reasons for adopting gasoline injection system

To have uniform distribution of fuel in a multi-cylinder engine.

To reduce (or) eliminate detonation.

To improve volumetric efficiency.

To improve fuel atomization by forcing fuel under pressure into

the cylinder.

To prevent fuel loss during scavenging in case of two-stroke

engines.


Fuel-injection system in SI engine can be classified as follows:

In indirect injection, fuel is injected into the air stream prior to entering

the combustion chamber. In direct injection, fuel is injected directly inside

the combustion chamber.

The gasoline fuel injection system used in a spark-ignition engine can

be either of continuous injection or timed injection.

1.4.1 Continuous fuel injection system (CIS)

In continuous injection system, the injection nozzle and its valves

are permanently opened while the engine is running, so that the

fuel is injected continuously into the combustion chamber.

This system usually employs rotary pumps for fuel injection.

This pump maintains a fuel line gauge pressure of about 0.75 to

1.5 bar.

The timing and duration of the fuel injection is determined by the

Electronic Control Unit (ECU) depending upon the load and speed.

1.4.2 Timed fuel injection system

In this system, the fuel is sprayed from the injector nozzle in

pulses at certain time i.e usually during the early part of the intake

(or) suction stroke.

Fuel-in jection system s

Indirect in ject ion (ID I)(M echanical (o r) e lectron ic contro l)

D irect injection (D I)(M echanical (o r) e lectron ic contro l)

M ono-Point In jection(M PI)

M ulti-po in t fuel injection(M PFI)

Continuous ContinuousTimed Timed

Timed


This system has a fuel supply pump which sends fuel at a low

pressure at about 2 bar when the engine is running at maximum

speed.

The length of time for fuel injection is determined by Electronic

Control Unit (ECU) depending on input signals from various

engine sensors.

1.5 MONOPOINT FUEL INJECTION SYSTEM

A monopoint injection system is also called as throttle body injection

system (TBI).

In this system, the injector nozzle is mounted just above the throat of

the throttle body as shown in Fig. 1.10.

Fig:1.10 Mono Point Fuel In jection(or) Throttle Body In jection (TBI)

A ir

Intake m anifold

Fuel

T hro ttle va lve

In jector

Engine

C 1 C 2 C 3 C 4

Thro ttle Body


The throttle body assembly resembles like a carburetor except that

there is no fuel bowl float (or) metering jets.

Here, the injector nozzle sprays gasoline into the air in the intake

manifold so that the gasoline mixes with air.

This air-fuel mixture then passes through the throttle valve and enters

into the intake valve.

Moreover, this system requires only one circuit in the computer to

control injection which simplifies the construction of electronic control unit

(ECU). Thus it reduces the cost of the system.

In this system, injection pressure is higher compared to carburetor

discharge pressure, which speeds up and improves the atomization of the

liquid fuel. However, maldistribution of fuel cannot be avoided. To overcome

maldistribution of fuel, multi-point fuel injection system can be used.

Advantages

1. Monopoint injection system meters fuel better than a carburetor.

2. Reduced fuel consumption.

3. Less expensive and easier to service.

1.6 MULTIPOINT-INJECTION SYSTEM

A multi-point fuel injection system is also called as port-injection

system. Refer Fig. 1.11.

In this system, the injector nozzle is placed on the side of the intake

manifold. Here each cylinder is provided with separate fuel injector.

The injector nozzle sprays gasoline into the air inside the intake

manifold so that the gasoline mixes with air.

This air-fuel mixture then passes through the intake valve and enters

into the each cylinder.

The seperate fuel injector used in this system supplies the correct

quantity of fuel to each of the engine cylinders by a fuel-rail according to

the Firing order or in a ‘particular sequence’. This system provides further

precision by varying the fuel quantity and injection timing by governing the


each injector separately and thereby improving the performance and

controlling the emissions.

This technology consists of following parts:

1. Injectors

2. Fuel Pump

3. Fuel Rail

4. Fuel Pressure Sensor

5. Engine Control Unit

Fig:1.11.Multipoint fuel injection (MPFI)

Fuel

Air

Throttle valve

Intake m anifold

In jectorsIn jectors

Engine

C 1C2 C3

C 4

Intakem anifold

Cylinders


6. Fuel Pressure Regulator

7. Various Sensors - Crank/Cam Position Sensor, Manifold Pressure

Sensor, Oxygen Sensor

The Fig. 1.12 shows the parts of MPFI system.

The advantages of Multi point fuel injection are

Increased power and torque through improved volumetric

efficiency.

More uniform fuel distribution to each cylinder.

More rapid engine response to changes in throttle position.

Further, MPFI systems are classified into D-MPFI system and L-MPFI

system.

1.6.1 D-MPFI system

D-MPFI system is the manifold fuel injection system.

Here, the vacuum in the intake manifold is first sensed. Then the

volume of air is sensed by its density in the intake manifold.

The block diagram of D-MPFI system is shown in Fig. 1.13.

Fig:1.12:Multi Point Fuel Injection system

Fuel Pressure Regu la tor

Fuel Tank

Filte r

Fue

l Pum

p

Return line

Fuel Rail

In le t M anifold

Engine

Pressure Sensor

In jectors

Fuel F ilter

1 2 3 4


As air from the atmosphere enters into the intake manifold, the

manifold pressure sensor senses the intake manifold vacuum and sends the

information to the electronic control unit (ECU).

Likewise, the speed sensor senses the rpm of the engine and sends the

information to ECU.

After collecting required information from various sensors, the ECU

then sends commands to the injectors in order to regulate the amount of

gasoline supply for injection.

Finally, the injector sprays the required amount of fuel into the intake

manifold, thereby the gasoline mixes with the air and the mixture enters into

the cylinder.

