Pdf_55_1.pdf

Advanced IC Engines As per Revised Syllabus of Leading Universities

Prof. R. Devaraj Dr. S. Ramachandran Dr. A. Anderson Professors School of Mechanical Engineering Sathyabama University Chennai - 600 119

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

First Edition

: 12-12-2016

ISBN : 978-93-84893-62-0

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 C.1

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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3 1.2.2 Mixture requirements at various loads . . . . . . . . . . . 1.4 1.2.1 Idling range . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5 1.2.2 Cruising/Normal range/Medium load . . . . . . . . . . . 1.6 1.2.3 Power range . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.6 1.2.4 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 . . . . . . . . . . . . . . . . . 1.8 1.3.2 Fuel Supply System in SI Engines. . . . . . . . . . . . . . 1.8 1.3.3 Carburetor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.9 1.3.4 Simple Carburetor . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.10 1.3.5 Various Compensation in Carburetors . . . . . . . . . . 1.11 1.3.6 Types of Carburetors . . . . . . . . . . . . . . . . . . . . . . . . . 1.14 1.4 Gasoline Injection System . . . . . . . . . . . . . . . . . . . . . . . . . . 1.14 1.4.1 Reasons for adopting gasoline injection system. . . 1.14 1.4.1 Continuous fuel injection system . . . . . . . . . . . . . . . 1.15 1.4.2 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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.19 1.6.2 L-MPFI system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.20 1.7 Direct Injection System . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.21

C.2 Advanced IC Engines

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 . . . . . . . . . . . . . . . . . . . . . . . . . 1.28 1.10.1.1 Factors affecting normal combustion in S.I Engines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.30 1.10.2 Abnormal Combustion . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.31 1.10.2.1 Pre-ignition. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.31 1.10.2.2 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 . . . . . . . . . . . . . . . . . 1.34 1.13 The Phenomenon of Knock in SI Engine . . . . . . . . . . . . 1.36 1.13.1 Effects of knocking in SI Engine . . . . . . . . . . . . . 1.37 1.14 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 . . . . . . 1.41 1.15.2 Important characteristics of SI Engine fuel. . . . . 1.41 1.15.3 Fuel properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.42 1.15.4 Octane Number. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.44 1.16 Anti-knock Additives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.45 1.16.1 Anti-knock Agents. . . . . . . . . . . . . . . . . . . . . . . . . . . 1.46 1.16.2 Effects of Anti knock additives . . . . . . . . . . . . . . . 1.46 1.16.3 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

Contents C.3

Chapter - 2

Compression Ignition Engines 2.1 Diesel Fuel Injection Systems . . . . . . . . . . . . . . . . . . . . . . . . 2.1 2.1.1 Fuel Pump (C.I. Engine) . . . . . . . . . . . . . . . . . . . . . . 2.1 2.1.2 Fuel Injection System . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 2.2 Electronically Controlled Diesel Injection Systems . . . . . . 2.4 2.2.1 Classification of Diesel Fuel Injection Pumps . . . . . 2.5 2.2.2 Rotary Distributor Type Fuel Injection System – Electronically Controlled . . . . . . . . . . . . . . . . . . . . . . . 2.5 2.2.3 Unit Injector System . . . . . . . . . . . . . . . . . . . . . . . . . . 2.7 2.2.4 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 . . . . . . . . . 2.14 2.5 Knocking (or) Diesel Knock . . . . . . . . . . . . . . . . . . . . . . . . . 2.15 2.5.1 Phenomenon of knock in CI engine . . . . . . . . . . . . 2.15 2.4.2 Comparison of knock on SI and CI Engines . . . . 2.17 2.4.3 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 . . . . . . . . . . . . . . . . . . . . . . . 2.21 2.6.2 Indirect injection system . . . . . . . . . . . . . . . . . . . . . . 2.22 2.7 Combustion Chamber Design for Compression Ignition Engine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.24 2.7.1 Open combustion chamber . . . . . . . . . . . . . . . . . . . . 2.24 2.7.2 Divided combustion chamber (or) Indirect combustion chamber . . . . . . . . . . . . . . . . . . . . . . . . . 2.26 2.7.3 Open and Divided combustion chambers . . . . . . . . 2.31

