-
`
`INTERNAL
`COMBUSTION
`ENGINE
`
`John B.LHeywood
`Professor of Mechanical Engineering
`Director, Sloan Automotive Laboratory
`Massachusetts Institute of Technology
`
`McGraw-Hill Series in Mechanical Engineering
`
`Jack P. Holman, Southern Methodist University
`Consulting Editor
`
`Anderson: Modern Compressible Flow: With Historical Perspective
`Dieter: Engineering Design: A Materials and Processing Approach
`Eckert and Drake: Analysis of Heat and Mars Transfer
`Heywood: Internal Combwtion Engine Fundamentals
`H i m : Turbulence, 2/e
`Hutton: Applied Mechanical Vibrations
`Juvinall: Engineering Considerations of Stress, Strain, and Strength
`Kane and Levinson: Dynamics: Theory and Applications
`Kays and Crawford: Convective Heat and Mass Transfr
`Mutin: Kinematics and Dynamics of Machines
`Pklan: Dynamics of Machinery
`Pbelan: Fundamentals of Mechanical Design, 3/e
`Pierce: Acoustics: An Introduction to Its Physical Principles and Applications
`Raven: Automatic Control Engineering, 4/e
`Rosenberg aod Karnopp: Introduction to Physics
`Schlichting: Boundary-Layer Theory, 7/e
`Shames: Mechanics of Fluiak, 2/e
`Shigley: Kinematic Analysis of Mechanisms, 2/e
`Sbigley and Mitchell: Mechanical Engineering Design, 4/e
`Sbigley and Uicker: Theory of Machines and Mechanisms
`Stoecker and Jones: Refrigeration and Air Conditioning, 2/e
`Vanderplaats: Numerical Optimization Techniques for Engineering Design:
`With Applications
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`

`

`INTERNAL COMBUSTION ENGINE FUNDAMENTALS
`
`This book was set in Times Roman.
`The editors were Anne Duffy and John M. M o m s ; the designer
`was Joan E. O'Connor; the production supervisor was
`Denise L. Puryear. New drawings were done by ANCO.
`Project Supervision was done by Santype International Ltd.
`R. R. Donnelley & Sons Company was printer and binder.
`
`See acknowledgements on page xxi.
`
`Copyright 0 1988 by McGraw-Hill, Inc. All rights rese~ed.
`Printed in the United States of America. Except as permitted under the
`United States Copyright Act of 1976, no part of this publication may be
`reproduced or distributed in any form or by any means, or stored in a data
`base or retrieval system, without the prior written permission
`of the publisher.
`
`ISBN 0-07-028637-X
`
`Library of Congress Cataloging-iP.Publication Data
`
`- Heywood, John B.
`
`Internal combustion engine fundamentals.
`(McGraw-Hill series in mechanical engineering)
`Bibliography: p.
`Includes index.
`I. Internal combustion engines.
`TJ755.H45 1988
`621.43
`
`I. Title. 11. Series.
`87-15251
`
`This book is printed on acid-free paper.
`
`ABOUT THE AUTHOR
`
`Dr. John B. Heywood received the Ph.D. degree in mechanical engineering from
`the Massachusetts Institute of Technology in 1965. Following an additional post-
`doctoral year of research at MIT, he worked as a research officer at the Central
`Electricity Generating Board's Research Laboratory in England on magneto-
`hydrodynamic power generation. In 1968 he joined the faculty at MIT where he
`is Professor of Mechanical Engineering. At MIT he is Director of the Sloan
`Automotive Laboratory. He is currently Head of the Fluid and Thermal Science
`Division of the Mechanical Engineering Department, and the Transportation
`Energy Program Director in the MIT Energy Laboratory. He is faculty advisor
`to the MIT Sports Car Club.
`Professor Heywood's teaching and research interests lie in the areas of ther-
`modynamics, combustion, energy, power, and propulsion. During the past two
`decades, his research activities have centered on the operating characteristics and
`fuels requirements of automotive and aircraft engines. A major emphasis has
`been on computer models which predict the performance, efficiency, and emis-
`sions of spark-ignition, diesel, and gas turbine engines; and in carrying out
`experiments to develop and validate these models. He is also actively involved in
`technology assessments and policy studies related to automotive engines, auto-
`mobile fuel utilization, and the control of air pollution. He consults frequently in
`&he automotive and petroleum industries, and for the U.S. Government.
`His extensive research in the field of eogines has been supported by the U.S.
`Army, Department of Energy, Environmental Protection Agency, NASA,
`National Science Foundation, automobile and diesel engine manufacturers, and
`petroleum companies. He has presented or published over a hundred papers on
`
`Page 2
`
`