1.6.2 L-MPFI system

L-MPFI system is a part fuel injection system.

Here, fuel metering is controlled by the engine speed and the amount

of air that enters the engine cylinder. It is also called as air-mass metering

or air-flow metering.

The block diagram of L-MPFI system is shown in Fig. 1.14.

Air

Intake M anifold Vacuum Sensor

Engine

In jection into

Intake M anifoldFuel In jector

Gasoline / P etrol

In jection Volum e Contro l

Electronic Control Un it (ECU)

RPM S ensor

M ixture ofAir and

Gasoline

Fig:1.13. D-MPFI Gasoline Injection System


As air from the atmosphere enters into the intake manifold, the airflow

sensors senses the amount of air. Then the sensed information is sent to the

ECU.

Likewise the speed sensor senses the engine rpm and sends the

information to the ECU.

After collecting required information, the ECU inturn sends commands

to the injector inorder to regulate the amount of gasoline supply for injection.

Then, the injector sprays the required fuel into the intake manifold

thereby the gasoline mixes with air and the mixture enters the cylinder.

1.7 DIRECT INJECTION SYSTEM

Direct Injection

In a direct injection engine, fuel is injected directly into the combustion

chamber. Modern gasoline engines may utilize direct injection using electronic

control, which is referred to as Gasoline Direct Injection (GDI). In IC engines,

GDI is also known as petrol direct injection, direct petrol injection, spark

ignition direct injection (SIDI) or fuel stratified injection (FSI). This is the

next step in evolution from multi-point injection. It reduces emissions and

fuel consumption.

Air

Engine

Fuel In jector

G aso line / P etrol

In ject ion Vo lum e C on tro l

E lectronic C o ntrol U n it (E C U )R PM

Sensor

M ixtu re ofA ir and

Fuel

Fig:1.14. L-M PFI Gasoline In jection System

Air Sensor D a ta in form ation

D a ta in form ation


1.8 ELECTRONICLLY CONTROLLED GASOLINE INJECTION SYSTEM

1.8.1 Description

The Bosch D-Jetronic electronic fuel injection system Fig. 1.16 is

composed of 3 major subsystems:

The air intake system

The fuel system, and

The electronic control system.

The D-Jetronic system uses constant fuel pressure and flow, so that

only injection duration time needs to be modified to control air/fuel mixture.

The D-Jetronic system measures incoming airflow by monitoring intake

manifold pressure. Engine speed, temperature, and other factors are monitored

for the purpose of fine-tuning injection duration. An auxiliary air valve, cold

start injector and thermo time switch are useful aid in cold starting and

operation. The simple layout of electronic fuel injection system is shown in

Fig. 1.17

1.8.2 Operation

1. Fuel systemAn electrically driven fuel pump forces fuel through a filter, into the

main system. Main system consists of one injector for each cylinder, a cold

start injector and a pressure regulator, which maintains fuel pressure at

Carburetor

Intake port

Carburetor

Injector

Fuel spray

Intake port

Port fuel injection(indirect injection)

Fuel spray

Injector

Direct injection

Fig 1.15


2 kg/cm2. A secondary system carries excess fuel from the pressure regulator

back to fuel tank.

2. Air systemIntake manifold, connected to an intake air distributor, supplies the

cylinders with air. A pressure sensor is connected to intake air distributor.

The pressure sensor operates according to difference in manifold pressure and

atmospheric pressure and signals control unit accordingly. A throttle valve,

operated by accelerator pedal, is located at the mouth of the intake air

distributor. The throttle valve and intake air distributor are connected to air

cleaner by an air duct elbow. The idling air system is in the form of a

by-pass system located between the air filter and air intake distributor. Its

size can be varied with an idling air adjusting screw. An auxiliary air line,

Filte r

Fuel tank

Fuel pum pRegula to r

A irtem perature

sensor Co ld startin jector

Fuelin jector

Extraa ir

valve

Thermotime

switch

Coolan ttem perature

sensorCoolan t

BatteryFrom

ign itionswitch

M ainrelay

Fuel pum prelay

Pressuresensor

E lectroniccon tro l unit

D is tributo r

Throttlepositionswtich

Fig:1.16.Bosch D-Jetronic Electronic Fuel Injection System


from air cleaner (auxiliary air valve), to intake air distributor forms the

warming-up air system. Its volume is varied, depending on engine

temperature, by the auxiliary air valve.

3. Electronic control system

Electronic Control UnitControl unit regulates the correct amount of fuel to be injected,

depending on engine speed, intake pressure and engine temperature. When

ignition is switched on, control unit receives its operating voltage directly

from battery, via voltage supply relay. It also controls the fuel pump, which

Fuel ra il

In jectorharness

Electronic control unit (E CU)

Return line

Fuel pressureregulator

Fuel filter Fuel pum p,high pressure

Fuel supply

Sensor harness

Thro ttleposition

Coolan ttem p

Oiltem p(if fitted)

Oxygensensor(if fitted)

Intake airtem p

Fig:1.17.Simple Layout of Electronic Fuel Injection System.


normally is provided with current from pump relay, only with engine running.

A time switch, in control unit, allows fuel pump to run approximately 1 to

1.5 seconds after ignition is turned on. The control unit is connected to all

sender units by a special wiring harness, coupled to a multiple plug. The

control unit is usually located inside vehicle under the dashboards, under one

of the seats or in the trunk.

Pressure SensorThe pressure sensor is located in the engine compartment and is

connected to the intake manifold by a vacuum hose. This sensor controls the

basic amount of fuel to be injected, depending on pressure in the intake

manifold and load on the engine

Air Intake Temperature SensorThe air intake temperature sensor provides control unit with

information about air temperature, so that control unit can increase the

injection quantity as necessary at low intake air temperature. This

compensation ceases when intake air temperature is greater than 20C.