C.4 Advanced IC Engines

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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.34 2.8.2 Fuel Rating for CI Engine . . . . . . . . . . . . . . . . . . . . 2.34 2.9 Fuel Spray Behaviour. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.35 2.9.1 Fuel injection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.36 2.9.2 Spray structure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.36 2.9.3 Spray penetration. . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.38 2.9.4 Droplet size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.40 2.10 Supercharging. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.41 2.11 Introduction To Turbocharging . . . . . . . . . . . . . . . . . . . . . 2.42 2.11.1 Principle of the turbocharger operation . . . . . . . . 2.42 2.11.2 Advantages of turbochargers . . . . . . . . . . . . . . . . . 2.42 2.11.3 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 . . . . . . . . . . . . . . . . . . . . . . . 3.1 3.2 Sources of Pollutants From IC Engine . . . . . . . . . . . . . . . . 3.2 3.2.1 Crankcase emissions . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 3.2.2 Evaporative emissions . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 3.2.3 Exhaust emissions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4 3.3 Carbon Monoxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5 3.3.1 Formation of CO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5 3.4 Unburnt Hydrocarbons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6 3.4.1 Formation of HC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6

Contents C.5

3.5 Oxides of Nitrogen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7 3.5.1 Formation of NO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7 3.6 Smoke . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7 3.6.1 Causes of smoke . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.8 3.6.2 Formation of smoke . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.8 3.6.3 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 . . . . . . . . . . . . . . . . . . . 3.10 3.9.2 Control of Nitrogen Oxides. . . . . . . . . . . . . . . . . . . . 3.11 3.9.3 Control of Carbon Monoxide and Hydrocarbon. . . 3.11 3.10 Emission Control in SI Engines . . . . . . . . . . . . . . . . . . . . 3.12 3.10.1 Engine design modifications. . . . . . . . . . . . . . . . . . 3.12 3.10.2 Operating parameter modifications . . . . . . . . . . . . 3.13 3.10.3 Treatment of exhaust products of combustion . . . 3.13 3.10.4 Fuel modifications . . . . . . . . . . . . . . . . . . . . . . . . . . 3.13 3.11 Control of Oxides of Nitrogen . . . . . . . . . . . . . . . . . . . . . . 3.14 3.11.1 Exhaust Gas Recirculation . . . . . . . . . . . . . . . . . . . 3.14 3.11.2 Catalyst . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.16 3.11.3 Water injection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.16 3.12 Emission Control Using Converters . . . . . . . . . . . . . . . . . 3.16 3.12.1 Thermal converter . . . . . . . . . . . . . . . . . . . . . . . . . . 3.17 3.12.2 Catalytic converter . . . . . . . . . . . . . . . . . . . . . . . . . . 3.17 3.12.2.3 Engine Emission Control by Three way catalytic converter system . . . . . . . . . . . . . . . . . . 3.20 3.12.2.4 Oxygen storage. . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.21 3.12.2.5 Diesel Oxidation Catalyst . . . . . . . . . . . . . . . . . . 3.21 3.13 Effect of Engine Emission on Human Health . . . . . . . . 3.22 3.14 Selective Catalytic Reduction. . . . . . . . . . . . . . . . . . . . . . . 3.24