`

`~i ABOUT THE AUTHOR
`
`his research in technical conferences and journals. He has co-authored two pre-
`vious books: Open-Cycle MHD Power Generation published by Pergamon Press
`in 1969 and The Automobile and the Regulation of Its Impact on the Environment
`published by University of Oklahoma Press in 1975.
`He is a member of the American Society of Mechanical Engineers, an associ-
`ate fellow of the American Institute of Aeronautics and Astronautics, a fellow of
`the British Institution of Mechanical Engineers, and in 1982 was elected a Fellow
`of the U.S. Society of Automotive Engineers for his technical contributions to
`automotive engineering. He is a member of the editorial boards of the journals
`Progress in Energy and Combustion Science and the International Journal of
`Vehicle Design.
`His research publications on internal combustion engines, power generation,
`and gas turbine combustion have won numerous awards. He was awarded the
`Ayreton Premium in 1969 by the British Institution of Electrical Engineers. Pro-
`fessor Heywood received a Ralph R. Teetor Award as an outstanding young
`engineering educator from the Society of Automotive Engineers in 1971. He has
`twice been the recipient of an SAE Arch T. Colwell Merit Award for an outstand-
`ing technical publication (1973 and 1981). He received SAE's Horning Memorial
`Award for the best paper on engines and fuels in 1984. In 1984 he received the
`Sc.D. degree from Cambridge University for his published contributions to
`engineering research. He was selected as the 1986 American Society of Mechani-
`cal Engineers Freeman Scholar for a major review of "Fluid Motion within the
`Cylinder of Internal Combustion Engines."
`
`' THIS BooK IS DEDICATED TO MY FATHER,
`Harold Heywood :
`
`I have followed many of the paths he took.
`
`.
`
`vii
`
`Page 3
`
`

`

`' CONTENTS
`
`Preface
`
`-
`
`Commonly Used Symbols, Subscripts, and
`Abbreviations
`
`xvii
`
`xxiii
`
`Chapter 1
`1.1
`1.2
`1.3
`1.4
`1.5
`1.6
`1.7
`1.8
`1.9
`
`Chapter 2
`2.1
`2.2
`2 3
`2.4
`2.5
`2.6
`2.7
`2.8
`2.9
`
`Engine Types and Their Operation
`Introduction and Historical Perspective
`Engine Classifiytions
`Engine Operating Cycles
`Engine Components
`Spark-Ignition Engine Operation
`Examples of Spark-Ignition Engines
`Compression-Ignition Engine Operation
`Examples of Diesel Engines
`Stratified-Charge Engines
`
`Engine Design and Operating Parameters
`Important Engine Characteristics
`Geometrical Properties of Reciprocating Engines
`Brake Torque and Power
`Indicated Work Per Cycle
`Mechanical Efficiency
`Road-Load Power
`Mean Effective Pressure
`Specific Fuel Consumption and Efficiency
`Air/Fuel and Fuel/Air Ratios
`
`Page 4
`
`

