Internal Combustion Engine Fundamentals

McGrawHill 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 Mass Transfer Heywood Internal Combustion Engine Fundamentals Himze Turbulence 2e 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 Transfer Martin Kinematics and Dynamics of Machines Phelan Dynamics of Machinery Phelan Fundamentals of Mechanical Design 3e Pierce Acoustics An Introduction to Its Physical Principles and Applications Raven Automatic Control Engineering 4e Rosenberg and Karnopp Introduction to Physics Schlichting BoundaryLayer Theory 7e Shames Mechanics of Fluids 2e Shigley and Mitchell Mechanical Engineering Design 4e Shigley and Uicker Theory of Machines and Mechanisms Stoecker and Jones Refrigeration and Air Conditioning 2e Vanderplaats Numerical Optimization Techniques for Engineering Design With Applications INTERNAL COMBUSTION ENGINE FUNDAMENTALS This book was set in Times Roman The editors were Anne Duffy and John M Morris the designer was Joan E OConnor the production supervisor was Denise L Puryear New drawings were done by Santype International Ltd R R Donnelley Sons Company was printer and binder See acknowledgments on page xi Copyright 1988 by McGrawHill Inc All rights reserved 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 14 15 16 17 DOCDOC 9 9 8 ISBN 007026837X Library of Congress CataloginginPublication Data Heywood John B Internal combustion engine fundamentals McGrawHill series in mechanical engineering Bibliography p Includes index 1 Internal combustion engines I Title II Series TJ755H45 1988 62143 8752551 This book is printed on acidfree paper ABOUT THE AUTHOR Dr John B Heywood received the PhD degree in mechanical engineering from the Massachusetts Institute of Technology in 1965 Following an additional postdoctoral year of research at MIT he worked as a research officer at the Central Electricity Generating Boards Research Laboratory in England on magnetohydrodynamic 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 Heywoods teaching and research interests lie in the areas of thermodynamics 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 emissions of sparkignition 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 automobile fuel utilization and the control of air pollution He consults frequently in the automotive and petroleum industries and for the US Government His extensive research in the field of engines has been supported by the US 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 CONTENTS Preface xvii Commonly Used Symbols Subscripts and Abbreviations xxiii HC Emissions from SparkIgnition Engines 601 Hydrocarbon Emission Mechanisms in Diesel Engines 620 Particulate Emissions 626 The author wishes to acknowledge the following organizations and publishers for permission to reproduce figures and tables from their publications in this text The American Chemical Society American Institute of Aeronautics Astronautics American Society of Mechanical Engineers Robert Bosch GmbH CIMAC Cambridge University Press The Combustion Institute Elsevier Science Publishing Company G T Foulis Co Ltd General Motors Corporation Gordon Breach Science Publishers The Institution of Mechanical Engineers The Japan Society of Mechanical Engineers MIT Press Macmillan Press Ltd McGrawHill Book Company Mir Publishers Mobil Oil Corporation MorganGrampian Publishers Pergamon Journals Inc Plenum Press Corporation The Royal Society of London Scientific Publications Limited Society of Automotive Engineers Society of Automotive Engineers of Japan Inc Society of Tribologists and Lubrications Engineers Department of Mechanical Engineering Stanford University 1 SYMBOLS a Crank radius Sound speed Specific availability a Acceleration A Area Ac Valve curtain area Aeh Cylinder head area Ae Exhaust port area Ae Effective area of flow restriction Ai Inlet port area Ap Piston crown area B Cylinder bore Steadyflow availability c Specific heat cp Specific heat at constant pressure cs Soot concentration massvolume cv Specific heat at constant volume C Absolute gas velocity n Number of moles Polytropic exponent nr Number of crank revolutions per power stroke N Crankshaft rotational speed Soot particle number density p Cylinder pressure P Power q Heattransfer rate per unit area Heattransfer rate per unit mass of fluid Q Heat transfer Q Heattransfer rate Qh Fuel chemical energy release or gross heat release Qhv Fuel heating value Qn Net heat release r Radius r Compression ratio R Connecting rod lengthcrank radius Gas constant Radius R R Oneway reaction rates Rl Swirl ratio s Crank axis to piston pin distance Specific entropy S Entropy Spray penetration Sb Turbulent burning speed SL Laminar flame speed Sp Piston speed t Time T Temperature Torque u Specific internal energy Velocity u Turbulence intensity ul Sensible specific internal energy ur Characteristic turbulent velocity U Compressorturbine impeller tangential velocity Fluid velocity Internal energy v Specific volume Velocity v Velocity vps Valve pseudoflow velocity Squish velocity Liquid 11 INTRODUCTION AND HISTORICAL PERSPECTIVE The purpose of internal combustion engines is the production of mechanical power from the chemical energy contained in the fuel In internal combustion engines as distinct from external combustion engines this energy is released by burning or oxidizing the fuel inside the engine The fuelair mixture