C.6 Advanced IC Engines

3.15 Particulate Traps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.25 3.15.1 Regeneration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.27 3.15.2 Other methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.28 3.15.3 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)3.29 3.16.2 Flame ionization detector . . . . . . . . . . . . . . . . . . . 3.30 3.16.3 Chemiluminescence analyzes - NOx detector . . . . 3.32 3.16.4 Smoke measurement . . . . . . . . . . . . . . . . . . . . . . . . 3.32 3.16.5 Measurement of particulate . . . . . . . . . . . . . . . . . . 3.37 3.17 Emission Norms (EURO and BS). . . . . . . . . . . . . . . . . . . 3.38 3.17.1 Euro Norms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.39 3.17.2 BS Norms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.39 3.18 Driving Cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.42 3.18.1 Constant Volume Sampler . . . . . . . . . . . . . . . . . . . 3.42 3.18.2 Indian Driving Cycle . . . . . . . . . . . . . . . . . . . . . . . . 3.44 Chapter 4

Alternative Fuels 4.1 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. . . . . . . . . . . . . . . . . . . . . . 4.2 4.4.2 Components of Natural gas . . . . . . . . . . . . . . . . . . . . 4.4 4.4.3 Characteristics of Natural gas . . . . . . . . . . . . . . . . . . 4.4 4.4.4 Production of Natural gas . . . . . . . . . . . . . . . . . . . . . 4.5 4.4.5 Types of Natural gas. . . . . . . . . . . . . . . . . . . . . . . . . . 4.6 4.4.6 Compressed Natural Gas . . . . . . . . . . . . . . . . . . . . . . 4.6 4.4.6.1 Compressed Natural Gas for SI Engines . . 4.7

Contents C.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 . . . . . 4.8 4.4.7 Liquefied Natural Gas . . . . . . . . . . . . . . . . . . . . . . . . 4.8 4.4.8 Applications of Natural gas . . . . . . . . . . . . . . . . . . . . 4.9 4.5 Biodiesel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.10 4.5.1 Production of Biodiesel . . . . . . . . . . . . . . . . . . . . . . . 4.11 4.5.2 Engine modifications for Biodiesel . . . . . . . . . . . . . 4.13 4.5.3 Merits of Biodiesel . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.13 4.5.4 Demerits of Biodiesel . . . . . . . . . . . . . . . . . . . . . . . . . 4.14 4.6 LPG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.15 4.6.1 Production of Liquefied Petroleum Gas . . . . . . . . . 4.16 4.6.2 Properties of LPG . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.16 4.7 Methanol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.18 4.8 Ethanol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.19 4.8.1 Production of Ethanol . . . . . . . . . . . . . . . . . . . . . . . . 4.20 4.8.2 Another methods of Ethanol production . . . . . . . . . 4.21 4.8.2 Merits of ethanol as a fuel . . . . . . . . . . . . . . . . . . . 4.24 4.8.3 Demerits of ethanol as a fuel . . . . . . . . . . . . . . . . . 4.24 4.8.4 Alcohol for SI engines . . . . . . . . . . . . . . . . . . . . . . . . 4.24 4.8.5 Alcohol for CI Engines . . . . . . . . . . . . . . . . . . . . . . . 4.25 4.9 Hydrogen. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.26 4.9.1 Hydrogen properties . . . . . . . . . . . . . . . . . . . . . . . . . . 4.27 4.9.2 Production of Hydrogen . . . . . . . . . . . . . . . . . . . . . . 4.33 4.9.3 Hydrogen storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.37 4.9.4 Engine modifications for hydrogen fuel in SI and CI engines . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.38 4.9.5 Merits of Hydrogen fuel . . . . . . . . . . . . . . . . . . . . . . 4.40 4.9.6 Demerits of Hydrogen fuel . . . . . . . . . . . . . . . . . . . . 4.40

C.8 Advanced IC Engines

Chapter - 5

Recent Trends 5.1 Air Assisted Combustion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1 5.1.1 Air Assisted combustion system . . . . . . . . . . . . . . . . . 5.3 5.2 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 . . . . . . . . . . . . . . . . . . 5.10 5.4.2 Working of CRDI . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.11 5.4.3 Benefits of CRDI . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.12 5.4.4 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 . . . . . . . . . . . . . . . . . . . . . 5.13 5.5.2 General Configuration of Electric Vehicle . . . . . . . 5.13 5.5.3 Advantages of Electric vehicle . . . . . . . . . . . . . . . . . 5.15 5.6 Hybrid Electric Vehicles . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.15 5.6.1 Series hybrid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.15 5.6.2 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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.20 5.9.2 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.