`

`X CONTENTS
`
`2.10
`2.11
`2.12
`2.13
`2.14
`2.15
`
`Volumetric Efficiency
`Engine Specific Weight and Specific Volume
`Correction Factors for Power and Volumetric Efficiency
`Specific Emissions and Emissions Index
`Relationships between Performance Parameters
`Engine Design and Performance Data
`
`Chapter 3
`3.1
`3.2
`3.3
`3.4
`3.5
`
`Chapter 4
`4.1
`4.2
`4.3
`4.4
`4.5
`
`Thermochemistry of Fuel-Air Mixtures
`Characterization of Flames
`Ideal Gas Model
`Composition of Air and Fuels
`Combustion Stoichiometry
`The First Law of Thermodynamics and Combustion
`3.5.1 Energy and Enthalpy Balances
`3.5.2 Enthalpies of Formation
`3.5.3 Heating Values
`3.5.4 Adiabatic Combustion Processes
`3.5.5 Combustion Efiency of an Internal Combustion Engine
`The Second Law of Thermodynamics Applied to Combustion
`3.6.1 Entropy
`3.6.2 Maximum Work from an Internal Combustion
`Engine and Efficiency
`Chemically Reacting Gas Mixtures
`3.7.1 Chemical Equilibrium
`3.7.2 Chemical Reaction Rates
`
`Properties of Working Fluids
`Introduction
`Unburned Mixture Composition
`Gas Property Relationships
`A Simple Analytic Ideal Gas Model
`Thermodynamic Charts
`4.5.1 Unburned Mixture Charts
`4.5.2 Burned Mixture Charts
`4.5.3 Relation between Unburned and Burned
`Mixture Charts
`Tables of Properties and Composition
`Computer Routines for Property and Composition Calculations
`4.7.1 Unburned Mixtures
`4.7.2 Burned Mixtures
`Transport Properties
`Exhaust Gas Composition
`4.9.1 Species Concentration Data
`4.9.2 Equivalence Ratio Determination from Exhaust
`Gas Constituents
`4.9.3 Effects of Fuel/Air Ratio Nonuniformity
`4.9.4 Combustion Inefficiency
`
`Chapter 5
`5.1
`5.2
`5.3
`5.4
`
`5.8
`
`Chapter 6
`6.1
`6.2
`
`6.4
`6.5
`6.6
`
`6.7
`6.8
`
`Ideal Models of Engine Cycles
`Introduction
`Ideal Models of Engine Processes
`Thermodynamic Relations for Engine Processes
`Cycle Analysis with Ideal Gas Working Fluid with c, and
`Constant
`5.4.1 Constant-Volume Cycle
`5.4.2 Limited- and Constant-Pressure Cycles
`5.4.3 Cycle Comparison
`Fuel-Air Cycle Analysis
`5.5.1 SI Engine Cycle Simulation
`5.5.2 CI Engine Cycle Simulation
`5.5.3 Results of Cycle Calculations
`Overexpanded Engine Cycles
`Availability Analysis of Engine Processes
`5.7.1 Availability Relationships
`5.7.2 Entropy Changes in Ideal Cycles
`5.7.3 Availability Analysis of Ideal Cycles
`5.7.4 Effect of Equivalence Ratio
`Comparison with Real Engine Cycles
`
`.
`
`Gas Exchange Processes
`Inlet and Exhaust Processes in the Four-Stroke Cycle
`Volumetric Efficiency
`6.2.1 Quasi-Static Effects
`6.2.2 Combined Quasi-Static and Dynamic Ekects
`'iming
`6.2.3 Variation with Speed. and Valve Area, Lift, and 'I
`Flow Through Valves
`6.3.1 Poppet Valve Geometry and Timing
`6.3.2 Flow Rate and Discharge Coefficients
`Residual Gas Fraction
`Exhaust Gas Flow Rate and Temperature Variation
`Scavenging in Two-Stroke Cycle Engines
`6.6.1 Two-Stroke Engine Configurations
`6.6.2 Scavenging Parameters and Models
`6.6.3 Actual Scavenging Processes
`Flow Through Ports
`Supercharging and Turbocharging
`6.8.1 Methods of Power Boosting
`6.8.2 Basic Relationships
`6.8.3 Compressors
`6.8.4 Turbines
`6.8.5 Wave-Compression Devices
`
`Chapter 7
`
`7.1
`7.2
`
`SI Engine Fuel Metering and Manifold
`Phenomena
`Spark-Ignition Engine Mixture Requirements
`Carburetors
`
`Page 5
`
`