before combustion and the burned products after combustion are the actual working fluids The work transfers which provide the desired power output occur directly between these working fluids and the mechanical components of the engine The internal combustion engines which are the subject of this book are sparkignition engines sometimes called Otto engines or gasoline or petrol engines though other fuels can be used and compressionignition or diesel engines Because of their simplicity ruggedness and high powerweight ratio these two types of engine have found wide application in transportation land sea and air and power generation It is the fact that combustion takes place inside the work Comparison of Otto fourstroke cycle and OttoLangen engines Internal combustion engine fundamentals The automotive urban airpollution problem Classification of reciprocating engines by application The fourstroke operating cycle Cutaway drawing of Chrysler 22liter displacement fourcylinder sparkignition engine Crosssection drawing of an ElectroMotive twostroke cycle diesel engine This engine uses a uniflow scavenging process with inlet ports in the cylinder liner and four exhaust valves in the cylinder head Bore 2302 mm stroke 254 mm displaced volume per cylinder 1057 liters rated speed 750900 revmin Courtesy ElectroMotive Division General Motors Corporation Cross section of singlebarrel downdraft carburetor Courtesy Robert Bosch GmbH and SAE Schematic drawing of LJetronic port electronic fuelinjection system Courtesy Robert Bosch GmbH and SAE cylinders per engine An upper limit on cylinder size is dictated by dynamic considerations the inertial forces that are created by accelerating and decelerating the reciprocating masses of the pistons and connecting rod would quickly limit the maximum speed of the engine Thus the displaced volume is spread out amongst several smaller cylinders The increased frequency of power strokes with a multicylinder engine produces much smoother torque characteristics Multicylinder engines can also achieve a much better state of balance than singlecylinder engines A force must be applied to the piston to accelerate it during the first half of its travel from bottomcenter to topcenter The piston then exerts a force as it decelerates during the second part of the stroke It is desirable to cancel these inertia forces through the choice of number and arrangement of cylinders to achieve a primary balance Note however that the motion of the piston is more rapid during the upper half of its stroke than during the lower half a consequence of the connecting rod and crank mechanism evident from Fig 11 see also Sec 22 The resulting inequality in piston acceleration and deceleration produces corresponding differences in inertia forces generated Certain combinations of cylinder number and arrangement will balance out these secondary inertia effects Fourcylinder inline engines are the most common arrangements for automotive engines up to about 25liter displacement An example of this inline arrangement was shown in Fig 114 It is compactan important consideration for small passenger cars It provides two torque pulses per revolution of the crankshaft and primary inertia forces though not secondary forces are balanced V engines and opposedpiston engines are occasionally used with this number of cylinders The V arrangement with two banks of cylinders set at 90 or a more acute angle to each other provides a compact block and is used extensively for larger displacement engines Figure 19 shows a V6 engine the six cylinders being arranged in two banks of three with a 60 angle between their axis Six cylinders are usually used in the 25 to 45liter displacement range Sixcylinder engines provide smoother operation with three torque pulses per revolution The inline arrangement results in a long engine however giving rise to crankshaft torsional vibration and making even distribution of air and fuel to each cylinder more difficult The V6 arrangement is much more compact and the example shown provides primary balance of the reciprocating components With the V engine however a rocking moment is imposed on the crankshaft due to the secondary inertia forces which results in the engine being less well balanced than the inline version The V8 and V12 arrangements are also commonly used to provide compact smooth lowvibration largerdisplacement sparkignition engines placement can deliver Figure 110 shows an example of a turbocharged fourcylinder sparkignition engine The turbocharger a compressorturbine combination uses the energy available in the engine exhaust stream to achieve compression of the intake flow The air flow passes through the compressor 2 intercooler 3 carburetor 4 manifold 5 and inlet valve 6 as shown Engine inlet pressures or boost of up to about 100 kPa above atmospheric pressure are typical The exhaust flow through the valve 7 and manifold 8 drives the turbine 9 which powers the compressor A wastegate valve just upstream of the turbine bypasses some of the exhaust gas flow when necessary to prevent the boost pressure becoming too high The wastegate linkage 11 is controlled by a boost pressure regulator While this turbocharged engine configuration has the carburetor downstream of the compressor some turbocharged sparkignition engines have the carburetor upstream of the compressor so that it operates at or below