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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 

mfuel 1  AFR 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 approximately 15:1 by weight. This is known as chemically correct (or) stoichiometric air fuel ratio. The normal range of Air-fuel ratio is in between 20:1 to 8:1 approximately. Air-fuel ratio during starting is approximately 10:1 - i.e., very rich mixture. 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 15:1 neither rich nor lean mixture. Air-fuel ratio for economic running (medium load), is approximately 17:1 - economic mixture. Air-fuel ratio during overtaking, is approximately 12:1 - rich mixture.

Spark Ignition Engines 1.3 Excess Fuel

Excess air

Too R ich

Too L ea n

9

15

19

Fig:1.1.Air Fuel Ratio

1.2.1 Mixture requirements at full throttle and constant speeds

S toich io m e tric M ixture

P ow e r o utpu t (kW )

Po

bs

we

ro

u tp

ut

bsfc (k g/kW h )

B e st P o w er

fc

B e st E con o m y 8

10

12

14

16

18

20

22

A /F R a tio (kg of air / kg o f F ue l ) Fig:1 .2

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.

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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

R ic h

P

5

S

P

M ulti C ylin de r S ing le C ylind er

S

B e st P o w er

10 Q

R

Q

20

R B e st E con o m y

L ea n

A /F R a tio (kg of air / kg of Fu el)

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

0

50 Th ro ttle O p en in g (% )

C h em ica lly C o rrect M ixtu re 1 00

Fig: 1.3 A nticipated Carb uretor P erfo rma nce to fu lfill En gin e R eq uirem ents

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

Spark Ignition Engines 1.5

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. Fuel Air  0.08 or  12.5 Air Fuel Refer Fig. 1.3. The curve PQ in the graph represents the idling range of SI engine.

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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 leaner A/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 is desirable that the running should be most economical in this condition. So a lean mixture can be supplied, as engine has low fuel consumption at 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

Spark Ignition Engines 1.7

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 the increase of fuel/air ratio for best economy. 1.3 FUEL INJECTION SYSTEMS FOR S.I. ENGINES To run S.I. engine, the petrol from the fuel tank must reach cylinder. The petrol vaporize easily at atmospheric condition, therefore the engine suction is sufficient to vaporize petrol. In petrol engine, the petrol from the fuel 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.

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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.

Sto rag e Tan k

Fu el Pu mp

Fu el filter

Carburettor

Fig 1.4 A S chem atic diagram o f fu el su pply system.

En gin e

Spark Ignition Engines 1.9

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. O utle t valve Strainer

D ia phragm

Pu m p C h amb er

Sp ring H in ged poin t G la ss bo w l

Fig.1.5

Fuel pump for SI E ngine

Cam

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.

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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. To E ngin e

Th rottle Valve

Fu el Inle t M ixtu re Ve nt

N e ed le valve

Fu el Jet Ve nturi

Float 2

x

2

Float cha m be r

C h oke Valve

Fig.1.6 Sim ple Carbu retor Air

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.

Spark Ignition Engines 1.11

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.

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1. Auxiliary (or) Extra air valve compensation Air Extra a ir valve Air

Pa rt of float cham ber Th ro ttle

To in take m an ifold

Fig: 1.7 A uxiliary (o r) Ex tra air valve com pensation

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 A ir

R e stric te d a ir b le ed o pe ning O uter e nc lo se r

Jet Tub e

P a rt o f flo at cha m be r

Th ro ttle

To intake m an ifo ld

Fig:1.8 R estric ted air-bleed com pensation

Spark Ignition Engines 1.13

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

To intake m an ifo ld Th ro ttle

O pe n to a tm osph ere P a rt o f flo at cha m be r c

A M ain je t B C o m p e nsatin g jet A ir Fig: 1.9 C om pensating jet C o m p ens ation

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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.