`

`xii CONTENTS
`
`7.3
`
`7.4
`7.5
`7.6
`
`7.2.1 Carburetor Fundamentals
`7.2.2 Modem Carburetor Design
`Fuel-Injection Systems
`7.3.1 Multipoint Port Injection
`7.3.2 Single-Point Throttle-Body Injection
`Feedback Systems
`Flow Past Throttle Plate
`Flow in Intake Manifolds
`7.6.1 Design Requirements
`7.6.2 Air-Flow Phenomena
`7.6.3 Fuel-Flow Phenomena
`
`Chapter 8
`8.1
`8.2
`
`8.3
`
`8.4
`8.5
`8.6
`8.7
`
`Chapter 9
`9.1
`9.2
`
`9.3
`
`9.4
`
`9.5
`
`9.6
`
`Charge Motion within the Cylinder
`Intake Jet Flow
`Mean Velocity and Turbulence Characteristics
`8.2.1 Definitions
`8.2.2 Application to Engine Velocity Data
`Swirl
`8.3.1 Swirl Measurement
`8.3.2 Swirl Generation during Induction
`8.3.3 Swirl Modification within the Cylinder
`Squish
`Prechamber Engine Flows
`Crevice Flows and Blowby
`Flows Generated by Piston-Cylinder Wall Interaction
`
`Combustion in Spark-Ignition Engines
`Essential Features of Process
`Thermodynamic Analysis of SI Engine Combustion
`9.2.1 Burned and Unburned Mixture States
`9.2.2 Analysis of Cylinder Pressure Data
`9.2.3 Combustion Process Characterization
`Flame Structure and Speed
`9.3.1 Experimental Observations
`9.3.2 Flame Structure
`9.3.3 Laminar Burning Speeds
`9.3.4 Flame Propagation Relations
`Cyclic Variations in Combustion, Partial Burning, and Misfire
`9.4.1 Observations and Definitions '
`9.4.2 Causes of Cycle-by-Cycle and Cylinder-to-Cylinder
`Variations
`9.4.3 Partial Burning, Misfire, and Engine Stability
`Spark Ignition
`9.5.1
`Ignition Fundamentals
`9.5.2 Conventional Ignition Systems
`9.5.3 Alternative Ignition Approaches
`Abnormal Combustion: Knock and Surface Ignition
`9.6.1 Description of Phenomena
`
`Chapter 10
`10.1
`10.2
`
`Chapter 11
`11.1
`11.2
`
`9.6.2 Knock Fundamentals
`9.6.3 Fuel Factors
`
`Combustion in Compression-Ignition Engines
`Essential Features of Process
`Types of Diesel Combustion Systems
`10.2.1 Direct-Injection Systems
`10.2.2
`Indirect-Injection Systems
`10.2.3 Comparison of Different Combustion Systems
`Phenomenological Model of Compression-Ignition Engine
`Combustion
`10.3.1 Photographic Studies of Engine Combustion
`10.3.2 Combustion in Direct-Injection, Multispray Systems
`10.3.3 Application of Model to Other Combustion Systems
`Analysis of Cylinder Pressure Data
`10.4.1 Combustion Efficiency
`10.4.2 Direct-Injection Engines
`10.4.3
`Indirect-Injection Engines
`Fuel Spray Behavior
`10.5.1 Fuel Injection
`10.5.2 Overall Spray Structure
`10.5.3 Atomization
`10.5.4 Spray Penetration
`10.5.5 Droplet Size Distribution
`10.5.6 Spray Evaporation
`Ignition Delay
`10.6.1 Definition and Discussion
`10.6.2 Fuel Ignition Quality
`10.6.3 Autoignition Fundamentals
`10.6.4 Physical Factors Affecting Delay
`10.6.5 Effect of Fuel Properties
`10.6.6 Correlations for Ignition Delay in Engines
`Mixing-Controlled Combustion
`10.7.1 Background
`10.7.2 Spray and Flame Structure
`10.7.3 Fuel-Air Mixing and Burning Rates
`
`Pollutant Formation and Control
`Nature and Extent of Problem
`Nitrogen Oxides
`1 1.2.1 Kinetics of NO Formation
`11.2.2 Formation of NO,
`11.2.3 NO Formation in Spark-Ignition Engines
`11.2.4 NO, Formation in Compression-Ignition Engines
`Carbon Monoxide
`Unburned Hydrocarbon Emissions
`1 1.4.1 Background
`11.4.2 Flame Quenching and Oxidation Fundamentals
`
`Page 6
`
`