Engine Temperature SensorThe engine temperature sensor provides the control unit with

information about coolant temperature (cylinder head temperature). This

enables control unit to adapt injection interval and determine how long the

cold start injector should remain open during cold starting.

Triggering ContactsThe triggering contacts are located in the distributor. They provide

signals which determine when and to which cylinder fuel is to be injected.

The contacts also supply information concerning engine speed to determine

the amount of fuel that needs to be injected into the engine.

Throttle Valve SwitchThe throttle valve switch is mounted on the throttle housing. This

switch signals the control unit of throttle position. During deceleration, above

1500 RPM, throttle switch cuts fuel supply off and below 900 RPM, fuel

supply is turned on.


Auxiliary Air ValveDuring cold starts, the auxiliary air valve opens to allow additional air

into the inlet duct. As engine heats up, a bi-metallic element expands and

closes this valve. At approximately 80C, the auxiliary air pipe is completely

closed by the valve.

1.9 STAGES OF COMBUSTION IN SI ENGINESA typical theoretical pressure-crank angle diagram during the process

of compression P Q, combustion Q R and expansion R S in an ideal

four-stroke spark-ignition engine is shown in Fig. 1.18. In an ideal engine,

the entire pressure rise during combustion takes place at constant volume i.e.,

at TDC. However, in practical cases this does not happen.

The pressure variation due to combustion in a practical engine is shown

in Fig. 1.19. In Fig.1.19, P is the point of passage of spark (say 20 before

TDC). Q is the point at which the beginning of pressure rise can be detected

(say 8 before TDC) and C the attainment of peak pressure. Thus PQ

represents the first stage and QR the second stage and RS the third stage.

There are three stages of combustion in SI Engine as shown Fig. 1.19.

0 180 360

P

Q

S

R

Pressure

TDC

Com pression

Expansion

C rank ang le in Degrees

Fig:1.18. p- Diagram (Theoretical)


1. Ignition lag stage (stage I)

2. Flame propagation stage (stage II)

3. After burning stage (stage III)

Ignition lag stage

There is a certain time interval between instant of spark and instant

where there is a noticeable rise in pressure due to combustion. This time lag

is called IGNITION LAG. Ignition lag is the time interval in the process of

chemical reaction during which molecules get heated up to self-ignition

temperature, get ignited and produce a self-propagating nucleus of flame. The

ignition lag is generally expressed in terms of crank angle as shown in

Fig.1.19.

The period of ignition lag is shown by path PQ. Ignition lag is very

small and lies between 0.00015 to 0.0002 seconds. An ignition lag of 0.002

seconds corresponds to 35 deg crank rotation when the engine is running at

3000 RPM. Angle of advance increase with the speed. This is a chemical

I II IIIR

P

Q

S

I Ignition lagII Propagation o f f lame III After burning

Spa

rk

TDC M otoring

100 80 60 40 20 8 0 20 40 60 80

0

10

20

30P

ress

ure

in B

ar

C rank angle in degrees

Fig:1.19. Stage of Combustion in an S I Engine


process depending upon the nature of fuel, temperature and pressure,

proportions of exhaust gas and rate of oxidation or burning.

Flame propagation stage

Once the flame is formed at Q, it should be self-sustained and must

be able to propagate through the mixture. This is possible when the rate of

heat generation by burning is greater than heat lost by flame to surrounding.

After the point R the flame propagation is abnormally low at the beginning

as heat lost is more than heat generated. Therefore pressure rise is also slow

as mass of mixture burned is small. Therefore it is necessary to provide angle

of advance 30 to 35 deg, if the peak pressure to be attained 5-10 deg after

TDC. The time required for crank to rotate through an angle II is known as

combustion period during which propagation of flame takes place.

After burning

Combustion will not stop at point R, but continue after attaining peak

pressure and this combustion is known as after burning. This generally

happens when the rich mixture is supplied to engine.

1.10 COMBUSTION PHENOMENON

1.10.1 NORMAL COMBUSTION

A high intensity spark is produced by a spark plug. This spark travels

through the air fuel mixture and leaves a thin thread of flame behind it. The

air-fuel mixture enveloped around the thin thread of flame gets ignited and

combustion commences. Since the air fuel mixture is in turbulent condition,

the surface area of heat transfer is more and combustion is speeded up

enormously.

In P- diagram (Fig. 1.20), we can see the stages of normal

combustion. LNQM is the normal combustion curve. At point N, the ignition

starts [N is the point 35 before TDC]. At point Q, pressure rise can be

noticed. From point M, sudden pressure rise occurs.


Ignition lag: The time period between first igniting fuel and commencement

of main phase of combustion is called ignition lag (or) period of incubation.The ignition lag is normally 0.0015 sec.

(Pre-ignition Detonation Engine failure)

Ignition Advance: The ignition actually starts at about 35 before TDC.

This angle of crank is called ignition advance.

Maximum pressure: The maximum pressure inside the cylinder is attained

at about 10to 12 after TDC.

After Burning: Once it reaches its maximum pressure, the ignition stops.

But at this point the whole heat of the fuel is not liberated. So the remaining

heat in the fuel is burnt after this maximum pressure point. This is called

‘after burning’.

The stages of normal combustion is shown in following Fig. 1.21.

M ax.P r.

40

P(bar) Fo r bes t pe rfo rm ance

a t 10 to 12o o

C om p ress ionN

MQ

S

BD C

Ignitio nadvan ce

150o

120o

90o

60o

60o

90o

120o 150o

30o

30o

TD C BD C

L

Expansion

Fig.1.20


1.10.1.1 Factors affecting normal combustion in S.I Engines

1. Induction pressure

As the pressure falls, delay period increases, and the ignition must beearlier at low pressures.