Spark Ignition Engines 1.15

Fuel-injection system in SI engine can be classified as follows: Fuel-inje ctio n system s

In direct injection (ID I) (M echanical (o r) electron ic contro l)

D irect injection (D I) (M echanical (o r) electron ic contro l) Tim e d

M ono-Point In je ctio n (M P I)

C o ntinu ous

Tim e d

M ulti-point fu el injectio n (M P F I)

C o ntinu ous

Tim e d

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.

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

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.

A ir

Fu el

In je ctor

T hro ttle va lve

Th ro ttle B o dy

Inta ke m a nifold

C1

C2

C3

C4

E n gine Fig:1.10 M on o P oin t F uel In jection(o r) Th rottle B ody In jection (TB I)

Spark Ignition Engines 1.17

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

1.18 Advanced IC Engines - www.airwalkpublications.com Air

Throttle valve

Inta ke m anifold

F uel

Inje ctors

Inje ctors

Inta ke m anifold

C2

C1

C3

E ngin e

C4

C ylind ers

Fig:1.11.M u ltip oint fuel injection (M PFI)

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

Spark Ignition Engines 1.19

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. Fuel F ilter

P ressure S enso r

Fuel P re ssure R eg u la tor

Fuel R ail

In je ctors

F uel P um p

R eturn lin e In le t M a nifold 2 1 3 4 Filte r E ngine

Fuel Tan k

Fig:1.12:M ulti Po int Fu el Injection system

The advantages of Multi point fuel injection are 

Increased power efficiency.

and

torque

through

improved

volumetric



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.

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Air

Inta ke M anifold Vacuum S enso r

G asoline / P etrol Inje ctio n into

M ixture of Air and G asoline

Inta ke M anifold

Fuel In je ctor Inje ctio n Volum e C on tro l

En gine

Electro nic C ontrol U n it (EC U )

R P M S en so r Fig:1.13. D-M PFI Gasoline Injectio n System

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.

Spark Ignition Engines 1.21 A ir

D a ta in form a tion A ir S e nso r G as o line / P etrol M ixtu re of A ir a nd Fu el

Fu el In je ctor In je ctio n Vo lu m e C on tro l

E n gine

E lectro nic C o ntrol U n it (E C U )

RPM S e nso r D a ta in form a tion

Fig:1.14. L-M PFI G asoline In jectio n S ystem

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.

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Inje ctor

C a rburetor

Intake port

Intake port Carburetor

Inje ctor

Fu el spra y Po rt fuel injection (in direct injection)

Fu el spra y Direct injection

Fig 1.15

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 system An 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

Spark Ignition Engines 1.23 Fuel ta nk Fuel p um p

R e gula to r A ir tem pe rature sen so r

C o ld start inje ctor

Filte r

Fuel inje ctor E xtra a ir valve Throttle p osition sw tich Therm o time sw itch

C o olan t tem pe rature C o olan t sen so r

B attery

Fro m ign ition sw itch

M ain rela y

P re ssu re sen so r E lectro nic con tro l unit D istrib uto r

Fuel p um p rela y

Fig:1.16.Bo sch D -Jetronic Electronic Fuel Injection System

2 kg/cm2. A secondary system carries excess fuel from the pressure regulator back to fuel tank. 2. Air system Intake 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,

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Fu el supply

Fu el filter

Fu el pum p, hig h pressure

Fu el pressure regulator

Fu el ra il

R e turn lin e Injector harness Se nsor harness

Th ro ttle position

Electro nic control unit (E C U )

O il O xyg en Intake air sensor tem p tem p (if fitted) (if fitted) Fig:1.17.Simple Layout of Electronic Fuel Injection System . C o olan t tem p

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 Unit Control 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

Spark Ignition Engines 1.25

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 Sensor The 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 Sensor The 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 20C. Engine Temperature Sensor The 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 Contacts The 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 Switch The 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.