`

`xiv CONTENTS
`
`11.5
`
`11.6
`
`11.4.3 HC Emissions from Spark-Ignition Engines
`11.4.4 Hydrocarbon Emission Mechanisms in Diesel Engines
`Particulate Emissions
`11.5.1 Spark-Ignition Engine Particulates
`11.5.2 Characteristics of Diesel Particulates
`11.5.3 Particulate Distribution within the Cylinder
`11.5.4 Soot Formation Fundamentals
`11.5.5 Soot Oxidation
`11.5.6 Adsorption and Condensation
`Exhaust Gas Treatment
`11.6.1 Available Options
`11.6.2 Catalytic Converters
`11.6.3 Thermal Reactors
`11.6.4 Particulate Traps
`
`Chapter 12
`12.1
`12.2
`
`12.3
`12.4
`
`12.5
`
`12.6
`
`12.7
`
`Engine Heat Transfer
`Importance of Heat Transfer
`Modes of Heat Transfer
`12.2.1 Conduction
`12.2.2 Convection
`12.2.3 Radiation
`12.2.4 Overall Heat-Transfer Process
`Heat Transfer and Engine Energy Balance
`Convective Heat Transfer
`12.4.1 Dimensional Analysis
`12.4.2 Correlations for Time-Averaged Heat Flux
`12.4.3 Correlations for Instantaneous Spatial
`Average Coefficients
`12.4.4 Correlations for Instantaneous Local Coefficients
`12.4.5
`Intake and Exhaust System Heat Transfer
`Radiative Heat Transfer
`12.5.1 Radiation from Gases
`12.5.2 Flame Radiation
`12.5.3 Prediction Formulas
`Measurements of Instantaneous Heat-Transfer Rates
`12.6.1 Measurement Methods
`12.6.2 Spark-Ignition Engine Measurements
`12.6.3 Diesel Engine Measurements
`12.6.4 Evaluation of Heat-Transfer Correlations
`12.6.5 Boundary-Layer Behavior
`Thermal Loading and Component Temperatures
`12.7.1 Component Temperature Distributions
`12.7.2 Effect of Engine Variables
`
`Chapter 13
`13.1
`13.2
`13.3
`
`Engine Friction and Lubrication
`Background
`Definitions
`Friction Fundamentals
`
`13.3.1 Lubricated Friction
`13.3.2 Turbulent Dissipation
`13.3.3 Total Friction
`Measurement Methods
`Engine Friction Data
`13.5.1 SI Engines
`13.5.2 Diesel Engines
`Engine Friction Components
`13.6.1 Motored Engine Breakdown Tests
`13.6.2 Pumping Friction
`13.6.3 Piston Assembly Friction
`13.6.4 Crankshaft Bearing Friction
`13.6.5 Valve Train Friction
`Accessory Power Requirements
`Lubrication
`13.8.1 Lubrication System
`13.8.2 Lubricant Requirements
`
`13.4
`13.5
`
`13.6
`
`13.7
`13.8
`
`Chapter 14
`
`14.1
`14.2
`
`14.3
`
`14.4
`
`14.5
`
`Modeling Real Engine Flow and Combustion
`Processes
`Purpose and Classification of Models
`Governing Equations for Open Thermodynamic System
`14.2.1 Conservation of Mass
`14.2.2 Conservation of Energy
`Intake and Exhaust Flow Models
`14.3.1 Background
`14.3.2 Quasi-Steady Flow Models
`14.3.3 Filling and Emptying Methods
`14.3.4 Gas Dynamic Models
`Thermodynamic-Based In-Cylinder Models
`14.4.1 Background and Overall Model Structure
`14.4.2 Spark-Ignition Engine Models
`14.4.3 Direct-Injection Engine Models
`14.4.4 Prechamber Engine Models
`14.4.5 Multicylinder and Complex Engine System Models
`14.4.6 Second Law Analysis of Engine Processes
`Fluid-Mechanic-Based Multidimensional Models
`- 14.5.1 Basic Approach and Governing Equations
`14.5.2 Turbulence Models
`14.5.3 Numerical Methodology
`14.5.4 Flow Field Predictions
`14.5.5 Fuel Spray Modeling
`14.5.6 Combustion Modeling
`
`Chapter 15
`15.1
`15.2
`
`Engine Operating Characteristics
`Engine Performana Parameters
`Indicated and Brake Power and MEP
`
`Page 7
`
`