2. Engine speed

When the engine speed increases, the delay period time needs morecrank angle and ignition should take place earlier.

3. Ignition timing

If the ignition takes place too early, then the peak pressure will occur

early and work transfer falls. If the ignition takes place too late, then peak

pressure will be low and the work transfer falls.

Tem

pera

ture

Idea l Com bustion

M axCombustion

w ith D issocia tion

W eak R ich

Fig:1.22

Air Fuel R atio.

Step 1 Step 2 Step 3 Step 4

Spark produced

Com bustionstarts

Com bustionspreads

Com bustioncom pleted

Fig. 1.21


4. Fuel choice

The calorific value and enthalpy of vaporisation will affect the

temperature achieved. The induction period of the fuel will affect the delay

period.

5. Combustion chamber

The combustion chamber should be designed to give shorter flame path

to avoid knocking and it should give proper turbulence.

6. Compression ratio

When the compression ratio increases, it increases the maximum

pressure and the work transfer.

7. Mixture strength

The rich mixture is necessary for producing the maximum work

transfer.

1.10.2 ABNORMAL COMBUSTION

The abnormal combustion deviates from the normal behavior resulting

in loss of performance and physical damage to the engine.

There are two types of Abnormal combustion.

1. Pre-ignition

2. Knocking (or) Detonation (or) Pinking

1.10.2.1 Pre-ignition

High temperature carbon deposits formed inside the combustion

chamber ignite the airfuel mixture before normal ignition occurs by spark

plug. This ignition due to hot carbon deposits is called pre-ignition. After

some time of Pre-ignition, the normal ignition starts and both the flames get

collided.

If Pre-ignition occurs much early in the compression stroke, the work

to compress the charge will be increased. So the net power output will be

reduced. Also this may cause crank failure due to high load to compress

charge. Pre-ignition causes very high pressure and temperature. It causes the

detonation. Thus, Pre-ignition is considered as abnormal combustion.


1.10.2.2 Knocking (or) Detonation (or) Pinking

There are two general theories of detonation:

1. The auto-ignition theory

2. Detonation theory

A sudden and violent noise (knock) experienced inside the engine

cylinder is known as Detonation. This detonation is due to high pressure

waves striking the cylinder walls, cylinder head and piston with loud noise.

When spark occurs, the combustion of fuel near the spark plug

commences. The flame travels through combustion chamber with high speed.

1. Spark produces

2. Combustion starts

3. Very h igh temp. flam e com presses the

remaining charge

4. Detonation

Fig:1.24 Detonation

....

... .. ..

.... . .

. .. . . . .....

. . . .......... .

. .. . ... . ..

..... .

. .. .

. ....... . ..

......... ..

..

.. .... ...

.

.

... .

..

.

Ignition startedby ho t carbon

deposits insidethe com bustion

cham ber

Spark p roducedby Spark P lug So

regula r ign itionstarts from the

righ t s ide. Ign ition because o f hot deposits

a lso spread from the left side .

Both flam esspread fast

Bo th flam esCo llide

Fig:1.23 Pre Ignition

..

1 . 2 . 3 . 4 .


The high pressure and high temperature gases produced by this ignition

compress the fresh charge in front of the moving flame. Hence the

temperature and pressure of fresh charge is increased beyond the limit and

a spontaneous ignition takes place in far away from spark plug. This zone,

far away from spark plugs where spontaneous ignition takes place is called

‘detonating zone’.

This auto ignition spreads throughout the air-fuel mixture making its

temperature and pressure rise further and produces loud pulsating sound called

‘pinking’ or ‘knocking’ or ‘hammer-blow’.

The temperature in the detonating zone is higher than the

non-detonating zone. More heat is lost in the surface of the combustion

chamber and as a result, the output of engine is decreased.

In mild detonation, the engine surface will be heated up. In severe

detonation, fracture may occur on the engine.

Due to detonation, carbon may be deposited inside the combustion

chamber. When this carbon deposit gets heated, its temperature will be very

high to preignite the fresh charge which is known as pre-ignition.

Detonation occurs after sparking and pre-ignition occurs before

sparking. One of the causes for pre-ignition is detonation.

The detonation can be reduced by properly designing the combustion

chamber so that there is always a turbulence of mixture.

1.11 FLAME FRONT PROPAGATIONThe concept of flame propagation speed is important in SI engines, as

it may lead to detonation.

Flame front is the front surface of the flame that separates the burnt

charges from the unburnt one.

The rate of movement of flame front across the combustion chamber

is based on reaction rate and transposition rate. The reaction rate is the result

of chemical reaction occurring within a region where unburnt mixture is

heated and converted into products.


The transposition rate is due to the movement of flame front relative

to the cylinder wall. It is also the result of pressure differences existing

between the burnt and unburnt gases in the combustion chamber.

1.12 IMPORTANCE OF FLAME SPEED AND EFFECT OF ENGINE VARIABLES

Flame speedFlame speed is the speed at which the flame travels.

Flame speed affects the combustion phenomena, pressure developedand power produced.

Burning rate of mixture depends on the flame speed and shape ofcombustion chamber.

1.12.1 Factors affecting flame speed

1. TurbulenceIt helps in mixing and boosts the chemical reaction. A lean mixture

can be burnt easily without any difficulties. The flame speed is quite low in

non-turbulent mixture and increases with increase in turbulence. Turbulence

consisting of many minute swirls increases the rate of reaction and produces

a higher flame speed than that of larger and fewer swirls.