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Auxiliary Air Valve During 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 80C, the auxiliary air pipe is completely closed by the valve. 1.9 STAGES OF COMBUSTION IN SI ENGINES A 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.

P re ssu re

R Exp

Q Com

P 0

pr

ans

io n

n e s s io

S TD C 1 80

3 60

C ra nk a ng le in D e g ree s Fig:1.18. p-  Diagram (Theoretical)

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.

Spark Ignition Engines 1.27

30 II

III R

20 S park

P ressure in Ba r

I I Ignition lag II Pro pagation o f flam e III After burning

10

S

Q P

M o to

TD C

r in g

0 1 00

80

60

40

20

8 0

20

40

60

80

C ra nk a ngle in d eg rees Fig:1.19. Stage of Co m bustion in an S I Engin e

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

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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.

Spark Ignition Engines 1.29 P (b ar)

Fo r b es t pe rfo rm an ce o o a t 1 0 to 1 2

M ax .P r. 40

Ig nitio n a dvan ce

C o m p re ssio n

E x pa n sion

Q

N

M S

L o

B D C 1 50 1 20

o

90

o

o

60 30

o

o

TD C 3 0 Fig.1.20

60

o

90

o

o

1 20 1 50

o

BDC



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 10to 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.

1.30 Advanced IC Engines - www.airwalkpublications.com Step 1

Step 2

Sp ark pro duced

Step 3

Step 4

C om bu stion C om bu stion spreads starts Fig. 1.21

C om bu stion com pleted

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 be earlier at low pressures. 2. Engine speed When the engine speed increases, the delay period time needs more crank 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 pe ra ture

Id ea l C om b ustio n

M ax C o m b u stion w ith D isso cia tio n

W ea k

R ich Air Fuel R atio. Fig:1.22

Spark Ignition Engines 1.31

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.

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

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

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

..... . .

2 . Sp ark p roduced 3 . Bo th flam es 1 . Ignitio n started b y S park P lug S o spread fast b y ho t carb on regula r ign ition d ep osits inside starts fro m th e the co m bu stio n righ t sid e. cha m be r Ign ition because o f hot deposits a lso sprea d from the left side .

.

4 . Bo th flam es C o llide

Fig:1.23 P re Ignition

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. Sp ark pro duces

2. C o m b ustion 3. Ve ry h igh tem p . 4. D eton atio n starts flam e com presses th e re maining ch arge Fig:1.24 D eton ation

Spark Ignition Engines 1.33

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 PROPAGATION The 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.

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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 speed Flame speed is the speed at which the flame travels. Flame speed affects the combustion phenomena, pressure developed and power produced. Burning rate of mixture depends on the flame speed and shape of combustion chamber. 1.12.1 Factors affecting flame speed 1. Turbulence It 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.

Spark Ignition Engines 1.35

0 .00 4

0 .00 2

Stio ch iom etric M ixture

Tim e in S e co nd s

0 .00 6

A 60 L ean M ixtu re

1 00

1 40 R ich M ixtu re

1 80

Fig:1.25 E ffect of M ixture Strength on Flame P ropagation Tim e

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.

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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.

Spark Ignition Engines 1.37



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’.

Co

Po

s io n m p re s

BDC

Ig nitio n we

TD C

r

C

BDC

ess om pr

BDC

Tim e N orm al C o mb ustion

Pre ssure

Ig nitio n

Pressu re

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.

Po

io n

we

r

TD C

BDC

Tim e K no ckin g C o mb ustion

Fig:1.26 R esults Plotted b y Pressu re Transdu cer.

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.