atmospheric pressure Figure 111 shows a cutaway drawing of a small automotive turbocharger The arrangements of the compressor and turbine rotors connected via the central shaft and of the turbine and compressor flow passages are evident Figure 112 shows a twostroke cycle sparkignition engine The twostroke cycle sparkignition engine is used for smallengine applications where low cost and weightpower ratio are important and when the use factor is low Examples of such applications are outboard motorboat engines motorcycles and chain saws All such engines are of the carburetor crankcasecompression type which is one of the simplest prime movers available It has three moving parts per cylinder the piston connecting rod and the crank The prime advantage of the twostroke cycle sparkignition engine relative to the fourstroke cycle engine is its higher power per unit displaced volume due to twice the number of power strokes per crank revolution This is offset by the lower fresh charge density achieved by the twostroke cycle gasexchange process and the loss of efficiency in the engines Also con turbinecompressor combination and supercharged engines where the air is compressed by a mechanically driven pump or blower are common Turbocharging and supercharging increase engine output by increasing the air mass flow per unit displaced volume thereby allowing an increase in fuel flow These methods are used usually in larger engines to reduce engine size and weight for a given power output Except in smaller engine sizes the twostroke cycle is competitive with the fourstroke cycle in large part because with the diesel cycle only air is lost in the cylinder scavenging process The operation of a typical fourstroke naturally aspirated CI engine is illustrated in Fig 115 The compression ratio is generally higher than typical SI engine values and is in the range 12 to 24 depending on the type of diesel engine and whether the engine is naturally aspirated or turbocharged The valve timings used are similar to those of SI engines Air at closetoatmospheric pressure is induced during the intake stroke and then compressed to a pressure of about 4 MPa 600 lbin² and temperature of about 800 K 1000F during the compression stroke At about 20 before TDC fuel injection into the engine cylinder commences a typical rate of injection profile is shown in Fig 115b The liquid fuel jet atomizes into drops and entrains air The liquid fuel evaporates fuel vapor then mixes with air to form combustible proportions The air temperature and pressure are above the fuels ignition point Therefore after a short delay period spontaneous ignition autoignition of parts of the nonburnt fuelair mixture initiates the combustion process and the cylinder pressure solid line in Fig 115c rises above the nonfiring engine level The flame spreads rapidly through that portion of the injected fuel which has already mixed with sufficient air to burn As the expansion process proceeds mixing between fuel air and burning gases continues accompanied by further combustion see Fig 115d At full load the mass of fuel injected is about 5 percent of the mass of air in the cylinder Increasing levels of black smoke in the exhaust limit the amount of fuel that can be burned efficiently The exhaust process is similar to that of the fourstroke SI engine At the end of the exhaust stroke the cycle starts again In the twostroke CI engine cycle compression fuel injection combustion and expansion processes are similar to the equivalent fourstroke cycle processes it is the intake and exhaust pressure which differ The sequence of events in a loopscavenged twostroke engine cycle is illustrated in Fig 116 In loopscavenged engines both exhaust and inlet ports are at the same end of the cylinder and are uncovered as the piston approaches BC see Fig 116a After the exhaust ports open the cylinder pressure falls rapidly in a blowdown process Fig 116b The inlet ports then open and once the cylinder pressure p falls below the inlet pressure p1 air flows into the cylinder The burned gases displaced by this fresh air continue to flow out of the exhaust port along with some of the fresh air Once the ports close as the piston starts the compression stroke compression fuelinjection fuelair mixing combustion and expansion processes act in the fourstroke CI engine cycle The diesel fuelinjection system consists of an injection pump delivery pipes and fuel injector nozzles Several different types of injection pumps and nozzles are used In one common fuel pump an inline pump design shown in Fig 117 a set of camdriven plungers one for each cylinder operate in closely fitting barrels Early in the stroke of change the inlet port is closed and the fuel trapped above the plunger is forced through a check valve into the injection nozzle The piston reaches a highspeed rate when the fuel starts into the injection line The injection nozzle Fig 118 has one or more holes through which the fuel sprays into the cylinder A springloaded valve closes these holes until the pressure in the injection line acting on part of the valve surface overcomes the spring force and opens the valve Injection starts shortly after the line pressure begins to rise Thus the phase of the pump camshaft relative to the engine crankshaft controls the start of injection Injection is stopped when the inlet port of the pump is uncovered by a helical