2. Engine speed

When engine speed increases, flame speed also increases due to the

turbulence inside the engine cylinder. The crank angle required for the flame

propagation during the entire phase of combustion, will remain constant at

all speeds.

3. Engine size

The time taken for flame propagation is smaller in small engines when

compared to larger engines.

In larger engines, the time required for complete combustion is more

because the flame has to travel a longer distance.

3. Compression ratio

A higher compression ratio increases the pressure and temperature of

mixture.


This reduces the initial phase of combustion and hence less ignition

advance is needed. High pressure and temperature of the compressed mixture

also speed up the second phase of combustion.

Increased compression ratio reduces the clearance volume. Thus

engines having higher compression ratio have higher flame speed.

A further increase in the peak pressure and temperature results in the

increase in the tendency of the engine to detonate.

4. Inlet temperature and pressure

When the inlet temperature and pressure increases, it results in better

homogenous mixture which helps to increase the flame speed.

5. Fuel-Air ratio

The highest flame speed obtained with slightly rich mixture gives

complete combustion. Lean mixtures have low thermal energy and hence have

low flame speed. A rich mixture burns readily and completely, resulting in

higher flame speeds. A stoichiometric air-fuel ratio is usually chosen to

prevent compromise on flame speed and air-fuel ratio.

0.002

0.004

0.006T

ime

in S

econ

ds

Stio

chio

met

ric M

ixtu

re

60 100 140 180Lean

M ixtu reR ich

M ixtu re

Fig:1.25 Effect of M ixture Strength on Flame Propagation Time

A


6. Engine output

When the engine output is increased, the cycle pressure also increases.

With the increased throttle opening the cylinder gets filled to a higher density

of mixture. This results in increased flame speed. When the output is

decreased by throttling, the initial & final pressure decreases.

Poor combustion at low loads and the necessity of mixture enrichment

causes wastage of fuel and discharge of products like carbon monoxide into

the atmosphere which are the main disadvantages of SI engines.

1.13 THE PHENOMENON OF KNOCK IN SI ENGINE

In spark-ignition engine, the combustion is initiated using spark-plug

electrodes which spread combustible mixture across the chamber. A flame

front is used to separate the fresh mixture from the product of combustion.

In combustion chamber, burnt part of mixture has higher pressure &

temperature than the unburnt mixture. To maintain a pressure equalization,

the burnt mixture will expand and compress the unburnt mixture adiabatically

thereby increasing its pressure and temperature. The flame front propagates

completely till the end of the cylinder, thereby leaving the unburnt mixture

at an increased pressure and temperature.

The temperature of unburnt mixture exceeds the self-ignition

temperature during preflame reaction and hence spontaneous ignition occurs

at various points inside the engine. This phenomenon is called knocking.

An important fact about knocking is that it is very much dependent

on the properties of the fuel.

Knocking does not occur when the unburnt charge does not reach the

auto ignition temperature, or in other words, in ignition lag period, if the

flame front takes more time to burn the unburnt charge, no knocking occurs.

But if the flame front takes less time to burn the unburnt charge, knocking

occurs [since the end charge will detonate]. Hence, fuels with high auto

ignition temperature and a long ignition lag are often used as fuels for S.I

engines, inorder to avoid detonation.

In summary, during auto ignition, two different cases are encountered.


A large amount of mixture gets autoignited leading to a very rapid

increase in pressure throughout the combustion chamber and there

will be a direct blow on the engine structure. This results in the

thudding sound and consequent noise from the free vibration of

the moving parts. These noises can be detected by human ears.

A large pressure difference may exist in the combustion chamber

and the resulting gas vibrations force the walls of the chamber to

vibrate in the same frequency as that of the gas. In this case, an

audible sound may be evident.

Normally knocking combustion in an engine is often detected by a

distinct audible sound. But a scientific method of detecting the phenomenon

of knocking involves the use of a ‘Pressure Transducer’.

The output of this pressure transducer is connected to a cathode ray

oscilloscope. The pressure-time traces obtained due to the presence or absence

of knock are shown in Fig. 1.26.

1.13.1 Effects of knocking in SI Engine

1. Noise and Roughness

Knocking produces a loud pulsating noise and pressure waves. These

waves vibrates back and forth across the cylinder. The presence of this

vibratory motion causes crankshaft vibration and thus the engine runs roughly.

Ignitio n Pre

ssu

re

C om pressionPower

BD C TD C BD C

Time

Norm al Combustion

Ignitio n Pre

ssur

e

P owe rC om pression

BD C TD C BD C

Time

Knocking Combustion

Fig:1.26 Results Plotted by Pressure Transducer.


2. Mechanical Damage

1. The high pressure wave generated during knocking can increase rate of

wear of parts in combustion chamber. Severe erosion of piston crown,

cylinder head and small holes created on inlet and outlet valves may

result in complete damage of the engine.

2. Due to Detonation, high noise level occurs in engine. In small engines,

the noise can be easily detected and corrective measures can be taken,

but in large engines, it is difficult to detect knocking noise and hence

corrective measures cannot be taken which results in complete damage

of the piston.

3. Carbon depositsDetonation leads to a huge amount of carbon deposition at the engine

outlet.

4. Increase in heat transferKnocking is accompanied with the increase in rate of heat transfer

across the combustion chamber walls.

5. Decrease in power output and efficiencyDue to increase in the rate of heat transfer, the power output as well

as efficiency of the engine decreases.

6. Pre-IgnitionThe increase in heat transfer on the walls causes local overheating of

the spark plug which may reach a temperature high enough to ignite thecharge before the passage of spark, thus leading to pre-ignition. An enginedetonating over a long period of time often results in pre-ignition which isthe real danger of detonation.