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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 deposits Detonation leads to a huge amount of carbon deposition at the engine outlet. 4. Increase in heat transfer Knocking is accompanied with the increase in rate of heat transfer across the combustion chamber walls. 5. Decrease in power output and efficiency Due to increase in the rate of heat transfer, the power output as well as efficiency of the engine decreases. 6. Pre-Ignition The increase in heat transfer on the walls causes local overheating of the spark plug which may reach a temperature high enough to ignite the charge before the passage of spark, thus leading to pre-ignition. An engine detonating over a long period of time often results in pre-ignition which is the real danger of detonation. 1.14 FACTORS AFFECTING KNOCK IN SI ENGINES It has already been established that the knocking of an engine typically depends upon either the quantity of the charge inside the chamber, the temperature of the chamber or the time of detonation. Hence, the different variables which affect knocking can be classified into 

Density factors



Time factors



Composition factors

Spark Ignition Engines 1.39

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

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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.

Spark Ignition Engines 1.41

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.

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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.

Spark Ignition Engines 1.43

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. O2   kJ HCV  33800 C  144000  H2    9270 S kg 8   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.

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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 15C. 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).

Spark Ignition Engines 1.45

The octane numbers measured under two different engine conditions in a standard “Cooperative Fuels Research (CFR)” engine has a variable compression 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] CH3CH24 Pb



Methylcyclopentadienyl CH3C5H4MnCO3



Ferrocene Fe C5H52

manganese

tricarbonyl

(MMT)

1.46 Advanced IC Engines - www.airwalkpublications.com



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) CH3CH24 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, headache and it causes difficulties in breathing. 

Ferrocene Fe C5H22 is an organometallic compound of iron. The iron contents in ferrocene forms a conductive coating on the spark plug.

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1.16.3 Factors affecting Detonation and Remedies 1.

2.

3.

4. 5. 6.

Factors Remedies The type of fuel used is the reason Fuel like alcohol and benzol do for detonation not cause detonation. Addition of a small quantity of tetraethyl lead with petrol will suppress the detonation. (This process is called doping). The position of spark plug in the Less distance reduces the chances combustion chamber determines the of detonation. A spark plug placed will reduce the distance the flame travels to reach centrally the detonating zone. More distance detonation. causes detonation High temperature combustion The cooling system should be chamber raise temperature of proper to maintain the cylinder cylinder wall and also detonating wall temperature at optimum level. zone. The compression ratio is the cause The compression ratio should not for detonation. More compression be increased beyond the limit. ratio will overheat the engine. The presence of carbon deposits Good quality fuel should be used. promote detonation. Excessive sparking temperature Ignition system voltage should be promotes detonation limited to produce spark with sufficient temperature to ignite.

1.17 COMBUSTION CHAMBER FOR SI ENGINES The design of combustion chambers for SI engines plays a very important role in the operation and performance of the engine. The design involves, the shape of the combustion chamber, location of 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:

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(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 on the cylinder head, so it is called overhead valve. This type of combustion chamber has two forms. Bath-tub form This type of combustion chamber, consists of oval shaped chamber with both valves mounted overhead. The spark plug is mounted at the side.

S park p lug

front in le t va lve (back exhau st valve is hid d en )

Bath - tub form o f com bustio n chamber

Fig:1.27

Wed ge form o f com bustio n chamber

Wedge form This type of combustion chambers also consist of oval shaped chamber with both valves mounted overhead at its side with slight inclination. The spark plug is mounted centrally. A few features of this combustion chamber are listed below: 1.

Less heat loss because of less surface to volume ratio.

2.

Less flame travel length and greater freedom from knock.

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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.

E xha ust Valve

Fig :1.28 T - H ead Type

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.

L - H e ad Typ es

Fig:1.29 L - He ad Types

Advantages 1.

Valve mechanism is simple and easy to lubricate.

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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 Sp ark p lu g

O verhead EV

IV engin e block R e cipro cating Piston

Fig:1.30 F - Head Type

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.