groove in the pump plunger because the high pressure above the plunger is then released Fig 118 The amount of fuel injected which controls the load is determined by the injection pump cam design and the position of the helical groove Thus for a given cam design rotating the plunger and its helical groove varies the load Distributortype pumps have only one pump plunger and barrel which meters and distributes the fuel to all the injection nozzles A schematic of a distributortype pump is shown in Fig 119 The unit contains a lowpressure fuel pump on left a highpressure injection pump on right an overspeed governor and an injection timer High pressure is generated by the plunger which is made to describe a combined rotary and stroke movement by the rotating eccentric disc or cam plate the rotary motion distributes the fuel to the individual injection nozzles Diesel fuel system with inline fuelinjection pump type PE 12 Courtesy Robert Bosch GmbH Diesel fuel system with distributortype fuelinjection pump with mechanical governor12 Courtesy Robert Bosch GmbH Details of fuelinjection nozzles nozzle holder assembly and fueldelivery control Courtesy Robert Bosch GmbH Fourcylinder naturally aspirated indirectinjection automobile Volkswagen diesel engine14 Displaced volume 147 liters bore 765 mm stroke 80 mm maximum power 37 kW at 5000 revmin V8 aircooled directinjection naturally aspirated diesel engine Displacement 134 liter bore 128 mm stroke 130 mm compression ratio 17 maximum rated power 188 kW at rated speed of 2300 revmin Courtesy KlocknerHumboldtDeutz AG15 and thereby avoid the fuel ignitionquality requirement of the diesel 3 control ling the engine power level by varying the amount of fuel injected per cycle with the air flow unthrottled to minimize work done pumping the fresh charge into the cylinder Such engines are often called stratifiedcharge engines from the need to produce in the mixing process between the fuel jet and the air in the cylinder a stratified fuelair mixture with an easily ignitable composition at the spark plug at the time of ignition Because such engines avoid the sparkignition engine requirement for fuels with a high antiknock quality and the diesel requirement for fuels with high ignition quality they are usually fueltolerant and will operate with a wide range of liquid fuels Many different types of stratifiedcharge engine have been proposed and some have been partially or fully developed A few have even been used in practice in automotive applications The operating principles of those that are truly fueltolerant or multifuel engines are illustrated in Fig 125 The combustion chamber is usually a bowlinpiston design and a high degree of air swirl is created during intake and enhanced in the piston bowl during compression to achieve rapid fuelair mixing Fuel is injected into the cylinder tangentially into the bowl during the latter stages of compression A longduration spark discharge ignites the developing fuelair jet as it passes the spark plug The flame spreads downstream and envelops and consumes the fuelair mixture Mixing continues and the final stages of combustion are completed during expansion Most successful designs of this type of engine have used the fourstroke cycle This concept is usually called a directinjection stratifiedcharge engine The engine can be turbocharged to increase its power density A commercial multifuel engine is shown in Fig 126 In this particular design the fuel injector comes diagonally through the cylinder head from the upper left and injects the fuel onto the hot wall of the deep spherical piston bowl The fuel is carried around the wall of the bowl by the swirling flow evaporated off the wall mixed with air and then ignited by the discharge at the spark plug which enters the chamber vertically on the right This particular engine is air cooled so the cylinder block and head are finned to increase surface area An alternative stratifiedcharge engine concept which has also been mass produced uses a small prechamber fed during intake with an auxiliary fuel system to obtain an easily ignitable mixture around the spark plug This concept first proposed by Ricardo in the 1920s and extensively developed in the Soviet Union and Japan is often called a jetignition or torchignition stratifiedcharge engine Its operating principles are illustrated in Fig 127 which shows a threevalve car bureted version of the concept A separate carburetor and intake manifold feeds a fuelrich mixture which contains fuel beyond the amount that can be burned with the available air through a separate small intake valve into the prechamber which contains excess air beyond that required to burn the fuel completely is fed to the main combustion chamber through the main carburetor and intake manifold After intake valve closing lean mixture from the main chamber is compressed into the prechamber bringing the mixture at the spark plug to an easily ignitable slightly rich composition After combustion starts in the prechamber rich burning mixture issues as a jet through the orifice into the main chamber entraining and igniting the lean main chamber charge Though called a stratified charge engine this engine is really a jetignition concept whose primary function is to extend the operating limit of conventionally ignited sparkignition engines to mixtures leaner than could normally be burned