1.14 FACTORS AFFECTING KNOCK IN SI ENGINES

It has already been established that the knocking of an engine typicallydepends upon either the quantity of the charge inside the chamber, thetemperature of the chamber or the time of detonation. Hence, the differentvariables which affect knocking can be classified into

Density factors

Time factors

Composition factors


1. Density factors

Density factors deal with the basic mass properties of the charge

present inside the cylinder. The properties include different thermodynamic

variables like the temperature of the charge, pressure, volume of charge,

density etc. It is evident that the auto ignition can be prevented if the

temperature of the charge entering the cylinder is minimum. Similarly, a

charge at lower pressure is less likely to cause a knock. This is due to the

reduced energy of the charge, disabling it from combusting automatically.

The different density factors which affect the knocking phenomenon are

discussed below.

Compression Ratio: Higher compression ratio simply implies that the

pressure of the air-fuel mixture is quite high. Hence, the temperature of the

gases at the end of compression is also high. Therefore, upon combustion,

there is a considerable decrease in ignition delay. This directly increases the

possibility of a knock. Hence, to prevent knocking, it is always wise to limit

the compression ratio to a lower value, but not low enough to drastically

decrease the efficiency of the engine.

Charge temperature: An increased inlet temperature of the air-fuel mixture

causes it to rise above the normal temperature at the end of the compression

stroke. Due to this increased temperature, the ignition delay is decreased,

resulting in knocking of the engine. However, a low inlet temperature could

result in vapourization and starting problems in an engine.

Mass of fuel injected: A reduced amount of charge experiences lower

pressure and has lower energy when compared to normal levels. Thus, the

temperature of the reduced amount of charge at the end of the compression,

is not high enough to cause knocking. Hence, the possibility of a knock is

directly proportional to the mass of the charge inside the cylinder.

Cylinder wall temperature: The combustion chamber is continuously

subjected to several frictional and thermal stresses during operation. Hence,

the walls of the chamber may develop minute hotspots which could ignite a

fuel before the anticipated time, thereby resulting in knocking. Hence, uniform

cooling of the walls using an efficient coolant is of paramount importance.

Moreover, the exhaust valves and the spark plugs are the most hottest regions


inside the cylinder. Hence, the concentration of the compression against these

regions, is to be avoided to reduce knocking.

Horse power: High powered engines operate at high temperatures and

pressures. Thus the chances of a knock to occur in a high powered engine

is greater than that of a low powered engine.

2. Time factors

Time factors play an important role in determining the chances of

knock in an engine. Some common time factors are flame speed, velocity of

the charge, engine speed etc. The effect of different time factors on the knock

of an engine is discussed below.

Velocity of the charge: A turbulent charge ignites much faster than a

non-turbulent charge. Thus, the flames propagate much faster, leaving little

margin for the end charge to auto ignite. Hence, the chances of a knock is

reduced effectively by increasing the velocity of the charge, above its

turbulent level.

Engine speeds: At higher engine speeds, the turbulence of the charge

increases greatly. This results in reduced knocking, as discussed above.

Flame travel distance: It has been well established that a faster flame

reduces knocking possibilities when compared to a slower flame. Hence, if

the time taken for the flame to travel across the chamber is reduced, knocking

can be prevented. This can be done by either decreasing the combustion

chamber size, or by repositioning the spark plug appropriately.

A centrally placed spark plug, or usage of two or more plugs, can

effectively reduce the knocking of an engine.

Combustion chamber configuration

A combustion chamber should be designed in such a way that it

promotes the turbulence of the particles inside. Moreover, the chamber should

be made as spherical as possible with the least possible height. These two

factors can effectively reduce the flame travel time, thereby preventing

knocking.


3. Composition factor

Composition factor deals with the flammability of the charge present

inside the cylinder. Air-fuel ratio and the octane number of the fuel are the

most important composition factors pertaining to the knocking phenomenon.

(i) Air-fuel ratio: Flame speed depends upon the air-fuel ratio. It varies

as per the type of fuel used. The flame temperatures and the reaction time

also vary based on the air-fuel ratio.

If a specific ratio can cause low reaction time, then this ratio can give

way to increased chances of knocking.

(ii) Octane value

Knocking can be reduced by either increasing the self-igniting

temperature of a fuel or by reducing its pre-flame reactivity.

In general, Aromatic hydrocarbons have the minimum tendency to

knock an engine, whereas the paraffin series are more likely to knock an

engine. Any appropriate compound with a compact molecular structure is less

prone to knock an engine.

1.15 FUEL REQUIREMENT AND FUEL RATING

1.15.1 Important properties of fuel in SI Engine

The fuel characteristics that are important for the performances of

internal combustion engines are

Volatility of the fuel.

Detonation characteristics.

Good thermal properties like heat of combustion and heat of

evaporation.

Sulphur content.

Aromatic content.

Cleanliness of fuel.

1.15.2 Important characteristics of SI Engine fuel

Every engine is designed for a particular fuel according to its desired

qualities.


For good performance of SI engine, the fuel used must have the proper

characteristics like,

It should readily mix with air to make an uniform mixture at inlet.

It must be knock resistant.

It should not pre-ignite easily.

It should not tend to decrease the volumetric efficiency of the

engine.

Its sulphur content should be low.

It must have adequate calorific value.

It must have proper viscosity.

1.15.3 Fuel properties

Brief explanation of fuel properties are given below.

1. Viscosity of Fuel

Viscosity is the resistance offered by the fuel to its own flow. Viscosity

decreases when the temperature of fuel increases and vice versa. Good fuel

should have proper viscosity.

2. Pour Point of Fuel

The pour point (freezing point) of fuel must be less than the lowest

climate temperature of atmosphere. In cold climate days, the fuel should be

in liquid state. So its pour point should be less sufficiently.