`

`xvi CONTENTS
`
`15.3
`
`15.4
`
`15.5
`
`15.6
`
`Operating Variables That Affect SI Engine Performance,
`Efficiency, and Emissions
`15.3.1 Spark Timing
`15.3.2 Mixture Composition
`15.3.3 Load and Speed
`15.3.4 Compression Ratio
`SI Engine Combustion Chamber Design
`15.4.1 Design Objectives and Options
`15.4.2 Factors That Control Combustion
`15.4.3 Factors That Control Performance
`15.4.4 Chamber Octane Requirement
`15.4.5 Chamber Optimization Strategy
`Variables That Affect CI Engine Performance, Efficiency, and
`Emissions
`15.5.1 Load and Speed
`15.5.2 Fuel-Injection Parameters
`15.5.3 Air Swirl and Bowl-in-Piston Design
`Supercharged and Turbocharged Engine Performance
`15.6.1 Four-Stroke Cycle SI Engines
`15.6.2 Four-Stroke Cycle CI Engines
`15.6.3 Two-Stroke Cycle SI Engines
`15.6.4 Two-Stroke Cycle CI Engines
`Engine Performance Summary
`
`Appendixes
`A Unit Conversion Factors
`Ideal Gas Relationships
`B
`B.l
`Ideal Gas Law
`B.2 The Mole
`B.3 Thermodynamic Properties
`B.4 Mixtures of Ideal Gases
`C Equations for Fluid Flow through a Restriction
`C.1 Liquid Flow
`C.2 Gas Flow
`D Data on Working Fluids
`
`Index
`
`-
`
`PREFACE
`
`Internal combustion engines date back to 1876 when Otto first developed the
`spark-ignition engine and 1892 when Diesel invented the compression-ignition
`engine. Since that time these engines have continued to develop as our knowledge
`of engine processes has increased, as new technologies became available, as
`demand for new types of engine arose, and as environmental constraints on
`engine use changed. Internal combustion engines, and the industries that develop
`and manufacture them and support their use, now play a dominant role in the
`fields of power, propulsion, and energy. The last twenty-five years or so have seen
`an explosive growth in engine research and development as the issues of air pol-
`lution, fuel cost, and market competitiveness have become increasingly impor-
`tant. An enormous technical literature on engines now exists which has yet to be
`adequately organized and summarized.
`This book has been written as a text and a professional reference in response
`to that need. It contains a broadly based and extensive review of the fundamental
`principles which govern internal combustion engine design and operation. It
`attempts to provide a simplifying framework for the vast and complex mass of
`technical material that now exists on spark-ignition and compression-ignition
`engines, and at the same time to include sufficient detail to convey the real world
`dimensions of this pragmatic engineering field. It is the author's conviction that a
`sound knowledge of the relevant fundamentals in the many disciplines that con-
`tribute to this field, as well as an awareness of the extensive practical knowledge
`base which has been built up over many decades, are essential tools for engine
`research, development, and design. Of course, no one text can include everything
`about engines. The emphasis here is on the thermodynamics, combustion physics
`and chemistry, fluid flow, heat transfer, friction, and lubrication processes rele-
`vant to internal combustion engine design, performance, efficiency, emissions, and
`fuels requirements.
`
`Page 8
`
`