3. Sulphur Content in the Fuel

Sulphur present in the fuel is dangerous to engine. During combustion,

the sulphur in the fuel become sulfuric acid. This acid causes corrosion of

engine parts. So the sulphur content in the fuel should be removed (or)

sulphur content should be kept as minimum as possible.

4. Volatility

The ability to evaporate is called volatility. If the fuel evaporates in

low temperature, then it has high volatility. The petrol and diesel should have

adequate volatility.


5. Flash Point and Fire Point

Flash point is the minimum temperature of fuel when the fuel gives

a momentary flame (or) flash.

Fire point is the minimum temperature of fuel when the fuel starts

continuously burning.

The flash point and fire point of fuels should be adequate so that it

is used in IC engine without any problem.

6. Calorific Value of Fuels:

The amount of heat liberated by burning 1 kg (or 1 m3 of fuel is

known as Calorific value of fuel (or Heating value of fuel).

For solid fuel, the unit for calorific value is expressed in kJ/kg. For

liquid and gaseous fuel, the unit is kJ/m3 measured in S.T.P. condition (i.e.,

Standard Temperature and Pressure 15 C and 760 mm of mercury).

Higher Calorific Value:

The amount of heat obtained by the complete combustion of 1 kg of

fuel, when the products of combustion are cooled down to the temperature

of the surroundings is known as Higher Calorific Value HCV of the fuel.

Here the water vapour formed by combustion is condensed and the

entire heat of steam is recovered from the products of combustion.

Dulong’s formula is used to determine HCV of a fuel.

HCV 33800 C 144000 H2

O2

8 9270 S

kJkg

where C, H2, S and O2 are the fractions of mass of carbon, hydrogen, sulphur

and oxygen in 1 kg of fuel.

Lower Calorific Value (LCV)

The amount of heat obtained by the combustion of 1 kg of fuel, when

the product of combustion is not sufficiently cooled down to condense the

steam formed during combustion is known as Lower Calorific Value (LCV)of the fuel.


So, LCV of the fuel

H.C.V Enthalpy of evaporation of steam formed

H.C.V. 2466 steam for med kJ/kg

H.C.V. 2466 9H2

where, 2466 kJ/kg is the specific enthalpy of evaporation of steam at 15C.

1.15.4 Octane Number (ON)

Octane Number (gaseous fuel) indicates the anti-knock properties of a

fuel, based on the comparison of mixtures of Iso octane and normal heptane.

Fuel rating for SI engine

Octane value is for SI engines

Octane Number: (Applicable for SI Engine) This is a number to rate the

petrol fuel according to its detonating tendency. If the fuel has the tendency

to detonate less, then it has high octane number and vice versa.

Iso-octane is a high rating fuel (i.e. detonation is less).

Normal heptane is a low rating fuel (i.e. detonation is more).

Iso-octane and normal heptane are mixed together and this sample

mixture is used for running a test engine.

The octane number of the fuel is the percentage of octane in this

sample mixture which detonates in similar way as the fuel under the same

condition.

High octane fuel’s number is 100. This type of fuel will not have

tendency to detonate. We can make given fuel into octane number 90 to 100

by adding Tetraethyl Lead. But this addition will reduce the engine life.

Fuels with a higher octane ratings are used in high performance

gasoline engines that require higher compression ratio.

Fuels with lower octane number are ideal for diesel engines, because

diesel engines do not compress the fuel but rather compress only air and then

inject the fuel.

Two methods that are employed for measuring octane number are

Research Octane Number (RON) and Motor Octane Number (MON).


The octane numbers measured under two different engine conditionsin a standard “Cooperative Fuels Research (CFR)” engine has a variablecompression ratio.

Research Octane Number (RON)

The most common type of octane rating is Research Octane Number

(RON). RON is determined by using the fuel in a test engine running at 600

rpm with the variable compression ratio under controlled condition, and

comparing the results with the mixture of iso-octane and n-heptane.

Motor Octane Number

Motor Octane Number is determined at 900 rpm engine speed instead

of 600 rpm used in RON.

MON testing uses a similar test engine used in RON testing but with

a preheated fuel mixture, higher engine speed and variable ignition timing.

Anti-knock Index RON MON

2

Advantages of High-Octane Fuel:

1. We can increase the compression ratio without detonation.

2. Engine efficiency can be increased without detonation.

3. Supercharging can be done without detonation.

So totally, the unwanted detonation can be reduced by using

high-octane fuel.

1.16 ANTI-KNOCK ADDITIVES

Anti knock additives are used to reduce engine knocking and to

increase the fuel’s octane rating by raising the temperature and pressure at

which auto ignition occurs.

The widely used antiknock agents are:

Tetraethyl lead [TEL] CH3CH24 Pb

Methylcyclopentadienyl manganese tricarbonyl (MMT)

CH3C5H4MnCO3 Ferrocene Fe C5H52


Iron pentacarbonyl

Toluene

Iso octane

1.16.1 Anti-knock Agents

Anti-knock agents are classified into high-percentage additives like

alcohol and low-percentage additives based on heavy elements.

Internal combustion engine discharges various substances to the

atmosphere. Some of these emissions are harmful to the environment such

as Carbon monoxide, Nitrogen oxides, unburnt hydrocarbons and certain

compounds of lead.

The catalytic converter is used to oxidize the unburnt hydrocarbons

and carbon monoxide to carbon dioxide and to decompose nitrogen oxides

into nitrogen and oxygen.

High percentage additives are those organic compounds that do not

contain metals, but require high blending ratios, such as 20-30% for benzene

and ethanol. Ethanol is inexpensive, and widely available but being corrosive

in nature, it is not used.

Tetraethyl lead (TEL) CH3CH24 Pb is a main additive and it is a

common anti knock agent.

Adding a small amount of Tetraethyl lead (TEL) improves the

anti-knock quality of fuel.

1.16.2 Effects of Anti knock additives

The main problem in using Tetra ehtyl lead is the lead content in

it since lead is extremely toxic and poisonous.

A manganese - carrying additive like methylcyclopentadienyl

manganese tricarbonyl (MMT) directly affects the humans.

The exposure of MMT results in eye irritation, giddiness, headacheand it causes difficulties in breathing.

Ferrocene Fe C5H22 is an organometallic compound of iron.

The iron contents in ferrocene forms a conductive coating on the

spark plug.


1.16.3 Factors affecting Detonation and Remedies

Factors Remedies1. The type of fuel used is the reason

for detonationFuel like alcohol and benzol donot cause detonation.Addition of a small quantity oftetraethyl lead with petrol willsuppress the detonation. (Thisprocess is called doping).

2. The position of spark plug in thecombustion chamber determines thedistance the flame travels to reachthe detonating zone. More distancecauses detonation

Less distance reduces the chancesof detonation. A spark plug placedcentrally will reduce thedetonation.

3. High temperature combustionchamber raise temperature ofcylinder wall and also detonatingzone.

The cooling system should beproper to maintain the cylinderwall temperature at optimum level.

4. The compression ratio is the causefor detonation. More compressionratio will overheat the engine.

The compression ratio should notbe increased beyond the limit.

5. The presence of carbon depositspromote detonation.

Good quality fuel should be used.

6. Excessive sparking temperaturepromotes detonation

Ignition system voltage should belimited to produce spark withsufficient temperature to ignite.

1.17 COMBUSTION CHAMBER FOR SI ENGINES

The design of combustion chambers for SI engines plays a veryimportant role in the operation and performance of the engine.

The design involves, the shape of the combustion chamber, locationof spark plug and the location of inlet and exhaust valves.

Important requirements of an SI engine combustion chamber.(i) To provide high power output with minimum octane requirement.

(ii) High thermal efficiency.

(iii) Smooth engine operation.

Factors to be considered while designing combustion chambers for S.I

engines include:


(i) Rate of pressure rise during combustion,

(ii) Temperature and pressure of the last part of the mixture to burn,

(iii) Location of hotspots on the combustion chamber wall (to locate spark plug)

1.17.1 Types of combustion chambers

1. Overhead or I - head combustion chamber

2. T - head combustion chamber

3. L - head combustion chamber

4. F - head combustion chamber

1. Overhead valve (or) I - Head combustion chamber

In this type of combustion chamber, both the valves are located onthe cylinder head, so it is called overhead valve. This type of combustionchamber has two forms.

Bath-tub form

This type of combustion chamber, consists of oval shaped chamberwith both valves mounted overhead. The spark plug is mounted at the side.

Wedge form

This type of combustion chambers also consist of oval shaped chamberwith both valves mounted overhead at its side with slight inclination. Thespark plug is mounted centrally. A few features of this combustion chamberare listed below:

1. Less heat loss because of less surface to volume ratio.

2. Less flame travel length and greater freedom from knock.

Bath - tub form of com bustion chamber

Wedge form of com bustion chamber

Sparkp lug

front in le t va lve(back exhaust

valve is h idden)

Fig:1.27


3. High volumetric efficiency from larger valve cylinder.

4. By keeping the hot exhaust valve in the cylinder head instead of

cylinder block, it reflects in confinement of thermal failure to cylinder

head.

2. T - Head combustion chamber

In this type of combustion

chamber, two valves are placed on

either side of the cylinder which

requires two camshafts. Fig. 1.28. In a

manufacturing point of view, providing

two camshafts is not recommended.

The distance across the

combustion chamber is very long so the

knocking tendency is high in this type of engine.

3. L-head combustion chamber

In L - head type, two valves are

provided on the same side of the cylinder

which can be operated by a single camshaft.

In this type, it is easy to lubricate the valve

mechanism, with the detachable head

provision. The cylinder head can be removed

without disturbing valves, gears etc.

In Fig. 1.29 the air flow has to travel

a longer distance to enter the cylinder. This

causes loss of velocity head and loss in

turbulence level. This design reduces knocking

tendency by reducing the flame travel length.

This type of combustion chamber gives

additional turbulence during compression stroke.

Advantages

1. Valve mechanism is simple and easy to lubricate.

ExhaustValve

Fig :1.28 T - Head Type

L - H eadTypes

Fig:1.29 L - Head Types


2. Detachable head-easy to remove for cleaning and decarburizing without

disturbing either the valve gear or main pipe work.

3. Valves of larger sizes can be provided.

Disadvantages

1. Lack of turbulence as the air has to take two right angle turns to enter

the cylinder.

2. Extremely prone to detonation due to large flame length and slow

combustion due to lack of turbulence.

F - head combustion chamber

In F - head type, exhaust valve is located in the cylinder head and

the inlet valve is located in the cylinder block. Here, the valves are actuated

by two camshafts which is a disadvantage.

Advantages

1. High volumetric efficiency.

2. Maximum compression ratio for fuel of given octane rating.

3. High thermal efficiency.

4. It can operate on leaner air-fuel ratios without misfiring.

Disadvantage

1. This design is a complex mechanism for operation of valves and

expensive special shaped piston.

Spark p lugOverhead

engine b lock

IV

EV

Fig:1.30 F - Head Type

ReciprocatingPiston


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Advanced IC Enginesairwalkbooks.com/images/pdf/pdf_55_1.pdfUNIT II COMPRESSION IGNITION ENGINES9 Diesel Fuel Injection Systems - Stages of combustion – Knocking – Factors affecting