`

`450
`
`INTERNAL COMBUSTION ENGINE FUNDAMENTALS
`
`COMBUSTION IN SPARK-IGNITION ENGINES 451
`
`A larger prechamber (5 to 12 percent) and larger orifice (orifice area/precham
`volume ratio of 0.04 to 0.2 cm-') gives a lower velocity jet which penetrates t
`main chamber charge more slowly, resulting in a slower burn.
`All these concepts extend the engine's lean stable operating limit, relative to
`equivalent conventional engines, by several air/fuel ratios. For example, the
`unscavenged cavity without auxiliary fueling can operate satisfactorily at part.
`load with air/fuel ratios of 18 (equivalence ratio 4 x 0.8, relative air/fuel ratio
`I x 1.25). The prechamber stratified-charge flame-jet ignition concepts can
`operate much leaner than this; however, the best combination of fuel consump.
`is obtained with 4 x 0.9 - 0.75,
`tion and emissions characteristics
`I x 1.1 - 1.3.70* One performance penalty associated with all these flame-jet
`ignition concepts is the additional heat losses to the prechamber walls due to
`increased surface area and flow velocities. The stratified-charge prechamber con.
`cepts also suffer an efficiency penalty, relative to equivalent operation with
`uniform airlfuel ratios, due to the presence of fuel-rich regions during the com-
`bustion process.
`
`9.6 ABNORMAL COMBUSTION: KNOCK
`AND SURFACE IGNITION
`9.6.1 Description of Phenomena
`Abnormal combustion reveals itself it many ways. Of the various abnormal com-
`bustion processes which are important in practice, the two major phenomena are
`knock and surface ignition. These abnormal combustion phenomena are of
`concern because: (1) when severe, they can cause major engine damage; and (2)
`even if not severe, they are regarded as an objectionable source of noise by the
`engine or vehicle operator. Knock is the name given to the noise which is trans-
`mitted through the engine structure when essentially spontaneous ignition of a
`portion of the end-gas-the
`fuel, air, residual gas, mixture ahead of the propagat-
`ing flammccurs. When this abnormal combustion process takes place, there is
`an extremely rapid release of much of the chemical energy in the end-gas, causing
`very high local pressures and the propagation of pressure waves of substantial
`amplitude across the combustion chamber. Surface ignition is ignition of the fuel-
`air mixture by a hot spot on the combustion chamber walls such as an overheat-
`ed valve or spark plug, or glowing combustion chamber deposit: i.e., by any
`means other than the normal spark discharge. It can occur before the occurrence
`of the spark (preignition) or after (postignition). Following surface ignition, a turb-
`ulent flame develops at each surface-ignition location and starts to propagate
`across the chamber in an analogous manner to what occurs with normal spark
`ignition.
`Because the spontaneous ignition phenomenon that causes knock is g
`erned by the temperature and pressure history of the end gas, and therefore
`the phasing and rate of development of the flame, various combinations of th
`two phenomena-surface ignition and knock-can occur. These have been
`gorized as indicated in Fig. 9-58. When autoignition occurs repeatedly, d
`
`Normal combustion
`A combustion p m s which is initiated
`solely by a timed spark and in which the
`flame front moves completely across the
`combustion chamber in a uniform manner at
`a normal velocity.
`
`I
`
`Spark knock*
`A knock which is recurrent and repeatable
`in terms of audibility. It is controllable by
`the spark advance; advancing the spark
`increases the knock intensity and reZarding
`the spark reduces the intensity.
`
`I
`
`Abnormal combustion
`A combustion process in which a flame
`front may be staned by hot combustion-
`chamber surfaces either prior to or after
`spark ignition, or a process in which some
`part or all of the charge may be consumed
`at extremely high rates.
`I
`
`I
`Swface ignition
`hot spots-combnstionehamber deposits
`Surface ignition is ignition of the fuel-air
`charge by any hot surface other than the
`spark discharge prior to the arrival of the
`normal flame front. It may occur before the
`spark ignites the charge @reignition) or
`after normal ignition (postignition).
`I
`
`Continuation of engine fYing
`after the electrical ignition is
`
`k m m a y surfnce ignition
`Surface ignition which
`occurs earlier and earlier in
`the cycle. It can lead to
`serious overheating and
`struchral damage to the
`
`Kwdring* surface ignition
`Knock which has been
`preceded by surface
`ignition. It is not controlla-
`ble by spark advance.
`
`1
`
`WUd ping
`Knocking surface ignition
`characterized by one or
`more erratic sharp cracks. It
`is probably the result of
`early surface ignition from
`deposit particles.
`
`Surface ignition which does
`not result in knock.
`
`Rumble
`A low-pitched thudding
`noise accompanied by
`engine roughness. Probably
`caused by the high rates of
`pressure. rise associated with
`very early ignition or
`multiple surface ignition.
`
`'Knock: The noise associated with autoignition of a portion of the fuel-air mixture ahead of the advancing flame front.
`Autoignition is the spontaneous ignition and the resulting very rapid reaction of a portion or all of the fuel-air mixture.
`
`FIGURE 9-58
`and abnormal (knock and surface ignition)--in
`Ddinition of combustion phenomena-no&
`spark-ignition engine. (Courtesy Coordinating Research Council.)
`
`a
`
`Page 9
`
`

`

`452 INTERNAL C O M B U ~ O N ENGINE FUNDAMENTALS
`
`COMBUSTION IN SPARK-IGNITION ENGINES 453
`
`otherwise normal combustion events, the phenomena is called spark-knock.
`Repeatedly here means occurring more than occasionally: the knock phenome.
`non varies substantially cycle-bycycle, and between the cylinders of a multi-
`cylinder engine, and does not necessarily occur every cycle (see below).
`Spark-knock is controllable by the spark advance: advancing the spark increases
`the knock severity or intensity and retarding the spark decreases the knock. Sina
`surface ignition usually causes a more rapid rise in end-gas pressure and tem-
`perature than occurs with normal spark ignition (because the flame either starts
`propagating sooner, or propagates from more than one source), knock is a likely
`outcome following the occurrence of surface ignition. To identify whether or not
`surface ignition causes knock, the terms knocking surface ignition and non-
`knocking surface ignition are used. Knocking surface ignition usually originates
`from preignition caused by glowing combustion chamber deposits: the severity of
`knock generally increases the earlier that preignition occurs. Knocking surface
`ignition cannot normally be controlled by retarding the spark timing, since the
`spark-ignited flame is not the cause of knock. Nonknocking surface ignition is
`usually associated with surface ignition that occurs late in the operating cycle.
`The other abnormal combustion phenomena in Fig. 9-58, while less
`common, have the following identifying names. Wild ping is a variation of knock-
`ing surface ignition which produces sharp cracking sounds in bursts. It is thought
`to result from early ignition of the fuel-air mixture in the combustion chamber by
`glowing loose deposit particles. It disappears when the particles are exhausted
`and reappears when fresh particles break loose from the chamber surfaces.
`Rumble is a relatively stable low-frequency noise (600 to 1200 Hz) phenomenon
`associated with depositcaused surface ignition in highcompression-ratio
`engines. This type of surface ignition produces very high rates of pressure rise
`following ignition. Rumble and knock can occur together. Run-on occurs when
`the fuel-air mixture within the cylinder continues to ignite after the ignition
`system has been switched off. During run-on, the engine usually emits knocklike
`noises. Run-on is probably caused by compression ignition of the fuel-air
`mixture, rather than surface ignition. Runaway surface ignition is surface ignition
`that occurs earlier and earlier in the cycle. It is usually caused by overheated
`spark plugs or valves or other combustion chamber surfaces. It is the most
`destructive type of surface ignition and can lead to serious overheating and struc-
`tural damage to the engine.74
`After some additional description of surface-ignition phenomena, the
`remainder of Sec. 9.6 will focus on knock. This is because surface ignition is a
`problem that can be solved by appropriate attention to engine design, and fuel
`and lubricant quality. In contrast, knock is an inherent constraint on engine
`performance and e5ciency since it limits the maximum compression ratio that
`can be used with any given fuel.
`Of all the engine surface-ignition phenomena in Fig. 9-58, preignition is
`potentially the most damaging. Any process that advances the start of com-
`bustion from the timing that gives maximum torque will cause higher heat rejec-
`tion because of the increasing burned gas pressures and temperatures that result.
`
`Higher heat rejection causes higher temperature components which, in turn, can
`advance the preignition point even further until critical components can fail. The
`parts which can cause preignition are those least well cooled and where deposits
`build up and provide additional thermal insulation: primary examples are spark
`plugs, exhaust valves, metal asperities such as edges of head cavities or piston
`bowls. Under normal conditions, using suitable heat-range spark plugs, preigni-
`tion is usually initiated by an exhaust valve covered with deposits coming from
`the fuel and from the lubricant which penetrates into the combustion chamber.
`Colder running exhaust valves and reduced oil consumption usually alleviate this
`problem: locating the exhaust valve between the spark plug and the end-gas
`region avoids contact with both the hottest burned gas near the spark plug and
`the end-gas. Engine design features that minimize the likelihood of preignition
`are: appropriate heat-range spark plug, removal of asperities, radiused metal
`cooled exhaust valves with sodium-cooled valves as an extreme
`
`Knock primarily occurs under wide-open-throttle operating conditions. It is
`thus a direct constraint on engine performance. It also constrains engine efi-
`ciency, since by effectively limiting the temperature and pressure of the end-gas, it
`limits the engine compression ratio. The occurrence and severity of knock depend
`on the knock resistance of the fuel and on the antiknock characteristics of the
`engine. The ability of a fuel to resist knock is measured by its octane number:
`higher octane numbers indicate greater resistance to knock (see Sec. 9.6.3). Gas-
`oline octane ratings can be unproved by refining processes, such as catalytic
`cracking and reforming, which convert low-octane hydrocarbons to high-octane
`hydrocarbons. Also, antiknock additives such as alcohols, lead alkyls, or an
`organomanganese compound can be used. The octane-number requirement of an
`engine depends on how its design and the conditions under which it is operated
`affect the temperature and pressure of the end-gas ahead of the flame and the
`time required to burn the cylinder charge. An engine's tendency to knock, as
`defined by its octane requirement-the octane rating of the fuel required to avoid
`knock-is
`increased by factors that produce higher temperatures and pressures
`or lengthen the burning time. Thus knock is a constraint that depends on both
`the quality of available fuels and on the ability of the engine designer to achieve
`the desired normal combustion behavior while holding the engine's propensity to
`knock at a minimum.74
`The pressure variation in the cylinder during knocking combustion indi-
`cates in more detail what actually occurs. Figure 9-59 shows the cylinder pressure
`variation in three individual engine cycles, for normal combustion, light knock,
`and heavy knock, re~pectively.~~ When knock occurs, high-frequency pressure
`fluctuations are observed whose amplitude decays with time. Figures 9-59a and b
`have the same operating conditions and spark advance. About one-third of the
`cycles in this engine at these conditions had no trace of knock and had normal,
`smoothly varying, cylinder pressure records as in Fig. 9-59a. Knock of varying
`severity occurred in the remaining cycles. With light or trace knock, knock
`